ionic polymerization and living polymers

Ionic Polymerization

and Uving Polymers Michael Szwarc

Marcel Van Beylen

m SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

© 1993 Springer Science+Business Media Dordrecht Originally published by Chapman & HalI, Inc. in 1993 Softcover reprint of the hardcover 1 st edition 1993

AlI rights reserved. No part of this book may be reprinted or reproduced or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or by an information storage or retrieval system, without permission in writing from the publishers.

Library of Congress Cataloglng-in-Publicatlon Data

Szwarc, Michael. Ionic polymerization and living polymers I Michael Szwarc and

Marcel Van Beylen. p. cm.

Includes index. ISBN 978-94-010-4649-7 ISBN 978-94-011-1478-3 (eBook) DOI 10.1007/978-94-011-1478-3 1. Additional polymerization. I. Van Beylen, Marcel. m. Title.

QD281.P6S99 1993 547'.28--dc20 92-2344

CIP

British Lil>rary Cataloguing in Publication Data also available.

to Marysia and Chris

Contents

Preface ix

1. Introduction 1 1. Comparison of Ionic and Radical Polymerizations 1 2. Developments of Ionic Polymerization 5 3. Living and Dormant Polymers 12

3.1. Definition of Living Polymers and Their Stability 12 3.2. The "Seeding" Technique 19 3.3. Dormant Polymers; Rate Constant of Propagation 21 3.4. Determination of the Concentration of Living

Polymers 22 3.5. Synthetic Value of Living Polymerization 24

4. Polymerizability and the Monomer-Living Polymer Equilibria 25

2. Ionic Species 39 1. Formation of Ions 39 2. Ionophores and Ionogens, Ion-Pairs, and Covalent

Species 41 3. Free Ions and Ion-Pairs 43 4. Different Types of Ion-Pairs 48 5. Equilibria Between Free Ions and Ion-Pairs:

Conductance Studies 58 6. Heat and Entropy of Dissociation of Ion-Pairs 64 7. Triple Ions 65 8. Higher Ionic Aggregates 70 9. Dynamics of Interconversions Between Ionic Species 72

3. Initiation and Propagation of Ionic Polymerization 87 1. General Remarks 87 2. Initiation and Propagation Induced by Salts 89

2.1. Initiation and Propagation of Polymerization by Free Ions 89

v

vi / Contents

2.2. Initiation and Propagation of Polymerization by Ion-Pairs 92

2.3. Initiation and Propagation of Cationic Polymerization by Free Ions and Ion-Pairs 130

2.4. Role of the Solvent 141 2.5. Initiation and Propagation of Ionic Polymerization

in the Gaseous Phase 144 3. Initiation and Propagation Involving Lithium Alkyls 147

3.1. Structure of Alkyl Lithiums 147 3.2. Initiation of Anionic Polymerization by Alkyl

Lithiums 150 3.3. Interaction of Alkyl Lithiums with Solvating or

Complexing Agents 157 3.4. Bifunctional Lithium Initiators 158 3.5. Propagation of Polystyrene Initiated by Alkyl

Lithiums 159 3.6. Propagation of Lithium Polydienes 162 3.7. Stereochemistry of Propagation of Lithium

Polydienes 165 3.B. Effect of Solvating Agents on Propagation Induced

by Alkyl Lithiums 169 4. Initiation of Cationic Polymerization by Protonic

Acids: Role of Esters (Including Halides) 172 4.1. General Observations 172 4.2. Self-Associations of Protonic Acids and the

Homoconjugation 174 4.3. Mechanism of Protonation of Monomers 177 4.4. Problem of Covalent-Ionic Equilibria 179 4.5. Esters Derived from Vinyl Monomers 186 4.6. Stopped-Flow Experiments 194 4.7. Living Cationic Polymerization 202 4.B. Uniformity of Polymers 210 4.9. Some Comments on the Kinetics of

Polymerization of Vinyl Ethers Initiated by the HI-I2 System 212

4.10. Some Experimental Results 213 5. Initiation of Polymerization by Lewis Bases and Acids:

Problem of Zwitterions 215 5.1. Lewis Bases 215 5.2. Lewis Acids 218 5.3. Spontaneously Initiated Zwitterionic

Copolymerizations 221 6. Initiation of Cationic Polymerization by Friedel-Crafts

Reagents in Conjunction with Co-initiators 223

Contents / vii

6.1. General Features of Friedel-Crafts Reagents 223 6.2. Complexes of FC Reagents with Protogens 225 6.3. Initiation of Cationic Polymerization by FC

Reagents in Conjunction with Cationogens 227 7. Initiation of Polymerization by Electron Transfer and

Related Topics 230 7.1. General 230 7.2. Solutions of Alkali Metals 232 7.3. Structure and Properties of Solvated Electrons

and Negative Alkali Ions 234 7.4. Reactivity of Alkali Metal Solutions 236 7.5. Homogeneous Electron Transfer; Initiation of

Anionic Polymerization 239 7.6. Initiation of Anionic Polymerization by a

Heterogeneous Electron Transfer 247 7.7. Formation of Radical-Cations 249 7.8. Initiation of Cationic Polymerization by Electron

Transfer 251 7.9. Initiation of Radical Chain Process by Electron

Transfer 257 7.10. Two Simultaneous Homopolymerizations 259 7.11. Field Emission, Field Ionization, and Electrolysis 262

8. Polymerization Initiated by High Energy Radiation 264 8.1. Ionizing Radiation 264 8.2. Radiolysis of Monomers 265 8.3. Kinetics of Radiation-Induced Polymerization 267 8.4. Solvent Effects in ,,(-Initiated Polymerization of

Vinyl Ethers 270 9. Initiators Involving AI, Fe, Zn, and Other

Metallo-Organics 274 9.1. Al-Zn Oxyalkoxides 274 9.2. Pseudoanionic Polymerization of E-Caprolactone 277 9.3. Metalloporphyrins-Catalysts 282 9.4. Stereoselection and Stereoelection 284 9.5. Group-Transfer Polymerization 288

10. Activated Monomer Mechanism 292 10.1. Polymerization of NCA Initiated by Primary and

Secondary Amines 292 10.2. Polymerization Initiated by Aprotic Bases 293 10.3. The Activated Monomer Mechanism in Lactam

Polymerization 295 10.4. The Real Systems 297 10.5. Cationic Polymerization of Lactams 298

viii I Contents

10.6. Activated Monomer Propagation in Cationic Ring Opening Polymerization of Cyclic Ethers 299

4. Elementary Steps of Polymerization Other Than Initiation or Propagation 319 1. Termination 319

1.1. General Remarks 319 1.2. Unimolecular Termination 320 1.3. Bimolecular Termination Involving Two Growing

Polymers 324 1.4. Other Bimolecular Terminations 325 1.5. Proper Termination Caused by Solvents or

Impurities 325 2. Intramolecular Proton or Hydride Ion Transfers and

Other Isomerizations 326 3. "Wrong" Monomer Addition 328 4. Chain-Transfer 330 5. Proton Traps 332 6. Polymerization of Isobutene Initiated by Cumyl

Chloride and Its Analogues 333 7. Ring-Chain Competitions 334

7.1. Branching and Ring Formation 334 7.2. Ring-Chain Equilibrium 335 7.3. Overview of Earlier Observations 336 7.4. Simultaneous Polymerization of Propylene Sulfide

and Cyclization of Its Polymer 339 7.5. Alteration of Growing Polymer End-Groups 341

S. Ionic Copolymerization 347 1. General Schemes of Copolymerization and the

Crossover Constants 347 2. Reactivity and Basicity of Monomers 354 3. Reactivity Ratios: Composition of Copolymer and Feed 354 4. Copolymerization and Homopolymerization 358 5. System Li-Polystyryl and Li-Polybutadienyl in

Hydrocarbon Solvents 359 6. Consequences of Equilibria Between Unreacted

Aggregates and Reactive Nonassociated Polymers: Mixed Dimerizations 361

7. Reversible Copolymerization: System 2-Vinylnaphthalene-Styrene 366

8. Cationic Copolymerizations 373

Index 377

Preface

Two basic questions, namely "why" and "how," dominate the treatment of any scientific or technological problem. In this work we are more often concerned with the question "why" than "how." We try here to rationalize and explain why the various ionic processes yielding polymeric molecules proceed as observed, and how their rates and the equilibria established in these systems are affected by the external conditions and by the structure of the reagents. Moreover, we look upon anionic and cationic polymerizations from a comparative point of view, stressing, wherever possible, the similarity and differences between these two modes of the reaction.

The scope of the known ionic polymerizations is too large to allow a comprehensive treatment of all of them in a volume of moderate size. Hence, some subjective selection of topics is inevitable and, not surprisingly, those studied in the course of our own investigations gained most of our attention.

The statements made in this monograph do not claim to be authoritative. We expressed our opinions concerning some controversial subjects, trying then to justify them by the results of reported experiments and by logical arguments. We proposed a few experiments that might clarify the problems under consideration or perhaps enlarge their scope. We did not hesitate to outline some hypothetical schemes in the hope that they might induce the reader to conceive better solutions for the debated questions.

It is not our intention to survey all the available literature. Some valuable papers dealing with important subjects have been omitted due to lack of space. For example, the microstructure of polymer chains and the pertinent statistical considerations might have been discussed more extensively, but the space restriction prevented further elaboration of this subject.

This monograph is composed of five chapters. In Chapter 1 the ionic addition polymerization is contrasted with the more widely known radical process. The historical development of ionic polymerizations is then briefly outlined. The terms "living polymers" and "living polymerization" are

be

xl Preface

defined, the significance of "living polymers" for polymer science is stressed, and its relation to the thermodynamic of propagation is elaborated.

We believe that a general discussion of the modes of formation of ions and of their behavior in solution would be helpful for the readers of this monograph. Therefore, we devoted Chapter 2 to a review of this subject, emphasizing the problems of association of free ions into ion-pairs and still higher aggregates.

The initiation and propagation of ionic polymerizations are the topics covered in Chapter 3. Combined treatment of the propagation processes and those concerned with the initiation seems justified. Indeed, propagation may be looked upon as the initiation of further growth of polymeric molecules induced by the active polymers. Other processes affecting the course of polymerization, such as termination, chain-transfer, and backbiting, are treated in Chapter 4, whereas Chapter 5 is devoted to ionic copolymerization.

Finally, we wish to thank our friends and colleagues, Dr. S. Bywater, Profs. K. Matyjaszewski, G. Olah, S. Penczek, A. Persoons, P. H. Plesch, Dr. M. Sawamoto, Prof. V. Stannett, Prof. J. P. Vairon, and Dr. O. W. Webster, listed here in alphabetic order, for their comments and discussions most helpful in writing this book. One of us (M.V.B.) wishes to thank the National Fund for Scientific Research of Belgium for their financial support during the preparation of this monograph.

1

Introduction

1. Comparison of Ionic and Radical Polymerizations

Ionic polymerization, like the well-known radical polymerization, is a chain polyaddition. It starts with a reaction of a monomer with a species capable of forming an electrically charged or highly polar active group on the added monomer molecule. Interaction of this group with another molecule of the monomer results in the formation of a covalent bond linking these two species, and simultaneously regenerates the active group on the newly added molecule. Such a repetitive covalent bonding of the monomer molecules to the active end-group, and simultaneously the repeated regeneration of the latter on the added unit, produces a growing polymer chain.

Ionic polymerization is referred to as a classic cationic when the active terminal group is positively charged, or as a pseudocationic (or a cationic coordination) if this group forms the positive end of an active dipole. By the same token we refer to ionic polymerization as a classic anionic when the charge of the active group is negative, or as a pseudoanionic (or an anionic coordination) when the active group forms the negative end of an active dipole. Whenever electrically charged end-groups are formed, suitable counterions have to be present in the polymerizing system to ensure its electric neutrality.

It is instructive to compare the radical and ionic modes of polymerization. Radical polymerization is initiated by adding to a molecule of monomer a small radical produced from a suitable initiator. The added radical then forms the tail-group of the ultimately produced polymer, but its nature, or the nature of the initiator, does not influence the propagation rate constant, the selectivity, or the stereochemistry of the ensuing propagation. All these features of radical propagation are determined by the nature of

1

2 / Ionic Polymerization and Living Polymers

the polymerized monomer and by the conditions prevailing in the reaction, such as temperature, pressure, and, to a lesser degree, the solvent's nature. Although the influence of some additives on the course of radical propagation has been discussed in the literature, l the pertinent effects are minor in most of the conventional radical polymerizations.

In contradistinction, the nature of the initiator does affect, and often in a most profound way, the rate constant and the mode of ionic propagation. The initiator, or some of its fragments, e.g., the counterion, is associated in some fashion with the growing center during the whole course of propagation. Hence, the character of ionic homopropagation may be altered substantially when the nature of the initiator is varied.

The active end-groups of radically growing homopolymers are uniform in their character, all of them being identical. In this respect ionically growing polymers are different. Their active end-groups often consist of a mixture of different species, even when the employed initiator has a unique structure. This peculiarity of ionic polymerization is due to the changeable character of their propagating groups. For example, the active end-groups of some ionically growing polymers may exist as ion-pairs. However, ionpairs dissociate into free ions with which they are in equilibrium, and then both species, the undissociated pairs and the free ions, contribute to propagation, each in its specific and distinct fashion. Since the observed rate and mode of polymerization depend on the mole fractions of these species in the reacting mixture, and the latter are affected by temperature, the nature of the solvent, and so forth, the course of ionic polymerization is more complex, but also more versatile, than that of radical polymerization.

Interaction of radicals with their surroundings is usually weak. Therefore, the nature of the solvent, or of the penultimate unit, affects marginally the rate of most of the radical polymerizations, and the resulting effects can be often ignored. This is not so in ionic polymerization. The solvent, penultimate units, and even some more remote units may strongly affect the course of ionic propagation. Indeed, the nature of the solvent is often a decisive factor in determining the mode and even the feasibility of some ionic polymerizations.

The class of monomers polymerized by radical mechanism is limited to the vinyl, vinylidene, and diene types, whereas many additional monomers, e.g., aldehydes, ketones, numerous heterocyclics, and so forth, not polymerizable by the radical technique, are polymerized by ionic procedures. The larger versatility of ionic polymerization results from the ability of ionic or polar end-groups of such polymers to polarize the oncoming monomer and facilitate its addition, or from the capacity of the counterions to coordinate with the monomer prior to its ultimate addition and thus make propagation feasible. Such interactions are especially powerful when the propagation involves a reaction of the active end-group with some highly

Introduction I 3

polarizable heterobonds of a monomer (e .g., C=O, C=N, etc.) with which radicals interact only weakly. Polarization of the reacting monomer reduces the potential energy barrier hindering the addition. For this reason, the propagation rate constant of any monomer is substantially higher for the polymerization induced by free ions than for those proceeding by a free radical mechanism.

Polymerization of iso-butene deserves special consideration. It is well known that this monomer is not polymerized either by a radical mechanism or anionically. A cationic mode of polymerization is the only one yielding high molecular weight poly-iso-butene. The inability of radical polymerization to produce such polymers is not caused by the inertness of this monomer toward free radical additions. In fact, the addition of methyl radicals to iso-butene is faster than to ethylene2 which yields high molecular weight polymer by a radical mechanism. The lack of radical polymerization of iso-butene stems from a relatively rapid chain-transfer to monomer compared with its rate of propagation. The former reaction has a higher activation energy than the latter. Hence, a high molecular weight poly-isobutene might be formed by radical mechanism at very low temperature, had the activation energy of its propagation been lower. The activation energy of propagation of cationically polymerized iso-butene is indeed low, hence the formation of its high molecular mass polymer becomes possible at low temperatures, e.g., at -70°C, whereas the radical propagation is then too slow to be observed.

Chain reactions are expected to be terminated by some events. Collision of two free radicals annihilates them through their coupling or disproportionation. Since both these reactions are very fast, virtually diffusion controlled, this unavoidable bimolecular termination of freely mobile radical polymers* makes their lifetime very short, often less than a few seconds. In contrast, in most of the ionic polymerizations the activity of growing polymers is not affected by their mutual encounters, hence their collisions with each other do not destroy their capacity of growth.

The unavoidable bimolecular termination, characteristic of radical polymerization, imposes a well-known relation of the rate of polymerization on the molecular weight of the resulting polymers, namely the higher the rate the lower the molecular weight. Although this kind of relation is observed in some ionic polymerizations, e.g., in the cationic polymerization induced in bulk monomers by 'Y radiation,3 this rule does not apply to the majority of ionic polymerizations.

"Diffusion of polymers is slowed down in viscous media and the termination is then retarded. Precipitation of polymers virtually prevents the encounters between their radical endgroups and hence their annihilation. Under such conditions, "buried" radicals are formed which, in the presence of monomer, grow virtually without being terminated.


The bimolecular character of the unavoidable termination of a conventional radical polymerization limits also the maximum concentration of growing polymers. Their stationary concentration rarely exceeds 10-7 M

whenever the reaction produces high molecular mass macromolecules. No restrictions of that kind are imposed on the concentration of growing polymers in ionic polymerizations. In fact, it is not uncommon to find ionic systems in which the concentration of growing polymers is as high as 10-2 M.

The inevitability of bimolecular termination in radical polymerizations demands an unceasing supply of small radicals that produce new growing polymers replenishing those that become terminated. This is necessary for the perpetuation of polymerization. It follows that most of the macromolecules present in a radical polymerization are dead, incapable of growing further, and only a minute fraction of them contributes to the propagation at any time. This accounts for the nearly constant degree of polymerization of the formed macromolecules established shortly after the onset of radical polymerization, and independent of the degree of monomer conversion.

In numerous ionic polymerizations it is possible to avoid termination and chain-transfer. Under such conditions the resulting polymers are living, i.e., they retain their capacity to add monomer and continue their growth for a long time. Their molecular mass increases then with the monomer conversion. For a rapid initiation such an increase is proportional to the degree of the conversion, and the resulting polymers have virtually uniform size, provided that the termination and chain-transfer are rigorously excluded. The uniformity of polymers sizes is lost, however, for slow initiation, since then new polymers are formed while those previously produced still continue their growth.

Anionic and cationic polymerizations have many common features. Some differences are caused by the much greater reactivity of the cationically growing polymers than of those propagated anionically. This is reflected in the much greater susceptibility of the growing cationic polymers to damaging impurities than that of the anionic ones, and it makes their shelf time much shorter than that of the anionically growing polymers.

The interest in ionic polymerization increased sharply during the last 40 years. Discovery of living polymerization4 and of the stereospecific polymerizations of isoprene induced by lithium metal,5 both proceeding by an anionic mechanism, led to a truly exponential growth of this field. Since many cheap monomers, unpolymerizable by radical methods, are polymerized ionically, a multitude of new, commercially valuable products became available after development of ionic modes of polymerization.

Utilization of living polymers led to preparation of numerous speciality macromolecules (e.g., a variety of block polymers, star-shaped polymers, etc.; see ref. 6) that fulfill the needs of modern technology as well as

Introduction / 5

research. Not surprisingly, these techniques became industrially valuable and are employed today on an ever larger scale. The field is still rapidly expanding as manifested by the numerous publications appearing each year in scientific and technical journals.

2. Developments of Ionic Polymerization

The modern ideas of polymers were developed in 1920 by Staudinger7

who was the first to propose the existence of linear macromolecules composed of monomeric units linked by covalent bonds. The likelihood of formation of large molecular aggregates, albeit not covalently bonded, was visualized even earlier; for example, in 1915 in a paper by Ostromysslensky,8 the viscous hydrocarbon resins were described as large aggregates produced by a sequence of many bimolecular reactions.

The idea of consecutive reactions leading to macromolecules found general acceptance from quite an early stage in the development of polymer chemistry. However, the exact nature of these processes was a subject of controversies. Staudinger9 advocated chain addition reaction as the mechanism yielding linear polymers, but opposing views were expressed by other investigators; for example, Whitby and KatzlO treated the polymerization of indene induced by SnCl4 as a stepwise process. The nature of the active species was again controversial. It has been easy to visualize a stepwise polymerization, but the explanation of chain polyaddition was confused in those days. In his first papers, Staudinger9 described the growing intermediates as chains possessing free valences at their ends, i.e., in our modern language as free biradicals, but the discussion of such a structure was couched in vague general terms. The first convincing evidence ofthe radical nature of ethylene polymerization initiated by the decomposition of diethyl mercury was presented by Taylor and Jonesll in 1930, and subsequent studies of many investigators fully verified this idea.

Although the soundness of the radical mechanism of many addition polymerizations is unquestionable, it was not realized in that early period that a monomer might be polymerized by two, or more, distinct reaction modes, each involving different kinds of intermediates. It was Whitmorel2

who first proposed in 1934 the formation of an ionic intermediate that accounts for the polymerization of isobutene initiated by acids; the alternative radical mechanism, conceptually still feasible, had not been considered.

The idea that two distinct kinds of reactions may lead to polymerization of the same monomer was suggested first by Williams.13 In a paper published in 1940, he stressed the profound difference between the polymerization of styrene initiated by SnCl4 and that induced by the decomposition of benzoyl peroxide. His results clearly demonstrate that the same monomer


is polymerized by two different reaction modes, later identified as cationic and free radical. 14 Thus the concept of cationic polymerization propagated by carbenium ions was soundly established.

The concept of anionic polymerization propagated by carbanions was developed later, although the feasibility of polymerization involving alkoxide groups was realized much earlier. Indeed, Staudinger's appreciation of the anionic character of formaldehyde polymerization initiated by bases such as sodium methoxide is evident from reading his paper, and a similar mechanism was contemplated for the polymerization of ethylene oxide. Condensations involving the -Q-alkali were palatable in those days, but a clear concept of carbanions did not exist yet. Interestingly, the polymerization of ethylene oxide catalyzed by alkali metals,15 although not recognized as such a reaction, was reported as early as 1878.

The first process being interpreted now as cationic polymerization16 was described in 1789 by Watson, who converted turpentine into a resin by the action of sulfuric acid. The first cationic polymerization of styrene induced by SnCl4 was achieved by Deville17 in 1839, and the condensation of benzyl alcohol into ill-defined branched polymer was reported by Canizzaro18 in 1854. The cationic polymerization of styrene by sulfuric acid was observed by Berthelot19 in 1866, and a year later vinyl ether was polymerized by Wislicenius20 by the action of iodine.

Oligomerization of isobutene and propene by BF3, obviously a cationic process, was described by Butlerov and Gorianov21 in 1873, and a few years later the formation of resins resulting from the addition of AICl3 to benzyl chloride was reported by Friedel and Crafts.22 Of course, none of the above authors referred to the reported products as polymers, nor was any attempt made to provide a mechanism for their formation.

As mentioned earlier, the deliberate and conscientious studies of cationic oligomerization of olefines induced by protic acids were reported by Whitmore12 in 1934. He was the first to describe the process in modern terms, namely as the addition of proton to a C=C bond leading to the formation of a carbenium ion which, in turn, adds the next molecule of olefin. Not surprisingly, similar processes, such as the polymerization of isobutene induced by BF3, were treated then in a somewhat analogous fashion, namely, as the additions of the initiator to a C=C bond resulting in the formation of zwitterions23:

which perpetuate the cationic propagation. This mechanism was challenged by Polanyi24 and his co-workers who stressed the difficulties of separation of electric charges in low polarity solvents, and demonstrated subsequently

Introduction / 7

the necessity of introducing a co-initiator, e.g., water, which induces the polymerization (see ref. 25). The subject of co-initiators is discussed in a later chapter (see p. 223).

Studies of the important cationic polymerizations of heterocyclic compounds, e.g., tetrahydrofuran, arose from the pioneering investigations of Meerwein26 started in the late 1930s but unknown to many western chemists until the end of the World War II. The discussion of this broad subject, revealing many mechanistic details of this class of reactions, is covered in Chapter 3.

The first reports describing processes classified today as anionic polymerizations appeared at the end of the last century. At that time, several authors reported the formation of gums and resins produced under the influence of alkali metals. Deliberate activities in this field began in the first decades of this century. A patent issued to Matthews and Strange27

claimed polymerization of dienes initiated by metallic sodium. In the following year, Harris28 published his pioneering studies of isoprene polymerization, and 3 years later, the formation of presumably high molecular weight polymers of styrene and of 1-phenylbutadiene was reported by Schlenk29 and his students who reacted these monomers in ether solution with sodium dust. Since the concepts of electron-transfer and radical-ions were unknown in those days, the pwducts of these reactions were described, in vague terms, as a kind of complex.

In the following years, the investigators of chain polyadditions were mainly concerned with the free radical reactions, the interest in anionic polymerization being marginal. The activities in this field centered at that time around Ziegler in Germany and Lebedev in Russia. Both groups were interested in polymerization of styrene and the dienes by metallic sodium, and their work led eventually to the industrial production of synthetic rubber marketed by I. G. Farben Industry as "Buna."

The results of the systematic studies of Ziegler3° led to his formulation of the first step of the reaction of sodium or lithium metal with olefins or dienes as an addition of two alkali atoms to unsaturated carbon atoms forming two C-Na or C-Li covalent bonds, e.g.,

Isolation of such adducts eluded him; nevertheless he argued for their existence by demonstrating30 the formation of 2-butene in the reaction of butadiene with sodium performed in the presence of an excess of methylaniline. Although the latter compound does not react with sodium directly, 2 moles of sodium anilide were produced for each mole of the formed


butene under the conditions of these experiments. These results were accounted for by the scheme:

Na-CH2CH:CHCH2-Na + 2 PhNH(CH3) ~

CH3CH=CHCH3 + 2 PhN(CH3)Na.

The concepts of carbanions, radical-anions, and ion-pairs were still in infancy at that time. In our modern description these reactions would be formulated, most probably, as a sequence of steps:

CH2:CHCH:CH2 + Na ~ PhNHCH3

(CH2:CHCH:CH2)T, Na+ ) CH3CH:CHCHi

+ PhN(CH3)Na,

followed by

PhNHCH3

CH3CH:CHCH; , Na + ) CH3CH:CHCH3

+ PhNCH3Na.

The ensuing propagation was described by Ziegler as an insertion of the monomer into the C-Na bond generating a new C-Na end-group, a description insignificantly different from our present formulation. Not surprisingly, it was superfluous to postulate any termination step in this mechanism; after cessation of the polymerization the resulting polymers still possessed the active C-Na bonds. Indeed, after consumption of the initially supplied monomer the polymerization resumed when fresh monomer was added to the reactor. 31

An alternative mechanism of the above reaction was advocated by Schlenk and Bergmann. In a paper29 reported in 1914, Schlenk described the addition of alkali metal to aromatic hydrocarbons in ether solution. The resulting intensely colored adducts were referred to as 1:1 complexes. A similar reaction of 1,I-diphenylethylene produced a colored dimer which, after carboxylation, yielded the 1,1,4,4-tetraphenyl adipic acid. These studies were resumed again in the 1920s when the concept of free radicals was well established. At that time Schlenk and Bergmann32 argued that the initially formed 1:1 adducts are free radicals, e.g.,

CH2:CHCH:CH2 + Na ~ NaCH2CH:CHCHi,

Introduction / 9

which initiate a free radical polymerization of the dienes. The success of the free radical theory of polymerization added credibility to their suggestion, causing some regression. Thus, Schulz33 in 1938 and Bolland34 in 1941 were still upholding the idea of the radical nature of the polymerization of dienes induced by alkali metals.

Nevertheless, slow progress was made. Abkin and Medvedev35 demonstrated the long lifetime of the growing centers formed in the above polymerization. The device used in their experiments is shown in Fig. 1.1. Metallic sodium was placed in bulb A and thereafter butadiene was distilled into it. After closing the upper stopcock, the progress of the ensuing reaction was monitored by observing the pressure drop on the attached mercury manometer. After a while the residual monomer was distilled into bulb B and the connecting stopcock was closed. The pressure in the system remained constant proving the absence of initiating species in B. Since the free radicals dimerize, they would disappear if kept in bulb A for a sufficiently long time. However, when the monomer was back-distilled into A, the polymerization resumed with its previous rate. The authors concluded, therefore, that the active species could not be the free radicals, but no alternative suggestion about their nature was proposed. The het-

B

Na metal

Figure 1.1. The apparatus used by Abkin and Medvedev for demonstrating the long-lived nature of the species responsible for anionic polymerization of butadiene.


erogeneous nature of the reaction could allow for an explanation invoking "buried" radicals, which, however, would be incorrect.

In the following years several processes were described as anionic polymerizations. Blomquist36 reported the anionic polymerization of nitro-olefins initiated by hydroxyl ions. Beaman37 recognized the anionic character of methacrylonitrile polymerization initiated in ether solution by Grignard reagents. Robertson and Marion38 reinvestigated the sodium-initiated polymerization of butadiene in toluene and isolated oligomers having benzyl moieties as their end-groups. The characteristic red color developed in the course of the reaction was interpreted as evidence of the formation of benzyl sodium through proton transfer from the solvent to the carbanionic end-group of the resulting polymers. Studies of homogeneous polymerizations induced by potassium, or potassium amide, in liquid ammonia solution, to be discussed later (see p. 93), left no doubt about their anionic character.

The final impetus for vigorous studies of anionic polymerization came in 1956 when two papers were published. Szwarc4 described the homogeneous electron-transfer-initiated polymerization of styrene and isoprene and conclusively demonstrated the lack of termination and chain-transfer in this process. This led him to introduce the concept of living polymers. Stavely5 reported the discovery of the stereospecific polymerization of isoprene initiated by metallic lithium in hydrocarbon solvents producing the all cis-1,4-polyisoprene. Since then the development of anionic polymerization progressed in a truly exponential fashion.

The development of coordination polymerization started conscientiously in the early 1950s after the epoch-making discovery by Ziegler,39 who induced by titanium complexes a low-pressure polymerization of ethylene into a very high molecular weight crystallinic polymer. The impoitance of this discovery was enormously magnified by the spectacular work of Natta and his co-workers,40 who demonstrated that the initiators developed by Ziegler induce stereospecific polymerization of propylene. He coined also the terms isotactic and syndiotactic polymerizations that denote the two possible stereoregular enchainments of vinyl or vinylidene units in such polymers.

The novelty of the mechanism of coordination polymerizations was recognized from the very start of development of this field, although its details were confused and miscomprehended at the beginning. For example, much of the early discussions were concerned with the question of whether the initiator is bimetallic or not. The basic features of this reaction were described correctly by Price,41 who proposed the coordination-rearrangement process for the propagation. In this mechanism a monomer becomes coordinated with the active center of the initiator, which need not be necessarily a transition metal although it is in the original Ziegler and Natta

Introduction I 11

initiators, and thereafter through a rearrangement becomes inserted into a covalent bond linking the active center to a growing polymer chain. As pointed out by Price,41 this mechanism applies also to ring opening polymerization of propylene oxide induced by some modified ferric chlorides.

In the following years numerous research studies led to the discovery of a wide spectrum of initiators capable of similar action. Various novel coordination polymerizations were reported, and interesting modifications of Price's scheme have been contemplated by many authors. Although the heterogeneous nature of the initiator, i.e., its solid insoluble state, was considered initially to be indispensable for inducing stereoregularity in the resulting polymers, soluble initiators endowed with this property were discovered eventually.42 Some soluble coordination initiators induce polymerization free of termination and chain-transfer, i.e., yield living polymers. The best example of such a process was reported by Inoue,43,44 who developed initiators based on aluminum complexes of porphirines capable of producing living polyoxiranes.

Finally, let us comment on the discovery of living polymers. In the course of his studies of polymerization of butadiene initiated by lithium alkyls, Ziegler31 observed the reinitiation of polymerization on the addition of fresh monomer to quiescent mass of polymers produced by the preceding polymerization that consumed the previously supplied monomer. Moreover, the molecular mass of the subsequently formed polymers was higher than that of the previous ones. This observation, that indicated lack of termination, did not arouse interest because the phenomenon of termination was not appreciated in those days (see p. 8), and the heterogeneous nature of the milieu hampered quantitative studies of this process.

It was 20 years later when one of us clearly and conclusively demonstrated the formation of polymers retaining their ability of growth in a homogeneous system48 and outlined all the ramifications arising from the lack of termination and chain-transfer. After quantitative polymerization of a batch of styrene, the reaction not only resumed on the addition of fresh monomer, but the molecular mass of the newly formed polymers was higher than that of the previous ones, proving that the active macromolecules are still capable of growing further. Moreover, examination of the product resulting from the addition of isoprene to the previously polymerized styrene demonstrated the formation of polystyrene-polyisoprene block copolymers with exclusion of the homopolymers. Thus, not only termination but also chain-transfer to monomer are absent in this system showing a remarkable retention of the activity of those macromolecules named, therefore, as living polymers.

Although the first living polymers were produced by anionic technique, within less than a decade the living character of cationic polymerization of tetrahydrofuran was demonstrated simultaneously by three research


teams,45-47 and a few years later, the living nature of coordination polymerization of caprolacton was proved by Teyssie and his co-workers.48 An exceptionally clean coordination process yielding living diblock and triblock polyoxiranes was described in the late 1980s by Inoue. 43,44

During the last 15 years several new living polymer systems were discovered. An interesting group-transfer polymerization yielding living polymethylmethacrylate was developed by Webster and his associates.49 Living cationic polymerization of vinyl ethers was reported by Higashimura and Sawamot050 in 1982, and 2 years later Kennedy and his co-workers51 developed living cationic polymerization of isobutene. A most promising living polymer obtained by the ring opening polymerization of strained cyclic olefines induced by the complexes of transition metals was described by Grubbs.52 Undoubtedly, many new systems yielding living polymers will be developed in the near future.

3. Living and Dormant Polymers

3.1 Definition of Living Polymers and Their Stability

Special attention is paid in this volume to living polymers and living polymerizations. Living polymerization provides the deepest insight into the mechanisms of ionic polymerizations, and allows the most detailed control of the size and the structure of the resulting macromolecules.

Originally, polymerization was considered to be living when growing polymers retain indefinitely their propensity of growth and their propagation proceeds with exclusion of termination and chain-transfer. 4 However, in time any living polymer eventually decomposes, or isomerizes, or reacts with its surroundings, and subsequently loses its activity. Therefore, the first condition has to be relaxed and we shall refer to polymers as living53,54 if their end-groups retain the propensity of growth for at least as long a period as needed for the completion of an intended synthesis, or any other desired task. Indeed, in the very first paper4a introducing the concept of living polymers, its author explicitly stated that "living polymers are not immortal. "4at Hence, the above pragmatic definition is justified and applicable to any real living polymer system.

It is inconceivable for propagation to proceed with rigorous exclusion of termination or chain-transfer. This again is an ideal requirement. A billion of monomer molecules could not be added to a growing center

tIn several papers43 Inoue refers to the living polymerization of oxiranes initiated by Al porphirines as "immortal" polymerization. In these systems alcohols act as reversible chaintransfer agents, and not as the terminating agents like in most common anionic polymerizations. This propensity of alcohols led Inoue to use this unfortunate term in describing his most interesting polymerization.

Introduction / 13

without inducing termination or chain-transfer. It suffices if propagation yields a requested maximum molecular mass polymer without being interrupted by termination or chain-transfer. In fact, even this requirement could be relaxed. One is often satisfied if not more than say 2% of the growing species are terminated, or undergo chain-transfer, when polymers of the demanded size are formed. The above limit is arbitrary; it depends on the precision imposed on the quality of the producing polymers by the user or producer.

The initiation process was not mentioned in the above definitions. It is often desired to produce polymers of an approximately uniform size, referred to as polymers of "narrow" molecular mass distribution. This is achieved in a living polymerization when the rate of initiation is faster than or at least equal to the rate of propagation. For example, polymers of uniform size are formed when the propagation is initiated by the synthetically prepared "living monomeric polymers." The above term refers to low molecular mass species having the structure of the last growing unit of the desired living polymers. Hence:

1. C6H5.C-(CH3bK+ (cumyl potassium) is a "living monomeric poly-a-methylstyrene" that initiates anionic growth yielding living poly-a-methylstyrene.

2. The CH3CH(Ph).CI,SnCI4 complex55 is a "living monomeric polystyrene." On addition of styrene it initiates cationic polymerization resulting in the formation of living polystyrene.

3. The metallated iso-butyric ester56 is a "living monomeric polymethylmethacrylate" initiating anionic polymerization of added methylmethacrylate that results then in the formation of a living polymethylmethacrylate.

4. The CH3CH(OR).1 complexed with a quaternary ammonium salt57

is a "living monomeric polyvinyl ether." A cationically growing living polyvinyl ether is produced when vinyl ether is added to this complex.

Obviously, these initiators yield living polymers of uniform size since the first step of those polymerizations is faster than, or equal to, the following steps.

Lack of termination and chain-transfer does not imply that living polymerization yields infinitely long polymers.4 In every polymerization procedure some finite amount of monomer is reacted with a finite amount of initiator. Denoting by Mo the initial concentration of the monomer, and by 10 that of the initiator, one finds, after quantitative conversion of the

14/ Ionic Polymerization and Living Polymers

monomer into living polymers, the number average degree of polymerization of the resulting macromolecules is given by

DPn = Mollo,

provided that all the initiator was utilized effectively. The resulting polymers should have Poisson molecular mass distribution provided that the propagation is irreversible, the polymerized solution or melt remain homogeneous during the whole course of polymerization, the termination and transfer are rigorously excluded, and the initiation is not slower than propagation. Although all these conditions are beneficial and desired, they are not necessarily demanded for achieving a living polymerization.

How does one ascertain whether an investigated polymerizing system is truly living? Different criteria of "livingness" are employed by various research teams. The proportionality of the experimentally determined number average molecular mass of the produced polymers with the degree of monomer conversion is frequently claimed as a sufficient evidence of "livingness" of an investigated polymerization. Such a relation by itself is not adequate to prove the lack of termination and chain-transfer. In fact, a polymerization involving a slow termination, but free of chain-transfer, does conform to this criterion. However, the above relation in conjunction with the experimentally verified "narrow" molecular mass distribution of the produced polymers provides a satisfactory evidence for the absence of termination and chain-transfer in an investigated process.

Although the formation of polymers of narrow molecular mass distribution is only possible in a rapidly initiated living polymerization, a broad distribution does not imply that the polymerization is not living since various incidental factors could lead to its broadening, even when the propagation proceeds with exclusion of termination and chain-transfer.

Rapidly initiated irreversible polymerization free of termination, but not necessarily of chain-transfer, proceeds as a pseudo-first-order reaction, i.e.,

In([M]o/[M]t) = const. t, const = kp[P*].

This kinetic characteristic of a propagation in conjunction with the proportionality of the number average degree of polymerization, DP m with the monomer conversion leads to a relation derived by Penczek and his colleagues,58 namely

illustrated graphically by the plots shown in Figs. 1.2 and 1.3. The linearity of such plots proves definitely the absence of termination and chain-transfer

Introduction I 15

Convers ion in % o 20 40 60 80 90

c

I~ ::::02,0 ,.........--.---~---.--~----.----:70

'i: "'-e',5 I~c ~ 1,0 .f I

0,5

a

b

10

800

600

400

200

0 20 30

Time in s .

Figure 1.2. Dependence of In(1-[11oDPnl[M1o} on time for [M1o = 1 M, [110 = 10- 3 M, kp = 102 M- 1 S-I, and k, = O. Variable k,/kp = 0,10-\ 5.10-\ 10-3, 2.10- 3,10- 2•

I~C ::0 2,0 ~ ~ 0

), 1.5 80 .£ 800 c - .Q

I~c 1.0 III

50 '-Q/ 600

.,..!.. > c c 0

I 0,5 40 0 400

20 200

0 0 0 0

10 20 30 Time in s

Figure 1.3. Dependence of In(1-[11oDPnl[M1o} on time for [M1o = 1 M, [110 = 10-3 M, kp = 102 M- 1 S-I, and k" = O. Variable k,lkp = 0, 10-4, 5.10-4, 10-3 ,

2.10- 3 , 10-2•

in the studied polymerization. However, no information about the lifetime of the formed polymers is provided by this criterion. Some independent test is needed to determine their longevity.

It is imperative to ensure a long shelf time of the formed polymers if we intend to produce living polymers. Their shelftime increases when a growing but labile polymer P* is in equilibrium with a stable dormant species D, i.e., D ~ P*, equilibrium constant K. Denoting by 'T the half-lifetime


of P* , and by Po the total concentration of the growing and dormant species, one finds the half-lifetime of the whole system to be T.(1 + K)/K, i.e., it is substantially longer than T for K < < 1. The cationically growing polymers terminated by carbenium ions (or their ion-pairs) remaining in equilibrium with the respective ester, e.g., PI ~ P + , 1-, exemplify such systems (see p. 186 for further discussion of this subject.)

Longevity of living polymers depends on the nature of the polymerized monomer, the choice of solvent, the temperature of the solution or melt, the concentration of the growing macromolecules, and so forth. On the whole, their life is longer at lower temperatures and, in fact, some living polymers have to be kept at very low temperatures all the time to prevent their decomposition or isomerization during their preparation and storage.

Generally, dilution of living polymers is detrimental to their stability. The dilution increases the ratio of the concentrations of damaging impurities present in the solvent to that of living polymers, an obvious and trivial factor shortening their lifetime. This difficulty could be avoided by adopting a special procedure, described elsewhere, in which a dilution is achieved without addition of any fresh solvent (see ref. 59, p. 177).

Another more serious factor decreases the stability of living polymers on their dilution. The destabilization is caused by the nature of their endgroups affected by the degree of their aggregation. Aggregation stabilizes, on the whole, living polymers and decreases their reactivity. For example, free ions coexisting in equilibrium with ion-pairs are usually more reactive, and therefore more labile, than the pairs. Their proportion in a system increases on dilution, shortening the lifetime of the respective living polymers. In many systems the aggregated polymers are inert, acting as dormant species, whereas the nonaggregated or less aggregated polymers are active and susceptible to damages caused by various parasitic side reactions. Since the proportion of the latter increases with decreasing living polymers concentration, dilution reduces again their lifetime. The stability of many living polymers is increased by their complexation with appropriate agents. The degree of complexation is reduced by dilution, again reducing the stability of the system.

It is up to the investigator to prolong the lifetime of living polymers by choosing the optimal conditions for the pertinent polymerization. The most rigorous purification of the reagents, solvents, walls of the reaction vessels, and so forth, is imperative. Use of high vacuum techniques is recommended and often essential. A detailed description of the vacuum techniques used when working with living polymers is published elsewhere (see ref. 59, pp. 152-209).

An intriguing question may be posed: is the lifetime of living polymers prolonged by the presence of monomer? The answer depends on the mech-

Introduction I I7

anism of propagation. The lifetime of living polymers growing by a direct monomer addition,

and destroyed by some parasitic unimolecular reaction,

P! (living) --+ P n (dead), kd'

is not affected by the presence or absence of the monomer, being determined by lIkd • However, if the growth takes place via some labile intermediate A~ being in equilibrium with a relatively stable living polymer P~ reformed by the monomer addition, i.e.,

followed by a direct conversion

then the presence of monomer stabilizes the system by reducing the stationary concentration of the labile intermediate. Ring opening polymerization of some strained cyclic olefins, to be discussed later (see p. 181), exemplifies such a system.

A somewhat similar situation could be visualized in a polymerization proceeding through a monomer-living polymer adduct less susceptible to destruction than the unassociated living polymer, i.e.,

The cationic polymerization of propylene induced by AlBr 3' HBr complexes studied by Fontana and Kidder,60 provides an example of such a process, as indicated by its rate of propagation independent of the monomer concentration. Recently, a clear example of stabilization of living polymers by monomer has been reported.61 The reaction of vinyl ether with HI yields CH3CH(OR)-1 that initiates the polymerization of the ether but only after activation of the -CH(OR)-I group by an added 12 or zinc halide. The ensuing propagation was shown to be living (see p. 202). The concentration of the -CH( OR)-I end-groups, whether activated or dormant, was determined by quenching the polymerization with sodiomalonic ester. 61 It was demonstrated in this way that the iodide end-groups remain


intact during the whole course of polymerization but gradually become destroyed (presumably due to the elimination of HI) when the monomer is exhausted. The rate of this living propagation was found to be independent of the monomer concentration, being proportional to [HI] and [12].62 It seems that a rapid and quantitative formation of a monomer-living n-mer complex propagates the polymerization through its unimolecular transformation into a living n + I-mer which, in turn, is rapidly associated with another molecule of the monomer.:t: The association generates a living monomer-n + I-mer complex and thus perpetuates the propagation. Such a scenario implies that the association of the monomer with living polymers stabilizes these labile species.

In closing this discussion, consider the following question. Does the addition of a monomer rejuvenate an "old" growing n-mer by converting it into a new "young" n + I-mer. The answer is an emphatic no! The rate of any elementary reaction of any species, whether unimolecular, bimolecular, or whatever, is determined by its concentration and not by its "age." The time of its creation has no bearing on the reactivity at the moment of observation, and therefore neither on its longevity. The reactivities of an n-mer and n + I-mer are identical and independent of the time of their formation whatever the molecular mass distribution of the studied polymers, provided that n > 3 or 4§.

It is common to refer to a solution of a reagent, e.g., a living polymer, as "aged," meaning that a large fraction of its original molecules were destroyed or isomerized. However, a molecule of a reagent cannot be "aged," either it is still there or otherwise it is not a molecule of the reagent. Although this comment is obvious, nonetheless it may be advisable to make this statement explicitly.

*To explain the zero order of this propagation, the authors of ref. 62 invoked the formation of a monomer-iodine complex and assumed it to consume most of the iodine added for the activation of the iodide. Two mechanisms of propagation were contemplated. Either the monomer-iodine complex is assumed to be the "monomer" that reacts with the nonactivated iodide, or the propagation supposedly results from a bimolecular reaction of the ordinary monomer with the iodine-activated complex (the active end-group) and concentration of the former is depressed by the competition of the monomer and the iodide (the inactive endgroup) for the iodine. In the latter case the rate of the bimolecular reaction accelerates on increasing the monomer concentration but simultaneously is retarded by the decrease of the concentration of the iodide-iodine complex caused by the enhanced competition of the monomer for the iodine. Both mechanisms account for the reported kinetics but only when the ratio [I21/[HI] < 1. This restriction is not required in the mechanism outlined above. In fact, it was reported62 that an excess of iodine over HI does not affect the rate of the polymerization nor the DP. of the formed polymer, contradicting therefore the mechanisms of Cho and McGrath.

§The intramolecular reactions of the growing polymers with their own ends, or any group in their chain, are exceptions.

Introduction / 19

3.2 The "Seeding" Technique

As remarked earlier, a slow initiation of living polymers broadens their molecular mass distribution. It was proposed that the following procedure should prevent the broadening. Mixing a slow initiator with a small fraction of the monomer to be polymerized was expected to lead, within some reasonable time, to a quantitative conversion of the initiating agent into living oligomers, the "seeds." Thereafter, the addition of the remaining monomer would result in a rapid polymerization yielding living polymers of uniform size. The most common polymerizations yielding living polymers are those involving irreversible initiation and propagation, rates of both being proportional to the monomer concentration. The procedure proposed above is futile in such reactions as evident from the following mathematical argument. 63

The various species participating in a living polymerization coexist in a rapid equilibrium with each other. Hence, the rates of initiation and propagation depend only on the total concentration of all the polymeric species P, all the initiating species I, and, of course, are proportional to the concentration of the monomer M, as demanded above. Since P = 10 - 1 (each reacted initiator yields one living polymer), the pertinent rates are functions of [I] and [1]0 - [I]:

-d[I]/dt = F([I],[I]o-[I]) . [M],

and

- d[M]/dt = {F([I],[I]o-[I]) + G([I],[I]o-[I])} . [M],

where F and G are some functions of [I] and [P] determined by the nature of the equilibrium established in the considered system between the initiating, propagating, and dormant species. It follows that d[M]/d[l] is independent of [M], and therefore [M]o is only an added constant in the equation yielding [M] as a function of [I].

Two kinds of curves, shown in Fig. 1.4, illustrate the functional dependence of [M] on [I]. Their shape is independent of [M]o. For any equilibrium system, i.e., for a fixed set of functions F and G and any chosen value of [1]0, the variation of the initial concentration of the monomer [M]o results only in shifting the curves parallel to the 1 axis. Therefore, the molecular mass distribution of polymers initiated in such a polymerizing system by some fixed amount of initiator is unaffected by the mode of the monomer addition, whether all of it is initially mixed with the initiating agent or is added in a piecemeal fashion. This conclusion becomes obvious on inspection of Fig. 1.4 (trace the process determined by the points


M

M

_ - - GA' I

A

c __ - -Q A' I

Figure 1.4. Curves depicting the dependence of [M] on [I] for a constant initial concentration of I, [1]0. Each curve corresponds to a different value of [M]o. Two cases should be distinguished. (1) All the monomer is polymerized while the initiator is still available (upper figure). (2) All the initiator was used up before all the monomer polymerized, provided that the initial concentration of the latter was sufficiently large. Note the critical initial concentration of the monomer, Mer. In case (1) some initiator remains always in the system after quantitative conversion of monomer into polymer, whatever its initial amount. In case (2) all the initiator is consumed only when [M]o > [M]cr.

A' -0 and that following A, B, C, arid 0), and was verified experimentally by Van Beylen and his colleagues.63

On the other hand, the seeding technique is applicable and useful when the propagation of living polymers is reversible and produces a sufficiently large amount of the monomer in a relatively short time to allow for a quantitative conversion of all the initiator into the living oligomers. An example of successful application of this approach is described in ref. 63.

Introduction / 21

3.3 Dormant Polymers; Rate Constant of Propagation

As stated repetitively in the preceding sections, the end-groups of living polymers exist frequently in a variety of distinct forms, each form corresponds to a species usually coexisting in equilibrium with all the others. For example, free ions are in equilibrium with ion-pairs, aggregates of polymers remain in equilibrium with the nonaggregated ones, covalently and ionically bonded species may be in equilibrium with each other, and so forth. The propagation constant of each of these species is different, and for some might be virtually zero. Although the latter do not contribute directly to the propagation, they are not dead either since a spontaneous and reversible interconversion transforms them into the propagating polymers. One refers to such temporarily inactive species as dormant polymers, and the dynamic equilibrium established between them and the active polymers allows all of them to participate in the polymerization.

Examples of systems composed of living and dormant polymers are numerous. They are discussed later in the chapter dealing with ionic propagation (see p. 151). It should be stressed that the presence of dormant polymers is revealed by the kinetics of the observed overall propagation and by the molecular mass distribution of the resulting polymers. The latter is especially significant when the rate of exchange between the active and dormant polymers is slow.

The difference between the dormant and dead polymers is profound. The dead polymers do not grow further after being terminated. The dormant polymers do not participate in polymerization during the period of their dormancy, but eventually they do react. The span of their dormancy is determined by the reciprocal of the rate constant of their conversion into the active form, whereas the extent of their contribution to the overall reaction depends on the relaxation time of the reversible process, dormant polymers ;;= active polymers.

The observed propagation constant, kp, of polymers composed of a variety of species endowed with different end-groups is given by the sum:

where /; denotes the mole fraction of the species possessing the end-group i, and kp ; is their respective propagation rate constant. In deriving the above equation it is assumed that the rate of interconversion between the various polymer species present in the system is much faster than the rate of propagation. There are systems where this assumption is not justified and then the overall propagation rate constant becomes a complex function affected by the rates of exchange.


3.4 Determination of the Concentration of Living Polymers

The term concentration of living polymers needs clarification. It designates the total concentration of all the interconvertible polymeric species which ultimately participate in the growth, even if one kind only, being in minute concentration, is growing while all the others are dormant. The polymers with inert end-groups, incapable of propagating and of being spontaneously converted into the active ones, are referred to as the dead polymers. These are not counted when the concentration ofliving polymers is determined.

A question arises. Since living polymers form a mixture of various interconvertible species, how is their concentration determined experimentally? In many systems, one kind of species, usually a dormant one, is dominating, the mole fraction of all the others being very small, often less than 1 %. In such a case the concentration of that dormant species, determined by the intensity of its UV-Vis spectrum or IR spectrum, provides a reliable measure of the concentration of living polymers. The spectrophotometric method is nondestructive, fast, and especially convenient for continuous monitoring of the concentration of living polymers in the course of reaction. The main obstacle in the use of optical photometry comes from the frequent presence of intensively colored impurities or by-products of the reaction. To avoid the errors the absorbance should be determined not at one wavelength only, but the whole spectrum should be scanned at frequent time intervals and compared with the spectrum of the original species.

In recent studies the intensity of a proper NMR line is used often for the determination of the concentration of living polymers. Its intensity should be measured by its area given by the integration of the line; the height of the signal might be misleading since the exchange with the other species, even if present in minute amounts, broadens the line and affects its height.

The "capping technique" is a convenient and reliable method for determining the concentration of living polymers. The addition of a judiciously chosen reagent to a solution of living polymers converts their active, and often labile, end-groups into new, usually stable groups that possess a property useful for analytic determination. For example, the addition of Michler ketone to a solution of living sodium polystyryl converts its endgroup into an alcohol readily oxidized into an intensively colored dye.64 The concentration of the latter is determined then spectrophotometrically. Similarly, the addition of sodium phenolate to a solution of living, cationically growing polytetrahydrofuran results in attachment of the phenolate moieties to its end, and subsequently the UV absorbance determines the living polymer's concentration.65 The same stratagem was adopted by

Introduction / 23

Higashimura for the determination of the concentration of cationically growing living polyvinyls or polyvinyl ethers. Quenching their polymerization with thiophen66 or sodiomalonic esters61 adds the respective moieties to the end-groups of the investigated polymers, allowing thus the determination of their concentration.

Although the capping method determines the concentration of the active end-groups, it does not provide information about their nature. Such information is available in a method developed by Penczek67 based on capping living polymers by phosphines (in cationic systems) or by CIPO(OPhh (in anionic systems) and investigating thereafter their 31p NMR spectrum. The 31p Ff NMR is not only a highly sensitive instrument, detecting species present at concentrations as low as 10-4 M, but the chemical shift of the 31p line depends strongly on the structural changes taking place even as far as four atoms away from the diagnostic P atom. This allows the differentiation between the various distinct end-groups.

An interesting capping method distinguishing between -CO"2 and -CH20- end-groups involves their reaction with 2,4,6-trinitroanisole. While the reaction with the carboxylate leads to its estrification by the methyl with formation of the trinitrophenolate, the reaction with the alcoholate leads to the formation of the strongly colored Meisenheimer complex

It is essential in a capping technique to ensure the complete conversion of the original end-groups into the modified ones, i.e., the eqUilibrium of the reaction - X + Y ~ - XY should be shifted far to the right. This is achieved by increasing the concentration of Y. Let us point out here a mistaken belief of some authors who expect a high degree of the conversion of - X into - XY for a large [Y]/[ - X] ratio. It is not the value of the ratio but the concentration of Y (of course higher than stoichiometric) that determines the degree of conversion of - X into - XY ([ - XY]/[ - X] = const. [YD.

An important advantage of the capping technique should be stressed. When living polymers are composed of a mixture of two or more species


present in comparable amounts, their spectra result from the superposition of the spectra of the individual components. Since the composition of such a mixture varies with the temperature and the degree of dilution (when the participating species differ in their degree of aggregation), the shape and the intensity of the spectrum varies also, complicating the determination of the concentration of living polymers. This difficulty is avoided in the capping technique. No matter which species reacts with the capping agent, all of them become capped due to their rapid interconvertibility.

Similarly, the problem of different reactivities of the various species composing living polymers is eliminated in the various titrimetric methods. For example, the titration of anionically growing sodium polystyryl with BuI yields an equivalent amount of NaI, the concentration of which is determined by the conventional analytic technique. It is again immaterial which of the species present in the mixture of living polymers is titrated, as long as the nonliving components of the solution remain inert.

An unconventional method providing the concentration of living polymers was developed by Schulz.68 The weight, W, of polymers formed per unit volume of their solution is proportional to their number average degree of polymerization, DPn determined by size exclusion chromatography. The plot of W vs. the fraction of the monomer-to-polymer conversion is linear if such a polymerization is living. Its slope, denoted by c*, provides then the value of the concentration of the investigated living polymers.

3.5 Synthetic Value of Living Polymerization

Living polymerization provides a synthetic polymer chemist with various unique opportunities. It gives him a perfect control on the molecular mass of the produced polymers. One feeds the living polymers with the monomer until they reach the desired size. Moreover, the resulting polymers have uniform size, provided that their initiation is fast and the reacting mixture is homogeneous. The living polymer technique allows also the preparation of block copolymers, free of homopolymers, having the desired sequence and size of the blocks. The reactivity of the end-groups permits their conversion into requested functional groups, i.e., allows for the preparation of functional and bifunctional (telechelic) polymers with desired end-groups. By this procedure, using the properly synthesized reagents, macromers were produced. The latter are the polymers endowed with end-groups mimicking the desired small monomer and capable of being copolymerized with the various small monomers, thus producing branched polymers with the requested branches. The use of "linking agents" allows the conversion of bifunctional linear living polymers into macrocyclics. Modification of these procedures yields star-shaped and comb-shaped polymers.

Introduction / 25

The application of the living polymer technique led to development of numerous new macromolecules of considerable technological value as well as provided researchers with materials allowing precise studies of various properties of polymers, especially those depending on their molecular mass.

Not surprisingly, studies of living polymers attracted the attention of many industrial and academic chemists.

4. Polymerizability and the Monomer-Living Polymer Equilibria

Ionic polymerization allows the preparation of living polymers, the macromolecules retaining their capacity of growth by the addition of the monomer whenever it is available in the system. The principle of microscopic reversibility demands then that the polymers be spontaneously degradable into shorter ones and the monomer. Hence, living polymers retain not only their propensity to grow but also to degrade, and their solutions reach a state of equilibrium in which the rate of polymerization is balanced by the rate of depolymerization. Under such conditions the monomer cannot be quantitatively converted into polymer, but its concentration reaches its respective equilibrium value ceo

Polymerizability of a monomer refers to its capacity to undergo polymerization. Its two aspects should be distinguished: thermodynamic and kinetic. Polymerization of pure liquid monomer to a high molecular mass amorphous polymer is thermodynamically allowed when the standard free energy change, AG~, of the conversion,

liquid monomer ~ monomeric segment of an amorphous polymer,

is negative. Since AGg = AHg - TASg, the polymerization is thermodynamically feasible only at temperatures T lower than the critical one, Te = AHg/ASg, provided that the standard heat and entropy change of the conversion are both negative. For the positive heat and entropy of the conversion the polymerization becomes possible only at temperatures higher than Te. The critical temperature Te is referred to as the ceiling temperature in the former case, and as the floor temperature in the latter case. 69

The polymerization is permitted at all temperatures for the negative heat and positive entropy of the conversion, and always prohibited when the heat of the conversion is positive and the respective entropy change negative. The above statements apply to every monomer-polymer conversion into high molecular mass polymers, whatever the mechanism of propagation.

Most of the common polymerizations are exothermic and reduce the entropy of the system. Hence, their propagation is thermodynamically allowed only at temperatures lower than the critical one. Polymerizations


exhibiting floor temperature are infrequent. The polymerization of Ss into the linear "plastic" sulfur exemplifies the latter case.

The relations are more complex for solution polymerization. The entropy of monomer, but not of high molecular mass polymer, increases on dilution. Hence, at every temperature below the ceiling one the polymerization becomes thermodynamically prohibited at monomer concentrations lower than a critical one, Ce , at which the living polymers reach the state of equilibrium with their monomer. In ideal solutions this monomer equilibrium concentration, Ce , is given then by the relation:

where Co denotes the concentration of the monomer in its pure liquid and aG~ the change in the free energy on conversion of 1 mole of pure liquid monomer into monomeric segment of amorphous polymer. In nonideal systems the interaction of the monomer with its polymer, gauged by the Flory parameters X, has to be taken into account, as well as the interaction of the monomer and the polymeric segments with the solvent. The term

RT( In <1>1 + X( <1>2 - <1>1»

accounts then for the monomer-polymer interaction. The interaction with the surroundings increases Ce for solvents solvating the monomer stronger than the monomeric segments of the polymer, and decreases it in the reverse case. An example showing the effect of monomer-polymer interaction on the monomer equilibrium concentration is illustrated by Fig. 1.5 (note the maximum characteristic for such systems).

The equilibrium concentration of a monomer depends also on its initial concentration when experiments are performed in a solution which is allowed to polymerize until the monomer-polymer equilibrium is attained. The activities of the components depend on the composition of the solution, and the final volume fractions of monomer, polymer, and solvent depend on the initial monomer concentration. These effects of solvents and of the initial monomer concentration on the equilibrium monomer concentration are shown in Fig. 1.6.

Not surprising, but neither immediately expected is the influence of the initiator upon the equilibrium concentration of the monomer in the experiments involving living polymers. The end-groups of living polymers are ionic (salts) and their concentration (living polymers) affects the activity coefficients of the growing end-groups of the living polymers. Neglect of that effect led in the past to a controversy. 70 Two groups of investigators claimed different values for the equilibrium concentration of a liquid monomer in the absence of solvents. The determination of this concentration

Introduction /27

0.7

0.6

O.S '-~

:= ...... 0.4 VI ~

0 E

- 0.3 ~

w

0.2

0.1

0.0 0 2 3 4

Co ' moles / l iter

Figure 1.5. The effect of polymer concentration on the equilibrium concentration of a-methylstyrene. Plot of c. versus the initial concentration of the monomer in tetrahydrofuran (THF) solution at constant temperature. Ce is very low for the equilibrium with a-methylstyrene dimer, increases as the dimer is converted into higher polymers, and should reach a plateau for high Co had it not been for the increase in the monomer activity coefficient caused by its interaction with the polymer. The point denoted by ~ gives the eqUilibrium concentration Ce reduced by the addition of dead poly-a-methlystyrene to a solution resulting from the polymerization of the monomer at the initial concentration Co.

was simple. Some amount of initiator was dissolved in a pure monomer and the polymerization carried out until the monomer-polymer equilibrium was reached. However, since the concentrations of the initiator in their respective experiments were substantially different, the ultimately reached equilibrium concentrations of the monomer were greatly diverse. The correct value of the equilibrium concentration of the monomer is obtained by extrapolation of the observed values to zero concentration of the initiator.

The most general treatment of the effects of monomer, polymer, and solvent interactions was outlined by Ivin and Leonard71 who derived the relation:


6

5

3

12 4 6 8 -1 10 [Ml (mo Ie I ) o

Figure 1.6. The concentration of THF in equilibrium with its living polymer in various solvents as a function of its initial concentration. The solvents: a, CCI4 ;

b, benzene; c, CH2CI2 ; d, CH3N02•

where the subscripts m, p, and s refer to monomer, polymer, and solvent, respectively, and the symbols <I> and X have their conventional meaning.

The values of ~H2 and ~S2 could be derived from plots of In Ce vs. lIT illustrated by Fig. 1.7. It is advisable in the experimental determination of the equilibrium concentration of monomer at temperature T to measure its value as the temperature of solution approaches T from below, and then repeat the measurements as the temperature approaches T from above.

The equilibrium between living polymer and monomer may be described by the scheme:

IM* + M ;:= IMM*, Kl>

IMM* + M ;:= IMMM*, K2,

Although Kl need not be equal to K2, nor K2 be equal to K3, it is expected for large i's K; = K;+l = ... = K. Without loss of generality one may

Introduction / 29

8 r-------------~------------~

6

5 4

9-::.::: OJ 2 High polymer 0

o

- 2

3.0 5.0

Figure 1.7. Plot of log K versus Iff for the equilibrium polymerization of a-methylstyrene. K = liCe is the equilibrium constant for the monomer-living polymer equilibrium. The living polymers are dimers, tetramers, and high molecular mass polymers, respectively.

assume that Kl = K2 = . . . = K; = ... K. This assumption does not affect the essential feature of this equilibrium and it leads to the relations72:

and

where the subscript in refers to the initial concentrations of the monomer and the initiator, whereas e refers to their equilibrium values. The above relation was confirmed by Leonard.73 The equilibrium concentration Ce of the monomer depends, therefore, not only on K but also on the initial amounts of the monomer and the initiator supplied to the system. The final result is given by the compact relation: Ce = (1 - lIDPn)/K, where DP n is the number average degree of polymerization of the produced

30 I Ionic Polymerization and Living Polymers

polymers. 72 At a high degree of polymerization this relation is reduced to Ce = 11K. The instructive plot giving the dependence of DPn of the equilibrated polymers as a function of temperature for a constant total amount of monomer and a constant concentration of the initiator is shown in Fig. 1.8. The remarkable increase of DPn on decreasing the temperature in the critical temperature region should be noted.

Interestingly, for [IM*] ~ 0 the equilibrium concentration of monomer is given by 11K for KCe > 1 but by Cin for KCe < 1. The system exhibits a second-order transition at the extremely low concentration of initiator, a discontinuity of dce/dT at the temperature at which KCe = 1. See refs. 74-76 for a discussion of this phenomenon.

The molecular mass distribution of the polymers present in an equilibrated monomer-polymer system has to conform to Flory distribution, i.e., [IMiM*] = [IM*].Ki. This conclusion is derived from the treatment

DP n

c

o 10 20 30 40 50

Temperature ·C

Figure 1.B. The number average degree of polymerization j of a living polymer as a function of temperature for a constant amount of the supplied monomer [M]o and a constant concentration of the initiator [1]0' As the temperature decreases, the fraction of the monomer converted into polymer increases.

Introduction /31

outlined above. However, a rapid propagation and slow depropagation yields living polymers having initially a Poisson molecular mass distribution. After completion of the polymerization a process resulting from a continual dissociation of i-mers into (i - 1 )-mers and a simultaneous growth of j-mers into (j + 1 )-mers broadens the molecular mass distribution and ultimately the Poisson distribution gives way to a Flory distribution. For high molecular mass polymers this process hardly affects the equilibrium concentration of the monomer, or the number average molecular mass of the formed polymers, but it increases its weight average molecular mass and the higher moments. The kinetics of such changes is described by an approximate treatment of Brown and Szwarc77 and subsequently a rigorous treatment was outlined by Miyake and Stockmayer. 78 In the first approximation, the initial rate of change, measured by d(DP wlDP n)/dt, is inversely proportional to DP n and proportional to the rate constant of depropagation.

Depropagation of polymers formed by ring opening polymerization of cyclic monomers may lead to the formation of larger polymerizable or unpolymerizable cyclic compounds. For example, polysiloxane formed by polymerization of the cyclic trisiloxane may depropagate into a cyclic tetrasiloxane, a monomer in its own rights. In fact, the polymerization followed by depropagation provides a mechanism for converting one cyclic monomer into another one, e.g.,

I I 4(SiOh ~ 3(SiO)4.

I I

The ultimate monomer-polymer eqUilibrium involves, therefore, both monomers maintained at eqUilibrium with each other, as well as in equilibrium with the high molecular mass polysiloxane.

An interesting situation arises in the polymerization of trioxepane:

This monomer is readily converted by a cationic ring opening mechanism into a living polymer. However, as reported by Schulz,79 the composition of the residual monomers being in equilibrium with the living polymer gradually changes, as depicted in Fig. 1.9. The amount of trioxepane decreases and dioxolane and formaldehyde are formed instead, i.e., the de-


1M)

~,-------------------T I

--f----------------------- 0 ,/" I

" ,... ... ·-f-·-·-·_·-·-·-·_·_·-·_·_·_·_·- F ,/ I

time

Figure 1.9. Changes in the composition of the monomers remaining in contact with living trioxepane as functions of time. The concentration of trioxepane (T) decreases, while dioxolane (D) and formaldehyde (F) are formed. The scale for formaldehyde concentration is greatly exaggerated.

propagation decomposes the trioxepane and yields three monomers, formaldehyde (CH20), ethylene oxide

and dioxolane

CH2-CH2 / '\ o 0,

" / CH2

each capable of being homopolymerized as well as copolymerized with the others. In fact, trioxepane may be treated as a small cyclic "copolymer" of ethylene oxide and formaldehyde in the stoichiometric ratio of 1:2, whereas dioxolane is a cyclic "copolymer" of these two monomers formed in the ratio of 1: 1. The depropagation of the living polytrioxepane into the above three copolymerizable monomers is responsible for Schulz's observations. The thermodynamic-statistical treatment of such an equilibrium was reported by Szwarc and Perrin80 who derived the expressions for the equilibrium concentration of the above monomers in terms of a parameter r given by the ratio of CH2CH20 and the CH20 units present in the system, i.e., [Trioxepane]e = KT.r(l - r), [Dioxolane]e = Ko.r, [Formaldehyde]e = KF.(l - r), where the factors K's denote the equilibrium constants of the ideal equilibria established for each of the above monomers

Introduction / 33

and their respective homopolymers, each homopolymer is assumed to depropagate into its own monomer only.

A spectacular observation was reported by Goethals81 who investigated the cationic polymerization of the four-membered cyclic sulfide. A rapid propagation converts virtually all the monomer into a linear high molecular mass polymer which slowly decomposes thereafter into the cyclic eightmembered dimers of the original monomer. Thus, the viscosity of the solution rapidly increases initially and later slowly decreases. This strange behavior of the system is due to the participation of two independent reactions in the overall process. A fast reversible propagation characterized by a large eqUilibrium constant is responsible for the rapid and virtually quantitative conversion of the monomer into the linear polymer. Its degradation arises from another reaction, namely the back-biting of the active endgroup of the living polymer yielding the cyclic unpolymerizable dimers. The latter reaction is virtually irreversible (the cyclic dimers do not polymerize).

Further complexities of the monomer-polymer equilibria result from the phase separation, crystallization of the polymer, and its variable structure, e.g., does a vinyl polymer form a pure isotactic or syndiotactic structure, or is it atactic?; does a polydiene have 1-4 or 1-2 enchainment?; and so forth. For a discussion of these topics the reader is referred to a monograph by Sawada.82

The first application of living polymers in the thermodynamic studies was reported by Worsfold and BywaterS3 and by McCormick84 who investigated the anionic polymerization of a-methylstyrene. The results led to aH2 = 25 kJ/mole for the reaction performed in tetrahydrofuran, with the respective aS2 = 29 e.u. The extensive investigations of the equilibria established in the cationically polymerized tetrahydrofuran were reported by Penczek and MatyjaszewskFo who determined the pertinent thermodynamic variables in a variety of solvents. Let it be emphasized again that all these results are independent of the mechanism of the reaction and obtained by allowing a mixture of living polymer and its monomer to reach the state of eqUilibrium. However, the results are affected by the nature of solvent and the composition of the investigated solution.

In contrast to thermodynamic polymerizability, the kinetic feasibility of polymerization depends on the mechanism of the process. Not every thermodynamically polymerizable monomer may be polymerized. For example, the highly strained cyclopropene should easily yield a linear (--CH2CH==CH-)n polymer, but no mechanism capable of inducing its polymerization is yet known. Some monomers, although copolymerized with other monomers, are unable to be homopolymerized. For example, maleic anhydride does not homopolymerize, although it is readily copolymerized with a variety of vinyl monomers.


Some monomers are polymerized by a cationic procedure but not by an anionic one and vice versa. As a rule the monomers endowed with electron withdrawing groups (e.g., nitroethylene, acrylonitrile, and acrylates, etc.) are polymerized by an anionic mechanism, whereas propagation of those possessing electron-donating groups (e.g., isobutene, p-methoxystyrene, and numerous heterocyclics) proceeds by cationic means. Although some strained heterocyclic monomers are polymerized by an anionic mechanism, e.g., ethylene oxide or propylene sulfide, the cationic mechanism of polymerization is preferred for this class of monomers. A cationic ring opening propagation requires the rupture of a weak carbon-to-onium bond, whereas a much stronger carbon-to-heteroatom bond has to be broken in the corresponding anionic process. This difference in the strength of the ruptured bond accounts for the preference for cationic mode of polymerization of these monomers.

The propagation rate constant is a poor measure of the reactivity of a monomer. As a rule, the more reactive monomer yields a less reactive polymer end-group, and this compensating effect prevents a correlation of the propagation rate constant with the monomer's reactivity. More significant is the correlation of the monomer structure with the crossover rate constant (the rate of addition of a monomer to the end-group of some living polymer chosen as a standard). However, the rate of crossover depends on the mechanism of reaction, and thus for two monomers, say M. and Mz, the first may be found to be more reactive than the other in respect to one standard, whereas the reverse relation may be observed for another standard. Furthermore, the "reactivity" may be affected by the conditions of the reaction. For example, in tetrahydrofuran solution the addition of styrene to lithium polystyryl is faster than that of butadiene, whereas the reverse is found in benzene solution. The mechanism of addition is different in these two solvents. A direct bimolecular addition of the monomer to the free anion accounts for the propagation in the ether, whereas its coordination with the cation of an ion-pair followed by a rearrangement and monomer insertion into the polymer chain is responsible for the propagation in the hydrocarbon milieu. Indeed, the term reactivity of a compound in a bimolecular reaction has no meaning without specifying the partner and the character of the process.

References

1. Eastmond, G.C. 1976. The kinetics of free radical polymerization. In C.C. Bamford and C.F.H. Tipper (eds.), Comprehensive Chemical Kinetics, Vol. 14a, pp. 80-85, Elsevier, New York.

2. Buckley, R.P., Rembaum, A., and Szwarc, M. 1957. J. Polym. Sci. 24, 135.

Introduction / 35

3. Williams, F. 1968. In P. Ausloos (ed.), Fundamental Processes in Radiation Chemistry, Interscience, New York.

4. (a) Szwarc, M. 1956. Nature 178, 1168.

(b) Szwarc, M., Levy, M., and Milkovich, R. 1956. J. Am. Chern. Soc. 78, 2656.

5. Stavely, F.W., et al. 1956. Ind. Eng. Chern. 48, 778.

6. Rempp, P., Franta, E., and Hertz, J.E. 1988. Adv. Polym. Sci. 86, 145.

7. Staudinger, H. 1920. Ber. 53, 1073.

8. Ostromysslensky, 1.1. 1915. Soc. Phys. Chim. Russe. 47, 1928.

9. (a) Staudinger, H. and Luthy, M. 1925. Helv. Chim. Acta 8, 41.

(b) Staudinger, H. 1925. Helv. Chim. Acta 8, 67.

10. Whitby, G.S. and Katz, M. 1928. J Am. Chern. Soc. 50, 1160.

11. Taylor, H.S. and Jones W.H. 1930. J. Am. Chern. Soc. 52, 1111.

12. Whitmore, F.C. 1934. Ind. Eng. Chern. 26, 94.

13. Williams, G. 1940. J. Chern. Soc. 775.

14. Plesch, P.H. 1963. The Chemistry of Cationic Polymerization, Pergamon Press, New York.

15. Wurz, A. 1878. C. R. Seanc. Acad. Sci. Paris 86, 1176.

16. Watson, R. 1789. Chern. Ess. Lond. 3.

17. Deville, M. 1839. Ann. Chim. France 75, 66.

18. Canizzaro S. 1854. Ann. Chim. Pharmacol. 90, 252 and 92, 113.

19. Berthelot, M. 1866. Bull. Soc. Chim. France 6,294.

20. Wislicenius, J. 1878. Ann. 92, 106.

21. Butlerov, A.M., and Gorianov, V. 1873. Ann. 169, 146.

22. Friedel, C., and Crafts, J.M. 1877. Cornpt. Rend. 84, 1450 and 85, 74.

23. Hunter, W., and Yohe, R.V. 1933. J. Am. Chern. Soc. 55, 1248.

24. (a) Evans, A.G., Polanyi, M., Holden, D., Plesch, P.H., Skinner, H.A., and Weinberger, M.A. 1946. Nature 157, 102.

(b) Evans, A.G., Meadows, G.W., and Polanyi, M. 1946. Nature 158,94.

25. Plesch, P.H. 1966. Pure Appl. Chern. 12, 117.

26. (a) Meerwein H. 1939. German Pat. 741, 478.

(b) see also Meerwein, H., Delfts, D., and Morschel, H. 1960. Angew. Chern. 72,927.

27. Matthews, F.E., and Strange, E.H. 1910. British Patent 24, 790.

28. Harris, G., Liebigs, A. 1911. Ann. Chern. 383, 213.

29. Schlenk, W., Appenrodt, J., Michael, A., and ThaI, A. 1914. Chern. Ber. 47,473.

30. (a) Ziegler, K., Colonius H., and Schafer, O. 1929. Ann. Chern. 473, 36.


(b) Ziegler, K., and Schafer, O. 1930. Ann. Chern. 479, 150.

(c) Ziegler, K., Jacov, L., Wollthan, H., and Wenz, A. 1934. Ann. Chern. 511,64.

(d) Ziegler, K. 1936. Angew. Chern. 49, 499.

31. Ziegler, K., and Bahr, K. 1928. Chern. Ber. 61-,253.

32. (a) Schlenk, W., and Bergmann, E. 1928. Ann. Chern. 463, 1.

(b) Schlenk W., and Bergmann, E. 1930.479,42,58, 78.

33. Schulz, G.V. 1938. Ergeb. Exact Naturw. 17,405.

34. Bolland, J.L. 1941. Proc. R Soc. Lond. A178, 24.

35. Abkin, A., and Medvedev, S. 1936. Trans. Faraday Soc. 32,286.

36. Blomquist, A.T., Tapp, W.J., and Jonson, J.R 1945: J. Am. Chern. Soc. 67,1519.

37. Beaman, RG. 1948. J. Am. Chern. Soc. 70, 3115.

38. Robertson, R.E., and Marion, L. 1948. Can. J. Res. 268, 657.

39. (a) Ziegler K., Holzkamp, E., Breil, H., and Martin, H. 1955. Ang. Chern. 67,541.

(b) see also Ziegler, K. 1954. Belg. Pat. 533,362.

40. Natta, G., Pino, P., Corradini, P., Danusso, F., Mantica, E., Mazzanti, G., and Moraglio, S. 1955. J. Am. Chern. Soc. 77, 1708.

41. Price, C.C., and Osgan, M. 1956. J. Am. Chern. Soc. 78, 4787.

42. Zambeli, A., Natta, G., and Pascon, I. 1964. J. Polym. Sci. C4, 411.

43. (a) Aida, T., and Inoue, S. 1980. Macromolecules 14, 1162, 1166.

(b) Inoue, S., and Aida, T. 1985. Am. Chern. Soc. Symp. Ser. 286, 137.

44. (a) Inoue, S., and Aida, T. 1988. Macromol. 21, 1195.

(b) Inoue, S., and Aida, T. 1986. Makromol. Chern. Symp. 6,217.

45. Bawn C.E.H., Bell, RM., and Ledwith, A. 1965. Polym. 6, 95.

46. Dryfuss, M.P., and Dryfuss, P. 1965. Polym. 6, 93.

47. Vofsi, D., and Tobolsky, A.V. 1965. J. Polym. Sci. A3, 3261.

48. Osgan, M., and Teyssie P. 1967. Polym. Lett. 85, 789.

49. Webster, O.W., Hertler, W., Sogah,D.Y., Farnham, W.B., and Rajen-Babu, T.V. 1983. J. Am. Chern. Soc. 105,5706.

50. (a) Miyamoto, M., Sawamoto, M., and Higashimura, T. 1984. Macromolecules 17, 265, 2228.

(b) Sawamoto, M., Fujimori, J., and Higashimura, T. 1986. Macromolecules 20, 916, 2693.

51. (a) Faust, R., and Kennedy, J.P. 1986. Polym. Bull. 15, 317.

(b) Mishra, M.K., and Kennedy, J.P. 1987. Polym. Bull. 17,7.

(c) Mishra, M.K., and Kennedy, J.P. 1987. J. Macromol. Sci. A24, 933.

Introduction /37

52. Gillion, I.R., and Grubbs, R.H. 1986. J. Am. Chern. Soc. 108, 733.

53. Szwarc, M. 1987. In T.E. Hogen-Esch and J. Smid (eds.), Recent Advances in Anionic Polymerization, p. 93, Elsevier, New York.

54. Szwarc, M. 1992. Makromol. Chern. Rapid Comm. 13, 141.

55. Ishikama, Y., Sawamoto, M., and Higashimura, T. 1990. Polym. Bull. 23, 361.

56. Lochmann, L., Kolarik, J., Doskocilova, D., Vozka, S., and Trekoval, J. 1979. J. Polym. Sci. 17, 1727.

57. Nuyken, 0., and Kroner, H. 1991. Makromol. Chern. 191, 1.

58. Penczek, S., et al. 1991. Makromol. Chern. Rapid Comm. 12, 77.

59. Szwarc, M. 1968. In Carbanions, Living Polymers, and Electron Transfer Processes, pp. 275-279, Interscience, New York.

60. Fontana, C.M., and Kidder, G.A. 1948. J. Am. Chern. Soc. 70, 375.

61. Choi, 0., Sawamoto, M., and Higashimura, T. 1990. Macromolecules 23, 48.

62. Cho, C.G., and McGrath, J.E. 1988. J. Macromol. Sci. A25, 499.

63. Van Beylen, M., Szwarc, M., and Van Hoyweghen, D. 1987. Macromolecules 20,445.

64. Trotman, J., and Szwarc, M. 1960. Makromol. Chern. 37, 39.

65. Saegusa, T., and Matsumoto, S. 1968. J. Polym. Sci. A6, 1559.

66. Higashimura, T., Kasano, M., Masuda, T., and Okamura, S. 1971. J. Polym. Sci. 89, 468.

67. Brzezinska, K., Chwialkowska, W., Kubisa, P., Matyjaszewski, K., and Pen-czek, S. 1977. Makromol. Chern. 178, 2491.

68. Barnikol, W.K.R., and Schulz, G.V. 1965. Z. Phys. Chern. 47, 89.

69. Dainton, F.S., and Ivin, K. 1958. Q. Rev. Chern. Soc. 12,61.

70. Penczek, S., and Matyjaszewski, K. 1976. J. Polym. Sci. Polym. Symp. 56, 255.

71. Ivin, K.J., and Leonard, J. 1970. Eur. Polym. J. 6, 331.

72. Tobolsky, A.V., and Eisenberg, A. 1962. J. Colloid Sci. 17,49.

73. Leonard, J. and Maheux, D. 1973. J. Macromol. Sci. 7, 1421.

74. Kennedy, S.J., and Wheeler, J.C. 1981. J. Chern. Phys. 75, 2021.

75. Wheeler, J.c., Kennedy, S.J., and Pfeuty, P. 1980. Phys. Rev. Lett. 45, 1768.

76. Wheeler, J.c., and Pfeuty, P. 1981. J. Chern. Phys. 74, 6415.

77. Brown, W.B., and Szwarc, M. 1958. Trans. Faraday Soc. 34,416.

78. Miyake, A., and Stockmayer, W.H. 1965. Makromol. Chern. 88, 90.

79. Schulz, R.c., et al. 1977. Am. Chern. Soc. Symp. 59, 77.

80. Szwarc, M., and Perrin, c.L. 1979. Macromolecules 12, 699.


81. Lambert, J.L., Van Ooteghem, D., and Goethals, E.J. 1971. J. Polym. Sci. 9,3055.

82. Sawada, H. 1976. Thermodynamics of Polymerization, Marcel Dekker, New York.

83. Worsfold, D.J., and Bywater, S. 1957. J. Polym. Sci. 26, 299.

84. McCormick, H.W. 1957. J. Polym. Sci. 25,488.

2

Ionic Species

1. Formation of Ions

Matter in bulk is electrically neutral and whenever it contains electrically charged species, i.e., ions or their aggregates, their opposite charges have to balance each other exactly. Conceptually, the simplest process converting electrically neutral molecules into electrically charged ions results from electron ejection (ionization) induced by photolysis or radiolysis (a process caused by the action of high-energy projectiles, e.g., 'Y rays, fast electrons, etc.). The ejected electrons are captured eventually by some neutral molecules yielding then the negative ions, while the ionized molecules constitute the positive ions.

A direct ionization of a neutral molecule requires a large amount of energy, about 9 eV/mole or more. Such amounts of energy are available in radiolysis and in photolysis induced by radiation of very short wavelength (vacuum UV), but not in photolysis induced by near UV or visible light. The latter facilitates electron transfer, provided that some electron acceptor of a sufficiently high electron affinity is available in the vicinity of the radiation-absorbing electron donor. Various species may act as electron acceptors; even some solvents may play this role. For example, the near UV irradiation of aromatic amines in water yields the respective radicalcations and solvated electrons; water acts here as an electron acceptor. However, in the media poorly solvating ions such as hydrocarbons, the same irradiation of the amines produces neutral radicals (ArNH· and H") and not ions. The nature of the solvent is important even if it does not act as an electron acceptor. For example, irradiation of triethylamine and biphenyl, B (the acceptor), in acrylonitrile solution yields an exciplex which dissociates into a pair of ions, Et3N;- and B:-, their solvation makes the process feasible. On the other hand, in toluene solution no electron

39


transfer takes place, ions are not formed, and the absorbed exitation energy is dissipated through fluorescence.

Although a monophotonic process resulting from irradiation by near UV or visible light does not provide enough energy for a direct ionization, ions may be formed by a biphotonic process. For example, the ionization potential of benzene being 9.2 eV/mole is too high to allow for the formation of its radical-cation by a monophotonic UV irradiation. However, its excitation to the singlet (4.84 eV/mole) followed by the intersystem crossing to the triplet (3.7 e Vlmole) may lead to ionization caused by the consecutive absorption of two photons, the first yielding the triplet while the second causing its ionization.

Encounter of a species of very low ionization potential with another of very high electron affinity might lead to electron transfer and the formation of oppositely charged ions even in the absence of radiation, provided that the reaction takes place in a well-solvating medium. Indeed, the solvation of the resulting ions contributes substantially to the driving force of the process. The system N ,N ,N'N' -tetramethyl-p-phenylenediamine-chloranil is a classic example demonstrating the importance of solvents in the formation of ions. The above pair forms a ground-state charge-transfer complex in low polarity solvents, e.g., in dioxane or benzene, and no total electron transfer occurs on their irradiation. However, in acetonitrile, a powerful solvent of high dielectric constant, the initially formed complex spontaneously gives way to the separate cation and anion radicals, each being identified by its absorption and electron spin resonance (ESR) spectra.

The reaction of alkali metals with aromatic hydrocarbons provides another example stressing the role of solvents in electron-transfer processes. In well-solvating media this reaction yields aromatic radical-anions and alkali cations. For example, sodium reacts with naphthalene (Nph), dissolved in an aprotic ether yielding a solution of Nph -=- ,Na + ion pairs. An equilibrium is eventually established when the ratio [Nph -=- ,Na + ]/[Nph]o reaches its equilibrium value, namely =0.9 for the reaction performed in tetrahydrofuran at ambient temperature, but only =0.02 when the reduction takes place in diethyl ether. This finding demonstrates again the importance of solvation in processes leading to the formation of ions. The former ether, being a more powerful solvating agent of sodium cations than the latter, allows for a higher degree of naphthalene reduction.

Alternatively, ions are formed on transfer of some charged moiety from one neutral molecule to another. Proton transfer from a covalent acid to a covalent base is the most common example of such a process. Thus anhydrous perchloric acid reacts with an equimolar amount of water yielding a crystalline ionic salt, H30 +, CI04"; the nonconducting solution of HBr in sulfur dioxide becomes conducting on addition of an cquimolar amount of water due to the formation of free H30 + and Br- ions. Transfer

Ionic Species I 41

of other charged moieties leads also to the formation of ions. For example, the transfer of Cl- from benzyl chloride to boron trichloride yields in methylene chloride solution the PhCHt, BCli ion pairs; the transfer of OH- from triphenylmethyl carbinol dissolved in liquid sulfur dioxide to a solvent molecule yields the Ph3C+ cations and HS0"3 anions, and so forth.

Finally, two oppositely charged ions may be formed through heterofission of a covalent bond of some molecules, e.g., trityl chloride dissociates into Ph3C+ cations and Cl- anions when dissolved in nitromethane. The nature of the solvent is important again because it is the solvation energy of the ions that provides the driving force favoring the hetero-dissociation of the covalent Ph3C-CI bond over its homo-dissociation. Heterodissociation of many C-C bonds has been extensively investigated by Arnett.!

2. Ionophores and Ionogens, Ion.Pairs, and Covalent Species

Many solids are built up of neutral molecules cemented together by van der Waals forces, while others, known as ionic crystals, are composed of ions as their building blocks. The latter, referred to by Fuoss2 as ionophores, dissociate quantitatively into free ions in media in which they are soluble. There are exceptions. For example, crystals of nitrogen pentoxide are composed of NOt cations and NOi anions,3 while their sublimation or dissolution in low-polarity solvents yields the covalent molecules of N20 s. Moreover, this nonpolar solute decomposes on heating into the neutral NO; and NO; radicals and and not into ions. A somewhat similar phenomenon is observed on sublimation of ionic crystals of ammonium chloride. Here again the electrically neutral Hel and NH3 molecules are the products of sublimation; the ionic ammonium chloride does not exist in the gas phase. It is the lattice energy that accounts in both these cases for the formation of ions in the solid state.

The dissociation of acetic acid in water exemplifies the behavior of compounds classified by Fuoss as ionogens. These nonionic molecules react with the solvent, forming then the oppositely charged ions.

The distinction between a pair of associated ions and a neutral molecule (or a complex) is blurred sometimes. For example, the adduct BF30(C2Hsh is not an ion pair. It does not dissociate into BF30C2H; anions and C2Ht cations, although it acts as if it were an ion-pair, e.g., it transfers the positively charged ~H; moiety to vinyl ethers, and thus initiates their cationic polymerization.4 On the other hand, the reaction of acyl fluoride with SbFs performed in freon at low temperature yields a crystalline salt,S RCO+, SbF6 , and its solution initiates cationic polymerization of tetrahy-


drofuran (THF). The conductance of this salt in CH2Cl2 demonstrates establishment of a binary equilibrium:6

R(CO)F + SbFs ~ RCO+ + SbF6 ,

i.e., the salt dissociates into ions as well as into its nonionic components. This is a general feature of similar salts, e.g.,

There is an important class of compounds that exist in two distinct forms: as covalently bonded molecules in equilibrium with the respective ion-pairs (equivalent to ionic bond). This was clearly visualized by Ziegler and Wollschit1 who studied the equilibrium established between Ph3CCl and Ph3C+ ,Cl- in liquid sulfur dioxide. The nuclear magnetic resonance (NMR) analysis facilitates the distinction between these two kinds of species, e.g., the chemical shifts are different for the Sp3 protons of the covalently bonded C-H's and the Sp2 protons attached to ionic carbanions or carbenium ions. Indeed, broadening of such lines allowed Kessler and Feigel8 to evaluate the height of the potential energy barrier hindering the covalent-into-ionic conversion.

Use of symmetry provides another interesting method discriminating between the covalent and ionic bonding. The electron cloud around the ionic Cl- or CIOi anion is spherically symmetric and therefore the respective35CI NMR lines are sharp. The symmetry is lost in the covalent esters, R-Cl or R-OCI03, causing a large degree of line broadening due to the rotation of the quadrupole of the 3sCI nucleus. This distinction showed that trityl perchlorate forms ion-pairs in a 1:1 mixture of CH2CI2/ MeCN, whereas the analogous silicon compound, SiPh30CI03, is covalently bonded.9

The above observations refer to ionizations taking place in solution where the interconversion process proceeds simultaneously with some reorganization of solvent molecules surrounding the interconverting species. It is interesting to know whether such interconversions take place in the gas phase. Unfortunately, our knowledge of such processes is limited presently to quantum mechanical calculations; no experimental data are available yet.

In the following chapters we will be dealing extensively with carbanions and carbeniums ions. Both these species are capable of forming salts, and a question could be raised whether there are two distinct species, ionic and covalent. As shown in the preceding paragraphs, such a distinction is possible and meaningful for the salts of carbenium ions and their parent compounds. Is such a distinction feasible for the analogous alkali or alkaline

Ionic Species / 43

earth compounds? Unfortunately, no soluble compounds containing covalent C-Na or C-K bonds are known. The aliphatic R-Na, R-K, and so forth are insoluble and infusible, and the solids have structures resembling those of ionic salts. The salts of the resonance stabilized carbanions, e.g., benzyl sodium, are soluble and their solutions are conducting. Their optical spectra leave no doubt about their ionic structure, covalently bonded isomers do not exist (see p. 100 for the discussion of tight and loose ion-pairs).

Soluble aliphatic R-Li compounds are well known. They exist in hydrocarbon solvents as aggregates bonded by multiple-center bonds (see p. 147 for the detailed discussion of their structure) remaining in equilibrium with the less aggregated species including monomers. However, their degree of association is lower in ethereal solutions, and in THF they exist as monomers. The resonance stabilized organolithium compounds, such as benzyl lithium, lithium diphenylmethyl, and so forth, although associated in hydrocarbon solvents, but not in THF, have optical spectra only slightly different from those of the respective unassociated salts of other alkali ions. Moreover, their THF solutions are conducting leaving no doubt about their ionic character.

In conclusion, the unambiguous distinction between covalent and ionic (tight ion-pairs) species useful in carbenium ion chemistry is hardly possible in the field of carbanions.

Equilibria between the two types of molecular species, the covalently and electrovalently bonded molecules, deserve further elaboration. The underlying ideas are clarified by the somewhat idealized energy-distance diagrams shown in Fig. 2.1. Fig. 2.1a depicts the most common case of a gaseous molecule X-Y which dissociates in its ground state into x· and Y· radicals (or atoms), while the dissociation of its excited state yields the corresponding ions, X+ and Y-. The energy level of the excited state is, on the whole, high enough to make its equilibrium concentration negligible at ambient temperature. In solvents strongly interacting with the free ions and the respective ions pairs, the energy relations may be reversed. This is shown in Fig. 2.1b. An interesting situation arises when a solvent strongly stabilizes the free ions but only weakly the ion pairs. The respective hypothetical curves, shown in Fig. 2.1c, are expected to cross, but due to the quantum mechanical restrictions they become separated. Thus the nonionic covalent molecule would dissociate into free ions, while the dissociation of the electrovalent ionic species yields neutral radicals. The dissociation of the hypothetic covalent molecule of sodium chloride immersed in water might exemplify such a case.

3. Free Ions and Ion-Pairs

The presence of free ions in a solution is manifested by ionic conductance. From its value the concentration of the free ions may be calculated provided


X·+Y -

E X+Y

X-y

X- Y distance (a)

X+Y

E X++ Y- solvated

X+. Y- solvated

X - Y d istance ( b )

x.y

~ E + -X + Y solvated _ ...

X· .Y- ,,(

X-Y

X- Y d istance (c )

E

X-Y X+ - Y-charge -transfer complex

X - Y d istance ( d)

Figure 2.1. Schematic representation ofthe energy-distance diagram for covalent and ionic bonds of XY and X +, Y - .

that their mobilities are known or can be estimated. This method of determination of the concentration of free ions was utilized in studies of cationic polymerization of styrene initiated by 'Y radiation of the bulk monomer.1O The experimentally determined conductance of the polymerizing solution, in conjunction with the observed rate of polymerization, per-

Ionic Species / 45

mitted evaluation of the absolute rate constant of propagation of the respective free macrocarbenium ions (see p. 267).

Ionophores supposedly dissociate quantitatively into free ions in dilute aqueous solutions. The dependence of the conductance of such solutions on the concentration of the dissolved salts is accounted for by the DebyeHuckel theory. On the other hand, the relation between the conductance and the concentration of dilute ionogens solutions is given by the mass law. Surprisingly, sodium chloride, a typical ionophore, behaves in liquid ammonia solutions as if it were an ionogen; the dependence of conductance on its concentration is given by the mass law. 11 This observation was rationalized by Bjerrum's hypothesis12 which assumes a reversible association of ions into neutral, nonconducting species, referred to as ion-pairs, a process taking place even in aqueous solutions of ionophores.

The electric neutrality of ion-pairs prevents them from conducting electric current. Nevertheless, during the 25 years following the original Bjerrum's paperp all the evidence for the existence of ion pairs was derived from studies of the property they did not possess, namely from measurements of the decrease of electric conductance caused by their formation. The first positive evidence for the existence of ion-pairs came from the study of the ESR spectrum of sodium naphthalenide,13 a radical-anion. Each of the 25 hyperfine lines of the free naphthalenide ion (see Fig. 2.2a) is split into four sharp components (see Figs. 2.2b and 2.2c). The splitting results from the interaction of the odd electron with the spin of the sodium nucleus. The sharpness of such lines demonstrates a relatively long lifetime (> 10-5 s) of the associate of the counterion with the radical-anion. Subsequent studies of optical, infrared, and notably Raman spectra of such solutions provided further evidence of two kinds of species in such systems: the free ions and ion-pairs.

Although the reality of ion-pairs is unquestionable, this concept, like many concepts of science, is justified and profitable within some range of temperature and concentration, but it loses its utility beyond these boundaries. For example, one cannot differentiate between free ions and ionpairs in molten sodium chloride. The melt conducts electric current and each of its sodium ions has some chloride ions as its nearest neighbors and vice versa. However, no two neighboring ions of the melt could be treated as an ion-pair. Such systems are best described by a distribution function giving the probability of ion separation, and not in terms of equilibrium between free ions and ion-pairs.

Some solutions acquire entirely new properties when the concentration of ions is too high. Consider the rather esoteric example of a solution of lithium in liquid ammonia. At low concentrations of the metal the electric conductance of such solutions is well accounted for by an equilibrium between the solvated electrons, alkali ions, and their pairs. As demanded


a

(NAPTHALENE )~

b

SGAUSS

Ionic Species / 47

c

5 GAUSS

Figure 2.2. The ESR spectra of sodium naphthalenide. a) The spectrum of a free anion in HMPA (the dissociation of ion-pairs is quantitative in this solvent). b) The spectrum of tight sodium ion-pairs in tetrahydropyran (the Na coupling constant is large, 1.4 G): upper, the recorded spectrum; [ower, the computer-simulated spectrum. c) The spectrum of loose ion-pairs recorded after addition of tetraglyme to the tetrahydropyran solution of sodium naphthalenide (the sodium coupling constant is small, 0.4 G): upper, the recorded spectrum; [ower, the computersimulated spectrum.

by this model, the equivalent conductance decreases with increasing concentration of the metal. However, such a relation is observed in the lower concentration range of lithium, whereas at higher concentrations the equivalent conductance reaches a minimum and thereafter steeply rises with further increases of the metal's concentration. The character of the conductance changes at that stage; it acquires the feature of a metallic conductance, making then meaningless any discussion of this system in terms of free ions-ion pairs equilibria.


The behavior of ion-pairs in solutions has been extensively investigated since 1960 by applying a variety of spectroscopic techniques. Electron spin resonance studies yielded probably the most detailed information about their structure, although unavoidably they are restricted to salts of paramagnetic radical-ions. This technique allows the determination of the position and of the character of motion of the counterion with respect to its radical-anion.

Optical spectroscopy, although less informative, is less restrictive and permits studies of a greater variety of salts. It is simple and does not require high concentrations of the investigated species, a condition often imperative in NMR studies. Moreover, it often permits easy discrimination between free ions and tight ion-pairs since the absorption peak, say of the anion, shifts towards shorter wavelength as it becomes paired with cations. Nonetheless, an unequivocal distinction between free ions and their pairs is provided only by the conductance studies, the spectral shifts, discussed in the following section, do not take place on the formation of loose ionpairs.

Recent developments in NMR techniques permit studies of such nuclei as 6Li, 7Li, 23Na, 137es, and so on. This led to interesting information about the nature of ion-pairs. A comprehensive review of this subject was published by Popov. 14

4. Different Types of Ion-Pairs

In 1954 Fuoss,15 and simultaneously but independently Winstein,16 suggested that ion-pairs may exist in two distinct forms, now referred to as tight and loose pairs, respectively. The reasoning of Fuoss that led him to this conclusion is instructive and interesting. It is presented here in a somewhat modified version. An ion in solution may approach its partner, surrounded by a tightly attached solvation shell, with no hindrance until it contacts the shell. Thereafter, the resulting associate may retain its structure of a loose, solvent-separated pair, or it may collapse, squeezing out the solvent molecule (or molecules) separating the ions. In the latter case a tight, contact pair is formed. In essence, these two alternative outcomes of the association are equivalent to the establishment of an equilibrium between the loose and the tight ion-pairs.

The two types of ion-pairs form thermodynamically distinct species, each characterized by its own specific properties. * For example, as shown by Figs. 2.2b and 2.2c the ESR spectrum of the paramagnetic sodium

*It is implausible to treat tight pairs as covalently bonded and the loose ones as ionic. The electronic structure of the anion is virtually the same in both pairs, although it is slightly perturbed by the cation, see p. 49. '

Ionic Species I 49

naphthalenide in tetrahydropyran changes drastically on addition of a drop of tetraglyme to its solution.17 In the absence of the glyme only tight pairs are formed in this ether since its solvating power is low. The glyme is a powerful solvating agent of the cation; it strongly interacts with the Na + ,

surrounds it, and thus converts the tight pairs into the loose ones. The ESR spectra of both pairs are resolved into 100 lines, whereas the spectrum of the free naphthalenide anions in hexamethyl-phosphoric-triamide, a powerful solvating agent dissociating quantitatively ion-pairs, is resolved into the expected 25 hyperfine lines as shown by Fig. 2.2a. The increase in the number of lines results, as pointed out earlier, from the splitting of each hyperfine line of the free naphthalenide into four, an effect caused by the coupling of the odd electron with the sodium nucleus possessing 3/2 spin. The respective coupling constant for the tight pairs is 1.4 G (strong interaction), but smaller, 0.4 G (weaker interaction), for the loose pairs. This accounts for the different shapes of the spectra of the two kinds of ion-pairs and justifies their assignments as the tight and loose pairs, respectively, since the tighter the association of the anion with the N a + the larger the coupling constant. t

Replacement of a small cation paired to an aromatic anion by a larger one leads to a bathochromic shift in the absorption spectrum of the anion. The shift results from the stronger stabilization by a small cation, rather than a large one, of the ground state of the anion relative to its FrankCondon excited state. The conversion of a tight pair into a loose one is tantamount to the increase of the size of the counterion, and thus it leads to a shift of the absorption spectrum to longer wavelength as observed in such a process. 18 The absorption spectra of loose pairs are virtually indistinguishable from those of free ions because the displacement of the cation from the vicinity of the anion is then sufficiently great to eliminate, virtually entirely, its perturbing effect on the absorbance of the anion.

Extensive studies of the spectra of fluorenyl salts by Smid and HogenEsch18,19 greatly contributed to our understanding of the relations between the tight and loose ion-pairs. 19 Two absorption peaks at 355 and 373 nm appear in the spectrum of lithium 9-(2-hexyl)-fluorenyl in dimethyltetrahydrofuran, as shown in Fig. 2.3. Their relative intensities are unaffected by dilution but vary with temperature, the 373-nm peak becoming higher on cooling. 18 It was shown that the 355-nm peak results from the absorption of the tight pair and the 373-nm peak corresponds to the loose one. The

tThe problem is more complex. The statement is correct when the cation hovers above the plain of the anion. Had it been placed in the nodal plane of the aromatic anion no splitting of the lines would be observed.


0.0

1.5

1,0

0.5 .

340

- 200( - 26° ( - 31° ( - 41 ° (

" j" . \ I .

. \ / . . .'. \

/ !' .. : : \ / : ,: ....

: ,~ : \

360 A in nm

I \'; I \ ;'

I \ : \

,/ \\'\ \ ; r,. ~ .. \ \:

380 400

Figure 2.3. Variations of the optical absorption spectra of 9-(2-hexyl)-fluorenyllithium in 2,5-dimethyltetrahydrofuran with temperature reflecting the changes of the equilibrium between the tight and loose ion-pairs. Note the isosbetic point.

ratio of their relative intensities yields the equilibrium constant of the conversion:

tight pair ~ loose pair, Kt •l ,

and from the temperature dependence of K t •1 the respective dH?,1 and dS~1 were calculated. The results show that the conversion of the tight into loose pairs is exothermic and leads to a decrease of entropy. This implies that the gain in solvation energy is greater than the electrostatic work required for the partial separation of the ions, and the decrease of entropy results from an increased degree of solvation of the cations that immobilizes solvent molecules. For the sake of illustration, the results reported by Hogen-Esch and Smid for a variety of solvents are given in Tables 2.1 and 2.2.

Similar spectroscopic observations allowed the investigation of the effects of pressure on the equilibrium established between the tight and loose ionpairs.20 The results, illustrated by Fig. 2.4, reveal a large decrease in the volume of the system on conversion in tetrahydrofuran of the tight sodium or lithium salts of fluorenyl anions into the loose pairs. The decrease of volume amounts to 24 and 16 mllmole, respectively. Solvent molecules are

Ionic Species 151

Table 2.1. Ratios of loose and tight ion pairs of Lithium fluorenyl (F- ,Li+) and 9-(2-hexyl)-fluorenyllithium (HeF- ,Li+) in various solvents

Solvent

Tetrahydrofuran (THF) 3-Methyl-THF 2-Methyl-THF 2,5-Dimethyl-THF 2,5-Dimethoxy-THF 2-(Methoxy ,methyl)-THF 3,3-Dimethyloxetane o-Dimethoxybenzene m-Dimethoxybenzene Hexamethyl-phosphoric-triamide

4.6 0.85 0.33 0.02 0.04

>50 1.2

>50 0.01

>50

>50 >50

1.50 0.07

>50

Table 2.2. Enthalpies and entropies of conversion of tight ion-pairs into loose pairs in a variety of solvents

Lithium Lithium fluorenyl 9-(2-hexyl)-fluorenyl

-I1H -I1S -I1H -I1S Solvent (kcal/mole) (e.u.) (kcal/mole) (e.u.)

2,5-Dimethoxy-THF 2.9 16 2,5-Dimethyl-THF 2.0 14 10.0 50 Dioxolane 3.5 17 112_ Dihydropyran 3.0 16 8.2 33 Tetrahydropyran 6.6 28 THF 7.5 22 2-Methyl-THF 7.5 27 9.8 32 DioxanelTHF 111 3.6 14 DioxanelTHF 2/1 11.0 36 Hexamethylene oxide 4.3 16

more tightly packed in the solvation shell than in the bulk of the solvent. Assuming that an additional three molecules of tetrahydrofuran become incorporated into the solvation shell of the Na + ion on the conversion of the tight sodium fluorenyl into its loose pair, one finds that the above reaction contracts the volume of this ether by =10% of its molar volume. Such a compression of tetrahydrofuran requires a pressure of -2000 atm. Hence the force compressing the solvent around aNa + cation is equivalent to that needed to produce a pressure of 2000 atm.


0.0.

t

300 350

- + F,No in THF

3900otm.

__ 2920olm.

1920

---940olm.

____ 101m.

400 ~ nm

450

Figure 2.4. Variation of the absorption spectra of sodium fluorenyl in tetrahydrofuran solution with hydrostatic pressure at constant temperature, 25°C. Note, the appearance of the loose ion-pairs at higher pressure.

The absorption spectrum of the presumably skewed tetraphenyl ethylene dianion (T2 -) salts reveals an interesting phenomenon.21 The two cations of this salt are coupled to the two Ph2C - chromophores. In tetrahydrofuran, both sodium ions of this salt (T2 - ,2Na +) appear to be fully solvated and form two solvent separated ion-pairs absorbing at 495 nm (see Fig. 2.5). On addition of LiCI the sodium ions are replaced by lithium. The absorption spectrum of the dilithium salt, shown also in Fig. 2.5, reveals two peaks of equal intensity; the Amax of one is still at 495 nm, whereas the Ama< of the other is at 385 nm. It seems that one of the Li + ions is still fully solvated and forms on association a loose ion-pair, while the other, poorly solvated, forms a tight pair. The intramolecular solvent exchange

Ionic Species I 53

4 .0

";~ 3.0

.. ~

... 2.0 0

'= UJ

1.0

500 600 700 800

Unm

Figure 2.5. Optical absorption spectra of the salts of tetraphenylethylene. Dotdash line: the spectrum of the sodium salt in THF. Solid line: the spectrum of the lithium salts obtained by adding the equivalent amount of Liel solution to the sodium salt solution. Dashed line: the spectrum of the solution after the addition of one half of the equivalent amount of Liel solution to the sodium salt solution. The resulting solution contains a 50:50 mixture of the sodium and lithium salts. The addition of Liel converts the loose disodium ion-pairs into a dilithium salt with one tightly and one loosely bonded Li +. Note the isosbetic point.

between these two cations is fast on the NMR scale as proved by the pertinent NMR spectrum showing only one line, its peak shifts as the proportion of Li + increases.

Tight ion-pairs formed in a poorly solvating medium may be converted into loose ones by the addition of a powerful solvating agent to their solution. As mentioned earlier, such a conversion occurs on addition of tetraglyme to a solution of tight sodium naphthalenide pairs in tetrahydropyran (see the ESR spectra shown in Fig. 2.2). The optical spectra, seen in Fig. 2.6, reveal another example of such a phenomenon. The addition of dimethylsulfoxide to a dioxane solution of lithium fluorenyl results in conversion of tight into loose pairs.22 In the absence of this solvating agent only the tight pairs are present in the solvent, but its addition led to the appearance of the loose pairs. The intensity of their absorption increases with increasing concentration of the additive. The equilibrium constant of the association

tight Li + ,fluorenyl- + Me2SO ~ loose Li + ,Me2S0, fluorenyl- , Ke,


" o

1.0

0.5

lDMSO lo >< 103

1 = 235 2 _ 5.10

J = 8.49

4 = '1 .4 5 K7

360 A,mu

380 400

Figure 2.6. The conversion of tight ion-pairs of sodium fluorenyl in dioxane solution into the loose ones on addition of dimethylsulfoxide as revealed by their optical spectra. Note the isosbetic point.

Table 2.3. Effect of solvating agents (A M) on the tight-loose pairs equilibria

Solvating agent

Oxetane THF THP Hexamethylene oxide 2,3-Dihydrofuran 3,4-Dihydropyran Dioxolane Dioxane

[F- llLi + ]I[F-,Li +]

>50 4.6 0.45 0.24 1.1 0.92 0.08 0.01

>50 20 2.3

50 0.14

10 0.3

was calculated then from the results derived from such observations. Similar studies performed with other solvating agents allow the determination of their relative complexing power leading to the data collected in Table 2.3.

More intricate relations were observed in the system potassium fluorenyl-tetraglyme in THF solution. The glyme may be complexed with the salt externally or internally yielding a loose pair in the latter case but

Ionic Species / 55

not in the former. Treatment of such a system is outlined in ref. 19, p. 121.

In solvents where the investigated salt exists as a mixture of tight and loose pairs, the simplistically computed complexation constants of some added coordinating agent differ from those obtained from the studies of their complexation taking place in poorly solvating media. In the former media the complexing agent competes with the solvent for the coordination sites on the cations and then, as deduced by Smid (see ref. 19, p. 128), two kinds of loose ion-pairs are formed: those with ions separated by the solvent molecule, and the other with ions separated by a molecule of the additive. Their proportions could be computed from the experimentally observed dependence of the complexation constants on the concentration of the complexing agent.

The extensive ESR studies of Hirota23 revealed the existence of two kinds of contact ion-pairs of the sodium salts of naphthalene and anthracene radical anions: those externally coordinated with the molecules of a complexing agent, and the other externally solvated by the solvent, both coexisting in the same solution. It is apparent, therefore, that more than two types of ion-pairs may coexist simultaneously in some solutions, their presence being revealed by their distinct properties.

The importance of applying different techniques in studies of structure of ion pairs cannot be overstressed. A change detected by one method of investigation, (e.g., ESR) , may remain undetected by the other study (e.g., optical spectroscopy).

Powerful cation complexing agents were developed by Pedersen24 and by Lehn,25 namely the crown ethers and the cryptands. Their complexation with inorganic salts considerably increases the solubility of the latter in organic solvents. The existence of such complexes in water and in methanol was demonstrated by potentiometric measurements. Complexation of fluorenyl salts by crown ethers is too extensive in tetrahydrofuran to permit the determination of the respective complexation constants by optical or NMR spectroscopy. However, the relative complexation constants of a crown with a variety of cations could be determined by recording the optical or NMR spectra of solutions containing salts of two different cations and a limited amount of crown ether. 26 The NMR studies led also to determination of the rates of crown exchange between two cations or the exchange between a free crown and the one complexed with a cation.

Some salts of divalent cations reveal the existence of two different kinds of association in the same complex. For example, two absorption maxima of equal intensity appear in the spectrum of the strontium salt of fluorenyl, Fl- ,Sr2 + ,FI-, recorded in tetrahydrofuran at - 25°C. This observation implies different degrees of attachment of the two anions to the cation,


one fluorenyl anion being strongly bound to the cation, whereas the attachment of the other is weak. 27

Judging by its absorption spectrum, the barium salt of fluorenyl, Fl- ,Ba2+ ,Fl- , forms in tetrahydrofuran only tight associates, both anions are strongly bound. Its complex with crown ether seems to be loosely bound; only one absorption peak at longer wavelength appears in its spectrum. Interestingly, this is a 1: 1 complex and its composition remains unchanged on addition of an excess of the crown.19 Apparently the complex has a sandwich structure with the cation oscillating rapidly through the ring of the crown. For the Ba2+ salt, the exchange between the bound and free crown is relatively slow compared with a similar process involving the sodium salt. In a 1:1:1 mixture of the crown, the sodium, and the barium salt, all the crown is bound to barium, confirming the much stronger association of the crown with Ba2+ than with Na + .

The complexing agents discussed above interact with the cations. An interesting complexing agent capable of interacting strongly with cation as well as with anions was synthesized recently by Reetz and his students.28

It resembles a crown ether, but it possesses a boron containing moiety attached to the phenyl ring, as shown in Scheme 2.1. The empty orbital of boron acts as a sink for the lone electron pairs of anions providing a bonding center for these species, whereas the (CH2CH20)n ring strongly binds the cations.

The relative solvating power of solvents may be gauged by determining the mole fraction of the reduced aromatic hydrocarbon A (Le., [A -:-]/[A] = K) when its solution in the solvent under investigation is kept in contact with an alkali metal until the eqUilibrium: "alkali metal + A (in solution) ~ A -:- , Metal + in solution" is established. The most extensive studies of such reductions were reported by Shatenstein29 who determined photometrically the concentration of the relevant radical-anions and calculated

Scheme 2.1. Crown ether-boronate has two types of binding sites.

Ionic Species 1 57

then the respective equilibrium constant. His results, summarized in Table 2.4, show the importance of steric factors, often the most decisive in making a solvent a powerful coordinating agent.

Shatentein's method does not reveal the number of solvent molecules composing the solvation shell. This information is provided by a modified approach.30 After establishing the equilibrium between an alkali metal and an aromatic hydrocarbon A in a solvent poorly solvating the salt of the resulting radical-anions, one determines the increase of the degree of their reduction caused by the subsequent addition of the investigated, better solvating solvent to the previous solution. Such experiments may reveal a stepwise character of the solvation process, a phenomenon observed during the studies of solvation of Na + ,AI(Butyl) 4" in hexane solution.31

The stepwise solvation of ions in the gaseous phase was extensively investigated by Kebarle32 who determined the heats of addition of the first, second, third, and so on, molecule of solvent to a gaseous ion. His technique allowed him also to investigate the competition of two solvating agents for an ion, and revealed the contrasting effects of basicity and bulkiness of the solvating molecules on their capacity to be incorporated into a partially formed solvation shell. For example, methanol is preferred over water in the formation of a monosolvated H+ ion, because in the gas phase the former is more basic than the latter. However, water is preferred to methanol in large solvation clusters, e.g., in that of 9 ligands, since the smaller size of water molecule reduces the steric hindrance caused by the crowding of ligands around the solvated ion.

In conclusion, this section reveals to the readers the complexity of interaction of ions with solvents and solvating agents. Numerous distinct species are formed in such systems, each characterized by its specific properties revealed by their spectra and, as will be seen later, they often differ

Table 2.4. Enthalpies and entropies of reactions of metallic sodium with biphenyl solutions: metallic Na + biphenyl;;: biphenyl:- ,Na +

Solvent T range in °C - l1H1kcal/mole l1S/e.u.

Tetrahydropyran -45 to 0 6.8 31 DiEt-ether -15 to 20 9.6 38

-30 to -15 22.0 86 Me-THF -53 to -25 9.9 43 THF -10 to 40 11.2 40 1,3-DiMeOPr -35 to -10 15.5 63 1,2-DiMeOPr o to 45 16.5 58 DiMeO Ethane o to 45 17.4 60 THF/Heptane 6/1 -10 to 10 8.5 54 Glyme-3lHeptane 111.4 25 to 45 31 100


greatly in their reactivities. As mentioned earlier, their distinction requires application of different techniques; they may appear to be identical when tested by one technique, but their distinction and individuality could be revealed through another experimental approach.

5. Equilibria Between Free Ions and Ion-Pairs: Conductance Studies

In solutions of partially dissociated salts the conducting free ions are in equilibrium with the nonconducting ion-pairs, e.g.,

The dissociation constant Kdiss is derived commonly from the dependence of the equivalent conductance of the studied solution A, on the concentration of the salt denoted by c, in conjunction with the value of the limiting conductance Ao,

Ao = limA (c) for c --+ O.

The latter is needed in the calculation of K diss . The simplest procedure, due to Ostwald, leads to the values of both constants, KdisS and Ao, derived from a linear plot:

Thus Ao is given by the reciprocal of the intercept, whereas the reciprocal of the slope provides the value of KdissAij from which Kdiss is computed.

The above relation is based on the mass law valid for sufficiently dilute solutions since then the activity coefficients differ only negligibly from unity. For real solutions, which deviate substantially from ideality, it is necessary to modify the above procedure. The best and most widely used modification,applicable to such systems, was developed by Fuoss. 33 It calls for plotting F/A vs. cA . FIF, where F and f are functions of the salt concentration c and of A, as well as of temperature, dielectric constant, and viscosity of the investigated solution. A computer program for calculation of F and f is available and the values of Ao and Kdiss derived from the Ostwald plot are used to start the computation. The intercept of the Fuoss plot gives again 1/Ao, and its slope yields 1lKdissAij.

In the course of studies of ionic polymerizations, dissociation constants as low as 10-12 M have to be determined. To get reliable values for the slope and intercept of the Fuoss plot, it is imperative to extend the measurements to very low concentrations of the salt, often lower than 10-6 M;

Ionic Species I 59

otherwise the slope is too low and the intercept too high. This is clearly shown by Fig. 2.7. Since both errors reinforce each other, the error in the such computed KdisS could be very large. Moreover, rigorous elimination of any impurities by the purging procedure described elsewhere34 is then imperative. Furthermore, even at these high dilutions the intercepts of the Ostwald or Fuoss plots are often too small to allow their reliable determination. Therefore other methods, to be discussed later, have to be employed to provide more accurate values of Ao. Let it be stressed that the value of Aij is needed for the calculation of Kdiss from the experimentally determined value of the slope, and any error in the evaluation of Ao is strongly magnified in the computation of the K diss '

The limiting conductance depends on the viscosity, 1'1, of the solvent. However, the product 1'I.Ao is approximately constant (Walden rule) and independent of the nature and temperature of solvents, at least for a series of similar liquids. The above relation is useful; it allows one to calculate Ao pertaining to a desired solvent and temperature either from its more reliable value determined at a different temperature or by utilizing some more accurate Ao values derived from the conductance study performed in another, but similar solvent.

The knowledge of Ao is also of considerable intrinsic interest. Its value is given by the sum of the equivalent conductancies of the pertinent cations, At, and anions, Ai, i.e.,

28

21. ..E A

20 - S~No· in THP I , o TWO-.ndM 16 o On.-.ndM

I. 8 12 16 20 21. 28 32 36

f2CN F x 10·

Figure 2.7. The Fuoss plot of FIA vs. flcAlF for the conductance of sodium polystyryl in tetrahydropyran at 25°C. Note the curvature of the plot and the error caused by failure to extend the measurements to very low salt concentration.


For a salt composed of ions of presumably of equal mobilities, the individual values of ~t and ~o are given then by one half of the limiting conductance of the investigated salt. Tri-iso-amyl-butyl ammonium tetraphenyl boride exemplifies such a salt.35 From its limiting conductance the ~o of the BPh4" anion is calculated and thereafter, by subtracting its value from the 1\0 of the investigated boride salt, the ~"6 of the respective cation is obtained. The resulting values of the ~t allow one to compute the values of the corresponding Stokes radii of the pertinent cations. The Stokes radius is the radius of a hypothetical, uncharged sphere that diffuses through a solvent equally fast as the investigated ion. Its value, rs, is proportional to the reciprocal of the mobility, ~t or ~o denoted by ~, of the ion under consideration, namely rs = O.819/~.TJ where TJ denotes the viscosity of the solvent.

The Stokes radius provides a measure of the size of a solvated ion, and, for the sake of illustration, the mobilities and Stokes radii of alkali cations in THF are listed in Table 2.5. Significantly, in that solvent the Stokes radii of the small Li + and N a + cations are larger than the Stokes radius of the bulky Cs+ ion,36 implying a more extensive solvation of the former ions than of the latter one. The gradient of the electrostatic field around the small Li + or Na + cations is large. Therefore, these cations strongly bind the surrounding molecules of solvent and form compact solvation shells which participate in their motion. The gradient is much lower around the larger Cs + cation; hence Cs + behaves in its motion as if it were a nearly bare ion, and its Stokes radius is only slightly greater than its crystal radius.

The relation between mobility and the size of an ion is more complex for the large and flat ions. The resistance of solvent to their motion should depend on their orientation, the biggest contribution to their mobility coming from the movement in the direction perpendicular to their smallest cross section. These considerations account for the observed mobilities of large, flat aromatic radical-anions,37 and large, flat aromatic cations.38

Table 2.5. Mobilities (A+) and Stokes radii (rs) of alkali cations in THF at 25°C

Cation

Li+ Na+ K+ Cs+ NH:

36.6 45.2 49.8 79.0 84.8

4.86 3.94 3.57 2.25 2.10

Ionic Species / 61

The Ostwald or Fuoss plots obtained for living polymers have intercepts often indistinguished from O. In such cases the needed Ao values may be calculated by summing the pertinent ~t and ~o. For example, in the conductance studies of the anionically growing salts of polystyrene in ethereal solvents the needed ~t is determined through the previously described procedure utilizing the conductance data obtained for the respective tetraphenyl boride salts, while the ~o is derived from the Stokes-Einstein equation relating mobility and diffusion constant. Since the diffusion constants of dead and living polystyrene of the same molecular weight are virtually the same, the mobility of the living polystyrene anion could be calculated from the experimental data reported for the dead polystyrene. An alternative method is described in Chapter 3 dealing with the propagation of living polymers.

The dissociation constants of ion-pairs composed of large, poorly solvated ions could be calculated by a theoretical approach. In Bjerrum's treatment, ions are visualized as rigid, electrically charged spheres of radius !a immersed in a continuous medium characterized by its dielectric constant e and viscosity TJ. Coulombic attraction of the oppositely charged ions leads then to formation of ion-pairs. The probability density (not normalized) of finding two oppositely charged ions separated by a distance r is 4-rrr2dr . exp( - e2lrekT) with a minimum at r = rB = e212ekT. The latter is known as the Bjerrum radius. It was proposed to consider two oppositely charged ions separated by a distance smaller than rB as an ion-pair, while those being further apart were counted as free ions. On this assumption the fraction 'Y of all the ions associated into pairs is given by the integral

IrB

'Y = a (4-rrr 2drN)· exp( -e2IrekT),

where a denotes the distance of closest approach of the ions (the distance at contact), and the dissociation constant is given therefore by the relation

Kdiss = C • (1 - 'Yf • P/-y.

This derivation is questionable because the proposed distribution function is faulty since it tends to infinity as r increases. Nevertheless, the values of the dissociation constants computed from the above relations for a equal to 6.4 A agreed surprisingly well with those obtained from the conductance studies of solutions of tetra-iso-amyl ammonium nitrate in a series of dioxane-water mixtures. These values vary within a factor of 1015 for different mixtures, and therefore the most impressive agreement of the computed constants with the experimental results was hailed as a strong evidence of the reliability of the proposed approach. 39


For aqueous solutions the Bjerrum radii have reasonable magnitudes of less than 10 A. However, in hydrocarbon solvents their values are implausibly high, exceeding 100 A. One cannot treat two ions separated by such a distance as an ion-pair, nonetheless the Bjerrum radius still has a physical significance. It defines the transition state of dissociation of ionpairs into free ions-a state of a pair when its diffusion-controlled dissociation is equally probable as the diffusion-controlled association of the resulting ions.

The unsatisfactory feature of Bjerrum's treatment, causing the divergence of the distribution function with the increasing interionic distance, was remedied by Fuoss,40 and subsequently by other workers, who removed the redundance of the partners associated with a reference ion. (For a simple treatment of this problem, see ref. 35, p. 220.) Still, the choice of rB = e2/2ekT as the critical distance is artificial and physically unrealistic.

An alternative approach was suggested.41 Two situations are contemplated: the two oppositely charged ions are either free, being far apart from each other, or are in contact, paired together with their centers at a distance r1 + r2 , where r denotes the judiciously chosen ionic radii. This is a plausible treatment for highly dilute solutions because under these conditions it is improbable for two ions to be separated by some intermediate and not too small distance. Obviously, when not paired together they are far apart.

On dissociation the electrostatic free energy of a pair increases by:

Neglecting the deviations of the activity coefficients from unity and taking the pre-exponential factor as the fraction of volume occupied by the pairs, one finds:

This approximate relation, reliable for bulky ion-pairs in poorly solvating media, is useful for the estimation of their interionic distance.

Finally, let it be stressed that the dielectric constant of a medium is not the only parameter affecting the degree of dissociation of an ion-pair in a solvent. For example, the dielectric constants ofTHF and dimethoxyethane (DME) are similar, namely 7.39 and 7.20, respectively at 25°C. Nonetheless, many ion-pairs are more extensively dissociated in DME than in THF. The bidentate nature of DME reduces the negative entropy of solvation of ions; it seems that 2 molecules of DME suffice for solvation of a cation

Ionic Species I 63

when 4 molecules of THF are needed. For the same reasons the dissociation of ion-pairs is very extensive in glymes, CH30(CzH40)nCH3, n = 2-4.

The necessity of considering other factors than dielectric constant in evaluation of the dissociation of ion-pairs was stressed in a later paper of Fuoss.42 As he pointed out, divergent values of the dissociation constant are observed for the same value of the dielectric constant and vice versa. This is illustrated by the results shown in Fig. 2.8 giving the pairing constants of potassium iodide in a variety of solvent mixtures. Moreover, the local dielectric constant of a mixed solvent in the vicinity of ions differs from the bulk constant due to demixing of the solution.

Some general remarks are appropriate. Divergent values of dissociation constants were reported in the literature for some salts of living polymers. The dissociation constant of the lithium salt of polymethylmethacrylate in THF at -70°C was reported as 200 . 10-10 M by one research group, whereas another team claimed a value of 4 . 10-10 M. As a rule, a lower value should be more reliable. The discrepancies may be caused by the presence of conducting impurities in the studied solutions, or could arise from faulty extrapolation to zero concentration of the salt.

o L-______ ~ ______ ~ ______ ~

45 60 D 75 90

Figure 2.B. The lack of dependence of 1nKdiss of ion-pairs on the dielectric constant of the solvent for a variety of solvents.


6. Heat and Entropy of Dissociation of Ion-Pairs

The equation deduced in the preceding section for Kdiss leads to:

Since dinE/dinT is usually smaller than -1, e.g., -1.16 for tetrahydrofuran, -1.33 for diethyl ether, and so on, the dissociation of bulky ionpairs is expected to be slightly exothermic. The values of aHo calculated from the above equation agree fairly well with the experimental ones for bulky ions in poorly solvating media. However, for small ions in wellsolvating media the exothermicity is substantially larger. Its increase arises from the more extensive degree of solvation of free ions than of the associated ion-pairs. Consequently, the dissociation of such pairs decreases the entropy of the system since it immobilizes the solvent molecules that become incorporated into solvation shells of the free ions.

The plots of InKdiss vs. lIT for sodium and cesium salts of fluorenyl carbanions in tetrahydrofuran are shown in Fig. 2.9. It is evident that the heat of dissociation is greater for the sodium than for the cesium salt, confirming the previously mentioned larger degree of solvation of the Na +

than of the Cs + ion and very large for Li + ions. Similar results obtained for the sodium and cesium salts of polystyryl carbanions are shown in Fig. 2.10.

The flattening of the above curves at lower temperatures deserves comment. Tight and loose ion-pairs are present in these systems. Hence, the equilibrium between the free and paired ions is given by the scheme:

K'I Kdiss I tight pairs ~ loose pairs ~ free ions,

and the observed Kdiss is given by the relation:

where Kdiss,1 denotes the eqUilibrium constant of dissociation of loose ionpairs, and Kt,1 is the eqUilibrium constant of conversion of the tight into loose pairs. At high temperatures when K t ,/ « 1, the slope of the van't Hoff line is equal then to:

where aH?,1 and aH~iss,1 denote the heat of conversion of tight into loose ion-pairs and the heat of dissociation of loose pairs, respectively. On the

2.5

2.0

I.~

1.0

o F: Li +

o f: NO+

- + D. F. C.

109 Kd

+7

i

(Li)

(No)

3D

Ionic Species / 65

log K +9 ,

2.0

1.0

_ I/T)( lOll

4.0 5.0

Figure 2.9. Plot of logKdiss vs. Iff for the ion-pairs of fluorenyl anions in TIIF. Note the difference between the exothermicity of dissociation, ~HdisS> of the Na+ and Cs + salts.

other hand, at very low temperatures when Kt,l » 1 the slope is lower being equal to dHY only. The curvature of the Van't Hoff plots is discussed again in Chapter 3 dealing with the propagation of living polymers. The equation for dH~.s., shown at the beginning of this section, applies to dH~.ss,I' since the bulky well-solvated ions are not further solvated on their dissociation.

7. Triple Ions

The treatment discussed in the section 5 predicts a decrease of equivalent conductance with increasing concentration of the salt. However, in the course of their extensive conductance studies, Fuoss and Kraus43 observed


a

2.4

2.0

1.6

1.2

0.8

0.4

o

b 1.4

~ 10 III VI

"0 ~

Ol o - 0.6

71 K "'v\tvS-,Na+inTHF / • 09 d·

3. 5

ISS /

DO ne l ivi ng end /

o Two I iving ends / /

/ D /

I I

/

/

/

4.5

0.29

o 0 .27 <

F

0.25

5.0

t:,. H= -0.9 k cal/mole

kcal/mole (b)

3.5 4.0 4.5 5.0

Figure 2.10. a) The Van't Hoff plot giving the dissociation constant of sodium polystyryl in THF as a function of lIT. Note the curvature of the line at low temperatures indicating a decrease of I1Hdiss • b) The Van't Hoff plot giving the dissociation constant of cesium polystyryl in THF as a function of lIT.

Ionic Species / 67

at sufficiently high salt concentration an increase of equivalent conductance. These findings were interpreted as evidence of association of ions with ion-pairs leading to the formation of charged triple ions capable of conducting electric current:

A - + B + ,A - ~ A - ,B + ,A - ,

or

For Kt- = Kt+ the above equilibria may be combined into:

3 A-,B+ ~ A-,B+,A- + B+,A-,B+.

The formation of triple ions requires modification of the relation between the equivalent conductance and the concentration of the salt. The approximate relation, valid at not too low salt concentration, becomes useful:

where Kt = Kt- = Kt+ is the equilibrium constant of association of free ions with ion-pairs into triple ions, Ao and An are the limiting conductances of the free ions and the triple ions, respectively, and c is the total concentration of the salt. This relation is valid for the range of concentrations of salts high enough to maintain the ratio [free ions]/c « 1, but sufficiently low to keep the ratio, [triple ions]/c « 1. Under such conditions (Kdiss1c)1I2 = [free ions]/c and Kt . (KdisS . C)1/2 = [triple ions]/c. At higher salt concentrations, but still for [triple ions]/c « 1, the conductance is proportional to Cl/2•

For K t- = Kt+ the concentration of free ions is not affected by the formation of triple ions, provided that their mole fraction is low, but for Kt- ~ Kt+ the formation of triple ions does affect the concentration of the free ions. This is seen clearly on inspection of an alternative scheme useful for the description of the eqUilibria between free ions, ion-pairs, and their triple ions,44 namely;

2 A - ,B + ~ A - ,B + ,A - + B + ,

and

2 A - ,B + ~ B + ,A - ,B + + A - ,


For KT+ = 0 and KT - »KT+ one finds [A-,B+,A-) = [B+), and at sufficiently high concentrations of the salt KT+ » Kdisi[A - ,B +). In this concentration range, generation of triple ions through the reaction of two ion pairs yields more B + cations than the direct dissociation of ion pairs into free ions. Moreover, under such conditions

i.e., the ratio [B+)/c is constant = K~':.. The buffering effect of B+ ions depresses the concentration of A - ions and makes it constant, equal to Kdis/K}~ and independent of the concentration of ion-pairs.

The reverse relations are valid for KT - = 0 and KT+ » KT _. Under the latter conditions the concentration of B + ions becomes constant, equal to KdissIK~~, and independent of the concentration of ion pairs. Finally, let us stress that the constants KT - and KT+ are dimensionless while the values of k t- and kt+ depend on the choice of the concentration units.

The conductometric evidence of formation of triple ions is supported by the direct observation of some of them through ESR spectra. For example, the ESR spectrum of sodium salts of the duroquinone radical anion is affected by the presence of an excess of sodium tetraphenyl boride. Each of the 13 hyperfine lines of this radical-anion becomes split into seven sharp lines,45 as shown by Fig. 2.11, implying that the spin of the odd electron of the radical-anion is coupled with the spins of two sodium nuclei (their total spin = 3). The sharpness of the lines implies a long lifetime of this aggregate, i.e., the lifetime of the duroquinone.- , 2Na + triple ion is much longer than 10-5 s. Interestingly, both cations retain their identity and follow the odd electron as it is transferred from one duroquinone to another in the exchange process. 46

The aggregates involving a bivalent cation and two monovalent anions resemble in some way the triple ions. However, they are electrically neutral and hence do not conduct electric current. Those composed of two different anions, namely A - ,Me2 + ,B - , could dissociate either into A - ,Me2 + cations and B - anions, or into B - ,Me2 + cations and A - anions. Usually, one of these modes of dissociation is strongly preferred to the other. For example, the aggregate of polystyryl anion and BPhi with Ba2 + dissociates in THF into polystyryl- ,Ba2+, and BPhi. The heteroaggregates are in equilibrium with their homoaggregates, i.e.,

2A - ,Me2 + ,B - ~ A - ,Me2 + ,A - + B - ,Me2 + ,B - .

The formation of heteroaggregates is favored by the increase of entropy of the system. Reaction of ion-pairs with the salts of bivalent metals yields

Ionic Species / 69

10MHz

Figure 2.11. The ESR spectrum of duroquinone radical anion recorded in the presence of an excess of sodium tetraphenyl boride. The formation of triple ions

is demonstrated by the splitting of each of the 13 hyperfine lines of the quinone into seven lines resulting from interaction of the odd electron with the two nuclei of sodium (total spin components 3,2,1,0, -1, - 2, - 3).


the mixed aggregates. For instance, lithium polystyryl reacts with MgBr2 in benzene solution yielding the mixed aggregate polystyryl- , Mg2+ ,Be.

In want of a better term the neutral aggregates such as A - ,Me2+ ,Aare referred to as triple ions, although their electric neutrality deprives them of the conducting property of the genuine triple ions.

8. Higher Ionic Aggregates

In many systems the dipole-dipole interactions lead to the formation of dimers of ion-pairs and still higher aggregates. These are readily formed in poor solvents, e.g., diethyl ether and hydrocarbons. In the extreme cases the formation of aggregates leads to micelles. The large aggregates provide a favorable environment for free ions and distort the free ions-ion-pairs equilibrium.

Typical examples of aggregates encountered in studies of ionic polymerization are the dimers of polystyryl lithium or the tetramers of polybutadienyl lithium, both formed in hydrocarbon solvents. The extensive aggregation of living polytetrahydrofuran and other alcoholates is observed even in good solvents.

On the whole, the aggregates are not reactive and do not contribute to initiation or propagation of ionic polymerization. However, they participate in the polymerization since they dissociate into a more reactive species with which they remain in equilibrium. They form, therefore, the dormant polymers.

The aggregates are often disrupted by various coordinating reagents, here denoted by C. On the addition of the latter, equilibria such as

(A - B +) + 2C --" 2A - B + C ,2 ~ "

are established. The pairs associated with the coordinating agent usually are more reactive than the homoaggregates, but not necessarily more than the nonaggregated and noncoordinated ion-pairs.

The formation of triple ions in a solution of a weak electrolyte (KdisS = 10-11 M) may be recognized from the plots of 11 A vs. Ac. The anticipated straight line appearing in the absence of triple ions becomes progressively curved downward as the proportion of the triple ions increases (see Fig. 2.12a). On the other hand, the formation of dimers of ion-pairs, but not the triple ions (a hypothetic case since triple ions are formed before the dimers) is revealed by an upward curvature (see Fig. 2.12b).

The formation of triple ions of both kinds (Kt+ = Ky_ = 1.105 M- 1)

is manifested also by the plot of log A vs. log c which appears as a curve with an initial slope of - ~ tending eventually to a slope of + ~ at higher

a

b

Ionic Species / 71

3.00 .--__________ ~--_:::'-----_,

N

<=> )(

2.00 .:§

1.00

0.00

0.00

4.00

b 3.00 . )(

:§

2.00

1.00

0.00 0.00

a

0.50 1.00

2.00 4.00

. : KI+= OM'! KI'= OM'! b: KI+: OM'! KI'= 100M" c: Kt+= OM'! Kt'= lOOOM' ! d: Kt+: OM'! Kt"= I OOOOM'! e: KI+=IOOOOM'! Kt' : lOOOOM' !

I\C XIOs

1.50 2.00

Kdim(M" ): a: 0 b: 30 c: 100 d : 300 e: 1000 f : 3000

I\C XI06

6.00 8.00 10.00

Figure 2.12. a) The effect of the formation of triple ions on the plot l/A vs. Ac for a weak electrolyte (Kdiss = 10- 11 M). b) The effect of formation of dimers of ions-pairs, but not triple ions, on the plot of VA vs. Ac for a weak electrolyte (Kdiss = 10- 11 M).

salt concentration (see Fig. 2.13a). The formation of the dimers distorts such a line downwards. For only one kind of triple ion (K y = 1.1()6 M- 1),

log A vs. log C becomes horizontal at high concentrations. Dimer formation again distorts the line downwards (see Fig. 2.13b).

The diagnostic value of such plots is obvious.


a ____________________________________

b

Kdim(M'I):

-1.50

·2.00 -7.00

0.00

-0.50

-1.00

Kdim(M'I):

-1.S0

a: 0 b: 100 c: 1.101

d: 1.10' .: I. lOs

-6.00

0: 0 b: 100 c: 1.101

d: 1.10' e: 1.I0s

c

d

L<>gC

·S.OO 4.00 ·3.00 -2.00

d

Loge -2.00 L-_ _ __ L--__ ----"L--_ _ -...l ___ ---i.:::....-__ ----'

-7.00 -6.00 -S.oo 4 .00 -3.00 -2.00

Figure 2.13. a) The effect of formation of dimers of ion-pairs on the plot of logA vs. log c in the presence of both kinds of triple ions (KT + = KT _ = 1.106 M-l). b) The same in the presence of one kind of triple ions (K,+ = K,_ = 1.106 M-l).

9. Dynamics of Interconversions Between Ionic Species

The equilibria established between the various types of ionic species are the subject of the preceding sections. Let us consider now the kinetics of their interconversion, the mechanisms of these processes, and the methods permitting determination of their rates. We begin the survey of this field by discussing first the conversion of covalent into ionic species.

Ionic Species I 73

Our knowledge of the rates of conversion of covalent species into ion pairs is meager. Two approaches allowing their determination are reported in the literature: one utilizes the dynamic NMR technique, i.e., the effect of the interchange between two or more species on the shape of their NMR lines,47 while the temperature jump technique was adopted in the other approach.

The application of the dynamic NMR technique is restricted by the limited choice of compounds adapted for such investigations. The suitable compounds have to ionize into stable ions, the relative concentrations of the covalent and ionic forms, influenced by the temperature and the polarity of the solvent should be comparable, and their solubility adequate to provide strong NMR signals. These conditions are met by trityl chloride and some of its derivatives in solvents such as sulfur dioxide, CD2CI2, CDCI3, CS2, CD3CN, and their mixtures. The interconversion of these compounds into their respective ion-pairs was investigated by Kessler and Feigel8• Separate NMR lines of these two species, the covalent trityl chloride and its ion pair (tight or loose?), appear in the spectrum when their solution in a mixture of S02 and CD2Cl2 is kept at temperatures lower than - 80°C. Coalescence was observed at somewhat higher temperature. The data thus obtained led to the value for the potential energy barrier of the ion pair collapse, namely aG* = 41.5 kJ/m, aH* = 44 kJ/m, and as* = 9.6 J/om at -70°C. The rate constants at this temperature are: k j = 40 S -1 for the ionization and kc = 67 S -1 for the collapse of ion-pairs.

A temperature jump method was used by Penczek and his associates,48,49 for studies of the systems: triflic esters (CF3S03 - R) in equilibrium with the respective cyclic oxonium ions, e.g.,

and the analogous reaction of the ester derived from oxepane (a seven membered cyclic ether). Note, not only a hetero-fission of the c-o bond takes place in the course of these reactions but also a ring closure.

The experimental procedure is simple. The ester and the pertinent ions resulting from its ionization are dissolved in a desired solvent. The solution is kept in a sealed NMR tube, equilibrated, say at 16°C, and then inserted in a spectrometer's cavity kept at the same temperature. The ratio of the concentrations of the ester and the ions is determined from the intensities of the pertinent NMR lines. Thereafter, the solution and the cavity are rapidly cooled to a lower temperature and the approach of the system to


the new equilibrium is followed by monitoring the intensities of the appropriate NMR lines. The process is slow, half lifetime is in the range of many minutes, while the thermal equilibrium is established in tens of seconds.

The relaxations were treated as reversible first-order processes, and since the required equilibrium constants were known, the forward and backward rate constants could be calculated from the rate of relaxation. The results shown in Table 2.6 were obtained for the tetrahydrofuran system at 25°C. Significantly, the rate of ionization increases with increasing polarity of the solvent, whereas the rate of ion collapse decreases.

The analogous processes are slower for the oxepane system (by about a factor of 1000 in the highly polar nitromethane). This is not surprising, the ring is larger hence the probability of its closure is lower. The above processes are discussed again in the chapter dealing with the propagation of cationic ring opening polymerization (see p. 179) where these spontaneous reactions are compared with those resulting from the monomer addition.

Let us consider now the kinetics of the interconversions between the various kinds of ion-pairs. For paramagnetic radical-anions the rates of conversion of tight into loose pairs may be studied by the ESR technique. As expected, the coupling constant of the odd electron to the nucleus of the cation is larger for the tight than for the loose pair (see Fig. 2.2). In the slow exchange region the presence of two types of ion pairs leads to spectra composed of two sets of lines corresponding to different coupling constants. Such a spectrum, recorded for lithium anthracenide in diethyl ether, was reported by Hirota50 who interpreted his findings as evidence for the presence of two distinct, slowly exchanging ion-pairs (lifetime » 10-5 s). Evidence for the presence of two types of rapidly exchanging ion pairs is provided by the line-width effect (broad, narrow, narrow, broad) first observed by Hirota,51 and subsequently reported by other workers.

The system sodium naphthalenide in tetrahydropyran with a judiciously added small amount of tetraglyme allows the determination of the rate constants52 of the reversible reaction,

tight ion-pair + glyme ;:= loose, glyme separated ion-pair,

from the shape of the ESR line. The bimolecular rate constant of the forward reaction is -107 M- 1 S-l and of the backward -105 S-l.

Table 2.6. Relaxation times for conversion of macroesters into macroions

Solvent

0.008 0.012

I1HllkJ/m

43.6 34.4

I1SjlJfOm

-138 -163

0.019 0.003

lliVkJ/m

50.7 57.8

-33.5 -79.6

Ionic Species I 75

The rates of ion-pair dissociation into free ions in aqueous solutions were extensively investigated by Eigen,53 and thereafter by other investigators, through the use of various jump techniques. Space limitation prevents us from reviewing the numerous results reported in the literature obtained by this technique.

The association of two oppositely charged free ions into a solvent-separated ion pair is treated as a simple diffusion-controlled process taking place under the influence of Coulombic force. Ritchie54 questioned whether the motion of ions in solution could be treated as independent of the motion of the molecules of solvent surrounding them, and whether the transition state of the association is "fully solvated" as usually assumed. These problems are important for the process of conversion of loose pairs into the tight pairs since then a partial desolvation of ions takes place. However, it seems that the association of free well-solvated ions into loose pairs could be still treated as a simple diffusion-controlled process.

A most versatile relaxation technique, which provides many results valuable for our discussion, was developed by Persoons.55 The measurements of conductance yields accurate information about the distribution between conducting and nonconducting species in a solution kept under any freely chosen but constant thermodynamic conditions. However, they tell us nothing about the processes leading to the conversion of one kind of those species into the other. Such a knowledge is gained from studies of rates of relaxation of the system after it has been perturbed from its steady state by some external action.

In Persoons's technique electric current flowing through a conductance cell is perturbed by a variable, high-strength electric field. The field affects the conductivity of the solution of ions and ion-pairs in two ways: by increasing the mobility of the fast-moving free ions due to their partial desolvation resulting from their high speed, and, to a much greater extent, by increasing the fraction of the free, conducting ions since the field enhances the rate of conversion of the nonconducting ion-pairs into conducting free ions. The former phenomenon is known as the first Wien effect, whereas the latter is referred to as the second Wien effect.

A successful treatment of the second Wien effect is due to Onsager56

who derived the following equation for the relative increase of the rate constant of ion-pair dissociation into free ions caused by the action of an electric field E.

kiE)lkiE=O) = 1 + 213q + l(2I3q)2 +

where q = -ele2/2EkT is the Bjerrum radius (see p. 61), and


with u denoting the mobilities of the ions. Because the rate of association of the ions is independent of the strength of the electric field,57 one derives for 1: 1 electrolytes the following approximate equation giving the relative increase of the dissociation equilibrium constant, K d , with increasing strength of the field, namely:

It follows that the field dissociation effect is most pronounced in low polarity media. Since 81n Kd is proportional to 81n (J' «(J' being the conductance of the solution), one concludes that at a very low degree of dissociation, the changes of Kd could be detected by observing the changes of conductance. Paradoxically, the lower the concentration of the conducting ions due to lower polarity of the solvent, the higher the sensitivity of this technique.

The electronic circuit developed for these studies is described in ref. 55. It applies a modulated square pulses of a high-frequency and a highamplitude electric field to a conductance cell containing the investigated solution. The cell acts as a nonlinear resistor and its response depends on the duration of the pulse, i.e., on the frequency of modulation, due to the time lag caused by the finite relaxation time of the system. The above method gives therefore the relative conductance, /l.(J'/(J', as a function of the frequency of the electric field modulation, i.e., it yields the dispersion curves, as exemplified by Fig. 2.14, from which the relaxation time could be calculated.

In the simplest case of a system composed of ion-pairs and free ions that remain at equilibrium with each other, the relaxation is described by the scheme:

and the relaxation time T, is given then by the relation

where Cion denotes the equilibrium concentration of the free ions. For low degrees of dissociation of ion pairs, the inequality ki « 2kacion is valid and Cion = (Kdiss • C )112, where c denotes the total concentration of the salt. Hence, the approximation

T- 1 = 2 . k (K . C)1/2 = 2 . (k.k )1/2 • Cl12 a dlSs IQ

Ionic Species / 77

10 r--=-:a:---""'"'O:a::---~:---:::::::::::-:""""'i::::::---------' tJ.(J

(T

O.B

0.6

0,4

0.2

x 'x

'x 'x

\ X

\ x

\ x

1.S

\ X \ X

\ x

\ , \

2,0

x 'x

...... x

2.S 3.0 3.5 4.0 log f

Figure 2.14. The dispersion curves resulting from modulation of the frequency of the field potential across the cell containing the solution of the investigated electrolyte, i.e., plots of !la/a vs. frequency of filed modulation.

is justified, and the plot of or-I Vs. cl12 yields a straight line through the origin. Such a plot, illustrated by Fig. 2.15, confirms the assumed simplicity of an investigated system. From its slope, in conjunction with the equilibrium constant of the dissociation derived from conductance studies, the rate constants of ionization, ki' and of ions recombination, ka, are calculated.

The reliability of the above technique was tested by determining the relaxation times of tetra butyl ammonium picrate dissociation in mixtures of benzene and chlorobenzene, and benzene and nitrobenzene. 58 The results are shown graphically in Fig. 2.15. It is remarkable how good is the agreement between the experimentally determined rate constants and those computed from the theoretical relations namely:

derived by Debye59 for the diffusion-controlled association of free ions, and

derived by Eigen60 for the diffusion-controlled dissociation of ion-pairs.


4r-----------------------------------------,

3

2

0~~~~~L-....L.-~ 0.2 0.4 0.6 0.8 1.0 1.2

102 ie

Figure 2.15. Dependence of the reciprocal of the relaxation time on the square of salt concentration. 0, lithium fluorenyl in diethyl ether-THF mixture; fl, tetrabutyl ammonium picrate in 50:50 chlorobenzene-benzene mixture; D, the same in 40:60 mixture; e, the same in 30:70 mixture.

The interionic distance a is calculated from Fuoss's equation giving the equilibrium constant of the experimentally determined dissociation constant (see p. 62). These results not only justify the assumed dynamic model for ion-pair dissociation, but also demonstrate the validity of the simple model of "sphere in continuum" for highly dilute solutions of large ions, even in mixed solvents.

Interesting findings were reported for solutions of fluorenyl lithium in diethyl ether.61 ,62 Only tight ion-pairs seem to be present in this solvent, no loose pairs could be detected by spectroscopic investigations. The plot of the determined T- 1 vs. CI12 yielded a straight line passing through the origin, as expected for a direct dissociation of the tight pairs into the ions and the direct association of the latter into the tight pairs. However, the rate constants of ion-pair dissociation k;, computed from the value of the conductometrically determined dissociation constant in conjunction with the value of the slope of the line T- 1 vs. CI12 , increase with decreasing temperature, being .73 s -1 at 20°C, 1.15 s -1 at - 20°C, and 1.45 s -1 at

Ionic Species / 79

- 40°C. Since activation energies of elementary reactions cannot be negative, this simple mechanism could not be correct.

It seems, therefore, that the solvent-separated loose pairs are present in this solution, although their concentration is exceedingly low, making them undetectable by the spectroscopic means. Only the loose pairs dissociate, and these are the primary products of association of the ions. The loose pairs, being in a rapidly established equilibrium with the tight pairs, are replenished on their dissociation through the fast tight-to-Ioose-pairs conversion, its equilibrium constant being denoted by Kt,l' The concentration of the loose pairs, Ch being minute compared with the concentration of the tight pairs, is given by Kt,1 • C because the concentration of tight pairs is virtually equal to the total concentration of the salt. The electric field does not perturb the fastly established tight-to-Ioose-pair conversion. Therefore, the relaxation time is given again by the previously deduced approximate relation, but with the rate constant ke,1 that of association of the free ions into the loose ion pairs, instead. of ke, i.e.,

The term due to the dissociation of loose ion-pairs, i.e., ki,I' could be neglected again, provided that Cion « C/, an acceptable assumption implying K diss ,/> the dissociation constant of loose ion-pairs, to be sufficiently low. Since

Cion = (Kdiss,I' Kt,1 • C)ll2 = (KdisS • C)ll2,

where Kdiss is the overall dissociation constant of the salt (for Kt,1 « 1 Kdiss = Kt,l • Kdiss,l), one gets:

-r- 1 = 2 . k . (K . )112 • Cl12 = 2k. . (K /K1I.2). Cl12 e,1 dlSS 1,1 t,1 dlSS •

The relaxation of this system resembles, therefore, that of a simple pairsinto-ions dissociation. However, the observed activation energy of the overall process (the conversion followed by the dissociation) is given by the positive activation energy Ediss,1 of the loose ion-pair dissociation, and the negative heat of conversion aHt,1 of the tight pairs into the loose ones from which is subtracted one half of the negative heat of dissociation of the salt into the free ions, aHdisS ' The previously calculated k i is given by ki,I' Kt,1 and its "activation energy" is given by the positive EdisS,1 and the negative aHt,I' For - aHt,1 > Ediss,1 the "activation energy" of k i is negative as was remarked earlier.

Let it be stressed again that the equilibrium between the tight and loose pairs is virtually unperturbed by the electric field because the rate of the


interconversion is much faster than the rate of dissociation of the loose pairs. On the other hand, had the dissociation been faster than the interconversion, then the relaxation of the system would be governed by the rate of conversion of tight into loose pairs and the observed activation energy would not be negative.

The above findings might indicate a general rule. In a system composed of tight and loose ion-pairs, the dissociation into free ions results entirely from the dissociation of the loose pairs, the tight pairs do not dissociate directly. This assumption is incorporated into the latest Fuoss theory of ionic dissociation.4z Moreover, the dissociation is slower than the interconversion of ion pairs, contrary to the conclusions of Schulz et al. 63

The power of the relaxation techniques is fully revealed when more complex dissociation processes are investigated. Two examples are appropriate. The field relaxation studies of solutions of cesium fluorenyl in dimethoxymethane,6zb,64 a solvent of very low polarity, E = 2.66 at 20°C, revealed that the reciprocal of the investigated relaxation time, or-I, is proportional to C3/Z as shown by Fig. 2.16. The conductance studies demonstrated that the triple ions, Fl- ,Cs+ ,Fl- and Cs+ ,Fl- ,Cs+, are virtually the only conducting species present in that system, because the equivalent conductance is proportional to CI/Z (see p. 67). Hence, the equilibrium between the conducting and nonconducting species is given by the scheme:

The triple ions are not formed by this implausible termolecular reaction. The most probable mechanism leading to their formation involves participation of the rapidly formed dimeric ion-pairs (Cs+ ,Fl-)z, i.e.,

followed by

The perturbation produces an excess of triple ions and the relaxation results from their recombination yielding the nonconducting ion-pairs and their dimers. Provided that the dimeric pairs form only a minute fraction of the nonaggregated pairs, i.e. Kd is very small, and that [triple ion] « c, then the first term in the expression giving or-I, i.e., 3·klKd ·cZ, being much smaller than the second one, 2·kb[triple ion], can be neglected. Hence,

or-I = 2k [triple ion] = 2k KI/Z • C3/Z b b trp •

Ionic Species / 81

4 10 C

5 10 15 20

-2 - I 14 -5 -1 10 t 10 t

12.5 12

10 10

8 7.5

6

5.0

4

2.5 2

Figure 2.16. Evolution of the concentration dependence of the reciprocal relaxation time for FlCs in DMM-THF mixtures. e, 3/2 order dependence in pure DMM at 23°C; 0, in pure DMM at 13°C; (), in pure DMM at 4.8°C. In a mixture with [THF] = 6.19 M (A, a, and _), a linear dependence on concentration of FlCs is found. A, 20°C; a, lOoc; _, O°C.

This result accounts for the observation. Since K trp is given by the conductance study, its value in conjunction with the slope of the line shown in Fig. 2.16 allows the computation of kb •

The relation 7- 1 - 212 still holds when small amounts ofTHF are added to dimethoxymethane. However, a linear relation is observed between 7- 1

and c, as shown by the upper curve of Fig. 2.16, when the proportion of THF in the solvent is higher.

In a more polar solvent one kind of triple ions is dominant, presumably the FI- ,Cs + ,FI- , and the conducting species are composed of an equimolar mixture of the triple ions and Cs +. Had the kinetics of conversion of the


nonconducting ion-pairs into the conducting Cs + and Fl- ,Cs + ,Fl- been described by the scheme:

kj 2· Cs+ ,Fl- ~ Cs+ + Fl- ,Cs+ ,Fl-,

ki,

with equilibrium constant K, then the reciprocal of the relaxation time, T-I, would be given by the relation:

T- 1 = 2·k;·c + 2·k~·[Cs+] = 2 . (k; + KII2 . k~)c.

In such a case the plot of T -1 vs. c would give a straight line passing through the origin. However, the intercept seen in Fig. 2.16 demands a more complex mechanism, presumably the one given by the scheme:

fast k'j

2· Cs+,Fl- ~ (CS+,Fl-)2 ~ Cs+ + Fl-,Cs+,Fl-, fast k'b

which leads to the relation

provided again that virtually all the salt is in the form of ion-pairs. The lCf is now comparable in its magnitude to k'b[CS+] because [(Cs+ ,F-h] is ofthe same order of magnitude as [Cs+]. Hence, the intercept of the lines shown in Fig. 2.16 gives lCf, while the value of k'b is derived from its slope. Note, lCf is distinct from k; and the ratio lCflk'b is not equal to K. The latter constant could be deduced from the conductance studies; it is dimensionless, whereas kjlk'b has dimension of molarity. The above examples show how some species undetectable by spectroscopic techniques, even if nonconducting, are revealed by the relaxation studies.

A general problem of relaxation of a system composed of ion-pairs, their dimers, free ions, and triple ions, was treated by Everaert and Persoons.65

They derived the following expression for the relaxation time:

T- 1 = {kd + (kt + ki)· 4eo/Ks + (9/16)· kf · Ks· (4eo/Ks)2}/(1 + 4eolKs) the dissociation term

+ 2 . {KJ~s . cb12 . (1 + eo/K+)112 . (1 + eo/K-)112 1 + 2eo/(K+ + K-) + 3~1K+ . K-

x {ka + 2(k;/K+ + k;/K-) . eo + 3 . kA/K+ . K-} the association term

Ionic Species I 83

where Kdiss refers to the equilibrium dissociation of ion-pairs to free ions, K + and K - to the equilibrium of association of free cation or anion, respectively, with their ion-pairs to the corresponding triple ions, ka and kd denote the rate constants of association of free ions into ion-pairs and dissociation of the pairs into free ions, k: and k;; the association rate constants of free ions with ion pairs into the respective triple ions, k;t and ki the reverse rate constants, and kfand kg the rate constants of association of the oppositely charged triple ions into an ion-pair and its dimer, and of the respective reverse process.

The following processes contributing to the relaxation were considered in this general treatment:

A + ,B - ~ A + + B - ,

(A+,B-)2~A+ + B-,A+,B- orB- + A+,B-,A+,

and

A + ,B - ,A + + B - ,A + ,B - ~ A + ,B - + (A + ,B - h.

By proper choice of the pertinent equilibrium and rate constants, the various limiting cases can be reproduced from this general treatment.

References

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3. Girson, E., Eriks, K., and deVries, J.L. 1950. Acta Cryst. 3, 290.

4. (a) Schildknecht, C.E., Gross, S.T., Davidson, H.R., Lambert, J.M., and Zoss, A.O. 1948. Ind. Eng. Chern. 40, 2104.

(b) Higashirnura, T., and Sawarnoto, M. 1978. Polyrn. Bull. 11.

5. Olah, G., Kuhn, S.J., Tolgyesi, W.S., and Baker, E.B. 1962. J. Am. Chern. Soc. 84, 2733.

6. Grattan, D.W., Plesch, P.H. 1979. J. Electroanal. Chern. 103,81.

7. Ziegler, K., and Wollschit, M. 1930. Ann. 90,479.

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12. Bjerrum, N. 1926. Kgt. Danske Vidensk. Selsk. 7, 9.

13. Atherton, N.M., and Weissman, S.1. 1961. J. Am. Chern. Soc. 83, 1330.

14. Popov, A.1. 1979. Pure Appl. Chern. 51, 101.

15. Sadek, H., and Fuoss, R.M. 1954. J. Am. Chern. Soc. 76, 5897, 5905.

16. Winstein, S., Clippinger, E., Fainberg, AH., and Robinson, G.C. 1954. J. Am. Chern. Soc. 76, 2597.

17. Hofelmann, K., Jagur-Grodzinski, J., and Szwarc, M. 1969. J. Am. Chern. Soc. 91,4645.

18. Hogen-Esch, T.E., and Smid, J. 1965,1966. J. Am. Chern. Soc. 87, 669, and 88,307318.

19. Smid, J. 1972. M. Szwarc (ed.), Ions and Ion-Pairs in Organic Reactions, vol. I, pp. 85-151, John Wiley, New York.

20. Claesson, S., Lundgren, B., and Szwarc, M. 1970. Trans. Faraday Soc. 66, 3053.

21. Levin, G., Lundgren, B., Mohammad, M., and Szwarc, M. 1076. J. Am. Chern. Soc. 98, 1461.

22. Hogen-Esch, T.E., and Smid, J. 1966. J. Am. Chern. Soc. 88,318.

23. (a) Hirota, N. 1967. J. Am. Chern. Soc. 89,32.

(b) Hirota, N. 1967. J. Phys. Chern. 71, 127.

24. Pedersen, e.J. 1967, 1970. J. Am. Chern. Soc. 89,7017,92,391.

25. (a) Dietrich, B., Sauvage, J.P., and Lehn, J.M. 1969. Tetrahyr. Lett. 34, 2885.

(b) Lehn, J.M., Sauvage, J.P., and Dietrich, B. 1970. J. Am. Chern. Soc. 92,2916.

26. Wong, K.H., Konizer, G., and Smid, J. 1970. J. Am. Chern. Soc. 92, 666.

27. Hogen-Esch, T.E., and Smid, J. 1972. see ref. 19, p. 135.

28. Reetz, M.T., Niemeyer, C.M., and Harms, K. 1991. Ang. Chem. 103,1515.

29. (a) Shatenstein, AI., et al. 1963. Dokl. Akad. Nauk SSSR 151, 353.

(b) Shatenstein, A.I., et aI. 1967. J. Polym. Sci. C 16, 1729.1967. Usp. Khim. 36,269.

(c) Shatenstein, AI., and Petrov, E.S., 1967. Russ. Chern. Rev. 36, 100.

30. Lee, L., Adams, R., Jagur-Grodzinski, J., and Szwarc, M. 1971. J. Am. Chern. Soc. 93, 4149.

31. Schaschel, E., and Day, M.e. 1968. J. Am. Chern. Soc. 90, 503.

32. (a) Kebarle, P. 1972. in M. Szwarc, (ed.), Ions and Ion-Pairs in Organic Chemistry, vol. I, pp. 27-83, John Wiley, New York.

(b) Kebarle, P. 1979. Pure Appl. Chern. 51, 63.

33. Fuoss, R.M., and Accascina, F. 1959. Electrolytic Conductance, Interscience.

34. Szwarc, M. 1968. in Carbanions, Living Polymes and Electron Transfer Processes, pp. 275-279, Interscience, New York.

Ionic Species / 85

35. (a) Coplan, M.A., and Fuoss, R.M. 1964. J. Phys. Chern. 68, 1177.

(b) Coetzee, J.F., and Cunningham, G.P. 1964. J. Am. Chern. Soc. 86, 3403.

36. Carvajal, C., Tolle, K.J., Smid, J., and Szwarc, M. 1965. J. Am. Chern. Soc. 87,5548.

37. Slates, RV., and Szwarc, M. 1965. J. Phys. Chern. 69,4124.

38. Ledwith, A., and Sherrington, D.C. 1975. Adv. Polym. Sci. 19, 1.

39. Fuoss, RM., and Kraus, C.A. 1933. J. Am. Chern. Soc. 55, 21.

40. Fuoss, RM. 1934. Trans. Faraday Soc. 30, 967.

41. Denison, J.T., and Ramsey, J.B. 1955. J. Am. Chern. Soc. 77, 2615.

42. Fuoss, R.M. 1978. J. Am. Chern. Soc. 100,5576.

43. Fuoss, R.M., and Kraus, C.A. 1933. J. Am. Chern. Soc. 55, 2387.

44. Szwarc, M. 1983. Adv. Polym. Sci. 49, 115.

45. Gough, T.E., and Hindler, P.R 1969. Can. J. Chern. 47, 1998,3393.

46. Adams, RF., Staple, T.L., and Szwarc, M. 1970. Chern. Phys. Lett. 5, 474.

47. Jackman, L.M., and Cotton, F.A. 1975. Dynamic Nuclear Magnetic Resonance Spectroscopy, Academic Press, San Diego, CA.

48. Buyle, A.M., Matyjaszewski, K., and Penczek, S. 1979. Macromolecules 10, 269.

49. Baran, T., Bzedzineka, K., Matyjaszewski, K., and Penczek, S. 1983. Mak-romol. Chern. 184,2497.

50. Hirota, N. 1968. J. Am. Chern. Soc. 90, 3603.

51. Hirota, N. 1967. J. Phys. Chern. 71, 127.

52. Hofelmann, K., Jagur-Grodzinski, J., and Szwarc, M. 1969. J. Am. Chern. Soc. 91, 4645.

53. Eigen, M. 1954. Z. Phys. Chern. 1, 176. 54. Ritchie, C.D. 1970. Acc. Chern. Res. 5, 348.

55. Persoons, A. 1974. J. Phys. Chern. 78, 1210.

56. Onsager, L. 1934. J. Chern. Phys. 2,599.

57. Langevin, P. 1903. Ann. Chim. Phys. 28, 433.

58. Persoons, A., and Evereart, J. 1981. Am. Chern. Soc. Symp. Ser. 166, 153.

59. Debye, P. 1942. Trans. Electrochem. Soc. 82, 265.

60. Eigen, M. 1954. Z. Phys. Chern. 1, 176. 61. Van Beylen, M., Persoons, A., and Uytterhoeven, H. 1974. International lSI

Symposium, Wepion-Leuven, Preprint, p. 190.

62. Uytterhoeven, H., Vanzegbroeck, J., Persoons, A., and Van Beylen, M. 1977. XXVI International Congress of Pure and Applied Chemistry, Tokyo, Japan, Abstr. p. 1173.

63. Bohm, L.L., LOhr, G., and Schulz, G.V. 1974. Ber. Bunsen Ges. 78, 1064.

64. Vanzegbroeck, J., and Van Beylen, M. 1978. IUPAC IV, Abstr. p. 106.

65. Everaert, J., and Persoons, A. 1981. Am. Chern. Soc. Symp. Ser. 166, 153.

3

Initiation and Propagation of Ionic Polymerization

1. General Remarks

The process yielding the first species capable of being linked with a monomer molecule by a covalent bond while simultaneously endowing the added monomer unit with the activity associated with the initiator is the initiation of addition polymerization. The addition of an anion A - to a vinyl monomer CH2=CHX yielding a new A·CH2CHX anion is an example of initiation of ionic polymerization. The addition of the next monomer molecule yields the same end-group but attached to a longer tail, i.e. A·CH2CHX·CH2CHX. The sequence of the latter events constitutes the propagation of polymerization.

Propagation perpetuated by a positively charged end-group, or induced by a positive end of a reactive dipole, is known as a cationic, or pseudocationic, polymerization. Similarly, an anionic or pseudo anionic polymerization ensues when a negatively charged end-group, or a negative end of a dipole, constitutes the active, propagating center. On the whole, the initiator, or its fragment, remains associated in some fashion with the active end of an ionically growing macromolecule during the whole period of its growth. For example, the counterion accompanying the initiating anion mentioned in the preceding paragraph becomes now a chaperoon of the newly formed polymer. Hence, the propagation constant, the mode of monomer addition, and the other characteristic features of ionic polymerizations are affected by the nature of the initiator. In this respect ionic propagation differs from radical propagation since the initiator does not affect the rate constant of propagation of the latter polymerization, a point previously stressed in Chapter 1.

The various processes that initiate ionic polymerization may be classified under the headings: the initiations by salts, i.e., by free ions and their

87


various aggregates; the initiations by protic and Lewis acids or by Lewis bases; the initiations induced by electron transfer; and the initiations caused by ionizing radiation. The propagation of polymerization, the process resembling the initiation by salts, is discussed in the first section of this chapter, and photochemical initiations are reviewed in the section dealing with electron transfer processes. This classification, although not sharp, helps to systematize the various phenomena observed in this large field of divergent chemical reactions.

Some initiators of ionic polymerizations require activation by reagents either present in the system prior to monomer addition or added subsequently. Such initiations are complex and often result in the establishment of an equilibrium between active and dormant species. Although the latter do not initiate polymerization, they do participate in the reaction, because, due to the dynamic nature of the equilibrium, they are ceaselessly and reversibly converted into the active species.

The simplest mode of initiation of polymerization arises upon the addition of an initiator or its active fragment, denoted by X, to a monomer M, Le.,

X + M~X·M*.

Such an addition could be reversible and then its rate is given by:

-d[X]/dt = k([X]'[M]'{kAM]/(k_i + kp[M])},

where kp denotes the rate constant of monomer addition to X' M*, and ki and k-i are the forward and backward rate constants of the X addition to the monomer. For k_ i « kAM] this equation yields the rate of initiation, first order in X and M. At the other extreme, for k-i » kAM], the - d[X]/dt pertains to the rate of initiator consumption while at equilibrium with the monomer. The consumption is then first order in X but second orderinM, Le., -d[X]/dt = kpKe'[X]'[M]2withKe = k;lk_i' The addition is always exothermic, and when its exothermicity exceeds the activation energy of the monomer addition to X· M* the polymerization speeds up on lowering the temperature and its apparent activation energy is negative.

It is beneficial when the rate of initiation is faster than, or at least equal to, the rate of propagation. Under such conditions polymers of narrow molecular mass distribution could be formed, provided that the system is "living," Le., the termination and chain-transfer are virtually avoided.

Most of the initiators of ionic polymerization are labile. Attention has to be paid to their integrity; their decomposition or isomerization must be prevented. For example, sodium naphthalenide, an initiator frequently used in anionic polymerization, isomerizes1 when kept in solution at am-

Initiation and Propagation of Ionic Polymerization / 89

bient temperatures for a day or two. Obviously, such an initiator has to be freshly prepared before being used. Such a precaution is advisable whenever other initiators or living polymers are handled. For instance, strange and inexplicable results were reported by a team that initiated polymerization of dienes with commercial t-butyl lithium.2 Subliming the aged reagent in vacuum was of no avail. 3 However, these artifacts were avoided by employing a freshly prepared initiator. 3 The most stringent purification of the reagents, monomers, solvents, the walls of the reaction vessel, and so forth, is imperative when investigating ionic polymerization, a point hardly to be overemphasized. The experimental techniques useful in such studies were described by Szwarc4 and more recently by Plesch.5

Numerous contradictory findings were reported in the literature describing the results of ionic, and especially cationic polymerizations. Most of the reported observations seem to be genuine. However, the precise conditions of the studied systems are often missing because the investigators either were unaware of some of the details of their experiments, or considered them as unessential, whereas they were critical. Such lack of some details causes probably most of the published controversies and confuses the issue.

An amusing incident illustrates the problems one might face on examining some of the published reports. A student who followed the progress of a polymerization by a dilatometric technique reported that the reaction stops whenever he goes home but resumes on his return. His statement was perfectly correct, but the additional fact that the light behind the dilatometer was switched off as he departed and switched on as he returned was not stated. The investigated reaction turned out to be photosensitized!

2. Initiation and Propagation Induced by Salts

2.1. Initiation and Propagation of Polymerization by Free Ions

Ionic polymerization may be induced by a sufficiently reactive free ion, as exemplified by the following idealized schemes:

x+ + C=C - X-C-C+ ,

or

X- + C=C- X-C-C-.

Slightly modified schemes describe the initiation of ring opening polymerizations of heterocyclic monomers by free ions. They proceed either by


the route:

or

These initiations resemble the respective propagations induced by the free ions of growing polymers, e.g.,

and

whereas in the ring opening polymerization they proceed according to the schemes:

or

with analogous schemes for the anionic polymerization. For the sake of brevity the notation

will be used in the later text. It should be stressed, however, that systems involving free ions, with exclusion of ion-pairs or other aggregates, are hardly known; polymerization induced by ionizing radiation is an example of polymerization involving only free ions (see p. 264). The above considerations are limited therefore to that part of the system that acts as the free ion. Since the free ions are formed by the dissociation of ion-pairs, the value of the dissociation constant of the latter is important in determining the fraction of the free ions participating in the overall process.

The dissociation of ion-pairs is favored by dilution, lower temperature, and the increasing solvating power of the medium. Typically, the dissociation constants of salts initiating cationic polymerisation in chlorinated


hydrocarbons vary from lO-5 to lO-4 M at ambient temperatures. They increase by a factor of 10 to 100 in nitromethane or nitrobenzene, whereas in hydrocarbon solvents the dissociation is too low to be detected, i.e., the respective constant is smaller than lO-15 M. The degree of dissociation of the salts initiating anionic polymerization vary widely depending on their nature and the nature of the solvent.

Solvation of the free ions increases the degree of dissociation of their ion-pairs but reduces the reactivity of free ions because at least one molecule of the solvent has to be removed from their solvation shell to allow for contact with the monomer. This is confirmed by the recent studies of the reactions of free ions with molecules in the gas phase;6 many reactions requiring activation energy when performed in solution proceed with no activation energy in the gas phase.

Free ions are often the most reactive initiating species. They initiate polymerization by direct attack on a monomer, electrophilic in cationic and nucleophilic in anionic processes. The intrinsic reactivity of an initiating or propagating ion depends on the charge density on that of its atom which ultimately becomes linked by a covalent bond to the polymerized monomer. Thus, a benzyl anion or cation is a better initiator than a diphenylmethyl ion, which, in turn, is faster than a triphenylmethyl anion. The reactivity of the latter ion is further diminished by the steric strain caused by its propeller-like shape.

The steric factors are often decisive in determining the rates of initiation or propagation. The substituents attached to the reactive center, especially when bulky, shield it and hinder the reaction.

The rates of free ion addition to a series of monomers vary with their nature. In cationic polymerization the rate increases with increasing monomer's nucleophilicity, e.g., it decreases in the order: N-vinylcarbazole > vinyl ethers> p-methoxystyrene > p-methylstyrene > cyclopentadiene > a-methylstyrene > styrene> indene. The reverse order governs the rates in anionic polymerization. In fact, N-vinylcarbazole and vinyl ethers cannot be polymerized anionically, whereas acrylonitrile, acrylates, and methacrylates are incapable of undergoing cationic polymerization, although they are readily polymerized by anionic techniques. Substitution of a-hydrogen of a monomer by a methyl group accelerates the cationic initiation or propagation, but retards the anionic one. On the whole, initiation is more efficient when the initiating ion is less stable than the one formed in the process.

The position of attack by an ion is uniquely determined in the initiation of vinyl or vinylidene monomers, but for unsymmetrical dienes two alternative sites of attack are possible, the preference being determined by the difference of the electron densities on the respective atoms. Obviously,


the preferred choice for the site of attack by an anion is the reverse of that observed for a cation.

More complex are the initiations of cyclic monomers. For example, anions initiating polymerization of cyclic ethers, sulfides, or amines attack the carbon atoms adjacent to the heteroatom, and thereafter the splitting of the carbon-heteroatom bond opens the monomer ring. A thiophilic attack was claimed for only one cyclic sulfide'? On the other hand, cations attack the heteroatom and yield then the onium ions. The initiation of polymerization of lactones can lead to two alternative modes of ring opening: one results in the formation of carboxylate ions, whereas the other yields alcoholate anions (see p. 113).

Let it be stressed, following Penczek,8 that virtually the same transition state describes the nucleophilic substitution reaction of an anion with the basic cyclic monomer as the electrophilic substitution reaction of a cation with the electron-rich monomer. Both kind of reactions are essentially SN2 processes proceeding through a linear transition state, irrespective of whether the charge resides on the initiator, growing polymer, or the monomer (see Table 3.1).

Table 3.1. Reaction types

Reagents Transition state Example

RX + Y- x-a ... R ... y-a EOx + O-CH2CHr(Anionic) RZ + X+ ZH ... R ... XH RX + Y XH ... R ... y-a EOx + CF3S02OCH3

+ /CH2 /CH2

RX+ + Y x+a ... R ... yH MACH20", I + 0", I (Cationic) CH2 CH2

RX+ + y- X+ ... R ... y-Zwitterionic RX- + y+ X- ... R ... y+

EOx denotes ethylene oxide.

2.2. Initiation and Propagation of Polymerization by Ion-Pairs

The simple previously discussed schemes need closer scrutiny. The condition of electric neutrality demands the presence of counterions in the polymerized solution in an amount equivalent to that of the initiating and propagating ions. The mutual attraction of the oppositely charged ions leads then to associations yielding a variety of species such as ion-pairs, triple ions, aggregates of ion-pairs, and so on, all coexisting in eqUilibrium with each other. Some of these associates act as initiators in their own right and contribute to the rate of initiation proportionally to their mole fraction /; weighted by their characteristic initiation rate constant kin ,i' The apparent


initiation rate constant, kjn,ap, derived from the experimental data is given, therefore, by the relation:

A similar relation provides the apparent rate constant of propagation of ionic polymerization since the propagating polymers may form also a mixture of distinct species, i. Hence, the experimentally derived overall propagation rate constant, kp,ap, is given by the relation:

where gj denotes the mole fraction of a respective kind of the growing polymers i's, and kp,j their propagation rate constant.

To illustrate the complexity of such systems let us consider the polymerization of styrene initiated in liquid ammonia by potassium amide.9

The amide is only slightly dissociated in this solvent, and thy degree of its dissociation is computed from the measured conductance of its solution. The rate of the ensuing polymerization is first order in styrene but half order in the amide, implying that the free amide anions are the actual initiators, while the undissociated ion-pairs, which form the bulk of the salt, are the dormant inactive species. The initiation proceeds then according to the scheme

k,

NHi + CH2=CHPh -- NH2·CH2CHPh,

followed by the propagation carried by the styryl anions. The growth of the polymeric anions is terminated by their reaction with the solvent

k,

"""CH2CHPh + NH3 -- """CH2CH2Ph + NHi,

regenerating the initiating amide anions and leaving behind the inactive "dead" polystyrene. Hence, the concentration of the free amide anions remains constant in the course of this reaction, being determined by the equilibrium:

The rate of initiation is given therefore by kjKY~s . [styrene] . [amide ]112, whereas K"J?sskp . [styreneJ2 . [amide)1/2Ikt[NH3] gives the rate of polymerization, in agreement with the reported observations.


More complex are the initiation processes taking place in the anionic polymerization of styrene or the dienes induced in hydrocarbon solvents by alkyl lithium. In view of the importance of these initiators in synthetic polymer chemistry, their reactions are described in a separate section following this one.

Ion-pairs are the simplest associates of free ions remaining in equilibrium with the latter. Since the association of the oppositely charged free ions is diffusion controlled (rate constant -1011 M-1S- 1), the equilibrium concentration of the free ions is virtually unperturbed by the initiation. The behavior of ion-pairs is different in anionic and cationic polymerization, therefore it is advisable to discuss separately their contribution to the initiation and propagation for the anionic and cationic polymerizations.

2.2.1. Anionic Systems

At least two kinds of ion-pairs participate in the reactions ensuing in many anionic systems: the highly reactive loose ion-pairs and the relatively unreactive tight contact pairs. The reactivity of loose ion-pairs depends on the nature of the species solvating the cations, although variation of the solvating agent rarely affects their reactivity by a factor greater than 10.

The most extensive data revealing the behavior of ions and ion-pairs in anionic systems were obtained through studies of anionic polymerization of styrene in ethereal solvents. In the absence of impurities, oxygen, moisture, and so forth, these reactions proceed free of termination and chaintransfer, i.e., the pertinent polymers are living.

The polymerization of sodium polystyrene in dioxane, a reaction studied by several research groups,10-13 displays simple behavior. The polymerizing solution does not conduct electric current, implying that the concentration of free ions is exceedingly low and their presence cannot be detected by this technique. The structure of the ion-pairs, gauged by the spectral observations, is that of the tight contact ion-pairs. The propagation proceeds in this solvent as a reaction first order in the living polymers and the monomer, i.e.,

where Sty denotes styrene. Its rate constant is reported to be 6.5 M-1S-1 at 25°C, with activation energy of 44 kJ/M corresponding to the A factor of 2.5 . 108 M- 1s- 1Y

The propagation rate constant is affected by the nature of the counterions. The lowest value is found for the Li+ pairs, kp = 0.9 M-1S- 1, but higher for the K+, Rb+, and Cs+, the respective kp's are 20, 21.5, and 24.5 M-1S-1 (slightly different values were reported by another team,12 somewhat higher for K + and Rb + , but lower for Cs + , namely 15 M -1s -1).


Initial state Transition state Final state

Figure 3.1. Schematic representation of a vinyl monomer addition to an ion-pair.

The increase of the cation's size seems to enhance the reactivity of tight ion-pairs, a plausible inference. The addition of a monomer to a tight ionpair has to proceed simultaneously with a transfer of the cation from the previously active center to the newly formed one, as shown schematically in Fig. 3.1.

Such a transfer is opposed by the Coulombic attraction between the ions of the pair. Since the attraction decreases with the bulkiness of cations, while their complete solvation is hindered by the crowdiness in the transition state, the change in the electrostatic interaction might be the cause of the observed enhancement of propagation. However, for a sufficiently large, but not too large, cation the transfer may take place without separation of the ions. The transition state of such a reaction is depicted in Fig. 3.2. This picture, although naive, conveys a plausible idea.

The cooperative action of both ions on the added molecule of the monorner, visualized in the transition state as shown in Fig. 3.2, is known as the push-pull mechanism and its discussion is postponed to page 111.

The character of bonding in tight contact ion-pairs has been discussed previously (see p. 43). The bonding is basically ionic as revealed by the spectrum of these species which closely resembles, apart from a small hypsochromic shift, that of the loose ion-pairs and of the free ions. On the other hand, the spectra of ionic and covalently bonded isomers, e.g., trityl chloride ion-pairs and the covalent triphenylmethyl chloride, are entirely different.

There is a significant covalent contribution to the bonding of some sodium or lithium tight ion-pairs revealed by the line splitting seen in the ESR spectra of the pertinent salts of radical-anions (see p. 46). Such a contribution, in conjunction with the extremely strong electrostatic attrac-

Figure 3.2. Schematic representation of a vinyl monomer addition to an ion-pair with a large cation. No partial dissociation in the transition state-the counterion interacts with the monomer and the growing polymer.


tion between the sodium or lithium cations and the relevant carbanions caused by their close proximity in the tight pairs, could account for the strikingly low reactivity of the tight sodium and lithium polystyryls. Their propagation rate constants in dioxane are by four powers of 10 smaller than those of the respective free ions in tetrahydrofuran (THF). Moreover, these constants might be still lower if a minute concentration of undetectable free ions or loose ion-pairs contributes to the observed reaction (see p.78).

Propagation of alkali polystyryls in THF reveals a new feature of this reaction. The polymerization is at least 10 times faster in this solvent than in dioxane. The monomer addition is first order in the monomer and the living polymers, but the apparent bimolecular propagation rate constant,

kap = (-dln[M]/dt)/[living polymer],

increases with decreasing concentration of the growing polymers. 14 Thus, in THF the kap of living polystyryl salts vary linearly with the reciprocal of the square root of polymer concentration as shown by Fig. 3.3. The following treatment, first described by two independent research teams, 15,16

rationalizes this finding. Solutions of polystyryl salts in THF are assumed to be composed of their

free ions and ion-pairs, both being in eqUilibrium with each other. Denoting their mole fractions by a and I-a, respectively, their propagation constants by Land k ±, the equilibrium constant of their interconversion (the dissociation of the pairs into free ions) by Kdiss , and the concentration of all the growing polymers by c, one deduces the relations:

kap = a . k_ + (1 - a) . k±,

and for a « 1 these equations lead to the approximation:

reduced to

for k_ » k±. This is indeed the observed relation. Hence, the slopes of the lines shown in Fig. 3.3 yield (k_ - k±)K~~s and the intercepts give the corresponding k ±. For the sodium salt in THF solution at 25°C an

8

'u 6 .. '"

' .. 0

E

~ 4

M ' 0

a. .x

2

o


U ' 1'1 .' K' LI'

n:~Rb: 0

0-<> C 5

0 25 [L Er1l2

50

•

100 200 1/ [L E] 1/2

Na'

Rb '

Cs'

300 400

Figure 3.3. A linear plot of the apparent propagation constant kap of living polystyrene vs. the reciprocal of the square root of living polymer concentration. Cations: Li+, Na+, K+, Rb+, Cs+. Solvent THF, T = 25°C. From ref. I5b.

identical value of 25 M- 1I2S- 1 was reported for that slope by both teams, in spite of the entirely different techniques employed in their work.

The variations of the slopes of the lines shown in Fig. 3.3 arise from changes of the cations. Their nature affects the dissociation constant of the respective salts, but not of the k _. The line corresponding to Li + is the steepest, whereas the flattest is the one pertaining to the cesium cations. This gradation seems justified since in THF, as in other ethereal solvents, the degree of solvation of alkali cations, that enhances the dissociation of their pairs, decreases from Li + to Cs + .1Sc

The gain in the solvation energy of a cation arising on the dissociation of a tight ion-pair exceeds the amount of electrostatic work needed for the separation of the ions. Therefore, the dissociation of tight ion-pairs is exothermic in most of the ethereal solvents. Indeed, the heats of dissociation of tight ion-pairs of sodium and cesium polystyryl in THF are - - 33 and - 8 kJ/M, respectively (see the pertinent Van't Hoffs plots shown on p. 66), demonstrating again that the solvation energy of the sodium cation is larger than that of cesium.


Similar investigations were carried out in other ethereal solvents, e.g., in tetrahydropyran (THP) , 2-methyltetrahydrofuran, dimethoxyethane (OME), and so forth. The linear dependence of the apparent bimolecular propagation rate constants on the reciprocal of the square root of polymers concentration is observed in all of them, indicating the participation of both free ions and ion-pairs in these propagations.

The absolute rate constant of propagation of the free ions is derived from the slopes of the lines shown in Fig. 3.3 in conjunction with the independently determined dissociation constant of the pertinent ion-pairs, whereas the relevant propagation constant of the ion-pairs is given by the corresponding intercept. The dissociation constants are obtained from conductance studies, technically difficult because the degree of dissociation of these salts is exceedingly low. Thus the propagation rate constant, L, of the free polystyryl anion in THF is found to be -1.105 M -is -1 at 25°C, a value by powers of 10 larger than the directly observed apparent propagation constant. Significantly, k _ seems to be independent of the solvent's nature (see Fig. 3.4 depicting the Arrhenius plot of k_ determined in a variety of ethers). The respective activation energy is 17 kJ/M and the A factor 108 M- 1S- 1•

The high reactivity of free ions accounts for the variations in the rates of anionic polymerizations caused by changes of cations and solvents. Even a small increase in the proportion of free ions leads to a large enhancement

75 50 25 0 -25 -50

5.5 -

. 0., .:,t- S.o 0>

~

• 1, .5

• 1, .0

3.0 3.5 -3

1. .0 1. .5

lIT 10

Figure 3.4. The Arrhenius plot of the propagation constant of the free ions of living polystyrene in a variety of solvents: HMPA, DME, THF, 3-MeTHF, THP, Oxepane. E = 16 kJ/mole, A = lOS M -IS -1.

Initiation and Propagation of Ionic Polymerization I 99

of the rate of the overall propagation. The degree of dissociation of ionpairs into free ions is therefore an important factor determining the rate of propagation.

The method outlined here yielding the absolute propagation rate constant of free anions requires knowledge of the dissociation constants of the pertinent polymer salts. Since the latter are extremely low, their values are somewhat uncertain. An alternative method allowing the determination of k _ or Kdiss is desired. Such a method is provided by the investigation of the effect of a common cation on the rate of propagation of living polymers. 15 For example, the addition of sodium tetraphenylboride, a salt readily dissociated in THF, to a THF solution of sodium polystyrene slows down its polymerization. The common ion effect reduces the degree of dissociation of the living polymers, decreases the concentration of the most reactive free polystyryl anions, and retards, therefore, the polymerization. Using the previously outlined treatment one deduces:

[Na+] being the total concentration of sodium ions formed by the dissociation of the sodium salts of polystyryl and the tetraphenylboride. * Thus, a plot of kapp vs. lI[Na+] is also linear, as shown by Fig. 3.5. However, while the slope of the line shown in Fig. 3.3 is equal to (k_ - k±) . KJ~s> the one shown in Fig. 3.5 is given by (k_ - k±) . Kdiss ' Hence, the square of the ratio of these two slopes yields 1lKdisS> a value that agreed satisfactorily with that derived from the conductance studies. Let us note that the value of k ± 's derived from Fig. 3.5 is more reliable than that given by Fig. 3.3, due to a larger scale of the former plot.

At high concentrations of sodium polystyryl or of the added sodium tetraphenyl boride, the propagation seems to be faster than expected.15h

The values of the observed kp passes through a minimum in this region and rise thereafter as the concentration of sodium ions increases. This phenomenon was tentatively accounted for by the formation of triple ions through reactions such as:

and

'For [NaBPh4] > [Na+ ,polySty-], the [Na+] = (KdissB • [NaBPh.])"2, since Kd/SsB' the dissociation constant of sodium tetraphenylboride, is about 1000 times larger than that of sodium polystyryl. The exact procedure needed for calculation of the total concentration of sodium ions is outlined on p. 95 of ref. 17.


32 0 r------------------------------,

"lj 01 > "-01 II> .c o

a.

2L0

"'" 160

80

o

'" /

/

o

/0 o

Figure 3.5. The plot of the apparent propagation constant kap of sodium living polystyrene vs. 1/[Na +]. Solvent THF. T = 25°C. The concentration of Na + was varied by the addition of NaBPh4 •

which produce additional highly reactive free polystyryl anions and thus increase the observed value of kp •

It is appropriate to mention at this place some contradictory claims pertaining to the values of k:!:; derived by the extrapolation of the plots of kp vs. [living polymer)1/2. As shown by Fig. 3.6, the experimental values of kp reported by the Syracuse and by the Mainz teams for sodium polystyryl in THF are concordant,18 nevertheless the different kinds of extrapolation led to divergent values of k:!:;'s, namely 80 M-1S- 1 (all the points considered) and 180 M -IS -1 (only the square points considered), respectively. 18 To reduce the ambiguity of extrapolation it is desired to extend the range of the plotted data as close as possible to the Y axis.

2.2.2. Role of Loose Ion-Pairs

The effects arising from the presence of free ions were discussed in the preceding section. Let us consider now the k:!:; constants that allow us to assess the contribution of all the ion-pairs to the propagation. Do the ionpairs consist of a single kind of species, or are they a mixture of two or more distinct associates? The temperature dependence of k:!:; points out

Initiation and Propagation of Ionic Polymerization 1101

600 kp Msec. /0 0 0

500 /""' ,/""'

. ~

400 0

>V" r

300

200

/"

100 0

lo"-,d l2 1/"-M

2 3 4 5 6 7 8 9 10 II 12 13 14

Figure 3.6. Plot of the observed propagation constants of sodium polystyryl in THF vs. the square root of its concentration. The values of the points shown as circles were reported by the Syracuse group, those shown by squares by the Mainz group. In spite of their consistency the extrapolation led to large discrepancy in their intercept. The dashed line represents the extrapolation based on the results of the Mainz group only.

to the latter case. For example, k± of sodium polystyryl determined in THF increases19 on lowering the temperature of its solution (see Fig. 3.7), and the Arrhenius plot, describing the temperature dependence of the propagation of ion-pairs proceeding in dimethoxyethane (DME) has a maximum20 shown also in Fig. 3.7. Since the activation energy of any elementary reaction cannot be negative, these observations, soon confirmed by an independent research team,21 imply that at least two kinds of ion-pairs participate in this propagation, say the sluggish tight pairs growing with a rate constant k" and the very reactive loose pairs, remaining in eqUilibrium with the tight ones, which propagate with rate constant k/. Thus:


DME

3.5

-.>< 01

~ 30

2 .5

3.0 3.5 L.5

o i n DME

o i n THF

THF

3.0 +,

.><

01 o

__ 2.5 -

---2.0

5.0

Figure 3.7. The Arrhenius plot of the propagation constant k± of sodium polystyryl ion-pairs in DME (upper curve) and in THF (lower curve). Note the "negative" activation energy in THF and the maximum in DME. From ref. 20.

where Kt ./ denotes the equilibrium constant of the interconversion of tightto-loose pairs, a reaction known to be exothermic (see p. 51). At ambient temperature Kt./ « 1, nevertheless k t seems to be substantially smaller than Kt./k/, implying a high value of k/. In such a case

Moreover, if the exothermicity of the tight -to-loose-pair conversion is greater than the activation energy of the loose pairs propagation, then lowering the temperature would increase k ±, i.e., its apparent activation energy would be negative. 19 This seems to be the case in the polymerization of sodium polystyryl in THF at temperatures lower than 25°C.

In DME, a more powerfully solvating agent than THF, the proportion of the loose pairs is much higher than in THF, i.e., the equilibrium between the tight and loose-pairs shifts substantially to the right when DME is substituted for THF. This accounts for the high k± values of sodium polystyryl in DME, namely, -2000 M-1S- 1 at 25°C compared to -80 M-1S-1 found in THF. The high fraction of the loose pairs in DME solution increases still further on cooling, and probably it reaches 95% at about - 40°C. Thereafter, further chilling of the solution insignificantly increases the proportion of loose pairs. Since k/ decreases with decreasing temperature, the k±, being then approximately equal to k/, decreases also on further chilling. This accounts for the maximum seen in Fig. 3.7.

As pointed out by Schulz,21.22 at sufficiently high temperatures Kt ./ might become small enough to make k t » k/Kt .[. The Arrhenius curve goes then


through a minimum, rises on further increase of temperature, and when drawn over the whole accessible temperature range it acquires an S-shape. Indeed, S-shaped Arrhenius plots, shown in Fig. 3.8, were obtained when the polymerization of sodium polystyryl was investigated in THF or in 3-Me-THF over a wide range of temperatures, from -110° to + 80°C. However, various technical reasons cast some doubts on the results obtained at higher temperatures (>sooq, as well as on their interpretations (see later).

., .>< 01

3 .0

2 2.0

3.0 i. .0

·c - 50 -75 - 100

103/ T

Figure 3.B. The Arrhenius plot of the propagation constant k± of sodium polystyrene ion-pairs in THF and in 3-MeTHF. Note the sigmoidal shape of the curves. From ref. 23.

The thorough and most extensive studies of polymerization of sodium polystyryl reported by Schulz and his co-workers are summarized in their two reviews23.24 and presented pictorially in Fig. 3.9. Its inspection reveals, as remarked earlier, that k[ seems unaffected by the nature of the solvent. This might be plausible. The reactive anion of a loose ion-pair well separated from its extensively solvated cation resembles a free anion. Since the anions interact only weakly with ethereal solvents, in contrast to cations, one might expect the nature of such solvents to affect only negligibly their reactivity as well as that of the weakly coupled loose ion-pairs (see, however, contrary evidence on p. 184).

On the other hand, the other generalization transpiring from Fig. 3.9, i.e., the lack of influence of the nature of ethereal solvents on the reactivity of tight ion-pairs, is questionable. The equilibrium constants of the conversion of tight-into-Ioose ion-pairs, Kt,l, are strongly affected by the nature of the ether, their values vary greatly, e.g., from 0.13 for DME, to 1.3 . 10-4 for tetrahydropyran, and to less than 10-5 for dioxane. 23,24 The strong effect of the solvents on Kt.l, but the lack of any on the reactivities of the tight ion-pairs, is hard to reconcile. Hence, in spite of the remarkable self-


5.0

4 ,0

3.0

-. , .><

:5' 2.0

1.0

o

, , , x,

HMPA ~, , , , , DME , ~~' ~~~ - . ~"

, ,

- 100 - 125

, k " ( ! ) s ·

,

- 1.0 L----l3 .-0 ---,.1..0--=--5-',-0 ---6.L..0----'7.0

1/T . 103

Figure 3.9. The Arrhenius plot of the propagation constant of sodium polystyryl ion-pairs in a variety of solvents. The curves seem to approach a common asymptote at lower temperatures interpreted as a linear Arrhenius plot referring to loose ionpairs, and again a common asymptote at higher temperatures interpreted as a linear Arrhenius plot referring to tight ion-pairs. Some critical comments pertaining to these plots are outlined in the text.

consistency of the data and the interpretation of the results reported in the refs. 23 and 24, the subject of reactivities of sodium tight ion-pairs of polystyryl deserves closer scrutiny.

The reactivities of loose ion-pairs are not readily determined. Undoubtedly they are high, at least 1000 times higher than those of the tight pairs. For example, k/ of sodium polystyryl in DME is estimated20 to be 20,000 to 30,000 M-1S- 1 at ambient temperature. In fact, Schulz implies that the reactivities of loose pairs are closely similar to those of free ions (see Fig. 3.9)_ Therefore, a question arises: are tight ion-pairs reactive at all? A minute fraction of the otherwise undetectable loose ion-pairs present at equilibrium with the tight ones might account for the observed reactivity attributed to the latter. This possibility should be kept in mind when the


effects of counterions and solvents on the reactivity of tight ion-pairs are discussed. Furthermore, in poorly solvating media, such as dioxane, the conversion of tight ion-pairs into loose ones might be endothermic and not exothermic as it is in the well cation solvating THF or DME. Then, the increase of reactivity of living polystyrene with rising temperature, reported by Schulz and illustrated by Figs. 3.8 and 3.9, could result from the endothermic conversion of tight into loose pairs. It is a speculative suggestion but the presently available data are too limited to resolve these questions, and further study of this subject might be advisable.

The complex behavior of ion-pairs is revealed also by the diverse temperature dependence of the conductance and the reactivity of lithium polystyrene in THF when compared with that of sodium polystyrene. While the dissociation of the latter salt is exothermic, it is thermoneutral for the lithium salt and, as shown by Fig. 3.10, it becomes even endothermic in

- 4.00

-5 .00

- 6.00

/,,;:i/ --,, ___ ,,- PSt Na in THF

------" ,,-

-7. 00

"C ~

01 - 8 .00 0

-9 . 00 •

-1 0.00

-11. 00 3.50 4.00 4. 50 5.00

lIT X 103

Figure 3.10. Temperature dependence of the dissociation constant, Kd/SS> of polystyryl lithium in different ethereal solvents. The dotted lines refer to the sodium salt; solid lines refer to the lithium salt.


oxepane solution.25 Similar behavior of sodium and lithium salts of fluorenyl carbanion was reported earlier by Hogen-Esch and Smid.26 Interestingly, at very low temperature a shoulder appears at -460 nm in the spectrum of sodium polystyryl in THF implying a substantial increase in the concentration of loose pairs. In contrast, no shoulder was observed in the spectrum of the lithium salt, even at -7(fC (unreported findings of Van Beylen). A diverse temperature dependence of the reactivities of sodium and lithium polystyryl is depicted in Fig. 3.11 showing the respective Arrhenius curves.

Tight ion-pairs do not dissociate directly into free ions. The dissociation proceeds via the loose pairs, even if the latter cannot be detected due to their exceedingly low concentration (see p. 78 for an example illustrating this point).

The pressure dependence of the reactivity of sodium polystyryl in THP and cesium polystyryl in DME was investigated by Schulz and his coworkers. 27 Two factors affect the pressure dependence of the rate of polymerization: one due to the difference of volumes of the initial and transition states of the tight and loose ion-pairs, i.e. d V; and d Vj, the other caused by the shift in the tight-to-Ioose ion-pairs eqUilibrium determined by the d VI,1 (see p. 50 for the discussion of this phenomenon). To simplify the study of the pressure effect the contribution of free ions to the reaction

4.0

3.0

... 2.0

1.0

3.0

k(! ) . Na+ . THF

---

4.0

103 , T

----- - ....

S.O

Figure 3.11. Temperature dependence of the propagation rate constants, k"" of the lithium and sodium polystyryls in THF.


was eliminated by adding a salt depressing the ionization of ion-pairs to the investigated solution.

The experimental results were presented in the form of plots of log k ±

vs. pressure at constant temperature, and vs. lIT at constant pressure. The directly determined propagation rate constants, k ±, of both kinds of ionpairs is given by the relation:

reduced to:

since under the chosen experimental conditions Kt,/« 1. The plots of log k ± vs. p were found to be linear; the respective overall !l. V1 was reported as - 22 mllmole for the Na salt in THF and - 28 mllmole for the Cs salt in DME. The linearity of such plots is only possible if either

or k t « k/ . Kk ,/. In their studies Schulz and his co-workers calculated their final results on the basis of the first alternative. However, such an assumption leads to improbably large values of !l. Vf. On the other hand, reasonable values of !l. Vf are obtained on the assumption k t « k/ . Kt /

which agrees with the conclusions derived from the previous considerations (see p. 51).

2.2.3. Effect of Solvating Agents

Although the suppression of the concentration of loose ion-pairs coexisting in equilibrium with the tight ones is not feasible, the increase of their proportion is readily accomplished by the addition of solvating or coordinating agents to their solution, thus producing the loose pairs separated by the added agent (see p. 53). A large enhancement ofthe rate of initiation or propagation is then achieved. For example, the reactivity of sodium polystyryl in tetrahydropyran solution is strikingly increased by the addition of tetraglyme, TG = CH30(~H40)4 CH3. This polyether becomes associated with Na + ions and converts the sluggish tight ion-pairs into the reactive and more extensively dissociated loose pairs. The following equilibria are established in this system:

Na+ + TG ~ Na+, TG

J08 / Ionic Polymerization and Living Polymers

as well as

Kdiss,l'

The effects caused by the relatively low concentrations of the glyme are depicted in Fig. 3.12 in which the observed apparent bimolecular propagation constant, kobs , is plotted vs. the reciprocal of the square root of living polymer concentration.28 The intercepts of the resulting lines yield the average propagation constants of ion-pairs composed of the tight pairs, Ml\Sty-, Na +, and the loose ones having their ions separated by THP (negligible amount) or by the glyme, ~ty-, TG, Na+. The slopes are related to the average of the dissociation constants of both kinds of pairs (compare this plot with that shown in Fig. 3.3), their steepness reflects the high degree of dissociation of Ml\Sty-, TG, Na+ pairs. The plot of the intercepts, denoted by kav' vs. the concentration of the glyme is linear, as shown in Fig. 3.13, and its slope gives kTG . K TG , where kTG denotes the propagation rate constant of the glyme separated loose pairs.

Extention of this work to higher glyme concentrations29 led to the determination of the absolute propagation rate constant of the loose glyme

5000

4000 -

'0

'" 3000 III

';" ~

OIl .0 0

::.:: 2000

o 50 100 150 200

~ J-1/2 -112 LLE M

Figure 3.12. The effect of glyme on the plots of the apparent biomolecular propagation constants kp of sodium polystyrene in THP at 25°C vs. the reciprocal of the square root of living polymer concentration. The half-shaded points are the results obtained in the presence of an excess of NaBPh4 •


1000 ,---------,..--.,

0..

~ 500

5 12 + 3.1 .10 [4 GJ

o 1.0 2.0 3.0

[ L.G].10 3 M

Figure 3.13. The plot of the intercepts of the lines shown in Figure 3.12, denoted as kaY> vs. the concentration of tetraglyme.

separated pairs, kTG. Obviously, kay = (1 - 'Y)kt + 'YkTG' where "I denotes the mole fraction of the glyme separated pairs, and kt is the propagation rate constant of the "ordinary" ion-pairs present in this solvent even in the absence of the glyme. The fraction "I tends to 1 as the concentration of the glyme increases, while kay asymptotically approaches the value of kTG • The plots of kay vs. [TG] are shown in Fig. 3.14 for four different temperatures, and the asymptotes provide the respective propagation rate constants of the glyme separated loose pairs.

The reliability of this approach is checked. Since "1/(1 - "I) = KTdTG] it follows that for kt « kTG

and then the plot of 1/(kav - kt ) vs. 1/[TG] should be linear. Such a plot is shown in Fig. 3.15, and its intercept and slope provide the values of 1/ kTG and 1/kTGKTG' respectively.

The propagation rate constant of the loose glyme-separated ion-pairs is substantially lower than the propagation rate constant of the free polystyryl anions, e.g., their respective values are 4.103 and -1.1OS M -IS -1 at 25°C. Nevertheless, even these loose pairs are at least 1000 times more reactive than the tight sodium pairs in THP. As shown by Fig. 3.9, there is a


4000 25°C

0° C

'u <I>

_'" 3000 - 20° C , ~ -<:> - 45° C :::;

2000 Q. <I> U

~ c: 1000

o 0. 4

Figure 3.14. The plots of the kav vs. the concentration of tetraglyme at various temperatures. The asymptotes correspond to the absolute propagation rate constants of the "glymated" sodium ion-pairs in THP.

substantial difference between the thus derived values for the propagation rate constant of loose glyme-separated ion-pairs and those attributed to the loose sodium polystyryl pairs formed in THF and other ethereal solvents. Judging from the dotted line seen in Fig. 3.9, the deviations are minor at the lowest temperatures but larger at ambient temperature due to the steepness of that line. However, the slope of that line is biased by the points representing the rate constant determined in hexamethylphos-

3. 00 u <I> <II

~

c 2.00

.x , >

'" .x 1. 00

M2 0.25

0 1000

Figure 3.15. The plot of lI(kav - k,} vs. lI[tetraglyme]. The intercept = lIkTG and the slope = lIkTGKTG.


phorictriamide (HMPA). This powerful solvating medium is known, from the conductance studies, to dissociate virtually quantitatively the ion-pairs into the free ions. Thus, the upper points of that graph represent the rate constants of free polystyryl anions, and not of their loose ion-pairs. t

In conclusion, the loose ion-pairs appear to be less reactive than the free ions, but probably only by a power of 10. Their reactivities seem to depend on the nature of the solvent or the solvating agent, although not to a large extent. It is still debatable how reactive are the tight ion-pairs, but their initiation and propagation rate constants are undoubtedly by several powers of 10 smaller than those of the loose pairs. The values attributed to the rate constant of tight ion-pairs could be misleading; they might result, at least partially, from the presence of minute amounts of the undetected loose pairs remaining in equilibrium with tight ones. This might be the case for the Li+ and Na+ salts, although the reported constants for K+, Rb+, and Cs+ tight pairs might be more reliable.

2.2.4. Role of the Push-Pull Mechanism

Investigation of anionic polymerization of heterocyclic monomers such as oxiranes and thiiranes revealed a new and interesting feature of anionic propagation. The gradation of the rate constants of ethylene oxide addition to the alkali salts of polyethylene oxide follows the conventional sequence:30 <0.01, 0.05, and 0.12 M-1S- 1 for Na+ K+, and Cs+ counterions (all determined in THF solution at 20°C). Under the same conditions the addition to the free alkoxide anions is still faster, the rate constant being 1.7 M -lS - 1. This is expected for the reactions requiring partial dissociation of ion-pairs in the transition state of the addition. However, further separation of ions caused by their complexation with coordinating agents retards this reaction. For example, the propagation rate constant of the potassium salt of polyethylene oxide complexed with cryptate is 0.025 M-1S-t, i.e., lower than 0.05 M-1s-1-the rate constant of the noncomplexed salt. This decrease of the reactivity on complexation of the salt is significant in view of the enormous increase of the dissociation constant of this salt resulting from the addition of cryptates, namely from 2.7 . 10-10 M

for the noncomplexed salt to 3 . 10-7 M for the complexed one, both determined in THF at 20°C.31

It seems that the partial dissociation of ion-pairs in the transition state of the addition is not the only factor increasing their reactivity. The initial

tDr. Axel Muller (private communication) proposes to differentiate between the loose ionpairs having fully solvated cations (spherically surrounded by solvent molecules) and those externally solvated ion-pairs, with cations being in contact with the anions. The latter might be less reactive than the former. This idea, held also by one of us (M. VB.), may explain the different behavior of sodium and lithium salt and account for the relatively low reactivity of glymated ion-pairs.


separation of the ions does not always accelerate the reaction. Apparently, the cooperative action of anion and cation, when jointly interacting with the monomer, is another important factor affecting the rate of addition. The latter is known as the push-pull mechanism but its accelerating effect wanes on the separation of the ions. Hence if its contribution to the reaction is large, then the separation of ions retards the monomer addition. Apparently, the coordination of the cryptate with the cation that separates the ions of a pair reduces the beneficial effect of the cation, and the resulting loss is greater than the gain arising from the reduced eletrostatic work required for the partial ion separation.

More convincing evidence for the operation of the push-pull mechanism is provided by the reaction of ethylene oxide with anions having a substantially delocalized charge. While polystyryPO or carbazyP2 free anions react faster with ethylene oxide than their ion-pairs, a reverse relation is observed in the initiation induced by the sodium or potassium salts of more delocalized carbanions.:I:

For example, the undissociated ion-pairs of sodium 9-methylfluorenyP3 or fluoradenyP4 cleave ethylene oxide faster than their free ions. These bimolecular reactions, which are first order in the initiator and ethylene oxide, are accelerated by the addition of salts sharing common counterions with the initiator and favoring the formation of ion-pairs. In contrast, the cleavage is retarded by the addition of crown ethers or cryptatates that encapsulate the cations and hinder its interaction with the monomer. 35 Thus, the cleavage of ethylene oxide by sodium fluoradenyl proceeds in tetrahydropyran with a rate constant of 1.4 . 10-3 M-1S- 1 at 30°C, but it decreases to 5 . 10-7 M-1S- 1 on the addition of cryptates.

An idealized scheme that stresses the cooperative action of both ions is shown below:

o Na+O X-, Na+ + /" ~ ,//" - XCH2CH20-, Na+.

CH2-CH2 X-CH2-CH2

Further evidence for the operation of the push-pull mechanism was reported by Sigwalt.33 The increase of the complexity and size of the initiator leads to a greater delocalization of the negative charge and to reduction of its density on the atom directly involved in the nucleophilic attack causing the cleavage. The reactivity of such an initiator arising from its capacity

*The lithium salt of polystyryl anion, but not the sodium one, is an exception.30 This salt reacts faster with ethylene oxide than its free anion. It seems that the reaction of this ionpair proceeds by a unique route. The Li cation becomes externally solvated by ethylene oxide and a fast unimolecular ring opening of the complex follows thereafter. The complexation constant is apparently very high.


to polarize a monomer decreases, whereas its push-pull efficacy (restricted to ion-pairs) is influenced to a lesser degree by the delocalization of charge. Eventually, the push-pull effect may become the dominant factor affecting the reactivity of the ion-pair. For instance, the density of the negative charge on the active N-center is lower in dibenzocarbazyl than in the carbazyl salt.

dibenzocarbazyl-

Hence, the latter salt is expected to be a faster initiator than the former, whether acting as a free ion or as an ion-pair. This indeed is observed. The bimolecular rate constants of ethylene oxide cleavage by the free anions, determined in THF at 20°C, are: 13 . 10-4 M-1S- 1 for the carbazyl anion, but only .038 . 10-4 M-1S- 1 for the anion of dibenzocarbazyl. The gradation is similar for the potassium ion-pairs, the rate constants are 3.5 . 10-4 M-1S- 1 for the carbazyl salt and again less, only 0.95· 10-4 M-1S- 1

for the salt of dibenzocarbazyl. However, the enhancing effect of the pushpull mechanism is dominant when the charge of the initiator is greatly delocalized. This makes the ion-pair of dibenzocarbazyl more reactive than its free ion. On the other hand, in the carbazyl initiator the gain of reactivity arising from the push-pull action is not sufficiently large to upset the loss due to weakening of the electric field resulting from the ion pairing and the free carbazyl anion is still more reactive than its ion-pair, although only by a factor less than 4.

In summary, the cation plays a dual role in the initiation and propagation of anionic polymerization. It hampers the reaction by reducing the strength of the electric field that polarizes, and hence activates the incoming monomer, but facilitates the process by its pull action, the most important for highly polar monomers. The relative contribution of the cation's pull action increases with decreasing charge density on the atom to be covalently bonded to the added monomer.

2.2.5. Solvation by the Monomer: Anionic Polymerization of Lactones

In a paper36 describing the anionic polymerization of o.-Me-o.-Pr-~-propiolactone proceeding in THF at - 20°C and initiated by potassium acetate complexed by [2,2,2]-cryptate, the authors reported that the ion-pairs propagate faster than the free ions. This finding was rationalized by assuming that the propagation proceeds by the push-pull mechanism.


An interesting alternative explanation has been offered by Penczek.37

His investigation38 of the kinetics of polymerization of \3-propiolactone, initiated by a complex of potassium acetate with a crown ether and proceeding in CH2CI2, covered temperatures ranging from - 20° to 35°C. The propagation constants of the free ions and the ion-pairs were computed by combining conductometric and kinetic data. The results appeared to be conventional, the free ions propagated faster than ion-pairs. However, the positive activation energy of free ion's propagation was found to be larger than that of the ion-pairs, although one expects the more reactive species to propagate with lower activation energy than the less reactive one. In accord with this finding the ratio k; Ik; decreases on lowering the temperature. Indeed, its value was reported to be 154 at 35°C, whereas it was found to be 5.6 only at -20°C. One expects, therefore, a reverse order of the reactivities at temperatures lower than - 30°C.

Penczek attributed these results to the extensive solvation of the free anions, but not of the ion-pairs, by the highly polar monomer, supposedly a better solvating agent than methylene chloride due to its large dielecric constant. Since some desolvation is expected to take place in the transition state of propagation, the AH*, and hence also the activation energy, of the free anions increases with increasing degree of their solvation by the lactone (the anions solvated by lactone are supposedly less reactive than those nonsolvated).

The above system showed another peculiarity. Although the reaction was proved to be living and internally first order in monomer, the propagation constant of the free ions, but again not of the ion-pairs, increased substantially on decreasing the initial monomer concentration, e.g., kp being 49 M- 1S- 1

at -15°C when [M]o = 3 M increased to 490 M -IS -1 as [M]o was reduced to 1 M. This peculiarity of the system corroborates the explanation offered for the high activation energy of the anions propagation. The fraction of the less reactive free anions solvated by lactone increases with its increasing proportion in the reaction medium. Consequently the respective propagation rate constant decreases accordingly.

Although both observations indicate a decrease in the free ion reactivity on its increasing degree of solvation by the lactone, the proposed explanation does not seem plausible. It would be satisfactory had the solvating agent been different from the monomer. However, since it is the monomer that retards the polymerization, some additional justification of these findings has to be proposed. For example, one may argue that the orientation of the lactone in the solvation shell is drastically different from that acquired in the transition state of propagation and its reorientation requires its prior desolvation.

The later work of Slomkowskp9 who repeated these experiments using dimethylformamide, instead of methylene chloride, as the solvent casts further doubt on the above explanation. The dipole moment of formamide,

Initiation and Propagation of Ionic Polymerization / II5

3.86 D, is only slightly less then that of (3-propiolactone, 4.18 D. Nonetheless, the AH~ (free ion) increased from 88 ± 9 kJ/mole to 114 kJ/mole as the concentration the lactone increased from 0.5 M to 1 M, whereas the AH~ (ion-pairs) remained virtually constant (the respective values are 52 ± 8 and 61 ± 9 kJ/mole). In fact, the k; of the free ions at -20°C and [M]o = 1 M is reported to be 3 . 10 - 2 M -1s -1 whereas the corresponding value for ion-pairs is given as 4.10-2 M-1s-l, i.e., under these conditions k; < k:, although for [M]o = 0.5 M the conventional relation is reported, i.e., k; = 9.10- 2 andk: = 3.6.10-2, both inM- 1s-1 units. Furthermore, at higher temperatures k; > k:, in conformity with the common case.

It seems, therefore, that the effects described above are specific for lactones, and not simply related to its dielectric constant. Indeed, we shall see later (see p. 190) that the propagation constant of some vinyl ethers decreases, and not increases, as a less polar solvent is added to a polymerizing medium. The lack of simple relations between the dissociation constant of ion-pairs and the dielectric constants of the medium is strikingly revealed by Fig. 2.8 on p. 63. The value of Kdiss of a salt may remain the same in media having different dielectric constants, and vice-versa. The dissociation constant may vary although the dielectric constants of the solvents are kept constant. The complexity of these relations is amplified in concentrated solutions since the laws of ideal solution become then invalid.

Another interesting feature of anionic ring opening polymerization of lactones deserves mentioning. The polymerization may be initiated by alkoxides, e.g., RO-K+ which attacks the carbon atom of the carbonyl end-group (linear transition state):

o ° II II RO-K+ + F-P --+ R0-P P-K+,

CH2--CH3 CH2--CH2

regenerating the propagating -0 - K + group by opening the CO-O bond, or alternatively the attack may take place on the carbon adjacent to the o atom followed by the rupture of the CH2-O bond and yields then a carboxylate ion as a new propagating entity:

The structure of the active end-group can be determined, e.g., by capping the growing polymer with ClP(O)(OC6HSh (see p. 22), since the chemical shift of the 31p does distinguish between the various possible end-groups.


For example, the &31P = -11.5 ppm for -CH20P(O)(OC6H5)2' whereas for -CH2CO'OP(O)(OC6H5h the respective & = -26.0 ppm. It was established, by utilizing this technique, that the propagation of ~-lactone initiated by alcoholate, -CH20 - , produces both, the polymers terminated by the -CH20 - and the -CH2COi end-groups, whereas the propagation proceeding via the -CH2COi produces polymers terminated by the -CH2COi end-groups only, as reported previously by Yamashita. 40

The difference in the nucleophilicity of RO- and ROi groups rationalizes these findings. The RO- end-group, being a stronger nucleophile, may attack both: the ester carbon adjacent to 0 and the carbonyl group. The RCOi, a weaker nucleophile than alcoholate, is capable of attacking only the ester carbon. The planarity of ~-lactone facilitates the attack on the carbonyl carbon, and since E-caprolactone is puckled this mode of propagation becomes impossible in its polymerization which can be initiated and propagated by alkoxide but not by carboxylates.

The sensitivity of the lactones propagation to small variations in the conditions of the reactions is well illustrated by the polymerization of 10-

caprolactone initiated by the salts of Me3SiO- anion.41 Its potassium salt in THF solution initiates a clean propagation proceeding via nonaggregated ion-pairs, whereas the aggregated sodium and lithium salts do not initiate its polymerization.

2.2.6. Participation of Triple Ions in Anionic Propagation

Two kinds of triple ions can participate in anionic propagation: A - ,B + ,A -and B + ,A - ,B + . The former is expected to be more reactive than the tight A - ,B+ ion-pair, but less than the free A-ion. Little is known about the reactivity of the B + ,A - ,B + ions, most likely they are unreactive.

In an interesting paper, Sigwalt et al. 42 suggested that an appreciable fraction of sodium poly-2-vinylpyridine is present in its solution in the form of triple ions. The conductance of these solutions is extremely low, 43,44 e.g., the dissociation constant of sodium poly-2-vinylpyridine in THF at 25°C is only 10-9 M, about 100 times lower than that of sodium polystyrene solutions. Significantly, a higher degree of dissociation is observed for sodium poly-4-vinylpyridine.45 It seems that the cation in poly-2-vinylpyridine is bound to the penultimate unit as well as to the terminal one. The proposed structure of the end-group of this polymer is shown below:


In accord with this argument, the degree of dissociation of living sodium polystyrene terminated by a single 2-vinylpyridine unit is higher, 44 the absence of the penultimate pyridine unit makes the additional binding of the cation impossible. This kind of cation binding is prevented also in poly-4-vinylpyridine by the unfavorable location of N, justifying the higher conductance of its solution.

The low degree of dissociation of sodium poly-2-vinylpyridine is conducive to the formation of triple ions. Kinetics of its propagation42,44 reveals some unconventional features. Its large bimolecular propagation constant ( - 2000 M -1s -1 in THF at 25°C) is virtually independent of the living polymer concentration. This is expected when the reactive anions are formed by a process leading to triple ions, i.e.,

2A - ,B + ~ A - + B + ,A - ,B + ,

(see p. 65 for a discussion of such systems), and not by the conventional dissociation of ion-pairs. The mole fraction of these most reactive A - ions is then independent of the concentration of ion-pairs, and their dominant contribution to the propagation makes the apparent bimolecular propagation rate constant of sodium poly-2-vinylpyridine unaffected by dilution, in accord with the observation.

The cesium salt of poly-2-vinylpyridine behaves conventionally.43 The large volume of the Cs + cation prevents its placement in the cavity formed by the penultimate and terminal units, and precludes the formation of triple ions.

Intramolecular formation of negative triple ions is observed in the cesium salt of polystyrene endowed with two active end-groups.15,46 Propagation of such polymers is slower than those possessing only one active group, whereas at the same concentration of active end-groups the conductance of their solutions is higher. These anomalies are accounted for by the mechanism outlined below:

Cs+,Sty--- Sty-,Cs+ ;:::= Cs+,Sty---Sty- + Cs+, ~iss.Cs

Sty-,Cs+-- Sty- ;::= ~,Kc.

Since the free -Sty- ion formed on the one end of a bifunctional polymer is bound to remain in the vicinity of the undissociated ion-pair located on its other end, the intramolecular association yielding triple ions by cyclization becomes highly probable. The cyclization upsets the dissociationassociation equilibrium, causing additional dissociation of ion-pairs into Cs + cations and the polystyryl anions. The latter replenish then the living polystyryl anions lost through the cyclization. Consequently, the concentration of Cs + ions, the main contributors to the conductance, exceeds that


expected on the basis of the dissociation-association equilibrium, while the concentration of the free, most reactive polystyryl ions is greatly diminished. Hence the reactivity of such polymers is lower than that of the monofunctional ones, and the conductance of their solution is higher. On the basis of that scheme the triple ion reactivity and the cyclization constant, Ke, were computed from the available experimental data.46 The propagation constant of the triple ions,

was computed as 2200 M -IS -1 in THF at 25°C. The cyclization constant depends on the length of the polymer chain, decreasing with its elongation as indeed was observed. For D P n of - 25 the cyclization constant was found to be 5.5.

2.2.7. Propagation of Living Polymers Associated with Bivalent Cations

There are two classes of living polymers associated with bivalent cations. These possessing only one growing end-group dissociate into two charged particles, e.g.,

and conduct therefore electric current, whereas the dissociation of the other kind endowed with two growing end-groups:

results in the opening of the ring and formation of only one nonconducting species. Higher associates could be visualized but are not considered here.

The behavior of the Ba2 + salt of living polystyrene endowed with one active end-group is unconventional. The first-order rate constant of its propagation, ku = - dln[Sty]/dt, is independent of the concentration of the salt47 as shown by Fig. 3.16. This peculiar behavior, contrasting that observed for living polymers associated with monovalent cations (see p. 96) is accounted for by the following mechanism.

Two modes of dissociation of the Ba2 + salt are postulated. A conventionalone,


10- 3 r----------------------, 6-

o o I 4 ~O 0

~ ~-~~-Tv~O~~vT--~-----o-~ U) 0 0 0

U) 2f-.0 o

~ 104 t-

I I 2 4

I I -4

8 10

[LP]/M

I 2

I 4 8

Figure 3.16. The plot of the pseudo-first-order propagation constant, ku = - dln[St]ldt of mono-ended barium polystyrene in THF at 20°C, vs. concentration of living Ba salt of polystyrene.

and an alternative one resembling the formation of triple ions,

The free polystyryl anion is assumed to be the only reactive species that contributes to the propagation, whereas all the aggregates containing the Ba2+ ions seemed to be unreactive. For a low degree of dissociation of the salt and for Kt,Ba[(~ty-hBa2+] » KdissBa these assumptions lead to the relation:

demanding the concentration of the polystyryl anion to be constant in such systems, independent of the salt's concentration (see also p. 116). Therefore, the first-order rate constant of polymerization, - dln[Sty]ldt = k_[-Sty-], is independent of the concentration of living polymers, as observed.

Studies of the effect of barium tetraphenylboride on the progress of this reaction48 fully confirmed the proposed mechanism, and the assumed inequality Kt,Ba[(""" StY)2, Ba2+] » KdissBa was justified by the values of the two equilibrium constants determined by measurements of the conductance. The kinetics of polymerization of the strontium salt of monofunctional polystyrene49 is accounted for by the same mechanism.


There are technical difficulties in the preparation of barium polystyrene endowed with two active end-groups. 50 The results of kinetic studies of their propagation are somewhat ambiguous. Nevertheless, the plots of conversion vs. time are sigmoidal, i.e., the rate of polymerization accelerates as the reaction progresses. This is expected. The equilibrium concentration of the active open chain polymers increases on their elongation.

2.2.8. Anionic Propagation of Polar Monomers

Studies of polymerization of polar monomers revealed some new features of anionic propagation. In contradistinction with the simplicity of polymerization of nonpolar monomers, the propagation of polar monomers, such as acrylates, methacrylates, acrylonitriles, vinyl ketones, and so forth, is complicated by a variety of side reactions caused by the presence of polar groups. Polar monomers are bidentate species, and the polar groups compete with the C=C bond for the reagents. Thus, the conventional monomer addition to the carbanion center is supplanted occasionally by the addition to the O-center,

1,2 CH3 CH I &-/ 3

~C-CH2-C

I ~ &-(:=0

R-C=O / R

1,4

The activation of a proton a to the carbonyl group leads to chain-transfer to polymer, e.g.,

-CHCO(OR) + -CH2CH(CO·OR)-~

-CH2CO(OR) + -CH2C(CO·OR)-.

Moreover, as revealed by the later studies, the living end-groups of methacrylates undergo various associations caused by the formation of

Initiation and Propagation of Ionic Polymerization / I2I

enolates. Examples of such reactions provided by the anionic polymerization of methylmethacrylate, the most extensively investigated polar monorner, are discussed below.

The electron-transfer initiation of anionic polymerization of methylmethacrylate, first investigated by Rembaum and Szwarc,51 proceeds rapidly and quantitatively in THF solution at -70°C but the resulting growing polymers are deactivated in a few minutes. The discovery of a stereospecific polymethylmethacrylate produced by anionic procedure, reported later by Fox et al.,52 stimulated interest in this reaction. However, a number of puzzling and often contradictory observations, reviewed by Bywater, 53 marred the early research of this process. Much of the difficulties arising in the course of the early studies resulted from the unfortunate choice of the experimental conditions, namely the use of alkyllithiums as the initiators and hydrocarbons as the solvents.

The pioneering work of Figueruelo and his colleagues54 started a new phase of this research. His work demonstrated that the meticulously purified methylmethacrylate yields polymers of narrow molecular mass distribution in a reaction initiated by alkali biphenylide in THF or DME solution at -70°C. This implied a living character of this reaction, a conclusion confirmed soon by Mita,55 and independently by Schulz56 whose methodical and consistent work led to a profound understanding of the intricacies of this polymerization.

The early kinetic studies of methylmethacrylate polymerization reported by LOhr and Schulz56.57 appeared to be self-consistent. The resulting polymers had the anticipated narrow molecular mass distribution, the number average molecular mass increased proportionally with the conversion, and the propagation was claimed to be first order in the monomer and living polymers. However, the subsequent work of Warzelhan and Schulz58 demonstrated the propagation to be more complex. The plots of log([Mo]/[M)) vs. time, originally claimed to be linear, turned out to be concave, as exemplified by Fig. 3.17. The propagation accelerates as progressed.

The cause of this anomaly was soon discovered. 59 The investigated polymerization was initiated by electron-transfer using sodium biphenylide as the electron donor, or alternatively by utilizing sodium a-methylstyrene "tetramer ,"

as the initiator. In both cases bifunctional polymers, endowed with two growing end-groups are formed. The two ends, bound to be in vicinity of each other, become associated and form a cyclic polymer having presum-


0,8

0 ,6 I ..... ,......., ~ --0 0.4 ...!,

01

~ 0.2

°

/

I //

~// ",,/ E

/ V'

10 20

tIs

30

/"

4 0

Figure 3.17. The time conversion curve for the anionic polymerization of the sodium salt of polymethylmethacrylate initiated by a bifunctional initiator in THF at -80°C. [Mo] = 0.2 M, [initiator] = 5.9' 10-4 M, [NaBPh4] = 2.10-3 M. Plotted as log[Mo]/[M] vs. time. Note the acceleration of the propagation.

ably the following structure: CH3 CH2 @"- /

CH30 0 ---- Na --- -C "-/,,,~

C iG G )C '\" @ -'/ "-

C ---- Na ---- 0 OCH3 /"-CH3 CH2 -------'

The associated end-groups are less reactive than the nonassociated ones. Both are in equilibrium with each other as described by the scheme:

~ -=*MM-MM*,

where -MM* denotes the -CH2C(CH3)(C:O'OCH3)'CH2C-(CH3) (C:O'OCH3),Na + end-group of sodium polymethylmethacrylate salt. The equilibrium between the cyclic and linear polymers depends on the length of the chain connecting the two end-groups, a phenomenon observed previously in the polymerization of living cesium polystyryl (see p. 117). The longer the chain, the larger the fraction of the open rings, and hence the higher the reactivity of the polymers. Therefore, as the polymerization proceeds and the chains become longer, the fraction of the linear polymers increases and the propagation speeds up. To verify the proposed explanation of the auto-acceleration, the polymerization of methylmethacrylate


was initiated by long oligomers of a-methylstyrene (DPn - 70 or 270) endowed with two active end-groups.60 These active end-groups are far apart from each other at the very start of the reaction, making the degree of cyclization negligible. The resulting propagation behaved then conventionally; the plot of In([MJo/[M]) vs. time was then perfectly linear. 61

The association of the polymer end-groups described above is not limited to intramolecular reaction of bifunctional polymers. It may take place also intermolecu1arly and, of course, the fraction of the intermolecu1arly associated polymers depends then on the concentration of living polymers. The equilibrium constant of the intermolecular association of living polymethylmethacrylate was found to be -200 M-1 in THF at ambient temperature,62 a value confirmed at higher polymer concentration by vapor pressure osmometry. 63

Sodium polymethylmethacrylate in THF dissociates into free ions to a lesser extent than the sodium polystyryl, its dissociation constant is 2.10-9

M at - 78°C. A higher value could be expected since this is an allylicenolate salt, more delocalized than a salt of a simple carbanion. It seems that some additional binding hinders its dissociation, apparently an intramolecular association with the keto-group of the penultimate segment.

The propagation rate constants of the monofunctional salts of polymethylmethacrylates increase with the size of the cation as shown by Fig. 3.18. However, the Arrhenius plots seen in Fig. 3.19 reveal that the rate con-

4 r--------------,

a. .:.:

3

o 2 Cl o

o

k p( - I

(Na' ,2221

Cs'

0,2

K+ Na+

• •

0.4 -1

(l/a) / A

Figure 3.18. The plot of the propagation rate constant of ion-pairs of the salts of living polymethylmethacrylate in THF.


stants, as well as the activation energies, are closely the same for the the Na+, K+, and Cs+ ion-pairs. The reason for this peculiarity is not clear.

The perfect linearity of the Arrhenius plots, down to temperatures as low as -100°C, is significant. It contrasts with the curvatures observed in the Arrhenius plots of the propagation constants of polystyryl ion-pairs (see Figs. 3.7 and 3.8) which revealed the participation of two kinds of ion-pairs in the polymerization of styrene. Hence, the linearity shown by Fig. 3.19 implies that only one type of ion-pair partakes in the methylmethacrylate propagation. Such pairs are, probably, externally solvated by the keto-groups of the penultimate, or ante-penultimate segments of the polymers, although a solvation by solvents such as THF or DME cannot be ruled out, especially at the very low temperatures when the loss of translational entropy is diminished.

The intramolecular solvation may lead to structures such as

or

suggested by Schuerch et al. 64 and deduced from the NMR spectra of the partially deuterated polymers. In this respect, the ion-pairs of living polymethylmethacrylate resemble those of poly-2-vinylpyridine (see p. 116).

The structures depicted above are supported by the results of the 13C NMR studies of the salts of iso-butyric ester, a model compound for the growing polymethylmethacrylic anion, reported by Vance a and Bywater.65

There is evidence for restricted rotation around the -C=C=O bond

OR at lower temperatures, and for the shift of the negative charge toward the carbon atom on increasing the bulkiness of the cation.

The most recent 7Li NMR studies of Teyssie and his co-workers374 of the Li salt of this model compound in THF revealed their aggregation into tetramers and dimers coexisting at low temperature (- - 60°C) within the concentration range .05 to 2 M. The pertinent spectra show well separated peaks resonating at & - 0.52 ppm (tetramer) and -0.71 pmm (dimer).

Two mechanisms of propagation of anionic polymerization of methylmethacrylate were proposed by Muller. 66 One postulates the coordination of the keto-group of the monomer with the counterion by displacing a solvent molecule from the cation's solvation shell. This step is energetically unfavorable in good solvents such as DME, or even THF, that solvate the cation better than ketones, but it is advantageous in poor solvents, e.g.,


THP. The subsequent events are depicted by the scheme:

'T 1°C

50 20 0 -20 - 40 -60 -80 -100

'\ I

3

....--- -

0

- - .

0,. o

30

, ,

C.""t .. , ion

C O'

K '

No

u ·

I I I I I

PMMA- i , " . ',:~cs

,,~

Na'""" '\a.-:,.K" I Na', 222) t

'~~

sJ' '~'\", ~ • ~:<, Li taU 1ft.1

19,5 7 ,3

\ 19 .3 -18 ,3 7.0

24 7,4

60

Figure 3.19. The Arrhenius plots of the propagation rate constants of ion-pairs of the salts of living polymethyl methacrylate in THF including the Li + salt and the result of the addition of cryptands. The above data result from the extensive work of A.H.E. Muller (see ref. 66).


Alternatively, the reaction proceeds by the conventional mode of propagation. The negatively charged carbon center interacts directly with the C=C bond of the monomer and the partial dissociation of the initial ionpair in the transition state of the reaction facilitates the transfer of the cation to the newly formed center. This mechanism operates, of course, independently of the previous one, and it accounts also for the growth of the free anion and for the reaction involving the salt of the cryptated Na +

which cannot be desolvated. Apparently, it governs also the polymerization of the lithium salt in DME since this ether solvates the Li + cations very strongly. Indeed, the respective propagation rate constant is very low in this medium, 16 M-1s-1, compared with kp = 100 M-1S- 1 determined at the same temperature in THF.

The two mechanisms discussed above differ considerably in their stereoregulating effects. As pointed out by Schuerch et al.,64 the first mechanism should lead to a meso placement, whereas no specific geometry is favored by the second mechanism, but for steric reasons the racemic placement takes place preferentially.

Some initiators of anionic polymerization, especially lithium alkyls, may react with methylmethacrylate in a detrimental fashion, e.g.,

R

RLi + CH2:C(CH3)(C:O·OCH3) --+ CH2:C(CH3)t:O + LiOCH3•

Such a reaction consumes the initiator without inducing polymerization, hence it reduces its efficiency. The produced vinyl ketone67 is an extremely reactive monomer and rapidly adds to the living polymethylmethacrylate. The stability of the resulting end-group is high, making it unreactive with respect to methylmethacrylate. Hence, the resulting polymer is pseudodormant, behaves as if it were terminated, or rather pseudoterminated. Eventually, a slow addition of a molecule of methylmethacrylate to this pseudoterminated polymer restores its conventional end-group and its normal propagation is then resumed.

The addition of various complexing agents led to some improvement in the performance of anionic polymerization of methylmethacrylate. The best results have been achieved with LiCI which forms a JJ.-complex with the living end-groups.375a-c Under such conditions it has been possible to produce pure block polymers, functional oligomers, star-shaped polymers, all with high efficiency (>90%) and very low polydipersity (down to MwlMn = 1.02).

The most recent 7Li and 13C NMR studies of the model Li salt of methyl iso-butyrate revealed the formation of 1:1 and 1:2 complexes of the above


salt with LiC}.376 The dynamic associations with LiCI accounts then for the substantial improvement of polymerization of methylmethacrylate.

2.2.9. Oligomerization

The addition of the first, and often also of the second or third, monomer molecule to a growing chain may proceed with a rate different from that of the following monomer molecules. It is interesting, therefore, to determine the rates of the early steps of polymerization yielding the low molecular mass oligomers.

Two studies of oligomerization are discussed here: the conversion of a difunctional dimer of a-methylstyrene into its trimer, and the oligomerization of methylmethacrylate initiated by the metallated iso-butyric ester.

Exhaustive reduction of a-methylstyrene, a, by metallic potassium performed in THF solution converts it quantitatively. into its dimeric dianion:

On the addition of a-methylstyrene the trimer, K+, -aaa- ,K+, is formed reversibly

2kf

K+, -aa- ,K+ + a ~ K+, -aaa- ,K+. kb

The rate constants, kf and kb' of the forward and backward reactions were determined by utilizing the stirred-flow reactor technique.68 Its principle is simple. The reagents are continually fed at a constant rate into a wellstirred reactor, and the reacting liquid is withdrawn from it simultaneously at the same rate. The volume V of the reacting solution remains therefore constant. The reaction in the outgoing liquid is quenched immediately and, after attainment of a stationary state, its composition remains constant too, independent of time. The kinetics of the studied reaction is described then by a set of algebraic balance equations. For example, the reversible reaction of a-methylstyrene with its dimeric dianion yielding the trimeric dianion is described by the following balance equations:

The Cod denotes the concentration of the dimeric dianions in the feed, and Cm , Cd, and Crr are the respective concentrations of the monomer, dimer, and trimer in the outflowing liquid. The last two concentrations are the same as the concentrations of the dime ric and trimeric anions in the reactor. Denoting by v the rate of flow of the solution in and out of the reactor and by V its volume, we find the resident time T of the reactants in the reactor to be V/v. Rearrangement of the above equation in conjunction


with the stoichiometric conditions Crr = Com - Cm = Cod - Cd, (Com is the concentration of the monomer in the feed) leads to the relation:

Hence, the plot of 117 vs. Cm{Cod/(Com - cm ) - 1} should be linear with a slope yielding 2kf and the intercept providing - kb • Indeed, the experimental data obtained for the reaction proceeding in THF at 25°C led to such a plot shown in Fig. 3.20. From its slope and intercept the following values were deduced: kf = 17.1 M-1s-l, kb = 0.052 s-l, and the equilibrium constant of trimerization = 331 M-1. The last value is substantially larger than the equilibrium constant of propagation of the high molecular mass polymer equal =0.5 M- 1. In the latter process the added molecule of monomer has to be squeezed between two adjacent units of the polymer, a process opposed by a large increase of steric strain. The strain is substantially reduced in the conversion of the difunctional dimer into a trimer, and this accounts for the large value of the equilibrium constant of the above trimerization.

The simplest mode of initiation of methylmethacrylate polymerization results from the addition of a monomer to metallo iso-butyric esters, P 1,

This reagent is formed by metallation of the iso-butyric ester in THF solution by alkali di-iso-propylamide,69 and a similar procedure yields the

-uMeS·oMeS-. aMeS =-(U MeS·l 3- or L 0,20 ,...------------------,

0.15 o

p -:::- 0,10

o 0,05

0.00 '---'-"-----'--'--''---'---'---'-----' o 5 10 15

[M] t{[LE)/([MJ n-rMJt l-~ . 103 20

Figure 3.20. Application of stirred-flow reactor technique to kinetic studies. The reversible conversion of potassium a-methylstyrene dimers into trimers. The plot of lIT vs. cm{coAcom - Cm) - I}. T is the residence time, C the concentrations of the pertinent reagents and products in the incoming and outgoing flows.


dimeric ester, P 2' The monomeric metallo-ester undergoes a slow reversible Claisen auto-condensation yielding a ~-keto ester, denoted by Ket:

2(CH3hC- 'C:O(OCH3),Cat+ ~

(CH3)2C- ,Cat+ = Ket, I C:O·C(CH3h·CO·OCH3 + CH30Cat.

However, in the presence of the monomer a reversible oligomerization of methylmethacrylate takes place, a rapid forward reaction but a very slow backward process. The consecutive steps of the oligomerization were investigated by Muller and his associates70 who described this process by the scheme:

+M +M +M +M +M

PI ~ P2 ~ P3 ~ P4 ~ Ps ~ "'""k-l "'k-2 k-3 "'k-4 k-5

+ PI jf k3c jf k4c jf ksc jf Kef + CH30Cat P3c P4c PSc

+ CH3OLi; + CH3OLi; +CH3OLi;

In addition to the reversible oligomerization, the trimers and higher oligomers undergo readily a cyclization yielding the cyclic oligomers P3c' P4c' PSc' and so forth by elimination of lithium methoxide. The latter oligomers are terminated by a six-membered ring, their formation, shown by the scheme

results from an intramolecular "back-biting," an important reaction in dilute solution accounting for the termination of this polymerization.

The oligomerization is very fast, being over in less than 1 s. Its progress was studied by quenching the reaction at a desired predetermined time


and analyzing the products by the vapor phase and the size exclusion chromatographic techniques. The resulting data points70 shown in Fig. 3.21a fit fairly well the computer-drawn curves derived by using the rate constants obtained by the computer best fit procedure. The reverse reactions are neglected since they are too slow to affect the results. Their sluggishness was confirmed by the independent studies of the disproportionation of the living dimers of methylmethacrylate.70

The following values of the propagation constants were reported: k1 = 5000, k2 = 215, k3 = k4 = k5 = k~ = 190, all in the units M- 1S- 1• PI is the only anion not stabilized by its intramolecular association with the penultimate unit, and this accounts for its high reactivity. The increase of the length of the oligomers does not affect their propagation constant for n > 2, even the conversion of dimer to trimer proceeds with a rate constant marginally different from k~.

The rate of cyclization of trimers is 50 times faster than that of the higher oligomers whose sluggishness is attributed to the steric hindrance caused by the "tail." The respective cyclization constants are: k3c = 23 s-1, k4c = 0.45 s -1, and k5c = 0.35 S -1. The rapid cyclization of the trimers accounts for the preferential formation of methoxide in the first stages of polymerization, a fact stressed by the early investigators of this reaction.

Studies of the slow disproportionation of the living dimers, summarized by Fig. 3.21b, provide the data leading to the equilibrium constants of conversion of an n-mer into n + I-mer. The most significant is the large value of K1- 2 = 2.107 M- 1 compared with K2 _ 3 = K3- 4 = K4- 5 = 700 M -1. This demonstrates the very large increase in the stability of the higher anions (n > 2) resulting from the coordination of their carbanion endgroup with the keto-group of the preceding polymer segment.

It is advantageous when an initiator resembles structurally the active end segment of the produced polymer. The resulting initiation is then clean and simple. Thus potassium cumyl is an excellent initiator of a-methylstyrene polymerization, sodium ethoxide rapidly initiates the anionic polymerization of ethylene oxide, metallo-iso-butyric ester is an efficient, rapid, and clean initiator of methylmethacrylate, I-phenyl-hexyllithium71 rapidly polymerizes styrene, and so forth.

2.3. Initiation and Propagation of Cationic Polymerization by Free Ions and Ion-Pairs

The characteristic features of cationic initiation and propagation differ from those observed in anionic systems. Most of the counterions participating in cationic polymerizations are bulky, as exemplified by the spherically shaped SbCI6" , AsF6" , and PF6" ions derived from the Friedel-Craft reagents. Their charge is evenly spread over their surface and they are only

0 (J

"-(J

Initiation and Propagation of Ionic Polymerization / I3I

Q

kl = 5000

0.8 oJ = 23

0.6

0° .... 0

O.~

0.2

1.0 b

k = I 4Il0 3

-4

0.8 ~I:': 2:.:10

K1= 2xl0 7

0.6

0.4

0.2

0

'2 = 215 kJ • k, , .. . = kp - 190 I. mo i l _.-1

k • 0 '5 k = 0.3; , .1 0, Os pC

J ~------------ ---~- -

tis

k = 165 - 4 -1

kCI =4IlO I mol 2

k = -2 0,25 kc3= 25 8-1

K = 2

7Il02 I moll

pC 3

pC • 1

-----:!. 2 3 4 5

t (lOSs) __

t6

. - 1

6

Figure 3.21. a, Calculated conversion curves vs. time and the expenmental points for the anionic oligomerization of methylmethacrylate m THF Imtiated by methyla-lithio-iso-butyrate. [MMA]o = 0.1 M, [initiator] = 0.05 M, 25°C. b, The calculated time-conversion curves and the experimental data pomt!> for the !>low decomposition of the living dimers of MMA.

132 / 10nie Polymerization and Living Polymers

weakly interacting with the surrounding solvent molecules. The association of the oppositely charged ions yields only loosely linked pairs, and no triple ions are formed in such systems. The push-pull mechanism responsible for the interesting phenomena observed in anionic polymerization does not seem to operate in cationic reactions. On the other hand, the reversible equilibria converting ionic into covalent species, implausible in anionic reactions involving alkali cations, play an important role in cationic processes.

2.3.1. Salts Active in Cationic Polymerization

Only a few relatively stable carbenium ion salts capable of initiating cationic polymerization are crystallizable and permit a leisure investigation of their properties in solutions. The salts of the trityl cation and its derivatives, e.g., (p-CIC6H4)3C+), diphenylmethylium, tropylium, and a few other aromatic cations associated with large and non-nucleophilic anions derived from the Friedel-Crafts reagents exemplify this class of initiators (for a review, see Ledwith and Sherrington72). The interesting and stable salt shown below,

is a bifunctional derivative of the trityl salt. It was synthesized by Sigwalt and his associates73 and allows the preparation of cationically growing polymers with two active propagating end-groups. Derivatives of oxocarbenium ions R·C=O+, alkoxycarbenium ions ROCH!, and the various oxonium, sulfonium, etc. ions participating in the numerous cationic ring opening polymerizations exemplify the class of other stable salts capable of initiating cationic polymerization.

The growing polymers formed in numerous cationic ring opening polymerizations are sufficiently stable to act as initiators of some other polymerizations, whether ring opening or of strongly basic monomers.

It was hoped that trityl and tropylium salts would initiate rapidly and quantitatively cationic polymerization of monomers such as N-vinylcarbazyl and vinyl ethers. Unfortunately, this is not the case and the published values of propagation rate constants computed on the assumption of a rapid and quantitative initiation have to be discarded. The reactivity of these salts was re-examined by Sigwalt and his colleagues74 who monitored simultaneously the initiation and propagation of cationic polymerizations of some vinyl monomers induced by these salts. The reaction was performed in a modified adiabatic calorimeter, originally developed by Plesch,75 equipped with windows allowing determination of the absorbance of the initiator. The initiation induced by trityl salts was followed by recording its absorbance at 413 nm (Amax of the trityl cation, E = 4· 104 M·cm), while the heat evolved in the reaction monitored the progress of polymer!zation.


The results demonstrated that the initiation is relatively slow and incomplete.

Most of the salts initiating cationic polymerization are labile and have to be prepared in situ. For example, the reaction of cumyl chloride with boron trichloride leads to the equilibrium

established in milliseconds (unpublished results of Vairon) and in the presence of a monomer, say iso-butene, a cationic polymerization ensues,76 yielding PhC(CH3)2-(CH2CMe2)n·CH2C(CH3)2Cl. Telechelic polymers bearing two -CMe2Cl end-groups are formed by a similar reaction employing a bifunctional analogue of cumyl chloride,77,78 and star-shaped polymers are produced when the analogous trifunctional initiators are used. 79

A most promising pulse-radiolysis technique, developed by Dorfman,so yields free carbenium ions on irradiation of suitable generators in chlorinated solvents by pulse of fast electrons. This procedure permits recording of the electronic spectra even of unstable carbenium ions and allows studies of the kinetics of their reactions with the previously added substrates. For example, the decay of electronic absorption of carbenium ions formed by a pulse of fast electrons in the presence of an olefine, monitors their addition to that substrate. Extension of this work should be profitable.

The salts even of stable carbenium ions associated with anions derived from the Friedel-Craft reagents are in equilibrium with their covalent components. For example81 :

Such equilibria, first studied by Plesch82 and later by Penczek,83 are known as the binary ionic equilibria. The degree of dissociation of ion-pairs into free ions is unaffected by dilution and its extent is comparable to that of dissociation into the covalent components. In many systems some parasitc processes take place and then the concentration of the light-absorbing ions slowly decreases. For example, the reaction

PhC( CH3)2Cl + BCl3 ~ PhC + (CH3bBCI4" ~

PhC(CH3)=CH2 + HCI + BCl3

slowly destroys the carbenium ions present in equilibrium with cumyl chloride. Such reactions have to be kept in mind when the rate of initiation is gauged by the rate of bleaching of the initiator.

The most detailed studies of ionization of substituted diphenylmethyl chlorides induced by BCl3 were reported by Mayr. 84 The investigated sys-


terns are described by the scheme:

On addition of increasing amounts of BCl3 the covalent chloride, denoted by ACI, is gradually converted into a mixture of A + ,BCI4" ion-pairs and their free A + and BCI4" ions. Eventually, the UV -visual absorbance of the solution, as well as its electric conductance, reaches its asymptotic value unaffected by further addition of BCI3 , implying a quantitative conversion of the covalent species into ion-pairs.

The conductometric titration of the substituted diarylmethyl chlorides, illustrated by Fig. 3.22, shows that the dissociation constants of the resulting ion-pairs and the mobility of the free ions are virtually unaffected by the nature of the substituents, the variation of the latter only slightly influences the height of the plateaus seen in Fig. 3.22. On the other hand, the degree

5

o

101. x Sm1

OCH3,OCH3

5 10

BC,:! /m mol

OPh, H

H

-0- 1 X ~ /, I\~C I

O,lmmol y y

15 20

Figure 3.22. Conductometric titration of Ar2CHCI solution in CH2CI2 by BCI3 at -70°C [Ar2CHCI] = 10-3 M.


of ionization of these chlorides depends strongly on the nature of the substituents, as evident by the large variations of the initial slopes of the titration curves.

To avoid any confusion let us state that the amount of ACI present in each of the titrations is very small, about 0.1 mmole. The ionization would be quantitative at the very start of the titration had the equilibrium constant of ionization been large. The excess of BC13 is needed because the equilibrium constant is relatively low for most of the chlorides.

The titration of the least dissociated diphenylmethyl chloride, X CH3,

Y H, in conjunction with the known dissociation constant of its ion-pair (determined by an independent conductance study) provides the absolute value of its free energy of ionization, namely aGo = -7 kJ/mole. The difference of the free energies of ionization of any two chlorides, say AlCl and A2Cl, is determined by the NMR spectra of their mixture containing a small amount of BC13• The equilibrium

is rapidly established in these systems, hence the intensities of the pertinent NMR lines allow the calculation of the respective equilibrium constants. The results of such studies are shown in Table 3.2. Significantly, the aaGo vary linearly with the respective pKR +.

Table 3.2. Free energies of ionization of the substituted diarylmethylchlorides

X OCH3

Y OCH3

J1J1ao 0.0

OCH3

OPh 7.2

OCH3

H 18.5

OPh CH3

19.7

OCH3

Cl 20.9

OPh CH3

H CH3

25.9 28.1

The .1.1Go values were determined at -70°C in methylene chloride and are given in kJ/moJe.

2.3.2. Comparison of Reactivities of Ion-Pairs and Free Ions in Cationic Polymerizations

The large difference in the reactivities of tight ion-pairs and free ions characteristic of anionic reactions involving alkali ions led to the expectation of similar differences in the cationic systems. However, the counterions involved in most of the cationic polymerizations are very large, e.g., the radius of SbCl6" anion is estimated at 4.2 A, and hence its center is relatively far away from the reacting cation. Therefore, the influence of the counterion on the reactivity of the initiating or propagating cation is small, making the reactivities of ion-pairs and free ions comparable, contrary to the past belief. Indeed, the relation between the reactivities of ion-pairs and free ions in cationic systems seems similar to those between


the loose ion-pairs and free ions in anionic polymerizations. In both cases the counterions are bulky and the respective ion-pairs are only slightly less reactive than their free ions.

The notion of tight and loose ion-pairs is inappropriate for cationic systems, a point stressed earlier. The separation of bulky ions by solvent molecules would lead to the dissociation of the pertinent ion-pairs. The "sphere in continuum" model provides their best description. It accounts for the relatively large values of the dissociation constants of bulky ionpairs, varying within a narrow range, and for the low exothermicity of their dissociation.

Evidence for the comparable reactivities of free cations and their ionpairs has been gradually accumulated. Sigwalt and his associates,74 who studied the initiation of polymerization of p-methoxystyrene induced in methylene chloride by Ph3C+ ,SbCI6", found the reaction to be first order in the monomer and the salt.74 Knowing the dissociation constant of the initiator, determined by conductance study, they calculated the mole fraction, 'Y, of the free ions present in the reacting solutions. The bimolecular rate constant of the initiation, ki' given by the relation

where ki+ and ki± denote the initiation rate constants of the free cation and its ion-pairs, was anticipated therefore to vary with 'Y. Surprisingly, it remained unchanged, in spite of the extensive variation of the dilution of the polymerized solution, or the addition of quaternary ammonium salt of SbCI6", both greatly affecting the value of 'Y, namely from 0.36 to 0.96. This result, illustrated by Fig. 3.23, demonstrates that ki + = ki ±. The same conclusion has been drawn from the results of the previous study85 of polymerization of cyclopentadiene performed under similar conditions.

The equal reactivities of the free diphenylmethyl cations and their ionpairs were demonstrated by Mayr86 who studied the kinetics of their addition to a model olefin, CH2=C(CH3)C3H7. He fortified this conclusion by a semitheoretical argument.84 A survey of published data shows only small variation in the values of the dissociation constants of a large number of ion-pairs composed of bulky organic cations coupled to bulky counterions, such as BCI .. , SbCI6" , and PF6" . In other words, under such conditions aGo of the dissociation,

R+,A- ~ R+ + A-,

varies only a little on changing the nature of R. Therefore, a comparable change of free energy is expected for the dissociations

(R+M,A-)*~(R+M)* + A- asfor(R+M,A-)~R+M + A-,

,. III


0.3 r- o • • o • _---- - - --0- --0 ....... .. - - - ---

0.2 -

o I

0.5 -y

• • ••

10

Figure 3.23. The effect of the degree of dissociation 'Y of Ph3C+, SbCi6 ion-pairs on the rate constant of their initiation of cationic polymerization of a-methylstyrene. The degree of dissociation 'Y was varied by changing the concentration of the salt (closed circles) or by the addition of Bu4N+, SbCI6" (open circles).

where (R +M·A -)* and (R +M)* denote the transition states of a monomer addition to ion-pair and free ion, respectively. Hence a comparable reactivity of an ion-pair and its free ion is justified. The same argument applies to anionic systems when the reactivities of loose ion-pairs and their free ions are compared.

Let us remark that this reasoning is justified by the property of the 1Ir function. It is obvious that its value varies only slightly with r when r is large.

Convincing evidence confirming the equal reactivities of free ions and their ion-pairs in cationic polymerizations involving large counterions is provided by Brzezinska, Matyjaszewski, and Penczek.87 They studied the cationic polymerization of oxepane initiated by the 1 ,3-dioxolenium + ,SbF 6 salt (a very rapid initiator) in methylene chloride or in nitrobenzene. The resulting polymers are living and the kinetic studies of their polymerization allow the determination of the respective rate constants of propagation at different concentrations of the living polymers, i.e., at different degrees of their dissociation into the free ions. Their degrees of dissociation were computed from the conductance data. Moreover, the degree of dissociation of the living polymers was modified also by the addition of salts sharing common counterions with the polymers. The results, presented in Fig. 3.24, provide indisputable evidence proving equal reactivity of the respective free ions and ion-pairs. Similar results were reported for the cationic polymerization of THF investigated in these two solvents.88


a 0 0.05 0.10 0,15 0.20

I I f I I

• •• • (1) - 6 ' Ill • • • ~

' .. 5 -0 0 0 (2 )

0 o °Cb 0 00 1 0 E - , r-

~

~

0. 3 c-o.

"'0. -"

2 I I I 0 0.2 O. , 0.6 0.8 1.0

a

Figure 3.24. The effect of the degree of dissociation a of macro-ion-pairs of living polyoxepane on the apparent bimolecular rate constant of their propagation, kap. Solvent CH2Cl2 (closed circles), nitrobenzene (open circles).

In conclusion, in cationic systems the reactivity of ion-pairs associated with the large anions, e.g., those derived from the Friedel-Craft reagents, is comparable to the reactivity of the respective free ions. In this respect the relation between the reactivities of free ion and ion-pairs in cationic polymerization resembles those between free ions and loose ion-pairs in anionic systems. No genuine low-reactivity tight ion-pairs are formed in the cationic systems, although the cationic ion-pairs exist at very low concentration in equilibrium with the covalent species.

2.3.3. Some Reactions of Trityl Salts: Transfer of Hydride Ions

Trityl salts are relatively stable but they react extremely fast with traces of moisture and hence have to be handled in rigorously dried solvents. The bimolecular rate constant of the reaction

is lOS M-1S- 1 and therefore it has been proposed to adopt this reaction as an analytic tool for determining the concentration of residual water in aprotic solvents.

On the other hand, the addition of trityl cation to vinyl monomers is slow and reversible. Although they are capable of initiating cationic polymerization, the initiation takes place by an alternative mode and not by simple addition of the cation to the vinyl carbon. For example, the rate constant of trityl addition to p-methoxystyrene is 0.27 M-1S- 1 at lOoC. The


slowness of this addition arises from two causes: 1) a less stable ion is formed from a more stable initiating ion, and 2) the propeller-like shape of the initiator sterically hinders the addition. Moreover, as pointed out previously, the addition is reversible and results therefore in incomplete utilization of this initiator; a large fraction remains intact at the completion of polymerization.

The slow addition of the trityl cation favors an alternative mode of initiation. Its reaction with a-methylstyrene89 provides an example:

followed by

Unfortunately, direct evidence for the formation of the above allylic cation is not available yet.

A similar hydride ion transfer accounts for the initiation of the polymerization of dioxolane or tetrahydrofuran by the trityl cation.90 The propagation is initiated again by the positively charged deprotonated monomer formed by the relatively fast and quantitative reaction:

followed by a slower addition


The kinetics and equilibria of these hydride transfer reactions were investigated for a series of ethers by a spectrophotometric technique91 and a polarographic method.92 The results obtained by both methods were concordant. The bimolecular rate constants are of the order of 10-2 M-1s-1 at 25°C. The reaction is exothermic and results in a large decrease of entropy91 implying a more extensive solvation of the positive ether ions than of the trityl cation. Independent experiments revealed the rate of this process to be insensitive to the pairing.91

The cation resulting from the abduction of a hydride from the dioxolane is stable if coupled with the stable counterions of low nucleophilicity such as SbF6 or AsF6 . Such pairs initiate a clean polymerization of cyclic acetals and ethers as exemplified above. However, the pairs with more nucleophilic counterions, e.g., SbCl6 or BFi give rise to side reactions due to their fragmentation and the formation of covalent bonds.91 These reactions are facilitated by the readily established equilibrium between the oxonium and alkoxocarbenium ions,

The formation of the reactive alkoxocarbenium ions facilitates the backbiting to be discussed later (see p. 334).

Failure of a monomer to be polymerized by an initiator does not necessarily indicate the absence of the addition reaction. For example, trityl cations do not initiate polymerization of iso-butene although their addition to the C=C bond does take place. The polymerization is prevented by a rapid rearrangement yielding eventually a derivative of indene93:

C4Ph + y ~[qtPhl ~ Ph

2.3.4. Fontana Mechanism

~'Y< +HX. ~Ph

Ph

An interesting problem was raised by the work of Fontana94 who studied the kinetics of cationic polymerization of propylene initiated by AlBr3 and HBr in hydrocarbon solvents. The rate of propagation formally described


by the scheme

-CH2CH+(CH3),AlBri + C3Hs ~

-CH2CH(CH3)·CH2CH+ (CH3),AlBri ,etc.

was found to be independent of monomer concentration, a result accounted for by a mechanism postulating a rapid and virtually quantitative formation of a 1T complex

P;; ,A - + M ~ (P;;·M),A -,

followed by a slow monomer insertion into the chain:

This idea, used subsequently by many authors studying cationic polymerization, was applied also for the explanation of some findings observed in anionic systems (see p. 156). The 1T complexes are not ad hoc inventions. Similar complexes have been formed and conclusively identified in the reaction of Ag+ ions with aromatic and olefinic hydrocarbons.95

An interesting extention of these ideas was reported by Higashimura and his colleagues,96 who suggested that two distinct kinds of complexes could be formed between cations and monomers endowed with two electron donating centers. For example, vinyl ethers may form two kinds of complexes: one arising from the presence of a C=C bond (a 1T complex), and the other resulting from the cation interaction with the 0 atom (a n complex). These two kinds of complexes may differ in their reactivities, e.g., the former would lead to propagation as a result of monomer insertion, while the latter could be an inert nonpropagating species. The significance of these suggestions was discussed by Plesch.97

2.4. Role of the Solvent

The role of the solvent deserves detailed examination. The most common solvents used in anionic polymerization are either neutral (hydrocarbons) or nucleophilic, e.g., tetrahydrofuran or other ethereal liquids, pyridine, acetonitrile, dimethylsulfoxide, and so forth. They interact strongly with the positively charged counterions, especially with the small alkali cations, and facilitate the dissociation of ion-pairs into free ions and the conversion of tight ion-pairs into the highly reactive loose ion-pairs. These factors make the nucleophilic solvents beneficial in the initiation and propagation of anionic polymerization. Their interaction with anions is weak and affects the reaction negligibly. Indeed, inspection of the following idealized and


simplified model,

vividly demonstrates the basic difference in the modes of solvation of cations and anions by the ethers such as tetrahydrofuran. While the cation interacts strongly with the adjacent dipole associated with the ° atom, the "wrong" orientation of the ether molecule makes its interaction with the anion weak.

The solvents preferred in cationic polymerization, such as chlorinated hydrocarbons, nitromethane, and nitrobenzene, are also slightly nucleophilic or neutral. Hence cations, rather than anions, are again preferentially solvated. However, since the cations are now the reactive species, while the anions act as the counterions, surrounding the former with solvation shells hinders, rather than facilitates, the initiation or propagation of cationic polymerization.

Some cation-anion pairs participating in cationic polymerizations tend to form covalent species, the esters. The solvents of high dielectric constant favors their ionization and dissociation into free ions, thence they speed the rate of cationic polymerization.

It would be interesting to develop solvents that interact with anions more strongly than with cations. Liquid sulfur dioxide might be an example. Indeed, in his review published in Progress in Polymer Science, Matsuda381

remarked: "The solvent (liq. S02) is far more effective in bringing about ionization than could be anticipated on the basis of its dielectric constant (£ = 12.4 at 22°C). It may be that liquid S02 solvates anions preferentially, its affinity for protons or carbenium ions is low and the carbenium ions might be "bare" in this medium." It has been reported that the dissociation constant of trityl chloride in liquid S02 is -3 . 10-3 M at O°C, by powers of 10 higher than in nitrobenzene, inspite of its much higher dielectric constant. Significantly, the molecular mass of polystyrene resulting from cationic polymerization performed in liquid S02 is higher than that obtained in other solvents under the same conditions. Moreover, the molecular mass of polystyrene increases in mixed solvents with increasing concentration of S02.

Solvents interacting with carbenium ions may convert them into unreactive or less reactive cations. For example, the polystyryl cation reacts with the nitro-group of nitrobenzene and forms an inactive oxonium ion.98

Not surprisingly, the propagation rate constant of the free polystyryl cations in this solvent was found to be -200 M-1S- 1 only,99 at least 1000 times


less than in hydrocarbons, since most of the original carbenium ions responsible for the reaction are converted into the inactive oxonium ions.

Transition state theory of ionic reactions in solutions treats the solvent as a continuous medium characterized by its dielectric constant and viscosity. Since charges are more diffuse in the transition state of an ionic initiation or propagation than in their initial state, the theory predicts a decrease of their rate with increasing dielectric constant of the solvent (the degree of solvation of transition state is less than of the initial one). Accordingly, the initiation or propagation of ionic polymerization becomes slower as the dielectric constant of the medium increases. However, the treatment ignores any strong specific interaction of solvent molecules with molecules of the reagents that may profoundly affect the rate of these reactions.

Many reactions studied with the intention of testing the predictions of transition state theory were performed in mixed solvents (see, e.g., refs. 100 and 101). The composition of such solvents in the vicinity of ionic reagents, and especially the composition of the solvation shell surrounding the ions, may differ substantially from the composition of the bulk of the medium. Therefore, the dielectric constant of the medium computed on the assumption of the additivity rule pertaining to the bulk composition is a parameter of questionable value. 102

The distinction between the composition of the bulk of a medium and that of the immediate surroundings of the ions becomes important when the polymerized monomer competes with the solvent for the sites in the solvation shell of the growing polymer. For monomers which are much better solvating agents than the solvent, the rate of polymerization may become independent of the monomer concentration when its volume fraction is sufficiently high. Under an extreme condition only the monomer would be present in the vicinity of the growing polymer. The kinetics of such a system would resemble that predicted by Fontana mechanism (see p. 140), the reaction would then be zero order in monomer. Let us stress that the ideal solution law is invalid in such systems and the monomer's activity should be used instead of concentration.

Furthermore, the rate constant of propagation or initiation is usually calculated by dividing the observed rate of the reaction by the monomer concentration in the bulk. A changeable millieu of the reaction, leaving nevertheless the monomer concentration above its critical value, creates then the impression of propagation rate constants varying with the nature of milieu and induces the investigator to correlate their values with some property of the solvent. This kind of erroneous relations of propagation rate constants with solvent's property was considered in ref. 102.

The geometrical structures of solvation shells surrounding complex, and especially unsymmetrical, ions or ion-pairs are not known. There are un-


doubtedly some preferred locations occupied by solvent molecules which have, on the average, some specific orientations. Such pseudo-rigid structures of solvation shells impose steric conditions on the molecule of a monomer to be incorporated into a polymeric chain and may affect its tacticity.

The structure of the solvation shell may affect the relative probabilities of reactions competing with the propagation. Consider, for example, the addition of a monomer to a polystyryl carbenium ion which possesses the acidic, positively charged 13 proton involved in chain-transfer. In a basic solvent one of the molecules composing the solvation shell may be preferentially located in the vicinity of that proton, being hydrogen bonded to it. Such a structure of solvation shell hinders the chain-transfer to monomer and increases the ratio kplktr•

2.5. Initiation and Propagation of Ionic Polymerization in the Gaseous Phase

Two interesting studies of ionic polymerization in the gaseous phase were reported recently: the anionic oligomerization of methylacrylate initiated by gaseous anions such as CFi, C3Fs, or NCCHi investigated by McDonald,103 and the cationic polymerization of olefins initiated by BF{ cations studied by Bohme.104 In both investigations the initiating ions were selected by a flow-after-glow technique105 and injected into the stream of helium carrier gas containing the monomer vapor at the desired partial pressure. The progress of the reaction was followed then by mass spectrometry. The initiating ions decay by a simple bimolecular process. The plots of In of the intensity of the pertinent elm signal vs. time are linear at constant monomer concentration, their slopes being proportional to the partial pressure of the monomer vapor.

In the anionic oligomerization of methylacrylate initiated by CFi (elm = 69), the other observed elm signals are: 155 (CF3Acr-), 241 (CF3Acrz), and 327 (CF3Acri). In addition a signal elm = 295 appears, the latter due to the loss of CH30H from the CF3Acri ion. Here Acr denotes methylacrylate (mass 86). The results are summarized by Fig. 3.25, and from the decay of elm = 69 the bimolecular rate constant of the reaction

is calculated as 1.5 . lO- to cm3/molec·s (1.1011 M-1S- 1), Le., the addition is diffusion controlled. The fact that a molecule of methanol is lost from the CF3Acri ion proves that this ion, as well as its precursors, are covalently bonded species and not loose aggregates. The rate of this decomposition is calculated as 75 s -1 from the loss of the trimer observed at 16.7


10 20 30 40 50

Figure 3.25. The semilog plots of the decay of the gaseous CFi ions and of the appearance of various negative ions resulting from the reaction of CFi ion with methylacrylate in He flow at 1.1 Torr pressure and at rate of flow of 36 mIh. elm of CFi 69, of CF3Acr- 155, of CF3Acri 241, of CF3Acri 327. The ion of elm =

295 results from the ion of elm = 327 by loss of methanol.

ms after mixing the reagents. This decomposition is too fast to allow the addition of another monomer molecule to the trimer in this diluted vapor. Indeed, no higher ions were observed. Examination of all feasible modes of the trimer's decomposition shows that the methanol is produced by the formation of a cyclic trimer (see p. 129 for the discussion of cyclization in anionic polymerization of methylmethacrylate).

The addition of BFi to ethylene is not simple, it leads to a loss of HF:

Its rate is fast corresponding to a bimolecular rate constant of -7 . 1010

cm3/molec·s. The resulting ion adds consecutively ethylene molecules yielding then the C4H7BF+ , CJ1uBF+ , CSHlSBF+ , and CIOH19BF+ ions. The results are summarized in Fig. 3.26.

Quantum mechanical calculations favor the open chain ion (1) to the cyclic one (2) by 71 kJ/mole and show the addition of BFi to be exothermic by 242 kJ/mole, sufficiently large to allow the elimination of HF. The charge density in the (C2H3BF)+ ion is the highest on the boron atom, and the calculation indicates that the addition of C2H4 to the boron center is en-


5 10 c-~---r--'---.---r-~--~

4 10

2 10 "--..L......L __ ---'-__ --'-__ .L..-__ I....---I. __ --.J

o 2 4 6

C2 H 4 FLOW f (MOLECULES 5-1. 1017 )

Figure 3.26. The semilog plot of the decay of BF{ ions and of the appearance of positive ions resulting from the reaction of BF{ with ethylene in a He + ethylene flow at 0.349 Torr pressure at 21°C. The solid lines are computer calculated for kl = 6.5 . 10- 10, k2 = 1.6· 10- 10, k3 = k4 = 1.3· 10- 10, all in units cm3/molec.s.

ergetically favored to that to the C center.

F, tF B+

c/ ""c

H~' '''H H H (2)

Even more complex are the additions of BFi ions to cis-2-butene or isobutene:

-+ C4Ht + HBF2,

BFi + C4HS -+ ~H4BFi + ~H4'

-+ C2Ht + ~H3BF2

(1)

(2)

(3)


These three channels account for -85% of the products of the initial reaction. Channel (1) results from hydride abstraction, and channel (2), resulting from ethylene elimination, might be pre-emptied by channel (3) if proton transfer takes place prior to the separation of the products. Interestingly, channel (1) is the most favored in the addition to cis-butene, whereas channel (3) is the dominant one in the addition to iso-butene. The simple addition yielding the C4HsBFt ion, although observed, accounts only for a minor mode of reaction. This kind of addition is, however, the dominant mode of the reaction with styrene. The simple kind of additions presumably requires a termolecular reaction with He atoms being the third body.

3. Initiation and Propagation Involving Lithium Alkyls

3.1. Structure of Alkyl Lithiums

The peculiarity of lithium compounds arises from the small size of its cation, its high electronegativity, and the availability of empty, low-energy p-orbitals. Lithium alkyls exist as large aggregates in the solid and liquid state, in solution, and even in highly rarified vapor, mostly as tetramers or hexamers. This high degree of aggregation accounts for their low volatility, e.g., the vapor pressure of n-butyllithium106a is only 10-4 Torr at 60°C. The lithium atoms form the kernels of the aggregates, while the shell surrounding them consists of alkyl moieties. Such a structure endows the aggregates with their lyophilic properties, making them soluble in hydrocarbon solvents.

Colligative properties of solutions of alkyl lithiums prove that the aggregates of ethyl and n-butyllithium are hexameric in hydrocarbon solvents but tetrameric in diethyl ether, 107 whereas a mixture of dimers and tetramers of n-BuLi is formed in THF. The bulkiness of the alkyl group seems to affect the degree of aggregation, e.g., in hydrocarbons n-butyllithium is hexameric, t-butyl lithium tetrameric, and menthyl lithium dimeric. lOS

Dilution of hydrocarbon solutions of alkyl lithiums does not affect their degree of aggregation, implying that each alkyl lithium forms in this medium one kind of aggregate only.

The degree of aggregation is lower for arylmethyllithiums. For example, polystyryllithium is dimeric in hydrocarbons. It seems also that the C-Li bond of this compound is more ionic than that in the alkyl lithiums, although the 13C NMR indicates Sp3 hybridization of the benzylic carbon.1OO

The structure of the monomeric gaseous alkyl lithium molecules is controversial. It is disputable whether their Li-C bonds are ionic or covalent, and the theoreticians dissent on this subject. l1o,m At present the ionic structure is favored. 112 The structure of the aggregates is governed by their


electron deficiency; the number of bonding orbitals exceeds that of the available valence electrons.

The X-ray investigation of crystalline methyl lithium tetramers,113 the only alkyl lithium insoluble in hydrocarbons, shows the four Li atoms placed at the apices of a tight tetrahedron with the Li-Li distance of 2.5 A, shorter than the length of the Li-Li bond of the Li2 molecule (2.67 A). The methyl groups are located above the tetrahedron faces, as shown in Fig. 3.27. The sharpness of the Li-C-Li angle is indicative of the electron-deficient bridging. Each Li atom donates three p-orbitals to the binding of the aggregate and is associated with the carbon atoms of three methyl groups, and each methyl group contributes a single Sp3 orbital and is linked to three Li atoms. The overlap of these atomic orbitals results in four molecular orbitals, each joining four centers. However, they are populated by four valence electrons only. 114 A similar structure is attributed to the tetramers of t-butyllithium.

The tightness of the Li core is manifested by the appearance of Li4Rt ions in the mass spectra of tetramers and the Li6Rt and Li6Rt ions in the mass spectra of hexamers.115 These are the most abundant ions resulting

Figure 3.27. The structure of the crystalline tetramer of methyl lithium.


from the impact of electrons accelerated to 70 eV. Nevertheless, the 6Li-7Li coupling is not seen in the NMR spectrall5 of alkyllithiums enriched by 6Li, suggesting weak Li-Li bonds.

The chemical shifts in the 7Li NMR spectra of alkyllithiums are determined by the "local" environment, i.e., by the nature of the three alkyl groups attached to a lithium atom. For example, four lines appear in the spectrum of an equilibrated mixture of methyl and ethyl lithium in ether solution, namely those due to lithiums linked to three methyls, two methyls and one ethyl, one methyl and two ethyls, and three ethyls, respectively. Such a spectrum is seen at - 80°C, but at higher temperatures the lines are broadened by an intramolecular exchange, and further broadening arises at still higher temperatures from the intermolecular exchange. The intramolecular scrambling is faster in hexamers than in tetramers. 112

The intermolecular exchange results from a dissociation of higher aggregates into lower ones. For example, tetramers seem to dissociate into dimers and their subsequent association yields mixed tetramers. Indeed, analysis of the products arising on mixing (R1Li)4 with (R2Li)4 reveals first the formation of the (RlLih(R2Lih aggregates, whereas species such as RlLi(R2Lih and (RlLihR2Li appear only later. This result confirms the dissociation of the homotetramers into the dimers and the formation of the mixed tetramers either by the association of the dimers or through the reaction:

The latter reaction is important when the concentration of the dimers is low.

The dissociation of the aggregates becomes more facile on their solvation by ethers or amines. Such a solvation yields mixed solvated aggregates of a well-defined stoichiometry. The unsolvated aggregates have vacant "faces" on which the molecules of the solvating agent become accommodated. For example, the six Li atoms constituting the kernel of hexamers form an octahedron. However, only six out of eight of its faces are occupied by the alkyl groups, and two faces remain vacant. Solvation by ether yields the complex (RLi)6Etr2, where Etr denotes a molecule of ether. Its fragmentation is facile and results in the formation of new coordination sites allowing for further solvation.

The dimers seem to be the first products of fragmentation. The dissociation of a tetramer into a monomer and a trimer is unlikely. Nevertheless, in aromatic solvents the monomers are present being in equilibrium with the aggregates. Their concentration seems to be exceedingly low, less than 10-7 M, but unfortunately their exact concentration is still unknown; no


experimental attempt to determine its value has been reported yet. Most probably the monomers are formed by the dissociation of the dimers.

3.2. Initiation of Anionic Polymerization by Alkyl Lithiums

Alkyllithiums are extremely reactive and versatile reagents. They rapidly react with oxygen, carbon dioxide, water, alcohols, ketones, ethers, and so forth, e.g.,

RLi

RLi + O2 --+ ROOLi ---+ 2ROLi,

RLi + CO2 --+ RC(O)OLi,

RLi + H20 --+ RH + LiOH,

RLi + EtOEt --+ RH + ~H4 + EtOLL

Not surprisingly, alkyllithiums readily add to C=C bonds, especially those of styrene, the dienes, and their derivatives, and initiate their anionic polymerization:

RLi + C=C --+ R'C-C'Li (or R'C-C- ,Li+).

Such an addition is slow compared to the rates of reaction of alkyl lithium with O2 , moisture, and CO2 , hence the monomers and solvents have to be purged even from traces of these compounds when alkyllithiums are used as the initiators. On the other hand, the reaction of alkyl lithiums with ethers is slow compared with the initiation, making it possible to carry out the polymerization in ethereal solutions, although the products are contaminated then by lithium alkoxides.

Anionic polymerization of styrene, the dienes, and their derivatives initiated by alkyllithiums in hydrocarbon solvents was first studied by Ziegler116 and subsequently by many other investigators. Two questions arose. What is the nature of the initiating species, and how is their concentration maintained in the reaction? The answer required development of a technique that allows simultaneous monitoring of the progress of initiation and propagation of the polymerization. This was achieved first by Worsfold and Bywater,117 who studied spectroscopically the polymerization of styrene initiated by n-butyl lithium in benzene solution. The initiation produces the lithium salt of polystyryl carbanions that strongly absorb light at Amax = 334 nm. The produced polymers are living; the 334-nm absorbance increases monotonically as the reaction proceeds, and reaches asymptotically a constant value as shown by curve B in Fig. 3.28. The consumption of styrene was monitored by its absorbance at 291 nm that decays continuously


0.8 A

0.6 ~ III C

~ 04 ;;; v

8- 0.2

o 50 100 ISO 200 Time ( mini

Figure 3.28. Typical conversion curves recorded spectrophotometrically for the initiation of styrene polymerization in benzene by n-BuLi in benzene solution. Curve A; the disappearance of styrene monitored at 291 nm. Curve B: the formation of polystyryl anions monitored at 335 nm. The initial conditions: [styrene] =

1.4 . 10-2 M, [n-BuLi] = 1.1 . 10-3 M.

to zero, as shown by curve A in the figure. Hence, the conversion of the monomer into polymer is quantitative.

The initial slope of curve B seen in Fig. 3.28 measures the early rate of initiation R,. The plot of log(R/[styrenelo) vs. log[n-BuLilo, given in Fig. 3.29, is linear for [BuLilo > -10- 4 M, implying that the concentration of the active species at the early stages of the reaction is proportional to

- 3.0 ,...-----------------,

Q;' c ~ >-2., - 3.5 .... S

g' 0 ..J

o _ 4. 0 '------'-_..I.....---l_....L..._.L....--L_...J..........J

- 5.0 -4.0 -3.0 -2.0 -1.0

Figure 3.29. The dependence of the initial rate of initiation of styrene polymerization in benzene on the concentration of n-BuLi. The log-log plot of R/[styrene]o vs. [n-BuLi]o.

152 / lonie Polymerization and Living Polymers

[BuLi]a, a being the slope of the line shown in Fig. 3.29. Since a was found to be about i, and n-BuLi is reported to be hexameric in benzene, it was proposed that the monomeric BuLi, present at equilibrium with the hexamers, is the initiating species. This mechanism implies that the rate of polymerization should be given by the relation

- d[ styrene]/ dt = kp ' (Kd/Ss/6) 1/6. [styrene]· [BuLi]I/6,

where kp denotes the rate constant of addition of the monomeric BuLi to styrene, and Kdlss is the equilibrium constant of dissociation of hexamers into monomers. The experimental results provide then the composite constant kp ' K~~s which might be too large if the rate of reaction of the monomeric BuLi with styrene were comparable or higher than the rate of its conversion into the aggregates. Unfortunately, the individual values of kp and Kdlss are still unknown.

The proposed mechanism of the above polymerization gains support from the results obtained with other alkyl lithium initiators. The polymerizations of styrene or isoprene initiated by sec-BuLi in benzene are by powers of 10 faster than those induced by n-BuLi. Significantly, as shown by Fig. 3.30, their initial rates are proportional to -~ power of the initiator

00

3.5 c:

.~

'" 3.0 .~ .. '0 w

'" ~ -'" - 3.0 c Cl

2.5 0 -'

- I. -3 -2 Log (sec - BuU)

Figure 3.30. The dependence of the initial rate of initiation of polymerization of styrene or isoprene in benzene solution on the concentration of sec-BuLi. The loglog plot of R/[styrene)o or R/[isoprene)o vs. [sec-BuLi). The slope = 1.


100 r---------------,

c o

60

-= 60 <-

'" <1> ct:: "o! 40

20

o 200 400 600 800

Seconds

Figure 3.31. Typical curves of styrene consumption in its polymerization initiated by see-BuLi plotted vs. time. Curve A: the initiation in benzene solution, [seeBuLi]o = 1.1 . 10-3 M, [styrene]o = 5.3 . 10-2 M, T = 30°C. Curve B: the initiation in eyclo-hexane solution, [see-BuLi]o = 1.3 . 10-3 M, [styrene] = 8.7 . 10- 2 M,

T = 40°C. Note the instantaneous (no induction period) initiation in benzene and the induction period in eyclo-hexane.

concentration, a result compatible with the tetrameric aggregation of this reagent.§

Further support of the proposed mechanism comes from the studies of other reactions than polymerization. For example, the formation of 1,1-diphenylhexyllithium by the addition of n-butyllithium to 1,1-diphenylethylene in benzene solution,118 or the metallation of fluorene by n-BuLi119 in this solvent, are both first order in the substrate but again about t order in n-BuLi. Significantly, the addition of the tetrameric sec-BuLi to 1,1-diphenylethylene,120 although first order in this ethylene, is about 1 order in the above lithium alkyl.

The polymerization initiated by n-BuLi proceeds in benzene without induction period. The monomer consumption is the fastest at the onset of the reaction as shown by curve A of Fig. 3.31. This observation is worth stressing since it implies that the initiating agent is available from the very start of the reaction and presumably is replenished faster than consumed by the initiation.

This is not the case in the polymerization proceeding in aliphatic hydrocarbons. The initiation is drastically different in these solvents as shown by the S-shaped curve B of Fig. 3.31 depicting the consumption of styrene in the polymerization initiated by sec-BuLi in cyclo-hexane. The process

§It is difficult to distinguish experimentally between a slope! or i of the In-In plot. Hence, as the authors remarked, one should not be dogmatic in claiming a specific value of a.


starts with an appreciable induction period and accelerates remarkably as th!! reaction proceeds. Obviously, the concentration of the initiating species is exceedingly low in those solvents at the onset of polymerization, and the rate of their formation probably is slower than their rate of consumption by the monomer. However, the rate of formation of the active species undergoes an autocatalytic acceleration after the polymerization ensued. The inflection point of curve B has no simple meaning; it does not indicate the onset of termination, as was claimed by the early workers.

The kinetics of the very early stages of the initiation in aliphatic solvents is difficult to study due to its extreme slowness. Some workers121 claim it to be first order in the initiator. This may be the result of a direct slow bimolecular reaction of the RLi aggregates with the monomer, or it may reflect the very slow rate of dissociation of the aggregates yielding the initiating fragments. The subsequent acceleration results probably from the formation of mixed aggregates of the growing polymers with the initiator. These are either more reactive or dissociate more readily into the reactive polymerization initiating fragments. Thus, the formation of growing polymers accelerates the initiation and the process proposed here accounts for its autocatalytic character.

The complexity of the initiation of polymerization by the alkyllithiums in aliphatic hydrocarbons makes it difficult to establish the order of their relative reactivities. This may be appreciated on examining the plot of the percentage of initiation vs. time given in Fig. 3.32. It is instructive, however, to determine the fraction of the unutilized initiator present in the system at various stages of polymerization. The plots illustrating such a relation are shown in Fig. 3.33. Undoubtedly, n-butyllithium is by far the slowest BuLi initiator. While virtually all sec- or t-BuLi is consumed before 10%, or less, of the monomer is polymerized, a substantial fraction of n-BuLi remains still unutilized and available for the initiation at completion of the polymerization. The ratios of the reactivities of the various alkyllithiums is greater, on the whole, for polymerizations proceeding in aliphatic hydrocarbons than in the aromatic ones.

The remarkably larger reactivity of alkyllithiums in aromatic hydrocarbons than in the aliphatic ones is revealed again by the rates of their intermolecular exchanges. The exchange between t-BuLi and trimethylsilyl lithium is 103 to 104 times faster in toluene than in cyclopentane.122 The lack of appreciation of these large increases in the rates of reactions of alkyllithiums resulting from the replacement of an aliphatic hydrocarbon as the solvent by an aromatic one led to some controversies fully reviewed in ref. 123.

The initiation of styrene polymerization in benzene by t-butyllithium is unconventional. The rate was proportional to the first power of t-BuLi but independent of the concentration of the monomer124 varied from 10-3 to

100

80

c: 0 60

'" -= 1. 0 ;-!

20

o


20 1.0

A

60 Minut es

(sec)

80

B

(no r I C

100 120

Figure 3.32. The percentage of the initiator consumption in initiating the polymerization of isoprene in n-hexane plotted vs. time. Curve A: t-BuLi, [t-BuLi1o =

1.0 . 10-3 M, [isoprene1o = 0.204 M. Curve B: sec-BuLi, [sec-BuLi1o = 1.10 . 10-3 M, [isoprene1o = 0.140 M. Curve C: n-BuLi, [n-BuLi1o = 0.75 . 10-3 M,

[isoprene1o = 0.335 M. Note the crossing of curves A and B.

100

80 ;;.'!

Ol 60 .= c

'" E 1.0 ~

;.:;

" 20 CO

0

Butad i ene at 50°C Isoprene at 30 QC

[M] 1.3 mIl LMJ 1.7 m Il -3

[BuLl] 2.6' 10 mI l

20 1.0 60 80

-3 [ BuLi ] 2.6·10 mIl

20 1.0 60 80 100

Convers ion 'I,

Figure 3.33. The plot of the percentage of the unutilized initiator vs. the percentage of the polymerized monomer. Left: polymerization of 1.7 M of butadiene at 500C, [initiator1o = 2.6 . 10-3 M. Right: polymerization of 1.3 M of isoprene at 30°C. Solvent is cyclo-hexane. The initiators are sec-BuLi, t-BuLi, n-BuLi.


-3

c: E

'"

- - L VI

'" (5

E

- 3 - 2 - 1

Log lO [t -BuLl)

Figure 3.34. The log-log plot of the initial rate of initiation vs. the concentration of t-BuLi used as the initiator in the polymerization of styrene and isoprene in benzene solution. Lower curve: styrene [t-BuLi1o = 4.5 . 10-4 M. Upper curve: isoprene [t-BuLi1o = 1.37 . 10- 3 M.

0.1 M. This strange behavior is illustrated by Fig. 3.34. The authors proposed that a slow dissociation of t-BuLi tetramers into some smaller but active fragments, e.g., dimers or monomers, is the rate-determining step of this addition. However, the analogous initiation of isoprene polymerization is more conventional at least at the lower monomer concentrations (see Fig. 3.34), although the rate seems to be constant again at its higher concentrations. Since the rates of initiation at the same concentration of t-BuLi are at least three times faster for isoprene than for styrene, the fragmentation of the tetramer cannot be the rate-determining step. An alternative explanation was propOSed,125a namely a rapid formation of a monomer-BuLi complex, for instance (t-BuLikM (M stands for the monomer) that undergoes an intramolecular rearrangement shown by the scheme

resulting in the formation of t-Bu-M- ,Li+ adduct. Apparently, the pertinent complexation constant is much larger for styrene than for isoprene. Therefore, while the complexation is virtually quantitative at styrene concentration of 10-3 M, it only approaches saturation at isoprene concentration of about 0.1 M.

The most convenient initiator of the polymerization of styrene or the dienes is sec-BuLi. The dimeric 1,I-diphenyl-n-hexyl lithium is a useful initiator for acrylic monomers, its relatively low reactivity is advantageous since the undesired side reactions that mar their polymerization are then minimized.


The unconventional initiation of the polymerization of propylene sulfide by ethyl lithium proceeds in two steps.7 The desulfuration of the monomer yields propylene and lithium mercaptide, i.e.,

and the latter is the actual initiator of the conventional anionic ring opening polymerization of thiiranes, i.e.,

However, alkyllithiums do not desulfurize the analogous four-membered cyclic sulfides and the initiation of their polymerization proceeds in a conventional way,7 e.g.,

1H3

CH-S I I

CH3

+ EtLi~ I Et·CH·CHz·CHz·SLi.

CHz-CHz

3.3. Interaction of Alkyl Lithiums with Solvating or Complexing Agents

The effect of Lewis bases on the rate of initiation was noticed by the early workers. For example, sec- or t-BuLi adds to ethylene only in the presence of ethers or amines. 1z6 The initiation and propagation of styrene or the diene polymerization induced by n-BuLi is dramatically accelerated by small amounts of THF, as reported first by Dolgoplosk.lz7 The slowly initiating hydrocarbon solution of n-BuLi initiates instantaneously styrene polymerization on addition of small amounts of THF (0.15 M). The acceleration of the initiation results, most probably, from a rapid fragmentation of alkyl lithium aggregates into smaller and highly reactive species when the former are solvated by this ether.

Mixed aggregates of alkyllithiums with lithium alkoxide are also more reactive than the homoaggregates. The addition of alkoxides speeds up the initiation and eliminates the induction period. These effects, first reported by Roovers and Bywater, 128 were systematically investigated by Guyot and Vialle. 129 They found the n-BuOLi is the most active of the alkoxides accelerating the initiation of isoprene polymerization induced by n-BuLi in cyclohexane. However, this agent retards the reaction at later


stages of polymerization. Obviously, these systems are complex and their understanding is still lacking.

The complexation of alkyl lithiums with multiamines, especially with tetramethylethylenediamine (TMEDA), was thoroughly studied. 130 It leads to extensive dissociation of the aggregates (MeLi being the only exception), and the fragments produced are exceptionally reactive. Their IH and 7Li NMR spectra indicate the formation of cyclic five-membered 1:1 complexes. The discussion of the reactivity of other complexes is continued on p.169.

3.4. Bifunctional Lithium Initiators

Much effort has been devoted in recent years to the development of dilithium initiators. Such compounds should allow easy preparation of triblock copolymers having a central polydiene block of high l,4-structure. The high degree of association of lithium alkyls in hydrocarbon solvents creates difficulties by making the dilithium alkyls insoluble in these media. The most attractive approach calls for reacting sec-BuLi with bifunctional monomers which, due to steric restrictions, cannot homopolymerize. The p- or m-di-isopropenyl benzene appears to be the most promising candidate for such synthesis,131 but various technical difficulties have prevented the achievement of desired results. The CH2=CH(Ph)-(CH2)n-CH(Ph)=CH2 precursor and the similar CH2=C(CH3)-C6H4-(CH2)n -C6H4-C(CH3)=CH2 hydrocarbon were prepared,132 as well as many difunctional "l,l-diphenyl ethylenes," e.g., 133,134

or

On the addition of sec- or t-BuLi they yield a fine dispersion of insoluble dilithium compounds. The excess of BuLi is extracted from the precipitate by washing it with the solvent (not reliable). The precipitate is solubilized by the addition of small amounts of butadiene or isoprene and thereafter the desired amount of the monomer is added. Some successful results were claimed.

The electron transfer initiation involving lithium metal dispersions yields difunctional dimers or tetramers (see p. 247). However, this method is not


practical since it requires the addition of small amounts of ethers to allow for the transfer. Unfortunately, the added ether cannot be removed quantitatively from the final product, and its presence affects the tacticity of the formed polydiene preventing the 1 ,4-cis-enchainments (even extremely low amounts of THF prevent the cis-enchainment). The claimed accelerated effect of anisol on the transfer is doubtful (it was not observed in an independent test).

3.5. Propagation of Polystyrene Initiated by Alkyl Lithiums

The early studies of the propagation of lithium polystyryl were confused by the lack of differentiation between the initiation and propagation steps and by the meager knowledge of the association phenomena that play an important role in all organolithium reactions performed in hydrocarbon solvents. To perceive clearly the kinetics of lithium polystyrene propagation proceeding in these solvents, one has to investigate the polymerization after complete depletion of the initiator. Otherwise, the formation of various mixed alkyl lithium-lithium polystyryl aggregates, as well as the initiation, distort the kinetics of the propagation.

The kinetics of this reaction studied under those conditions, when the distortions caused by the initiation were eliminated, was first reported by Bywater and Worsfold. ll7 The propagation was found to be first order in monomer but ! order in the growing polymers. It appears that lithium polystyrene in hydrocarbon solvents exists in the form of inactive dormant dimers in equilibrium with a minute amount of the active unassociated polymers. The alternative explanation invoking free ions and ion-pairs is unacceptable. Ionic dissociation is unlikely in hydrocarbon solvents, and the formation of free Li + cations is especially implausible. The kinetic results conform to the relation

_In(_1_ . d[Sty]) = k (K . 12)112'[LP]1I2 [Sty] dt p d/Ss ,

i.e., the plot of

( 1 d[Sty]) In [Sty]' ----;Jt vs. -In[LP]

is linear with a slope of 1 as shown by Fig. 3.35. Here Sty denotes styrene, LP the living lithium polystyrene, kp the propagation rate constant of the unassociated LP, and K diYs the dissociation constant of the dimeric polymers into the monomeric ones. The experimental data provide the value of kpKdiYs! = 0.93 . 10-2 M- 1I2S- 1 determined for the reaction proceeding in


15 r----- ---------,

' c E

~2. 5 " I;; ~IE ~

0>

.3 3_5

5 4 3 2

- Log (polystyryi ll th ium )

Figure 3.35. The log-log plot of the rate of propagation of polystyrene polymerization after completion of the initiation vs. the concentration of the living polymers. The results are reported by Worsfold and Bywater (solvent benzene) by Spirin et al. (solvent toluene), and by Johnson and Worsfold (solvent eye/o-hexane).

benzene or toluene solutions at ambient temperatures, whereas its value is lower in cyclo-hexane, but only by a factor of 3. 136 The comparable rates of propagation of the reaction proceeding in aromatic or aliphatic hydrocarbons contrast with the dramatically different rates of initiation when an aromatic hydrocarbon solvent is replaced by an aliphatic one.

The value of Kdiss is unknown but it has to be smaller than 10-7 M,

otherwise a deviation from the linearity would be seen in Fig. 3.35 at the concentration of lithium polystyryl of -10-5 M. For Kdiss = 10-7 M the respective kp is 40 M -IS -1 and it would be implausibly higher had the dissociation constant been much lower. Hence Kdiss of lithium polystyryl in benzene cannot be smaller than 10-8 M- 1s-1, and kp has to be equal to or larger than 50 M- 1s-1, i.e., substantially higher than that ofthe other "tight" lithium ion-pairs.

The monomer forms apparently a complex with the growing polymer prior to its insertion into the C-Li bond. The complexation might be quantitative in the range of monomer concentrations used in these experiments, and the insertion would result from a reaction of the complex with another molecule of the monomer. This suggestion might account for the large value of kp- Evidence for a reversible complexation of the monomer with the Li cation, followed by its ultimate insertion into the chain, is provided by Busson and Van Beylen137 who investigated the addition of a variety of disubstituted 1, I-diphenylethylenes to an excess of alkali salts of polystyryl anions dissolved in benzene.

Propagation of lithium poly-o-methoxystyrene in toluene138 proceeds in a fashion similar to that of lithium polystyrene. These living polymers are also aggregated into dormant pairs, but their degree of aggregation is much

Initiation and Propagation of Ionic Polymerization 1161

lower than that of lithium polystyrene. Consequently, the plot of

( 1 d[OSty]) In [oSty]· dt vs. -In[LP]

is slightly curved because the degree of dissociation, f, is relatively high making the approximation

unacceptable. Hence, f has to be computed from the exact equation

2[2/(1 - f) = Kdissl[LP],

invalidating the simple proportionality of f with [LP] 112 . The curvature of the plot allows one to determine the absolute value of

Kdiss from the experimental data using the curve fitting procedure. A rather high value of to- 3 M was computed for the dissociation constant of these dimers, and an extremely low value of 50 M -1min -1 for the propagation constant of their monomeric polymers. The increase in the degree of dissociation is attributed to the intramolecular solvating power of the methoxy group shielding apparently the active carbanion ion. The relative slowness of the propagation results from the deactivating (electron donating) effect of the methoxy group on the monomer.

The existence of dimers of lithium polystyrene in hydrocarbon solvents was confirmed by the viscometric studies of Morton. 139 The viscosity of solutions of living, high molar mass lithium polystyrene decreases by a factor of -to on the addition of a drop of methanol which converts the dimeric polymers associated through the -CH2CH - Ph,Li + end-groups into the "dead," non associated polymers terminated by the inactive -CH2CH2Ph groups. The viscosity of concentrated solutions of high mass polymers increases proportionally with (X power of their molar mass, MWn •

Since (X = 3.4, the to-fold decrease of the viscosity of solutions of living polystyrene on its conversion into the "dead" one implies a decrease of MWn by a factor of 2 (23 .4 = to), confirming, therefore, the dimeric nature of living lithium polystyrene in hydrocarbon solutions.

Significantly, such dimers dissociate in THF solution. The rate of propagation of lithium polystyrene in this solvent is proportional to its concentration, and the viscosity of its THF solution is not affected by the addition of methanol. Solvation of lithium polystyryl pairs by THF disrupts the aggregation.

The nature of the C-Li bond of lithium polystyrene in hydrocarbon solvent is disputed; it is unsettled whether it is covalent or ionic. It should


be stressed that the electronic spectrum of its solution in hydrocarbons or in THF is similar to that of sodium polystyrene in THF, and the latter is undoubtedly ionic. The similarity of these spectra contrasts with the greatly different spectra of, say, the covalent triphenylmethyl chloride and the ionic trityl chloride, Ph3C+ ,Cl- .

3.6. Propagation of Lithium Polydienes

The aggregation of lithium polybutadiene or polyisoprene in hydrocarbon solvents is higher than that of lithium polystyrene. The kinetics of their propagation shows their rates proportional to the ! power of their concentration,140-143 suggesting the tetrameric nature of their aggregates which are in eqUilibrium with a minute proportion of the active monomeric polymers. lI

This proposal was criticized by the workers who initially claimedl44 a ! order kinetics of this propagation with respect to the growing polymers, although later, after correcting their own data, they confirmed the! order. 145 Strangely enough, this group still upholds their claims of the dimeric nature of lithium polydienes in hydrocarbon solutions, quoting the results of their viscometric findings as the evidence in its favor. However, their viscometric findings were shown to be frequently incorrect and self-contradictory as demonstrated in ref. 146 and again in ref. 147 on pp. 124-125.

The coexistence of dormant tetrameric and active monomeric polydienes raises the question of whether the equilibrium between them is maintained by a direct dissociation, tetramer ~ 4 monomer, or whether some intermediates are involved, e.g.,

K, Kd

(Li,LP)4 ~ 2(Li,LPh ~ 4(Li,LP),

where LP denotes a living polydiene. The evidence for the tetramer-dimer equilibria in the lithium polyisoprene systems has been gradually accumulated. The first indications were provided by the UV spectra of low molar mass lithium polyisoprene in hydrocarbon solutions. On their dilution a shoulder appears in their spectrum, as seen in Fig. 3.36, and this suggests the formation of a lower kind of an aggregate. l48 Although a fourfold aggregation of polyisoprene is demonstrated by light scattering of concentrated solutions of lithium polystyrene capped by isoprene, some decrease of the degree of aggregation was noted on their dilution. 149 Significantly, neither change of the spectrum nor of the character of light scattering was observed on dilution of lithium polybutadiene solutions. The

'The experimental data do not distinguish between exponents 1,!, or t However, the kinetic data are compatible with the! exponent, confirmed by other evidence.


0.9 1 - - -----------,

0. 6

w

0.3

o 300 350 nm

Figure 3.36. The ultraviolet spectra of living end-groups of low molecular mass lithium polyisoprene in benzene solution recorded at various concentrations of polymers. Note the isosbestic point implying a stoichiometric relation between the initially and the finally formed species.

association of these polymers into tetramers appears to be much stronger than of lithium polyisoprene. The methyl group of isoprene seems to cause a strain that weakens the aggregation.

Evaluation of the spectral data led to the determination of the equilibrium constant of the dissociation of tetrameric lithium polyisoprene into the dimeric associates. 15o Its value, determined in cyclo-hexane solution, depends on the molar mass of the polymers, it increases from 1.6 . 10-6

M for MW 500 to 35-50.10-6 M for MW < 104• The dissociation is more extensive in benzene than in cyclo-hexane, indicating the importance of solvation of the end-groups by the former solvent.

In view of the above findings, one expects the kinetic order of the propagation to vary with the concentration of lithium polyisoprene. The effect is imperceptible in aliphatic solvents but clearly noticed in benzene as revealed by Fig. 3.37 giving the rate constant of propagation kp as a function of the polymer concentration. The change of the slope from! to ! indicates that the unassociated monomeric polymers are the only propagating species, neither the dimers nor the tetramers contribute significantly to the polymerization.

Some numerical calculations are instructive. Say Kt = 10-2 mM and Kd = 0.5 . 10-5 mM. Then at the total concentration of 16.4 mM of Li polyisoprene in whatever form, the concentration of tetramers is 4 mM, that of the dimers 0.2 mM, and of the nonassociated polymers 0.001 mM,


-1.5

.><0. -2 .0 C\ o

- 2.5

- 6 10

RATE OF PROPAGAT I ON

30 0 BENZENE SOLN

- 5 10

- G 10

-3 10

CONCENTRAT ION

-2 10

Figure 3.37. The log-log plot of the propagation rate constant kp of lithium polyisoprene polymerization in benzene at 30°C vs. the living polymer concentration. Note the change in the slope from 1 to ! as the concentration of the living polymers decreases, indicating that the predominant tetrameric aggregates are replaced by the dimeric ones.

i.e., the tetramers are by far the dominant species in the solution. The concentration of the monomeric polymers is given by the fourth root of the total concentration of the polymers with the overall equilibrium constant KtK~ = 0.25 . 10-12 mM.** However, at the total concentration of 5.7 . 10-3 mM of polyisoprene, the concentrations of the aggregates are 0.4 . 10-3 mM for the tetramers, 2 . 10-3 mM for the dimers, and 0.1 . 10-3 mM for the monomeric species. Now, the dimers are the most abundant species and the concentration of the active, nonassociated polymers is given by the square root of the total polymer concentration with an equilibrium constant of 0.38 . 10-5 mM, insignificantly different from the assumed value of Kd = 0.5 . 10-5 mM.

The apparent activation energy of propagation was determined at 71.9 kJ/mole at high concentration of the polymers when the tetramer-monomer equilibrium governs the propagation process. At low concentration of the polymers, when the dimers dominate, the apparent activation energy is

··The excluded volume affects the rates and equilibria of polymeric reagents making them dependent on molecular weight, although negligibly at low values of molecular weight.


60.2 kl/mole only. From the difference one computes the heat of dissociation of tetramers into dimers, since 71.9 = i . aHt - m + Ep , whereas 60.2 = ! . aHd _ m + Ep, where Ep denotes the activation energy of propagation of the monomeric polymers and the aH's have the obvious meanings of the respective heats of dissociations. Hence, the heat of dissociation of tetramer into two dimers, aHt - d , given by the relation

is equal to 46.8 kl per mole of tetramer. It is reasonable to assume that two monomeric polymers are more strongly

bounded into a dimer than two dimers into a tetramer. On this assumption one finds Ep < 36.8 kJ/mole.

3.7. Stereochemistry of Propagation of Lithium Polydienes

The allylic end-groups of lithium polydienes exist in two stereo-forms: as the cis group,

and the trans group,

/CH2 - •

,<;H ell .,yCH

CH2

These can be differentiated through NMR or UV analysis. Fig. 3.38 provides an example showing the UV spectra of cis and trans isomers of sodium polybutadienyl in THF. 151

The following questions call for answers: 1) What is the eqUilibrium ratio of the cis to trans end-groups in a living lithium polydiene?; 2) What are the rate constants of monomer addition to the cis and trans ends, respectively, and in what proportion are the cis and trans isomers produced by the addition of monomer to the cis and trans end-groups, respectively?; 3) What are the rate constants of the reversible cis-trans isomerization? The sterochemistry of this polymerization is further complicated by the participation of two modes of the monomer addition to an allylic end-


E cis

8000

6000

l.OOO

2000

0 250 300 350 400

A ( n m )

Figure 3.38. The spectra of the cis and trans end-groups of living sodium polyisoprene in THF solution.

group, namely one leading to 1,4 addition and the other causing 1,2 (or 3,4) addition.

The new C-C bond is formed by the monomer addition to the most reactive carbon atom of a monomer, referred to as 1. In this process the stereo-structure of the penultimate unit is frozen and it remains fixed in the resulting polymer. The proportion of the cis and trans units in a high mass polymer could be determined by its analysis. The modern assignments of the segment structures of polydienes is based on 1H and 13C NMR analysis, and the pertinent data reported in the literature were compiled by Yudin.152

Lithium polydienes form aggregates in hydrocarbon solutions and only a minute fraction of the polymers, being in a monomeric form, participate in the propagation. NMR reveals the structure of the aggregated endgroups, but this is also the structure of the active nonaggregated end-groups that determines the mode of monomer addition because the exchange between the nonassociated polymers and the aggregates is much faster than the isomerization of the end-groups. NMR or spectroscopic observations show the trans isomer to be preferred at equilibrium in hydrocarbon solvents,153 although the cis is more abundant during the polymerization. Hence, the monomer addition preferentially forms the cis isomer which subsequently isomerizes into the trans, provided the addition of the next monomer is not too rapid.

The aggregation of lithium polydienyls is disrupted in ethereal solvents and the studies of their solutions provide information about the configu-


ration of the ether solvated end-groups, relatively stable at low temperatures. In the ethereal solvent the situation is reversed. The cis isomer is preferred at the equilibrium and the trans one is preferentially formed during polymerization.154

To determine the cis to trans ratio in the high molecular mass polymer produced in hydrocarbon solvents, the following rate constants are needed:

-cis,Li + monomer ~ -cis'cis,Li kpcco

-cis,Li + monomer ~ -cis·trans,Li kpct>

-trans,Li + monomer ~ -trans·cis,Li kptc ,

-trans,Li + monomer ~ -trans·trans,Li kpu>

-cis,Li ~ -trans,Li kiso'

The isomerization takes place mainly in the aggregates, but, as remarked earlier, the exchange between the monomeric and aggregated polymers is much faster than the isomerization and hence the cis to trans ratio is the same in both kinds of polymers. Denote now by a the fraction of all the growing polymers possessing the cis end-groups in the course of polymerization, and by ao their fraction maintained at the equilibrium. The rate of isomerization of the cis end-groups into the trans ones in the course of polymerization is:

kiso·[LP]·(a - no), where [LP] denotes the concentration of all the polymers in whatever form. On the other hand, the rate of formation of the cis end-groups due to the reaction is:

where (4Kdiss[LP])1I4 is the concentration of the growing monomeric polymers. As the reaction proceeds a stationary state is attained when the rate of formation of the cis end-groups becomes equal to the rate of their disappearance, L e. ,

Examination of the experimental data pertaining to lithium isoprene polymerization in aliphatic hydrocarbons shows154 that kpct and kptt are negligible compared with kptc, Le., the addition of the monomer to any active end-group, whether cis or trans, yields a cis end-group. Moreover, kpcc -


8kpts , and then the preceding equation is reduced to

leading to

The fraction ~ of the trans units incorporated into the polymer is given by

Substituting a for the expression derived above and denoting the ratio kpcc1kptc by R one finds

~ = lI{A + B·[monomer]/[LPJ3/4},

where A = 1 + CXo·(R - 1) and B = kpcc(4Kdiss)1/4IkisO are constants. A plot of the experimentally available 'Y vs. the ratio [monomer]/[LP]3/4

is shown in Fig. 3.39. The solid curve in this figure is calculated from the above equation by using independently determined constants.154 It follows that the highest proportion of cis segments in a polymer is attained by increasing the concentration of monomer and decreasing that of the initiator. Indeed, nearly pure cis-polyisoprene is produced when the poly-

30 r-----------------------------~

Ul 20 c ra ~ ...

10

Figure 3.39. The variation of the microstructure of polyisoprene resulting from changing the ratio of monomer concentration to [living polymers )3/4. The curve is computer calculated; the points are the experimentally determined percentages of the trans units in the investigated polymers.


merization is performed in bulk monomer at a concentration of the initiator less than 10-4 M.

3.8. Effect of Solvating Agents on Propagation Induced by Alkyl Lithiums

The effects of the solvating or coordinating agents on the propagation induced by alkyllithiums are still vigorously investigated. The magnitude of these effects is determined by the concentration of the solvating agent and not by the ratio of [solvating agent]/[growing polymers] as is so often wrongly presumed in many papers.

The solvating agents may interact with the aggregates as well as with the fragments formed on their dissociation. On the whole the solvating agents stabilize the fragments to a greater extent than the fully aggregated polymers, and in such a case their addition favors the fragmentation. Note, however, that the solvating agents affect the reactivity of the growing polymers in two ways: by increasing the proportion of the less aggregated polymers, and by modifying the reactivity of the latter species by their solvation. A simple example helps to understand the complexity of these effects.

Let us consider a system composed of dimeric species at equilibrium with the monomeric ones:

Assume that a solvating agent, denoted by S, interacts with P and not with P2, and let Ks be the equilibrium constant of the solvation:

P + S ~ PS Ks.

Moreover, let kl be the propagation rate constant of the nonassociated polymers P, and k2 that of the solvated polymers PS, whereas the dimeric polymers P2 are assumed to be inactive. The rate of propagation is given by the relation:

R = -d[M]/dt = (k1 + k2Ks·[S))·[P]·[M].

Obviously, the rate increases on the addition of solvating agent when k2 > kb but this need not be so when k2 < k1• The mathematical treatment of this problem was outlined in ref. 155 where it was shown that the initial rate increases on the addition of small amounts of the solvating agent for:


and decreases for

Significantly, the result is independent of the value of Ks. The value of [P] is given by the stoichiometric relation:

where c denotes the total concentration of the polymers in whatever form. Extention of this treatment to a system involving complexation of the monomeric as well as dimeric polymers is outlined in the ref. 155.

An illustration of this treatment is provided by the behavior of propagation of lithium polystyryl in cyclo-hexane on the addition of tetramethylethylenediamine (TMEDA). The rate of propagation increases then for c ::::; 8.3 mM but decreases for c ::::; 0.93 mM156 as shown in Fig. 3.40.

(a)

"111 15

~

S! ><

-I~ 10

)(

~I-"0"0 5

0.5 1.0 1.5 2.0

Figure 3.40. The effect of the ratio r = [TMEDA]/[Li polystyryl] on the rate of polymerization of Li polystyryl in cyelo-hexane at 23°C. (a) At Li polystyryl concentration of 8.3 mM the rate increases on the addition of TMEDA. (b) At concentration of Li polystyryl of 0.92 roM the rate decreases on the addition of TMEDA. In either case the rate approaches asymptotically the value corresponding to that of the fully complexed Li polystyryl. TMEDA = tetramethylethylenediamine.

200 T " :: 160

>--; 120 ~

~ 80

~ c:

.", ,

o


0 .05 0.10 [THF]

0.15

Figure 3.41. The effect of TIIF addition on the apparent bimolecular rate constant of styrene polymerization initiated by LiBu in benzene. [Li polystyryl] = 1.4 . 10-4 M (crosses), [Li polystyryl] = 1.1 . 10- 3 M (open circles). Note the sharp increase in the rate on the addition of very small amounts of TIfF followed by a decrease on further TIfF addition.

The interesting behavior of propagation of lithium polystyryl in benzene on the addition of THF is shown by Fig. 3.41. This effect was attributed to the formation of two THF kinds of complexes, the very reactive monosolvated one and the much less reactive disolvated polymer.

A simple behavior is observed in the system Li polystyryl-TMTCflS7 (tetramethyltetraaza-cyclo-tetradecane). This very powerful coordinating agent is relatively resistant to strong bases, in contrast to crown ethers. It forms a 1:1 complex with Li polystyryl and greatly increases its first-order rate constant of propagation. Thus, the plot -dln[styrene]ldtvs. the ratio [TMTCT]/[Li polystyryl] shown in Fig. 3.42 is linear up to the ratio 1, and thereafter the line sharply becomes parallel to the x axis.

TMTCT

The above result proves that the equilibrium constant of TMTCf complexation with Li polystyrene is very large and virtually all of this com-


0.3

' ",

_1~0. 2

~I - 01 "0"0 .

0.0 0.5 1.0 1.5 2.0

[n-ncr] [SL i]

Figure 3.42. The influence of the ratio [TMTCf]/[Li-polystyryl] on the rate of polymerization at 20°C. TMTCf = tetramethyltetraazacyclotetradecane.

plexation agent is associated with the polymer when its added amount is less than stoichiometric.

Much of the interest in the complexation of living polymers with a variety of agents is generated by the fact that their addition affects the microstructure of the resulting macromolecules. This is especially important in the lithium polydiene systems. For example, small amounts of TMEDA added to hydrocarbon solution of polymerizing lithium polyisoprene modify its structure from mainly 1,4 enchainment to 80% of 1,2 polymer.1ss In this respect the addition of 1,2-dipiperydinoethane (DIPIP) is most effective, it produces virtually 100% of the vinyl units when present in equivalent amount to the lithium initiator. IS9 Combined studies of the rate of polymerization and the microstructure of the polymers strongly suggest that the solvation involves aggregates as well as the dissociated species. l60

4. Initiation of Cationic Polymerization by Protonic Acids: Role of Esters (Including Halides)

4.1. General Observations

Protonic acids appear to be the most direct initiating agents of cationic polymerization. A straightforward transfer of a proton from an acid, say HA, to a molecule of an appropriate monomer, M, may start a polymerization by yielding a protonated HM+ cation, and its A - counterion. The


cation propagates then the reaction by adding further molecules of the monomer, e.g.,

HA + CH2=CXY ~ CH3-C+XY + A - ,

CH3-C+XY + CH2=CXY ~ CH3-CXY-CH2-C+XY, etc.

Protonation of a monomer by a protonic acid is energetically unfavorable for reactions proceeding in the gas phase. For most of the conceivable systems such a process is endothermic, although it might become exothermic for some very strong acids protonating very basic monomers. On the other hand, protonation in the liquid phase might be feasible since the solvation energy of the formed ions compensates for the endothermicity of the gaseous reaction.

The simple scheme displayed above is deceptive. Most of the studied protonations seem to be more complex than shown above and the observation reported by one of US161 as early as 1956 illustrates some of its intricacies.

A polymerization is expected to produce polymers of high molecular mass when a small amount of initiator is introduced into a large excess of monomer, whereas a polymerization resulting from mixing small amounts of monomer with a large excess of initiator is anticipated to yield low molecular mass oligomers. Surprisingly, the slow addition of trifluoroacetic acid ( an initiator) diluted with ethylbenzene to a large excess of well-stirred styrene (a monomer) produced oligomers only, while high molecular mass polymers (MW - 15,000) resulted from the dropwise addition of styrene (the monomer) to well-stirred trifluoroacetic acid (the initiator), in spite of the large excess of the initiator with respect to the monomer.

Two factors account for these seemingly paradoxical results: the high polarity of the medium (trifluoroacetic acid) in which the high molecular mass polymers are formed, and the self-association of the acid molecules yielding the dimeric, or perhaps even trimeric, acid aggregates. The unassociated acid molecules yield on proton transfer the CH3CHPh + ,CF3COi ion-pairs, and eventually polymers endowed with the -CH2CHPh + ,CF3COi endgroups. The lifetime of such pairs is short, their prompt collapse into esters terminates their growth and prevents the formation of high molecular mass polymers. On the other hand, the reaction of the dimeric acid molecules with styrene yields the much more stable CH3CHPh+ ,H(CF3C02)i ionpairs which produce during their substantially longer lifetime polymers of high molecular mass before collapsing into esters.

The equilibrium of protonation shifts to the right when the less nucleophilic homoconjugated counteranions are formed instead of the unassociated ones, implying an amplification of the acid's strength on its dimer-


ization. Indeed, the nucleophilicity of the counterion is an important factor affecting the outcome of polymerization. Anions of high nucleophilicity associate with the carbenium ions into esters as soon as the appropriate pairs are formed. For example, the addition of HCI to isobutene produces only t-butyl chloride; the initially formed cation combines with the highly nucleophilic Cl- anion into a covalent chloride before any monomer molecule has a chance to be added. In contrast, the dimers and trimers of this monomer are formed when sulfuric or phosphoric acid are the protonating agents because the anions derived from these acids are less nucleophilic than the CI- anion.

The participation of aggregated acid molecules in the protonation raises several questions. How large are the aggregates formed in an investigated system? What is the effect of the acid's concentration, the temperature of the solution, the nature of the solvent, and other thermodynamic parameters on their mole fractions? We have to inquire also about the thermodynamics and dynamics of ion-pairs-ester equilibria. How rapid is the collapse of the pairs into the esters? Is this process reversible, and in that case what is the value of the pertinent equilibrium constant and how rapid is the ionization of esters into ion-pairs? The answers to all these questions depend on the nature of the acid and the polymerized monomer, as well as on the properties of the solvent. A variety of systems may be visualized and their studies, hopefully, should allow us to gain a broad understanding of this kind of initiation.

4.2. Self-Associations of Protonic Acids and the Homoconjugation

The association of protonic acids results from hydrogen bonding that links them together. The strength of such a bond depends on the nature of the acid. The weaker it is and the more basic its anion the stronger the bond. On the other hand, the loss of entropy is less in the formation of weaker associates since they are more "floppy."

The dimerization of carboxylic acids into cyclic or linear dimers, i.e.,

O····H-O / "-R'C C'R "- / O-H .... O

or ~

R·C-Q-H .... O=C(R)Q-H,

was investigated extensively. Their structure and properties were reviewed by Eberson. 162 The dimerization of acetic acid attracted early attention. Its dimerization constant is 800 M -1 at ambient temperature in acetic anhydride or in acetone (i.e., 46% is dimerized in a 10-3 M solution) and the equilibrium constant of dimerization is still higher in dioxane, namely


4000 M -I. Such dimers exist mainly in their cyclic form; the percentage of the linear form was determined by Belamy, 163 namely about 20% at ambient temperature. The rate constants of the forward and backward conversions of the cyclic-to-linear form of glacial acetic acid was found to be 106 and 5 . 106 S -I at 30°C by the ultrasonic technique. 164

The dimerization constants decrease with increasing strength of the acid, e.g., they were reported to be 3400 M- 1 for acetic acid in CCl4 at 25°C but only 430 M -I for trichloroacetic acid, the heats of dimerization being - 45 kJ/mole and - 33 kJ/mole,165 respectively. The heats and entropies of dimerization of several carboxylic acids in the gas phase were reported by Clague and Bernsteinl66 for several carboxylic acids, e.g., aHo :::: -43 kJI mole and aso :::: -160 kJ/Ko for acetic acid, and similar values were reported for its homologues. The high values of - aso indicate the rigidity of the cyclic dimers.

The above data reveal a substantial influence of the solvent on the dimerization constant and the heat of dimerization, but the nature of the solvent does not affect the strength of the hydrogen bond. The competition of the solvent and the acid for the acid molecules accounts for the observations, i.e.,

HA + Solvent;;:=: HA,Solvent,

competes with

HA + HA ;;:=: (HAh

Hence, the proportion of the acid available for the dimerization is reduced and the heat of dimerization lowered by the heat of solvation of the monomeric acid.

The dimerization of acids is diffusion controlled. For example, the kinetics of dimerization of benzoic acid in benzene was investigated and the rate constants of the forward and backward reactions were determined at 5-8 . 109 M-IS- I and 7 . 105 to 7 . 1()6 s-t, respectively. These values correspond to a dimerization constant of 103 to 104 M- I at 25°C.

The self-association of strong acids in solvents used in cationic polymerizations was investigated only for one system, trifluoroacetic acid in CH2CI2 , and its dimerization constant was reportedl67 to be 2 . 1 M- I at 35°C. Assuming a low value of 21 kJ/mole for its heat of dimerization one computes its association constant to be -200 M -I at - 80°C and thence 23% of the acid would be dimeric in its 10-3 M solution kept at this temperature.

Our knowledge of the association of anions with their acid, a process referred to as homoconjugation, 168 is extensive. Quantitative data for many


associations of this kind are available. The homoconjugation of the CF3SOi anion with its acid was observed in methylene chloride solution by the 1 Hand 19F NMR technique and its association with two and three molecules of acid has been reported. 170 In fact, the homoconjugation of most of the strong acids in nonaqueous solvents was studied and quantified. These associations are very strong and the pertinent equilibria are displayed far to the right as demonstrated by the sharp end-point of the titration of perchloric acid with perchlorate anions in methylene chloride.106

The homoconjugation competes efficiently with the protonation of many monomers. The anions are by far more basic than most of the monomers; they bind the acids strongly and make them unavailable for protonating the monomer. In other words, homoconjugation decreases the amount of acid available for any other reaction.l7l For example, each molecule of triflic acid that initiates the polymerization of alkene monomers yields a CF3SOi anion that binds, due to homoconjugation, one or two other acid molecules. Consequently, the maximum percentage of triflic acid available for the monomer protonation varies from 33% to 50%, depending on the conditions prevailing in the reaction. 169 Further protonation of the monomer ceases thereafter, in spite of the availability of the monomer and the acid. A similar restriction of the yield was observed in the reaction of strong acids with carbinols,169 the fraction of the utilized acid was again low.

In earlier and, unfortunately, even in some recent papers, it is common to describe the dissociation of an acid as HA ~ H+ + A -. No free protons could be present in the condensed phase. Proton affinities, i.e., the heats of proton addition to substrates, are exceedingly high even for molecules believed to lack any basic character. For instance, the proton affinity of He is - 160 kJ/mole, of H2 - 280 kJ/mole, and it is - 500 kJ/mole for methane. Therefore, the association of protons with the molecules surrounding them is quantitative in any condensed phase at any reasonable temperature.

Proton affinities of strong acids are greater than those of most of the molecules of solvents employed in cationic polymerization. Hence the ionic dissociation of acids in these media is better represented by the schemes:

(1)

or

(2)


than by the commonly written scheme appearing in many books

HA + S (solvent) ~ HS+ + A -. (3)

The latter scheme is applicable to the dissociations taking place in more basic solvents, e.g., alcohols, ethers, and so forth. The equilibrium described by scheme (1) is known as a binary ionic eqUilibrium. While the degree of ionization increases with dilution in the common ionic equilibria (3), it remains constant in binary dissociations. 172 The pertinent equilibrium constants were reported by Plesch172 who utilized the polarographic technique in his work, e.g., their values are 7.2' 10-3 for CF3S03H and 9.8 . 10-2 for FS03H, both determined in CH2Cl2 at 23°C. The square roots of these constants give the fraction of the dissociated acid molecules, i.e., the ratio [H2A +]/[HAh for an acid present in a solution.

The self-solvation of acids does not eliminate the effects exerted by solvents on their strength as gauged by their pKa. Even if a solvent is a poor acceptor of protons it still interacts with the formed ions and affects the ionization equilibria. For example, the pKa of triflic acid is 7.3 in 1,2-dichloroethane, 3.0 in nitromethane, and 2.6 in acetonitrile.

4.3. Mechanism of Protonation of Monomers

The protonation of monomers results from a reversible reaction:

HA + M~ HM+,A-,

and the knowledge of its equilibrium constant and of the relevant forward and backward rate constants is important for the understanding of the ensuing polymerization. The presence of the various kinds of acid aggregates and of their respective homoconjugated anions complicates further the course of protonation. Information about the extent of their contribution to the overall process is therefore desired.

The dimerization or trimerization of acid molecules, as well as the homoconjugation of the A - anions by the acid, seem to be diffusion controlled since no bonds are broken in these steps. On the other hand, the previously mentioned equilibria (1) and (2) are established less rapidly; some bonds are ruptured in the formation of, e.g., H2A + ions.

Still slower is the establishment of the equilibriatt :

M + HA ~ HM+ ,A - ~ HM+ + A - .

ttRecent stopped-flow work of Vairon 198.1988 implies a rapid establishment ofthe monomeracid equilibrium, at least for the styrene-triflic acid system.


Their rates are influenced by the strength of the acid, the basicity of the monomer, and the polarity of the solvent. Obviously, the increase of solvent polarity favors the protonation. The addition of salts sharing common anions with the acid has a dual effect on this process. It depresses the dissociation of the pairs into the free ions by the conventional mass law effect, and it reduces the available acid because the added anions sequestrate it by homoconjugation, making it unavailable for the protonation.

It has been proposed173 that the protonation may proceed in two steps:

Ka k,

M + HA ~ M,HA (a "IT complex?) ~ HM+ ,A -.

The idea of participation of a transient monomer-acid complex in the reaction does not seem helpful. It was invoked in some mechanistic schemes dealing with the protonation of olefins in order to account for the stereochemistry of these reactions174 (see, e.g., the review by Gandini and Cheradame175). The protonation would be still a second-order reaction, first order in the monomer and in the acid with a second-order rate constant given by k,Ka, and provided that Ka is very small, the relaxation time of this equilibrium is short, and k, large.

The linear dimeric acid molecules are expected to be faster protonating agents than those unassociated. The protonation by such dimers might be, say, 50 or more times faster than that by the monomeric acid, and then even 1 % of the dimers in the protonating solution would appreciably affect the rate of the process. Furthermore, since the rate of the dimerization is diffusion controlled, those dimers that reacted and protonated a monomer are replenished by the rapid dimerization, making their proportion constant in the acid-containing system.

The homoconjugated ion-pairs HM+ ,HAi may be produced by the fast direct protonation of the monomer by the dimeric acid present in a relatively low concentration, or could be formed by the slower reaction of the nonaggregated but more abundant HA with the monomer, followed by the diffusion-controlled association of the resulting HM+,A - with HA. The rate of the protonation by the dimeric acid is given by kpd· Kd·[M]·[HA]2, where kpd denotes its protonation rate constant and Kd the equilibrium constant of the acid's dimerization, whereas the rate of formation of the HM+ ,HAi ion-pairs through the sequence of steps,

kpm

M + HA ~ HM+,A-,

HM+,A- + HA~ HM+,HAi is determined by the rate of monomer protonation by the monomeric acid


since the subsequent step inevitably follows, being diffusion controlled. The relative values of k pm and kpdKd' [HA] would determine which of these two processes is faster. The suggestion of some authors176 that the formation of an acid-monomer complex is the rate-determining step is unlikely.

Finally, the reaction of monomer with an H2A + ion or an H2A + ,A -ion-pair may be invoked as a possible step of protonation, but such steps seem to be improbable in view of strong homoconjugation.

4.4. Problem of Covalent-Ionic Equilibria

The interaction of acids with monomers results in two kinds of ion-pairs, the nonhomoconjugated -M+ ,A - and the homoconjugated -M+ ,HAl" pairs, both collapsing eventually into the covalent esters. The nonhomoconjugated pair collapses fast and usually irreversibly* into the conventional ester molecule, whereas the collapse of the homoconjugated pair is slower and yields the acid activated ester, Ml\MA,HA, readily ionizes into a pair, a cation, Ml\M+, and a homoconjugated anion, HAi. The question arises whether a covalently bonded ester, Ml\M-A, could react with monomer and insert it into the M-A bond without being first ionized. The problem is still controversial, and in our opinion the answer should be negative. However, we postpone its discussion to a later section and consider first the coexistence of covalent and ionic forms of molecules.

The simultaneous presence of covalent species and their ion-pairs resulting from a heterofission of a covalent bond was observed by Kessler177

who determined by the dynamic NMR technique the potential energy barrier of this reaction. His study was limited to the derivatives of triphenylmethyl esters in judiciously chosen solvents, a class of compounds having extremely weak C-X bonds.

The equilibria between covalent and ionic entities are more common in the systems involving heterocyclics. Their behavior differs in some respect from that of the common esters. Therefore, these two kinds of systems are discussed separately.

4.4.1. Covalent and Ionic Species in Cationic Ring Opening Polymerization

In the cationic ring opening polymerization an ester bond is cleaved and simultaneously, as the charges become separated, a new bond is formed

'* A process is considered as irreversible when the probability of the reverse reaction occurring during the observation period is negligible.


at the heteroatom, e.g.,

ku

-CH2-0-(CH2hCH2-0S02CF3 ~ ku

This process resembles an SN2 reaction as may be seen by inspecting its modified scheme:

kjj ----

whereas the conventional fission of the ester bond that resembles an SNl reaction would produce a fleeting carbenium ion, which in turn would close the ring by attacking the preceding heteroatom yielding then the same final ion-pair, i.e.,

The above SN2 process could be distinguished from the hypothetical SNl rupture of the C-O bond followed by ring closure by determining the stereostructure of the product resulting from a chiral reagent.

The early evidence for the simultaneous participation of covalent and ionic species in polymerizing systems was reported by Saegusa178 who investigated the polymerization of oxazoline initiated by methyl tosylate or by methyl iodide. The profound disparity in the character of these two reactions implied a basic difference in the nature of the propagating species. It was realized that the tosylate initiates ionic propagation, whereas some covalent species participates in the reaction initiated by methyl iodide. These ideas, developed further by Penczek,179 led to a detailed under-


standing of the roles of ionic and covalent species in the cationic ring opening polymerizations of heterocyclic monomers.

The participation of two distinct kind of active end-groups in the polymerization of tetrahydrofuran initiated by triflic acid, CF3S03H, was revealed by the 1H NMR spectra180 of the polymerizing solution:

macro-esters

macro-ions

The spectrum reported by Matyjaszewski and Penczek180 is shown in Fig. 3.43. The triplets attributed to the macro-ester (upfield) and the other one characterizing the ion-pairs (downfield) are clearly resolved and identified by the studies of model compounds. Thus the simultaneous presence of these two distinct species in the reacting solution was established, and subsequently confirmed by the 19F NMR work of Saegusa181 and the 13C studies of Pruckmayr and WU. 182 Similar findings were reported later for the solutions of other polymerizable heterocyclics, e.g., oxepane,183 oxazoline and its various model compounds,184 and so forth.

The integration of such spectra allows the determination of the equilibrium constants of the respective covalent-ion-pairs equilibria. For example, for the triflic ester of polytetrahydrofuran, K = [ionic]/[covalent] varies with the nature of the solvent, being at 25°C: 0.06 in CCI4, 0.58 in CH2CI2, and 42.0 in nitromethane,185 a remarkable increase with the increasing solvation power of the medium. As expected, the ionic forms are favored by the higher polarity of the solvent.

The above ionizations are exothermic. For example, the exothermicity of the conversion of the linear triflic ester of polytetrahydrofuran into its isomeric oxonium ion-pair amounts to 24 kl/mole when performed in CH2Cl2 solution, and the entropy of the system decreases then due to the high degree of solvation of the resulting ions. The exothermicity is caused by the heat of solvation of the ions although it is reduced by the decreased strength of the 0 + -C bonds compared to the O-c.

The effect of the nucleophilicity of the anion on the ionization equilibrium is revealed by Saegusa's study of polyoxazoline and its model compounds. Inspection of the NMR spectra of the model systems shown below (A = CI, Br, and I) demonstrates that in solutions the chloride is purely

182 !Ionic Polymerization and Living Polymers

(a)

( b )

( c)

5.0 4.9 4.B 4.7 4.6 4.5 6 (ppm )

Figure 3.43. The IH NMR spectra (300 MHz) of growing polytetrahydrofuran initiated by triflic acid. (a) At -18°C in CH2Clz/CCI4 ; (b) in CCI4 ; (c) at + 18°C in CH2CI2• [CF3S03H]o = 0.1 M.

covalent, the bromide forms a mixture of covalent and ionic species, whereas the iodide is entirely ionic§§:

ionic

H3C-7-CH2-CH2-A .

/C~ H3C ~O

covalent

"There is no contradiction between these findings and the previous observation of Saegusa. The methylation reduces the density of the positive charge, and thus the ionic form of the iodide is enhanced.


4.4.2. Kinetics of Interconversion of Macro-Ester and Macro-IonPairs

The rates of interconversion of macro-esters into macro-ion-pairs were determined by applying the temperature jump technique. Only two systems were investigated: the triflic macro-esters of tetrahydrofuran185 and of oxepane. 186 These conversions are slow, and the comparison of their rates is instructive.

The ionization rate constant, k,,, amounts to -10- 2 S-1 for polytetrahydrofuran ester in nitromethane. This process produces a planar, virtually strainless five-membered ring. Hence, not surprisingly, it is 500 times faster than the ionization of the polyoxepane ester in which a puckled, strained seven-membered oxepane ring is formed. The rate constants of the reverse reactions, the conversion of the cyclic ion-pairs into the linear covalent esters, are comparable, namely kit's are 3· 10-3 S-1 and -3· 10-4 S-1 for the tetrahydrofuran and oxepane system, respectively. The strain of the puckled seven-membered polyoxepane ring is released on its conversion into the ester. One might expect, therefore, the conversion of the ionic ring into a linear ester to be faster for the polyoxepane than for the polytetrahydrofuran. However, this is not the case. Apparently, the puckled shape of the polyoxepane ring hampers the approach of the AO- anion to the pertinent carbon atom, making the conversion slower in the polyoxepane system than in the polytetrahydrofuran where the formation of linear transition state is allowed.

The spontaneous intramolecular ionization of the macro-esters results from the attack of the penultimate ethereal oxygen on the carbon atom of the terminal-CH20A group. A similar intermolecular reaction involving the oxygen of a cyclic monomer leads to propagation with the macro-ester being the propagating species, i.e.,

+

In this bimolecular reaction an n-meric ester is converted into an (n+ 1)meric ion-pair. Its reverse results from a nucleophilic attack of the AOion on the exo-methylenic group of the terminal cyclic oxonium ion causing the detachment of a neutral molecule of the cyclic monomer that constituted previously the terminal ionic end-group of a polymer chain. This intramolecular depropagation converts an n + 1-meric ion-pair into an nmeric ester.


The latter bimolecular (intermolecular) ionization of the macro-ester represents a mode of propagation of macro-esters derived from heterocyclic monomers. An alternative, conventional mode of propagation is shown below:

--CHrO-A kpc ~

+ C..H2-0-C~

A I OC~

In this route an n-meric macro-ester is converted into n + I-meric macroester. This is a slow process, since a direct addition (a four-center transition state) is prevented by the orbital symmetry restriction, and the formation of transient ions by a hetero-fission of the o-A bond requires a high activation energy. Indeed, the initiation of polymerization of heterocyclic monomers by the ethyl ester of triflic acid, a reaction resembling the one considered above, is very slow, e.g., its rate constant is -10-4 M-1S- 1 for the addition of this ester to THF in CD3N02•

Although cyclic ethers were used to illustrate the covalent - ion-pair interconversion, the phenomena discussed here are general, and were observed in the reactions of polymers derived from other classes of heterocyclic monomers, e.g., cyclic sulfides, imines, and so forth.

The availability of two routes for the reversible conversion of the covalent macro-esters of heterocyclic monomers into their macro-ion-pair deserves clarification. 187 The intramolecular reactions governed by the rate constants k;; and kit (see the scheme on p. 188) does maintain the equilibrium between the macro-esters and macro-ion-pairs even in a polymerizing system. It does not involve the monomer and therefore is unaffected by its concentration, provided that the rate of propagation is independent of the size of the polymers. This condition is fulfilled for high molecular mass polymers. On the other hand, the behavior of the bimolecular intermolecular conversion is different. This reaction does involve the monomer and the eqUilibrium between the living polymers and their monomer

ke; • [any n-meric macro-ester] . [monomer] ~

kit' . [any (n+l)-meric macro-ion-pair],

is maintained only when the concentration of the monomer equals its eqUilibrium value, i.e., the value at which the rate of propagation is exactly balanced by the rate of depropagation. This follows from the consideration


of the three reversible reactions:

n-meric macro-ester ~ n-meric ion-pair,

n-meric ion-pair + monomer ~ (n + 1 )-meric ion-pair,

n-meric macro-ester + monomer ~ (n + l)-meric ion-pair.

Obviously, these equilibria cannot be maintained in a polymerizing system since then the rate of the forward reaction governed by kei has to be greater than the rate of the backward reaction determined by kit" Let it be stressed that the bimolecular reaction that converts through the monomer addition the n-meric macro-esters into the (n + 1) me ric ion-pairs perturbs the equilibrium maintained by the unimolecular process governed by the rate constants k ii and ktt• The proportion of ion-pairs increases then over and above their equilibrium value.

A stationary state maintaining a constant concentration of all the macroesters and all the macro-ion-pairs may be attained at an approximately constant concentration of the monomer. In such a state the ratio:

[all the macro-ion-pairs]/[all the macro-esters]

is equal to

(kii + kedMD/(k" + ktt·) ,

and it increases with increasing monomer concentration. It is interesting to compare at this point the propagation steps of the

cationic and anionic ring opening polymerizations. The transition states of both reactions are similar, both may be treated as SN2 reactions as exemplified by the schemes depicted below:

+ -CH2-0-~ -CH2-?-~

9H2 ---? bH2 + I cationic propagation,

o-~ +-~ bH2 bH2

and

~CH2:; -CH2:;

O-CH O-CH I 2 I 2

+ I anionic propagation. I

CH2:; ~

CH2:;

6-CH2 -~CH2


In the cationic system the rupture of the weak C-O bond of the growing polymer takes place simultaneously with the formation of a new C-O + bond and with the transfer of charge, whereas in the anionic process the formation of the stronger C-O bond takes place simultaneously with the rupture of the strong C-O bond of the monomer and again with the transfer of charge. These processes are described shortly by the symbolic schemes shown beneath:

+~----o~ o----C-o+ or -O----C-o~ ~----O-.

Since a stronger bond has to be ruptured in the anionic process but a weak one in the cationic, the latter polymerization proceeds more readily than the former.

4.5. Esters Derived from Vinyl Monomers

Let us review briefly the salient features of the protonation of the vinyl monomers. The unassociated molecules of acid yield the HM+ ,A - ionpairs which grow into the -M + ,A - macro-ion-pairs. Eventually, either of them collapses reversibly or irreversibly into the macro-esters. The role of the esters in such a polymerization is controversial. It is debatable whether a vinyl monomer may be inserted into an ester bond. The insertion might appear plausible in some special cases, e.g., when a six-membered transition state could be visualized:

No evidence of such a transition state is available yet. The insertion proceeding through a four-membered transition state is precluded by the orbital symmetry restriction, a point repeatedly stressed. The results of an experiment reported by Bossaer et al.188 has been hailed as the convincing evidence for the ability of the ester to initiate and propagate a polymerization by a direct monomer insertion into the ester bond. However, an alternative mechanism, not requiring the insertion of the monomer into the ester bond, accounts equally well for their observations presented below.

The polymerization of styrene induced at - 8O"C by perchioric acid yielded a low molecular mass polymer (Mn - 15(0) terminated by a perchloric ester group. After completion of the reaction, t-Bu-aziridine was added and the solution allowed to warm up to ambient temperature. A polymerization ensued then again, and the resulting product was shown to be a block copolymer of polystyrene and polyaziridine.188 Such an outcome


does not necessarily prove the capacity of an ester to propagate the polymerization directly; a reversible ionization of the esters into reactive ionpairs could induce the propagation of aziridine, a much more reactive monomer than styrene.

Interestingly, a similar experiment performed with triflic acid as the initiator did not produce a two-block-polymer. 189 The perchlorate ester of styrene is a relatively stable compound. It was synthesized and identified at low temperature by its NMR spectrum, whereas the styryl ester of triflic acid could not be prepared even at - 78°C, although the polymerization of styrene is initiated by this acid.

Polymerization induced by dimeric acids yields more stable homoconjugated ion-pairs, HM+ ,HAi, than the reaction initiated by the nonassociated acid that produces the HM+ ,A - pairs. The former reaction competes efficiently with the protonation by the monomeric acid, and the collapse of the homoconjugated -M+ ,HAi pairs is slower than that of the simple -M+ ,A - pairs. Hence, the lifetime of the homoconjugated pairs is longer than that of the nonconjugated ones, allowing the former to attain a higher molecular mass than the latter. In the case of a relatively fast exchange,

all the ester molecules have an equal chance to grow. The growing ion-pairs dissociate into free ions. Although the latter prop

agate polymerization with a rate comparable to that of the pairs (see p. 186), their lifetime is longer, a point stressed by Matyjaszewski,189 allowing them to produce polymers of still higher molecular mass.

To understand the problems concerned with the molecular mass and molecular mass distribution of polymers, it is advisable to consider here the factors affecting the lifetimes of the species participating in the polymerizations. The collapse of-M+ ,A - pair into its esters may be reversible or irreversible (irreversible implying a negligible chance of return during the time of observation). In a reversible collapse the ester acts as a reservoir, supplying the active propagating ion-pairs and the products of their dissociation. Hence, an ester may be treated then as a dormant species in equilibrium with the active ion-pairs and their free ions.

The equilibrium constants of ionization of esters are low, probably about 10-6 for the common esters at ambient temperature, but they increase with the polarity of the solvent and decreasing temperature. The rate constants of collapse of homoconjugated ion-pairs may vary from 10 to 1()3 S-1 and the rate constants of ionization are then in the range 10-5 to 10-3 s-1.


The ionization rate constants of esters are greatly enhanced on their activation by acids presumably bound to the esters by an hydrogen bond. A "dead" inactive ester becomes active again and capable of adding monomer whenever fresh acid is supplied, but the polymerization ceases on its exhaustion. Even a small amount of the added acid may be sufficient to allow all the ester molecules to participate in the growth. In such a case only a small fraction of them, namely those converted into ion-pairs, propagate at any time. However, since the binding of the acid is reversible and the exchange between the activated and inactive ester molecules relatively rapid, the momentarily dormant esters have also a chance to propagate.

A striking example of "revival" of the activity of an inert ester, illustrated by Fig. 3.44, was reported by Gandini (see ref. 175, p. 88). Significantly, the added acid could be quite weak, although incapable on its own of protonating a monomer, nevertheless it does activate the esters.

An interesting ramification follows from this observation. A strong acid protonates the monomer molecules and is consumed but yields ion-pairs which might collapse irreversibly into esters. The propagation by the ionized macro-ester proceeds then as long as the acid activating them is still available, but it ceases when the acid becomes exhausted by the protonation, even if the monomer is still available. In contrast, a mixture of strong and weak acids may polymerize all the available monomer. The strong acid initiates the reaction by forming the growing ion-pairs. After its exhaustion no further macromolecules are formed, but the propagation does continue due to the activating effect of the weak acid which remains since its weakness prevents it from protonating the monomer.

c: o

-iii

30

Q; 20 > c: o

U o

0' 10

o

o o o

_________ -t:..

CF3 COOH

II sec

Figure 3.44. The time conversion plot for the polymerization of styrene in EtCI at ambient temperature initiated by H2S04 = 8.10-4 M. Effect of the addition of CF3COOH.


To appreciate the complex behavior of the acid-ester system it is helpful to consider a few conceptual examples. Let us first examine the three cases:

+M +M 1. M + HA ~ HMA ~ HMMA ~ HM3A, etc.

active active active

+M 2. M + HA ~ HMA ~ HM+,A- ~ HMi,A-, etc. ~

inactive active active

inactive

+HA +M -HA

3. M + HA ~ HMA ~ HMA,HA ~ HM2A,HA, etc. ~ inactive active active

Here, the rate of polymerization does depend on the concentration of the formed ester. Its presence affects the polymerization, even for an intrinsically inactive ester as shown by examples 2 and 3, since it is in equilibrium with the active ion-pair. On the other hand, in cases 4 and 5,

+M 4. M + HA~ HM+,A-~ HMi,A-, etc. ~ HMnA.

active active inactive ester

+M 5. M + (HAh ~ HM+ ,HAi ~ HMi ,HAi, etc. ~

active active

HMnA +HA, inactive ester

the rate of polymerization is independent of the concentration of the formed ester or macro-ester assumed to be inactive and incapable of being activated.

The problem of covalent-ionic equilibrium of conventional esters is confusing. The dual role of the acid, whether acting as a protonating or activating agent, complicates the issue, and the ambiguity of designation of an acid as "strong" or "weak" adds to the confusion. The demarkation lines are vague and depend on the condition of the system.

Some pertinent experimental findings are now considered. The polymerization of styrene induced by trifiuoroacetic acid in dichloromethane at 50°C was investigated by Sawamoto et al. 190 who monitored its progress by 19F NMR. This allowed them to observe the continuous depletion of the acid and concurrent formation of the macro-esters. Since no other signals appeared in the spectrum, it was concluded that the concentration of the CF3COi anions, and hence also of the growing polymers, is very low. The inertness of the ester in the absence of acids was demonstrated by independent experiments.

The monomer consumption was found to be first order in the monomer and the acid, a conclusion verified by the mathematical treatment of the


kinetic data. The progress of the reaction was accounted for by a mechanism postulating a bimolecular initiation (kdHA]'[M)) , a bimolecular propagation (kp·[growing polymers]'[M]), and a spontaneous unimolecular termination, an irreversible collapse of the active species into an inactive ester (kt·[growing polymers)). The concentration of the growing polymers (a mixture of the ion-pairs and the free ions) was assumed to be given by the stationary state approximation, i.e.,

[growing polymers] = kdHA]' [M]/ kt.

The excellent agreement of the observations with the conclusions derived from this mechanism suggests the constancy of the postulated rate constant of propagation, kp, given by the expression:

where 'Y denotes the fraction of the free cations and k; and kj the respective propagation constants. The concentration of the growing polymers decreases as the polymerization progresses causing an increase of 'Y. Provided that the values of k; and kj remain constant, one concludes, in concordance with the other previously discussed evidence (see p. 136), that k + "'" k± p p'

The molecular mass distribution of the resulting polymers was found to be bimodal and affected by the polarity of the solvent, the high molecular mass fraction gaining as the solvent polarity increases. Moreover, this fraction was eliminated on the addition of CF3CO·OAg, a salt sharing a common anion with the growing polymers. One infers, therefore, that two slowly exchanging species participate in the propagation, the longer polymers being formed by the free cations. In view of the approximate equality of k; "'" kj , the observed distribution of molecular mass demands a longer lifetime of the free cations compared with the ion-pairs. This is not surprising. The cations have to recombine with their counterions, or undergo exchange with the pairs, before collapsing into the inactive ester; therefore they last longer than the respective ion-pairs. The very low concentration of the free cations and their ion-pairs makes the exchange between them sufficiently slow and its influence on the molecular mass distribution is therefore negligible.

In another paper Higashimura and his associates191a,191b reported a considerable narrowing of the molecular mass distribution of the produced pOly-t-butoxystyrene caused by the addition of quaternary ammonium salts sharing a common anion with the growing ion-pairs. Significantly, such an addition led only to a slight change in the rate of propagation, revealing


again the approximate equality of the propagation rate constants of the free cations and their pairs while their lifetimes are different.

The findings reported by Sawamoto et aJ.190 conform to the fourth of the previously listed mechanistic schemes: the formation of the protonated monomers that propagate only as long as the acid is available. Thereafter the polymerization ceases and the remaining macro-ion-pairs collapse irreversibly into the macro-esters. ~~

The proposed mechanism accounts then for the kinetics of the polymerization and for the observed molecular mass distribution.

The role of esters in the cationic polymerization of styrene initiated by trifluoroacetic acid was studied also by Matyjaszewski.193 Their presence seems to retard somewhat the reaction (sequestration of the acid by the ester?). However, the addition of nondeuterated acid to a mixture of nondeuterated ester with only a tenfold excess of deuterated styrene produced oligomers, most of which (>90%) had the CH3CHPh-CD2CDPh- endgroups, and not the CHD2CDPh-CD2CDPh- groups. The latter endgroups were expected to be formed on initiation of polymerization of the deuterated monomer by the nondeuterated acid. Apparently, the ester, presumably activated by the acid, is a faster initiator of polymerization than the acid alone. This surprising observation could be rationalized by considering the following two reactions:

and

Since the second reaction yields a more stable homoconjugated anion, while both produce the same initiating cation, CH3CH(Ph) +, the latter might be faster and competes effectively with the monomer for the added acid. Indeed, the bonding of an acid to the acid residue of an ester, via a hydrogen bond, should be very fast.

The evidence for a reversible ionization of an activated ester into a flat carbenium ion is provided by the experiments performed with a chiral ester. On the addition of acid the ester racemizes faster than it initiates polymerization. Obviously, not every event resulting in a reversible ionization of the ester is followed by the initiation of polymerization.

The results reported by Matyjaszewski conform to the third of the listed mechanisms. The inert ester becomes activated by the acid and then propagates the polymerization. The reaction ceases when the acid is consumed.

"This study was repeated by another research teaml92 who reported puzzling and unexplained results conflicting with those described in ref. 190.


The apparent discrepancies between the results reported by Matyjaszewski and those claimed by Higashimura and Sawamoto are accounted for by the difference in the experimental conditions maintained in the respective studies. The large excess of the monomer in the experiments of the latter group made it much more efficient in the competition with the ester for the acid, whereas the monomer to ester ratio was only 10 in Matyjaszewski's study. The evidence for the feasibility of the reverse activation of the ester by the acid shown by the latter work seems to be convincing and undeniable (see also p. 188 and Fig. 3.44), and indeed the activating effect of the acid on the ester reactivity was confirmed by Higashimura and Sawamoto (private communication).

The reaction of the stronger triflic acid with the more basic a-methylstyrene monomer performed at low temperature in CH2Cl2 solution yields quantitatively the respective protonated monomer associated with the homoconjugated (CF3SOi, 2CF3S03H) counterion.194 Both these entities were identified through their 1H and 19F NMR spectra. These results demonstrate that the protonated species derived from more basic monomers and coupled with less nucleophilic anions are stable under these conditions. They do not collapse into an ester, i.e., the ionization equilibrium lies far to the right. Indeed, no lines characterizing the ester could be detected in the NMR spectrum which revealed, significantly, the presence of stable dimeric and even trimeric cations. This proves that ionic propagation does take place under such conditions.

Many attempts to prepare the triflic ester of styrene failed. 189 The ester is not formed even if the preparation was performed at -70°C, showing its lability and the ease of its decomposition even at these very low temperatures. Unfortunately the products of the decomposition are unknown. Is it possible that the primary ion-pairs isomerize into a tropylium derivative?

The attempt to synthesize the triflic ester of styrene by reacting I-phenyl ethanol with triflic anhydride is instructive.189 The reaction was expected to proceed according to the scheme:

but instead the ether, (CH3CHPh)zO, was produced. Apparently, the initially produced ion-pair, (CH3CHPh+ ,CF3SOi) or (CH3CHPh+, CF3SOiCF3S03H), reacts with I-phenyl ethanol faster than the ethanol with the triflic anhydride. The reaction proceeds apparently via a planar carbenium ion since the ether formed from a chiral ethanol, say R, was composed of a nearly equimolar mixture of RR and SR enantiomorphs. 193

The nature of the activated esters is unknown, although much speculation on that subject appeared in the recent literature. Its complexation with the


acid affects the ester bond as revealed by the NMR spectra. Nevertheless, it is questionable whether one may invoke an equlibrium between the ester having a "normal" C-A bond and another in which this bond is "stretched," as repeatedly proposed by some workers. The complexed ester is a stoichiometrically different species from the uncomplexed ester. Moreover, one cannot visualize a hump in the potential energy curve plotted as a function of the C-A distance that would separate such two implausible species. In view of the plethora of the proposed structures let us add one more, undoubtedly highly speculative.

The simplistic explanation of an enhanced reactivity of the activated esters assumes a shift to the right of the ester-ion-pair equilibrium resulting in acceleration of its ionization. This would account for the faster initiation and propagation of polymerization induced by them, but it would imply a fleeting formation of carbenium ions having their usual absorption spectrum. Moreover, the ratio of the rate of transfer to the rate of propagation should remain the same as in the conventional polymerization propagated by the pertinent ion-pairs. It seems, however, that the rate of transfer is impeded by the activation. We suggest therefore the following hypothetical structures for the activated esters, e.g.,

H H / "'" / "'"

-HC 0-CI03 -HC 0-CI03

I I ~ I I , XYC H XYC+ H " ,/"

0-CI03

H / "'"

-HC Cl I I

XYC BC12 '" ,,," Cl

\........ /'/

-0-CI03

H / "'"

-HC Cl I I

XYC+ BC12

"::'Cl'/

Such structures could account for the increased reactivity and for the retarded chain transfer.

In conclusion, the results of the studies discussed here illustrate the different aspects of protonation of monomers by acids and show how the


character of the reaction varies with the nature of the reagents and the conditions maintained in the process.

4.6. Stopped-Flow Experiments

The exploration of the early stages of the protonation was achieved through application of the stopped-flow technique.195-198 Although the results reported by the different investigators diverge in some respects, there are common features that persist and these are considered now.

In the stopped-flow experiments, solutions of a monomer and of an initiator are rapidly mixed in a few milliseconds. Thereafter, the optical spectrum of the reacting mixture, or its conductance, is recorded as a function of time. The formation of species absorbing in the near UV, not observed in the previous studies, is the most significant finding revealed by all the reported stopped-flow experiments. Their spectra are independent of the nature of the initiator, e.g., the normalized spectra recorded on mixing p-methoxystyrene with 12, CH3S03H, BF30(CzH5b or SnCl4 are all superimposable on each other .196 On the other hand, the nature of the monomer does affect the spectrum, e.g., Anuu: = 340 nm for the styrene system,195,197,198 whereas it appears at 380 nm for the p-methoxystyrene.196

The polystyryl cation is expected to absorb at Anuu: = 340 nm. *** It is plausible therefore to attribute the appearance cif the 34O-nm transient in the experiments performed with styrene to the formation of that cation, whether free or coupled with some anion. This conclusion is corroborated by the observation of a transient conductance which has been in phase with the optical absorbance. 195 Hence the free cation, or its precursor remaining with it in a rapid equilibrium, seems to be the first product of these reactions.

The rapid rise of the transient absorbance followed by its decay (the maximum of the absorbance is reached within a few seconds) is the other significant observation reported by all the investigators of these reactions. This is depicted by Figs. 3.45 and 3.46 showing the time evolution of the absorbance at 340 nm for the systems: styrene-perchloric acid195 and styrene- triflic acid.198 Obviously, the fast reaction of acid with the monomer yields an ionic transient which rapidly decays into some apparently nonionic species, since it neither absorbs in the near UV nor conducts electric current. At its maximum the concentration of the ionic transient amounts to a few percent only of the initiating acid.

Remarkably, as shown by Fig. 3.45, the maximum of the transient absorbtion wanes on increasing the monomer concentration and appears then at shorter time. This result seems to be caused by the faster acid con-

** *The A""", of this cation formed by 'V-irradiation of styrene in glassy butyl chloride appears at 350 nm. l99


80

"'0 - -- - ------- --------------g >-

20 - ' - '-' - --' - ' -

0

8 .. 0.0"' ~

'" 0

"' ... 0 -.jo .,

0.02 -C

0 o 2

"' 6 8 10

time/s

Figure 3.45. Stopped-flow polymerization of styrene by HCl04 at - 80°C. Lower: the time dependence of the transient concentration. Upper: the percent of styrene conversion. For [styrenelo = 0.1 M. The initial concentration of the acid decreases (up to down) from 6.9 mM, 6.1 mM, to 4.5 mM.

sumption due to its increasing initial concentration. Acid is needed for the formation of the transient, and the faster the acid disappears the less time is available for the production of the transient. This argument is supported by a computer simulation (unpublished results of one of us).

The strong absorbance of styrene at 291.5 nm, shown by the spectra seen in Fig. 3.47, allows monitoring of its concentration. Similarly, the concentration of p-methoxystyrene may be monitored by its absorbance at 295 nm. The decay of the monomer absorbance determined in this way is depicted in Fig. 3.48. Unfortunately, no method is available yet to monitor the consumption of the acid, but independent experiments200 revealed that the acid is consumed within a few seconds after mixing the reagents.

The initial slope of the curve giving the absorbance of the transient as a function of time at some standard conditions provides a measure of the relative rate constants of the initiation of polymerization by the acid. The results obtained for p-methoxystyrene at 30°C ShOW196 the rate of initiation by triflic acid or acetyl perchlorate being - 106 times faster than the rate of initiation by CH3S03H or SnCl4 (+ moisture ?) or BF30(CzHsh.

The dependence of the initial rate of initiation, R" on the acid concentration was examined for the styrene-triflic acid system. 198 The results


St/TfOH ,85 343um ,- 112 C

,711

[TfOH]o = 2 .05 mY

[St ]0 = 9.65 mY ,55

T= - 62· C ,311

,16

-,Oil 0 ,II 1,:1 2 2,5

1:";5

3<l3um

~ ,7118

'0; 1::1 ,50 " 'C

-.; (J

;l Q.,

0 ,III

-,Oil 0 2,00 4,111 11,24 11,32 10,4

~

S4Sum

Ipn+ at 340nml ,788

,50

- ,Oil 0'---2J..0- -1.40----'1I0:----::8':-0---'100

Figure 3.46. Stopped-flow experiments. The dependence of styrene concentration on time plotted at various time scales. Initiation by triflic acid. Temperature - 62°C.

were puzzling. The plots of In(RJ[MDo vs. In[ Acid]o were perfectly linear hut their slopes varied from 4.5 at - 62°C to 3.3 at - 23°C. It is tempting to justify these results by postulating the participation of the aggregates of the acid in the initiation, their degree of association rising with decreasing temperature. However, the existence of the reactive tetrameric acid aggregates is unlikely. The results may reflect some technical problems. For example, R j is proportional to the concentration of the active species present at the onset of the reaction, and the latter is proportional to the


.95 ""-------------,

.75 1.05 8

.15

280 304 328 352 376 400 >.(nm)

Figure 3.47. Transient spectra observed in flow experiments. Polymerization of styrene initiated by triflic acid at - 65°C. At longer wavelength the absorbance of the transient cation, at shorter wavelength the absorbance due to the disappearance of styrene.

[Acid]a'Td where Td denotes the "dead" time, the time needed to mix the reagents. The Td' in turn, depends on the viscosity of the reacting solution. The viscosity of the solvent, CH2Cl2, is 0.393 cp at 30°C increasing to 0.449 cp at 15°C. Unfortunately, the viscosity of triflic acid is unknown, but the viscosity of acetic acid is reported to be 1.04 at 30°C and 1.31 at 15°C. It is not only much larger than the viscosity of the solvent but it increases more steeply with the falling temperature than the latter. The increase of

1.0 2.0

.8 1.6

.6 1.2

.4 .8

.2 .4

0 0 0 2 4- 6 8 10

Time (8)

Figure 3.48. The appearance and decay of the transient cation (P+, 340 nm) and the consumption of styrene as a function of time.


'Td with increasing acid concentration might lead to the reported results. The above problem is discussed at length to impress upon the reader the importance of some factors, not mentioned by the experimenter, which could affect the results (see also p. 89).

Indeed, in his latest paper198 Vairon proposed two other factors as possible causes of the strange dependence of Rj on the acid concentration, namely an uncontrolled adsorption of the acid on the glass walls, or the brisk dilution of the acid solution by the monomer at the onset of the experiment resulting in a momentary nonequilibrium excess concentration of the higher aggregates of the acid.

The rate of initiation measured by the rate of growth of the peak of the transient was found, by two independent groups of investigators,195,198 to be strictly proportional to the square of the initial acid concentration, whether perchloric or triflic, while keeping the other conditions constant. This finding implies that the dimeric acid is the actual initiator of the polymerization, a proposal made previously by Kuzera.201 No dimerization of perchloric or triflic acids was observed directly. Nonetheless, small amounts of such dimers are expected to coexist in equilibrium with the monomeric acid. Their reactivity apparently exceeds that of the monomeric acid, probably by a large factor, and the diffusion-controlled dimerization replenishes those that reacted reversibly with the monomer. The resulting homoconjugated ion-pairs seem to collapse eventually into the colorless ester.

The collapse of ion-pairs into esters could be questioned in view of the findings of Matyjaszewski and Sigwalt.189 Their inability to synthesize the styryl ester of triflic acid, (see p. 192), even at temperatures as low as -70°C, may be interpreted as evidence for the far left position of the eqUilibrium

implying an extremely rapid ionization of the ester, and an exceedingly slow, as well as thermodynamically unfavorable, collapse of its ion-pair into the ester. This behavior seems, however, to be unique for the styryl ester of triflic acid, not revealed by the other styryl esters, even not by the styryl ester of perchloric acid. Moreover, it is debatable whether the equilibrium for such esters as

is displaced so far to the left. Indeed, the protonation of styrene by triflic acid does take place, and the subsequent propagation yields polystyrene terminated by the ethylenic end-groups, as demonstrated by Vairon. 198


The process yielding the polystyrene terminated by the -CH=CHPh group is a chain-transfer-to-monomer. The eliminated triflic acid may reinitiate the polymerization of styrene provided this monomer is still available. Therefore, the transient could be still observed by the most sensitive device of Vairon, even after total consumption of the monomer and the decay process may be long (see Fig. 3.16).

The final decay of the transient appears to result from two simultaneous reactions, a faster one proceeding with a rate proportional with the monomer concentration, and a slower bimolecular one, first order in the transient and the monomer. 198 The author attributes the unimolecular reaction to the fast elimination of the acid resulting in the formation of polystyrene terminated by the """CH CHPh group, whereas the faster process was assumed to reflect the bimolecular reaction:

yielding an indanyl derivative. The most detailed quantitative data pertaining to the styrene-triflic acid

system were reported by Vairon198 who developed a highly sophisticated stopped-flow technique (see ref. 198a). Fig. 3.49 illustrates some of his findings and the numbers at the arrows give the apparent propagation rate constants computed by the relation

for the early times of the reaction. The striking decrease of kp,apt calls for comments. The computation of the respective DPt shows them to be low, about 4 or 5. Hence, the corresponding values of kp,aPt are affected by the fast rate of the initiation and the rate of the addition of the first (or perhaps even the second) monomer molecule, both of which might be substantially larger than the rate of the monomer addition to longer growing polymers. The substantial contribution of the initiation and the addition of the first styrene molecule to HM* might account then for the high values of kp,apt

computed at the onset of the polymerization. The linear dependence of 1/[M] on time at the early stages of the reaction (see Fig. 3.49) supports this explanation.

It has been frequently argued that some "invisible" species contribute to the propagation. To appreciate the reasons for this suggestion it is necessary to review the earlier work of Pepper, 202,203 accomplished by the conventional batch technique. His findings are summarized in Fig. 3.50. Mixing perchloric acid with styrene leads to a rapid and quantitative con-


1.3

ci 0

1.0.

.78

.26

o

~ .6 1'1 o

':Jj~

.9 -.02

- .64

- 1.26

-1.88

- 2 .5

Ipn+= t(t) I

2 3 " t (8) 5

,---------------------------,7

~ 0-

5.6 S

•. 2

2.8

1..

L---~ ____ _L ____ ~ ____ ~ __ ~ O

o " t (8) 5

Figure 3.49. The computer-calculated rate constant of styrene decay at various times. The same results plotted as the first-order or second-order reaction. Note the linear portions of the experimental curves.

sumption of the acid exhausted in few seconds.2°O The ensuing polymerization of the monomer continues and proceeds at O°C as a first-order reaction, suggesting a constant concentration of the growing polymer species, i.e., the absence of their termination. Undoubtedly, most of the polymers, if not all, are terminated by ester groups. Hence either the esters are the propagating species or they are in equilibrium with a minute amount of the homoconjugated ion-pairs which propagate at this temperature at a measurable albeit very slow rate. The latter seems to be the case.

This reaction is too slow to be observed at - 97°C. However, the first products of the protonation of the monomer are the homoconjugated ion-


°1 .. o=o,.u(a) (c) y

~~ 0,8

.a 1,5 ftc r 0,17 0,7

1,0

VlO' 6,1 0,6

0,5 0 ,5 0,4

3,6

0,2

50100150 50100 150 20 40 0

80 min min min

Figure 3.50. The result obtained by batch technique for the polymerization of styrene investigated at different temperatures: (a) at O°C, (b) at -60°C, (c) at -97°C. Plots rate vs. time. Note the different qualitative behavior of the system at these different temperature ranges.

pairs, lasting only for few seconds, but propagating rapidly even at this low temperature. Hence, a small fraction of the monomer (this fraction increases with the increasing initial concentration of the acid) polymerizes virtually instantaneously during the fleeting lifetime (few seconds) of the homoconjugated pairs which are converted into ordinary pairs as the acid is consumed, and finally into the ester. The reverse reaction, the ionization, is too slow at this temperature and thus the reaction stops. The lifetime of the homoconjugated ion-pairs is much shorter at O°C and therefore their contribution to the polymerization is then negligible.

At an intermediate temperature ( - 60°C) both processes contribute to the reaction (see Fig. 3.50). The equilibrium concentration of the nonhomoconjugated and homoconjugated pairs and their rate of propagation are sufficiently high at this temperature to allow for a slow polymerization proceeding after the burst of polymer formation resulting from the shortlasting reaction due to the short-lived excess of homoconjugated pairs. Let it be stressed that the polymerization initiated at - 97°C resumes when the temperature of the cold quiescent solution is raised to - 60°C.

Most of the above findings are consistent with those observed in the stopped-flow experiments and therefore it is unnecessary to postulate the existence of some "invisible" species. However, the termination of the polymerization appears to proceed differently in Vairon's and Pepper's experiments. Since triflic acid was the protonated agent in the former study, whereas perchloric acid was used in Pepper's work, it is advisable to repeat the stopped-flow work of Vairon with the latter acid as the protonated reagent.


The molecular weight distribution of the resulting polymers is bimodal, an observation first reported by Higashimura204 who investigated the polymerization of styrene initiated by acetyl perchlorate. The bimodal distribution could be accounted for by the different lifetimes of free ions and ion-pairs, a point to be discussed later.

4.7. Living Cationic Polymerization

Gandini and Plesch205 were the first to treat a polymerization propagated by esters or halides as an insertion of a vinyl monomer into the ester bond, and coined the term pseudocationic polymerization for such a process. Their idea seemed to gain some support when the discovery of living cationic polymerization was reported by Miyamoto et al.206a in 1984, and 2 years later by Kennedy and his associates.207 Indeed, the polymerizations investigated by these workers are initiated by esters, incuding halides, activated by some suitable activating agents. For the sake of illustration let us consider the first polymerization of vinyl ethers classified as "living cationic polymerization. "

Higashimura and his colleagues206b,206c described the polymerization of vinyl ethers initiated by an equimolar mixture of HI and 12 which readily proceeds in nonpolar solvents at low temperatures «O°C), yielding quantitatively polymers of narrow molecular mass distribution. Their molecular mass increases proportionally with the degree of monomer conversion. In fact, the DP n at time t is given by ([M1o - [M1t)/[HI1o, where [M1o and [M1t denote the initial concentration of the monomer and its concentration at time t, whereas [HI1o is the initial concentration of HI. Significantly, after quantitative conversion of the monomer into polymer the polymerization may be resumed by the addition of fresh monomer, and the molecular mass of the resulting polymer increases then still further as depicted in Fig. 3.51. These features of the reaction demonstrate that the formed macromolecules retain their capacity of growth during the whole course of the polymerization, as well as in the time period elapsing between the consecutive monomer additions. Furthermore, the proportionality of the molecular mass of the formed polymers with the degree of monomer conversion proves a virtual lack of chain-transfer in this reaction. Hence, the reported process was rightly described as a living cationic polymerization.

The polymerization process induced by the HI-I2 mixture proceeds in two steps. The rapid addition of HI to the vinyl ether (the monomer) takes place in the first step and yields an iodide CH3'CH(OR)-I, a compound inert by itself and incapable of initiating a polymerization. On the addition -of iodine a 1:1 complex is formed between the iodide and iodine which propagates the polymerization with no induction period. It seems that the C-I bond has to be "activated" to allow for the addition of the monomer


L

Monome r 3 Addi I i on

0

0 0 .."

'2 2 " c

I~

0

0 Cony,"!.

Figure 3.51. Polymerization of isobutyl-vinyl ether initiated by HI-I2 mixture at low temperature. Note the proportionality of the degree of polymerization with conversion. The addition of the second batch of monomer initiates further polymerization with the same rate and the molecular mass of the polymers increases accordingly.

through its insertion into the C-I bond,

CH3'CH(OR)-1,12 + CH2=CH(OR) ~

CH3'CH(OR)'CH2'CH(OR)-1,12, and so on,

and at constant [HI]o the higher the concentration of iodine (but [12]0/[HI]0 < 1), the faster the polymerization. Indeed, the rate of polymerization, -d[M]/dt, has been reported208,209 to be proportional with [12]0' and not with [HI]o, for [I2]0/[HI]0 < l,ttt

The first step of this process may be circumvented by preparing a solution of the iodide prior to polymerization. Its CH2Cl2 solution is stable at least for a month when kept in dark at temperatures below - 40°C. 210

The number of polymer molecules produced in the above process is uniquely determined by the initial concentration of HI,z06b i.e., by the amount of produced iodide, CH3CH(OR)-1. This implies that every formed molecule of the iodide yields one polymer chain endowed with a -CH2CH(OR)-1 end group, even when the ratio [12]0/[HI]0 is less than 1, i.e., when not every macromolecule possessing the iodide end-group is activated by iodine. Nevertheless, the molecular mass distribution of the

tttThe polymerization is more complex when [12]0 exceeds [HI]o. The free iodine, not complexed with the iodide, reacts then with the monomer, and yields the diiodide which decomposes into CH2=C(OR)I and HI.


resulting polymers is still narrow. This is only possible if the exchange

"""CH2'CH(OR)-I,I2 + """CH2CH(OR)-I ~

"""CH2'CH(OR)-I + """CH2'CH(OR)-I,I2

does take place and its rate is much faster than the rate of propagation. The nature of the proposed bond activation will be considered later (see

p. 208), but the activation seems indispensable for the start of this polymerization. In fact, a toluene solution of the monomer and HI does not polymerize until the iodine is added. Only then does the reaction ensue.

Subsequent studies showed that iodine is not a unique activator of this reaction, ZnI2 or ZnCl2 are even better activators. 191 Their action results in faster polymerization than that induced by iodine, as shown by Fig. 3.52, and still yields polymers of narrow molecular mass distribution even at temperatures above the ambient one. A minor difference between the ZnX2/HI and IzfHI systems may be mentioned. ZnX2 at concentration 50 times lower than that of 12 is equally effective for the polymerization as the latter activator. On its own, ZnX2 is ineffective and does not initiate polymerization in the absence of HI.

The polymerization system developed by Kennedy207 resembles the one reported by Higashimura and Sawamoto.206 A tertiary ester or ether, a compound independently synthesized and inherently inert, induces the polymerization of isobutene on the addition of an excess of BCI3 • After cessation of the ensuing rapid reaction the polymerization was repeatedly reinitiated by the addition of fresh monomer. The molecular mass of the resulting polymers increased steadily as the conversion of the monomer

100~---.----.-----,--,

60

o ~--~----~--~~~ o 60 120 160

Time, min

Figure 3.52. The conversion curves in polymerization of isobutyl-vinyl ether initiated by the indicated initiators. [M]o = 0.38 M, [HI]I) = [10] = 5.0 mM, [ZnI2]o = [HI]o = 0.2 mM. Note the large differences in the rate of conversion.


into polymer progressed. The resulting polymers had again a fairly narrow molecular mass distribution.

Both systems have common features. The potentially polymerizable mixtures remain inert until activated by a suitable reagent and only then does the polymerization ensue. The inert -CH2CH(OR)-1 become activated on the addition of 12 or zink halide, whereas the inert tertiary isobutyl acetate, ether, or chloride end-groups require for their activation an excess of BC13 or TiC14 • Continuation of these studies led to the development of similar living cationic polymerization of a variety of other vinyl monomers, e.g., styrene and its derivatives.

Why does the polymerization described here exhibit the features of living polymerization? What makes it different from the common cationic polymerization?

A polymerization acquires the features of living polymerization when the growing macromolecules retain their capacity of growth for a relatively long time, sayan hour or more (see p. 12 for the definition of living polymers), and when the propagation yields polymers of the desired maximum molecular mass with virtual exclusion of termination and chaintransfer.

A direct insertion of a monomer into an ester bond does not seem likely. The quantum mechanical restrictions forbid the formation of a four-membered cyclic transition state needed for such a process. Instead, the ionization of the ester bond yields an ion-pair and the resulting cation reacts with the monomer and perpetuates the propagation.

The conventional lifetime of a cationic ethylenic ion-pair (a growing polymer) in meticulously purified systems is usually not longer than a few seconds. The ensuing growth is terminated when the active end-group becomes associated with its counterions yielding an ester, an event referred to as a collapse of an ion-pair:

P+ ,A - ~ ester.

Alternatively, the active end-group is destroyed by the loss of a j3-proton either in an encounter with a molecule of the monomer (a direct chaintransfer, the most likely mode of termination of the free cations), or through an intramolecular decomposition of an ion-pair into a dead polymer and an HA acid (A - is the counterion). The acid reacts then with the monomer and initiates another chain process (an indirect chain-transfer).

In principle, the collapse of an ion-pair into the respective ester is always reversible,

kc

P+ ,A - ~ ester, k.


However, this process is treated as irreversible when the reciprocal of the ester ionization rate constant, k;, is longer than the time of completion of polymerization. Speeding up the ionization of the ester bond makes an irreversible collapse a reversible process, and then the ester acts as a dormant species and not as a product of irreversible termination.

The reversibility of collapse is achieved by a proper choice of the counterion, which could be further modified by some added complexing agents, that participates again and again in the propagation. Thus, the magnitude of the ionization equilibrium constant, Kf , and the rate constant of ionization, k;, could be adjusted to fit our needs by proper choice of the activating agent.

Some revealing conclusions may be deduced by examining the numeric value of Kf . Let the dissociation constant, Kdiss , of the pertinent ion-pairs be 4 . 10-5 M, and the concentration of the growing and dormant polymers (Le., of the initiator) be c = 5 . 10-3 M. These are typical values for the systems under our consideration. The concentration of the ion-pairs is given then by Kf . c (for Kf « 1) and that of the free growing cations by (Kdiss . Kf ' C)ll2. It follows that:

for Kf = 10-6, [ion-pairs] = 5 . 10-9 M and

[free cations] = 4.5 . 10-7 M, whereas

for Kf = 10-2, [ion-pairs] = 5 . 10-5 M and

[free cations] = 4.5 . 10-5 M.

The above results are significant. For Kf = 10-6 only one kind of growing polymer contributes to the propagation, namely the free cation. The ionpairs represent then only 1 % of the growing macromolecules and their contribution to propagation is therefore negligible, especially since the propagation rate constant of ion-pairs is somewhat smaller than that of the free cations. The molecular mass distribution of the resulting polymers is then unimodal.

On the other hand, the concentrations of ion-pairs and free cations are comparable for Kf = 10-2 , both are -5 . 10-5 M, and both contribute then significantly to the propagation. The average molecular mass of polymers formed by the free cations is larger than that produced by the ionpairs, because the growth period of the free ions is longer than that of ionpairs (a conclusion deduced by Matyjaszewski189) while their propagation rate constants seem to be comparable (see p. 136). Therefore, for Kf = 10-2 the molecular mass distribution would be bimodal, hence much broader than that obtained for Kf = 10-6 •


We realize now that a mere change of the magnitude of the ionization constant, Kf , may drastically affect the molecular mass distribution of the resulting polymer. The magnitude of Kf affects also the shelftime of the polymers125a as shown by the following arguments.

Let kt be the termination rate constant of the growing polymers due, e.g., to chain-transfer, and denote by T the time interval needed for termination of 50% of all the polymers, whether growing or dormant. The time interval T == 0.69/kt • Kf , because at any time only the growing polymers are terminated, the ester is inert. Since the spontaneous ionization replenishes those ion-pairs terminated previously, therefore any polymer is terminated eventually. For kt == 10 S-1 and Kf ~ 10-2, the period T

amounts to 6.9 s only, but it increases to 19 h for Kf == 10-6• By the same token, the rate of polymerization sharply decreases with decreasing Kf .

From the data reported in the literature the values of the initial rates of polymerization, R, could be calculated. For example, for isobutyl vinyl ether polymerized at -15°C in a reaction initiated by the respective iodide activated by ZnCI2, the rate of polymerization, R, calculated from the experimental data, is about 10-2 M S-1 for the monomer concentration c of -0.4 mM. This rate is given also by {kp • c . Kf + k~ (KdiYs . C •

Kf )112} • Mo, where kp and k~ denote the propagation rate constants of the ion-pairs and the free cations, c and Mo are the initial concentrations of the initiator and the monomer, and Kf and KdiYs have their previous meaning. The KdiYs could be determined experimentally by a conductance study, the propagation rate constants of the ion-pairs and the free cations are probably of the order of 106 M- 1s-1, and therefore the ionization equilibrium constant, Kf , of the ester could be calculated from the relation

Experimental R == {kp • c . Kf + k~ . (KdiYS • C • Kf )ll2} . Mo.

The ionization equilibrium constant depends on the bond dissociation energy of the properly activated ester bond, ionization potential of the ester residue, electron affinity of the activated moiety split from the ester, and the solvation energies of the resulting ions. Its value could be somewhat adjusted by the proper design of the system.

The reversibility of the ion-pair collapse is essential for prolonging the shelftime of growing polymers, a condition, although not the only one, required for the establishment of living polymerization.125a The value of Kf does matter, as has been shown above. However, as the shelftime of the growing polymer increases with decreasing value of Kf , the rate of polymerization decreases sharply. These two opposing outcomes call for a compromise, as thoughtfully stated by Kennedy and Marechal (ref. 214, pp. 234-239). Hence, the choice of the ester and of the activating com-


plexing agent is dictated by the desire to find the best optimum for Kf and for the rate constant of ionization k j of the studied system.

The chain-transfer is the other obstacle hampering the achievement of the conditions needed for living polymerization. Its rate constant, k,n depends on the strength of the HA bond, on the basicity of the A - anion, and on the density of the positive charge on the ~ proton of the ion-pair. For example, the polymerization of vinylcarbazole initiated by HCI is not living; the elimination of HCI, converting a growing polymer into a dead one, is too fast. However, a living polymer is formed on initiation of the reaction by HI, since the HI bond is weaker by -100 kl/mole than the HCI bond, making the elimination of HI much slower than the elimination of HCI.

Let us compare now the polymerizations of vinylcarbazole and vinyl ether, both initiated by HI. The nucleophilicity of vinylcarbazole moiety is greater than that of the respective moiety of vinyl ether, therefore Kf is greater for the former than for the latter. This makes the rate of polymerization of vinylcarbazole reasonably fast, whereas the rate is too slow to be conveniently followed for the vinyl ether. By the same token, the higher nucleophilicity of vinylcarbazole reduces the density of the positive charge on the ~ proton, decreases therefore the k,n and imparts a living character on its polymerization.

The apparently very low value of Kf of the polyvinyl ethers possessing the iodide moiety as its end-group makes its propagation imperceptibly slow. The complexation of the iodine atom with, sayan iodine molecule, decreases its nucleophilicity, increases Kf , and results in an acceptable rate of propagation, still maintaining a reasonable stability of the system in respect to chain-transfer.

Does the complexation of the anion affect ktr? Apparently it does. The reduced basicity of the anion, Ii, slows down the elimination of HI and prolongs the stability of the growing polymer. Analogous problems affect the polymerization of isobutene propagated by the t-C-Cl end-group. The complexation of the CI by BCl3 decreases the basicity of the anion, increases the Kf of the complexed t-chloride, and reduces k,n thus making this polymerization living.

The reverse problem is faced in the polymerization of, for example, vinyl ethers initiated by AlEt2Cl. Its nucleophilicity is too high, the propagation and chain-transfer too fast, and the stability of the growing polymer too low, resulting in the formation of polymers of broad molecular mass distribution due to the chain-transfer and some other factors to be discussed later. Let it be stressed that a fast polymerization results in a broader molecular mass distribution than a slow one performed under otherwise the same conditions.


The fast polymerization could be retarded by the addition of dioxane191

or other Lewis base, and then produces polymers of uniform size. Most probably, this action of Lewis bases results from the conversion of the highly reactive carbenium ions into the sluggish onium ions. The equilibrium, such as

seems to be established rapidly, making the concentration of the carbenium ions very low. Since the onium ions do not initiate the propagation of vinyl monomers, the rate of propagation is greatly reduced, and the molecular mass distribution of the resulting polymers becomes sharper because as 1 % of the monomer polymerizes more exchanges take place.

A similar observation was reported by Webster and his colleagues215 who initiated the polymerization of vinyl ethers with triflic acid. Due to the very low nucleophilicity of triflate ions, the polymerization is fast, yielding polymers of broad molecular weight distribution. The reaction is retarded and the distribution markedly narrowed on the addition of dialkyl sulfides that converts the carbenium into sulfonium ions.

It is not known whether the complexation of a Lewis base with the carbenium ion affects the ratio of the rate constants of the transfer to the rate constant of propagation of the residual carbenium ions, but a speculative discussion of this subject might be of some interest.

The molecules of a medium surrounding an ion or ion-pair occupy, on the average, the energetically most favorable locations. A molecule of a Lewis base present in the solvation shell of a carbenium ion becomes covalently bounded to the carbon atom and then forms the onium ion, as depicted in the previously drawn scheme. Due to the thermal motion, the bounded molecule of the Lewis base jumps occasionally to another location where it resides for a while before returning to its previous location. During this fleeting time the reactive carbenium ion is exposed and could propagate. The second best location of the displaced base is in the vicinity of the ~-H-atom to which it could be attached by a hydrogen bond, especially since this atom bears a relatively large fraction of the positive charge. Hence, while the carbenium ion propagates, the ~-H-atom is partially blocked by the base that hinders the transfer. In this scenario the introduction of a Lewis base affects the ktr of the carbenium, reduces its value, and increases the kplktr ratio, a desired modification of the system.


4.8. Uniformity of Polymers

Much attention has been paid recently to the uniformity of polymers produced in cationic living polymerization. Their lack of uniformity does not necessarily imply the "nonlivingness" of their polymerization. There are several factors that affect the molecular mass distribution of living polymers. Let us consider, to begin with, the system ester-ion-pairs. The propagation arises from the reaction of ion-pairs; the polymers bearing the ester end-groups are dormant. The uniformity of polymers requires a rapid exchange between the esters and ion-pairs. A numerical example should be helpful.

Let us retain the symbolism of the preceding section and assume a value of 10-6 for Kfand 106 M- 1S- 1 for kp' For the chosen value of Kf , the rate constant of the collapse of ion-pairs into esters, ke, varies probably from 1()4 to 106 s -1. Hence, the ionization rate constant, k j , varies then from 0.01 to 1 S-l, and the lifetime of ion-pairs, 'T, from 10-4 to 10-6 s. For 1 M concentration of the monomer each ion-pair adds on the average 100 molecules of the monomer before it collapses into the ester, provided that the lifetime of ion-pairs, 'T = 10-4 s (note, the number of monomer molecules added to a growing ion-pair before its collapse is given by kp . 'T . [MD. However, for'T = 10-6 s only one monomer molecule is added, on the average, to an ion-pair before its collapse. Stating it differently, it takes 100 s, on the average, to allow all the ester molecules to be once ionized when'T = 10-4 s, and for the [Ester] = 5 mM and [monomer] = 1 M a large fraction of the latter (39% ) would be polymerized in this period. The molecular mass distribution of the polymers formed under such conditions would obviously be broad.

On the other hand, for 'T = 10-6 s, i.e., for k j = 1 S-l, all the ester molecules would have chance to be once ionized in 1 s, and only 5% of the available monomer would be polymerized in that time. The inolecular mass distribution of the polymers formed under such conditions should be narrow. Stating the above results still differently: in the time required for 90% of the monomer to be polymerized (500 s) each ester molecule would participate, on the average, 5 times only in the propagation when k j = 0.01 s-t, but 500 times when kj = 1 S-l.

Although the values of the constants were chosen arbitrarily they are realistic and acceptable. The purpose of the above numerical calculation is to show how a problem concerning us presently could be treated, and to what extent the frequency of the conversion of ester into ion-pair and vice versa depends on the numerical values of these constants, which in principle could be determined.

It is possible to increase the frequency of the conversion discussed above by adding a Lewis base to a polymerized system. As pointed out by Pen-


czek,218a the conversion proceeds then in two steps: the conversion of the ester into onium ion-pair, and the dissociation of the latter into a carbenium pair. The activation energy of a direct conversion of an ester into a carbenium pair is relatively high, whereas that of the intermediate steps is expected to be lower. Hence, the ionization of an ester is accelerated by the base acting as a catalyst in an SN2 kind of a process.

Numerous other factors broaden the molecular mass distribution of living cationically growing polymers. The effect of slow initiation is well known and appreciated. There are other purely physical causes. For example, the planar structure and the high polarizability of the carbazole moiety enhances the attraction between the vinylcarbazole units, promoting precipitation of its polymers. The precipitation affects the living, HI-initiated polymerization of this monomer in an undesired way. The aggregation of the polymer units becomes very tight, e.g., in toluene at low temperatures. The growing ends are buried then in the precipitated particles and those located deeply grow sluggishly, being starved of monomer due to its slow diffusion into the particles, whereas the others close to the surface grow fast. Subsequently, the molecular mass distribution of the resulting polymers becomes broad.216

The precipitation is avoided in methylene chloride solutions, a better solvent than toluene, but its higher polarity favors the formation of high proportion of free ions. Their lifetime is longer than that of the ion-pairs, since a free ion has to be converted into a pair before collapsing into an inert ester, and therefore, as remarked earlier, the molecular mass distribution becomes broad.

The complex behavior of vinylcarbazole polymerization is discussed here in order to illuminate the effects of incidental factors depriving a basically genuine living propagation of its capacity to produce polymers of uniform size. The disturbing factors have to be recognized and the means of reducing their effect on the distribution of the molecular mass has to be explored.

Let us summarize now the conclusions of the preceding discussion. For living polymerization to yield polymers of narrow molecular mass distribution the following conditions have to be met:

1. The initiation has to be faster than, or at least equally fast as, the propagation.

2. It is beneficial if each active (growing) polymer adds, on the average, not more than one monomer molecule before it collapses into its dormant state.

3. In a time period allowing each dormant polymer to be, on the average, once converted into its active form, the fraction of the polymerized monomer should be small.


4. The exchange between the polymers bearing different end-groups (e.g., the esters, ion-pairs, free cations) should be fast compared to propagation.

5. The polymerizing mass should be homogeneous and any gradient of concentration or temperature should be very small.

6. The polymerization should be reasonably slow, since the gradients of concentration or temperature are readily produced in a fast, exothermic reaction.

4.9. Some Comments on the Kinetics of Polymerization of Vinyl Ethers Initiated by the HI-I2 System

The kinetics of the polymerization of vinyl ethers initiated by HI-12 was reported by several research groups.208,209,212 All the investigators agree that the rate of the monomer polymerization is first order in HI and 12, provided that [12JO/[HIJo :s;;; 1. Surprisingly, for the n-Bu and Et vinyl ethers in hexane208 and 2-chloroethyl vinyl ether in toluene209 the rate of the reaction was reported to be independent of the monomer concentration (conversion linear with time). Two mechanisms were postulated to account for this result,208 both assume the formation of a monomer-12 complex which consumes nearly all of the iodine present in the system. One mechanism postulates that the propagation arises from a reaction of the monomer-iodine complex with the nonactivated polyvinyl ether iodide, regenerates the 12 which rapidly reacts with the monomer, and produces another monomer-12 complex. The other proposed mechanism assumes that the propagation results from the reaction of the noncomplexed monomer with the iodine activated -CH2CH(OR)I,12 (the activated iodide). However, since the monomer and the iodide compete for the iodine, the former complexing most of it, the concentration of the activated growing polymers is inversely proportional to the concentration of the monomer. As a result, the rate of polymerization becomes independent of the monomer concentration.

Either mechanism accounts for the independence of the rate of polymerization on the monomer concentration; nevertheless they are not acceptable. It is well known that iodine reacts with vinyl ether yielding diiodide which decomposes by elimination of HI. The postulated complex is assumed to behave differently, a debatable assumption. The stronger evidence contradicting both mechanisms is their limitation requesting the concentration of iodine not to exceed the concentration of HI. Neither of the two mechanisms demands such a limitation which is natural for Higashimura's mechanism postulating the activation of -CH2CH(OR)-1 by


iodine. In fact, the authors postulating the iodine-monomer complex report a puzzling behavior of the system when [I2]/[HI] > 1.

The independence of the rate of polymerization on the monomer concentration seems to indicate a complex mechanism of the propagation resembling the Fontana mechanism.94 The growing polymer -CH2CH(OR)I,I2 seems to form rapidly and quantitatively a complex with the monomer which slowly, in the rate-determining step, inserts the monomer into the C-I bond, thus converting an n-mer into an n + I-mer. This suggestion gains support from the recently reported study of the lifetime of the polyvinyl ether iodide.

An end-capping technique allows the determination of the concentration of the growing polymers.218 It demonstrates that their concentration remains constant during the polymerization, i.e., as long as the monomer is still available. However, after its exhaustion a slow decay of the active groups ensues, its rate is reported to be first order in the growing polymers. It seems that the monomer "protects" the active polymers and prevents their decay. This suggests a rapid and quantitative formation of a monomer-n-meric living polymer complex, which is slowly decomposed by inserting the monomer into the C-I bond, yielding thus an uncomplexed n + I-mer. The latter reacts again with the monomer and forms the n + I-meric complex. Such a mechanism accounts for the reported kinetics with no mentioned earlier restrictions.

Finally, it should be stressed that the recent studies of Higashimura and Sawamot0218b revealed that the order of the living polymerization with respect to monomer depends on its nature and the nature of solvent. The polymerization of ethyl and n-butyl vinyl ether is zero order with respect to monomer in hexane, but first order in toluene or CH2CI2, whereas the polymerization of 2-chloroethyl vinyl ether is zero order in toluene, but first order in CH2CI2. No explanation was proposed for the reported variations of the order.

4.10. Some Experimental Results

The initial studies of Higashimura and Sawamoto dealt with polyvinyl ether possessing the C-I bond activated by 12, Extension of this work demonstrated that the polyvinyl ethers terminated by OR bonds behave similarly when activated by ZnCI2. Fig. 3.53 presents a plot of Mn vs. % conversion for polymers of t-butylvinyl ether initiated by a variety of oxy acids and activated by ZnCh.

Examination of the sharpness of molecular mass distribution of polyvinyl ethers initiated by various derivatives of acetic acid shows an interesting correlation with the strength of the initiating acid (Fig. 3.54).


8

0 : HI

6 • : HOS0 2CH)

• D. : HO P ( O)( OPHI

" 2

.... . : HOC ( OICF) '0 Ca(ed

" c: 1:;[

2

o 50 100 Co nv ers i on ' f.

Figure 3.53. Plot of Mn vs. conversion for the polymerization of iso-Bu-vinyl ether initiated in toluene by a variety of protonic acids in conjunction with ZnCI2 •

0

G

M I S1 4 ~

c: I~

2

1.4 c:

I ~ 1.3

I~ 1.2

1.1

- R

0, c.: - CCI3

(),~ :- CHC '2

e , A :-cH2c 1

1.0 '--_--'-_ _ -1.. __ -'-_---'

o 50 Convor9lon, %

-cel,

-c r.,

1.31

1.20

]t1.11

1.07

1.00

10' 5 <103 MW (PSt ) I I

I I 25EV(ml)

Figure 3.54. The relation between the sharpness of molecular mass distribution and the strength of the initiating acid.

Initiation and Propagation of Ionic Polymerization /215

S. Initiation of Polymerization by Lewis Bases and Acids: Problem of Zwitterions

5.1. Lewis Bases

The initiation of anionic polymerization by uncharged nucleophiles has been observed in several systems. The first example of such a process was reported by Horner and his colleagues,219 who initiated the polymerization of nitroethylene and of acrylonitrile with trialkyl phosphine. They proposed the formation of zwitterions as the first step of the reaction:

The propagation, assumed to ensue from the negative end of the resulting dipole, was expected to yield the macro-zwitterions.

The same mechanism was postulated by Katchalsky220 who studied the kinetics of the polymerization of nitroethylene initiated by pyridine, and followed its progress by a calorimetric technique. The reaction performed in methyl-ethyl ketone is slow at 20°C and yields only oligomers of OP n

- to-IS. High molecular mass polymers are formed in tetrahydrofuran or dimethylformamide at low temperatures, e.g., polymers of OP n - 900 were obtained when the reaction proceeded at -104°C. Unfortunately, it was not checked whether or not any pyridine moiety was incorporated into the polymer.

The formation of macro-zwitterions raises some questions. The attraction of the oppositely charged end-groups of each polymer should favor the formation of macro-rings which, however, were not observed. Alternatively, one could anticipate the formation of very long linear chains arising from coupling of the initially produced zwitterions or macro-zwitterions. Such a product was not observed either, and hence it became debatable whether zwitterions are formed at all in these reactions.

The proposed zwitterionic mechanism calls for the presence of the initiator in the resulting polymers. With this in mind, Jaacks and his coworkers221 reinvestigated the polymerization of acrylonitrile induced by triphenylphosphine. The absence of the phosphine in the precipitated and meticulously purified polymers, as well as the recovery of 99% of the initiator in the form of a phosphonium salt, Ph3P+(·CH2CH2CN)-, invalidated the zwitterionic mechanism, contrary to the claims of some investigators. 222 An alternative mechanism, postulated by Jaacks, assumed


that the initiation proceeds through three steps:

PPh3 + CH2:CHCN ~ Ph3P+-CH2CHCN,

Ph3P+-CH2CHCN + CH2:CHCN ~

Ph3P+-CH2CH2CN + CH2 :CCN,

CH2 :CCN + CH2 :CHCN ~ CH2 :C(CN)CH2CHCN.

An intermolecular proton transfer, proposed as the second step of the initiation, requires this reaction to be faster than the addition of the monomer to the primary zwitterion:

Ph3P+CH2CH(CN) + CH2=CHCN

~ Ph3P+CH2CH(CN) . CH2CH(CN).

Apparently, the carbanion of that zwitterion, partially neutralized by the neighboring phosphonium cation, is a stronger base than a nucleophile. On the other hand, the vinyl carbanion, CH2=CCN, and the macrocarbanion, -CH2CH(CN)-, are stronger nucleophiles allowing for faster monomer addition than proton transfer.

The above mechanism is applicable also to the polymerization of nitroethylene; apparently macro-zwitterions are not formed in this reaction either. The latter polymerization involves probably the polynitroethylene carbanions coupled to the pyridinium counterions, and proceeds as a conventional anionic polymerization.

The initiation of the polymerization of acrylonitrile by triethylphosphite, P(OEth, was claimed223 to produce polymers endowed with phosphite endgroups. Since this polymerization was carried out in the presence of an enormous excess of the phosphite (in some runs the phosphite/monomer ratio was 1:1) the inadequate purification of the polymer could lead to its contamination by the initiator. The claimed formation of a charge transfer complex seems to be irrelevant to the observed polymerization. Indeed, many reports claiming the initiation of polymerization by charge-transfer complexes appear to be unsubstantiated. For example, the interaction of acrylonitrile with triphenylphosphine results in the formation of a chargetransfer complex with a spectrum closely similar to that of the acrylonitriletriethylphosphine complex but polymerization is not induced in this system.224

The mechanism of initiation proposed by laacks requires the availability of a-protons in the polymerizing monomer. No a-protons are available in methylene-malonic ester, hence the formation of zwitterions seems unavoidable in its polymerization induced, say, by triphenylphosphine. Such a polymerization was studied by laacks and Franzmann.225 The re-


action was terminated by the addition of K2S20 7 and the resulting oligomers of DP - 35 were isolated and rigorously purified. Their spectrum, having Amax at 268.6 nm, was identical with that of a model salt, Ph3P+CH2C(COORh,Bc, confirming therefore the formation of the phosphonium end-groups in the produced oligomers. Their proportion was high, equivalent to -60% of the total amount of the introduced initiator, while the residual unutilized phosphine and the primary zwitterions were detected in the low molecular mass residue.

The feasibility of zwitterionic polymerization is therefore demonstrated, provided that no a-protons are present in the polymerizing monomer. The conversion of a primary monomeric zwitterion into a dimeric one is slow, slower than the subsequent propagation. This is expected; the electrostatic work needed for separation of the oppositely charged ions is the largest when they are close together. After their initial separation further displacement is relatively easy. Hence the polymerization induced by the dimers does take place readily even in low polarity solvents and yields oligomers as well as high molecular mass polymers.

Polymerization of j3-propiolactone initiated by t-amines provides another example of a reaction propagated by macro-zwitterions. The first step yields a betaine226

that propagates the zwitterionic polymerization by a nucleophilic attack on the oncoming monomer. The presence of the ammonium end-groups in the resulting polymers was verified by J aacks and Mathes227 through NMR and chemical analyses of the fractionated and rigorously purified oligomers. However, the ammonium end-groups are readily lost on the prolonged storage of the oligomers, presumably due to Hofmann degradation.

Further evidence of the feasibility of zwitterionic polymerization of this monomer is provided by its initiation by the Me3N+CH2COO- betain. The absence of a removable proton prevents the Hofmann degradation and facilitates studies of the propagation. The kinetics was investigated in ethanol and the progress of the reaction was followed by the IR technique. This polymerization is homogeneous since the initiator, monomer, and polymer are all soluble in the alcohol. The rate of initiation is about four times slower than the rate of propagation. 227

The zwitterionic polymerization of cyanoacrylate esters induced by triethyl- or triphenylphosphine was investigated by Pepper. 228 The irreversible fast initiation is followed by a propagation exhibiting all the fea-


tures of a living polymerization free of termination and chain-transfer, provided that the reaction is performed at low temperature.

5.2. Lewis Acids

The cationic polymerizations initiated by Lewis acids are more complex than the anionic polymerizations induced by Lewis bases. Moreover, they often require the presence of some additional reagent, referred to in the older literature as co-initiators, for inducing the propagation. The processes involving the co-initiators are discussed in the next section. Here we focus our attention on the "direct" initiations supposedly taking place in the absence of co-initiators.

Conclusive evidence for the absence of co-initiators in a polymerization induced by Lewis acid is hard to get. It is difficult to demonstrate conclusively the absence of any impurity that could act as a co-initiator in a system undergoing a slow polymerization. A trace of ubiquitous water in conjunction with Lewis acids is a most powerful initiator of cationic polymerization. Exclusion of traces of moisture calls for a most elaborate drying of the solvents, reagents, and walls of the equipment. It demands the employment of the high vacuum technique and other elaborated procedures. However, after taking all these precautions, it is still debatable whether all the traces of moisture or any other adventitious impurities were indeed eliminated.

It is difficult to be convinced that some observed slow polymerization, supposedly proceeding in the "absence" of any impurities, is induced by a Lewis acid only. The famous "stop" experiments (see p. 235) demonstrate unambiguously the unquestionable requirement of some specified reagent needed for inducing polymerization in a system involving a monomer and a Lewis acid. In the absence of such a reagent, the polymerization does not start. However, such a result does not prove the reverse, the inability of a Lewis acid alone to start a polymerization. If a polymerization does ensue, especially a slow one, one may always argue that some undetectable impurity is still present in the investigated system and it acts as a coinitiator.

Experimental evidence in favor of "direct" initiation comes from the observation of high rates of polymerization and a quantitative conversion of monomer into polymer under the most rigorous exclusion of any conceivable cooperating reagent. Several systems were claimed to fulfill these conditions, e.g., styreneffiCl4 in CH2Cl2 at -90°C,229 iso-butene/AICl3 in CH2Cl2 at O°C,230 iso-butene/AlBr3 in heptane at -lOoC,231 or BCl3 in CH2Cl2 at - 30°C.232

Alternatively, the formation of a large concentration of stable carbenium ions of a non polymerizable "monomer," i.e., a model compound incapable


of undergoing polymerization due to steric hindrance, may serve as evidence of a "direct" initiation, provided that the conditions of high purity are met. For example, the reaction of l,l-diphenylethylene with TiCl4 performed in CH2Cl2 at - 30°C yielded the respective cations in concentrations substantially larger than that of any conceivable impurity or of the residual water which still could be detected.233 This observation was claimed to provide evidence of a direct initiation of cationic polymerization by TiCl4 in the absence of any co-initiator.

Although the progress of some polymerizations of this kind was monitored, any conclusions about their kinetics of initiation derived from the data provided by such experiments are at least ambiguous. Indeed, the mechanism of the direct initiation is still unknown. Several suggestions were made but none was proved convincingly.

It was argued that the chlorinated hydrocarbons, used most frequently as the solvents, could be the co-initiators. However, only in a few cases were the solvent fragments identified in the polymers, and even then their presence could arise from a chain-transfer to solvent and not from the initiation. Abstraction of allylic hydride from the monomer by the Lewis acid could yield carbenium ions and lead to a subsequent cationic polymerization,234 but this is a special case because such a mechanism is applicable only to the monomers possessing the desired kind of hydrogens. Moreover, this process was never proven and strong evidence contradicting it was quoted in the literature. A direct addition of Lewis acid to monomer was contemplated. This would start a zwitterionic polymerization yielding polymers with carbon-metal bonds, such as

Treatment of these polymers with tritiated water or alcohol would make them radioactive, but only a negligible radioactivity was observed in most of the polymers which supposedly contained carbon-metal end-groups.

Finally, the self-ionization of a Lewis acid might account for a direct initiation. For example,

The addition of AIBr{ to a C=C bond could start a polymerization, and such a process was claimed in the literature,235 but convincing evidence substantiating its reality is still lacking. The self-ionization of some Lewis acids is undeniable. It was demonstrated in several systems by observing the resulting conductance,236 but it is debatable whether such ions are


capable of initiating polymerization. A reaction described by the scheme

was claimed by Sigwalt237 but again without adequate proof. Interestingly, the most pronounced self-ionization takes place in mixtures

of two Lewis acids of different strength, e.g., TiCl4 and AlBr3.238 An example of a most complex self-ionization,239 proposed but not proved, is given below:

Had the polymerization been initiated by self-ionization of a Lewis acid, it would have introduced a carbon-metal bond into the resulting polymer. Unfortunately, no evidence for the formation of such a bond has been presented yet.

A most intriguing observation was reported by Cheradame and Sigwalt. 240 A slow incomplete polymerization takes place in the most meticulously dried system: iso-butenelTiCl4 in CH2CI2• It is caused, presumably, by a trace of residual moisture. After its cessation the polymerization is reinitiated by distilling in vacuum the volatile contents of the quiescent solution, but not the solvent, into a side vessel. Polymerization ensues on condensation, even when the walls of the side vessel are coated by a metallic sodium film and the system is kept in darkness. Similar results were obtained on condensation of a gaseous mixture of iso-butene and TiCl4 on a surface cooled to - 20°C. Polymerization ensued immediately and the condensed liquid turned yellow. This strange phenomenon is not unique to iso-butene, a similar behavior was observed in the system, l,l-diphenylethylene and TiC14,241 indenelTiCl4,242 and in cyclopentadiene with TiC130Bu.243

It is difficult to explain these observations which imply that initiation takes place in the gas phase only. A complex and not very convincing mechanism of this initiation was proposed by Sigwalt.240 It postulates the formation of a zwitterion that isomerizes to a covalent species which, in turn, reacts with another TiCl4 molecule yielding an ion-pair:

TiCl4 +CH2 :C(CH3h - Ti-Cl4'CH2C+(CH3h, Ti-Cl4'CH2C+(CH3h - TiCI3·CH2CCl(CH3h,

TiCI3·CH2CCI(CH3)2 + TiCl4 - TiCI3·CH2C+(CH3h, Ti-CI5•

The last step is then followed by propagation. Alternatively, it was proposed244

that the initially formed zwitterion is active but becomes inert on association


with another molecule of monomer. Apparently, some dissociative process generating HCI in the gas phase is depressed in the condenced phase. The formation of HCI is most likely and its polymerization initiating propensity is well known making the above suggestion plausible. A convincing explanation of this puzzle is still desired.

A clear example of direct initiation by a Lewis acid is provided by the initiation of the polymerization of 1,3-dioxolane by BF3,

CH2-CH2 CH2-CH2

/ '" + / '" + dioxolane BF3 + 0 O~ BF3-0 0 --~

'" CH2 / '" CH2 /

CH2-CH2

+/ '" BF3-O-CH2-CH2-O 0 O.

"'CH2/ '" CH2 /

The resulting onium ion propagates the cationic polymerization of dioxolane. Of course, the above polymerization proceeds in the conventional way, discussed in the next section, and yields in the presence of water polymers having a tail OH end-group and BF30H- counterion.

In summary, a "direct" initiation of cationic polymerization by Lewis acids was demonstrated in the systems involving the transfer of electron or a positive moiety (see p. 251). In all the other cases, except the polymerization of oxolanes initiated by BF3, a "direct" initiation was postulated but not proved satisfactory. Notwithstanding their academic interest, it has to be admitted that such an initiation plays only a minor role in cationic polymerizations, provided that it does occur, and even then it offers little advantage from the synthetic point of view. Hydrogen halides may act as co-initiators and they could be readily formed by a variety of reactions in mixtures of halogenated Lewis acids and monomers. Hence, a solution initially free of hydrogen halides, or of any other co-initiator, may become contaminated by them in the course of the investigated process. A review of such conceptually possible reactions was published by Gandini and Cheradame245 who critically discussed the various observations reported in the literature and their proposed explanations.

5.3. Spontaneously Initiated Zwitterionic Copolymerizations

A most interesting kind of copolymerization yielding alternating copolymers was developed by Saegusa.246 He discovered that some cyclic monomers, classified as nucleophiles, undergo spontaneous association with others having electrophilic character and then yield the so-called genetic


zwitterions, e.g.,

or

CH-N

I 2 " CH2 CH "0/

+ CH2-N-CH2CH2CO-

-- ~H2 + )JH II "y o

Denoting by N a nucleophilic monomer and by E an electrophilic one, the formation of the genetic zwitterions is represented by the scheme:

N + E~ +N-E-.

A slow dimerization of genetic zwitterions into dimeric zwitterions was postulated: .

2 +N-E - ~ +N-E·N-E - ,

and the latter are assumed to react rapidly with the abundant genetic zwitterions yielding the first macro-zwitterion (a trimer) which reacts again with other genetic zwitterions in a process leading to polymeric macrozwitterions:

+N-E·N·E·N ...... E·N-E- .

The above scheme resembles a well-known polycondensation of an equimolar mixture of bifunctional monomers AA with BB. There is, however, an important difference between these two reactions. In the classic polycondensation the monomers virtually disappear before the high molecular mass polymers are formed, since the probability of reaction of a functional group A with a B is independent of the size of the species terminated by these groups. For this reason the low molecular mass oligomers are formed first and the high molecular mass polymers are produced later, mainly by the condensation of the oligomers. On the other hand, in the ionic system of Saegusa, the dimeric zwitterions, the seeds from which the ultimately formed macro-zwitterions grow, are created slowly but react rapidly by accretion of the abundant monomeric zwitterions present at high concentration. Only towards the end of the reaction, when the genetic zwitterions are depleted, the association of the macro-zwitterions becomes prominent and then the viscosity of the polymerizing solution increases steeply.


Some questions call for answers. Why is the dimerization of the genetic zwitterions slow, while the formation of trimers, and still higher oligomers and polymers, is faster? The separation of the opposite charges is most difficult when they are close together, thereafter it becomes relatively easy. This is the argument used previously when the protonation of zwitterions and their growth were discussed (see p. 215). What prevents the formation of rings? The rigidity of the chains hinders the ring closure for short chains, and for long chains the probability of intermolecular reaction is higher than that of an intramolecular closure. How does the termination occur? Apparently, there are some side reactions that destroy the active ends, e.g., protonation. Whatever the answers to these questions, this versatile and most interesting copolymerization proceeds readily and provides new and valuable materials.

Finally, another interesting reaction resulting in oxidation of one monomer and reduction of another deserves to be mentioned.247 Its course is outlined below:

~/\ -Ph + 0=0=0 ---> CH2

'b The nucleophilic monomer is oxidized (P3 -+ P5) while the quinone is reduced. The scope of this process awaits further developments.

An extensive review covering numerous examples of polymerization of zwitterions was published by McEwen248 to which the interested reader is referred.

6. Initiation of Cationic Polymerization by Friedel-Crafts Reagents in Conjunction with Co-initiators

6.1. General Features of Friedel-Crafts Reagents

The possession of vacant orbitals is a common feature of Friedel-Crafts (FC) reagents. Thus the boron and aluminum FC reagents possess a vacant p-orbital and the FC reagents based on transition metals have vacant d-orbitals. This property of FC reagents allows them to form bonds with a variety of electron-donating species yielding complexes, some of which are capable of initiating cationic polymerization. Some FC reagents, notably aluminum halides and their organic derivatives, may dimerize. In fact, Alel3 exists in solutions, and even in the gas phase, as a chlorine-


bridged dimer,

and its reactions with some substrates are hampered by the need for its dissociation into the monomeric AICI3• Indeed, there are claims that the dissociation of the dimers is the rate-determining step in some initiation processes.

The formation of zwitterions resulting from a reaction of FC reagents with monomers is discussed in the preceding section. An alternative kind of association is also conceivable-the formation of 1T-complexes which by themselves do not initiate polymerization but readily react with, say, weak acids, AcOH, or suitable halogen derivatives, RHI, yielding the polymerization initiating ions:

or

CH2=CXY + MetZn ~ (CH2-CXY,MetZn) +

(CH2=CXY,MetZn) + AcOH ~ CH3-CXY,MetZnOAc,

+ (CH2=CXY,MetZn) + RHI ~ RCH2-CXY,MetZnHl.

Such processes, as discussed in the preceding section, yield carbenium ions coupled to anions derived from the FC reagents. The process leading to the formation of the first carbenium ions, referred to as a cationation, is followed then by propagation. The formation of monomer-FC complexes is frequently postulated but only in a few systems has their existence been demonstrated by spectroscopic, or other, techniques.

Alternatively, the cationation may result from the association of a FC reagent with a weak protonic acid, AcOH, or a suitable halogen derivative, RHI, a co-initiator. Such a sequence of events is described by the schemes:

AcOH + MetZn -- MetZnOAc,

k +

MetZnOAc + CH2=C(XY) -- CH3 • C (XY),MetZIlOAc,

k


or

+ -RHI + MetZn ~ R ,MetZnHl,

+ - + R,MetZnHI + CH2=CXY ~ RCH2·CXY,MetZnHI.

The formation of complexes of FC reagents with weak protonic acids was conclusively proved in only a few cases, the BF3·H20 complex being the classic example of an unambiguously defined adduct with well-characterized properties.249 Significantly, it reacts with water, forming the inactive BF30H- ,H30+ pairs.

Since the initiation of cationic polymerization may proceed by the one or the other of the two routes described above, it is not surprising that the order of addition of the reagents does affect, and sometimes considerably, the course of polymerization, although the ultimate outcome should be, in principle, identical. Examples of such phenomena are described in the literature, e.g., the initiation of iso-butene polymerization by HCI assisted by AICll50 or by AI(CH3h.251a The rate of a simple ideal initiation would be expected to be proportional with the concentration of the FC reagent, of the agent donating the proton (protogen) or the R + cation (cationogen), and of monomer. However, numerous side reactions make this process much more intricate.

6.2. Complexes of FC Reagents with Protogens

The second scheme of the previously discussed mechanisms of initiation, first proposed by Polanyi,251b reveals the role of the FC reagents in the initiation process. They facilitate the transfer to the monomer of the positively charged moieties, protons or R + cations, by binding the residual negatively charged fragments and thus producing the counterions. In this scheme the weak protonic acids or the halides act as the initiators, referred to as the protogens or the cationogens, respectively, and the FC reagents are then the co-initiators.

Water is the most common and usually the most powerful initiator of cationic polymerization. Numerous polymerizations initiated by the FC reagents are induced by moisture, i.e., by a small, often unknown amount of water still present in the investigated system in spite of their intensive "drying." The removal of the last traces of moisture is a difficult task; it requires an elaborate and time-consuming procedure. The work of Evans and Meadows252 is a classic example of a successful procedure "stopping" the polymerization of a mixture of dry BF3 and iso-butene. Reading their paper is illuminating; one then appreciates all the problems associated with this tedious and laborious task.


In spite of its catalytic effect at low concentrations, water inhibits polymerization at higher concentrations. It reacts with the growing carbenium ions and destroys their activity,

converts the powerful monohydrate, BF3'H20, initiating polymerization into an inert dihydrated complex, hydrolyzes many FC reagents, and so forth. Indeed, water-FC initiators reach a maximum efficiency at some specific ratios of the reagents. For example, for the BF3-water system the maximum efficiency is achieved at the H20IBF3 ratio of about 2:3249 (the equilibrium constant for H20 + BF3·H20 ~ BF3·2H20 is larger than that for H20 + BF3 ~ BF3·H20), whereas for the H20-BCI3253 system the maximum is attained at a H20IBCl3 ratio of 1, whether determined at -78° or -50°C.

A purely physical phenomenon that causes some confusion should be mentioned. Since most of the studied cationic polymerizations were performed at very low temperatures, water, if introduced as a vapor with the monomer, may freeze and form microcrystals of ice. On the addition of a FC reagent, the initiation is retarded by the slow sublimation of ice, causing an induction period of polymerization. On the other hand, the formation of ice is avoided when water is premixed with a FC reagent at ambient temperature and subsequently the chilled mixture is introduced into the cooled reactor. In such a procedure, the freezing of water is prevented by the formation of the H20-FC complex, and the polymerization ensues immediately on the addition of the monomer. This artifact led some workers to believe that the sluggish reaction of water with the FC reagent causes the induction period.

The most commonly used FC reagents in conjunction with protogens are: BF3, BCI3, SnCI4, TiCI4, SbCIs, and AICl3 and its alkyl derivatives, e.g., AIEt2Cl. The protogenic action of HCI is observed on its association with SnCi4, AICI3, and its organic derivatives, but to a much lesser extent with TiCI4• In the SnClc HCI system the SnCls anion seems to be the counterion, although it was proposed that HCI; might act as the anion and SbCl4 is a true catalyst facilitating the transfer of a proton without being consumed by the reaction. The nature of the counterion produced in the initiation induced by the TiClc HCI system is still disputed. This is a poor initiating system and its use is not recommended.

The anions generated by the interaction of water with the FC reagents, such as SnCI40H- and TiCI40H-, are stable and chaperon the growing carbenium cations formed in the initiation. The OH groups, and to a lesser extent the CI atoms, are transferred to the carbenium ions in the course


of termination forming the integral part of the end-groups of the dead polymers.

Let us consider now some examples of polymerizations induced by the water-FC reagents mixtures. The polymerization of isobutene initiated in the presence of unspecified amounts of water by BF3, BCl3 and BBr3 was reinvestigated by Kennedy and his co-workers.254 A striking difference in the behavior of these halides was noted. The polymerization is readily initiated by the BF3-water system, whereas BCl3 does not initiate the reaction in low polarity solvents even at -70°C, although the polymerization is observed in CH2Cl2 and accelerated by the addition of alkyl halides. Their hydrolysis often complicates the ensuing process. Finally, BBr3 is found to be entirely inert under all of the studied conditions.

The polymerizations induced by AICl3-water systems are complex and the produced polymers frequently precipitate, making the kinetic studies of this reaction impossible. On the other hand, the di- and tri-organoaluminum compounds in conjunction with protonic acids are the most useful initiators, yielding very high molecular mass polymers even at relatively high temperatures. Indeed, Saegusa et al.255 prepared a remarkably high molecular mass polystyrene (Mn - 200,000) by reacting the monomer with a premixed AlEt3-water initiator (the ratio AIEt3/H20 varied from 0.6 up to 1.0) at -78°C. This initiating system permits the polymerization to be performed in open reactors under a nitrogen blanket, therefore simplifying the synthetic work.

The use of other weak protonic acids, such as alcohols and phenols, leads to similar results as those observed with water. Their utilization modifies the structure of the counterions and of the end-groups of the ultimate dead polymers.

The kinetics of polymerization of several monomers induced by this kind of initiating systems were reported by many research groups. Some of the reported results were poorly reproducible, and the observed kinetics were complex. Moreover, the observations reported by different research groups often contradict each other. It is doubtful, therefore, whether reliable conclusions about the nature of initiation and its mechanism could be deduced from such data.

6.3. Initiation of Cationic Polymerization by FC Reagents in Conjunction with Cationogens

Trivial CH3 tail-groups are produced when polymers are formed by the initiation induced by protonic acids, whereas a variety of functional groups could be attached to the polymer's ends when judicially chosen cationogens are utilized as the initiators in conjunction with FC reagents. In fact, groups such as CH3CO- and CH3COCH2- were introduced into polystyrene,


as early as 1964,255 by initiating the styrene polymerization with a mixture of AIEt3 and CH3COCl or CH3COCH2Cl. This procedure may be adopted for other functional groups.

The relative efficiency of a large variety of organic chlorides, RCI, in the initiation process performed in conjunction with AlEt2CI was investigated by Kennedy and his colleagues.256 The weight of the polymers formed per mole of RCl served as a measure of the efficiency of R + in the initiation. The results were presented as a graph shown on p. 108 of ref. 214 having a maximum for the (CH3hC- alkyl group. The "maximum" revealed by this graph has a doubtful meaning since the sequence of the plotted points is determined by some arbitrarily assumed "stability" of the pertinent carbocations and not by any directly measured and quantized property. Moreover, the parameter evaluating the "efficiency" of the cation (the yield of polymer in grams per mole of halide) depends rather on the rate of the unspecified termination of the observed polymerization than on the rate of its initiation. This deficiency of the evaluation method described above could be eliminated by utilizing the number of the formed polymers, instead of their weight as the criterion of efficiency.

The initiation induced by a mixture of a FC reagent and a RCI halide is visualized as a sequence of two reactions:

followed by

(FC reagent) + RCI ~ (FC reagent,CI) - ,R + ,

(FC reagent,Cl)- ,R + + CH2=qXY) ~

RCH2·qXY)+ ,(FC reagent, Cl)-.

Does the addition of R + to a monomer proceed like that of the other R + ,A - ion-pairs discussed in the Section 2 of this chapter? This is probably the case. However, the reaction induced by the FC-RCI complex could be complicated by a variety of exchanges involving the organic moieties. For example, the complex of AIR2Cl with R'CI could be transformed into a complex of AIRR'CI and RCI, and then the R + , instead of R' + , would be added to the chain of monomers. Even a more complex reaction could be visualized. Thus, a reaction of AIR2CI with R'CI might yield AIRCl2

and RR'. Such an exchange could substantially affect the polymerization since it produces a stronger Lewis acid from a weaker one. Further complications arise from the dimeric nature of the aluminum FC reagents, whereas the simple scheme outlined above refers to its monomeric form.

Let us stress in closing this discussion that the initiation by the above adducts introduces an R-moiety as the tail-group of the resulting polymer.


This is indeed demonstrated, e.g., by the previously mentioned work of Saegusa, Furukawa, et al. 255

Much of the confusion in elucidating the mechanisms of initiation by the FC reagents-R-halide complexes results from the presence of moisture or some other impurities, not suspected by the investigators, in the supposedly "dry" and "pure" reagents. For example, it was noted that traces of some drying agents, such as P20 S, could be involuntarily introduced into the investigated system with the "dried" solvent or monomer and their presence could affect the behavior of these extremely sensitive reactions.

Purification of a reagent may lead to new kinds of impurities. For instance, in the course of purification of cumyl chloride, a-methylstyrene could be produced. The problems are intrinsic, the co-initiator-FC reagent systems are too sensitive, and the carbenium ions are too reactive. Remarkable progress has been made with less reactive systems, e.g., our understanding of cationic ring opening polymerization is more advanced due to the higher stability of the onium ions. Furthermore, the information resulting from the studies of the rates of monomer consumption (polymerization) and the molecular mass of the produced polymers are affected to a large degree by the termination process mostly not specified in the reports. More direct approaches are needed to unravel the mechanisms of these complex polymerizations, e.g., such as developed by Dorfman268a.268b who studied the addition of organic cations to a variety of substrates by pulse radiolysis. This approach allows monitoring the reaction by the optical absorbtion of the reagents. Alternatively, the recently reported stoppedflow investigations are most informative in such studies (see p. 194).

Strangely enough, in attempts to simplify the investigated systems some researchers made them more complex. The use of the heralded "proton traps" is an example. The sterically hindered pyridines, e.g., 2,6-di-tbutylpyridine, are effective bases but poor nucleophiles. It was anticipated2S7

that their addition to a polymerizing system would prevent the chaintransfer caused by the transfer of proton from the growing polymer to the monomer without affecting the initiation that is also caused by a proton transfer. The results raised more questions than answers, e.g., how could a proton trap distinguish between the protons initiating a polymerization and those causing the chain-transfer? Although the use of proton traps was beneficial in some synthetic studies, the results of such studies did not advance our understanding of cationic polymerization.

Recently, the role of proton traps in cationic polymerization was examined by Faust (a paper deliverd at the ACS meeting in San Francisco, April 1991). His studies demonstrated that the reaction of proton traps with any impurities providing potential protons cleans the investigated system, anihilates the proton donating species, and simplifies therefore the subsequent reactions. The cleansing is slow, it requires about 15 minutes.


The traps react with the potential protons responsible for the initiation of polymerization as well as with those involved in chain transfer. Their action reduces the chain-transfer process responsible for termination or initiation.

7. Initiation of Polymerization by Electron Transfer and Related Topics

7.1. General

Transfer of charge from one molecular species to another occurs frequently during their interaction. For example, the encounter of a molecule of a relatively low ionization potential with another one of a high electron affinity may result in electron transfer from the former, referred to as the donor, to the latter, known as the acceptor. In such a process two oppositely charged ions are formed. The transfer proceeds usually through some intermediate stage. The partners may first unite forming a polarized associate, known as a ground state charge-transfer complex, in which some negative charge is shifted from the donor to the acceptor. The binding of the complex arises from lowering the energy of the system caused by its polarization or, in more general terms, from spreading of electrons through a larger space. Subsequently, the complex might dissociate, yielding two oppositely charged free ions.

The partial separation of charges in the complex allows us to treat it as a hybrid of two entities: an associate of two neutral molecules and an aggregate of two oppositely charged ions, the contribution of the former being dominant in the ground state. Its excited state is represented as another hybrid of the same two entities, but the contribution of the two aggregated ions is dominant now. The electronic transition between these two states caused by irradiation results in light absorption distinct from the absorbance of the individual partners and referred to as the charge-transfer absorption band.258

The nature of the neutral partners and of the ions determines the stability of the complex, whether it is formed and, in the positive case, whether it would dissociate into the separate ions. For example, ammonia and boron trifluoride combine into the stable NH3BF3 complex. The molecular orbital resulting from the overlap of the sProrbital of nitrogen with the vacant orbital of boron accommodates the two electrons of the lone nitrogen pair which become shared by both atoms. The N-B bond is strong, and the complex does not dissociate into ions. On the other hand, the association of tetramethyl-p-phenylenediamine and chloranil yields a charge-transfer complex stable in low polarity solvents (e.g., in benzene, acetone, or dichloroethane), but it dissociates spontaneously in acetonitrile. In the latter solvent the initially formed and unabiguously identified complex gives way


gradually to the separated cation and anion radicals.259 The importance of the solvent's nature in this reaction deserves stressing.

The formation of charge-transfer complexes is not always followed by an unavoidable spontaneous generation of ions. In the majority of cases these reactive intermediates are produced from charge-transfer complexes only on their photochemical activation. For example, a charge-transfer complex stable in its ground state dissociates into the oppositely charged ions on its irradiation in the charge-transfer band. The photo-excited, more polarized species might overcome the potential energy barrier hindering the dissociation before it loses the acquired excitation energy, especially if the irradiation took place in a medium that solvates the resulting ions well.

Some ionizations take place in a direct process. For example, a neutral alkali atom in a'polar solvent is ionized spontaneously. Its electron is transferred into the liquid, becoming a solvated electron trapped in a cavity formed in this medium. Here the atom is the donor and the liquid is the acceptor; no intermediate charge-transfer complex is formed. The dissolution of alkali metal kept in contact with a solvent continues until the saturated solution of the neutral alkali atoms comes to equilibrium with the products of their ionization: the respective cations and the solvated electrons.

Most of the encounters between two ground state molecules result in the formation of very weak, not viable complexes, whereas an electronically excited molecule, whether a potential donor or acceptor, may interact strongly with a ground state molecule and produce a complex known as excimer260 (for identical partners) or as exciplex261 (when the partners are different). In such systems the irradiation of the initially noninteracting molecules might yield ions, as shown by the following scheme:

hv +A

D~D*~ (D*,A)~(D+,A-)~D+ + A-,

or

hv +D

A ~ A * ~ (D,A *) ~ (D + ,A -) ~ D+ + A - .

In the presence of monomers the variants of the above reactions may initiate ionic polymerization.

In the following sections the various modes of initiation of ionic polymerization by the electron-transfer process are reviewed. Our discussion begins by describing the preparation and properties of the alkali metal solutions and of the solvated electrons, as well as the means by which they are utilized for the initiation of anionic polymerization. Much thought is


devoted to the modes of formation of radical-anions and their chemistry, especially of those derived from the vinyl, vinylidene, and the diene monomers. Thereafter the preparation of radical-cations and their chemistry and photochemistry are described. The methods of initiation of cationic polymerization by radical-cations and their precursors are treated extensively, and the role of charge-transfer complexes in those processes are critically surveyed. The simultaneous co-initiation of anionic and cationic polymerization is outlined and discussed. Finally, some physical methods leading to the electron-transfer initiation of polymerization are described.

7.2. Solutions of Alkali Metals

Several distinct species coexist in eqUilibrium with each other in the solutions of alkali metals. A suitable solvent kept in contact with a clean surface of an alkali metal becomes saturated with the neutral alkali atoms, denoted here by Met. The concentration of Met in its saturated solution is exceedingly low, hardly measurable, but the neutral atoms are in equilibrium with the products of their ionization: the alkali cations and the solvated electrons, i.e.,

Met ~ Met+ + solvated electrons e-, K 1e•

The association of neutral alkali atoms and the solvated electrons results in the formation of the negative alkali ions262 and eventually the equilibrium

is established. Finally, the negative ions and the solvated electrons form the respective ion-pairs by association with the alkali cations and then the following equilibria are established:

and

Higher aggregates need not be considered because most of the pertinent solutions are highly dilute.

Let it be stressed that the eqUilibrium concentrations of all these species are uniquely determined for each solvent and at each temperature by the saturation concentration of the neutral alkali atoms. These are constant and therefore, as long as the solvent remains in contact with a clean surface


of the metal, their values depend only on the nature of the alkali metal, of the solvent, and on the temperature of the solution.

The total concentration of the dissolved metal is given by the sum:

It is very low for most of the solvents; only in liquid ammonia and in hexamethylphosphorictriamide (HMPA) are higher concentrations of the alkali metals attained.

The addition of the salts of the pertinent alkali cations decreases the concentrations of the solvated electrons and the negative alkali ions due to the common ion effect, whereas the concentrations ofthe e- ,Met + and Met- ,Met+ ion-pairs remain unaffected, namely:

and

The saturation concentration of a neutral alkali atom depends on the heat of sublimation of the respective metal and on the small energy of solvation of its atom. For most solvents the gradation of the concentrations of saturated solutions is: [Cs]e > [Rb]e > [K]e > [Na]e. The saturation concentration of Li atoms is too small to be determined by the available techniques.

The ionization constant, K 1e , of any alkali atom increases with the dielectric constant of the solvent; however, even more important is the ability of the solvent to solvate the resulting cation. For example, Kt • is substantially larger for dimethoxyethane (DME) than for tetrahydrofuran (THF), although the dielectric constant of the latter solvent is slightly greater than that of the former. The bidentate nature of DME makes it a much better cation solvating agent than THF since it reduces the entropy of solvation.

The capacity of solvents to enhance the ionization of an alkali atom decreases along the series: liquid ammonia > HMP A > methylamine -ethylenediamine - DME > ethylamine > THF » diethyl ether. The reverse order favors the association of the free ions into the ion-pairs.

The nature of the metal and the solvent determine which species predominates under a particular set of conditions. For example, in methylamine solution of sodium, the pairs Na + ,Na - dominate at ambient temperature, while in the corresponding solution of cesium, the Cs + cations and solvated electrons form the bulk of the solute.263 While solvated electrons and their ion-pairs are the most abundant species in liquid ammonia or HMP A, the negative alkali ions and their ion-pairs dominate in ethereal


solvents. In fact, pulse radiolysis of THF solution of alkali tetraphenyl borides revealed that the r ,Met + ion-pairs, observed immediately after a pulse, rapidly decay264 in a bimolecular reaction yielding by the disproportionation process the negative Met- ions and the Met+ cations, as well as their respective ion-pairs. These are then the main components of the equilibrated solution.

The concentration of solvated electrons and the negative alkali ions enormously increases on the addition to their solutions, kept still in contact with an alkali metal, of powerful coordinating agents such as crown ethers or cryptands.265 The increase results from the coordination of the cations with these agents yielding very stable 1:1 complexes***:

Met+ + cryptand or crown ~ Met + ,cryptand (or crown), K5e'

The coordination depletes the concentration of the noncoordinated cations which become then replenished by further dissolution of the alkali metal, provided that it remains still in contact with the solvent. Denoting the concentration of the coordinating agent by X, we find:

For K5eX » 1 (a most frequent case) the increase of the concentration of the solvated electrons and the negative alkali ions caused by the addition of crowns or cryptands is proportional with X1I2.

7.3. Structure and Properties of Solvated Electrons and Negative Alkali Ions

An electron trapped in a polar liquid is stabilized by creating a cavity surrounded by the solvent molecules having their dipoles directed towards its center. This creates a nearly spherical electric field within which the electron moves. The energy levels of the trapped electron become, therefore, quantized, allowing it to absorb radiation in the visible down to the near infrared range. The respective absorption band is broad and structureless, showing a marked asymmetry on its high energy side. The maximum of the absorbance depends on the solvent, being independent of the nature of the alkali metal; it appears at Amax = 630 nm in water, at 1480 nm in liquid ammonia, and at -2020 nm in THF.

The size of the cavity is interpreted as the size of the solvated electron. The extent of cavitation, revealed by the dilation of the medium upon injection of electrons,266 is measured by flV,

a v = V solution - (V solvent + V metal) per gram atom of metal,

mIn some systems 1:2 complexes are formed, e.g., Cs+ ,2crown or Ba2+ ,2crown.


and from its value the radius of the solvated electron is calculated, e.g., its radius in liquid ammonia is 4.5 A. The cavitation makes the saturated solution of lithium in liquid ammonia the lightest liquid at ambient temperature, its density being 0.48 g/ml only.

The solvated electrons are mobile and conduct electric current. Their mobility is higher than those of the ions of comparable size, although not by a large factor. Both the "hopping" mechanism and the migration of the entire cavity contribute to their conductance, and in this respect solvated electrons differ from the very rapidly moving "dry" electrons formed in the course of radiolysis.267 The solvated electrons are paramagnetic, their ESR spectrum reveals a single sharp peak at the expected g value of a free spin.

The optical spectra of the r ,Met + ion-pairs resemble those of the solvated electrons. Their Amax are shifted to shorter wavelength and the shift depends on the nature of the alkali metal. The optical spectra of the r ,Na+, e- ,K+, and e- ,Cs+ pairs in THF are shown in Fig. 3.55. These pairs are paramagnetic, similarly as the solvated electrons, but their ESR spectra are structured. The splitting of the electron signal is caused by the coupling of its spin to the spin of the alkali atom nucleus. The coupling constant increases enormously with rising temperature, an effect accounted for by the dynamic equilibrium supposedly established between the pairs and the un-ionized atoms, the coupling constants of the latter being very large. The equilibrium shifts towards the un-ionized atoms as the temperature rises. Since the interconversion is rapid on the ESR time scale, the spectra are averaged and from their width the coupling constants are calculated. For some systems the interconversion is slow and then their ESR spectra consist of two sets of lines corresponding to different coupling constants.

:n 2 ;u c

.~ 1

'" "t)

"' u

600 1000 1400 ).. (nml

1800 2200

Figure 3.55. Spectra of the solvated electron and of alkali cations pairs. Na+, K + , Cs +. Recorded immediately after a pulse.


The negative alkali ions are diamagnetic. They were described incorrectly in the older literature as electron dimers (e-h. Their optical spectra are shown in Fig. 3.56 together with the spectrum of the free solvated electron. They are sharper than those of the ion-pairs, and their absorption peaks are shifted slightly towards shorter wavelength. The spectrum of Li - was not observed, although the spectrum of e- ,Li+ was reported.268

5 ~-------------------------------.

'" ~ 2

"' u

0-o

.:~.' A X,X ... · .. 1 .f .• ! ~

/i \\ I ~ . . . • ' I : 1. .\ ~

// XI xl \ \ !I / \ /\ '\

/ xV \ \ / \. \ 0

700 1100 1500

AI nm)

1900 2300

Figure 3.56. Spectra of the negative alkali ions and of solvated electron in THF. Na-,K-,Cs-,e-.

The pairs Met + ,Met - are stoichiometrically identical with the diatomic alkali molecules observed in the gas phase. The crystaline salts of Met- ,Met + solvated by a proper coordinating agent were prepared and their X-ray spectra were reported. 269

7.4. Reactivity of Alkali Metal Solutions

The extremely high reactivity of alkali metal solutions calls for the utmost care in their handling. They must not be exposed to the air; in fact, the solutions of potassium in HMP A are pyrophoric.

The stability of alkali metal solutions depends on the nature of the solvent and its purity. The HMP A solutions are relatively stable, although some decomposition yielding alkali hydrides is noted after a few days. Liquid ammonia solutions kept at low temperature hardly evolve any hydrogen gas, the direct reaction

does not take place. However, solvated electrons react rapidly with the NHt ions formed by the self-ionization of the solvent and yield then H


atomsZ70

r + NHt ~ NH3 + H.

The slow rate of hydrogen evolution is governed, therefore, by the rate of the self-ionization of the solvent, hence this rate is independent of the metal's concentration and retarded by the addition of amide ions. A similar mechanism accounts for the hydrogen formation caused by the addition of water or alcohol to the alkali metal's solutions in liquid ammonia since such an addition produces ammonium ions through a proton transfer

ROH + NH3 ~ RO- + NHt.

The conversion of solvated electrons into amides or alkoxides is reversible. Therefore, solvated electrons are formed when a solution of amides in liquid ammonia, or hydroxides in water, is pressurized by Hz gas. The equilibrium

has to be established whatever the mechanism. Indeed, solvated electrons were detected under these conditions by the ESR technique.271

A slow bimolecular disproportionation of electron-cation pairs yields the Mer- ,Met + pairs,

The alternative association of solvated electron-cation pairs with the neutral metal atoms is insignificant due to the extremely low concentration of the latter. The disproportionation becomes even slower in the presence of crown ethers or cryptands since the powerful coordination of Met + with these agents has to be destroyed to allow for the conversion of the coordinated Me+ into the uncoordinated Met-. On dissolution of alkali metal in solvents, the electron-cation pairs are formed initially and subsequently they are converted into the negative alkali ions. This sequence of events was observed.272

The solvated electrons and negative alkali ions are powerful reducing agents converting aromatic hydrocarbons and many monomers into the respective radical-anions. The reduction of a monomer into its radicalanion starts its anionic polymerization.

An aromatic hydrocarbon A dissolved in a solvent remaining in contact with a solid or liquid alkali metal is only partially reduced since the


equilibrium

Alkali metal + A(solution) ~ A - ,alkali metal +

keeps the ratio [A - ,alkali metal+]/[A] constant, independent of the concentration of A. This ratio is uniquely determined by the nature of the aromatic hydrocarbon to be reduced, of the alkali metal, the solvent, and the temperature of the solution, and its value provides a measure of the solvating power of the solvent.

The heat of reduction of a hydrocarbon by an alkali metal into the respective radical-anion is given by

where the symbols denote the heat of sublimation of the metal, its ionization potential, the electron affinity of the hydrocarbon, and the heat of solvation of the reSUlting cation (the main contribution to aHr) and of the radical-anion. The last term denotes the energy of separation of the charges in a medium of dielectric constant E. The ionization potentials and the heats of sublimation of various alkali and alkaline earth metals are given in Table 3.3.

Only a few investigations of polymerization initiated by the solvated electrons or by the negative alkali ions have been reported in the literature. Exploratory studies of such systems stabilized by the addition of cryptands were reported by Boileau273 and by Schue.274 The results were conventional, no new phenomena were observed.

Are the negative alkali ions stronger reducing agents than the solvated electrons? Does a simultaneous two-electron transfer take place in the interaction of the negative alkali ions with a potential electron acceptor? The only evidence of an unusual behavior of the negative alkali ions was

Table 3.3. Some physical data on alkali and alkaline earth metals

Metal Heat of fusion, caVmole Boiling point Ionization potential, eV/mole

Li 1100 1347°C 5.39 Na 630 883 5.14 K 574 774 4.34 Rb 525 688 4.12 Cs 500 678 3.89 Mg 2160 1090 7.64 Ca 2236 1484 6.16 Ba 1830 1640 5.21


reported by lediinskF75 who studied the interaction of K - with ~-propiolactone in THF. The claimed formation of methylacetate in a high yield ( -80%) implies a surprising occurrence of a reaction arising from the fission of the C-C bond of the lactone ring. Confirmation of this unusual result is desired.

7.5. Homogeneous Electron Transfer; Initiation of Anionic Polymerization

Electron transfer from a suitable electron donor, AT, to a monomer, M, leads to the reduction of the latter to its radical anion:

Usually, the species A is an aromatic hydrocarbon, and M refers to a vinyl, vinylidene, or diene monomer. The counterions are omitted here for the sake of briefness. The radical-anions of such monomers dimerize or react with the unreduced monomer yielding the dimeric species as exemplified below276:

or

The dimeric dianions initiate anionic propagation by adding monomers to both of their carbanionic ends, whereas the dimeric radical-anions could, in principle, start an anionic propagation from one of their ends and a radical addition from the other. However, the radical termini either undergo a rapid dimerization yielding tetrameric dianions, or they disproportionate. The latter reaction results in the formation of a dimeric dianion and two molecules of the monomer. Therefore, the ultimate polymerization is always induced by the anionic termini, being propagated from both ends of the initiating dianions.

The dimerization of the radical anions or their association with the monomer arises from a tail-to-tail coupling yielding the thermodynamically most probable product which, apparently, is also preferred kinetically. For example, carboxylation of the dimeric dianions of styrene yields 2,4-diphenyladipic acid, a product resulting from a tail-to-tail coupling. No other isomers were detected. Coupling of some sterically hindered radical-anions led to exceptional products of dimerization. For example, the protonation of the dianions formed by dimerization of radical-anions of t-butylphenylacetylene


yields the following hydrocarbons, all identified by their NMR spectra277:

(/-Bu) H

H-l \f-(t-BU)

\-@-J C 0 C / \

Ph H

lh H\ H-C f - (I-BU)

\----foL! /~\

(t-Bu) H

(t-Bu) (t-Bu) I \

H- C\ ~-;:;L f- H

/C-0 .. C\ Ph H

Ph (I-Bu) / \

H-C f-H

\----foL! /~\

(t-Bu) H

Such modes of dimerization resemble that operating in the dimerization of the sterically hindered triphenylmethyl radicals.278

The studies of two systems: the dimerization of radical-anions of amethylstyrene279 (denoted by a), and of those derived from l,l-diphenylethylene280 (denoted by D), allowed the determination of their absolute dimerization rate constants. The following approach was adopted in these investigations.

Flash photolysis of -10 - 6 M THF solutions of the potassium salt of the dimeric dianions of a-methylstyrene results in their photodissociation279:

The - aa - denotes the dime ric dianions of a-methylstyrene, aT its radicalanion, whereas the K+ cations are omitted for the sake of brevity. The identity of the produced aT radical-anions was established by spectroscopic observations. In the dark period following the flash, the system relaxes to its initial state; the radical-anions dimerize and regenerate the original dimers. The rate of the dimerization was monitored by the increase of the absorbance at 340 nm (~max of the dimers), or by the decay of the absorbance at 400 or 600 nm caused by the disappearance of the transient. The recorded spectral changes are shown in Fig. 3.57. The isosbestic point at 390 nm proves that the transient is converted directly and stoichiometrically into the dimer, and the identity of the final spectrum with the one observed initially demonstrates the absence of any side reactions.

The spectrum of the transient was obtained from the recorded difference spectrum by adding to it the known absorbance of the bleached dimer. Thus constructed spectrum, shown in Fig. 3.58, agrees with the reported spectrum of aT radical-anion obtained either by the pulse-radiolysis of a-


Fia sh photolysis of K+ .-aa; K+ in THF

0,1

a 0 0 <J

5ms

0.1

• 80 m s

300 GOO 500 600 700 800 A/nm

Figure 3.57. Difference spectra recorded in flash photolysis of K+, -a<C, K+ at increasing time intervals after the flash.

0.2

0.1

g 0 <J

-0.1

-0.2

300

Fiashphoto lysis of K :-aa-,K + .n THF

'x

--- -x~.~.:.::x:::.:-:: . r. ,.., ~x--x- - -x-

.. ... x·,x

l :'1.:

o 0.-

• Observ ed diff erenc e spectrum (5ms af ter flash I

x a a.-

500 600 700 800

A/nm

Figure 3.58. Derivation of the spectrum of a - , K + , from the difference spectrum and the spectrum of the bleached K+, -aa-, K+.


methylstyrene solution, or by 'V-irradiation of a-methylstyrene in frozen glass.

The linear plots of lILlOD vs. time, shown in Fig. 3.59, confirm the second-order character of the studied reaction. Their slopes, in conjunction with the knowledge of the respective extinction coefficients, provide the absolute value of the bimolecular rate constant of dimerization of the aT radical-anions, namely 1.0 . 107 M-1S- 1•

Similar experiments performed with the Li+, Na+, K+, and Cs+ salts of the dimeric dianions of 1,1-diphenylethylene280 (-DD-) led to the respective bimolecular rate constants of dimerization of the alkali salts of the DT radical-anions. The values of the latter vary from 1.2 . lOS M-1S- 1

for the Li+ salt to 30 . 108 M-1S- 1 for the Cs+ salt. Interestingly, the dimerization of radical-anions is slower than the dimerization of free radicals.

The rates of dissociation of the dimeric dianions into the respective radical-anions were determined by two approaches. Two solutions of the dimeric dianions of a-methylstyrene were prepared; one produced by the reduction of a conventional monomer, while a-methylstyrene per-deuterated in the phenyl rings was used in the preparation of the other solution. The above solutions were mixed in the 1: 1 molar proportion, and thereafter aliquots were removed at the desired times. These were protonated by the addition of methanol, and the composition of the resulting hydrocarbons

F lllshphotolysis of K+"OlOl - K· in TH F •

400nm ./1 20 /.7 '/·40

./ ././340nm

15 ./ / 30 0 ;17(. • 0 0 0 <I

" ./ .7 /wonm ~

10 ././ ~. 20 .... / "f. / •

• 10 ./ .'

0 0 0 20 40 60 80

t/ms

Figure 3.59. Plot of 1I0D vs. time for the dimerization of IX - , K + in THF recorded at different wavelengths.


was analyzed by mass spectroscopy. The appearance of the dimers containing five deuterium atoms, i.e., H·(X(XSd·H, is evidence for the dissociation of the homodimers followed by the association of some of the resulting radical-anions into the mixed dimers. Their rate of formation therefore monitors the rate of dissociation of the homodimers into the radical-anions.281 The pertinent rate constant of the dissociation was found to be 6· 10-8 S-1 (only!% of the homodimers dissociates in a day).

A different approach was used in the study of dissociation of the sodium salt of the dimeric dianions of 1,1-diphenylethylene.282 The monomer, 0, labeled by 14C, was added to a solution of the nonradioactive dimers, Na + , -00-, Na+, and the rate of incorporation of the radioactivity into the dimers was investigated. Two reaction modes may lead to the incorporation of the radioactivity. One results from a direct dissociation of the dimers into the radical-anions followed by the very rapid radical-anion-monomer electron transfer

and the subsequent association of the radical-anions. Alternatively, the incorporation of the radioactivity may result from the establishment of an equilibrium between the dimeric dianions and the monomer

that maintains a minute stationary concentration of the labile dimeric radicalanions, -000 (the counterions are omitted again for the sake of brevity). The latter, being in equilibrium with the products of their dissociation,

-000 ~ OT + 0, k-2

undergo exchange with the radioactive monomer. The incorporation of the radioactivity by this route depends on k-2 and the dissociation constant of the dimeric dianions into radical-anions, K d • The results led to the upper limit of 1O-7M for K d , and to the approximate value of -300 M-1S- 1 for the bimolecular rate constant, k-2' of the radical-anion-monomer association. Its smallness, compared with the large dimerization rate constant ofradical-anions (> 10S M - 1s -1), shows that the reaction of radical-anions with the monomer is of no significance in the initiation of anionic polymerization by electron-transfer process. Even in the bulk of the monomer this reaction is too slow to affect the results, as confirmed by the data derived from the study of flash-photolysis of the dimeric dianions of 1,1-diphenylethylene.28o


It is beneficial when the electron affinity of an aromatic hydrocarbon donor is lower than that of the monomer to be polymerized, although this is not essential. The unfavorable equilibrium between an electron donor and a monomer,

is still shifted to the right by the virtually irreversible dimerization of the monomer radical-anion. However, the electron affinity of the donor must not be too high either. For example, the polymerization of styrene cannot be initiated by the anthracene radical-anions, the electron affinity of anthracene is too high. In fact, anthracene (At) present during the anionic polymerization of styrene (S) adds reversibly to the growing polymers,

-S- + At ~ -S'At-,

yielding dormant polymers.283 Under these conditions the rate of polymerization is retarded, being inversely proportional to the concentration of the added anthracene, whereas the fraction of the living polystyrene unassociated with the anthracene is very low.

Anthracene as well as other aromatic hydrocarbons of high electron affinity decompose the dimeric dianions derived from monomers of not too high electron affinity, e.g.,

-DD- + 2At ~ 2D + 2AC,

(-DD- denotes the dime ric dianions of diphenylethylene, the counterions are omitted again for the sake of brevity.) The equilibrium and the kinetics of this reaction were investigated.284 The decomposition of the dimers is governed by the reversible electron transfer

-00- + At ~ -00· + Af,

followed by the rate-determining decomposition of the dimeric radicalanions and the subsequent extremely rapid electron transfer from 0"" to At.

The interesting modification of this scheme285 accounts for the reaction of anthracene with the dimeric dianions of a-methylstyrene, - aa -. The quantitative complexation of anthracene with the dimers, a reaction prevented in the -OD- system by the steric hindrance, yields rapidly, in less than a second, the adducts

VI c: II> "0 2 01 u

0.1 o

o


Figure 3.60. Optical density of the -acC dimer-anthracene (A) adduct for increasing ratio [A]/[-aa-]. Note the saturation at the ratio equal to 2.

or

characterized by their absorption band at 451 nm. Its intensity increases with an increasing ratio [At]/[-aa-] and, as shown in Fig. 3.60, it reaches a plateau for the ratio = 2, implying the eventual quantitative conversion of - aa - into - AtaaAt -. The adduct decomposes into the anthracene radical-anion, At'", and the dimeric -aa· radical-anion, its initial rate, i.e., the rate of formation of At'" (~max = 720 nm), depends on the initial ratio [At ]/[ - aa -]. The rate increases with the increasing ratio of [At ]/[ - aa -] but, as shown in Fig. 3.61, it reaches a maximum at the ratio of 1 and thereafter decreases, virtually to 0, as the ratio of [At]/[-aa-] approaches

c: .~ 3 0; ,~ E (; /1

/ I -2 I <t

'0 ., '"

1

'" C

0 6

Figure 3.61. The initial rate of decomposition of the adduct of the dimeric dianion of a-methylstyrene and anthracene obtained at different ratios of the components. The dotted line represents the expected rate of decomposition of the adduct had the anthracene addition taken place at one end of the dimer only.


the value of 2. The experimental points, shown in Fig. 3.61, fit a parabola

rate = const.ratio.(1 - ratio).

The probability of addition of anthracene to either end of the - aa - dimer seems to be independent of the fate of the other end, but while -aa.Atdecomposes rapidly yielding the Af radical-anion and the labile - aa. (which readily disproportionates), the adduct - Ataa.At- is relatively stable and its decomposition is very slow. This observation shows the effect of electrostatic repulsion between the negatively charged terminal units. Its effect decreases with their separation making the -aa.At- less stable than the - At.aa.At-.

The examples discussed above show the complexity of the initiating systems when donors of high electron affinity are used as the electrontransfer initiators. Not surprisingly, the polymerization of styrene cannot be achieved with benzophenone ketyl286 (electron affinity of benzophenone is high), but the ketyl is a good initiator of the polymerization of acrylonitrile, a monomer of much higher electron affinity than styrene. The electron-transfer initiation of the anionic polymerization of styrene and the dienes is most conveniently accomplished by the naphthalenide or biphenylide radical-anions.

The electron-transfer mode of initiation of a polymerization could be adopted for cationic polymerizations, provided that radical-cations of a sufficiently high oxidation power are available. To our knowledge such an initiation has not been reported yet, although a model reaction discussed on p. 251 is known.

Radical-ions may initiate polymerization by other than electron-transfer modes of reactions. For example, they initiate polymerization of some cyclic monomers by acting as nucleophiles. The reaction of sodium naphthalenide with ethylene oxide proceeds as shown below:

Its bimolecular rate constant was determined at -1 M -IS -1 .286 The resulting radical is reduced further by another naphthalenide radical-anion and then yields a carbanion that reacts with ethylene oxide. Thus a polymer is formed having a dihydronaphthalene moiety in its middle.287 A similar process initiates the polymerization of the octamethylcyclotetrasiloxane.288


7.6. Initiation of Anionic Polymerization by a Heterogeneous Electron Transfer

Any heterogeneous surface reaction is favored by a large surface area of the heterogeneous reagent. The heterogeneous reduction by alkali metals performed on a laboratory scale is best achieved with an alkali mirror produced by evaporating the metal in vacuum onto the walls of the reactor. Such a mirror has a large, clean, and shiny surface allowing for a rapid reaction of the alkali with organic substrates dissolved in a suitable solvent. Lithium mirrors cannot be produced by this procedure in a glass vessel. Heating this metal in glass cracks the container, and therefore quartz vessels have to be employed, as described by Favier and Fontanille,289 in the preparation of lithium mirrors. Alternatively, such mirrors could be formed by the thermal decomposition of recrystallized lithium azide in evacuated vessels. An elaborate technique developed by Francois290 yields highly dispersed pyrophoric lithium dust, as well as pyrophoric dispersions of alkaline earth metals.

Extensive studies of competing surface reactions leading to polymerization were reported by Richards and his colleagues.291 Solutions of alkyl bromides in THF react vigorously with alkali metals, say lithium, yielding the Wurtz coupling products. Such violent reactions are slowed down by the addition of aromatic monomers, e.g., styrene, and the nature of the products is then drastically altered. While butane results from the reaction of ethyl bromide with the lithium surface, an ethyl-capped dimer, i.e.,

is the product formed after the addition of styrene.292

The absorption of the reagents on the active surface is the first step of any heterogeneous reaction. An aromatic reagent, e.g., styrene, possessing polarizable 1T electrons is more strongly adsorbed on a surface of an alkali metal than an aliphatic one like ethyl bromide. Thus the addition of styrene to an ethyl bromide solution kept in contact with a clean surface of an alkali metal leads to replacement of the latter by the former on the surface of the lithium, hindering the Wurtz coupling. The adsorbed styrene acquires an electron from the metal and thus is reduced to its radical anion attached to the positively charged metal surface that acts as a giant counterion. The radical-anions, being mobile on the surface, become coupled and form the dimeric dianions which, on their desorbtion, react, in turn, with the dissolved ethyl bromide. Thus, the observed product is formed.

The proposed mechanism gains support from the results of experiments293

performed with phenyl bromide instead of ethyl bromide. The polarizable 1T electrons of this aryl compound allow it to compete effectively with


styrene for the sites on the lithium surface, and the Wurtz coupling reaction becomes again important.

An interesting extension of this approach is provided by the reaction of butadiene, BD, and p-xylene dibromide, both dissolved in THF, on the surface of metallic lithium. This reaction yields an unusual copolymer294:

Its composition is determined by the initial ratio of the reagents in the feed, and the NMR analysis confirms the proposed structure and shows that the p-xylene moieties exist in the polymer chain exclusively as the C-C linked dimers.

The vicinal dibromides react differently. 295 An equimolar mixture of styrene and 1,2-dibromoethane reacting on a lithium surface yields a headto-head-tail-to-taillinked polystyrene, together with ethylene and lithium bromide. Apparently, the adsorbed styrene yields the dimeric dianions which react with the dibromide, eliminate ethylene and lithium bromide, and subsequently become coupled forming this unconventional polymer.

Other linking agents were investigated, e.g., dibromodimethylsilane and dichlorophenylphosphine. An interesting product was obtained with diepoxides,295 namely

-CHPh·CH2CH2·CHPh·CH(OLi)·R·CH(OLi)·CHPh·CH2CH2CHPh-,

that yields the respective diols on hydrolysis. Different monomers are adsorbed on alkali metal surfaces to different

degrees. This differentiation accounts for some confusing results reported in the literature. The homogeneous anionic copolymerization of an equimolar mixture of styrene and methylmethacrylate initiated by alkyllithiums yields polymethylmethacrylate contaminated by about 1 % of styrene. However, when a dispersion of lithium metal was used for the initiation, the resulting copolymer contained a large proportion of styrene.296 It has been argued that the polymerization is propagated by the anionic and the radical ends of the initially formed dimeric radical-anion. Hence the resulting product was claimed to be a block polymer, a homopolymethylmethacrylate block produced by the anionic polymerization, and a block of an -1: 1 copolymer of styrene and methylmethacrylate arising from the conventional free radical copolymerization. §§§ However, this claim has been questioned by Overberger297 whose NMR investigation demonstrated that the resulting

§§§The radical termini do not contribute to polymerization, but this was not yet fully realized at the time the above claim was made.


polymers are composed of a short block of homopolystyrene followed by a block of homopolymethylmethacrylate; no styrene-methylmethacrylate copolymer was found.

Overberger's results may be rationalized in terms of a preferential adsorption of styrene on the lithium surface with virtual exclusion of methylmethacrylate. The sorbed styrene perpetuates the surface polymerization propagated by the carbanionic ends of the living oligomers of styrene. The less readily adsorbed methylmethacrylate remains in the solution, having hardly any access to the growing centers. Eventually, the living oligomers are desorbed, the driving force being provided by the gain of configurational entropy of the desorbed oligomers. The desorbtion requires removal of a cation from the metal,298 a process associated with a high potential energy barrier but facilitated by the solvation of the formed cation. Therefore, the lower the solvating power of the solvent, the longer the growing carbanion ends remain associated with the lithium surface, and the higher is the molecular weight of the formed styrene oligomers. On its desorption a living oligomer of styrene reacts with the methylmethacrylate present in the solution and initiates its homopolymerization yielding eventually a block copolymer. Hence, the fraction of styrene units in the copolymer produced in a medium poorly solvating the cations is expected to be higher than in the copolymer produced in a medium well solvating the cations. 298

This prediction is confirmed by the results of Overberger. 297 The percentage of styrene in the polymer was found to increase with decreasing solvation power of the medium. Moreover, since removal of a sodium cation from a sodium metal is more facile than removal of a lithium cation from a lithium dispersion, a homopolymethylmethacrylate containing negligible amounts of styrene is formed when a sodium dispersion, instead of lithium, initiates the polymerization.

7.7. Formation of Radical-Cations

Radical-cations are formed on removal of an electron from a neutral molecule. Such one-electron oxidation may be achieved with a variety of chemical oxidants, as well as by physical means, e.g., by photoionization, pulse radiolysis, anodic oxidation, and so forth. The ease of removal of an electron from a neutral molecule is determined by its ionization potential and by the solvation energy of the resulting ions (the solvation energy of the neutral molecule is usually negligible).

During early ESR studies of aromatic radical-cations, these were produced by dissolving the parent compound in a concentrated sulfuric acid. Although the detailed mechanism of such an oxidation is still unknown, the protonation of the substrates undoubtedly facilitates this process, ap-


parently proceeding through the following steps:

ArHi + ArH ~ ArH; + ArH+o,

Alternatively, radical-cations are formed by oxidizing a suitable aromatic compound with Lewis acids such as SbCIs. The resulting radical-cations were conclusively identified by their ESR spectra, whereas the expected radical-anions of the Lewis acids have never been detected. Apparently, the latter are destroyed by some unidentified reaction, most probably by their disproportionation299:

2SbCI; ~ SbClt;- + SbCli.

The isolated crystalline salts produced by the oxidation, incorrectly described

as AH+o,SbCI;, seem to be mixed salts, (2ArH+o,SbCli ,SbCli). In his classic studies reported at the beginning of this century Wieland3°O

described the preparation of the "aminium salts," recognized now as radicalcations. The "aminium" salts were prepared by the low temperature oxidation of tertiary amines with bromine, e.g.,

Oxidation by iodine or chlorine was not successful, the former being too weak an oxidant, while chlorine is engaged in electrophilic substitution. Interestingly, Wieland reported also the decomposition of this salt into a mixture of the brominated and the unsubstituted tertiary amines, demonstrating the feasibility of disproportionation of the participating radicalcations.

The phenomenon of a complete transfer of an electron via a chargetransfer complex resulting in the formation of radical-cations is illustrated by the previously mentioned reaction of tetramethyl-p-phenylenediamine (TMPD) with chloranil. 301 A stable ground state charge-transfer complex is formed in low polarity solvents (e.g., in dioxane, benzene, or dichloroethane). However, in the polar acetonitrile the initially formed complex, identified by its absorption spectrum, decomposes slowly and gives way to the separate radical-cation and radical-anion, each being unambiguously identified by its ESR and absorption spectra. The dissociation is relatively slow since the separation of the partners proceeds through a configuration in which the bond between them is stretched while the solvent molecules,


eventually solvating and stabilizing the ions, cannot yet be squeezed in. This is therefore the configuration of the transition state (see p. 48 for the description of an analogous conversion of tight ion-pairs into loose ones).

Photolysis induced by the near UV or visible light cannot produce radicalcations from most of the irradiated substrates since their ionization potential is too high. However, although the monophotonic ionization is prevented, some radical-cations and the solvated electrons could be produced through biphotonic processes. The first absorbed photon produces an excited singlet that rapidly decays through intersystem crossing into the respective triplet. The relatively long lifetime of the latter allows it to absorb another photon and to be ionized, yielding then the pertinent radical-cation and a solvated electron. The lifetime of such pairs depends on the energy of the photons causing the ionization. The larger their energy, the further away from the cation travels the ejected electron, and hence its return requires a longer time.

The nature of the solvent affects such a photoionization in two ways. First, it solvates and stabilizes the cation and the trapped electron and thus facilitates the ionization and hinders the return of the ejected electron. Second, it affects the probability of the intersystem crossing yielding the triplets and it influences, therefore, the quantum yield of the triplet formation. As a rule, the higher the polarity of the solvent, the lower the efficiency of the intersystem crossing. 302

7.B. Initiation of Cationic Polymerization by Electron Transfer

Oxidation of a monomer to its radical-cation provides the simplest mode of initiation of cationic polymerization through an electron-transfer process. One may expect, in analogy with the anionic polymerization, that the produced mom>mer radical-cations would dimerize, yield dimeric dications, and then the addition of monomer to their positively charged ends would start the propagation.

Convincing evidence demonstrating the feasibility of such a mode of initiation was reported by Szwarc and his associates303 who oxidized by an excess of SbCIs a model monomer, 1,I-diphenylethylene, in methylene chloride at - 80°C. The progress of the reaction was followed spectrophotometrically. First an absorbtion band, attributed to the formation of a charge transfer complex, appeared at Amax = 465 nm. Within 15 min this band was replaced by the ultimate absorption with the maxima at 435 and 310 nm characterizing the final product. The latter was unequivocally identified as the dimeric dication of 1,I-diphenylethylene,

+ + CPh2CH2CH2CPh2,


because its optical spectrum is identical with the spectra of the products formed on reacting l,l-diphenylethanol or 1,1,4,4-tetraphenylbutane-l,4-diol with SbCIs. The absorption of the latter cations is due to the presence of the -C + Ph2 chromophores and therefore the identity of these three spectra confirms the proposed structure of the investigated oxidation product. Further evidence for its structure is provided by the NMR spectra of all the three cations discussed above; the significant lines appear in all of them.

The above kind of oxidations seems to be restricted to Lewis acids of high electron affinity such as SbCIs. The same procedure utilizing TiCI4, a less powerful oxidizing agent, failed to produce the pertinent tail-to-tail dimer.304

The reactions of SbCIs with l,l-diphenylethanol or 1,1,4,4-tetraphenylbutane-l,4-diol produce the SbCIsOH- counterions neutralizing the positive charge of the formed carbenium ions. Quenching these salts with methanol yielded the stoichiometric amounts of the respective methoxy derivatives identified by their NMR spectra, an observation confirming the proposed structures of the parent cations. In contrast, a mixture of some unidentified products was formed on quenching the dimeric dications produced by oxidizing the l,l-diphenylethylene with SbCIs. Apparently, the counterions formed in its oxidation differ from those produced in the reactions of SbCIs with the alcohols. These unidentified counterions seem to interfere, in some unknown way, with the quenching process.

The problem of the structure of the counterions is stressed here since it did not get sufficient attention in many studies of the kinetics of the cationic polymerization. In many of these studies the nature of the primarily formed products was postulated and discussed without considering the structure of counterions, whereas, as shown by this example, they may play an important role in determining the course of the subsequent reactions.

The formation of a similar dimeric dication through the electron-transfermediated oxidation of (MeOC6H4hC=CH2 by the aminium salt derived from N(C6H4Br)3 was reported by Ledwith.30s

The simple electron transfer is not the only process by which SbCIs may yield carbenium ions. For example, (p-CH30C6H4hC+ ·CH2CI carbenium ions are formed as the product of the reaction of SbCIs with (pMeOC6H4)2C:CH2, an olefin of very high electron affinity.306 The respective carbenium ions were identified by their absorbtion and NMR spectra and by the product of their deprotonation, namely the corresponding chlorinated ethylene derivative. Apparently, such ions, coupled to SbCli anions, are formed by transfer of CI + from SbCls to the above electron-rich olefin.··· It is debatable whether this process proceeds in one or two steps.

mAn alternative mechanism was proposed: chlorination of the monomer by SbCls yielding a chlorinated olefin and SbCI3, followed by its reaction with a second SbCls resulting in removal of Cl- and formation of SbCli.


Numerous cationic polymerizations were claimed to be initiated by chargetransfer processes. However, their mechanisms were not fully elucidated, nor was any hard evidence produced supporting the proposed course of the observed events. Moreover, many of the reported results could be bedevilled by various impurities. For example, oxygen is known to act as an electron acceptor, whereas most of the pertinent experiments described in the literature were performed in open reactors and not in high vacuum.

The formation of a charge-transfer complex between a cationically polymerizable monomer and a nonpolymerizable acceptor need not necessarily result in a polymerization, even when oppositely charged ions are formed. The chain propagation might be prevented by the annihilation of the growing centers by their charged companions, the counterions. Hence, a successful initiation of polymerization yielding a high mass polymer requires a low nucleophilicity of the counterions in conjunction with a high reactivity of the growing positive centers.

The initiation of polymerization of N-vinylcarbazole (NVC) by traces of organic electron acceptors, such as chloranil, tetracyanoethylene, and so forth, was attributed to an electron-transfer process resulting in the formation of the monomeric radical-cations.307,308 The reaction was claimed to ensue spontaneously on the addition of these agents to the monomer solution. Since N-vinylcarbazole polymerizes readily by a radical or cationic mode, but not by anionic mechanism, the formation of its radical-cations seemed plausible under such conditions. The fate of the radical-anions expected to be formed simultaneously with the cations was not discussed. The radical-anions produced by the reduction of the above electron acceptors might act as the counterions; their oxidation could be prevented by their high electron affinity, especially since the electron affinity of Nvinylcarbazole is very low. This suggestion was explicitly adopted by Stille,322 who investigated a similar cationic polymerization of vinyl ethers induced by dichlorodinitrile-quinone, a compound of a very high electron affinity.

Doubts were expressed about the soundness of the electron transfer initiation, in spite of its apparent plausibility. For example, Shirota309 found the meticulously purified chloranil ineffective as the initiator of N-vinylcarbazole polymerization, contradicting its claimed activity. It was concluded that some other mechanism, and not the direct electron transfer, is responsible for the initiation of this most intriguing polymerization.

Several suggestions were made. A monomer and an electron acceptor capable of being copolymerized may form a zwitterion, a kind of a composite monomer that polymerizes spontaneously and yields an alternating copolymer. Of course, this mode of polymerization requires equivalent amounts of both ingredients, and cannot account for the polymerization initiated by small amounts of the acceptors. Remar~ably, a radical-induced polymerization of mixtures of N-vinylcarbazole and fumaronitrile yields an


alternating 1:1 copolymer, whereas only the homopoly-N-vinylcarbazole is formed in the absence of a radical source.

The most attractive is the suggestion that some impurities either present in the supposedly active initiator, or formed on mixing it with the monomer, are the actual polymerization inducing agents. Since N-vinylcarbazole is readily polymerized by protonic acids, attention was drawn to the reactions leading to the acids' formation. A specific process, described by the fol-

lowing scheme in which )N-CH=CH2 denotes N-vinylcarbazole, was

proposed by Shirota31O:

, EEl ~ [crC]~ ie CH2-CH ~ polymer '... I Cl ! I N

o ./ .......... !

X<0 H=~H ./ +M

N .......... + HCl~ polymer CI I

o

The formed HCI initiates the reaction and, as is known, even small traces of this acid can polymerize large amounts of N-vinylcarbazole. The absence of chloranil in the produced polymers, if rigorously proved, would support the proposed mechanism. On the other hand, a gelation, if observed on the addition of vinylcarbazole to a solution of a polymer having attached to it chloranil moieties, would provide evidence for a mechanism postulating propagation ensuing from the positive end of the initially formed chloranil--NVC + zwitterion.

Elimination of HO, or some other small molecule, from a donor-acceptor complex may be a common feature of these reactions. Thus Ledwith311

accounted for the reaction of N-vinylcarbazole (denoted again by

)N.CH=CH2) with tetracyanoethylene acceptor by the scheme:

~ ~ + /N.CH=CH2 + (CNhC=C(CNh -+ /N. CH~H2

(CN)2C~(CN)2 -+ )N.CH=CH-CCCN)=C(CNh + HCN.

A similar elimination was observed earlier by Foster312 who reacted dimethylan-


iline with tetracyanoethylene and isolated the (CH3)2N·CJi4·C(CN)=C(CNh as the reaction product. Both eliminations proceed via a charge-transfer complex.

Most of the polymerizations initiated by some fragment eliminated from charge-transfer complexes require its irradiation; on the whole they do not take place in darkness. An example is provided by the complex of amethylstyrene and tetracyanoethylene.313 Its solution in dichloroethane remains quiescent in the dark but polymerizes at - 30°C on irradiation by light of A > 300 nm, and then yields a highly syndiotactic polymer of high molecular weight. (The propagation constant of this reaction was determined by flash photolysis in the nanosecond region.314)

The reaction of tetranitromethane with 1,1-diphenylethylene315 provides another well-documented system of that kind. A charge-transfer complex absorbing at A=410 nm is formed instantaneously on mixing these reagents in nitrobenzene. Its slow decomposition changes the absorbance and eventually a band appears at A=385 nm. Yellow crystals were isolated from this solution and identified as 1,l-diphenyl-2-nitroethane. Their formation is accounted for by the scheme:

Ph2C:CH2 + C(N02)4 ~ Ph2C+·CH2·N02 + C(N02h ~

Ph2C:CH·N02 + HC(N02h,

implying the transfer of the positive N02 + moiety from tetranitromethane to the olefin, followed by the deprotonation of the intermediate carbenium ion and the formation of nitroform. The postulated formation of this intermediate carbenium ion was verified by performing the above reaction in the presence of methanol. Under these conditions the expected methoxy derivative

was produced and isolated, therefore confirming the proposed scheme. The transfer of NOt moiety from tetranitromethane to an olefin seems

to be a general reaction.316 For example, the addition of tetranitromethane to a solution of a-methylstyrene in deuterated methanol yields quantitatively the respective nitro-olefin.315 The course of this process was followed by NMR. It proceeds presumably via the intermediately formed

+ 02NCH2· C ·CH3,C(N02)3

I Ph

carbenium ion which is rapidly deprotonated.


Further evidence for the formation of carbenium ions in the reactions of tetranitromethane with olefins was provided by the studies of oxetane polymerization induced by the mixture of tetranitromethane and 1,1-diphenylethylene. Tetranitromethane alone does not induce the cationic polymerization of this cyclic ether, whereas the addition of 1,1-diphenylethylene to a quiescent nitrobenzene solution of oxetane and tetranitromethane initiates the polymerization.315 Apparently, either the carbenium ion produced by the NOt transfer to 1,1-diphenylethylene or, more likely, the nitroform resulting from the deprotonation of the carbenium ion is the initiating species.

The polymerization ensuing on the addition of tetranitromethane to a solution of N-vinylcarbazole in nitrobenzene was studied by Pac and Plesch.317

The rate of this polymerization, measured by the rate of the monomer consumption, was reported to be first order in the monomer and the initiator, and the molecular mass of the resulting polymer was found to increase with the initial monomer concentration, being independent of the initiator's concentration. The latter observation was accounted for by a frequent chain-transfer to monomer. A conventional charge-transfer mechanism was postulated for this polymerization, namely, the oxidation of the monomer to its radical-cation by tetranitromethane, followed by the addition of the excess of monomer to the positive end of the formed radicalcation (propagation). The radical end of the reaction initiating radicalcation was assumed to be trapped by the excess of tetranitromethane (?), while the fate of the tetranitromethane radical anion was not discussed. Finally, the propagation was assumed to be terminated by the "wrong" monomer addition.

Although this mechanism accounts for all the observations it does not seem plausible. It postulates the questionable "trapping" of the radical end and does not account for the fate of the anion. An alternative mechanism postulated by US318 and supported by the previously discussed findings accounts for the facts reported by Plesch, and for some new observations. Since N-vinylcarbazole is readily polymerized by protonic acids, and nitroform is a relatively strong acid, the observed polymerization of vinylcarbazole seems to be initiated by nitroform produced through the sequence of events described in the preceding paragraphs. The plausible deprotonation of the positively charged end-group of the growing polymer by its counterion, C(N02h, produces the acid that initiates a new chain, thus accounting for the chain-transfer.

Strong protonic acids seem to be formed often in the course of the oxidation of monomers to their radical-cations, and then the acid, and not the radical-cation, may initiate the observed polymerizations. The polymerization of vinylcarbazole initiated by chloranil, discussed on p. 253, seems to exemplify such a process.


7.9. Initiation of Radical Chain Process by Electron Transfer

Some highly polar organic salts are still not sufficiently reactive to initiate polymerization by oxidizing monomers. However, they may oxidize radicals and convert them to diamagnetic cations since the ionization potential of radicals is substantially lower than that of aromatic or olefinic compounds. The resulting cations may, in turn, initiate cationic polymerization. The ionization potential of radicals possessing electron-donating substituents is especially low. This makes them the most desired substrates for such electron-transfer reactions.

Polymerization of tetrahydrofuran induced by the photolysis of diazonium salts319 provides an example of such a process. It leads to polymers of DPn - 70 and its mechanism is described by the following scheme:

o OCH3 II . I

Ph-C-C-Ph ~ I OCH3

~ iCH3

Ph-C· + t::-Ph I OCH3

OCH3 gcH3

Ph-f +a-C6H4-N2+'PF6-~Ph-C\~ +CI-C6I4 +PF6-+N2.

OCH3 OCH3

and

+ PhC=O + PF6" + CI-C6H: + Nz.

The transfer of a hydride ion from THF to the resulting cations initiates the cationic polymerization of that ether. Let it be noted that each formed radical produces a carbenium ion and another radical which may perpetuate a chain of the initiation process by decomposing another molecule of the diazonium salt. The quantum yield of such an initiation may be high and it is amplified by the high quantum yield of the subsequent propagation.

Halonium or sulfonium salts with bulky counterions, such as PF6", AsF6" , and SbF6" , react in a similar way.320 These salts are thermally stable, in contrast to the diazonium salts. Their action is illustrated by the


scheme,

hv

R2 ~ 2R·, (R2 is a photolyzed radical generating species).

followed by propagation resulting from the addition of R + to the monomer. Alternatively, the initiation of propagation may result from the formation of a strong protic acid, e.g.,

hv

Ph2·COH-COH· Ph2 ~ 2Ph2COH,

and the addition of the proton to the monomer initiates again its cationic polymerization.

The photodecomposition of the onium salts can be sensitized to allow for the photolysis at wavelength longer than 360 nm. Dyes such as acridine orange and benzoflavine are useful for this purpose.320c The sensitization of triaryl sulfonium salts by perylene and other polynuclear hydrocarbons permits the photolysis to be performed by incandescent light sources.32Od

The feasibility of amplification of the events initiated by the reactions of the photochemically produced radicals with the onium salts is the most interesting feature of these processes. A photo-initiated radical polymerization amplifies the quantum yield since a propagation, a chain reaction, ensues after each successful absorption of a photon. In the process described here a double amplification is possible. The formation of an active species, whether R + or a strong acid that initiates the cationic propagation, is accompanied by the creation of another free radical capable of producing another species initiating the cationic propagation and, simultaneously, another free radical is formed. Such a possible chain of initiation steps is unique for this kind of process, being impossible in the conventional radical chain reaction where only one radical is formed as the other one disappears. Both chain reactions eventually have to be terminated by some events.

The radicals formed in the above process may also initiate a conventional radical polymerization provided that a suitable monomer is available. This was demonstrated320e by the irradiation of Ph3S + ,SbFi in an equimolar mixture of 1,4-cYclohexene oxide and methylmethacrylate. Two homopolymers were produced: the cationically growing polycyclo-oxide and the radical propagated polymethylmethacrylate. The same system containing


2,6-di-t-butyl-4-methylphenol, a radical inhibitor, yielded polycyclo-oxide only, whereas only polymetylmethacrylate was formed in the presence of trimethylamine, an inhibitor of cationic polymerization. The monomers such as glycidylacrylate or methacrylate, capable of undergoing cationic or radical polymerization, yield a cross-linked insoluble gel under these conditions.

A direct photolysis of, say, an iodonium salt, is a more conventional process. Its course is described by Crivello and Lam320 by the sequence of steps:

hv

Ar2I+,X- ~ (Ar2I+,X-)* ~ ArI+' + X- + Ar·,

followed by some reaction which converts the radical-cation into a strong protic acid, e.g.,

ArI+·,X- + SvH ~ ArI + HX + Sv·,

where SvH denotes the solvent. The acid, in turn, initiates a cationic polymerization. Note, however, since a radical, Ar' , is formed in the course of these reactions, the previously discussed chain initiation might still ensue.

7.10. Two Simultaneous Homopolymerizations

In a paper devoted to the investigation of radical copolymerization of vinylidene cyanide with a variety of monomers, Gilbert and Miller321 reported an unusual behavior of the pair, vinylidene cyanide-vinyl ether. The authors proposed a mutual co-initiation of anionic polymerization of vinylidene cyanide and cationic polymerization of the vinyl ether, both proceeding in the same solution. This phenomenon was reinvestigated and confirmed by Stille322 who was astounded by the simultaneous growth of two homopolymers, one anionic and the other cationic, in the same reactor.

Notwithstanding the novelty and the interesting features of this reaction, it should be stressed that the coexistence of two oppositely charged species in the same solution is a common and necessary feature of all ionic reactions. The lack of a permanent linkage between a well-solvated anionic end of a growing polyvinylidene cyanide and a solvated cationic end of a growing polyvinyl ether might be not as surprising as it appears. Moreover, it was suggested323 that the two incompatible polymers may form separate micelles and thus a microphase separation could take place.

The thorough studies of Stille322 revealed the following facts. 1) Mixing of vinylidene cyanide with vinyl ether, or with some similar monomer, e.g., dihydropyran, results in the spontaneous formation of three products: a homopolyvinylidene cyanide, homopolyvinyl ether, and a cycloaddition dimer, the latter being produced in a relatively low yield. 2) NMR analysis


proved the head-to-tail structure of the homopolymers, and the head-tohead bonding of the cyclodimer,

/CN CH2-C",

I I CN.

CH2-CHOR

3) No block copolymers were produced. 4) Neither the dead polymers nor the cycloadduct initiate polymerization of either of the monomers. 5) Increasing the concentration of one of the monomers increases the weight of its respective homopolymer without affecting the yield of the other one. It appears that some event produces two independently growing centers.

A comprehensive review of reactions taking place between electron-rich and electron-poor monomers has been published recently. 324 As pointed out by the author of this review, substituted tetramethylenes are produced in these processes. Tetramethylenes are extremely reactive intermediates existing as hybrids of diradicals and zwitterions that could cyclize and form four-membered rings, subject to restrictions imposed by the parity rule and steric considerations. The diradicals are coplanar and relatively readily cyclized, while the nonplanar zwitterions are prompt to form alternating copolymers. Table 3.4 shows the transition from the systems yielding diradicals to those favoring the formation of zwitterions.

It seems that another process may still occur on the association of two monomers of vastly different polarity, namely the intramolecular transfer

Table 3.4. Acceptor-donor relationship

E

O==<S=O r< o E E

~ DR DR

\ DR 00Me ...

\OR § DR

\ : DRIZI \NCz .~

~ NR2 ~ S

.S ~

J ECNNCCN \J "-I

1\ 1\ 1\ E CN E CN NC CN

increasin acceptor ability

DR DR

DR DR ZI

DRIZI ZI

DR ZI ZI

ZI ZI

E = COOCH3 ; DR = diradical; NCz = N-carbazolyl; ZI = zwitterion


of a hydride ion may yield two diamagnetic but oppositely charged ions. For example, the associate of vinylidene cyanide and vinyl ether could produce an (ROC:CH2)+ cation and the (CH3C(CN)2)- anion. The high electron affinity of vinylidene cyanide promotes its addition to the negative anion producing a polymer chain possessing the dead methyl group on one of its ends and the active carbanion on the other. The easily oxidized vinyl ethers react with the positive cations, and again a polymer chain is formed with the dead vinyl end-group, whereas the active carbenium ion forms its other end. This scheme accounts then for the two homopropagations that ensue in this process with the respective homopolymers growing as free ions. Their association into the reversibly formed ion-pairs are hindered by the incompatibility of the polymers, as suggested by Chapiro.323

Alternatively, the two components of the complex may form a cyclic dimer; however, a high activation energy of this process, due to the steric strain, reduces its yield.

The anionic character of the vinylidene cyanide polymerization was confirmed by its inhibition on addition of phosphorus pentoxide, a test verifying the anionic nature of a polymerization, whereas the inhibition of the polyvinyl ether growth by pyridine, a typical inhibitor of cationic polymerizations, confirmed the cationic character of the latter polymerization.

Similar systems may behave differently. For example, addition of trioxane to vinylidene cyanide initiates the polymerization of the latter monomer but not of the former. Apparently, the cation formed from trioxane by a loss of a hydride ion is too stable to induce the polymerization of the parent monomer.

The interaction of N-vinylcarbazole and oxetane in nitrobenzene solution was reported to yield two homopolymers and not a block copolymer. 318

The reaction was initiated by the addition of tetranitromethane, but the eventually formed nitroform seems to be the actual initiator. The progress of the reaction was followed by NMR, showing a simultaneous consumption of both monomers. In spite of some questionable problems, the proposed mechanism supported by the results of this work deserves consideration. Two, not copolymerizable monomers may compete for the same initiator, in this case for nitroform. The resulting polymers may grow independently of each other, each being fed by its respective monomer. For example, the growing poly-N-vinylcarbazole does not react with oxetane, and the growing polyoxetane does not interact with vinylcarbazole. Consequently, the rate of polymerization, but not the molecular weight, of polyvinylcarbazole decreases on the addition of oxetane because then a large fraction of the initiator is consumed then by the latter monomer.

A tentative answer was given to the question of why a growing polyvinylcarbazole does not react with oxetane and the growing polyoxetane does not interact with the carbazole. The growing polyoxetane is an ox-


onium ion, not sufficiently reactive to add a vinyl monomer although its nucleophilic addition to the very basic oxetane does take place. Polyvinylcarbazole could be treated as an ammonium ion, highly inert. Nevertheless, it initiates the polymerization of vinylcarbazole due to the operation of an additional driving force arising from the high polarizability of the planar vinylcarbazole molecule by the planar polyvinylcarbazole cation. No interaction of that kind is taking place between polyoxetane cation and vinylcarbazole, and hence the copolymerization is again prevented.

The lack of cross-propagation in the system N-vinylcarbazole and oxetane was confirmed by the interesting results reported later by Turchi and his colleagues.325 They initiated a rapid polymerization of N-vinylcarbazole in methylene chloride by an oxonium salt, 0+(~H5h,PF6 labeled by 14C.

No radioactivity was found in the produced polymer initiated by a radioactive oxonium salt, demonstrating that the polymerization was not initiated by the expected transfer of C2Ht moiety to the monomer. However, repetition of this reaction performed in tetrahydrofuran led to the formation of polytetrahydrofuran, as well as polyvinylcarbazole, but not to any copolymer. Neither polymer showed any radioactivity. The initiation of tetrahydrofuran polymerization resulted from a hydride abstraction from the monomer by the ~Ht ion, and the polymerization of the vinylcarbazole seem to be initiated by PFs formed by the spontaneous decomposition of the oxonium salt. The lack of cross-propagation between THF and N-vinylcarbazole is confirmed.

This work is instructive. It shows how the apparently convincing evidence could be misleading. The inability of the oxonium salt to initiate polymerization of vinylcarbazole, a reaction initiated by carbenium ions,311 implies that the ethylation by the oxonium salts is a concerted reaction, the C2Ht moiety is not a fleeting intermediate formed in this process.

7.11. Field Emission, Field Ionization, and Electrolysis

We close this survey of electron-transfer initiation of ionic polymerization by considering the effects of strong electric fields on a monomer. The high gradient fields, > 107 V/cm, developed around a negatively charged sharp needle or blade act as an electron source, i.e., we deal here with field emission. On the other hand, a positively charged needle or blade behaves as an electron sink and the field ionizes the neighboring molecules, i.e., now we deal with field ionization.

The field emission device is preferred for the initiation of anionic polymerization; monomers of high electron affinity capture the electrons formed in the vicinity of the electrode and are reduced to the radicalanions. The question arises as to how the counterions are formed. The emitted electrons and the repelled anions have a sufficiently high kinetic


energy to ionize or rupture the molecules surrounding the electrode, and positive ions, radicals, and so forth, are formed in the process, creating a plasma. Indeed, the charges in the overall neutral space arise from the ionization processes, while the field gradient supplies the needed energy. The emitted electrons are removed, directly or indirectly, through the electric current. Hence, a cationic polymerization, as well as an anionic one, could be induced, provided that a suitable monomer is available.

The field ionization dissociates the molecules around the electrodes by its powerful polarizing effect, creating a current of positive ions and a sink for the removed electrons. Again, the space around the electrodes is neutral but the direction of the current of the created ions is reversed. The energy for the various fission events comes from the kinetic energy of the ejected cations, as well as from that of the accelerated anions, both processes being facilitated by the high degree of polarization of the participating molecules. It is not surprising, therefore, that both kinds of polymerization are inhibited by scavengers of cations (H20, Et3N), as well as those of anions (SF6, N20).

To summarize, the discharge resulting from the electric gradient produces the fast moving ions and electrons, and their kinetic energy, supplied by the externally maintained electric potential gradient, permits them to ionize and rupture the molecules in the vicinity of the electrode. Hence ions and radicals are formed and they, in turn, may initiate the respective ionic or radical polymerizations whenever a suitable monomer is available. The results depend on the lifetime of the produced oligomers. The conditions are drastic and many modes of termination are possible. Similar conditions are encountered in a glow discharge, and indeed ions, as well as radicals, are formed by this technique.

The electrolytic initiation resembles the previous technique but the conditions are milder and amenable to rigorous control by varying the electrode potential. The anodic and cathodic compartments have to be separated by a porous membrane especially if formation of living polymers is envisaged.326 The participation of a supporting electrolyte in this process endows it with a new variable feature and allows for further modification of the overall reaction. Radical-anions and radical-cations were formed and studied by an electrolytic technique (see, e.g., the review by Bard and his colleagues, ref. 327). The application of electrolysis for polymerization has been reviewed by Breitenbach and Olaj.328

In spite of some of its attractive features, the electrolytic technique has many restrictions that limit its usefulness. The solvent must be capable of dissolving the monomer, the polymer, and a supporting electrolyte, and should allow for a measurable dissociation of the latter into free ions. It is beneficial if the solvent does not destroy the growing polymers, a necessary condition for the preparation of living polymers. In view of all these


difficulties the electrochemical initiation is confined at present to the realm of laboratory research, although future technological applications are not excluded.

8. Polymerization Initiated by High Energy Radiation

8.1. Ionizing Radiation

Irradiation of matter, whether gaseous, liquid, or solid, by'Y rays, hard X rays, or fast electrons leads to its ionization. The energy of these ionizing agents, known collectively as the ionizing radiation or high energy radiation, amounts to hundreds or even thousands of electron volts and exceeds by powers of 10 the amount of energy needed for the ionization of any molecule. Therefore, the high energy radiation is indiscriminate in its action and ionizes every encountered molecule of the irradiated matter. In this respect radiolysis, the process induced by the 'Y irradiation, differs from the conventional photolysis of a solution. In the latter reaction the light is absorbed by a solute, whereas in radiolysis the energy of the radiation is absorbed mainly by the solvent.

The ionizing corpuscle loses only a small fraction of its energy as it penetrates the irradiated matter. A 'Y photon passing through a liquid collides, on the average, with the molecules of the irradiated fluid every 10-15 s, and deposits in each collision 50-100 eV of its energy into the encountered molecule. Such a collision results in the ejection of an electron from one of the deep electron shells of the encountered molecule, leaving behind a positively charged molecular ion. The ejected electron, referred to as the primary electron, is endowed with a large kinetic energy and may ionize on its way the neighboring molecules, producing in this process secondary elel:trons and the respective positive ions. The secondary electrons still possess a substantial kinetic energy and may continue the ionization process, but ultimately all the ejected electrons are trapped by the surrounding molecules or molecular fragments of positive electron affinity, yielding the negative ions, or they are stabilized by "bubbles," the empty spaces between the molecules of the irradiated liquid, becoming solvated electrons.

In summary, a 'Y photon traversing a liquid leaves along its track sparingly distributed clusters of solvated electrons and positive and negative ions, 5-50 A in diameter, known as spurs.

Most of the electrons and ions in a spur are annihilated within 10-7 s by the geminate association of the oppositely charged particles, and the energy liberated in this process causes the excitation and fragmentation of the molecules of the irradiated matter. Thus free radicals are formed and many of them diffuse into the bulk of the liquid. On the other hand, only


very few of the initially formed ions have a chance to diffuse out of the spurs into the bulk of the liquid, and these are referred to as the free ions capable to initiate ionic reactions.

At a steady rate of irradiation, an equal number of positively and negatively charged species diffuse out of the spurs and become homogeneously distributed throughout the liquid which has to be always electrically neutral. The lifetime of the free ions is much longer than that of the ions in the spurs because their stationary concentration is exceedingly low. In fact, at the conventional rate of irradiation the stationary concentration of the free ions in the bulk of a meticulously dried and purified liquid is about 10-10 M and their lifetime, T, determined by the rate of diffusion, is about 1 ms.

It is customary to measure the intensity of the 'Y radiation, or the dose rate, in Mrad/h or rad/s, 1 rad = 100 ergs of the absorbed 'Y radiation per gram of irradiated substance (the modern unit is joules per kg). The irradiation causes the formation or destruction of some products, its yield being given as its G value, i.e., the number of molecules of the product formed or destroyed on absorbtion of 100 eV of'Y radiation. The G(i) denotes the number of free ions formed by the absorbtion of 100 eV of radiation; their values, specific for each substance, were tabulated by Allen.329

8.2. Radiolysis of Monomers

The irradiation of liquid monomers by 'Y rays leads to polymerization. This reaction, studied since, at least, 1938 by Dainton and others, was proved to proceed by radical mechanism. It was surprising, therefore, when Davison et al.330 reported in 1957 a successful polymerization of liquid isobutene initiated at -78°C by 'Y rays. Isobutene polymerizes only by a cationic mechanism, neither radical nor anionic propagation of this monomer is possible. Hence the above report demonstrated, for the first time, the feasibility of a cationic polymerization induced by 'Y rays at very low temperatures.

Considerable research by a number of workers, notably Williams, Okamura, Hayashi, and Metz, led to a theory of this kind of polymerization. 331 Briefly, it has been proposed that those few positive molecular ions that escaped the mutual annihilation in the spurs and diffused out into the bulk of the monomer initiate its cationic polymerization propagated by the free cations. Ion-pairs do not participate in this polymerization, as was pointed out by Szwarc,332 since the stable pairs are formed only with the judiciously chosen anions of low nucleophilicity, whereas the anions present in the irradiated system readily destroy the cations and neutralize their charge. Two modes of termination are visualized for the ensuing polymerization: the association of the growing polymers with the anions, or their reaction with impurities, notably with moisture, that converts the reactive cations


into inert dead polymer, e.g.,

+ IW\CH2·C+Me2 + H20 -+ IW\CH2·CMe20H2 -+ IW\CH CMe2 + H30+ ,

followed by the regeneration of water on reaction of H30+ with anions, e.g.,

The latter reaction compounds the deleterious effect of water; its traces cannot be removed by the mere irradiation of the polymerizing system. In this respect the action of water is unique, distinct from the retardation of radical reactions by a variety of radical inhibitors. The latter are consumed, and not regenerated in the course of the inhibition process. However, at higher concentrations of water the initial adduct may react with another water molecule and in this process the concentration of water is reduced:

+ IW\CH2·CMe20H2 + H20 -+ IW\CH2·CMe20H + H30 +.

It may be remarked that under conventional conditions water is dimeric, (H20)z, and at equilibrium its dissociation provides minute amounts of the monomeric H20 at exceedingly low concentration. The dimeric water is partially scavenged by the growing carbenium ions:

and the remaining dimers, at the concentration of about 10-10 M or lower, are virtually quantitatively dissociated into the monomeric water.

In view of the exceedingly low concentration of the growing cations, the polymerizing system has to be rigorously dry to suppress the termination by any residual water. The "super" dry technique must be applied; the reactor has to be baked in high vacuum at 500°C for a day or two to desorbed traces of moisture from its walls. In the inadequately purified system, the rate of polymerization is proportional to the intensity of radiation, since the termination arises from the reaction of growing polymer cations with the water present in a large excess ([H20] » 10- 10 M). Indeed, as the drying of the reagents has been improved, the poorly reproducible (due to the varying amount of moisture) G(m) of the monomer converted into its polymer increased substantially.

The lack of appreciation of the effects caused by traces of water led to an interesting controversy. It arose from the observation334 of a marked


enhancement of the polymer yield on incorporation of a fine suspension of inorganic oxides, especially zinc oxide, into the irradiated monomer. It was argued334 that the suspended oxide traps the free electrons and thus prolongs the lifetime of the growing cations. It was realized, eventually, that the finely dispersed oxides adsorb the traces of water that retard the polymerization and thus "accelerate" its rate.

Under the most meticulous drying conditions the yield of the polymer is proportional to the square root of the radiation intensity, implying a termination resulting entirely from the collisions of the growing cations with the solvated electrons or the oppositly charged ions. On the other hand, in a wet monomer the rate is proportional to the first power of the radiation intensity since the termination is first order in the monomer. Therefore, the proportionality of the rate of polymerization with the square root of 'Y rays' intensity is considered as the evidence of purity of the reagents.

The presence of the long-lived ions or solvated electrons is evident from the electric conductance of an irradiated liquid. The conductance provides a measure of the ion concentration, namely the specific conductance K is proportional with the number n of the free ions present in unit volume of the irradiated liquid:

K = n.e.(J..L+ + J..L-).

Unfortunately, the limiting conductances J..L+ and J..L- of the pertinent ions are unknown and have to be estimated. Their reasonable estimate335 allows one to calculate the yield of the free ions formed on absorbtion of 100 e V of radiation. For common hydrocarbon monomers the respective G(i) -0.1, compared to the G value of 2-3 for the ions produced in the spurs.

8.3. Kinetics of Radiation-Induced Polymerization

The rate of polymerization, Rp , of a steadily irradiated and meticulously purified monomer is given by the Hayashi-Williams equation:

where kp is the propagation rate constant, kim the termination rate constant, and R; the rate of initiation equal to the steady rate of formation of the free ions:

R; = 6.24 . 1014 • G(free ions) . I . 8/N,

I being the intensity of the radiation in radls; 8, the density of the monomer;


and N the Avogadro number. The coefficient 6.24 . 1014 gives Ri in units M·S- 1•

Alternatively,

where T, the liftime of the free ions, is given by lIktm . n = lI(Ri' ktm)ll2. The lifetime T is related to the specific conductance K of the irradiated

liquid by the Langevin equation:

K = E/( 4'lT . T),

where E denotes the dielectric constant of the liquid. Hence, the lifetime of the free ions T and the termination rate constant krm could be deduced from the conductance study. In the systems rigorously free of impurities, the value of ktm can be estimated also from the simplified Oebye relation:

where e denotes the electron charge, and D is the diffusion constant of the recombining ions.

The contribution of the ions formed in the spurs to the overall polymerization is negligible, their lifetime, -10- 7 s, is too short to allow for the formation of a long polymer.

The nature of the initiating cations is unknown. To our knowledge there are no reports on studies of the end-groups. Such investigations would be difficult and probably not conclusive. However, since the yield of free radicals, R', is substantially larger than the yield of the free ions, it is probable that the reaction of the radicals with the primary or oligomeric cations produces the R + or RM + cations which propagate and continue to grow.

The irradiation of the hydrocarbon monomers in bulk results in long kinetic chains of propagation, although the degree of polymerization of the resulting polymers is relatively low, indicating a large contribution of chain-transfer to the overall reaction. For example, the irradiation of liquid isobutene at the dose rate of 70 rad/s resulted in the kinetic chain length of - Hf, while the degree of polymerization was -104• It should be stressed that the determination of OPn of the produced polyisobutene requires extrapolation to zero intensity of irradiation to correct for the degradation and perhaps also for a possible grafting of the polymers caused by the irradiation.

Radiation-induced polymerization of cyclopentadiene revealed an interesting pitfall. The irradiation at -78°C of samples of monomer left standing


for a while at room temperature led to abnormally low yield. This phenomenon is caused by the spontaneous thermal dimerization of this monomer yielding the dimer known to be an efficient retarder of its polymerization.

The cationic polymerization is inhibited by bases such as trimethylamine, ammonia, diethyl ether, and water, listed in the order of decreasing efficiency. They act as proton acceptors converting the growing N\i\CH2C+XY polymer into a dead N\i\CH=CXY one. Trimethylamine totally inhibits the cationic polymerization of vinylmonomers at a concentration as low as 10-3 M. Since it does not affect either the anionic or radical polymerization, its inhibitory action proves the cationic nature of the investigated, 'Y rayinduced polymerization of the meticulously purified monomers such as styrene and a-methylstyrene.

Studies of retardation of polymerization induced by 'Y rays provide data helpful in the determination of the rate constant of propagation, kp' For a retarded system the Hayashi-Williams equation acquires the form:

where X denotes the inhibitor and kx the rate constant of its inhibitory action. At constant rate and time of irradiation, inversion of the above equation yields

where (Rp)o is the rate of polymerization in the absence of the inhibitor. It follows that a plot of lIRp vs. [X]/[M] is linear with slope (k)kp)/Ri' Hence kp could be evaluated provided that Ri and kx are determined independently.

Polymerization of super-dried styrene and a-methylstyrene initiated by 'Y rays proceeds to a large extent by a cationic mechanism.335 This is evident from the pronounced effect of trimethylamine on its rate of polymerization. However, styrene polymerizes also by the radical and anionic mechanisms. A question arises as to why the cationic mechanism is favored in the polymerization induced by 'Y radiation. It seems that the propagation constant is by far the largest for the free cations, much larger than for the free anions or radicals. In other words, the interaction of the 11' system of those monomers with the free carbenium ion is much stronger than its interaction with a free carbanion or a free radical. This seems plausible. Therefore, although an equal number of carbenium ions and carbanions are formed by the 'Y radiation, the weight of the cationically growing polymers exceeds by far (at least 1O-fold) that of those formed anionically.


The bimolecular propagation constant of the free cations of styrene in the bulk of the monomer is reported336 to be 3.5 . 106 M-1S- 1 at 15°C, whereas the respective constant of the free radical propagation is less than 40 M- 1S-t, and that ofthe free carbanions in ethereal solvents -lOS M- 1S- 1•

The propagation rate constant of the free carbanions in the bulk of styrene is unknown, but its value may be estimated from the data reported by Ueno et al.,335 who determined Rp of styrene polymerization when its cationic growth was totally inhibited by trimethylamine. The polymerization observed under such conditions should be attributed to the anionic growth. It is -100 times slower than that of the uninhibited reaction, and hence the kp of anionic propagation of styrene in its bulk is - 3 . 1()4 M -IS -1 ,

apparently slightly slower than in ethereal solvents. The molecular mass of the polystyrene produced by cationic growth of

the irradiated monomer is low in spite of the long kinetic chain of propagation, e.g., DPn = 500 at ooe was reported for a meticulously dried styrene, and lower -50 for a-methylstyrene polymerized under the same conditions. This is the result of a ready proton-mediated chain-transfer. The activation energies of these propagations are very low, virtually zero for hydrocarbon monomers but higher, about 20 kJ/mole for vinyl ethers.

The cationic propagation constant of a-methylstyrene is reported336 to be slightly higher than that of styrene, a strange finding since the considerable steric strain hindering the addition of this monomer to its polymer makes the propagation constant of anionic growth of a-methylstyrene substantially smaller than that of styrene. The propagation of isobutene is -40 times faster than of styrene (kp = 1.5 . 108 M- 1S- 1) and it is still faster for cyclopentadiene (kp = 6 . 108 M-1S- 1).

The 'Y irradiation of the monomers that are incapable of polymerizing by the cationic mechanism but readily grow by the anionic mode of propagation leads to their anionic polymerization. An example is provided by the polymerization of nitroethylene.337

8.4. Solvent Effects in 'Y-Initiated Polymerization of Vinyl Ethers

Solvents affect the 'Y-initiated polymerization in a complex and not easily discerned manner. Dilution of a monomer by a solvent might affect the G j because, on the whole, the G j is different for the monomer and the solvent. The dilution may influence also the rate of escape of solvated electrons and positive ions out of the spurs since the viscosity and the dielectric constant, E, of the milieu is altered by the addition of solvent. For the same reason the termination rate constant, kim, governing the mutual annihilation of the ions, is also modified. Finally, the addition of solvent affects the rate constant of propagation of the growing cations, kp,


by altering the nature of their solvation. This last effect is the subject of the following discussion.

The solvation of the ions affects their reactivity, sometimes profoundly. Therefore, the propagation constant kp is expected to vary as the composition of the milieu in which the polymerization proceeds undergoes a change. Such effects were systematically studied by Stannett and his coworkers338 who investigated the 'Y ray-induced polymerization of vinyl ethers in the bulk and in a variety of solvents. Unfortunately, no direct observation provides the value of kp, but Rp, the rate of conversion of monomer into polymer under the influence of 'Y radiation, is a readily accessible and experimentally reliable quantity that might reflect a change of kp •

The 'Y ray-induced polymerization of ethyl vinyl ether in bulk was investigated by Stannett's group.338 They found the plot of In([M]o/[MD vs. time, computed from the experimental values of Rp , to be linear up to 50% of monomer conversion. It is tempting to interpret this result as evidence of a simple pseudo-first-order polymerization proceeding with a constant kp at a constant stationary concentration of the cations. However, it is instructive to scrutinize this assertion.

The Rp is given by the Hayashi-Williams equation, Rp = kp[M] . (R;I ktm)ll2 and the linearity of the In plot could be expected provided that (R;I ktm)1I2 and kp are constant. The former factor gives the stationary concentration of the free cations responsible for the propagation. Had their concentration been truly constant as the monomer is converted into the polymer, then the linearity of the plot In[M]o/[M] vs. time would imply the constancy of kp. The putative constancy of kp, in spite of the continually increased volume fraction of the polymeric segments caused by the polymerization, is justified if the growing cations are solvated by the polymeric segments only, as claimed by the investigators, or when the presence of the polymeric segment in the solvation shell affects the cation's reactivity in the same way as the presence of the monomer.

The above conclusions hinge on the assumed constancy of the stationary concentration of the ions formed by 'Y rays. However, examination of the data reported by Stannett's group338 indicates that the G; varies with the composition of the irradiated liquid. Hence, the linearity of the In([M]ol [MD vs. time plot could be a result of a compensation of independent factors and therefore kp need not be constant. Moreover, the polymerization need not be a pseudo-first-order reaction. In fact, its kinetics could be zero order in monomer, provided that the cation forms rapidly and quantitatively a 'IT complex with the monomer, and the unimolecular rearrangement of the 'IT complex of an n-mer into (n + 1 )_mer339 is the ratedetermining step of the process.

The results of Stannett's studies of the polymerization of vinyl ethers initiated by 'Y rays in various solvents are the most intriguing. The addition


- 9

- 10

- 11

"" ~ .9

- 12

-13

-14

0.5 1.5 2 2.5

Figure 3.62. Dependence of the rate of polymerization on the concentration of ethyl vinyl ether (EVE) in CH2Ci2 solution. Open circles in bulk, closed circles in isodielectric mixture of EVE-CH2CI2-benzene. E = 5.0, T = 22°C, dose rate =

71 rad/s.

of small amounts of a diluent to a vinyl ether results in a relatively steep decrease of Rp , whereas further dilution causes a less drastic and apparently linear decline of the rate of polymerization. This is illustrated by the most striking plots of In Rp vs. In[MJo shown in Fig. 3.62 pertaining to the system ethyl vinyl ether-in methylene chloride.340 Apparently a small amount of this diluent saturates a critical site of the solvation shell of the growing cation and then strongly retards the propagation, whereas the remaining amount simply dilutes the system. This specific effect of methylene chloride may be appreciated on inspection of Table 3.5.

Table 3.5. Rates of polymerization of ethyl vinyl ether in bulk or in benzene solution on addition of small amounts of methylene chloride

[M]o·lIm [cI>H] . 11m [CH2Ci2] • 11m 106 ·Rp ·m/l·s DPn

5.4 5.5 0 8.3 85 5.4 5.4 0.25 4.9 29 5.4 4.4 1.5 4.5 26 5.3 0 7.7 4.4 21


Alternatively, the diluent may disrupt an ordered structure of the solvation shell that favors propagation. Stannett and his colleagues suggest that an intramolecular coordination of the growing carbenium ion with the o atom of the polymer segment, second one from the active end, results in a six-membered ring favoring the propagation. The addition of the diluent disrupts this structure and thus retards the growth. Although the formation of the proposed six-membered complex is plausible, it is doubtful whether its formation favors the propagation. On the contrary, the resulting carboxonium ion should be less reactive than the original carbenium ion. Indeed, the formation of such ions accounts for the extremely slow polymerization of ethyl or isopropyl vinyl ether diluted by nitromethane.341

The complex nature of the dilution is evident from the slopes of the lines previously discussed. The slope of one is expected for a simple dilution of a polymerization being first order in monomer, whereas the slopes of such lines were found to be larger, e.g., 1.4.

It should be stressed that the experiments marked by black circles in Fig. 3.62 were performed in different but isodielectric mixtures, proving that the effects discussed here are not caused by a change of the dielectric constant of the milieux.

Interestingly, the action of diglyme341 differs from that of all the other investigated diluents whose addition retards the polymerization. In contrast, the addition of small amounts of diglyme to ethyl vinyl ether accelerates its polymerization, although further addition leads to retardation. This unusual behavior of diglyme, contrasting the action of diethyl ether, is illustrated by Fig. 3.63. It seems that the coordination of diglyme with the growing cations weakens their interaction with the anions. Consequently, the stationary concentration of the ions initiating and propagating the polymerization is increased and therefore the rate of polymerization is accelerated. Indeed, the values of the Rp/[C+]'s computed for ethyl vinyl ether diluted by diethyl ether or diglyme fit the same line in the plot of In RJ[C+] vs. In[M]o·

In spite of the large variations of the rates of polymerization of 'Y-irradiated solutions of ethyl vinyl ether in different diluents, the respective activation energies are similar. The Arrhenius plots of In Rp determined for the polymerization of ethyl vinyl ether in bulk and those pertaining to the polymerizations performed in benzene solution or methylene chloride solutions produce virtually parallel lines. The respective activation energies were reported to be 5.3, 5.4, and 4.9 kcallmole, whereas the A factors calculated from the data obtained at 30°C are 8.2 . 109 , 4.2 . 109 , and 6.8 . 107. The solvent seems to affect strongly the entropy of activation, but only slightly the energy of activation.

Although the extensive work of Stannett's group revealed the intriguing features of the 'Y-induced polymerization of vinyl ethers performed in a


-10

~ ... I .,

! -11 Q" ~

.8

-12

Figure 3.63. Dependence of the rate of polymerization of EVE on its concentration in diglyme (~), in mixture of benzene and diglyme (~), and in bulk (e).

variety of solvents, still more work is needed to understand the mechanism of the action of the diluents.

9. Initiators Involving AI, Fe, Zn, and Other Metallo-Organics

9.1. Al-Zn Oxyalkoxides

A controlled hydrolysis of iron salts or of some metallo-organic compounds of AI, Zn, and so forth results in the formation of efficient and stereospecific initiators of epoxide, lactone, or lactam polymerization. Comprehensive studies of propylene oxide polymerization induced by such catalysts were first reported by Price and Osgan342 and independently by Tsuruta343 and by Vandenberg. 344 Subsequently, Teyssie and his co-workers345 embarked on a systematic investigation of the structure of these catalysts and concluded that a superior activity is achieved on the formation of bimetallic oxyalkoxy associates having the general structure345:

Met! stands here for AP + or Ti4 + , Met2 represents Zn2 + , crZ + , M02 + ,


Fe2+, or Mn2+, the subscript pis equal 2 for Al and 3 for Ti, and R is an alkyl group. These bimetallic associates aggregate further and their degree of aggregation depends on the nature of the R group and on the solvent, increasing with its decreasing polarity. For example, the AI,ZnlBu oxyalkoxides exist in benzene or cyclohexane solutions as octamers, whereas they dissociate in butanol into monomeric species. Their compact oxide nucleus surrounded by the lipophilic layer of the alkyl groups accounts for their remarkable solubility in hydrocarbons.

For the sake of illustration the degrees of aggregation of some oxyalkoxides in benzene and the colors of their solutions are given in Table 3.6.

The preparation of these bimetallic catalysts requires special procedures described by Teyssie,345 e.g., a thermal condensation of the respective alkoxides and carbonates performed at 200°C in a solvent such as decaline yields the active products. Thus prepared alkoxyoxides rank among the best catalysts of the ring opening polymerization of oxiranes, thiiranes, and lactones. These formed from AI(OBu)3 or AI(iso-propoxide)3 and zinc acetate appear to be the best.

The polymerization initiated by the oxyalkoxide has the character of an anionic-coordination propagation. The coordination of the monomer with the metallic center starts the process which is followed by the insertion of the coordinated monomer molecule into the aluminum-alkoxy bond simultaneously with the opening of the CH2-O bond of the monomer's ring:

RO 0 0 OR "'-./ "'-. /"'-./

Al Zn Al /1"'-. / "'-. / "'-.

RO 6 0 0 OR / "'-.

CH2-CH2

RO 0 0 OR "'-. /"'-. /"'-. /

Al Zn Al / "'-./ "'-./ "'-.

CH2-O 0 0 OR /

CH2-OR

The reactivity and selectivity of the catalyst may be substantially modified by varying the solvent and the R group.

The rate of polymerization decreases with increasing polarity of the solvent,346 a behavior reverse of that observed for the alkali alkoxides initiators, indicating the pseudoionic character of the reactions of AI-O derivatives. For example, while the polymerization of one half of the mon-


Table 3.6. Mean association degree of some bimetallic oxyalkoxides in benzene

Metals R group Mean degree of association Color

AlIZn n-Bu 8 for fresh solution, 6 for Pale Yellow solution aged 2 days

AlICo n-Bu 4.1 Blue AlICo iso-Pr 2.0 Red-violet

omer dissolved in toluene is accomplished in 180 s, it takes 1470 s to achieve the same result in chloroform, all the other conditions remaining the same. The reaction is 17 times slower in the presence of alcohol than in its absence. These observations imply that the monomer and the solvent compete for the active sites of the catalyst and therefore the lower the solvating power of the solvent the faster the reaction.

The rigidity of these catalysts imparts on them properties intermediate between those of the soluble catalyst and the solid ones, supporting the idea of topochemical control of these reactions. Their kinetics is simple, the rate is first order in the monomer and in the soluble catalyst, but it is second order for the highly aggregated catalysts. The DP n of the resulting polymers is high, e.g., a polymerization of methyl oxirane yields polymers of D P n - 105• The presence of 0-Bu and hydroxyl end-groups in hydrolyzed polymer chains confirms the assumed mode of monomer insertion into the AI-OR bond. The formation of cyclic oligomers, abundantly produced in the course of classic anionic polymerization, is prevented in this pseudoanionic polymerization, confirming its assumed character.

A thorough investigation of the action of these kind of catalysts revealed the participation of three distinct kinds of active sites in the propagation: the nonselective centers that produce oligomers by random opening of the rings, the still nonselective sites but yielding living polymers of high molecular mass, and the stereoselective sites producing high molecular mass isotactic polymers, the degree of their isotacticity varying from 50 to 80%, depending on the structure of the catalyst and the conditions prevailing in the reaction. The fraction of each kind of the active centers was determined by gradually poisoning them with lithium chloride.

The polymerization of e-caprolactone induced by the Al-Zn catalyst was proved to be living.346 The degree of its polymerization increases with the conversion, and the polymerization resumes when fresh monomer is added to a quiescent solution of polymers left after exhaustion of the previously supplied monomer. The reaction carried out in butanol, a solvent that dissociates the aggregates, yields polymers of a narrow molecular mass distribution (MwlMn -1.05). Under these conditions all four alkoxy groups


participate in the propagation and, therefore, the DPn of the product is given by [MJo/4[Zn].

Finally, the living character of this polymerization allows the preparation of block copolymers. The consecutive addition of two different lactones, e.g., caprolactone and j3-propiolactone, yields the expected two-block copolymer. The conversion of the monomers into the block copolymer is quantitative, and the two-block copolymer is formed with exclusion of either homopolymer, indicating the monomeric character of the catalyst. Otherwise, some sterically hindered and inactive OR groups could become available for the initiation as the second monomer is added, and then some homopolymer could be formed. Block copolymers built up of monomers belonging to different classes of compounds could also be produced. For example, consecutive addition of e-caprolactone followed by the addition of propylene oxide produces the relevant block copolymers.

Interestingly, these bimetallic catalysts are capable of incorporating carbon dioxide into the polymer chain, e.g., the incorporation of CO2 into a chain of polyoxirane yields some carbonate segments.

9.2. Pseudoanionic Polymerization of e-Caprolactone

Polymerization of e-caprolactone leads to the commercially important polyester whose most valuable properties are: its compatibility with a variety of polymers facilitating the formation of polymer blends and its biodegradability. Not surprisingly, much work has been devoted recently to the studies of the mechanisms of its formation.

The polymerization of this cyclic ester is readily initiated by a variety of bases such as carbanions, alkoxides, and HO- anions. Under such conditions the propagating end-groups are the carboxy or carboxylate anions or their ion-pairs. Such anionic end-groups attack the polymer segments of their own chain and then form the cyclic polyesters, e.g.,

X--CO'O<fH2~

+ -a.co)

The incorporation of the cyclic polyesters into the linear ones deteriorates the beneficial properties of the latter polymers, and therefore many attempts were made to minimize the cyclization process. The proportion of the cyclic polyesters decreases with increasing initial concentration of the monomer, but even in a polymerization proceeding in the bulk, i.e., in a undiluted monomer, their proportion is still too high. Several investigators reported that the polymerization of e-caprolactone initiated by the catalysts possessing aluminum oxide bonds lessens the extent of this undesired cyclization. For example, Teyssie reported345 that the polymerization of e-


caprolactone initiated by AI(O-iso-Prh yields polymers with very low proportion of the cyclic polyesters, and a product free of cyclics is produced by the bimetallic catalysts yielding living polymers. A similar claim was made by Inoue349 who utilized the derivatives of aluminum porphirines as the initiator (see p. 382). Let it be stressed, however, that any alteration of the catalyst cannot change the ultimate, i.e., the equilibrium, composition of a mixture formed by some competing reversible reactions. Nonetheless, when two or more reactions are involved in a reversible polymerization yielding living polymers the choice of the catalyst may affect the composition of the product formed at the time of a virtual exhaustion of the monomer. Hence kinetic studies are helpful in controlling the outcome of such a process.

A thorough kinetic study of the polymerization of e-caprolactone initiated by dialkyl aluminum alkoxide, R2AlOR, was reported by Penczek and Duda.347 The ensuing polymerization, performed in toluene solution at 20°C, yields living polymers as indicated by the proportionality between the calculated DPn = [M1o/[AI1o and the experimentally determined one, as well as by the relatively narrow molecular mass distribution (DPwIDPn

< 1.15). It was established that the initiation and propagation involve the insertion of the monomer into the AI-O bond since the respective alkoxy groups appeared at the end of the formed polymer chains, a feature, as stated earlier, common to all the polymerizations initiated by the catalysts possessing the AI-OR or Zn-OR groups. The lack of the cyclic products (even at 99% of monomer conversion) implies the absence of the terminal ions or ion-pairs in the ensuing process, since these are the species engaged in the intra- or intermolecular reactions resulting in the formation of the cyclic polyesters. Indeed, while the reaction

is plausible, no feasible reaction of these catalysts with any segment of a polymer chain could be proposed that would lead to cyclization.

The kinetics of polymerization of e-caprolactone initiated by dialkyl aluminum alkoxide was studied by the dilatometric technique.347 Each molecule of the catalyst produced one polymer chain only, a fact confirmed by the observed relation DPn = [M1o/[AI1o. In view of the aggregation of the low molecular mass aluminum alkoxides, it was necessary to determine the degree of association of the alkoxides endowed with a long polymer


chain. This was accomplished by investigating the kinetics of propagation of E-caprolactone induced by the above catalyst. 347

Assuming that the polymerization is propagated only by the unassociated polymeric species,

with a rate first order in the monomer and the unassociated polymers, the authors concluded that the pseudo-first-order rate constant of propagation, r, is given by

r = -dln[monomer]ldt = kp · [unassociated polymers],

where kp denotes the bimolecular propagation rate constant of the unassociated polymers. Provided that the association of the Al catalyst obeys the ideal mass law, the equilibrium relation between the non associated and aggregated polymers is given by the equation

[unassociated polymers]n . [aggregated polymers] = K = const.

At sufficiently high concentration of the catalyst, i.e., for

f = [nonassociated species]/(totai concentration of all the polymers) « 1,

the approximation

f = (K/n)lIn/[AI]lIn-l

is permissible, where [AI] denotes the concentration of all the polymers, whatever their state of association. Hence,

In(r) = (lin) . In(Kln) + lnkp + (lin) . In[AI],

and a plot of In(r) vs. In[AI] should be linear with a slope of lin. This indeed is the case as shown by Fig. 3.64 reproduced from the paper by Duda and Penczek.347b

Remarkably, the degree of association, n, of the polymers attached to Et2AIO- is 3, those attached to (iso-BuhAIO- is 2, and the aggregation is disrupted, as indicated by n = 1, on the addition of a secondary, but not tertiary, amine. Apparently, the amines are hydrogen bonded to the alkoxide and their presence causes the dissolution of the aggregates. The reported gradation of the association agrees with the known degree of aggregation of the small aluminum alkoxides. It seems that the degree of


~ " . _ e--x- e - ~ To? u

lo~me-~ ..... ~p 8 ". - 2 ~

" 1" 6

) 4 5 6 - In(1 )0

Determination of the aggregation degrees from the external orders

in initiators concentrations. THF, 2SoC.

(.):(C2I1S)2AI-OCH3' (A): (C2I1S)2AI-OCII2CIIDCII2; l!]o - 3,0 molll

(0): O-C4 H9)2AI-OCH3; l!]o· 1,0 molll

(e):(C2HS)2AI-OCH2CHmCII/(CH3)2NCH2CH2NHC2I1s' initiator to amine

1:2 molar rlltio; (.) (C2115)2AI-OCII{II-CII/(C:2I1S)2NII initiator to

amine I: 18 molar rntio; uJ· 2 moi/i.

Figure 3.64. Determination of the aggregation degrees from the external order in initiator concentrations. THF, 25°C. e, (C2HshAI-OCH3;Ll, (C2HshAIOCH2CH=CH2; [1]0 = 3.0 moles/liter; 0, (i-C4~)2 AI-OCH3; [1]0 = 1.0 moleliiter; 0, (C2Hs)2AI-OCH2CH=CHj(CH3)2NCH2CH2NHCzHs, initiator to amine 1:2 molar ratio; ., (C2Hs)2AI-OCHz,CH=CHi(CzHs)zNH initiator to amine 1:18 molar ratio; [1] = 2 moles/liter.

association of the aluminum alkoxides is determined by the size of the groups neighboring the AI center, whereas the length of the polymer chain protruding from the aggregates does not affect the degree of their association. Most probably the aggregation results from the formation of

The treatment leading to the plots shown in Fig. 3.64 provides also the value of kp . (Kin ) lin given by the intercepts of the lines shown in that figure. However, the separate values of kp and of K cannot be derived from the data deduced from the experiments performed at higher concentrations of the AI, i.e., when f« 1, although such information can be


derived from the data obtained at high dilutions of the catalyst when K = r . [AI)n-l/(l - f) and 1 - f cannot be approximated by 1.

A useful graphic solution of the above equation was developed by Penczek and Duda.347 Accepting the validity of the mass law one presents the relation for the K in the form:

n[AI)n-l/K = lIr - lIr-I,

and since the pseudo-first-order rate constant of polymerization r = kp . f· [AI), one gets a linear relation

lIrn- 1 = kp . [AI)/yn - nlK· k';-l.

Hence a plot of lIyn-l vs. [AI)/yn (n being determined from the plots shown in Fig. 3.64) should be linear, its slope yielding kp and the intercept the nlK . k';-l.

Obviously, the data derived from the experiments performed with the addition of a secondary amine provide directly the value of kp = 0.045 M-1S-1 since then n = 1. Accepting the same value for the propagation of the other nonassociated polymers, an assumption justified by the above discussed graphically derived k/s, the respective K's could be calculated.

It is interesting to compare the propagation rate constant of the pseudoanionic polymerization with those governing the propagation of the free alkoxyanions and their ion-pairs. The pertinent data were reported by Penczek's group.39,41 The former propagate about 100 times faster than the latter, which in turn are 'about 100 times faster propagators than the pseudo anionically growing diakyl aluminum alkoxides.

It is also instructive to compare schematically the propagation scheme of these three systems, as presented by Penczek.347b It appears that the progress of all these reactions follows the SAC2 scheme of Ingold:

a. Propagation on ions I I

Cc=o e

9 d 0

I

I

(=0 ~ Cc=o

e o


b. Propagation on ion-pairs:

c. Propagation on covalent bond: I

I I

c=o C /R 0-1\1 .. \ ~ d OR

9.3. Metalloporphyrins-Catalysts

A variety of cyclic monomers are cleanly and readily polymerized by tetraphenylporphinatoaluminum chloride and its analogues having the following structure:

TPPAI-X

1 a X=Cl b X=OR c X=02CR d X=OAr e X=C2Hs

Initiation and propagation involve the insertion of the monomer into the AI-X bond, a reaction discussed in the preceding section, resulting in linear polymers terminated by X.348 The initiation is fast, its rate being equal to or faster than the rate of propagation, and since the termination and transfer are rigorously avoided in this system, the produced polymers have very narrow molecular mass distribution.

The living character of this polymerization permitted the preparation of di- and triblock polymers, all composed of uniform size blocks. This ini-


tiating system, similar to the one previously discussed, is capable of incorporating carbon dioxide or anhydrides, e.g., phtalic anhydride, into the polymer chains. Although the resulting polymers were not rigorously alternating, their molecular mass distribution was claimed to be narrow. The reactivity of this porphinatoaluminum systems is greatly improved by the addition of quaternary ammonium or phosphonium salts that apparently act as the ligands of the Al center. 349

On further extension of this work it was demonstrated that this initiator is capable of polymerizing lactones. The IR and NMR spectra of the equimolar mixture of this catalyst and the lactone proved that the ring of this monomer is cleaved at the CH2-O bond yielding the carboxylate group attached to AI. The carboxylate group is regenerated as the polymerization of lactone proceeds further. The sharpness of the molecular mass distribution of the resulting polymers attests to their living character.

The addition of alcohol to the polymerizing solution does not terminate the polymerization. Alcohol acts in this system as a chain-transfer agent, e.g.,

@-OCH2-X + ROH~ @-OR + HOCH2-X,

@-OR + n oxiranes ~ @-O(oxirene)n-R.

The inability of alcohol to terminate this polymerization led the author to coin the term immortal polymerization, which may be misleading.

The most remarkable feature of the system involving alcohols arises from the rapid exchange between the growing polymers attached to the catalyst and the dead ones possessing the OH end-group:

@-OCH2-CH2X + HOCH2-CH2Y ~

@-OCH2-CH2Y + HOCH2-CH2X.

The exchange seems to be much faster than the propagation, hence the molecular mass distribution of the resulting polymers remains narrow, in spite of the chain-transfer. However, their DPn = [MJo/([AlJ + [ROH)) is smaller than [MJo/[AIJ anticipated for the polymerization proceeding in the absence of alcohol.

The porphyrin catalyst with X = CH3 is capable of polymerizing methylmethacrylate. However, the polymerization taking place under the irradiation by visible light is slow, e.g., in CH2Cl2 solution at 35°C only 6% of the monomer is polymerized in 2.5 h. The initiation and propagation are basically nucleophilic reactions in which the growing polymer (the nucleophile) attacks the monomer (the electrophile). Thus, the growing


center in the polymerization of methylmethacrylate is the enolate:

@-O-C=QCH3)·CH2-(CMe.CH2)n-CH3'

6CH3 to(OCH)3

It is anticipated that activation of the monomer by a judiciously chosen Lewis acid should enhance its reaction with the porphyrin catalyst, provided that the chosen acid does not polymerize the monomer. This condition is fulfilled by the aluminum organo-compound with bulky substituents which sterically prevent the polymerization. This reasoning turned out to be correct. The addition of

to a mixture of the catalyst and methylmethacrylate is claimed to cause a 50,OOO-fold acceleration of the polymerization yielding a narrow molecular mass distribution polymer with MwlMn = 1.07. The methylaluminum phenoxide with bulky substituents accelerates also the Al-porhinato-initiated polymerization of epoxides and lactones by a factor of about 500.

9.4. Stereoselection and Stereoelection

It might be appropriate to define here the terms stereospecific and stereoelective polymerization. A polymerization, and especially a polymerization of a chiral monomer, is considered to be stereoselective if the resulting polymer, when produced from the racemic mixture of respective monomers, forms a racemic mixture of the pertinent polymers. In other words, an equimolar mixture of rrrr . . . and ssss . . . polymers is produced from a racemic monomer. The initiator inducing such a polymerization could be either achiral or a racemic mixture of two chiral initiators. In the former case the selectivity of the polymerization process could be induced by the handedness of the first formed polymer segment produced by the addition of the first molecule of a chiral monomer (chain-end control).

Stereotactic polymers may be produced also from a prochiral monomer, an achiral molecule of monomer which forms, however, on polymerization a chiral polymer segment. In the course of propagation the incorporated monomer may occasionally acquire the form of the other enantiomer than those of the preceding segments. In such a case multi-stereoblocks are formed, e.g., rrrrrssssssrrrrr, and so forth. Note, this is not possible for a


chiral initiator. Even if a "wrong" monomer has been added, the preference for the "right" one is still maintained.

The ring opening polymerization of di-substituted heterocyclic monomers, e.g., methyl oxirene, should be mentioned here. Such monomers exist as two, inconvertible isomers: trans and cis, i.e.,

\ J \ J trans /C,f", (r or s) and cis /C,f", (meso).

HOR HOH

The cationic or coordination polymerization of the former yields mesodi-isotactic polymer, rs-rs, whereas the polymerization of the latter may produce either di-syndiotactic rr-ss polymer, or a racemic mixture of diisotactic rr-rr and ss-ss polymers. This general observation leads to the important conclusion that Walden inversion of the configuration takes place in the initiation and propagation steps of these SN2 reactions proceeding through a linear transition state. The stereospecificity is accounted for by the transfer of monomers from one to the other metal center (flip-flop mechanism382) •

A polymerization is stereoelective when the initiation and propagation processes preferentially utilize only one of the two enantiomers of racemic mixture of the monomers. Such a reaction yields an optically active polymer, and the residual unpolymerized monomer, although initially racemic and optically inactive, becomes enriched in the other enantiomer and therefore gets optically active. The first stereoelective polymerization, namely of propylene sulfide, was described in 1967 almost simultaneously by Spassky and Sigwalt378 and by Furukawa and his co-workers. 379

The optical activity should be distinguished from the tacticity of the polymer. The latter refers to the relative configuration of the monomeric segments and this concept, as is well known, applies to achiral, e.g. planar, as well as to chiral monomers. Examples 'of tacticity are discussed in the preceding paragraph.

Energywise the distinction between the isotactic and syndiotactic placements is small, e.g., ~~G* of the respective activation energies is typically about 1-2 kJ/mole for the meso and racemic diads.

The first case of a stereospecific polymerization, namely of propylene oxide initiated by a catalyst resulting from the reaction of this monomer with FeCI3, was described by Pruit and Bagget380 as early as in 1955. The stereoselective behavior of zinc alkoxide catalysts was reported by Tsuruta and his colleagues in the late 1960s.350 Their systematic studies of organozinc compounds as the initiators of polymerization led to isolation of a crystallinic, colorless compound Zn(OMeh·(Et·Zn·OMe)6 exhibiting catalytic and stereoselective activity in the polymerization of methyloxirane,


contrasting with the polymerization initiated by potassium alkoxide that yields an atactic polyoxypropylene.

Eventually, single crystals of the above 1:6 complex were isolated and their structure was determined by X-ray analysis.351 The results are shown in Fig. 3.65. This centrosymmetric complex consists of two enantiomorphic distorted cubes sharing one zinc corner (the center of symmetry). The Zn atoms of each cube form the apices of a tetrahedron and are linked through oxygen atoms, whereas the remaining three exomethoxy groups protrude to the outside. This aggregation is retained in solution as proved by cryoscopic measurements in benzene, and by the 13C NMR spectrum that confirms the proposed structure.

A similar complex Zn(OCHzCHzOMeh·(EtZnOCHzCHzOMe)6 was also prepared and its structure, determined again by X-ray analysis, is analogous

Figure 3.65. Molecular structure of {Zn(OMe)J2·{EtZn OMe}6 complex.


to that of the previously described complex. It is also soluble in benzene and displays a similar catalytic activity.

The progress of the polymerization initiated by the 1:6 complex is visualized as a linkage of the oxygen atoms of the monomer with the central Zn atom of the catalyst taking place simultaneously with the dissolution of the respective central Zn-O bond. Thereafter, the nucleophilic attack of the methoxy group, previously forming the Zn(central)-O(CH3)-Zn bridge, on the carbon atom of the monomer opens its ring while its oxygen atom restores the previously disrupted bridge. The sequence of these events is depicted in Fig. 3.66.

Inspection of Fig. 3.65 shows that the Zn(central)-o-Zn units form a chiral hole that accommodates preferentially one of the enantiomorphs of the racemic monomer. In view of the enantiomorphic structure of the catalyst there is another hole that preferentially accommodates the other enantiomorph of the monomer. Thus both enantiomorphs of the monomer participate in the polymerization and yield isotactic polymers. The relation d[d]/d[l] = [d]/[l] was confirmed by the results of copolymerization of the pure enantiomorphs of methyloxirane.

On continuation of these studies an entirely different zinc complex was prepared.352 It is composed of Zn(OCH(CH3)CH20CH3h and

( ,..Me ~O~

Me /~~- z./ ..... O~\/

I c4 °f.e p-. -z--- ® til) R

!

!

Figure 3.66. A possible mechanism of stereoselective polymerization of methyloxirane.


EtZnOCH(CH3)CH20CH3 in 2:2 proportion. Its structure, again derived from the X-ray studies, is shown in Fig. 3.67. The four Zn atoms and the four oxygen atoms linking them acquire the shape of a chair-like skeleton. This complex is also stable in benzene solution, as confirmed by the 13C NMR. The rate of polymerization of propylene oxide by the 2:2 complex is much faster than the rate observed in the reaction catalyzed by either of the two 1:6 complexes.353 While 93% of the monomer is polymerized by the 2:2 complex in 100 h at 80°C, only 2% is polymerized by the 1:6 complexes under the same conditions. The faster polymerization induced by the 2:2 complex than by the 1:6 complex is attributed to the easier access of the monomer to the active Zn centers in the former catalyst than in the latter one. The active Zn atoms of the 2:2 complex are apparently located at the middle corners of the chair-like skeleton. The cleavage of the (O-CH2) bond of the monomer is highly regioselective. The mode of the addition is depicted by the following scheme:

r-r,..~-:-(oo'd,~,(j\~ --------:

i IS) - CO,OCH"o.""'cX:>-- I '-- _____________________ J r-,:;o ---coo. "-,ro iid- --------

l_(~!..:_c:c?:>..~~~~ ___ .

Since the sites controlling the propagation are enantiomorphic, the population of the I, H, and S triads of the resulting polymers is given in terms of a single parameter (1, namely I = 1 -3(1·(1 - (1), H = 2(1·(1 - (1), and S = (1·(1 - (1). The parameter (1 is larger for the 2:2 complex, namely 0.86 to 090, whereas it is 0.73 to 0.76 for the 1:6 complexes.

9.5. Group-Transfer Polymerization

Group-transfer polymerization, reported by the DuPont team in 1983,354 allows for a living polymerization of acrylic monomers, especially methacrylates, to proceed at ambient or at elevated temperatures (-80°C). This is an improvement over the classic anionic living polymerization that requires low temperatures (- - 60°C) for its successful operation. The group-


@ Zn

o 0

• c

Figure 3.67. Configuration of asymmetric carbon atoms in the 2:2 complex.

transfer process involves two reagents: an initiator-namely a silylketene acetal which on its own is inactive and does not initiate the polymerization; and an activator-a nuc1ephile that energizes the growing end-group allowing it to add the monomer.

The mechanism of the group-transfer polymerization proposed in the original papers of the DuPont team is depicted as follows:

R Nu 1 R-__ 1 e

R-Si-R -Si-R 1. I R""I 2.

o + Nu e ----t 0 1 1

/C~ /C~ CHO C CHO ~C/

3 \' 3 .........

Nu R-__ I ",R 3.

-sr ""1 --R '\9, ~CH3

0) C 1 1

/C\,~ ~7-CH3 CH30 f---- CH

\ 2

1

2. +M


4.

In this scheme 1 denotes the initiator, a stable and unreactive compound that can be stored indefinitely under conventional conditions. On the addition of a nuc1eophile, e.g., HFi, a complex, 2, is formed which interacts with the methacrylate, M, as shown by 3. It is debatable whether 3 should be treated as the transition state of the propagation leading to 4 (one step propagation), or as an intermediate having its own lifetime. The present consensus favors the latter interpretation. The insertion of the monomer yields an adduct 4 identical with 2, but by one unit longer. The 2, as well as 4, are in dynamical equilibrium with the noncomplex 1 or 5, the inert dormant species. The repetition of the steps 1-5 results in propagation yielding a longer and longer polymer. Under the common conditions used by the DuPont team the concentration of the activator is -1 % of that of the initiator. The concentration of the initiator exceeds that of the activator by a large factor, therefore the equilibria 1 + Nu- ~ 2, or 5 + Nu- ~ 4 are shifted far to the right making any free activator undetectable.

This ingenious mechanism, known as the associative one, could be treated as a pseudo anionic process since the propagation results from the insertion of the monomer caused by rearrangement of covalent bonds, and not from the addition to an ion. The propagation seems to be first order in the monomer and the activator which is rapidly transferred from a living, activated polymer (2 or 4) to a dormant, nonactivated one (lor 5). Hence, all the polymers grow and their molecular mass distribution is sharp (provided that the exchange between the growing and dormant polymers is much faster than the propagation) and the DPn equals (total polymerized monomer)/(total initiator). The polymerization resumes when the monomer is added to the quiescent solution of the polymers and the conversion of the monomer to the perfectly stable living polymers is quantitative.

A detailed study of the kinetics of this polymerization,355 revealed that the concentration of the free activator, remaining in equilibrium with the initiator, is measurable when the ratio [activator]/[initiator] > -0.2. Then the kinetics show a slight induction period (observed for the ratio > 2) indicating participation of some termination in this process. The infrared studies of oligomers formed in this polymerization led Brittain and

(4)


Dicker356 to conclude that the major mode of its termination results from back-biting yielding the cyclic trimeric end-groups with elimination of trimethylsilylmethoxide (see p. 129).

The Arrhenius plots derived from the kinetic study of the group-transfer propagation of methyl- and t-butylmethacrylates are significant. As seen from the examination of these plots, the kinetic constants of the grouptransfer polymerization are very similar to those derived from the kinetic data of the classic anionic polymerization. Such a similarity seems unlikely for the processes governed by the entirely different mechanisms, and therefore this observation casts doubt on the originally proposed mechanism of group-transfer polymerization and suggests that both processes might proceed through some common intermediate.

An alternative mechanism involving such an intermediate has been developed.357 It justifies the similarity of the kinetics of group-transfer and anionic polymerizations by postulating the following set of steps:

(3)

The enolate (3 or 4), capable of adding the monomer, is formed by the dissociation of the monomeric (2) or polymeric (5) complex releasing the Me3SiNu fragment. The nucleophile accelerates the formation of the enolate. Hence one expects the Me3SiNu moieties to be exchanged rapidly between the different polymeric chains, while the double-labeling experiments reported by Sogah358 demonstrated the lack of such a process. However, later work casts doubts on these findings and the dissociative mechanism cannot be ruled out in spite of the calculation demonstrating the stability of the penta-coordinated species 2, and revealing that its negative charge is mainly located on the ~-carbon.


Another source of possible difficulties marring the results of the "double exchange" experiments was pointed out by Matyjaszewski,359 who suggested that the incompatibility between the polymethyl- and polybutylmethacrylate may slow down the expected exchange.

Studies of some side reactions leading to the formation of cyclic oligomers identified previously by Lochmann69 and observed in the course of anionic polymerization of methylmethacrylate by Piejk0 360 provide additional support for the dissociative mechanism of group-transfer polymerization. If one adds the close identity of polymer microstructure and its temperature variation between the samples produced by group-transfer polymerization and that resulting from the anionic polymerization involving large counterions, it is difficult to rule out the idea of an active enolate anion being the common intermediate in group-transfer polymerization and in the classic anionic polymerization. Further studies of this most interesting reaction continue.

10. Activated Monomer Mechanism

10.1. Polymerization of NCA Initiated by Primary and Secondary A mines

In most of the addition polymerization it is the end-group of a growing polymer that interacts with a monomer and allows for its addition to the polymer chain with simultaneous regeneration of the reactive entity on the added unit, perpetuating the preceding reaction. This mechanism, accounting for the growth of many polymers (the propagation step), is not unique and an alternative one will be discussed now.

Polymerization of Leuchs' anhydrides, briefly known as NCA's

3 2

R'CH-CO", I 0 1

NH-CO/ 4 5

involving decarboxylation of the intermediate product, yields polypeptides, -(CO'CH(R)'NH)n-, valuable model polymers for biochemical studies. This polymerization, investigated by many researchers, shows several peculiarities deserving discussion (see, e.g., a relatively recent review by SekiguchP61). It may be initiated by a variety of agents such as primary, secondary, and tertiary amines, bases like methoxy anions, salts such as LiCI, but not LiCI04, etc. The amines react preferentially with the 2-CO


group of NCA yielding zwitterionic adducts

+ HNR'

21 +

HNR'R" 1

R'CH-~ I :}O NH-Co

or R'CH-~

I jP NH-CO.

whereas their much less frequent reaction with the 5-CO group terminates the polymerization.

On opening the ring of the adduct, a carbamic acid, NHR'--CO-CHR-NH--COOH, is formed which, in turn, readily decarboxylates yielding a primary amine, NHR'--CO-CHR-NH2 , capable of reacting with another NCA molecule. The repetition of the above steps yields then to the dimer, NHR'--CO-CHR-NH--CO-CHR-NH2,

again terminated by an amine group. The repetition of the cycle described here gives rise to a polypeptide.

This so-called "simple" or "normal" NCA polymerization might be eventually terminated either by a wrong monomer addition, i.e., the addition to the 5-CO group resulting in an ureido-acid, NHR'--Co-NH--cHR--COOH, that does not decarboxylate to an amine, or by some other reaction taking place during or after completion of the polymerization. In a favorable case, a living polymer terminated by an amino group is the final product of such a polymerization.

As demanded by the mechanism outlined above, the "normal" NCA polymerization yields polypeptides of DPn equal to or smaller than the [NCA]o/[Initiator]o ratio when a primary or secondary amine, or any other suitable proton donating base-H, such as, moisture, acts as the initiator. Indeed, the ideal NCA polymerizations yielding living polypeptides of DPn = [NCA]o/[Initiator]o were described in the literature, proving the validity of the "normal" NCA mechanism. It has been surprising, therefore, when Blout reported362 polymerizations yielding polymers of DPn > [NCA]o/[Ilo. Even the polymerization initiated by n-hexylamine could form such a polymer under appropriate conditions. Furthermore, the "normal" polymerization does not account for the initiation by tertiary amines or aprotic bases, e.g., the methoxide anion, whereas such initiators result in polymerization/aster than the normal and yield polypeptides of DP n greatly exceeding the ratio of [NCAlo/[Ilo.363 It became obvious that some other mechanism operates in the NCA polymerization.

10.2. Polymerization Initiated by Aprotic Bases

The comprehensive studies of NCA polymerizations initiated by aprotic bases led to the following observations:


1. The initiator seems to be regenerated in the course of the reaction. After cessation of polymerization the reaction ensues again on addition of fresh monomer.

2. The reaction is faster and yields polymers of higher DPn than the polymerization initiated by primary amines. In fact, the DP n of the resulting polymer may be substantially greater than the ratio [NCA[o/[Initiator ]0'

3. The increase of the concentration of the initiator accelerates the reaction although the concentration of the growing species does not necessarily increase.

4. The DPjDPn may be as large as 10.

5. The formed polymers may be dead and could even precipitate, nevertheless the polymerization ensues again on the addition of fresh monomer.

Observations (1) and (5) are the most significant. It seems that the initiator, or a product formed in the course of the initiation, interacts with the monomer, activates it, and then as the activated monomer is incorporated into the polymer chain the activating agent is regenerated.

This idea, somewhat modified, was outlined by Bamford and Block364

who visualized the abstraction of a proton from NCA by the base to be the step activating the monomer. Thus the following mechanism was outlined:

R'CH-CO R'CH-CO" I /"0 + Base~ I /0, Base'H+,

NH-CO ~-co

R'CH-CO" I ° Base'H+ + ~-CO/

/OC-CH-R o I

"OC-N-CO-CH'R-NH-COOH, + Base.

followed by the decarboxylation of the carbamic acid which yields the

referred to as the dimer, whereas the regenerated base converts another molecule of NCA into its "activated" form which may react with the dimer


by attacking the 2-CO group (propagation of the polymerization) or with another NCA molecule initiating a new polymer chain. It should be stressed that the dimer (as well as the trimer, tetramer, etc.) seems to be more reactive than the unsubstituted NCA molecule, due to the presence of an activating substituent attached to its N atom.

The proposed reactions result in propagation of the polymerization or in the formation of additional growing centers. The latter reaction might account for the acceleration frequently observed in these polymerizations. The role of the monomer in these processes justifies their designations as the "activated monomer mechanism." 365

The amines catalyze a variety of reactions, e.g., the hydrolysis of carboxylic anhydrides. The catalysis may result from their action as nuc1eophiles or as Bronsted bases. To distinguish between these two alternative actions in the above hydrolysis, Gold and Jefferson373 studied the catalysis by pyridine, 2-picoline, and 2,6-lutidine. The basicity of these amines increases along the above series due to the electron donating action of the methyl group. On the other hand, their role as a nuc1eophile is diminished along this series because the increasing steric hindrance caused by the substituents hinders their action. In the Gold and Jefferson study pyridine was found to be the most effective catalyst, implying that these amines act there as nuc1eophiles in catalyzing the hydrolysis. However, a reverse order of their activity was observed in their effect on the rate of NCA polymerization, confirming the proposed mechanism of initiation by tertiary amines acting as Bronsted bases and not as nuc1eophiles. Moreover, since the polymerization initiated by diethylamine is faster than that initiated by n-hexylamine under comparable conditions, one may infer that the activated monomer mechanism is more efficient than the "normal" amine mechanism. ****

10.3. The Activated Monomer Mechanism in Lactam Polymerization

The concept of the activated monomer mechanism is outlined here, in a somewhat idealized fashion, by considering the anionic ring opening polymerization of lactams. The reaction ensues on the addition to a lactam monomer of an appropriate base, not necessarily negatively charged:

NH-CO+B ~N-CO+BH, ~ '--...../

(1)

In this step an inactive molecule of the monomer is transformed into an activated monomer capable of reacting with an ordinary monomer molecule

"""The activated monomer mechanism of NCA polymerization was criticized by Harwood.377


forming an intermediate dimer:

NH-CO+N-CO~NH CO-N-CO. ~ '---'" '---" '---'"

(2)

This step is slow but is followed by a rapid abstraction of a proton from another molecule of the monomer by the dimer:

-~O-t-L.S0 + NtLS0 ~ N!&.30 . Jt.3.0 + N~O, (3)

in the course of which another activated monomer molecule is formed. The repetition of these steps leads to the formation of a high molecular mass polymer:

-NH2 CO·N-CO+N-CO~NH2 CO·N CO·N-CO, (4) ~ '---'" '---'" ~ ~ '---'"

followed by:

NH2 CO· N CO· N-CO+ NH-CO ~ ~ '--/ -"-./ '--" NH2 CO·NH CO·N-CO + N-CO, ~ '----'" '--" '--"

(5)

-N~CO. ~O. t,90 + N----S0 ~

~O.~O'1'l.50.~O (6)

An important distinction between steps (2) and (6) should be stressed. As remarked earlier step (2) is slow, whereas step (6) is fast. In step (2) the opening of the N!!::>0 ring, caused by the addition of the lactam anion,

involves a "bare" lactam, whereas in steps (4) and (6) the same reaction takes place on a substituted lactam moiety, i.e., on a

-CO·N-CO '---'" .

The acylation makes the ring moiety more reactive, its opening on the addition of a lactam anion is faster. The difference in the rates of reactions (2) and (6) imparts on the overall process some autocatalytic character. As the reaction progresses, more of the rapidly growing """CO· N-CO.

'--./

end groups are formed and the propagation accelerates. This undesired feature of the process may be eliminated by invoking a "promoter," a compound, e.g., such as CH3CO·Cl or PhCO·Cl, rapidly acylating some of the monomer prior to the initiation. Step (2) of the preceding scheme is


modified therefore, e.g.,

PhCO . N-CO + ~ <;0 -+ PhCO . N CO· N-CO '--.-/ -- '--.-/ '--.-/

followed by:

PhCO . N CO· NH-CO + NH-CO -+ ~~ ~

PhCO . NH CO· NH-CO + N -CO. ~~ ~

The propagation starts at once, without the induction period observed in the earlier discussed reaction and the autoacceleration is eliminated.

Let us summarize the modified scheme of the activated monomer mechanism as now presented. The reaction involves two reagents in the addition to the monomer: a promoter, e.g., PhCO·Cl, and an activator-a properly chosen base, e.g., MeO-. The promoter determines the number of the polymer molecules to be produced by the polymerization, and therefore the ratio [consumed monomer]/[promoter] provides (ideally) the degree of polymerization, DPm of the resulting polymers. The initiator (the base) determines the rate of propagation per polymer molecule, without affecting its degree of polymerization. The termination and chain-transfer are excluded in this scheme, but it is not correct to refer to the produced polymers as living ones. The polymers:

PhCO . NH CO· NH CO· NH-CO, ~ '-.-7 ~

albeit activated by the carbonyl substituent, are inert and do not grow on the addition of lactam, the monomer. Neither is the monomer active; its mixture with the polymer remains quiescent and no reaction ensues until the activator (a base) is added, only then the polymerization starts proceeding with the regeneration of a fresh activated monomer in each propagating cycle.

10.4. The Real Systems

In the above presentation of the activated monomer mechanism we deliberately idealized the system in order to expose clearly the fundamental features of this kind of polymerization. The real systems are much more complex. To begin with, we presented the activated monomer polymerization of lactams as the reaction of free ions. Indeed, the contribution of the free ions to the polymerization of many lactams is often decisive although not unique. Under the polymerization conditions, the lactamate salts, as well as the corresponding salts of the polyamides, enter into complicated equilibria involving various aggregates, ion-pairs, triple ions, and


so forth, which in turn may interact with the other donors and acceptors present in the system. At the catalytic concentrations of the activators used in the bulk polymerization, the aggregates and the triple ions seem to dominate, and the contribution of the free lactam ions of the alkali salts starts to be appreciable at high temperatures (> 150°C) and at dilutions lower than 5.10-4 M. Hence, the nature of the counterions matters, often to a great extent, as well as the dielectric constant of the medium. Significantly, the dielectric constant of the molten polyamides is substantially higher than that of the monomer, hence the permitivity of the milieu varies in the course of bulk polymerization with the degree of the monomer conversion. It should be remarked also that impurities, especially traces of water, strongly affect the behavior of these systems. For example, the reported decrease of the rate of polymerization of caprolactam with increasing concentration of sodium caprolactamate seems to be caused by the increased concentration of the water introduced with the activator.

Many side reactions accompany the polymerization process, especially when it proceeds in bulk at high temperatures (sometimes as high as 210°C). For example, at 150°C a large number of the growing centers are converted within a few seconds into the derivatives of the ~-ketoamides. The latter are much more acidic than caprolactam and their presence causes an appreciable decrease in the concentration of the lactamates. The ~-ketoimines, as the derivatives of diacylamines, are strong acylating agents in their neutral form, and may act as the growth centers from which the linear or branched molecules may protrude.

The complex dependence of the rate of polymerization of some lactams on their structure is exemplified by the behavior of a,a-disubstituted propiolactams. The rate of polymerization decreases with the size of the substituents when the reaction proceeds in THF, whereas in the reaction performed in DMSO the reverse is observed, the rate increases with the substituent's size.367

The evidence for the activated monomer mechanism of NCA polymerization initiated by tertiary amines or by the bases such as methoxides is well established in spite of the complex features of Leuch's anhydrides. The long-standing debate, whether tertiary amines deprotonate the monorner, was positively resolved by Kricheldorf.368 The controversy concerning the merit of the activated monomer mechanism vs. the carbamate propagation369 has been clarified also, see ref. 370 for further discussion of this subject.

10.5. Cationic Polymerization of Lactams

The cationic polymerization of lactams is more complex than the anionic one, although all types of lactams, unsubstituted as well as substituted, are


polymerized by this technique, whereas N-substituted lactams cannot be polymerized by the anionic mode of the reaction.

The peculiarity of the cationic lactam polymerization is revealed by the nonmonotonic dependence of its rate on the ring size of the monomer. It is assumed generally, that the N-substitution of the monomer decreases its polymerizability, but examples to the contrary are known.

The polymerization proceeds again by the activated monomer mechanism, but the monomer is activated now by the strong protic or aprotic acids. The activated acid protonates the carbonyl group of the unsubstituted or substituted monomer, facilitating the initiation as well as the propagation. The first step, common for the substituted and unsubstituted monomers, is

+ ~O+HA~ ~--OH + A-,

and is followed by the formation of the dimer

+ NH-CO=OH+~O ~NH CO'N-CO+"H+" '-../ ~~'

and the propagation takes place on the amino end-group:

+ -NH2 + HO==CO-NH ~ -NH' CO NH2 + "H+" '--""" ~ ,

(regeneration of another activated monomer). There are some termination reactions resulting from the formation of

strongly basic acyl amidine groups that neutralize the activating acid. Let it be stressed again that the activity responsible for the polymer growth resides not on the end of the polymer chain but is associated with the activated monomer.

10.6. Activated Monomer Propagation in Cationic Ring Opening Polymerization of Cyclic Ethers

An interesting approach to the cationic ring opening polymerization of cyclic ethers and acetals was developed recently by Penczek and his associates.366 The conventional polymerization of these monomers yields eventually long polyether chains terminated by a hydroxyl group. This reaction proceeds through a tertiary oxonium ion which reacts with the monomer, say ethylene oxide, e.g.,


and the termination, say by water, produces eventually the terminal hydroxyl group,

+ /CH2

N\l\OCH2CH2·OCH2CH2·O" I + 2H20 ~ CH2

N\l\OCH2CH2·0CH2CH2·0CH2CH20H + H30 +.

In the above process the activity resides with the oxonium end-group of the growing polymer. However, these groups interact also with the preceding segments of the polymer, yielding then the undesired cyclic oligomer. In an attempt to avoid this cyclization a new ingenious polymerization process of oxiranes was developed.

The reaction starts with the formation of the first oxonium ion, e.g.,

In the presence of a deliberately added alcohol the oxonium ion yields a protonated associate:

Provided that the monomer (oxirane) is a stronger base than the polyether, another activated monomer is regenerated by the reaction:

+ /CH2

-CH2CH20H·CH2CH2·OH + 0" I ~ CH2

+ /CH2

-CH2CH20·CH2CH20H + H.O" I . CH2

Repetition of these last two steps yields a longer and longer polyether terminated by the hydroxyl group. Thus, the terminated polymer does not react with the preceding segments of the polyether and the formation of the undesired cyclic oligomers is prevented. However, the conventional polymerization involving the tertiary oxonium ions proceeds simultaneously with the above described process referred to as the activated mon-


omer propagation. Indeed, at the very onset of the polymerization two reagents compete for the protonated (activated) monomer, namely

Provided that the alcohol is more basic than oxirane and reacts faster with the activated monomer, then the first of the last two competing reactions prevails and provides the preferred route of the overall process.

Such a preference is amplified when the ratio

kpa . [alcohol]/kiox . [oxirane],

is large, i.e., the concentration of the alcohol should greatly exceed that of the monomer.

The initial concentration of the alcohol is limited by the desire to produce polymer chains of the requested degree of polymerization, DPn • Indeed, had the reaction proceeded entirely via the activated monomer mechanism, then the DPn of the resulting polymers would be given by the ratio:

total amount of the supplied oxirane the stationary concentration of the alcohol'

To keep the stationary concentration of the oxirane low and to allow for its total amount supplied to the system to be sufficiently large, one has to add it continually at a low rate as the reaction proceeds, and not to provide its total amount at the start of the polymerization.

To determine the ratio of the constants kpjkiox the authors noted that the first 01, i.e., RO·CH2CH20H, formed on the addition of the activated monomer to the mixture of the chosen ROH alcohol and the monomer in the desired proportion is produced only by the activated monomer mechanism. Hence, the

lim . [RO·CH2CH20H]/([Oxirane]o - [Oxirane],)t=o

gives the ratio (kpjkiox) . ([ROH]o/[Oxirane]o). Therefore, the extrapolation of the plot of

[RO'CH2CH20H],!{[oxirane]o - [oxirane]t} vs. t


to t = 0 provides the value of kpjkiox• The [RO·CH2CH20H], and the {[oxirane]o - [oxirane]J are obtained experimentally using, e.g., gas-liquid chromatography analysis.

An alternative method, rather complex, of evaluation of the above ratio is outlined in ref. 366 to which the interested reader is referred for further details. It is claimed that both methods lead to concordant results.

The actual behavior of this system is more complex than suggested by the above description. The formation of ion-pairs is neglected, only the free ions are considered. Moreover, the protonation is restricted to monomer only, while the protonation of the ethereal oxygen is hardly mentioned, although it is quite important. Let it be stressed that as the polymer chains become longer the protonation of ethereal oxygens gets more significant. Hence, the fraction of activated monomer is smaller than assumed.

Let it be emphasized again that the beneficial feature of the activated monomer mechanism arises from its ability to eliminate, or at least to minimize, the formation of the cyclic oligomers that deteriorate the properties of the linear polyethers.

The formation of the cyclic oligomers is favored by the reactions induced by strong protic acids. Indeed, Ito et al. 371 described the conditions under which ethylene oxide is quantitatively converted into dioxane via polyglycol. This reaction is depicted below (back-biting):

The protonation of the ethereal oxygen atoms does not yield the cyclic oligomers, because the positive charge is concentrated on the H atoms and not on the C atoms which are engaged in the intramolecular formation of the tertiary oxonium ions.

The extension of this study to propylene oxide372 show further complications arising in this approach. There are now two modes of opening the oxirane ring, leading to two kinds of activated monomer and to two kinds of ultimately formed alcohols. Hence, four elementary reactions participate in the growth.

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307. Scott, H., Miller, G.A., and Labes, A. 1963. Tetrahedron Lett. 1, 1073.

308. Elinger, L.P. 1963. Chern. Ind. 1982.

309. Shirota, Y. 1977. Chern. Commun. 189.

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311. Bawn, C.E.H., Ledwith, A., and Sambhi, M. 1971. Polymer 12, 209.

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315. Penczek, S., Jagur-Grodzinski, J., and Szwarc, M. 1968. J. Am. Chern. Soc. 90,2174.

316. Patterson, J.M. 1955. J. Org. Chern. 20, 1272.

317. Pac, J., and Plesch, P.H. 1967. Polymer 8,252.

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319. Abdoul-Rasoul, T.A.M., and Ledwith, A. 1978. Polymer 19, 1219.

320. (a) Crivello, J.V., and Lam, J.H.W. 1976. J. Polym. Sci. Symp. 56, 383.

(b) Ibid. 1977. Macromolecules 10, 307.

(c) Ibid. 1980. J. Polym. Sci. 18,2441.

(d) Ibid. 1980. Macromolecules 14, 1141.

(e) Ibid. 1979. J. Polym. Sci. Lett. 17,759.

321. Gilbert, H., Miller, F.F., Averill, S.J., Carlson, E.J., Folt, V.L., Heller, V.D., Stewart, F.D., Schmidt, R.F., and Trumbull, S. 1956. J. Am. Chern. Soc. 78, 1669.

322. Stille, J.K., and Chung, D.C. 1975. Macromolecules 8,83, 114.

323. Chapiro, A. 1980. J. Polym. Sci. 18,327.

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155. 328. Breitenbach, J.W., Olaj, O.F., and Sommer, F. 1972. Adv. Polym. Sci. 9,

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330. Davison, W.H.T., Piner, S.H., and Worrall, R. 1957. Chern. Ind. 1274.

331. (a) Williams, F., Hayashi, K., Ueno, K., Hayashi, K., and Okamura, S. 1967. Trans. Faraday Soc. 63, 1501.

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332. Szwarc, M. 1960. Makromol. Chern. 35a, 123.

333. Stannett, V. 1964. Int. J. Appl. Rad. 15,747.

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3161 Ionic Polymerization and Living Polymers

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339. Plesh, P.H. Proc. Roy. Soc., in press.

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(b) Hirota, M. and Fukuda, H. 1987. Makromol. Chern. 188,2259.

342. Price, C.C., and Osgan, M. 1959. J. Polym. Sci. 34,153.

343. Sakata, R., and Tsuruta, T. 1960. Makromol. Chern. 40, 164.

344. Vandenberg, E.J. 1960. J. Polym. Sci. 47,486.

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349. Inoue, S. 1985. ACS Symp. Ser. 286, 137.

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354. Webster, O.W., Hertler, W.R., Sogah, D.Y., Famhem, W.B., and RajanBabn, T.V. 1983. J. Am. Chern. Soc. 105,5706.

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356. Brittain, W.J., and Dicker, LB. 1989. Macromolecules 22, 1054.

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358. Sogah, D.Y. 1987. Macromolecules 20, 1473.

359. Matyjaszewski, K. 1988. Makromol. Chern. Symp. 13/14,389,433.


360. Piejko, K.E. 1982. Ph.D. Thesis, Bayreuth.

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364. Bamford, C.H., and Block, H. 1961. J. Chern. Soc. (Lond.) 4989,4992.


366. Penczek, S. 1986. Makromol. Chern. Symp. 3, 203.

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4

Elementary Steps of Polymerization Other Than Initiation or Propagation

1. Termination

1.1. General Remarks

Propagation of polymerization continues until the activity of the growing ends of polymeric molecules is lost or the supply of monomer exhausted. The reactions that irreversibly deprive the growing polymeric molecules of their propensity for growth are known as terminations. They are classified as proper terminations resulting in cessation of all processes that consume the monomer by extending the length of the formed macromolecules, and as chain-transfers-the events irreversibly terminating the propagation of some of the polymeric molecules growing at that time while simultaneously starting the growth of new ones. Both these processes should be distinguished from the formation of dormant polymers-the events temporarily terminating the growth of some polymeric molecules which, however, is resumed spontaneously at a later stage of the reaction.

Termination and chain-transfer affect the degree of polymerization of the resulting macromolecules as well as their molecular mass distribution. Denoting by [M]o and [M]oc the initial and final concentrations of the monomer, and by [1]0 the initial concentration of the eventually fully consumed initiator, we find for a unimolecular termination:

where the k's refer to the rate constants of propagation and termination, respectively. The degree of polymerization, DPm is reduced by chain-transfer but not by proper termination. For example, the increase in the number of all polymers, growing and dead, resulting from a chain-transfer first

319


order in the growing polymers and in the monomer is given by (k"lkp)·aM, aM being the amount of polymerized monomer per liter.

In polymerizations proceeding with termination the number-average degree of polymerization at time t is

where Rp = kP[M]C;" , and Rtr = ktrC;" , with C;" denoting the concentration of the polymers still growing at time t, and k's being the respective rate constants of propagation and termination averaged for all kinds of growing species. In the derivation of the above equations it is assumed that the termination is first order in the growing polymers. In such a system P n,t

increases asymptotically with time towards a limiting value of P n,"'" whereas in the absence of any termination the P n,t increases with time as

Termination, including chain-transfer, as well as slow initiation, strongly affect the molecular weight distribution of the resulting polymers. This subject has been treated by many authors who also considered other factors affecting the distribution of molecular weights. Most of the pertinent references were collected in MOller's review, "Carbanionic Polymerization: Kinetics and Thermodynamics. "la

1.2. Unimolecular Termination

Radical polymerizations are most frequently terminated by bimolecular collisions, whereas the most common terminations of ionic polymerization take place by unimolecular reactions. The spontaneous decomposition of macro-ion-pairs yielding irreversibly the inert, electrically neutral products is the commonly observed mode of termination of cationic polymerization. For example, the schemes

or

+ """CH2'QCH3boS020H-

~ """CH=C(CH3)2 or """CH2'QCH3)=CH2 + H2S04,

+ """CH2'CH(Ph),SbC16 ~ """CH CH(Ph) + SbCIs + HCI,

Elementary Steps of Polymerization Other Than Initiation or Propagation / 321

or

+ I\N\CH2·C(CH3bBCI4 ~ I\N\CH2·C(CH3)2Cl + BCI3,

illustrate typical irreversible terminations in cationic polymerization of vinyl monomers. It should be remarked, however, that the first two reactions could be treated also as chain-transfers in so far as the liberated acid (H2S04 or SbCIs + HCI) could initiate the polymerization of the still available monomer, whereas the last reaction could be reversible since the association

+ I\N\CH2C(CH3hCI + BCl3 ~ I\N\CH2C(CH3bBCI4

takes place readily. Similar decompositions, causing termination, are observed in anionic

polymerization. An example of such a termination of the classic anionic polymerization of styrene was reported by Spach et al. lb Living sodium polystyrene undergoes a slow spontaneous decomposition:

yielding the insoluble sodium hydride and a dead polymer possessing an extremely acidic 'Y proton (conjugated to a phenyl and a vinyl group). Subsequently, a very rapid proton transfer takes place from this dead polymer to a still living polystyrene,

converting the former into a highly conjugated, stable, but unreactive anion absorbing light at 535 nm. T~s reaction manifests itself by a slow change of color of the original living polystyrene solution from cherry-red to purple as shown by Fig. 4.1. The expulsion of NaH is first order in the living polymer. It seems to be catalyzed by sodium dust, suggesting the following mechanism:

NlACH2CH(Ph)-CHCHPh, Na+ ~ ~HPh·CH=CHPh + NaH

----~-l- Solution

Na Na Na Na Na Na Na Na Na Metallic sodium.


-; (J

:l Q,

1.5

o 0.5

350 400 500

Waveleocth (om)

600 700

Figure 4.1. The slow spectral changes of solution of sodium polystyrene caused by its aging. '

Changes of colors in aging solutions of living polymers are frequently observed. Such reactions are faster the more polar the solvents, e.g., they are virtually instantaneous in hexamethylphosphorictriamide (HMPA).2 They were investigated by Comyn and Glasse,3 who accounted for these processes by various complex reactions, often involving the residual monorner, enhanced by UV irradiation.

The decomposition of lithium polybutadienyl into lithium hydride was observed on heating its hydrocarbon solution. It yields a polymer mimicking the monomer, e.g.,

The reaction of the latter with a still available living lithium polybutadienyl results in the formation of branched polymers.

In cationic systems some complex counterions, especially those involving Al derivatives, e.g., Me3AICI- , interact with carbenium ions transferring to them the organic moieties, e.g.,

Such reactions terminating the propagation allow the introduction of some desired organic functional group as the end-groups of terminated polymers.


Thus, vinyl end-groups could be introduced by adopting (CH2=CHhAICIas the counterions. Similarly, cyclopentadienyl end-group is introduced when Me2·CsHs·AICl- is the anion, and so forth (see ref. 4, p. 222 for other examples). However, in many of these reactions a hydride transfer takes place instead.s For example:

Alternatively, a complex counterion decomposes with transfer of a halogen anion, e.g.,

This kind of termination may be avoided by using complex ions with very strongly attached ligands, e.g., AsF6 or SbF6 . The stability of the complexed anions decreases along the series: AsF6 - SbF6 > PF6 > SbCl6 > BFi > AICli.

The irreversible collapse of ion-pairs into a covalent species, observed in many cationic polymerization, is the only spontaneous proper termination of such reactions. The reversible collapse was treated extensively earlier (see p. 186), and therefore is not discussed again at this place.

The cyclization (back-biting) involving the last three monomeric units of the growing polymer chains, in conjunction with the elimination of alkoxides, seems to be the termination step of anionic polymerization methacrylates. This reaction is fully discussed in Chapter 3 (p. 129). Apparently, the same reaction is responsible for the termination of group-transfer polymerization. It should be stressed, however, that the catalyst allowing for the propagation is also required for the termination of group-transfer polymerization.

Interesting mathematical treatment of this process was reported by Hocker, 6

who systematically investigated the termination of methylmethacrylate polymerization. He concluded that the termination schemes reported in the literature cannot account for the dependence of the rate of termination on the initial concentration of the reagents that was observed by him, and assumed, therefore, that some unknown substance S is formed during mixing the monomer and the initiator. The unknown S supposedly reacts with the growing polymers P* in a second-order reaction:

P* + S - dead polymer; rate constant k'.

'The association with a complex counterion could be more intricate. For example, the association of a proton with SbCl;; yields HCl and SbCls.


Integrating the equation - d[S] = k'[S] . [P*] . dt and - d[P*] = k'[S] . [P*] . dt led to the expression

([P*]o - [S]o) -1 • In([P*] . [S]oI[P*]o . [S]) = k' . t

with [S]o = [P*]o - [P*], which accounts quantitatively for the observations. Unfortunately, the nature of S is still unknown.

1.3. Bimolecular Termination Involving Two Growing Polymers

The termination resulting from a collision of two polymer radicals leads, in a successful encounter, to their combination or disporportionation. Both these reactions, unavoidable in radical polymerizations, destroy the growth capacity of the radical end-grops and constitute, therefore, the most common proper termination of such polymerizations. No analogous process takes place in ionic polymerizations. Two growing anions, or cations, cannot annihilate their growth capacity by being mutually associated and linked by a covalent bond, although a looser association of two ion-pairs may lead to the reversible formation of some inert dimers. However, the formation of such dimers results in the creation of dormant polymers and not in termination, because their reversible disaggregation converts them spontaneously into active growing polymers. The disproportionation of two anions or cations is also unfeasible since no simple mechanism is available for destruction of their electric charges. The claimed disproportionation of the anionically growing polymers formed on the initiation of ethylene polymerization by a soluble complex of bis-cyclopentadienyltitanium dichloride with dimethyl aluminum chloride might be an exception. The termination of this polymerization, investigated by Chien,1 was reported to be second order in the growing polymers. Its kinetics were accounted for by the speculative scheme:

Additional evidence is needed for its verification. The disproportionation of lithium polybutadienyl on heating its hydro

carbon solution was investigated by Antowiak8 who described its course as:

The formation of ~2CH=CHCHD2 and its isomer ~CH=CHCH2D


on deuteration of the polymers resulting from heating a solution of lithium polybutadienyl is claimed as evidence supporting the proposed mechanism.

1.4. Other Bimolecular Terminations

Polymerization induced by "I-ray irradiation of meticulously purified monomers, a reaction discussed previously in Chapter 3 (see p. 264), is terminated by bimolecular combination of the oppositely charged ions since their association results in electron transfer annihilating their charges. Their equivalent concentration is responsible for the observed linear relation of the rate with the reciprocal of "I-ray's intensity.

The termination caused by some soluble terminating agents present in the polymerizing solution has, of course, the character of a bimolecular reaction when the polymerization takes place in the presence of these agents. Such a reaction is discussed in the next section.

1.5. Proper Termination Caused by Solvents or Impurities

The reactions of anionically growing polymers with a solvent or impurities may lead to the transfer of some oppositely charged moieties from the latter to the growing polymer ends. The neutralization of the negative charge of the polymer end-groups converts the growing species into the dead ones. The transfer of protons is the most common example of such a termination in the anionic polymerizations. It is illustrated by the schemes:

H20

Ml\CH2CHPh + THF ~

Ml\CH2CH2Ph + CH2=CH2 + CH2=CH-0- (CH3CHO + OH-)

yielding ethylene and acetaldehyde, or alternatively leading to the formation of an alcoholate

The appearance of acetaldehyde in the products implies at least some contribution of the first route to the termination. The proton transfer termination is expected whenever the solvent or the impurity is sufficiently acidic and the growing anions are sufficiently basic. The reactivity of the anions produced in such a process is usually low, preventing the subsequent initiation of polymerization; otherwise one would deal with a chain-transfer via solvent. The free anions are stronger bases than their ion-pairs, and therefore their contribution to proton transfer causing the termination is enhanced by dilution of the polymerized solution since the fraction of the free ions increases then. This indeed is observed.1


Alternatively, the reaction of long-lived polymers with tetrahydrofuran (THF) leads to a slow feasion of the C-O bond of THF and the addition the resulting moiety to the polymer end:

The association of the thus produced sodium alcoholate end-groups results in a substantial increase in the viscosity of the solution when kept for a few hours at ambient temperature. This explanation of the dramatic gelation of solutions of dianionic sodium polystyrene offered by Fetters,9 corrects the questionable account of this phenomenon proposed by one of us. 10

The reactions of anionically growing living polymers with the deliberately added substrates forming bonds with the carbanions allow the functionalization of their end-groups. For example, polymers terminated by hydroxyl end-groups are formed on the addition of ethylene oxide to living polymers, i.e.,

HA

~ N\l\CH2CHPh·CH2CH20H.

Analogously, the addition of carbon dioxide yields, under proper conditions, polymers terminated by carboxylic groups. A review of the various functionalization procedures of anionically growing polymers was published recently by Rempp and his colleagues. 11

In the cationic polymerization a similar kind of termination takes place when the solvent or impurities are sufficiently nucleophilic. For example, ammonia and amines are efficient terminating agents for most of cationic polymerizations. In contrast to anionic polymerization, water is a relatively less effective terminating agent than ammonia or amines. Nevertheless, the reaction such as

rapidly quenches cationic polymerization.

2. Intramolecular Proton or Hydride Ion Transfers and Other Isomerizations

Intramolecular hydride transfer is often taking place in the course of cationic polymerization of vinyl monomers. A spectacular example of such


a transfer was reported by Kennedy and Langer,12 The conventionally initiated cationic polymerization of 3-methyl-l-butene performed at low temperature yielded a pure

instead of the expected

In this system the hydride transfer, faster than propagation, converts a less stable secondary carbenium ion into a thermodynamically more stable tertiary ion. An extensive review of similar isomerization processes was published by Kennedy Y

Isomerization of cationically formed trimers of poly-p-MeO-styrene was observed by the NMR technique.14 Again, the terminal secondary carbenium ions

PhOMe PhOMe PhOMe I I I

XCH2 CH-CH2·CH-CH2CH+

capable of propagation isomerize into the unreactive tertiary ions

PhOMe PhOMe PhOMe I I I

XCH2·CH-CH2·C+ -CH2·CH2•

The strong interaction of the middle positively charged phenyl group with both of their neighboring phenyls contributes to the driving force of this isomerization.

The proton transfers operating in anionic systems resemble the discussed above hydride tranfers observed in cationic polymerizations. For example, the rapid proton transfer taking place in the anionic polymerization acryl


amide described by Breslow et al. 15

yields segments of Nylon-3 dispersed in chains of polyacrylamide. A plausible isomerization of the anionically growing methylmethacrylate

has not been observed yet. The above isomerization, had it occurred, would lead to termination of the polymerization since the carboxylate anion, being a relatively weak base, does not propagate the polymerization of methylmethacrylate.

Still another kind of isomerization leads to the termination of cationic polymerization of styrene and its derivativatives; namely the Friedel-Crafts cyclo-addition converts the benzylic carbenium ion into an inactive cation derived from indene.16

In the course of propagation of lithium polymethylmethacrylate, a cyclization, combined with the expUlsion of lithium methoxide (see p. 129), yields an inactive six-membered ring. This reaction, which puzzled the early investigators of anionic polymerization of methylmethacrylate, contributes to the termination of this polymerization.

3. "Wrong" Monomer Addition

The addition of a monomer to a growing polymer may take place in more than one fashion. While the "conventional" mode of the addition perpetuates the propagation, the alternative modes produce unreactive ions, incapable of inducing further polymerization and therefore their formation terminates the growth. Some examples clarify the idea of a "wrong" monomer addition.


The conventional cationic propagation of vinyl carbazole polymerization requires the addition of the growing carbenium ion to the C=C bond of the monomer. In contrast, the addition to the nucleophilic N-atom leads to the formation of an unreactive quaternary ammonium ion:

terminating the growth ( N denotes the carbazyl moiety). /".

The conventional anionic propagation of 9-vinylanthracene proceeds according to the scheme:

where A denotes the 9-anthracyl moiety. However, the attack, depicted below, of the growing carbanion on the 10-carbon atom of the monomer (9-vinylanthracene) yields an extremely stable carbanion that propagates the polymerization but only very slowly.

The resulting chain of poly-9-vinylanthracene contains, therefore, the

segments resulting from the infrequent 1-6 addition, the latter being separated by the conventional units of the 1-2 adduct. The "wrong" monomer


addition is faster than the conventional one but nevertheless retards the polymerization by slowing the conventional 1-2 addition of the next monomer.

Attention was drawn by Schreiber17 to the reaction of PhLi with methylmethacrylate leading to the elimination of LiOCH3 and the formation of:

CH2=CH·(CH3)COPh ketone.

He suggested that a similar process may account for the termination of methyl methacrylate propagation, i.e.,

J\N\CH2C·(CH3)(CO·OCH3),Cat+ + CH2 QCH3)(CO·OCH3)

~ J\N\CH2C·(CH3)(CO·OCH3)-CO·QCH3)=CH2 + CatOCH3·

This wrong addition of the growing polymer to the carbonyl group of the monomer terminates the propagation (see p. 120).

4. Chain-Transfer

Chain-transfer is the most significant mode of termination of cationic polymerization of vinyl monomers. Its activation energy is higher than that of propagation, at least for this class of monomers. Consequently, the molecular mass of the formed polymers increases with decreasing temperature, an observation first stressed by Flory18 and illustrated by Fig. 4.2. Interestingly, for the isobutene system, and perhaps also for the cationic polymerization of other vinyl monomers, the respective "activation energy," Edp , decreases in the lowest temperature range. 19 The degree of polymerization of the produced macromolecules is determined by the ratio kplkm and the observed "activation energy" Edp = Ep - Etr is negative. Most probably, for the systems discussed here the decrease of its negative value is caused by the decrease of Etr• It may be that two distinct reactions contribute to the chain-transfer, one proceeding with high activation energy and high A factor, whereas the activation energy and A factor are low for the other. Alternatively, the high activation energy of the unimolecular reaction of the growing carbenium ion-pairs, yielding the HA acid protonating the monomer, may dominate at higher temperatures, whereas the low activation energy bimolecular reaction of the free carbenium ion with the monomer might be responsible for the second kind of deprotonation. The contribution of the latter to the overall transfer process is enhanced at low temperatures by the mere increase in the fraction of free ions, since the heat dissociation of ion-pairs is negative. Indeed, the aHdi>s of trityl


f o

100 r------r------,------.----,-----,---,

80

60

40 I

I 1--/ ,----r

20

-;; 10 I II. Q 8 I

6

2

1 L-____ -L ____ ~ ______ ~ ____ L_ __ ~ __ ~

4.0 5.0 6.0 7.0 8.0 9.0 10.0

Figure 4.2. The Arrhenius plot of molecular weight of polyisobutene. Note, the rather abrupt change of slope at the very low temperature range implying a change in the mechanism of polymerization.

hexachloroantimonate in methylene chloride was reported20 to be -3 kJ/ mole.

The direct bimolecular transfer of ~ proton to monomer is restricted to free carbenium ion. In fact, no direct decomposition of the free ion into HA is possible due to the absence of counterion. On the other hand, the bimolecular deprotonation of the carbenium ion-pair through its collision with the monomer is unlikely. The monomer is less basic than the Acounterion and hardly can compete with the latter for the ~ proton. Hence, the chain-transfer by ion-pairs results from their unimolecular decomposition yielding HA acid, which in tum, protonates the monomer and ini-


tiates the propagation of a new growing center. It would be interesting, therefore, to investigate the effect of dilution, at a constant monomer concentration, on the molecular mass of the resulting polymers produced by the chain-transfer.

Conceptually the simplest chain-transfer process results from the reactions of the growing polymers with solvents, impurities, and so forth, discussed in Section 1.5, provided that the resulting ions are sufficiently reactive and capable of initiating another chain addition. For example, cationic polymerizations performed in chlorinated hydrocarbons are accompanied by chlorine transfer to the polymers yielding aliphatic cations capable of sustaining further polymerization.

s. Proton Traps

The sterically shielded amines such as 2,6-di-t-butylpyridine, referred to as proton traps, exert a complex influence upon the course of cationic polymerization. These agents are strong bases, react readily with any protondonating species, but do not act as nucleophiles since the bulky substituents prevent any electrophile of approaching the nucleophilic nitrogen. The presence of proton traps in a system cleanses it, and removes the impurities which otherwise would initiate delayed polymerization, albeit slowly. Such a slow and delayed initiation broadens the molecular mass distribution of the produced macromolecules.

Does the presence of proton traps prevent the chain-transfer to monorner, as claimed by some investigators? The delayed initiation resulting from the formation of the AH acid produced in the course of the indirect chain-transfer is prevented indeed by the traps which react with the acid faster than the monomer. Prevention of this repetitive process greatly retards the polymerization and sharpens, as stated above, the molecular weight distribution of the formed polymers. The traps compete also with the monomer for any cationating initiator which starts the polymerization by protonating the monomer, but they do not affect other cationating agents. Since the free carbenium ions may act as protonating agents, their direct interaction with monomer may be also prevented, or at least slowed down, by the traps.

In conclusion, the proton traps cleanse the system by removing the impurities acting as proton donors, eliminate the contribution of free ions to the initiation, and prevent the initiation by the HA acid resulting from the decomposition of growing polymers possessing 13 protons. As a result the polymerization is drastically retarded and the molecular mass of the formed polymers is somewhat sharpened.

Elementary Steps of Polymerization Other Than Initiation or Propagation I 333

6. Polymerization of Isobutene Initiated by Cumyl Chloride and Its Analogues

This reaction has been singled out for a detailed discussion in view of its considerable value for the synthetic polymer chemistry.

The polymerization of isobutene initiated by the BCl3-activated cumyl chloride or its bi-functional analogue, CIC(CH3h,C6H4C(CH3)2·CI, proceeds according to the scheme21 :

kr

Ph'C(CH3hCI + BCl3 ~ Ph·C+(CH3b BCli , (1) kb

ki

Ph·C+(CH3)2,BCli + CH2=C(CH3)2 ~ (2) Ph·C(CH3h~H2C+ (CH3)2,BCli ,

kp

Ph'C(CH3h-CH2C+(CH3h,BCli + CH2=C(CH3)2 ~ (3) Ph·C(CH3h-CH2C(CH3h·CH2C+(CH3bBCli, etc.

The propagation is terminated by the reaction of growing polymers with the initiator generating a new cumyl-like cation that initiates the formation of next polymer molecule:

k tr

Ph·CMe2·(CH2CMe2)n·CH2C+Me2,BCli + CICMe2·Ph ~ (4) Ph·CMe2·(CH2CMe2)n·CH2CMe2CI + Ph·C+Me2,BCli·

The DP n of the produced polymers is determined, therefore, by the rate of propagation (kp'[M)) and the rate of transfer (ktr·[Initiator)), i.e.,

DPn = (kplktr)'[M]/[Initiator],

a relation verified by the experimental data. The repetition of such reaction cycles was attributed to the chain-transfer.

However, as shown by the detailed calculations,22 the stationary concentration of the growing polymers is maintained mainly by the dynamic equilibrium (1) slightly perturbed by initiation (2) and transfer (3). The perturbation is insignificant at the short relaxation times 7 of the equilibrium (117 = kr[BC131 + kb ) determined recently by Vairon23 and found to be a few milliseconds only.

The synthetic value of the above procedure gains appreciably by its extension to the bi-functional initiators. Under comparable conditions te-


lechelic polymers are produced then, i.e., polyisobutylenes terminated on both their ends by t-butyl chloride groups:

Procedures for the conversion of t-butyl chloride groups into various useful terminal groups were developed by Kennedy21b and thus the preparation of numerous telechelic polymers by cationic polymerization became feasible.

7. Ring-Chain Competitions

7.1. Branching and Ring Formation

An ionically growing macromolecule may abstract a charged moiety, e.g., a proton or a hydride ion, from a small molecule in a process terminating its growth and producing a new ion capable of initiating the growth of another macromolecule. This is a typical chain-transfer process discussed previously. A similar transfer may take place in a reaction between the growing end-group of a polymer and some of its segment reached through bending the polymer chain. Such an event results in branching, e.g.,

I\I\I\CH2CHPh-CH2CPh + I\I\I\CH2CHPh-CH2CPhl\l\l\

I

H I

- H

I\I\I\CH2CHPh-CH2CPhl\l\l\ + I\I\I\CH2CHPh-CH2CPhl\l\l\.

The ion formed in the midst of a polymer chain may react with the still available monomer and subsequently a branch grows from the newly created reaction center. Similar branching, although rare in the case of living polystyrene, does take place often in the high-pressure/high-temperature radical polymerization of ethylene.

The interaction of the growing end-group of a polymer with some of its segments may lead to another outcome than branching. The growing endgroup may become bonded to the attacked segment, while its linkage with the following segment is ruptured, e.g.,


Such a reaction results in the formation of a cyclic oligomer and in shortening the length of the original polymer.

It should be stressed at this point that another reaction may lead to the ring formation, namely the linkage of both ends of a macromolecule. This reaction, referred to as end-biting, is distinct from the previously mentioned one known as back-biting. Both are approximately thermoneutral, because as a bond is formed in producing the ring, another bond is ruptured in the separation of the latter from the residual linear polymer.

Initiator AB converts monomer M into a linear polymer.

A·M·M ... M·B,

where B denotes the growing, active end-chain. The following scheme represents, therefore, the end-biting regenerating the initiator:

A·M·M ... M'B~AB+~M .. ~

described later for the sake of briefness as

where Mn denotes an n-neric ring. Alternatively, the end-biting may yield A·B·Mm an n-meric ring with one segment possessing the active center from which a branch may grow:

The formation of cyclic oligomers in the course of polymerization, especially cationic polymerization of heterocyclic monomers, was reported by several investigators in the early 1950s. In fact, under some conditions the cyclic oligomers were the main products of the reaction. Their formation was intriguing and questions were raised about the modes of their formation. This phenomenon was, and still is, not only interesting per se, but important from the practical point of view, because some valuable properties of linear polymers are lost on formation of rings. On the other hand, some macrocyclics, e.g., crown ethers, are valuable materials and polymerization may provide a convenient route to their synthesis. Let us consider, therefore, the general problem of the chain-ring competition.

7.2. Ring-Chain Equilibrium

The question arises, "What are the proportions of the cyclic oligomers of various sizes formed by the back-biting?" There are two aspects of this


question that need to be distinguished: the thermodynamics of back-biting and its kinetics. The classic treatment of the thermodynamics of ring formation was developed by Jacobson and Stockmayer,24 who considered the thermoneutral equilibrium:

under the following assumption: (1) The conformations of linear polymers obey the Gaussian statistics. (2) The rings are strainless. (3) The probability of opening an n-meric ring is proportional to n. The conversion of chains into rings and vice versa are thermoneutral. Two basic conclusions were derived from this treatment:

1. The relative concentration of i-meric rings is given by const. (i - 5/2).

2. At and below some critical initial concentration [M]Crit only rings are present in the equilibrated mixture.

These conclusions were verified experimentally by several investigators, their results being reviewed by Semlyen.25 Let it be stressed, however, that Jacobson-Stockmayer theory assumes the cyclic oligomers to be strainless, and hence, it applies only to large rings where the strains caused by the bond bending, twisting, intramolecular repulsion, and so forth, could be neglected.

7.3. Overview of Earlier Observations

Studies of formation of cyclic oligomers led to many conflicting observations and diverse theories reviewed by Goethals.26 It was reported that these materials are not only formed during some polymerizations but they are also the products of some polymer degradation, raising the question whether the cyclic oligomers may be produced directly from the monomers or originate entirely through the degradation of polymers. For example, polyoxirane, formed by anionic polymerization of ethylene oxide is degraded through a chain process into dioxane by triflic acid, as shown by the following scheme:


Repetition of these events leads eventually to a quantitative degradation of the polyoxirane into dioxaneY

An important feature of the back-biting is revealed by this example: i.e., the feasibility of a complete degradation of a linear polymer into small cyclic oligomers through a kind of a chain reaction. Such a degradation resembles the depropagation, e.g.,

(discussed on p. 183). However, while the product of depropagation is a polymerizable monomer, the product of degradation in the previous example is a cyclic oligomer not capable of polymerization, i.e., dioxane. In other words, depropagation is a reversible reaction, whereas the degradation could be irreversible.

The degradation need not ensue from the active end of a growing polymer. For example, triflic acid may induce the degradation by reacting with a middle segment of a polyoxirane

-cH2CH20CH2CH20CH2CH20CH2CH2G- -cH2CH20CH2CH20S02CF3 tt --+

and the resulting ester would be degraded further through the previously described process.

The conversion of a monomer into a polymer followed by the degradation of the latter into a larger cyclic oligomers is frequently reported in the literature. For example, substituted thiiranes rapidly polymerize (within few minutes), and subsequently slowly degrade into a mixture of cyclic tetramers (-50%) and a smaller amount of cyclic pentamers.28

A striking example of such a behavior is provided 2,3-butene episulfide. Polymerization performed at O°C yields quantitatively linear polymers within few minutes. However, after few hours, the polymers are partially degraded, and the reaction performed at 25°C yields only cyclic oligomers.29

Knowledge of stereochemistry of the products of degradation is helpful in unraveling the mechanism of this reaction. However, the complexity of the problem is well illustrated by the isolation of 22 isomers of the cyclic tetramers of propylene oxide, out of 23 theoretically possible. These differ in the head-to-tail and head-to-head structure, as well as in the cis and trans isomerism. 30


The importance of configuration of the recurring units of a polymer chain is demonstrated by the different products of degradation of cis and trans poly2,3-butylene sulfide.

With some monomers, cyclic oligomers are formed during polymerization but not after its cessation. This indicates that polymers and cyclic oligomers are formed by two independent reactions, e.g., by direct backbiting, or by numerous trans-alkylations yielding the thermodynamically most stable products.

An instructive review paper31 shows how confusing could be the correct experimental evidence in unraveling the mechanism of polymerization of cyclic oligomers, especially in studies of cyclic oligomeric acetals. For example, polymerization of acetals initiated by protic acid gives linear chains terminated by OH and OCHi end-groups, the latter being the growing, active one. The propagation competes with cyclization arising from endbiting and back-biting. The end-biting produces only cyclic oligomers, but it wanes as the polymerization proceeds and the polymers get longer. The back-biting persists but it yields linear polymers as well as the cyclics. Thus, the linear polymers appear at later stage of the reaction, especially when ke » kb' whereas the small cyclic oligomers are the dominant products at earlier stages. This phenomenon is referred to as the kinetically enhanced formation of cyclics.

Still further complications may arize from changeable nature of the endgroups. This seems to be the case in polymerization of acetals, and model experiments allowed the investigation of the details of the end-groups interconversion.

In conclusion, the competition of propagation with the end-biting and back-biting greatly complicates the kinetics of these reactions. The results may be qualitatively different, depending on the initial conditions of the system and on the relative values of the rate constants kp, kemand kbm see the scheme shown below:

kp

A·Mn·B + Ml ~ A·Mn+1·B,

ken

A·Mn·B ~ AB + Mn ,

and

kbi

A·M·n·B ~ A·Mn_;,B + Mj •

for i < n. For a greater generality we may allow also for the incorporation of the n-meric ring into a chain, i.e.,

kpi

A·Mn·B + M j ~ A·Mn+jB.


The dependence of the rate constants on the size of the reacting species complicates every calculation. For steric reasons some sizes may be strongly preferred or avoided, e.g., in the polymerization of hexamethylcyclosiloxane cyclic oligomers of ring sizes from 6 to 48 (in SiO units) were detected, but those with ring sizes 6, 12, 18, 24 were 10 to 100 times more abundant than the other. 32 This fact strongly suggests that in this reaction the oligomers are formed from the monomers. The distinction between the end-biting and back-biting is justified by the different character of these reactions.

Various kinetic schemes of chain ring-competition were proposed. The competition between back-biting leading to the ring formation and the propagation of polymerization depends, first of all, on the concentration of the monomer. The back-biting and end-biting being unimolecular reactions, are independent of the monomer concentration, whereas the propagation is a bimolecular reaction and its rate increases with the monomer concentration. In addition, the factors such as the nucleophilicity of the monomer and the polymer segments, the stereochemistry of the ring, the orientation of the colliding partners, and the flexibility of the chain all have bearings on the rates of these processes.33 Some kinetic schemes of polymerization proceeding with macrocyclization are described in the literature (e.g., refs. 33 and 34) and the detailed computer studies of some cyclizations were reported by Slomkowski. 35

Detailed explanations of how to produce a program for computer calculations were outlined in a paper by Penczek and his colleagues.36

7.4. Simultaneous Polymerization of Propylene Sulfide and Cyclization of Its Polymer

We close this discussion by describing the cationic polymerization of propylene sulfide proceeding simultaneously with macrocyclization. This reaction, investigated by Van Ooteghem and Goethals,37 illustrates some interesting features of this process and shows its diversity.

The kinetics of this polymerization initiated by triethyl oxonium ions proceeds in two stages. A fast, virtually instantaneous polymerization consumes all the initiator but only a small fraction of the monomer, converting it into growing polymers that are rapidly terminated. Thereafter, the monomer consumption continues, but only slowly, until its exhaustion. Significantly, the addition of a fresh monomer to the quiescent solution left after the exhaustion of the previous monomer batch, reinitiates the slow polymerization. Its rate is comparable to that recorded at a similar condition during the previous slow reaction, and this process continues until the exhaustion of the second monomer batch.

The first stage of the overall reaction, consisting of a fast, rapidly terminated polymerization, implies a very fast initiation and a relatively fast


propagation:

CH3·CH-CH2 + Et30+ ~ CH3CH-CH2 + Et20, "'-/ "'-/ s s+

I Et

For geometrical and other reasons, the back-biting attack of the active three-membered sulfonium ion takes place preferentially on the CH2 group of the polymer chain by four units remote from the growing end-group, producing a 12-membered sulfonium ion ring at the end of the polypropylene sulfide:

This is the pseudotermination step since the resulting polymer reacts, albeit only slowly, with the original monomer and reforms then a new threemembered active sulfonium ion on the polymer end, simultaneously with the expulsion of an inert 12-membered cyclic oligomer:

+/ C1

H2

--<:H2CH(CH3)'SCH2C(CH3)S"

CH·CH3

+

H3C

" CH-S-H2C

/ " CH2 CH--cH3 I I S S I I CH CH2

/ " / H3C CH2-S-HC

" CH3

Elementary Steps of Polymerization Other Than Initiation or Propagation 1341

The newly formed active polymer grows for a while until its growth is terminated by the back-biting yielding the terminal 12-membered ring. Thus the repetitive cycle of these reactions degrades the polymers (at low propylene sulfide concentration the addition of one monomer molecule removes four of them as a 12-membered oligomer).

The polymer terminated by the 12-membered sulfonium ion cannot be referred to as a dormant polymer. Its reactivation takes place by a bimolecular reaction and not by a unimolecular rearrangement. Surely, it cannot be treated as a living polymer either because the addition of the monomer leads to degradation. This example shows the existence of still new kinds of species, neither growing, nor dormant or dead, still participating in ionic polymerizations.

7.5. Alteration of Growing Polymer End-Groups

The active end-group of polymers, say of a living poly-A, may be converted by an appropriate synthetic procedure into another active end-group capable of continuing the polymer growth but by a mode of propagation different from the previous one. For example, a carbanion end-group allowing for anionic propagation may be converted into a group permitting cationic growth. Such a procedure is helpful in the preparation of block copolymers, free of homopolymers, composed of two monomers polymerizing by different propagation modes.

A conventional and efficient method of preparation of AB copolymers, with exclusion of homopoly-A and homopoly-B, calls for sequential addition of the two respective monomers to a system polymerizing under living conditions. Thus, polystyrene-polyethylene oxide diblock is readily prepared by sequential anionic polymerization. However, this method fails for the anionically polymerized styrene and oxepane or tetrahydrofuran. The latter two monomers do not polymerize anionically, although they are readily polymerized by cationic initiators. Hence the anionically prepared polystyrene of uniform size, desired molecular mass, and free of side products, is converted into a bromide by reacting it with an excess of m-xylylyl dibromide

Ml\CH2CHPh,Na+ + BrCH2·C6H4 ·CH2Br

Ml\CH2CHPhCH2·C6~·CH2Br + NaBr.

The bromide is isolated and purified and subsequently reacted with AgPF6. The ultimately formed carbenium ion initiates then cleanly the polymerization of the above cyclic ethers.

Development of such techniques of conversion of one kind of active endgroup into another was pioneered by Richards and his co-workers,38 who


described in a brief article the conversion of an anionic end-group into a cationic one, a cationic end-group into an anionic one, a cationic end-group into a radical one, and so forth.

The active end-groups may spontaneously isomerize. The conversion of E form into Z or vice versa observed for some classes of active polymer end-groups is an example of such a process. The monomers, such as dienes, acrylates, methacrylates, and 2-vinyl pyridene, are virtually planar, and therefore acquire either a cis or a trans form in respect to the central single C-C bond which possesses some double bond character. For example, the following are the cis and trans isomers of methylmethacrylate:

and

cis trans

Their cis-trans isomerization, resulting from rotation around the central C-C bond is substantially faster than the propagation. Hence, we may assign a probability for the approach of each form to the active center of a polymer.

An allylic or enolic anion formed on the addition of such a monomer to the active center of a growing polymer acquires the Z or E structure:

Me I

"VV'CH2-C ~'C=O /

MeO Zanion

Me I

"VV'CH2-C '\.

-,;C-OMe

o E anion

The Z - E isomerization is slow compared to the propagation, hence the structure acquired by the enolic anion on the addition of the monomer to the active center of the polymer is preserved during the time interval elapsing between the two successive monomer additions.

The probabilities of formation of the Z and E stereoisomers affect the microstructure of the ultimately formed polymer chain. The ratio of E/Z, as well as the ratio mezolracemic end-groups was determined by analyzing the 13C NMR spectrum of polymethylmethacrylate polymerized anionically in THF and terminated by the addition of CISiMe3.39 The results revealed the influence of the counterion. For Li + and Na + the s-cis approach of


the monomer to the enolate is favored due to the coordination of these small cations with the carbonyl group of the monomer. This, as indicated by the following scheme, leads to the formation of Z group on addition of the monomer to the Z end-group.

MeO~O ~ ~~OMe Z isomer + s-cis MMA

H Zisomer

The coordination is not feasible for the bulky Cs + cation due to steric hindrance, and then the s-trans monomer approach is favored, as shown by the scheme given below.

MeO

MeO CS+ • .o

"rOMe

~ PMMA ~'Me ~O

H ~

PMMA:

H E isomer + s-trans MMA E isomer

Let it be stressed that spontaneous isomerization of E into Z or Z into E through rotation around the C-C axis is too slow and hardly affects the microstructure of polymer chain. However, such a transformation does take place in the course of propagation. The addition of s-trans monomer to Z enolate yields the E isomer, and the addition of s-cis monomer to E isomer yields the Z anion.

References

1. (a) Muller, A.H.E. 1989. in G. Allen, J.C. Bevington (eds.), Polymer Science, Pergamon Press, New York.

(b) Spach, G., et al. 1962. J. Chern. Soc. (Lond.) 355.

2. Fontanille, M. unpublished results.

3. Comyn, J., and Glasse, M.D. 1983. J. Polym. Sci. 21, 209.

4. Kennedy, J.P., and Marechal, E. 1982. Carbocationic Polymerization, John Wiley, New York.


5. Kennedy, J.P., and Renegachary, S. 1974. Adv. Polym. Sci. 14, 1.

6. Gezner, F.J., Hocker, H., Miiller, A.H.E., and Schulz, G.V. 1984. Eur. Polym. J. 20, 349.

7. Chien, J.C.W. 1959. J. Am. Chern. Soc. 81, 86.

8. Antowiak, J.A. 1971. Polym. Prep. Am. Chern. Soc. 12 (2), 393.

9. Fetters, L.J. 1964. J. Polym. Sci. B2.

10. Szwarc, M., et al. 1958. Chern. Ind. (Lond.) 1473.

11. Rempp, P., Frenta, E., and Hertz, J.A. 1988. Adv. Polym. Sci. 86, 145.

12. Kennedy, J.P., and Langer, A.V. 1964. Adv. Polym. Sci. 3, 508.

13. Kennedy, J.P. 1967. in Encyclopedia of Polymer Science and Technology, Vol. 7, p. 754, Interscience, New York.

14. Moreau, M., Matyjaszewski, K., and Sigwalt, P. 1987. Macromolecules 20, 1456.

15. Breslow, D.E., Halse, G.E., and Matlock, A.S. 1957. J. Am. Chern. Soc. 79,3760.

16. Pepper, D.C., and Reilly, P.J. 1961, 1966. Proc. Chern. Soc. 460. Proc. R. Soc. (Lond.), A291, 41.

17. Schreiber, H. 1959. Makromol. Chern. 36, 86.

18. Flory, P.J. 1953. Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY.

19. Kennedy, J.P., and Thomas, R.M. 1960. J. Polym. Sci. 45,227,46,223.

20. Kalfoglu, N., and Szwarc, M. 1968. J. Phys. Chern. 72, 2233.

21. (a) Kennedy, J.P., and Smith, R.C. 1979. Polym. Prep. Am. Chern. Soc. 20, 316.

(b) Kennedy, J.P. 1970. J. Org. Chern. 35, 532. 22. Szwarc, M. 1990. Macromolecules 23, 4616.

23. Vairon, J.P., Rives, A., Burel, C. 1992. Makrornol. Chern. Symp. 60,97.

24. Jacobson, H., and Stockmayer, W.H. 1950. J. Chern. Phys. 18, 1600.

25. Semlyen, J.A. 1976. Adv. Polymer. Sci. 21, 41.

26. Goethals, E.J. 1977. Adv. Polym. Sci. 23, 103.

27. Kobayashi, S., Morikawa, K., and Saegusa, T. 1975. Macomolecules 8, 952.

28. Lambert, J.L., Van Ooteghem, D., and Goethals, E.J. 1971. J. Polym. Sci. 9,3055.

29. Van Craeynest, W., and Goethals, E.J. 1976. Eur. Polym. J. 12, 859.

30. Katnik, R.J., and Schafaer, J. 1968. J. Org. Chern. 33, 384.

31. Szymanski, R., Kubisa, P., and Penczek, S. 1983. Macromolecules 16, 1000.

32. Chojnowski, J., Mazurek, M., Scibiorek, M., and Wilczek, L. 1974. Makromol. Chern. 175, 3299.

33. Penczek, S., and Slomkowski, S. 1989. in G.c. Eastmond et al. (eds.) Comprehensive Polymer Science, Vol. III, p. 725, Pergamon Press, New York.


34. Penczek, S. et al. 1980. Adv. Polym. Sci. 37, 104-119.

35. Slomkowski, S. 1984, 1985.1. Makromol. Sci. A21, 1383. Makromol. Chern. 196,2586.

36. Matyjaszewski, K., Zielinski, M., Kubisa, P., Slomkowski, S., Chajnowski, 1., and Penczek, S. 1980. Makromol. Chern. 181, 1469.

37. Van Ooteghem, D., and Goethals, E.l. 1974. Makromol. Chern. 175,1529.

38. Richards, D.H. 1985. ACS Symp. Ser. 286, 87.

39. Baumgarten, 1.L., Milller, A.H.E., Hoggen-Esch, T.E. 1991. Macromolecules 24, 353.

5

Ionic Copolymerization

1. General Schemes of Copolymerization and the Crossover Constants

A review of anionic copolymerization, covering the material available in 1968, was published by one of us. 1 It dealt with the question of how to modify the concept of reactivity ratios to make it adaptable to ionic copolymerization, and how solvents and counterions affect the composition of the copolymers. Their magnitudes illustrated by Table 5.1 were surprising at that time.

The copolymerization involving lithium counterions is greatly affected by small amounts of polar agents in hydrocarbon solvents. The presence of 4% of diethyl ether in a hydrocarbon solvent led to a copolymer virtually identical with that formed in the pure ether. Tetrahydrofuran (THF) is even more effective. At the concentration providing only one molecule of THF for each polymer molecule the copolymer is identical with that formed in the bulk of THF.

These effects were attributed to the change of the structure of the polymers from covalent to ionic. However, Worsfold et aU pointed out that the spectra of lithium polystyrene and sodium polystyrene are similar, whether in hydrocarbon or polar solvent, implying their ionic structure.

The irreversible copolymerization of two monomers A and B involves four steps, namely:

-A* + A~ -A'A*, kPA'

-A* + B ~ -A'B*, kA*B'

-B* + A~-B'A*, kB*A'

-B* + B ~ -B'B*, kpB '

347


The star denotes the active end-group of the growing polymers. The k's

are the respective rate constants referring to the anionic or cationic polymerization. A further notation is needed to specify the nature of the active end-groups involved in the reaction, i.e., whether the rate constants pertain to free ions, ion-pairs, or any other kinds of active species.

The magnitudes of the rate constants of the homopropagation, kpA and kpB, have been amply discussed in the previous chapters. Presently, our attention should be focused on the magnitudes of the kB*A'S and kA*B'S,

the so-called crossover constants. A simple method for determining the absolute values of the crossover constants is provided by the living polymerization technique. Monitoring some waning feature of the end-groups of a living poly-A * caused by the addition of monomer B, in the absence of monomer A, to a solution of poly-A * allows the evaluation of the respective kA*B' For example, the optical absorbance of poly-A * at its Amax decays under such conditions, and the rate of its decay allows the evaluation of the respective kA*B,

kA*B poly-A * + B -----+ poly-AB*.

Alternatively, one may monitor in a similar experiment the waxing of the optical absorbance of poly-B* at its Amax, and evaluate from such data the same constant, the observable kA*B' In either case the spectra of the homopoly-A * and homo-poly-B*, as well as their respective extinction coefficients, have to be known, and the contribution of the absorbance of polyA * to the optical density of pOly-B* at its Amax, and vice versa, have to be properly allowed for. However, the observable kA*B, similar to the observable k pA , is given by ".£-y;kA*B,;, whenever more than one species of - A * are present in the polymerizing system.

Note that this method of determining kA*Bis not affected by the reaction:

-AB* + B -+ -ABB*, etc.

Table 5.1. Percentage of styrene in a copolymer produced from a 50:50 mixture of isoprene and styrene (extrapolated to 0% conversion)

Initiator

Solvent Metallic Li ,I

n-BuLi Metallic Na

Benzene 15 18 66 Bulk 15 17 66 Et3N 59 60 77 Et20 I 68 68 75 THF 80 80 80

Ionic Copolymerization / 349

provided that [B] » [poly-A *] and no penultimate effect affects the spectrum or reactivity of poly-AB* (see p. 366 for the example to the contrary). The above method could be adopted, of course, to any other spectroscopic technique provided that the appropriate feature of the pertinent spectrum has been judiciously chosen.

It is convenient to determine the crossover constants by utilizing the stopped-flow technique. This method was chosen by Shima et al. 3 to determine the crossover constants of living sodium polystyrene into a variety of derivatives of styrene or some other monomers in THF solution. The results are collected in Table 5.2 and presented in Fig. 5.1 as a plot of [og.ksty*.x vs. Hammett's (J.

At the time of this study the distinction between the propagation of ionpairs and free ions was not yet appreciated, and the constants quoted in Table 5.1 are, therefore, the overall constants arising from the reactions of the free ions and ofthe ion-pairs, i.e., 'YkA"*B + (1 - 'Y)kX*B' However,

Table 5.2. Values of crossover constants for sodium polystyryl in tetrahydrofuran (THF) solution

Monomer

Vinyl mesitylene ~-Methylstyrene a-Methylstyrene p-Methoxystyrene p-t-Bu-styrene

1 18 27 50

110

[-Sty- ,Na+] = -3.10- 3 M; T = 25°C.

Monomer

Styrene 1,1-Diphenylethylene 4-Vinylbiphenyl p-Fluorostyrene 1-Vinylnaphthalene

5,...--,,----.--.----.,.--....-----,,----,

1

O~~--L--~-~--~-~~--~

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 a

950 2400 1700 1800 8000

Figure 5.1. The Hammett (J" - P plot for the addition of p-substituted styrenes to living sodium polystyryl in tetrahydrofuran at 25°C. Note the positive value of p.


in view of our present knowledge of these systems we realize that these data are only by about 10% lower than the expected k;'*B'S of the crossover constants of the free anions.

The same approach was adopted in the determination of all four constants of the anionic copolymerization (Na + counterions, THF solvent) of the pairs: styrene-p-methylstyrene, styrene-p-methoxystyrene, and styrenevinylmesitylene. The results4, listed in Table 5.3, show the inclination of the above anionic copolymerization to produce block, rather than some random copolymers.

Examination of the plot shown in Fig. 5.1 reveals that the linear Hammett's relation between the In(kA*B)'s (constant A* and variable B's) and (J results in a positive p constant of + 5.0. This high positive p value should be compared with the positive p = + 0.5 characterizing the same series of monomers in a radical copolymerization.5 They demonstrate that the free styryl carbanion and the styryl radical behave as nucleophiles in those reactions, the anion, as expected, being a stronger nucleophile than the styryl radical. On the other hand, the copolymerization of the same monomers initiated by a Natta catalyst6 is characterized by a negative p of -0.95, as shown in Fig. 5.2. Hence, in this intrinsically anionic copolymerization the coordination of the monomer with the positively charged transition metal of the catalyst seems to be the rate-determining step. Indeed, a question may arise whether in an anionic polymerization propagated by ion-pairs the rate-determining step is the association of the monomer with the cation (the counterion) or its addition to the negatively charged growing end-groups. The sign of p discriminates between these alternatives, pointing out to the first alternative when p is negative, and to the second one when its value is positive. Of course, this rule with reverse signs would apply to cationic copolymerization.

Table 5.3. Copolymerization constants

Monomer 1 kpJ kl ,2 Monomer 2 kp2 k2,1

Styrene 950 180 p-Methylstyrene 1150 210 rl = 5.3 r2 = 0.18

rl ,r2 = -1 Styrene 950 50 p-Methoxystyrene 1100 50

rl = 19 r2 = 0.045 r1.r2 = 0.85

Styrene 950 0.9 Vinylmesitylene 77 .3 r1 = 1060 r2 = 4.10- 3

r1.r2 = -4.0

Ionic Copolymerization 1351

0.5

t p=-O.IIII

~

-- 0 0

i

-0.6

-0.3 -0.2 -0.1 o 0.1 0 .2 0.3 0 .•

Figure 5.2. The Hammett (J' - P plot for copolymerization of substituted styrenes with styrene in a polymerization initiated by Ziegler-Natta catalyst. Note the negative value of p.

A positive sign of the p parameter was found again in the analogous investigation of the addition of the disubstituted 1,1-diphenylethylenes to living lithium polystyrene in benzene or cyclohexane,7 although its value is substantially lower than that characterizing the substituted styrenes (see Fig. 5.3). The living lithium polystyrene in benzene or cyclohexane is dimeric (see p. 159), but the polymerization results from the presence of a minute fraction of the active unassociated ion-pairs coexisting in equilibrium with the dimers. The former propagate the reaction. The lower p value ( -1. 7 -1. 9) is due to the influence of the counterion on the monomer, and apparently also to higher reactivities, and hence lower selectivities, of 1,1-diphenylethylene when compared with that of styrene. A positive p value was reported also for the initiation of polymerization of substituted styrenes with n-butyllithium in benzene.s

The decrease of the p values may be accounted for by two alternative explanations: a decrease of selectivity due to the increased reactivity of the monomers in a process involving their direct attack on a living polymer, or the formation of a complex of the monomer with the counterion of the growing polymer, followed by the monomer addition:


0.5 101 lip: _. 41 lie-

o " 41-t-Boa pp' 41-11e- 8

0.5101~ o kpH

- 0.5

o 1.9

o a

(A) 0.25

a

° (8)0.25

- 0.25

2.3- + 2 .•

° 0.5 1 .2x.

o Of)[pa

-0.5

o CAIoCULlftD wnw -1 -1 ....... M _

x CAIoCULlftD wnw ... 1 -

:U -.. 10M. -:-

o (D)O.16

Figure 5.3. The Hammett (J' - P plot for the addition of substituted diphenylethylenes to lithium polystyrene in benzene and in c-hexane. From ref. 7.

As evident from the inspection of Fig. 5.3, the decrease of the value of p in a series of ion-pairs of the growing polymers with the cations varying from Cs + to Li + may be attributed to the decrease of the monomers nucleophilicity caused by their increasing polarization by the cation.

Shima et al. 3 investigated also how the substituents placed on the living polystyrene affect the crossover rate constant (variable growing anions, while the polymerized monomer is kept constant). Their data, collected in Table 5.4, show that a substituent affects the crossover rate constant to a lesser degree when placed on the electrically charged growing anion than when it modifies the electrically neutral monomer. This had to be expected. Note, however, that the modification of the anion affects not only its reactivity but also the dissociation constant of its ion-pair into the free anion which is the main contributor to propagation. These effects are antagonistic. For example, an electron-donating substituent decreases the reactivity of a monomer in anionic copolymerization but by the same token


Table 5.4. Effects of substituents placed on living polystyrene on rate constants of the crossovers (constant monomer-styrene)

Substituent

p-Methoxy p-Methyl

1100 1150

Substituent

H a-Methyl

950 -500

Substituent

p, 0, 0-Trimethyl ~-Phenyl

70 0.3-1.0

increases the degree of dissociation of the respective ion-pair. Consequently, judging by the behavior of copolymerization, lumping together the contribution of ion-pairs and the free ions, such a monomer may appear to be more reactive or less reactive than the unsubstituted monomer depending which contribution is larger. This ambiguity does not occur in radical copolymerization.

Monomers M1, Mz, ... Mi, reacting with a growing polymer-X*, may be arrayed in a series of increasing reactivities with respect to -X*, i.e., by definition, Mi is more reactive than Mj , if kXi > kXj for i > j. The ordering is determined not only by the nature of the monomers but also by the nature of -X*, as well as by the conditions prevailing in the reaction. On the whole, whenever Mi is more reactive than Mj in an anionic copolymerization, a reverse inequality is expected in cationic copolymerization. Thus, the electron withdrawing substituents increase the crossover rate constant of a monomer in anionic copolymerization, but reduce them in cationic copolymerization. For example, in a copolymerization with an anionically growing polystyrene, isoprene is less reactive than butadiene by a factor of about 2 (the reactivity of one end of isoprene is greatly reduced by the methyl substitution9), and the reactivity of 2,3-dimethylbutadiene is reduced9 by a factor of -70 (the reactivity of both ends of this monomer is strongly reduced). By the same token, a-methylstyrene is less reactive than styrene4 in anionic copolymerization, methacrylonitrile is less reactive than acrylonitrile, and methylmethacrylate is less reactive than acrylate, and so on. Of course, the reverse behavior is found in cationic copolymerization where styrene is less reactive than a-methylstyrene and butadiene less reactive than isoprene.

In contradistinction to radical polymerization, the order of reactivity of monomers in ionic polymerization may be reversed by a change of solvent. For example, in tetrahydrofuran solution styrene is more reactive than butadiene towards sodium polystyrene, whereas in hydrocarbon solution the reverse is reported, butadiene is more reactive towards lithium polystyrene than styrene. This reversal of reactivities is caused presumably by a change of the mechanism of monomer addition. In THF the direct addition of monomer to the free polystyryl anion provides the largest contribution to the observed reaction, whereas in hydrocarbon solution the


complexation of the monomer with the lithium counterion of the living lithium polystyrene seems to be the first and decisive step of the ultimate addition.

2. Reactivity and Basicity of Monomers

The addition of numerous anions, derived from the substrates listed in Table 5.5, to four typical monomers (acrylonitrile, methylmethacrylate, styrene, and butadiene) was examined by Wooding and Higginson,lO who concluded that such a process may be compared with acid-base reactions: the more basic the anion, the more active it is in the addition. Their qualitative results are presented in Table 5.5.

The correlation of the basicity of the anion with its ability to be added to the monomer is not always justified. ll In the acid-base reaction, the process involves a proton transfer, a step affected by the bond dissociation energies of A-H and B-H and by the electron affinity of A· and B· radicals, whereas an A-B bond is formed in the copolymerization with the simultaneous dissolution of the second half of the C=C bond. In essence,the series of basicities are not identical with the series of nucleophilicities.

The ability of a monomer B to be added to a living polymer -A * is not always reciprocal. For example, methylmethacrylate readily adds to an anionically growing polystyrene. However, styrene is inert toward anionically growing polymethylmethacrylate. Hence, a diblock of polystyrenepolymethylmethacrylate is readily prepared by the addition of methylmethacrylate to living polystyrene, whereas a diblock polymethylmethacrylate-polystyrene cannot be produced by a sequential addition of styrene to living polymethylmethacrylate. On the other hand, diblocks of polystyrene-polyisoprene could be prepared in THF solution either by the addition of styrene to living polyisoprene or vice versa. In such systems it is possible to form a random copolymer by initiating the polymerization of a mixture of both monomers slowly added to the reactor containing an initiator.

3. Reactivity Ratios: Composition of Copolymer and Feed

In the classic studies of the radical copolymerization the reactivity ratios of two monomers were deduced by the conventional method based on the comparison of the composition of the copolymer obtained from a mixture of two monomers with the composition of the feed. Such a technique is acceptable in ionic copolymerization of two similar monomers, provided that the molecular mass of the copolymers is high, the tendency of block

Tab

le 5

.5.

Abi

lity

of

anio

ns t

o in

itia

te a

nion

ic p

olym

eriz

atio

n o

f so

me

repr

esen

tati

ve m

onom

ers

Rea

ctio

n pe

rfor

med

in

eth

er s

olut

ion

at 2

0°C

.

Ani

on d

eriv

ed f

rom

pK

A

cryl

onit

rile

M

ethy

lmet

acry

late

S

tyre

ne

Bu

tad

ien

e

Met

hano

l 17

+

E

than

ol

18

+

Ace

toph

enon

e 19

+

P

h 3C

OH

19

+

In

dene

21

+

+

P

heny

lace

tyle

ne

21

+

+

Dip

heny

lam

ine

23

+

+

Flu

oren

e 25

+

+

A

nili

ne

27

+

Xan

then

e 29

+

+

+

+

T

riph

enyl

met

hane

33

+

+

+

+

Rea

ctio

n pe

rfor

med

in

liqu

id a

mm

onia

sol

utio

n at

-33

°C.

Ani

on d

eriv

ed f

rom

A

cryl

onit

rile

M

ethy

lmet

acry

late

S

tyre

ne

Bu

tad

ien

e

Eth

anol

18

+

P

h]C

OH

19

+

D

iphe

nyla

min

e 23

+

+

F

luor

ene

25

+

Ace

tyle

ne

26

+

+

Ani

line

27

+

+

+

X

anth

ene

29

+

+

+

Tri

phen

ylm

etha

ne

33

+

+

+ T

he

addi

tion

doe

s ta

kes

plac

e; -

, no

add

itio

n is

obs

erve

d.

Not

e th

e se

quen

ces

in t

hese

tw

o se

ries

are

no

t id

enti

cal;

the

nat

ure

of

the

solv

ent

does

mat

ter.


formation low, and the effects caused by the initiator are distinguished from those arising from the propagation.

The extensive studies of Hatada and his colleagues12 of copolymerization of methylmethacrylate (MMA) with a series of methacrylate derivatives were accomplished by this classic approach. The copolymerizations were initiated by n-butyl lithium and performed in toluene or in THF at -70°C. The results were presented in the form of plots of the composition of the copolymer vs. composition of the feed shown in Fig. 5.4. The conversion was limited to a few percentages of the feed in order to maintain

0.5

M2=I -PrMA

0.5 .... ... '" cu E E ~

M2=t- BuMA ~ 0 0 8- 0 "'-

0 0 u u c 1.0 c 1.0 < < :: ::E ~ ~

0.5 0.5

0.5 1.0 U.S 1.0

MMA in Monome r MMA in Monomer

Figure 5.4. Composition of the feed and of the copolymer resulting from anionic copolymerization of monomers with methylmethacrylate (M1).


its constant composition in each experiment. Inspection of Fig. 5.4 shows that the change of solvent from THF to toluene only marginally affected the course of the copolymerization, the pairs MMA-dimethylbenzylmethacrylate and MMA - tritylmethacrylate were the only exceptions where the fractions of the second monomer in the copolymer were substantially higher for the copolymerization performed in toluene compared to that accomplished in THF.

The classic treatment of radical copolymerization leads to the well-known Mayo-Lewis equation,

d[A *]/d[B*] = ([A]/[B]}·{('1[A] + [B])/([A] + '2[B])}.

Here, [A] and [B] are the concentrations of the monomers in the feed, whereas d[A *] and d[B*] denote the momentary increase of the concentrations of A and B in the copolymers. The constants'1 = kpA/kA*B and '2 = kpB/kB*A' known as the reactivity ratios, are meaningful only for systems involving one kind of copolymer terminated by A * and only one kind of copolymer terminated by B*, say ion-pairs only. Whenever two or more copolymers coexist in the system, e.g., ion-pairs and free ions or living and dormant polymers, the'1 and'2 are undefined for such an ionic copolymerization and the Mayo-Lewis equation is then inapplicable. Unfortunately, this point has been not appreciated and the values of '1 and '2 quoted in the literature dealing with ionic copolymerization are probably incorrect.

In some ionic "copolymerizations" the product consists of two homopolymers. Two independent homopropagations proceed in such systems. An example is provided by the polymerization of mixtures of vinylcarbazole and oxetane (see p. 259). Such a situation arises when kll and k22 are relatively large whereas k12 and k21 are virtually zero. A common initiator may induce both polymerizations, and in the simplest case the relative rates of initiation are proportional to the ratio of the monomers concentrations. For living polymers the composition of the mixture, which could be mistaken for a copolymer, is proportional then to ([M1]/[M2D2, [Md and [M2] being the concentrations of the respective monomers in the feed. Such a situation was discussed by O'Driscoll. 13

A mixture of two or more components undergoes some demixing in the vicinity of ions or ion-pairs. Hence, in the vicinity of growing polymers the composition of the feed may differ from its composition in the bulk of the liquid. This phenomenon distorts the value of the reactivity ratios calculated from the nominal compositions of the feed and of the copolymer. Such a distortion could be quite substantial when the polarities of the comonomers differ greatly. It might be interesting, therefore, to compare


the reactivity ratios determined in this way with those computed from the directly determined propagation and crossover rate constants.

4. Copolymerization and Homopolymerization

The reactivity of a monomer cannot be judged by its rate of homopropagation. As a rule, a more reactive monomer yields a less reactive polymer end-group. This compensating effect upsets any expected correlation between the structures of monomers A and B and the propagation constants of their homopolymerization. However, one is justified in expecting such a correlation when the structures of the above monomers are compared with the ratio of the rate constant of the crossover A to B and the rate constant of homopropagation of the monomer A (the reciprocal of the reactivity ratio).

For the sake of illustration consider the reactivity of butadiene and isoprene with respect to either lithium polybutadiene or lithium polyisoprene in hexane. The results deduced from the study of McGrath14 showed that in hydrocarbon solvents and lithium counterion butadiene is more reactive than isoprene whether with respect to lithium polybutadiene or lithium polyisoprene, i.e.,

kp,Bu,obs > kBullsp,obs as well as klsplBu,obs > kp,lsp,obs'

Here the subscript obs refers to the observed rate constants, the subscript p to the rate constant of homopropagation, and Bu/lsp and lsp/Bu to the rate constants of crossover from the polybutadiene end-segment to isoprene and from the polyisoprene end-segment to butadiene, respectively.

Living lithium polybutadiene and lithium polyisoprene are tetrameric in hydrocarbon solutions (see p. 162) and only the monomeric species, present in minute concentrations in equilibrium with the tetramers, are capable of propagating the polymerization or of the crossovers. Hence,the directly determined crossover constants, or the homopropagation constants, are the products of the respective rate constants and the pertinent 114 roots of the equilibrium constants,

and

K-l 4-B - L' + ~ ( B - L' +) U , 1 ~ - U ,1 4,

K-l 4-1 - L' + ~ ( 1 - L' +) sp , 1 ~ - sp , 1 4,


Thus, the crossover constants reported by McGrath acquire the form:

and the respective rate constants of homopropagation are

One notes that the ratio

kp,Bu,ob)kBuIISp,Obs = kp,BJkp,Isp > 1,

as well as

kISpIBU,ob)kp,ISp,obs = kp,BJkp,Isp, > 1,

since the respective K's cancel. However, the ratio

because the ratio (KBulKIsp)1I4 is much smaller than 1 (the aggregation of lithium polybutadienyl is much stronger than that of lithium polyisoprenyl. Lack of understanding of these simple relations led some authors to the incorrect conclusion that butadiene is less reactive than isoprene in anionic copolymerization with Li + counterion (see ref. 15, p. 132 for the discussion of this erroneous claim).

5. System Li-Polystyryl and Li-Polybutadienyl in Hydrocarbon Solvents

The copolymerization of styrene and butadiene initiated in hydrocarbon solvents by lithium alkyls was first investigated by Korotkov16 who reported an unexpected phenomenon. The polymerization of, say, a 50:50 mixture of these two monomers starts slowly and initially consumes butadiene. As this monomer becomes depleted, the reaction speeds up and styrene then polymerizes rapidly. This peculiar behavior of this system is depicted in Fig. 5.5.

Korotkov offered an ingenious explanation of this phenomenon. Although shown later to be incorrect, it is worth describing because a similar system might be observed in the future. The monomers were treated as the solvents of the growing species, butadiene being a preferential solvent, whereas the more reactive styrene appeared to be a poorer solvating agent. Thus, butadiene is expected to be, virtually exclusively, in contact with the growing lithium end-groups and, being a less reactive monomer in hydro-


~

~ --j;f

.2 II ... G> I> j;f 0 u

100

80

60

.0

20

a

0/100 85/ 15

Parte "'onomer 100 Cyclohenne 1000 n-ButyWthlum 0. 15 (20 '" molel)

60 120 160 2.0 300 360 420

Time (min)

Figure 5.5. Copolymerization of butadiene and styrene initiated by BuLi in cyclohexane at 50°C. Note the slow initial polymerization of butadiene followed by the fast polymerization of styrene.

carbon solution, it propagates slowly. Only after its consumption, styrene gets a chance to come in contact with the growing centers and, being more reactive in hydrocarbon solution, it then propagates rapidly. * Indeed, this scenario was described recently by Teyssie. 17 He observed polymerization of oxiranes initiated in nonpolar solvents by AI-Zn oxyalkoxide. The less reactive methyloxirane was found to be more strongly absorbed by the catalyst, preventing therefore the access to it of the more reactive, but less strongly coordinated epichlorhydrine. Hence, the polymerization of a mixture of these two monomers speeds up after depletion of methyloxirane.

The correct explanation of the phenomenon reported by Korotkov was provided by O'Driscoll and Kuntz.Is As they pointed out, styrene homopolymerizes faster than butadiene under the conditions of the above experiments. However, the rate of crossover from lithium polystyryl to butadiene is much faster than the homopropagation of styrene. Hence, the addition of styrene is followed by the rapid addition of butadiene, whereas the addition of butadiene is not followed by the fast addition of styrene. *

This idea was confirmed by the experimental data of Morton and Ells19 and of Worsfold20 who determined the respective rate constants. Morton and Ells followed the addition of styrene to lithium polybutadiene in ben-

·In THF solution styrene is more reactive and competes effectively with butadiene for the active end-group, in hydrocarbon solvents styrene still homopropagates faster than butadiene but it is less reactive than butadiene towards the Li active center. Apparently, in THF the monomers react directly with the carbanions, whereas in hydrocarbon solvents they become first complexed with the Li + counterion and thereafter incorporated with the polymer chain.

Ionic Copolymerization I 361

zene by observing the waxing of the optical density at 436 nm on mixing the reagents. The choice of the wavelength was poor since the Amax of lithium polystyrene in this solvent is at 335 nm. The reaction is first order in styrene but 1/4 order in lithium polybutadiene (contrary to claims by Morton and Ells). The reverse addition was too fast to be followed by their experimental technique and the value of the respective pseudo-first-order constant was only estimated. The determination of the rate of addition of butadiene to lithium polystyryl was reported by Harris (unpublished data of Syracuse Group). His rates were about 100 times higher than those estimated by Morton and Ells, and his kinetics results demonstrated the 1/2 order character of the addition of butadiene to lithium polystyryl, confirming the suggestion of O'Driscoll and Kunz. 18

An interesting problem was raised by Worsfold. Why does butadiene or isoprene add so rapidly to lithium polystyrene, faster than styrene? Does their "vice-like" shape allow them to interact strongly with the Li cation and form a complex that eventually is converted into lithium polydiene? To answer this question, Worsfold investigated the rate of styrene addition (present in a large excess) to lithium polyisoprene in the presence and absence of isoprene. The rate of styrene addition was unaffected by the presence of the monomeric isoprene; hence the formation of the complex, if any, is disproved.

The four rate constants pertinent for styrene-isoprene copolymerization in cyclohexane at 47°C were reported by Worsfold and their values are displayed in Table 5.6.

6. Consequences of Equilibria Between Unreacted Aggregates and Reactive Nonassociated Polymers: Mixed Dimerizations

As mentioned before, the living lithium salts of polystyrene and polydienes are aggregated in hydrocarbon solutions into the unreactive associates. The latter are in rapid eqUilibrium with the reactive nonassociated polymers. Let us examine now the kinetic consequences of such associations.

Lithium polystyrene and the polymers terminated by --cH2C(Ph)i ,Li +

form in benzene the nonreactive dimers remaining in eqUilibrium with minute concentrations of the reactive nonassociated polymers. Let us denote by u and v the concentrations of the latter, and by (S - ,Li + h,

Table 5.6. Rates of crossover in the styrene isoprene system

kss = 2.3.10- 2 M- 1I2S- 1

kls = 2.4.10- 4 M- 1I4.S- 1

kSI = 51.10-2 M- 2S- 1

kll = 34.10- 4 M- 1I4S- 1•


(SD - ,Li +)z, and (S - ,Li + ;SD - ,Li +) those of the respective homo- and mixed dimers. The following equilibria are assumed to be established:

2(S- ,Li+) = 2u ~ (S- ,Li+)z !K1,

2(SD- ,Li+) = 2v ~ (SD- ,Li+h !K2,

and

Hence,

(S - Li+) =!K ·u2 , 2 2 1 ,

(SD- Li+) =!K 'v2 , 2 2 2 ,

and

Consider the kinetics of addition of 1, I-diphenylethylene (Do > > [S - ,Li + ];nitial) to lithium polystyrene in benzene solution. A reaction ensues converting the polymers terminated by the S - ,Li + end-groups (Amax = 340 nm) into those terminated by the SD- ,Li+ groups (Amax = 470 nm). Steric hindrance prevents the homopropagation of D, and therefore the resulting process involves only a simple bimolecular reaction assumed to proceed according to the mechanism:

Surprisingly, this reaction was found to be first order in lithium polystyryl, proportional to D = Do, but the first-order rate constant was invertially proportional to the square root of the initial concentration of lithium polystyryl, i.e.,

where [S - ,Li + ];nitial denotes the initial concentration of lithium polystyryl. The proposed above mechanism accounts for this result provided that K12 is equal to (K1·K2)112. Under such a condition the total concentration of S - ,Li + whether in the form of dimers or mixed dimers is given by:


and that of SD - ,Li + by

2'(SD- ,Li+h + (S- ,Li+;SD- ,Li+) = KY2·V·(K1'2·U + K!/2·V) ,

Since

and

2'(S- ,Li+h + 2'(SD- ,Li+h + 2'(S- ,Li+;SD- ,Li+)

= (K1'2u + K!/2V)2 = (S- ,Li+)initial

we deduce that:

i.e., the slope of the first-order plot is given by kIK1'2·[S- ,Li+ ]lMtial in agreement with the observations.21

The interesting feature of this reaction is its "memory." The rate is determined not only by the concentration of lithium polystyryl at a particular time of the process, but also by its initial concentration. Hence, the rates of the reaction at some stage of the process may be different in two experiments, although the concentrations of the reagents, lithium polystyrene and 1 ,l-diphenylethylene, are the same at that stage. This is illustrated by Fig. 5.6.

A similar behavior, accounted for by the same mechanism, was observed in the reaction of styrene with the dimeric -CH2CPhi ,Li + in benzene solution.22

The importance of mixed dimerization is revealed also by the results of copolymerization of styrene and p-methylstyrene initiated by lithium alkyls in benzene. O'Driscoll and Patsiga23 who investigated this reaction reported a linear decrease of logarithms of concentration of each monomer with time, a relation depicted in Fig. 5.7. As shown elsewhere,24 the previous scheme accounts for these relations.

u

v


n Memory" effect

initia l concentrat ion

Rate I a

Concentration of living polymers

Figure 5.6. The plot of the rate of addition of l,l-diphenylethylene to lithium polystyrene in benzene solution vs. the residual concentration of the living polystyrene, [M], resulting from the addition of a large excess of l,l-diphenylstyrene Do. Note the rate constant, -d[M]/[M]o/dt, i.e., the slope of the line, decreases as [M]o increases. For the same instantaneous value of [M], the rate depends on [M]o. The reaction "remembers" the initial concentration of living polystyrene; it has "memory."

and

u v

with the assumed kinetic steps:

kll U + S ---. u,

k21 V + S ---. u,

k u + MeS ---.!.4 v,

kn v + MeS~v.

The above symbols have a self-evident meaning. The linear relation reported by O'Driscoll and Patsiga implies that the slopes of the respective


1n [s] or

1n [pMe S]

Slope Az ; p Me S

Time

Figure 5.7. The plot of ln[ styrene] or (p-methylstyrene] vs. time for copolymerization of these two monomer initiated by BuLi in benzene. Both monomers disappear in a pseudo-first-order fashion. The respective first-order constants, Al and A2 , depend on the concentration of the initiator.

first-order plots shown in Fig. 5.7 and denoted by Al and A2 are:

and

The total concentration of living polymers, whether terminated by S or by MeS, is constant and equal to 10 (due to the absense of termination) since u and v are minute compared with [S] and [MeS]. Hence

Note, u and v are two independent variables, not related by the conventional equation of stationary state

valid for radical copolymerization. Therefore the above three equations (a), (b), and (c) have to be identical, i.e.,

to allow for the linear relation reported by O'Driscoll and Patsiga.23 The last relation need not be assumed, as was done in the previous treatment.


It comes as the consequence of the experimentally observed linear relations displayed in Fig. 5.7. Moreover, the proposed mechanism predicts the proportionality of Al and Az with IA'z, i.e., Al ::: "fl' IAI2 and A2 ::: "12' JA'z with "II and "Iz constant and independent of 10, This relation, un noted by O'Driscoll and Patsiga, deserves verification.

7. Reversible Copolymerization: System 2-Vinylnaphthalene-Styrene

Addition of styrene to a THF solution of living poly-2-vinylnaphthalene leads to a unusual phenomenon. The spectrum of the sodium salt of living poly-2-vinylnaphthalene in THF solution has its maximum at 558 nm (curve A in Fig. 5.8), whereas the Amax of THF solution of sodium polystyryl appears at 340 nm. Unexpectedly, the addition of a slightly larger amount than equimolar of styrene to a solution of living poly-2-vinylnaphthalene instantaneously changes its absorption spectrum (see curve B in Fig. 5.8), its Amax being at 440 nm is different than that of living poly-2-vinylnaphthalene or living polystyrene. The rate of its appearance was determined by the stopped-flow technique, as shown by Fig. 5.9, and the second-order rate constant of the reaction proceeding at 25°C was found to be 30 M -IS -1 .

However, the odd spectrum seen on the addition of styrene slowly changes

~ .. d

" "" ;; " :l

'" 0

2.5

2.0

1.5

1.0

0.5

[~(S)D-- Ne 1o, 11.6x10- 311

[slo, 26. 5x1 0-311

c, -------------------I I I I I I

B II I " ~ I ~ ... -{-, 1' - "'----"" · ,./ I \ 1 '..... ....'" I I .... ---__ y" I I -~- __ - ... , I 1 • I '\

: '--T-...........\ : : ~\ , I . ... ....

O ~ __ L_ __ _L ________ L_ __ ~ __ ~ ___ ~~~~~~~ ______ ~

300 340 400 500 600 700 800

A,mp.

Figure 5.8. The addition of styrene to living 2-polyvinylnaphthalene in tetrahydrofuran at 25°C. A, The spectrum of living 2-vinylnaphthalene; B, the spectrum observed immediately after addition of slightly larger than equimolar amount of styrene to living poly-2-vinylnaphthalene; C, the spectrum observed 24 h after mixing the reagents. The addition of the first styrene molecule to polyvinylnaphthalene is faster than the addition to the adduct.

0.8

0.7

~ 0.8 .. 0.5 CI ., .., 0.'

tl <> 0 .3 :;3 ...

0 .2 0

0.1

0 0

\ \ \ \ \ \ \ \ \ \ \

10 20


(u"", OIlU]O' 3 .411,,10- 311:

[S]O' e .2'h:10- 311: k l • 27.2 UI.e. molo- I .. o- I

30 '0 50 80

t. sec

Figure 5.9. The rate ofthe appearance ofthe new spectrum as studied by stoppedflow technique.

and after about -20 h the spectrum of living poly-2-vinylnaphthalene reappears but with lower intensity.

On the other hand, the addition of a large excess of styrene to living poly-2-vinylnaphthalene results in the appearance of the expected stable spectrum of living polystyrene. The following explanation was offered for these strange phenomena. 25

Aromatic moieties attached to the chains of living polyvinyls interact with their neighbors and such interaction affects the spectrum of their negatively charged end-groups. Thus the interaction of a naphthyl moiety with the single negatively charged styryl terminal following it added to a living poly-2-vinylnaphthalene yields a species having a spectrum different than that of a conventional living polystyrene (where the styryl carbanion is preceded by a styryl segment). Curve B shown in Fig. 5.8 illustrates such a spectrum. However, when two or more styrene molecules are added to a living polyvinylnaphthalene the resulting copolymer has the conventional spectrum of living polystyrene.

The propagation of polymerization of living polymers is reversible and leads eventually to equilibrium between living polymers and their monomer (see p. 88). Note, however, that Cl' the equilibrium concentration of styrene in the reaction

should be larger than C2, the equilibrium concentration of styrene in the


reaction

because the splitting of styryl moiety from the adduct produced in the former reaction yields a more stable-C-H'Nph ion, whereas an ion of equal stability is formed by splitting a styryl moiety from the product of the latter reaction (i ;::= 1). Hence a continuous, although slow, splitting of styryl molecules from the polymers endowed with a single styrene segment does take place, while their addition to those polymers possessing two or more styrene units then restores the equilibrium. The overall process therefore resembles a slow distillation of water from a container holding pure water (higher vapor pressure) to another filled with a salt solution (lower vapor pressure). Thus the monostyrene adducts are converted back into homopolyvinylnaphthalene, whereas the low proportion of those polymers possessing two or more styrene units become longer forming the two blocks of vinylnaphthalene-styrene copolymers (rich becomes richer and poor gets poorer). As the amount of the supplied styrene increases, this process converts more and more of the homopoly-2-vinylnaphthalene into the stable poly-2-vinylnaphthalene-styrene di-block polymers. Thus the reported observations are rationalized.

As stated earlier, the rate constant kl of the first step of the overall process

was determined by the stopped-flow technique. The stirred-flow reactor technique was utilized in the determination of the rate constant k2 of the second step


The pertinent balance equations are:

(klX + k:zA + k3y)-[Sb :;; [So] - [S],

k2·X·[S]·T :;; Xo - X,

(k}"X - k3·A)·[S]·T :;; A,

k3·A ·[S]·T :;; y,

where X denotes the concentration of living homopolymers terminated by a vinylnaphthalene end-group, A is the concentration of the monostyrene adduct, y is the concentration of polymers possessing at least two styrene segments, [S] the concentration of styrene, and T the residence time of the reagents in the reactor. All the concentrations are those maintained in the reactor at its stationary state, whereas the subscript "0" denotes the initial concentrations of styrene and the living poly-2-vinylnaphthalene fed into the reactor. The constants are: kl' the rate constant of the addition of the first molecule of styrene to polyvinylnaphthalene; k2' the rate constant of the addition of styrene to the adduct; and k3' the known propagation rate constant of living polystyrene. This set of equations leads to the relationst:

and

Plotting the left side of the first equation vs. r should lead to a straight line, its slope giving k2k3. The value of the left side was obtained by a series of successive approximations. The results of six experiments in which [S]o and Xo were varied led to a constant value of k2k3 as exemplified by Fig. 5.10.

The proposed kinetic scheme may be verified. The addition of a finite amount of styrene to living poly-2-vinylnaphthalene cannot convert all of it into polystyrene, even after consumption of all the styrene. The addition of the first styrene molecule to polyvinylnaphthalene competes with the addition to polystyrene (an initiation followed by propagation, see p. 88). Hence the fraction of the complex converted to polystyrene is uniquely determined by [S]o, Xo, and kjk2• Since k3 is known, such experiments

tEach experiment gives [S] for a chosen and known values of [S]o, Xo, and T. These data are used to compute A and y. The dissociations are too slow and may be ignored.


800 r-----r-----r-----r-----r-----r---~

100 200 300 400 500 800

Figure 5.10. Kinetics of the addition of styrene to living poly-2-vinylnaphthalene as studied by the stirred-flow technique. The slope of the lines gives k2k3' The lines corresponding to separate experiments are shifted one in respect to the other to avoid overlap.

provide the value of k2 in fair agreement with the value determined by the stirred-flow technique. This approach was used successfully by Ureta26 to determine the rate constant of addition of styrene to carbanions derived from 1,1-diphenylethylene (again a very slow initiation followed by a rapid propagation) .

Consider now a monomer, e.g., a-methylstyrene, for which its equilibrium concentration with its own living polymer is relatively high, say larger than 1 M at ambient temperature. Its addition to living polyvinylnaphthalene again leads to the formation of the analogous adduct, as revealed by the spectrum. Provided that the concentration of the pertinent monomer in equilibrium with the adduct is low, say less than 0.1 M, the adduct remains stable and does not dissociate. On the contrary, the living poly-


vinylnaphthalene is converted into the adduct in the presence of living poly-a-methylstyrene, the latter depolymerizes and the produced monomer converts the poly-2-vinylnaphthalene into the adduct. This indeed was verified by Stearne27 for the poly-2-vinylnaphthalene-a-methylstyrene system.

Addition of styrene to the lithium salts of carbanions derived from 1,1-diphenylethylene in benzene solution results in spectral changes similar to those observed in the living poly-2-vinylnaphthalene-styrene system. The absorbance of the anion's 1,1-diphenylethylene disappears on addition of a small excess of styrene, but it reappears after a few hours.28 The observed spectral changes are illustrated by Fig. 5.11.

A different behavior was observed in the system living polystyreneanthracene. Their mixing leads to a dormant adduct, ---ClI2CHPh'anthracene(linked at the 9 C atom of the anthracene) being in equilibrium with a

~iD mIll -soo 350

001

(DPHIJ] = 9,14 . 10-3

[Sty] = 0,95 • 10- 2

T = 22·C

500

time = 1 : 0 hr. 2 : Ll5hr.

3 : 5 hr.

4 : 7 hr.

5 : 23 hr.

II : 43 hr.

7 : 290hr.

550 1100

Figure 5.11. The spectral changes observed in a polymer terminated by -CH2·CPh2·CH2CHPh- anion resulting from its dissociation into the more stable -CH2·CPh2 anion and the styrene that adds to styryl anions. Curve 1 is the original spectrum of the -CH2CPh 2 anion prior to the addition of styrene.


small fraction of polystyryl anions.29,3o The rate of the adduct dissociation was determined by labeling the free anthracene with 14C and found to be 3.10-3 S-1 at 28°C. In this reaction a more stable ion is formed from a less stable one, a reverse of the relation observed in the previous case. Therefore the tendency to follow this addition by the subsequent addition of styrene is low. Nevertheless, whenever such an addition does take place, it is rapidly followed by the addition of another anthracene molecule, provided that this hydrocarbon is still available. Hence, an alternating copolymerization ensues because anthracene does not add to its own anion.

An interesting observation may be mentioned here. The kinetics of anthracene addition to living polystyrene was investigated by the stoppedflow technique.31,32 The results were most intriguing. The anthracene addition to living polystyrene endowed with one active end-group proceeds rapidly and quantitatively with a rate constant of 2000-3000 M -1 s -1 •

However, the addition to a polymer endowed with two active end-groups, although equally rapidly proceeding at the onset of the process, becomes -1000 times slower after one half of the polystyryl end-groups reacted. This suggests that the anthracenated end-group, denoted by -SA -, is intramolecularly associated with the other nonanthracenated end-group, yielding rapidly a relatively inert associate

which adds the anthracene very slowly. The degree of polymerization of the investigated polymer was about 25, and the concentration of the remaining anthracene -10- 4 M. The two ends of the bifunctional polymers were, on the average, less than 15 A apart, whereas the free anthracene was separated from the nonanthracenated polystyryl end-groups by 200 A on the average. Therefore, the intramolecular association of the two endgroups of the same polymer competes favorably with the intermolecular reaction between the free anthracene and the unassociated polystyrene end-groups. The spectrum of an equimolar mixture of anthracenated and nonanthracenated polystyrenes, each endowed with one active end-group only, was identical with that of a half-anthracenated polystyrene endowed with two active end-groups. This indicates that the intra- and intermolecular associates have the same nature. The evidence for an association is undeniable because the addition of anthracene to the above-mentioned equimolecular mixture of the two kinds of single-ended polystyrenes, one complexed with anthracene and the other not, results also in a slow reaction, in spite of the large excess of anthracene.


8. Cationic Copolymerization

Our knowledge of cationic copolymerization is meager. Study of the cationic copolymerization of ethyl and isopropyl vinyl ethers induced by ionizing radiation was reported by Stannett et al. 33 Under 'Y irradiation the propagation involves only the free cations. In spite of the four times faster homopropagation of the isopropyl than of the ethyl vinyl ether, the degree of incorporation of both monomers into the polymer is comparable, e.g., for the feed of 5.3:4.4 of ethyl to isopropyl vinyl ether, the percentage of isopropyl in the copolymer was 45%. This implies reciprocal values of the crossover constants, and therefore a random copolymer is produced as deduced from Fig. 5.12. Further studies of such systems are desired to clear up this peculiarity.

Large number of monomers copolymerize cationically under conventional initiation procedures; however, for most of them the crossover constants differ substantially in their magnitude, preventing the formation of truly random copolymers. The substituents on the growing ions affect their reactivity to a greater extent than of the reactivity of the growing radicals. Hence, the ratio of rate constants of homopropagation of two monomers

100

3' 'j:j ~ 75 ~

'" 'II

! 0 j:I., 0 50 tl

'II

:S .8

~ ~

25

~

100

? IPVE in the feed (moles)

Figure 5.12. The composition of the copolymer vs. the composition of the feed in a random copolymerization of ethyl and isopropyl vinyl ethers initiated 'Y radiation.


endowed with different substituents is much larger in the cationic than in the radical polymerization.

The ease of copolymerization of some pairs of monomers is surprising. For example, 2-chloroethyl vinyl ether and 4-methylstyrene readily copolymerize.34 So does the pair 4-vinylbiphenyl and 2-vinylfluorene,35 or styrene and trioxane.36 The nature of cation and solvent alters greatly the reactivity ratios in cationic copolymerization, whereas the surroundings have only a minor influence on the outcome of radical copolymerization. The molecular mass of copolymers is, on the whole, lower than that of homopolymers formed under comparable conditions, especially at low temperature.

The industrially most important cationic copolymer is butyl rubber, a polyisobutene containing a small but critical amount of dienes, typically 0.5-2.5%, that allow its cross-linking. This elastomer (MW - 100,000-150,0(0) is mainly used for inner tubes in automobile tires, but also for cable insulation, gaskets, and sealants. It is produced at very low temperature, - -100°C, in order to prevent the termination and chain-transfer that limit the degree of polymerization.

References

1. Szwarc, M. 1968. Carbanions, Living Polymers and Electron Transfer Processes, pp. 520-557, Interscience, New York.

2. Worsfold, D. J., Johnson, A.F., and Bywater, S. 1964. Can. J. Chem., 42, 1255.

3. Shima, M., Bhattacharyya, D.N., Smid, J., and Szwarc, M. 1963. J. Am. Chem. Soc. 85, 1306.

4. Bhattacharyya, D.N., Lee, C.L., Smid, J., and Szwarc, M., 1963. J. Am. Chem. Soc. 85, 533.

5. Walling, e., and Mayo, F.R. 1948. J. Am. Chem. Soc. 70, 1537

6. Natta, G., Danusso, F., and Sianesi, D., 1959. Makromol. Chem. 30, 238.

7. Busson, R., and Van Beylen, M. 1977. Macromolecules 10, 1320.

8. Burnett, G.M., and Young, R.N. 1969. Eur. Polym. J. 2,329.

9. Shima, M., Bhattacharyya, D.N., Smid, J., and Szwarc, M. 1964. J. Polym. Sci. Lett. 2, 735.

10. Wooding, N.S., and Higginson, W.C.E. 1952. J. Chem. Soc. (Lond.) 774.

11. Szwarc, M. 1960. Makromol. Chem. 35, 132.

12. Yuki H., Okamoto, Y., Ohta, K.,and Hatada, K. 1975. J. Polym. Sci. 13, 1161.

13. O'Driscoll, K.F. 1962. J. Polym. Sci. 57, 721.

14. Wang, I.C., Mohajer, Y., Ward, T.e., Wilkes, G.I., and McGrath, J.E. 1980. Am. Chem. Soc. Symp. Ser. 166,529.


15. Van Beylen, M., Bywater, S., Smets, G., Szwarc, M., and Worsfold, D.J. 1988. Adv. Polym. Sci. 86, 132.

16. Korotkov, A.A., Mitzengendler, S.R., Dornutzig, L.L. 1966. J. Polym. Sci. Lett. 4, 901.

17. Teyssie, P., Bioul, J.P., Conde, P., Drwet, J., Henschen, J., Jerome, R., Ouhadi, T., and Warin, P. 1985. Am. Chern. Soc. Symp. Ser. 286, 97.

18. O'Driscoll, K.F., and Kuntz, J. 1962. J. Polym. Sci. 61, 19.

19. Morton, M., and Ells, F.R. 1962. J. Polym. Sci. 61, 25.

20. Worsfold, D.J. 1967. J. Polym. Sci. A5, 2783.

21. Laita, Z., and Szwarc, M. 1969. Macromolecules 2,412.

22. Yamagishi, A., and Szwarc, M. 1978. Macromolecules 11, 504.

23. O'Driscoll, K.F., and Patsiga, R. 1965. J. Polym. Sci. A3, 1037.

24. Yamagishi, A., and Szwarc, M. 1978. Macromolecules 11, 1091.

25. Bhasteter, F., Smid, J., and Szwarc, M. 1963. J. Am. Chern. Soc. 85, 3909.

26. Ureta, E., Levy, M., and Szwarc, M. 1966. J. Polym. Sci. A4, 2219.

27. Stearne, J., Smid, J., and Szwarc, M. 1964. Trans Faraday Soc. 60,2054.

28. Dils, J., and Van Beylen, M. 1977. Proc. IntI. Symp. Macromol., Dublin I, 69.

29. Medvedev, S.S. et al. 1961. Dokl. Akad. Nauk SSSR 139, 1351.

30. Khana, S.N., Levy, M., and Szwarc, M. 1962. Trans. Faraday Soc. 58, 747.

31. Lipman, R., Jagur-Grodzinski, J., and Szwarc, M. 1965. J. Am. Chern. Soc. 87,3005.

32. Stearne, J., Smid, J., and Szwarc, M., 1966. Trans. Faraday Soc. 62, 672.

33. Deffieux, A., Stannett, V.T., Wang, A., Yung, J.A., and Squire, D.R. 1983 Polymer 24, 1469.

34. Masuda, T., and Higashimura, T. 1971. Polymer J. 2,29.

35. Cohen, S., and Mareshal, E. 1975. J. Polym. Sci. Symp. 52. 83.

36. Kern, W., Chezdron, H., Jearks, V., Bender, H., Deibig, H., 1961. Ang. Chern. 73, 177.

Index

Acrylates and methacrylates, 10, 12, 120-131

"Activated monomer" mechanism, 292 in cyclic ethers, 299-302 in lactam, 295 in NCA, 202

Activation energy of initiation, apparent negative values of, 88

Aggregation alcoholates, 43, 70, 278-281 ion-pairs, 21, 69, 70, 147, 152, 159

Aging, 18 Alkali metal solution, 232-234

negative alkali metal ions, 232-234 reactivities, 236-239 structure and properties, 234-236

Alkyl lithium desulfuration, 157 initiation of polymerization, 150 structure and aggregation, 147-150

Aluminum counterions in cationic polymerization, 323

Aluminum porphyrines, 11-12, 282-284

Aluminum-zinc oxyalkoxides, 274-277

Aminium salts, 250 Anionic polymerization

acrylates, 34 styrene in ethereal solution, 94-100

Anthracene, 55 in hydrocarbons, 159

Association of ions and ion-pairs, 45, 48,49,56,74,77,82

with crown ethers and cryptands, 56 with glymes, 107

377

with other complexing agents, 169 rate of, 75

Back-biting, 335 Bifunctional lithium initiators,

158-159 Bimetallic oxyalkoxide catalyst, 274 Binary cationic equilibria, 133 Biphotonic ionization, 40, 251 Bivalent cations, 118 Block polymers, 4, 11,24 Butadiene

initiation with lithium alkyls, 11, 150-157

initiation with sodium, 7, 9 t-Butyllithium, 156

Capping technique, 22-24 e-Caprolactone, 277

kinetics and mechanism of, 278-281 living polymerization of, 12,

277-282 Carbanions, 6, 8, 42, 64 Carbazyl anion, 112-113 Carbenium ions, 6, 42 Cationic propagation, 130,202-208,

213-214 of vinyl ethers, 202-208

mechanism of, 212-213 Cationogen and protonogen, 225 Chain-transfer, 3,11-14,209,319,

330-332 temperature dependence, 330-331

Charge-transfer complex, 40, 230, 253-255

378/ Index

Charge-transfer complex-continued absorption band, 230 dissociation on irradiation, 231 example of,

tetramethylphenylenediaminechloranil, 230

Co-initiation, 224 Co-initiator, co-catalysis, 7 Conductance, 43-47, 58-61, 71-82 Coordination polymerization, 10-12.

274-277 Copolymerization. 346-350

anionic, effect of equilibria of aggregated polymers. 358

anionic styrene-butadiene with Li counterion in benzene, 359

anionic styrene-p-methylstyrene, alkali lithium initiated. 363

cationic, 373-374 Covalent-ionic equilibria

in heterocyclic polymers. 179-183 kinetics of interconversion.

183-189 Crossover rate constant. 34. 348 Crown ethers and cryptands, 55 Cumyl chloride. homologues. 333

Depropagation, 25, 31 Dimerization, 8, 239

kinetics, 242 Dioxolane, 31. 139-140 1,l-Diphenylethylene,8 Dissociation

of ion-pairs, 58, 62, 64, 77, 82, 90 of radical-anions, 239-243

Dormant polymers, 12, 16, 19,21, 70 Duroquinone, 68

Effect of ether on crossover rate constant of anionic polymerization, 348

E-isomer and Z-isomer, 342 Electrolysis, 263 Electron transfer, 7, 10,39,244-246,

257 heterogeneous, 248-249 homogeneous,239-240

Esters derived from vinyl monomers activation by acids, 188-189 monomer insertion, 186 role in propagation, 191-193

Ethylene oxide cleavage, 112 depropagation, 32 polymerization, 4, 34

Excimer and exciplex, 231

Field dissociation, emission, effect, 76, 262

Flash photolysis of -aa-, 240-242 of -DD-, 242

Fluorenyl salts, 49, 53, 56, 64, 78, 80 Fontana mechanism, 17, 140 Formaldehyde, 6, 32 Free ions, 3, 16, 21, 39-45, 48,

62-64, 76-79, 82 conductance and mobility, 59-61, 67 reactivity and place of attack, 91

Friedel-Crafts reagents, 223 complexes with cationogens,

227-229 complexes with protogens, 225-227

Functionalization of living polymers, 326

Ground state charge-transfer complex, 230

Group-transfer polymerization, 12, 288

Head-to-head-tail-to-tail polystyrene, 248

Indene, 5 Initiation of polymerization, 4, 13, 19,

24,87 by alkyllithiums, 150-157 by electron transfer, 230-264 by free ions, 89 by Friedel-Crafts reagents, 223-230 in the gaseous phase, 144-147 by ion-pairs, 92 by Lewis acids and bases, 215-221 of oxirane by naphthalenide radical-

anion, 246 by protonic acids, 172-194 reversible, rate of, 88 by salts, 89 by trifluoroacetic acid, 173-174,

189-191 Initiation and propagation of ionic

polymerization

definition and examples, 87, 89 by electron transfer, anionic, 239 by electron transfer, cationic, 251 by tetranitromethane, 255-256 of vinylcarbazole, 253

Initiation of radical chain process by electron transfer, 257 - 259

reversibility of, 88 Intramolecular proton or hydride ion

transfer, 326 Ionization

aryl chloride, 134 conductometric titration, 134

Ionizing radiation, 264 polymerization induced by and its

kinetics, 264 effects of water traces, 266 solvation agents effects, 270

Ionophores and ionogenes, 41, 45 Ion-pairs, 2, 8, 16, 21, 41-43, 45,

47-49,50,53,58,61,64,67, 70, 73-76, 81

equilibria of dissociation, in cationic systems, 135

heat and entropy of dissociation, 64 interconversion, 72, 74 loose and tight, 48, 55, 100-107 solvation, 43

Isobutene, 3, 5, 12, 34 Isomerization, 326 Isoprene, 4, 7, 10, 153-155, 163-165

Kinetically enhanced formation of cyclics, 338

Lactone, polymerization of, 92, 113-116

Lewis bases, effect on initiation, 157 Living polymer-monomer equilibria,

25,30 Living polymers

definition and tests, 4, 10-15, 19, 21,25,30

longevity of, 16

Macromer, 24 Memory effect, 364 Metalloporphyrines, 282 p-Methoxystyrene, 34, 349, 350, 363 a-Methylstyrene, dimerization of,

127-128

Indf>x /379

Mixed dimerization, 361-366 Modification of polymer end-groups,

341-343 Molecular mass distribution, 13, 18,

30,210

Naphthalenide radical anion, 40 NCA polymerization, 292 Nitro-olephines, 10, 34

Oligomerization, 127 Overall (apparent) rate constant of

initiation or propagation, 93, 96 Oxiranes, 11

Penultimate unit, 2 Phenylbutadiene,7 Polar monomers, 120-127 Polymerizability, 25 Pressure effects, 50 Propagation of polymerization, 87

by lithium alkyls in hydrocarbon solvents, 159-165

overall rate constant, 93 Propene, 6, 17 Propylene oxide, 11 Propylene sulfide, 34

polymerization and cyclization of, 339-341

Proton affinity, 176 Proton transfer, 40 Proton trap, 229, 332 Protonation mechanism, 177-179 Push-pull mechanism, 95, 111-113

Radical-cations, formation of, 249-251 Reactivity of ions and ion-pairs in

cationic polymerization, 135-138

Reactivity ratios in anionic copolymerization, 354

Reversible copolymerization, 366-370 Ring-chain competition, 334-339

Salts active in cationic polymerization, 132

Scheme of copolymerization, crossover constant, 346-348

Seeding technique, 19 Self-association of protonic acid, 174

380/ Index

Simultaneous homopolymerizations, 259-262

Solvated electron, 45 Solvation, 40, 50, 60, 141-144

effect in polymerization of lithium living polymers, 169-171

Stereochemistry of propagation of lithium polydienes, 165-170

Stereoselection and stereoelection in ring opening polymerization, 284-285

Stirred-flow technique, 127-128 Stopped-flow technique, 194

results of, 194-200 Styrene

anionic polymerization in dioxane, 94 effect of pressure on, 106-107 effect of solvating agents on,

107-111 in liquid ammonia, 93-94 in THF and other ethereal

solvents, 96-100 role of loose ion-pairs in, 100-106

cationic polymerization initiated by perchloric acid, 199-201

living cationic polymerization effect of Lewis bases on, 208-209 ionization of esters and collapse

of resulting ion-pairs, 205-208 of isobutene, 204-208 of vinyl ethers, 202-212

Synthetic value of living polymers, 24

Temperature jump technique, 73 Termination, 3, 8, 11-14, 319-326

bimolecular, 324-326 collapse of ion-pair, 322-323 elimination of alkyl hydride,

321-322 unimolecular, 320-324

Tetrahydrofuran, cationic polymerization of, 7, 22, 33, 42

Tetraphenylethylene salts, 53 Transfer of NO:!,", 255-256 Trioxepane, cationic polymerization

of,32 Triple ions, 65-70, 81-83, 116-118

Uniformity of polymer sizes, 4, 210-212

Vinylcarbazole, polymerization of, 208 Vinyl ethers, 202

"Wrong" monomer addition, 328-330

Zinc complexes, stereocatalytic action, 285-288

flip-flop mechanism, 285 mechanism of polymerization, 287 structure, 286

Zwitterions, 6, 215-218, 221-223, 260

ionic polymerization and living polymers

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