review on making polymer macro cycles
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Synthetic approaches for the preparation of cyclic polymers
Boyd A. Laurent and Scott M. Grayson*
Received 9th February 2009
First published as an Advance Article on the web 17th April 2009
DOI: 10.1039/b809916m
Despite decades of studies devoted to the unique physical properties and potential applications
of cyclic polymer topologies, their exploration has remained limited because of synthetic
inefficiencies and acyclic impurities. Many recently developed synthetic techniques offer efficient
routes to well-defined cyclic macromolecules to answer this need. This tutorial review aims to
provide a concise overview of the most significant synthetic contributions in this field, and
highlight the relative advantages and disadvantages of each approach.
1. Introduction
Tailored control of polymer architecture has been a goal of
polymer chemists since it was first understood that a polymer’s
physical properties are inherently dependent on its nanoscopic
architecture. Synthetic exploration of a number of architec-
tures such as linear polymers, polymer brushes, star polymers,
ladder polymers, dendrimers, hyperbranched polymers, and
network polymers has enabled a detailed understanding of
specifically how covalent architecture affects their observed
macroscopic properties. The effect of a ‘‘continuous’’ cyclic
topology on polymer properties is of significant interest because
the end-groups of non-cyclic architectures have demonstrated
a significant role in their material properties. However, a
detailed physical understanding of cyclic polymers has been
limited largely by synthetic complications. In addition to
difficulties in preparing large scales of cyclic polymers, most
methods yield materials with at least trace amounts of linear
polymer impurities, which can jeopardize the validity of physical measurements. For these reasons, better synthetic
techniques that yield high-purity cyclic materials have been
sought.
During the first synthetic explorations of cyclization
reactions in small organic molecules, Paul Ruggli,1 and later
Karl Ziegler et al.2
demonstrated that high dilution could beused to favor the formation of cyclics. This results from the
fact that even under high dilution, when intermolecular reactions
are disfavored, the effective molarity of reactive groups for the
cyclization reaction remains high because they are covalently
tethered to each other. While small rings (3–4 covalent bonds)
are disfavored due to Baeyer strain, intermediate rings
(5–6 covalent bonds) are favored because of low strain, and
slightly larger rings (7–13 covalent bonds) are disfavored due
to Pitzer and transannular strain; the conformational flexibility
in significantly larger rings results in negligible strain energies.
As early as 1935, Ruzicka predicted that the increasing
entropic penalties expected for larger cyclization reactions
would become a significant complicating factor.3 However,
the discovery and structural determination of cyclic peptides,
such as gramicidin S,4 and cyclic DNA,5 have since verified the
synthetic feasibility of cyclic macromolecules.
The first examples of synthetic cyclic polymers were prepared
via the ring-chain equilibrium of poly(dimethylsiloxanes) and
polyesters.6 These early synthetic methods typically yielded
Department of Chemistry, Tulane University, New Orleans,LA 70118, USA. E-mail: [email protected];Fax: +1 (504) 865-5596; Tel: +1 (504) 862-8135
Boyd A. Laurent (left) and Scott M. Grayson (right)
Boyd A. Laurent earned his BS in Chemistry in 2005 fromLouisiana State University in Baton Rouge, LA, USA where heworked in the labs of Professor Graca Vicente and Professor KevinSmith. He is currently a Louisiana Board of Regents fellow, pursuing a PhD in Chemistry at Tulane University under the
guidance of Professor Scott M. Grayson. His research is focused on the exploration and synthesis of novel polymer architecturesbased on cyclic polymer substrates.Scott M. Grayson received a BS from Tulane University (1996)and a PhD in Chemistry from the University of California,Berkeley (2002), studying the role of polymer architecture fordrug delivery under Jean M. J. Fre chet. Following post-doctoral studies in the laboratories of C. Grant Willson at the University of Texas, he accepted a position as an assistant professor in thedepartment of Chemistry at Tulane University in New Orleans. Hespent his first semester on ‘‘hurricane sabbatical’’ at Washington
University in St. Louis and returned to Tulane by January of 2006 to continue his research, focusing on the synthesis, characterization,and application of complex, yet well-defined polymer architectures with a particular focus on cyclic polymers.
2202 |Chem. Soc. Rev.,
2009,38
, 2202–2213 This journal isc
The Royal Society of Chemistry 2009
TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews
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impure materials, with significant amounts of linear
by-products. The differences in physical properties between
the desired cyclic product and linear by-products have enabled
the isolation of macrocyclic fractions. Most commonly, if the
impurities have a significant difference in molecular weight,
fractional precipitation has been used to isolate cyclic materials.
In addition, because the cyclic polymers have a reduced hydro-
dynamic volume, preparative gel permeation chromatography(GPC) provides an effective means for isolating minute
quantities of polymer macrocycle. Alternatively, Beckham and
coworkers have taken advantage of the ‘‘open-ends’’ present in
the linear by-products to form inclusion complexes between
poly(ethylene oxide) and a-cyclodextrin, which enables the
selective precipitation of the linear impurities.7 In addition,
the use of liquid chromatography at the critical condition
(LCCC) has proved to be a very effective technique for isolating
cyclic polymers particularly at higher molecular weights, as
samples are separated by architecture and functionality rather
than molecular weight.8
While these and other rigorous methods of purification have
aided in the isolation of cyclic polymers, and have provided
the first physical data verifying the unique properties of
polymer macrocycles, improved synthetic approaches have
been developed to enable the production of cyclic molecules
with narrow polydispersities and vastly improved cyclic purity.
The aim of this tutorial review is to highlight the most successful
synthetic advances in the preparation of well-defined cyclic poly-
mers as well as to show the evolution of the field as a result of
modern advances in synthetic polymerization techniques. A
broader perspective of cyclic oligomers and polymers, including
detailed discussion of the physical and theoretical aspects, has been
reviewed elsewhere.9 While a diverse range of more complex cyclic
topologies such as tadpoles, figure-eights, theta-shaped,
mannacles, and sun-shaped polymers have been reported, these
polymeric architectures will not be discussed as they are outsidethe fundamental scope of this review (Fig. 1). Likewise, while there
is a diversity of polymerization and coupling chemistries that
have been applied to preparing cyclic polymers,10 this review
attempts to focus on the scope and limitation of those syntheses
believed to be the most representative of the general techniques,
and those believed to be the most versatile for future
investigations.
2. Synthetic approach
Synthetic strategies for the formation of cyclic polymers can be
divided into two main categories: (1) ring-closure techniques
(Fig. 2) and (2) ring-expansion techniques (Fig. 3). The ring-
closure method involves the coupling of a linear polymer’s
end-groups to yield a cyclic polymer while the ring-expansion
technique involves the insertion of cyclic monomer units into
an activated cyclic chain. The synthetic aspects of both
approaches will be discussed in detail below.
The verification of a polymer macrocycle’s cyclic architec-
ture is critical to judge the success of a synthetic route. A
number of methods have been used to verify the formation of
cyclic polymers based on the known differences in theirphysical properties both in solution and in bulk, relative to
linear analogues. Polymer macrocycles exhibit a longer
retention time by GPC compared to linear analogues due to
their more compact cyclic topology, and therefore exhibit
Fig. 1 Representations of more complex cyclic polymer topologies.
Fig. 2 Schematic representation of the three most common ring-
closure techniques for the preparation of cyclic polymers.
Fig. 3 Schematic representation of the ring-expansion technique for
the preparation of cyclic polymers.
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smaller hydrodynamic radii. This technique, when compared
with a more direct method of determining molecular weight
(e.g. MALDI-TOF MS) is the most common technique for
verifying cyclization. In addition, the glass transition (T g)
temperatures of cyclic polymers typically increase with mole-
cular weight faster than their corresponding linear polymers
due to a decrease in chain mobility of the cyclic species.
Likewise, the intrinsic viscosities of cyclic polymers are con-sistently lower than linear analogues and their self-diffusion
coefficients, as measured by pulsed-gradient spin echo NMR
techniques,11 resemble those of smaller linear polymers. It has
also been shown that cyclic poly(styrene) exhibits an enhanced
fluorescence emission intensity relative to its linear analogues
at molecular weights less than 20 kDa.12 Although no single
analytical technique can definitively prove a polymer’s cyclic
topology, a combination of multiple techniques can provide
strong evidence.
2.1 Ring-closure techniques
The first successful approach developed for the preparation of
relatively pure cyclic polymers utilized ring-closure techniques.These ring-closure techniques can be broken into two major
categories: (1) the ring-closure of homodifunctional polymers
and (2) the ring-closure of heterodifunctional polymers. Initial
investigations focused primarily on the homodifunctional
ring-closure of living dianionic polymers due to their synthetic
accessibility. One of the critical features of all ring-closure
techniques is that they require high ‘‘Ruggli–Ziegler’’ dilution
to favor intramolecular cyclization over intermolecular
coupling. For homodifunctional couplings that are bimolecular
in nature, it is difficult to prepare extremely high purity cyclic
polymers. This results from the fact that the first coupling
requires an intermolecular reaction whereas the second
coupling involves an intramolecular reaction and typically
optimized conditions that favor one, disfavor the other. On
the other hand, with appropriately efficient unimolecular
coupling reactions, either with homodifunctional or hetero-
difunctional polymers, this approach can yield high purity
cyclic macromolecules under high dilution.
2.1 (i) Ring-closure of a,x-homodifunctional polymers
Homodifunctional bimolecular coupling. Since the bimolecular
cyclization reaction of homodifunctional polymers is first order
in both polymer and the bimolecular coupling agent, the use of
exact stoichiometric quantities of reagents is crucial (Fig. 4). If
an excess of the bifunctional linking agent is added to the
homodifunctional polymer, this can result in both ends of a
polymer end-group reacting with different linking agentspreventing cyclization (Case I). If insufficient bifunctional
linking agent is used, the product will be dominated by linear
polymeric dimers (Case II).
Even with 1:1 stoichiometry, excluding these two side
reactions is difficult. This complication is elucidated by a
detailed examination of the competitive kinetics between
intramolecular cyclization and intermolecular oligomerization
(Fig. 5). The first step in the formation of cyclic polymers is the
reaction between the polymer and the linking agent (Rc1),
however, once the intermediate is formed, the rates of secondary
couplings (Rc2, Rc2’) and oligomerizations (Rolig) compete
with the rate of cyclization (Rcyc). Because the rate of each
reaction is merely the product of the rate constants ( k) and
the concentration of the reactants, the concentration of thebis-functional polymer and coupling agent must be kept low to
favor cyclization over the many by-products. However, in
order to provide sufficient precursor for the cyclization, there
must be an appreciable concentration of the starting materials.
Therefore, unless a synthetic procedure can be developed that
provides a vastly larger rate constant for the cyclization
relative to the by-products, linear contaminants are inevitable.
As a result, tedious purification techniques must be employed
to isolate pure cyclic product.
Because living anionic polymerization allows for the
formation of living polymers with well-defined molecular
Fig. 4 Schematic representation of the by-products formed by
inexact stoichiometries during the bimolecular homodifunctional
cyclization.
Fig. 5 Schematic representation of the competing coupling reactions
and by-products formed during the bimolecular homodifunctional
cyclization.
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weights and low polydispersities, this approach has been used
extensively to produce precursors for the bimolecular
homodifunctional coupling. Three research groups, those of
Ho ¨ cker,13 Rempp,14 and Vollmert,15 published nearly simul-
taneous reports for the preparation of cyclic polymers via
this route. In these procedures, the authors used sodium
naphthalenide (1) to generate a bis-anionic initiator (2) for
the polymerization of styrene. The anionic end-groups (3) were
eventually terminated by the addition of a,a0-dihalo- p-xylene
(4) linking agent to produce the corresponding ring-polystyrenes
(5) (Fig. 6). However, after adding 1 equiv. of linker, thepresence of styryl anion was still evident by colorimetric
means. Therefore, to enable the facile separation of the presumed
linear by-products, excess difunctional linking agent was
added to functionalize all remaining linear chain-ends,
followed by the addition of high molecular weight living
anionic polymer. The resultant disparity in molecular weight
between the high molecular weight by-products and relatively
low molecular weight cyclic polymers allowed facile separation
of the cyclic and linear compounds via fractionation. In these
initial reports, cyclic poly(styrene) with molecular weights
between 3 kDa and 60 kDa was synthesized with poly-
dispersity indices typically below 1.2 but the yields of cycliza-
tion were often low (o50%). GPC analysis of the reactionproduct showed the expected shift to longer retention times
when compared to the corresponding linear precursors. Since
these first reports, similar techniques have been used to
synthesize cyclic poly(styrene) with molecular weights up to
450 kDa.16
A number of groups have utilized analogous techniques to
prepare cyclic polymers with a diversity of monomers including
styrenics, vinylpyridines, ethylene oxides, and dienes. Typically,
anionic polymerization is initiated by reaction of sodium
naphthalenide (1) with either styrene or 1,1-diphenylethene
to produce bifunctional initiators 2 and 6 respectively.
Bifunctional living initiators 7 and 8 can also be formed by
reacting alkyl lithium nucleophiles with the alkene groups of
1,2-bis(isopropenyl-4-phenyl)ethane (9) or 1,3-bis(1-phenyl-
ethylenyl)benzene (10) (Fig. 7). Different electrophilicbifunctional linking agents that have also been used for the
ring-closure step include dichlorodimethylsilane (11), dichloro-
methane (12), and a,a0-dichloro- or a,a0-dibromo- p-xylene
(13). In addition, reaction of the dianionic polymer chain with the
1,2-bis(isopropenyl-4-phenyl)ethane (9) and 1,3-bis(1-phenyl-
ethylenyl)benzene (10) linking agents generates relatively
stable dianions which can either be quenched or reacted with a
different monomer to yield more complex cyclic topologies such
as tadpoles or figure-eight polymers. Since these initial publi-
cations, a number of groups have used this approach to obtain
a diversity of cyclic homopolymers as well as block copoly-
mers. Some examples of these polymer compositions include
cyclic poly(butadiene), poly(2-vinylpyridine), poly(isoprene),and cyclic block copolymers comprised of poly(styrene)-b-
poly(dimethylsiloxane), poly(styrene)-b-poly(2-vinylpyridine),
poly(styrene)-b-poly(ethylene oxide), poly(propylene oxide)-
b-poly(ethylene oxide), poly(styrene)-b-poly(isoprene), and
poly(butadiene)-b-poly(styrene). Further details for the
synthesis of these cyclic polymers and other complex
architectures using anionic polymerization has been
thoroughly reviewed by Hadjichristidis et al .17
In order to minimize the linear impurities produced during
the bimolecular coupling, Ishizu et al. investigated a bi-phasic
coupling reaction. The poly(styrene) bis-anion (3) was reacted
Fig. 6 The first syntheses of cyclic poly(styrene) using anionic poly-
merization and a bimolecular homodifunctional coupling with
dihalo- p-xylene linking agents.
Fig. 7 Structures of common bifunctional initiators used to generate
dianionic living polymers as well as common linking agents used for
ring-closure.
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with a large excess of 1,4-dibromobutane so that the
dihalogenated linear polymer intermediate (14) could be isolated,
purified, and characterized before cyclization. The ring-closure
to yield 15 was performed by the reaction of a stoichiometric
amount of 1,6-diaminohexane with the bromo-end-capped
poly(styrene) polymer (Fig. 8).18 In contrast to previous
methods, the authors confined the reaction to the organic-
aqueous interface by utilizing the solubility differences of both
components. The bromo-end-capped poly(styrene) (14) was
dissolved into a mixture of toluene and DMSO while the
1,6-diaminohexane coupling agent was dissolved into a basic
aqueous phase. The authors demonstrated extremely high
cyclization efficiencies (490%) to yield cylic poly(styrene)
(15) with linear precursor concentrations as high as 10À3 M.
Previous coupling strategies required much lower concentra-
tions of 10À5 M and 10À7 M in order to repress intermolecular
coupling and achieve similar yields of cyclic polymer. This
observed result is presumably caused by the reduced rate of the
intermolecular coupling because it must occur at the interface
between the two phases while the rate of cyclization is not
inhibited because of its intramolecular nature. In a subsequentpublication, the authors extended this technique to synthesize
cyclic block copolymers that consist of poly(styrene)-b-poly-
(isoprene) and compared the phase segregation properties
in the bulk of these materials to that of linear tri-block
analogues.19 As predicted, the covalently forced loop
configuration leads to a narrower lamellar spacing than the
linear analogues.
Oike et al. provided an alternative solution for minimizing
linear by-products by using electrostatic interactions to template
the cyclization.20 Using anionic polymerization and subsequent
end-group modification, polymers with reactive cationic termini
can be prepared. Small molecule anions such as di-carboxylates
and di-sulfonates readily form dimeric ion pairs in tetrahydro-furan and template their subsequent coupling to form cyclic
polymer. Because of the strength of the electrostatic attraction,
high dilution can minimize the interaction between polymer
chains (reducing Rc2, Rc20, Rolig), while the ionic pairing main-
tains a high rate of initial coupling and cyclization (Rc1, Rcyc).
The authors initially demonstrated this technique using
poly(tetrahydrofuran) polymers end-capped with strained
N -phenylpyrrolidinium groups and bifunctional carboxylate
anions to pre-assemble the cyclic ion-pair precursors (16). After
the assembly process was completed at the desired concentration
of polymer, the system was heated, triggering the nucleophilic
attack of the carboxylate on the pyrrolidinium salts to produce
the neutral cyclic diester species (17) (Fig. 9). The authors have
shown this to be a highly efficient process at concentrations as
high as 4.6 Â 10À5 M for poly(tetrahydrofuran) of 4.3 kDa.
Characterization of the product by GPC verified its smaller
hydrodynamic radius while viscosity measurements determinedthe inherent viscosity ratio [Zcyc.]/[Zlin.] to be 0.647, which is in
agreement with previously reported values. The authors have
since extended this same methodology to successfully produce
cyclic poly(ethylene oxide)s, cyclic poly(styrene)s,21 and a diverse
library of cyclic polymer architectures, but the molecular weights
of the produced polymers were generally limited to below 5 kDa.
Homodifunctional unimolecular coupling. An alternative
approach that overcomes the complications of the bimolecular
coupling reaction while using homodifunctional polymers
involves the direct coupling reaction between identical
polymer end-group functionalities. In this unimolecular case,
dilution of the linear precursors represses the oligomerization,but will not reduce the rate of intramolecular coupling since
the reactive groups are tethered to one another. Therefore,
if a high efficiency homocoupling reaction is applied to this
approach, very well-defined cyclic polymers can be made while
suppressing oligomeric by-products.
One early example of an efficient homocoupling cyclization
reaction was demonstrated by Tezuka and Komiya using ring-
closing metathesis of allyl end-functionalized polymers.22
Initially, poly(tetrahydrofuran) was polymerized, end-capped
with allyl groups, and cyclized under ultra-dilute conditions with
Grubb’s Ru-based metathesis catalyst. The authors found that
Fig. 8 The interfacial condensation cyclization technique employed
by Ishizu and co-workers to minimize intermolecular oligomerization
during bimolecular coupling.18
Fig. 9 Cyclization via ionic pre-assembly followed by a covalent
fixation used by Tezuka and co-workers to minimize intermolecular
oligomerization during bimolecular coupling.20
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concentrations below 4 Â 10À5 M were necessary to assure
cyclization over oligomerization. MALDI-TOF MS verified a
shift in mass corresponding to the loss of ethylene (À28 Da)
while GPC verified a reduced hydrodynamic volume. This
coupling technique is also compatible with atom transfer radical
polymerization (ATRP) if Keck couplings were used to install
the required alkene end-groups on both bromo-functionalized
polymer chain ends. Acrylate monomers were polymerized from
a bifunctional ATRP initiator (18) and then the halide end-
groups were converted with allyl tributyl tin (19) to the desired
allylic end-groups (20).23 Ring-closing metathesis to form cyclic
poly(methylacrylate) (21) was performed using analogous
conditions as described for the poly(THF) (Fig. 10). The cyclic
topology was again confirmed by the expected shift to longer
GPC retention times relative to linear precursors, and by the
characteristic loss of ethylene in the MALDI-TOF massspectra.
An additional unimolecular coupling technique that has been
applied to cyclize homodifunctional polymers is the reversible
oxidation of thiol terminated polymers to yield a disulfide
linkage. Monteiro and coworkers utilized reversible addition
fragmentation chain transfer (RAFT) polymerization from a
bifunctional initiator to synthesize a linear poly(styrene) with a
molecular weight of 3.7 kDa and a polydispersity of 1.1.24
During aminolytic removal of the dithioester end-groups, the
corresponding poly(styrene) di-thiols (22) were oxidized by
either aerial oxidation or by a strong oxidizing agent (Fe(III)Cl3)
to form the cyclic disulfides (23) using a slow feed addition
(2.4 Â 10À4
M final concentration) in order to favor theintramolecular cyclization product (90%) (Fig. 11). Alternatively,
if the oxidative coupling was performed at higher concentra-
tion or without the slow feed process, the reaction yielded
predominantly long linear oligomers of the poly(styrene)
precursor with smaller amounts of cyclic product. In addition,
both the cyclic and oligomeric products could be readily
converted back to the original linear precursors by reduction
with Zn. This approach is unique in that the polymer
sample can be rapidly and reversibly converted from cyclic
poly(styrene) to linear poly(styrene) by controlling both the
redox environment and concentration.
2.1 (ii) Ring closure of a,x-heterodifunctional polymers.
a,o-Heterodifunctional polymers, those bearing differentfunctional groups at opposite chain-ends, provide an effective
route to cyclic polymers as long as complimentary end-groups
can be efficiently installed and coupled at high dilution. A
significant advantage of this process over the bimolecular
homodifunctional coupling procedure is that the complimentary
functionalities are tethered to one another, eliminating
problems resulting from inexact stoichiometries. This
technique also avoids the need for an initial intramolecular
coupling and therefore high dilution can be used to prevent
intermolecular oligomerization without slowing the rate of
cyclization (Rcyc) (Fig. 12). A final minor advantage over the
unimolecular homodifunctional approach is that because hetero-
difunctional polymer precursors contain different end-groups,
the effective concentration of complimentary functional
groups on different chains is reduced by half, further slowing
the rate of intermolecular oligomerization (Rolig). However,
this approach is more synthetically demanding as it involves
Fig. 10 The homodifunctional unimolecular cyclization of poly-
(methyl acrylate) used by Tezuka and co-workers via an olefin
metathesis coupling.23
Fig. 11 Reversible cyclization and ring-scission utilizing the
reversibility of thiol/disulfide oxidation and reduction by Montiero
and co-workers.24
Fig. 12 Schematic representation of the competing coupling
reactions during unimolecular heterodifunctional cyclization.
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complimentary end-group functionalities, which usually require
a near quantitative post-polymerization end-group trans-
formation. Incomplete end-group transformations will reduce
cyclization yields and generate linear impurities.
The earliest work utilizing the heterodifunctional cyclization
technique was performed by Schappacher and Deffieux, by
utilizing the living nature of cationic 2-chloroethyl vinyl ether
(CEVE) polymerizations to synthesize extremely well-defined
linear polymer precursors with polydispersities typically less
than 1.2 (Fig. 13).25 The polymerization was initiated from a
styrene functionalized vinyl ether (24) via the addition of
hydroiodic acid followed by a ZnCl2 catalyzed polymerization.
The iodo end-group (25) can be efficiently abstracted by the
addition of SnCl4 and the resultant terminal carbocation
coupled to the opposite styrenic end-group to form the
cyclized polymer with a stabilized benzylic cation. Addition
of an alkoxide to the cation yields a stable cyclic polymer ( 26).
Crude GPC traces confirmed the generation of significantamounts of the cyclic product and smaller amounts of an
intermolecular condensation product (B20%), which ranged
in molecular weights from 1 kDa to 3 kDa. The glass transi-
tion temperatures (T g) were also used as evidence for the cyclic
structure since cyclic polymers are predicted to demonstrate a
higher glass transition temperature when compared to their
linear counterparts. Since this initial publication, Deffieux and
coworkers have successfully carried out the synthesis of figure-
eight, tadpole, and theta-shaped poly(CEVE) polymers
through similar polymerization and coupling chemistry.
Rique-Lurbet et al . then extended this work to the living
anionic polymerization of styrene using the previously described
cyclization chemistry because of its demonstrated efficiency.26
Because styrene was used as the monomer, the styrenic end-
group had to be added post-polymerization in order to prevent
cross reactivity. This was achieved by the anionic polymeriza-
tion of styrene from a protected diethyl acetal followed by
quenching the reactive end-group first with 1,1-diphenylethene
followed by p-chloromethylstyrene. The iodo functionality
was then added to the diethyl acetal end-group by reaction
with trimethylsilyl iodide before employing the analogous
cyclization procedure with SnCl4 as described above. In the
case of styrene, molecular weights ranged from 2 kDa to
12 kDa and polydispersities for both the linear and cyclic
polymers were narrow (o1.2). This synthetic approach affords
efficient yields of high purity cyclic polymer (495%) without
tedious purification techniques such as fractionation or
preparative GPC. The ratio of the Mn’s (Mn,cyc/Mn,lin) calculated
by GPC, when calibrated against linear poly(styrene)
standards, was determined to be 0.85 which is in agreement
with measurements previously reported for cyclic poly(styrene)
prepared via bimolecular coupling. In addition, this techniquehas been amenable to the preparation of block copolymers
comprising of both poly(styrene) and poly(CEVE).
A variety of alternative coupling chemistries have been
successfully applied to polymers generated via living anionic
polymerization for ring-closure following the hetero-
difunctional approach. Using linear poly(styrene) precursors
containing a-geminal diethyl ether and o-diol functionalities,
Schappacher and Deffieux have successfully carried out an
acid-catalyzed transacetalization reaction under high dilution
to produce the analogous cyclic polymer with B85% yield.27
Schappacher and Deffieux also used another similar acetaliza-
tion reaction between the pendant alcohols and pendant vinyl
ethers on relatively short ‘‘A’’ and ‘‘C’’ blocks of an ABC
triblock copolymer to form extremely high molecular weight
cyclic poly(CEVE) (84 kDa). The poly(CEVE) ‘‘B’’ block was
then grafted with poly(styrene) chains to form cyclic polymer
brushes.28 Atomic force microscopy images of the cyclic
polymer brushes provided the first clear visual evidence of
the ring-shaped polymers, as well as identified smaller
amounts of the expected linear and tadpole by-products.
Kubo et al. prepared cyclic poly(styrenes) through the use of
a high efficient amide bond forming reaction.29 The authors
initiated the polymerization by reacting ortho-ester 27 with
lithium followed by polymerization of styrene and quenching
the active anion with 2,2,5,5-tetramethyl-1-(3-bromopropyl)-
1-aza-2,5-disilacyclopentane (28) to give the requisite linear
protected poly(styrene) precursor (29) (Fig. 14). Hydrolysisafforded the a-carboxylic acid and the o-amino functionalities
(30) which were coupled via activation of the carboxylic acid
with 1-methyl-2-chloropyridinium iodide (31) to give the cyclic
poly(styrene) (32). In subsequent publications, the amide
linkage was reduced to the amine in order to enable
Fig. 13 Cyclization of poly(CEVE) via the ring-closing addition to a
terminal carbocation used by Schappacher and Deffieux.25
Fig. 14 Cyclization via amide bond formation by activation of
carboxylic acid end-group performed by Kubo et al.29
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subsequent functionalization by the same activated carboxylic
acid approach to yield both tadpole and figure-eight
poly(styrene)s.
One elegant approach addressing the removal of linear
impurities was developed by the Semlyen group by growing
the linear precursors from a solid support.30 The 11-bromo-
undecanoate monomer was bound to a tetraalkyl ammonium
functionalized support via an electrostatic interaction.Polymerization of the monomer could occur, but polymer
product would only be released from the solid support
upon neutralization via cyclization. This approach however,
involves a step-growth mechanism which led to low degrees
of polymerization (DP = 5–15), and high polydispersities.
The authors used this method to synthesize cyclic polymers
of 11-undecanoic acid, however fractionation was still
necessary to produce pure cyclic materials with narrow
polydispersities.
Controlled radical polymerization (CRP) approaches,
such as nitroxide mediated polymerization (NMP), atom
transfer radical polymerization (ATRP), and reversible
addition-fragmentation chain transfer (RAFT) polymerization,
are particularly attractive for preparing cyclic polymers
because of the ease with which end-group functionality can
be modified as well as their low PDI and broad functional
group compatibility.
Lepoittevin et al. utilized NMP to form a linear polymer
precursor which contained an a-alcohol functionality from the
stable nitroxide radical (4-hydroxy-TEMPO) (33) and an
o-carboxylic acid from the initiator 4,4 0-azobis(4-cyanovaleric
acid) (34).31 Activation of carboxylic acid functionalized
polymer 35 following the procedure of Kubo et al.,29 enabled
the ring-closure to yield the macromolecular lactone 36
(Fig. 15). However, this procedure was only efficient for the
production of low molecular weight cyclic poly(styrene)
(o4 kDa) and yielded increasing amounts of oligomeric con-taminants in larger polymers. This was attributed to the
thermal instability of the nitroxide group present in the
backbone of the cyclic polymer.
A particularly versatile technique for the preparation of
cyclic polymers is the combination of controlled radical
polymerization and the 1,3-Huisgen dipolar cycloaddition
‘‘click’’ reaction to provide a highly efficient yet functional
group tolerant synthetic procedure. Laurent and Grayson first
reported the combined use of CRP and ‘‘click’’ during their
synthesis of cyclic polystyrenes by a combination of ATRP
and the ‘‘click’’ coupling of complementary azide and alkyne
end-groups.32 Polymerization from an alkyne functionalized
initiator provides a benzyl bromide end-functionalized (37)
polymer which can be efficiently transformed to the requisite
azide functionality (38) (Fig. 16). The polymer was subsequently
cyclized via a drop-wise addition of the polymer to a Cu(I)
catalyst to yield high purity cyclic polymers containing a
triazole linkage (39). Characterization by MALDI-TOF MS,
GPC, 1H NMR, and FT-IR suggests that with the appropriate
rate of addition, negligible amounts of linear oligomer are
produced foregoing the need for rigorous purification. Theversatility of ATRP has also enabled the preparation of
cyclic poly(N -isopropylacrylamide)33 as well as cyclic block
co-polymers consisting of poly(methyl acrylate-b-styrene).34
The combination of RAFT polymerization and the ‘‘click’’
cyclization also provides an efficient route to cyclic polymers.
Winnik and coworkers prepared cyclic poly(N -isopropylacryl-
amide) via RAFT initiation from an azide functionalized chain
transfer agent (40), followed by aminolysis of trithiocarbonate to
yield a thiol terminated polymer (Fig. 17).35 An alkyne
functionality was then incorporated by the Michael addition of
an a,b-unsaturated propargyl acrylate ester (41) to yield a
bisfunctional polymer with complimentary functionalities at
opposite ends (42). The ‘‘click’’ cyclization reaction was thenperformed by the in situ reduction of Cu(II)SO4 by sodium
ascorbate in aqueous solution under extremely dilute conditions
to afford cyclic poly(N -isopropylacrylamide) (43) with molecular
weights up to 19 kDa and excellent control over polydispersity
(o1.2). Goldmann et al. used an alternative end-group modifica-
tion approach to produce cyclic poly(styrene) from an azide
functionalized RAFT agent. The end-group was exchanged
by quenching with an excess of propargyl functionalized
azo-bis(4-cyano valeric acid). The ‘‘click’’ cyclization was then
performed using analogous procedures described above to yield
the corresponding cyclic poly(styrene).36
Fig. 15 Synthesis of cyclic poly(styrene) by Lepoittevin et al. via
activation of the terminal carboxylic acid and subsequent coupling
with pendant hydroxyl on the initiator.31
Fig. 16 Synthesis of cyclic poly(styrene) via a combination of ATRP
and ‘‘click’’ coupling by Laurent and Grayson.32
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2.2 Ring-expansion techniques
Ring-expansion polymerizations typically involve a catalyst orinitiator that yields a growing cyclic polymer chain, held
together by a relatively labile bond (e.g. organometallic or
electrostatic). Propagation by insertion of new monomer into
this weak bond is driven by thermodynamic factors, such as
ring strain in the monomer. The cyclic polymer will either
retain this initiating species, or in some cases, expel the catalyst
by a back-biting or ‘‘intramolecular chain transfer’’ reaction.
The critical advantage of the ring-expansion technique is
that high dilution is not required to yield cyclic polymers. As a
result this technique, when optimized, is amenable to larger
scale syntheses. Also, because the cyclic structure is maintained
throughout propagation, high molecular weight polymers can
be easily prepared without the entropic penalty associated withthe ‘‘ring-closure’’ approaches. In addition, the propagating
cyclic structure will prevent the formation of linear
by-products, as long as the monomer and initiator are
rigorously purified to remove any linear contaminants. The
significant complication of this approach is that the formation
of stable cyclic polymer is based on inherent rates of
polymerization, depolymerization, and back-biting; therefore,
the catalyst and reaction components must be carefully
selected to ensure high molecular weight, low polydispersity,
and (if desired) complete removal of the catalyst from the
cyclic product.
Kricheldorf and Lee carried out the initial work on ring-
expansion polymerization using lactide monomers and cyclic
tin oxide catalyst.37 Modifying their previous work on
generating linear polymers from the alkoxide ligands of tin
catalyst, the authors demonstrated the synthesis of cyclic
polymers using a cyclic tin initiator (Fig. 18). Initially, the
authors successfully polymerized both cyclic b-butyrolactone
and cyclic e-caprolactone using the cyclic tin oxide initiator
2,2-dibutyl-5,5-dimethyl-1,3-dioxa-2-stannane (44). In this
process, propagation occurs by the insertion of monomer into
the tin–oxygen bond to produce a ring-expanded macrocycle
(45). The resultant macrocyclic architectures were verified by
generation of the linear analogues via competitive ligand
exchange between the alkoxide ligands on the tin species
within the polymer macrocycle, and dimercaptoethane.
Characterization using GPC demonstrated a shorter retention
time for the linear by-product and 1H NMR verified theremoval of the tin initiator to yield hydroxyl end-groups.
Analogous techniques have been used by Kricheldorf and
co-workers to prepare a variety of complex cyclic polymer
topologies based on lactone and lactide monomers.38
One disadvantage of this approach is that the hydrolytically
labile tin oxide bond is present in the cyclic topology of the
product.
In order to stabilize the macrocyclic products, a number of
groups have developed techniques for removal of the tin
catalyst while retaining the cyclic topology. Kricheldorf and
co-workers demonstrated that a bis-functional phthalate
thioester can replace the tin oxide initiator after polymeriza-
tion to yield a more stable polymer macrocycle.39
Aring-insertion/ring-elimination process allows the esterifica-
tion of both of the alkoxide ligands on the tin catalyst to
afford the corresponding diester linkage without intermediate
ring-opening. An alternative macrocyclic stabilization
approach was developed by Li et al. in which a macrocyclic
block co-polymer was prepared from a similar cyclic tin initiator
(46), caprolactone and a short photo-crosslinkable lactone
block with acrylate side-chains (47).40 After preparing the
cyclic block copolymer (48) and UV crosslinking, the tin
oxide initiator can be purposely hydrolyzed while maintaining
the integrity of the macrocyclic structure (49) (Fig. 19). This
Fig. 17 Synthesis of cyclic poly(N -isopropylacrylamide) via RAFT
polymerization and subsequent ‘‘click’’ cyclization by Winnick and
co-workers.35
Fig. 18 First example of the ring-expansion polymerization via
b-butyrolactone insertion into a cyclic tin initiator by Kricheldorf
and co-workers.37
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procedure produced pure cyclic poly(e-caprolactone) with a
molecular weight of 24 kDa and 1.45 polydispersity. No
inter-macrocycle crosslinking was noted during the UV cross-
linking process, as long as the concentration remained below
2.8 Â 10À4M.
Another early example of ring-expansion polymerization
was provided by Shea and co-workers, utilizing insertion of
methylene units into the carbon–boron bonds of 50 by the
polyhomologation of dimethylsulfoxonium methylide.41
Thisprocess occurs by the attack of the methylidene on the electron
deficient boron center followed by a concerted migration of
the alkyl group adjacent to the borane to the methylide (51)
and release of dimethylsulfoxide. Because polyhomologation
can occur along any of the carbon–boron bonds, the sterically
bulky thexyl group was attached on the boron center to direct
methylene insertion only along the two less hindered
carbon–boron bonds, yielding cyclic poly(methylene)s (52)
(Fig. 20). In these studies, the molecular weights of
poly(methylene)s generated were below 2 kDa, but poly-
dispersity indices were also low (o1.2). Attempts to replace
the boron atom with a ketone functionality showed only
modest yields (30%).
Bielawski et al. demonstrated an elegant application of ring-
opening metathesis catalysts to enable cyclic ring-expansion by
use of a cyclic Ru catalyst (Fig. 21).42 In this process, the cyclic
Ru catalyst (53) inserts itself into the unsaturated alkene of
cyclooctene (54) to initiate the ring-expansion polymerization.
Propagation occurs through repetition of the Ru insertion
into additional monomer; however, the polymerization is
complicated by two competing reactions. As the monomer is
consumed, the likelihood of depolymerization increases.
Likewise, at low monomer concentration, the Ru catalyst
can undergo intramolecular chain transfer to regenerate thecyclic catalyst and an inactivated cyclic poly(octenamer) (55).
Initial reports described the synthesis of cyclic poly(octenamer)
with molecular weights up to 1200 kDa and polydispersities
typically around 2.0. Additionally, the authors showed similar
results in the cases of both 1,5-cyclooctadiene and 1,4,9-trans-
cis-trans-cyclododecatriene, producing cyclic poly(1,4-buta-
diene) with molecular weights of 2.3 kDa to 145 kDa.
Typically, polydispersities were lower for low molecular
weight polymers (Mn = 2.3 kDa, PDI = 1.6) and increased
with higher molecular weight polymers (Mn = 145 kDa,
PDI = 1.8). High molecular weights were noted in the order
of minutes; however, once a significant amount of monomer
had been consumed, back-biting reactions led to a rapiddecrease in molecular weight of the polymer as well as an
increase in polydispersity.43 This technique has yielded some
of the largest molecular weight cyclic polymers to date. With
an appropriately tuned catalyst, the rates of polymerization,
depolymerization, and intramolecular chain transfer can be
optimized to yield well-defined cyclic polymers. Recent
optimization by Boydston et al. investigated the role of the
size of the catalyst heterocycle as well as the electronics of the
N -heterocyclic carbene ligand (saturated vs. unsaturated) on
the Ru center.44 The inherent rates of propagation, intra-
molecular chain transfer, and catalyst stability were studied
Fig. 19 UV crosslinking by Je ´ rome and co-workers of the shortacrylate functionalized A block within a cyclic ‘‘ABA’’ triblock-
copolymer to stabilize the macrocycle and allow removal of the tin
initiator.40
Fig. 20 Polyhomologation by Shea and co-workers of cyclic boranesused to form cyclic poly(methylene).41
Fig. 21 Olefin metathesis by Grubbs and co-workers utilizing a cyclic
Ru-catalyst to produce macrocyclic poly(octene).42,44
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in the polymerization of cyclooctene and these studies suggest
that cyclic Ru complexes with 5- and 6-carbon heterocycles as
well as saturated N -heterocyclic carbene ligands to be the best
catalysts for balancing these kinetic factors and producing
pure cyclic poly(olefins).
Kudo et al. extended their previous synthesis of linear
poly(ethylenesulfide) polymers from a thiocarbamate initiator
to cyclic analogues using the ring-expansion of a cyclic
thiocarbamate initiator.45 Using the cyclic thiazolidine-2,4-
dione (56) as an initiator, phenoxypropylenesulfide (57) was
polymerized via a tetrabutylammonium chloride catalyst to
yield well-defined cyclic poly(sulfide)s (58) with polydispersities
around 1.3 for molecular weights below 8 kD (Fig. 22).
However, at higher molecular weights, polydispersities increased
substantially (B1.9).
Recently, Waymouth and co-workers have reported cyclic
polymers by utilizing N -heterocyclic carbenes to polymerize
both lactide and lactone monomers in the absence of the
alcohol initiators typically used for preparing linear
poly(esters).46 Ring-expansion polymerization occurs via
N -heterocyclic carbene (59) attack of the carbonyl of the
lactide yielding a zwitterionic active polymer chain (60). When
sufficient monomer has been consumed by the growing
polymer chain, the proximity of the alkoxide anion to the
imidazole cation favors eventual back-biting to yield a cyclicpoly(lactide) (61) while regenerating the N -heterocyclic
carbene catalyst (59) (Fig. 23). Through this technique, the
authors were able to synthesize extremely pure cyclic polymers
of both lactide and b-butyrolactone ranging in molecular
weights from 7 kDa to 26 kDa. In all cases, extremely narrow
polydisperse materials were produced (e.g. o1.3). As a result
of the electrostatic attraction, the monomer concentration
during polymerization could be relatively high (0.6 M)
while still yielding high purity cyclic polymers without linear
by-products.
Because controlled radical polymerizations involve a rever-
sible homolytic bond cleavage, polymerization, and then
radical recombination, this polymerization technique is alsoamenable to ring-expansion polymerization if a cyclic initiator
is used. In order to prevent the formation of linear impurities,
the active radicals on opposite chain-ends must recombine in
an intramolecular fashion after propagation to reform the
macrocycle. Pan and co-workers have demonstrated such a
system using a modified RAFT technique wherein 60Co g-rays
were used to induce polymerization of methyl acrylate from a
cyclic dithioester initiator (62) at a temperature (À30 1C) low
enough to prevent intermolecular recombination of active
radicals.47 This contrasts traditional RAFT polymerizations
which involve a thermal cleavage of the thioester which in turn
leads to diffusion of the chain ends and the potential for
intermolecular recombination of radicals. This g-ray technique
was used to prepare high purity cyclic poly(methyl acrylate)
(63) with molecular weights of 8 kDa and polydispersities
typically around 1.3 (Fig. 24). Because of the monomer
versatility of controlled radical polymerizations, block
co-polymerization of N -isopropylacrylamide and methyl
acrylate was also achieved to produce cyclic ABA block
co-polymers. This approach is particularly attractive because
it allows for a diverse number of olefin monomers includingacrylates, methacrylates, and styrenics and is amenable to a
range of side chain functionalities.
Fig. 22 Ring-expansion polymerization of thiiranes by Kudo et al .
from a cyclic thiocarbamate initiator.45
Fig. 23 Zwitterionic ring expansion polymerization of lactide usinga N -heterocyclic carbene catalyst performed by Waymouth and
co-workers.46
Fig. 24 Ring-expansion of a cyclic dithioester initiator used by Pan
et al. to produce cyclic poly(methyl acrylate).47
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3. Conclusion and future outlook
Significant advances in synthetic methodologies promise to
improve the availability of a wide-range of cyclic polymers for
further study. In particular, the application of controlled
radical polymerization techniques and highly efficient coupling
reactions, such as the Huisgen 1,3-dipolar cycloaddition,
enable the preparation of high purity cyclic polymers. While
success of this technique requires quantitative end-group
functionalization and highly dilute conditions, the approach
enables the incorporation of a broad diversity of monomer
and sidechain functionalities onto the cyclic product. On the
other hand, using catalytic ring-expansion techniques, if the
relative rates of each competing reaction (intiation, propagation,
and back-biting) are optimized, offers a scalable route to cyclic
macromolecules with high molecular weight as well as control
over polydispersity.
With further optimization of the above techniques, access to
high purity cyclic polymers should provide the required data
to further our understanding of the physical properties of
cyclic topologies, and polymers in general. In addition, the
inclusion of functionality along the backbone of cyclicpolymers provides access to a diversity of more complex
topologies. These cyclic substrates are of particular interest
where the attachment of functional moieties onto the cyclic
topology is expected to have a unique effect on their synergistic
relationship.
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