Carbocationic polymerization

“Onium ions”

Carbenium ions

The kinetic chain carriers (active species) are positively charged.

Heterocycles ; Z= O, S, N, P, Si

Vinyl monomers

O

Cl

O

O

O O

OO

O O

(CH2)n

n=2, 4, 5

O

O OS

N N N O

R

P

N

P

N

P

N

N O

R

Heterocyclic monomers

Cationic

O S S OO

O

O

O

O

O

NH

O

O

Si

O

Si

O

Si

O

Si

O Si

O

Si

OSi

Cationic / anionic

Vinyl Monomers

Initiation reaction called as “cationation of the monomer”Propagation reaction: a nucleophilic attack of the monomer

Elementary steps in cationic polymerization

Initiation- A Bronsted acids: highly nucleophilic counterion Early termination

-Stable carbenium ions: too stable to initiate aliphatic olefins

- Friedel- Crafts acid systems cation sources (HX, X2, R-X etc.) + MXn

Quasi-living or pseudo-cationic

Photoinitiation by Onium Salts*

Aryldiazonium (ArN2 +Z -), diaryliodonium (Ar2I+Z-), triarylsulfonium (Ar3S+Z-) and

N-alkoxy pyridinium salts, where Z is a nonnucleophilic and photostable anion such as tetrafluoroborate(BF4), hexafluoroantimonate (SbF6 ), tetraperfluorophenylborate [(C6F5)4B], and hexafluorophosphate (PF6 ), are effective photoinitiators of cationic polymerization

Aryldiazonium salts have limited practical utility because of their inherent thermal instability. Diaryliodonium and triarylsulfonium salts are very stable—so stable that their mixtures with highlyreactive monomers do not undergo polymerization on long-term storage.

Some of these initiators have found commerical application in the photocrosslinking of epoxy resins through cationic polymerization.

*Please refer “EXTERNALLY STIMULATED INITIATOR SYSTEMS FOR CATIONIC POLYMERIZATION” YUSUF YAGCI, IVO REETZ Prog. Polym. Sci., Vol. 23, 1485–1538, 1998

Regarding onium salts, which are the most prominent latent cationic initiators, direct and indirect acting systems can be differentiated. This strict differentiation is reasonable, because the initiating species produced by either of these systems are often not the same. In direct acting systems, the energy is absorbed by the onium salt and this leads to its decomposition.In contrast, with indirect acting systems, the energy is absorbed by an additional component. After absorbing the energy, the additives can either react with the onium salt thus producing initiating species, or transfer their energy to the onium salt molecules. By changing the additives, one can often easily adjust to various temperature ranges or wavelengths for thermo- and photolabile systems, respectively

-Direct photolysisIf onium salt initiators, I, absorb light, electronically excited initiator, I*, species areproduced. The latter undergo a heterolytical or homolytical bond rupture leading to cations C+ or radical cations C+., respectively.

Aryldiazonium saltsUpon irradiation, these salts with complex metal anions undergo a fragmentation generating aLewis acid, which can initiate cationic polymerizations directly or react with a hydrogendonating constituent of the polymerization mixture yielding protons

Advantages: The decomposition quantum yields of aryldiazonium salts are relatively high, generally between 0.3 and 0.6. Disadvantages: -The thermal instability of the salt prevents long term storage and therefore limits practical applications. - Another disadvantage derives from the evolution of nitrogen. The evolved gas leads to gas bubbles in the hardening coatings, thus making the material porous.

Diaryliodonium salts

Upon UV irradiation of diphenyl iodonium salts, the Ar–I bonds are ruptured both hetero and homolytically. Whilst the heterolytic pathway generates a phenyl cation and an iodobenzene molecule, eqn (8), a phenyl radical and an iodobenzene radical cation are formed by homolytic cleavage, eqn (10). Both mechanisms involve the interaction with a hydrogen donating solvent or monomer yielding Brønsted acid which initiates the polymerization.

The spectral sensitivity of diaryliodonium salts is relatively poor. For example, the simplest salt, diphenyl iodonium, possesses an absorption maximum at 227 nm.

Sulphonium salts

Regarding the photolysis mechanism of triarylsulphonium salts, both heterolytic eqns (13) and (14) and homolytic eqns (15) and (17) bond rupture of one sulphur–carbon bond is evidenced. In direct irradiation of triphenylsulphonium salts, the heterolytic bond cleavage starting from the excited singlet state is the preferred reaction pathway

Cationic polymerizations following the direct photolysis of triarylsulphonium salts havebeen used for the industrially important UV curing of epoxy coatings

Phosphonium salts

Benzyl or pyrenylmethyl groups containing phosphonium salts produce the respective carbon centered cations after a heterolytic bond rupture according to eqn (20). These cations are assumed to be the initiating species in cationic polymerization.

The excellent initiating ability of phosphonium salts containing pyrenylmethyl groups hasbeen demonstrated for epoxides and vinyl monomers

In the case of phenacyltriphenyl phosphoniumsalts, however, Brønsted acid is most likely the initiating species. Upon photolysis of these salts, the resonance stabilized ylide and protons are formed according to eqn (21) Phenacyltriphenyl phosphonium salts were used for the cationic polymerization of cyclohexene oxide, styrene, and p-methyl styrene.

N-alkoxy pyridinium salts

When absorbing UV light in the presence of a cationically polymerizable monomer, pyridinium type salts do readily initiate polymerization. The two initiation mechanisms described are depicted in eqns (22) and (23) for N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP+PF6

-)

Upon photolysis, the nitrogen–oxygen bond of the salt is ruptured forming a pyridinium type radical cation and an alkoxy radical.

In addition to the radical cation, Brønsted acid formed in the presenceof hydrogen donors (monomer, solvent) may initiate the polymerization, as illustrated in eqn (23).

The absorption of the pyridinium based photoinitiator lies in the far UV region. As canbe seen in Table 5, phenyl substituents shift the absorption maximum towards higherwavelengths by ca. 40 nm.

As shown above, the spectral response of simple onium salts is only rarely acceptable for their practical application. One possible pathway in tackling this dilemma is the chemical attachment of chromophoric groups to the onium salt making it absorb at higher wavelengths (see, for example, Table 2).

Besides that, appropriate chemicals may be added to the polymerization mixture. Some aromatic compounds, like 1,2,4-trimethoxybenzene or hexamethyl benzene areable to form charge-transfer (CT) complexes with pyridinium salts. Being formed in the electronic ground state, these complexes exhibit higher optical absorptions than the pyridinium salt alone. In these circumstances, the incident light is absorbed by the CT complexes.

charge-transfer (CT) complexes

For example, the complex formed between N-ethoxy-4-cyano pyridinium hexafluorophosphate and 1,2,4-trimethoxybenzene possesses an absorption maximum at 420 nm. The absorption maxima of the two constituents are 270 nm and 265 nm for the pyridinium salt and trimethoxybenzene, respectively.

Sensitization by classical energy transfer

This mechanism involves the electronic excitation of the sensitizer (S), a molecule possessing a suitable absorption spectrum, to its excited state. Energy may be transferred from the sensitizer (S*) to the onium salt (I) by either resonance excitation or exchange energy transfer.

Depending on the two components involved, the energy transfer may proceed either in the excited singlet or in the triplet state. However, in all examples discussed in this chapter, triplet energy is transferred.

In consequence of the transfer process, the sensitizer returns to its ground state and excited onium salt species (I*) are formed. The further reactions may also differ from those, taking place when the onium salt is excited by direct absorption of light.

A sufficient energy transfer requires the excitation energy of the sensitizer E*(S) to be atleast as large as the excitation energy of the photoinitiator E*(I).Ep(S) > Ep(I)

Oxidation of free radicals

Onium salts can only rarely initiate a cationic polymerization by themselves. Instead, they may be used to oxidize free radicals according to reaction eqn (29), thus generating reactive cations.

This so-called free radical promoted cationic polymerization is an elegant and fairly flexible type of externally stimulated cationic polymerizations

Free radicals may be produced by various modes: photochemically, thermally or by irradiating the system with high energy rays.

Table 9. continue

The efficiency of onium salts as oxidizing agents is related to their electron affinity.

The higher the oxidation power of the onium salt, the higher (more positive) is the reduction potential E1/2 red (On+).

As seen in Table 10, aryldiazonium salts are most suitable for the oxidation of radicals.However, their practical application is hampered by the lack of thermal stability.Diphenyliodonium salts have also a relatively high reduction potential. Being very suitable for the oxidation of free radicals, these salts have been most frequently used for the oxidation of photogenerated free radicals1.On the other hand, triphenylsulphonium salts have only limited potential for radical induced cationic polymerizations due to their low reduction potential. However, some highly nucleophilic radicals could be oxidized with sulphonium salts.

Even though they do not possess an optimal reduction potential Ered1/2 (On+),

pyridinium salts may also be used for oxidizing carbon centered free radicals. Carbocations formed with the aid of N-ethoxy-2-methyl pyridinium (EMP+) were used to initiate the polymerization of butylvinyl ether and cyclohexene oxide.

Sensitization via exciplexes

Electron transfer via exciplexes. Sensitizers such as anthracene, perylene orphenothiazone form exciplexes with onium salts. Being formed in the consequence oflight absorption by the sensitizer, these energy rich complexes consist of non-excitedonium salt and electronically excited sensitizer molecules. In the complexation state,electron transfer to the onium salt is observed, giving rise to positively charged sensitizerspecies.The excitation of the sensitizer is followed by the formation of a complex between excitedsensitizer molecules and ground state onium salt. In this complex, one electron is transferred from the sensitizer to the onium salt giving rise to the generation of sensitizer radical cations.

The electron transfer (right part in eqn (52)) is energetically allowed, if DG calculated by eqn (56) (extended Rehm–Weller equation) is negative.

According to eqn (56), the requirements are low oxidation potentials, Eox½ (S),

and relatively high excitation energies, E(S*), of the sensitizer. Besides that, only onium salts with high (low negative) reduction potentials Ered

½ (On+ ) (see Table 10), such as diphenyliodonium or alkoxy pyridinium salts are easily reduced by the sensitizer.

The sensitization of onium salts (especially diphenyliodonium salts)by anthracene

Addition fragmentation reactions

The mechanism of the addition fragmentation type initiation is depicted on the example ofETM+ SbF-

6 and benzoin.

The first step consists in the photogeneration of free radicals Virtually any photolabile compound undergoing homolytic bond rupture may be used as a radical source. The radicals add to the double bond of the allylonium salt thus producing a radical in b position to the heteroatom of the onium salt cation. Consequently, the molecule undergoes fragmentation yielding initiating cations.

Notably, upon irradiation of monomer solutions containing EPP+ SbF-

6 at 270 nm in the absence of any additional radical source, cationic polymerizations take place

THERMOLATENT SYSTEMS

In many curing applications, the hardening of monomer containing curing formulas by heat is desired.

Concerning energy absorption, thermal and photolytical initiations differ for the following. In the case of thermal initiation, all chemical bonds absorb energy, whereas for photoinitiation, the photon energy is absorbed only by suitable chromophoric groups

Sulphonium salts

Alkyl substituted sulphonium salts are thermally unstable, decomposing sometimes already at room temperature. The reason, why alkylsulphonium salts are more thermally reactive than arylsulphonium salts is that the former are stabilized only by hyperconjugation, whereas the latter are stabilized by resonance.

Due to the stabilization of the leaving benzyl cation by resonance,alkylbenzyl sulphonium and alkylarylbenzylsulphonium salts possess a high thermal sensitivity. In Table 18, the alkylbenzylsulphonium initiators which have been used so far are compiled

Regarding the effect of the substituent in the aromatic ring, R2, it turned out clearly thatelectron donating substituents enhance the initiation activity by stabilizing the benzyl cation evolved. For example, with the p-OCH3 derivative, for a 50% conversion in styrene polymerization 50oC were sufficient, whereas with the unsubstituted derivative, as much as 120oChad to be applied for obtaining the same conversion in the same time (30 min)

Other highly thermosensitive benzylsulphonium salts are benzyl phenylalkyl sulphoniumsalts

As far as the substituents in the benzyl group, R2, are concerned, electron donatingsubstituents were found to enhance the thermal sensitivity by stabilizing the benzyl cation. The reactivity diminishes in the order CH3 > H = Cl > NO2

N-containing onium salts

N-Benzyl pyridinium salts, the N-containing onium salts most frequently used for thermal cationic polymerization.

The activity for pyridinium ring p-substituents was found to decrease in the order p-CN > H > p-CH3 > p-N(CH3) 2. This order indicates that electron accepting groups stabilize the cation on the pyridinium N-atom, thereby diminishing the leaving ability of the pyridinium moiety. If the pyridinium ring is substituted by o-CN instead of p-CN, the thermal sensitivity is drastically improved, increasing the activity by a factor of 20–30. The effect of the o-CN substituent was attributed to both steric and electronic factors.

Other N-containing onium salts

Besides N-benzyl pyridinium salts, further salts with positively charged nitrogen atoms have been utilized as thermolatent cationic initiators.Recently, the thermal initiation by benzyl ammonium salts having the following general structure, with R = OCH3, t-C4H9, CH3, Cl, were investigated in detail.

The yield with which initiating cations are formed falls in the order CH3O > t-C4H9 > CH3 > Cl, what is explainable in terms of the stabilization of ensuing benzylradicals by electron donating groups.

Hydrazinium salts have been introduced recently as thermally latent Brønsted acid generating initiators.

The evaluation of the initiator activity in glycidyl vinyl ether polymerization revealed that the p-NO2–C6H4 substituent (R1) is most effective in increasing the thermal sensitivity. With initiators containing this substituent, polymerizations were performed at 50oC.

Phosphonium salts

Substituted benzylphosphonium salts of the general structure with R being NO2, Cl,H,CH3, OCH3, have been synthesized by the reaction of correspondingly substituted benzyl chlorides with triphenylphosphine. In a second reaction step, the counter anion has been exchanged for the low nucleophilic SbF-

6These salts have been used for the thermal polymerization of glycidyl phenyl ether and cyclohexene oxide at temperatures in the range between 100 and 170oC.

Interestingly, the activity of these compounds was found to raise with changing for more electron withdrawing substituents; the order of reactivity observed is OCH3 , CH3 , Cl , NO2. This is in sharp contrast to benzylsulphonium and benzylpyridinium salts, where electron withdrawing substituents reduced the thermal sensitivity by destabilizing the benzyl cation formed upon thermolysis

Indirect acting thermolatent systems

Regarding thermal activation, the radical sources listed in Table 20 have been successfully applied in cationic polymerization.

Thermally induced addition fragmentation reactions

In the presence of certain onium salts, the radiation induced cationic polymerization of cationically polymerizable monomers is strongly accelerated, i.e. polymer is obtained with relatively low exposure doses. Initiating cations may be produced by two different ways.

high energy irradiation

(a) Onium salts may scavenge solvated electrons. Solvated electrons are generated by the interaction of ionizing irradiation with matter. These reactive species lead to the decomposition of onium salts and the formation of Brønsted acid.

(b) oxidation of radiolytically formed radicals (stemming from monomer or solvent) to produce initiating entities. In eqn (93), the a-ether radical (derived from vinylether)/ diphenyliodonium salt system is depicted as a typical example.