Carbocationic polymerization

1. Carbocationic polymerization “Onium ions” Heterocycles ; Z= O, S, N, P, Si Carbenium ions Vinyl monomers The kinetic chain carriers (active species) are positively charged.

2. Heterocyclic monomers Cationic Cationic / anionic

3. Vinyl Monomers

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

5. 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

10. Quasi-living or pseudo-cationic

15. 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. Diaryliodoniumand triarylsulfonium salts are very stable—so stable that their mixtures with highlyreactive monomers do not undergo polymerization on long-term storage. Some of theseinitiators 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 REETZProg. Polym. Sci., Vol. 23, 1485–1538, 1998

16. Regarding onium salts, which are the most prominent latent cationic initiators, direct andindirect acting systems can be differentiated. This strict differentiation is reasonable, becausethe initiating species produced by either of these systems are often not the same. In directacting 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 producinginitiating species, or transfer their energy to the onium salt molecules. By changing theadditives, one can often easily adjust to various temperature ranges or wavelengths forthermo- and photolabile systems, respectively -Direct photolysis If onium salt initiators, I, absorb light, electronically excited initiator, I*, species are produced. The latter undergo a heterolytical or homolytical bond rupture leading to cationsC+ or radical cations C+., respectively.

17. Aryldiazonium salts Upon irradiation, these salts with complex metal anions undergo a fragmentation generating a Lewis acid, which can initiate cationic polymerizations directly or react with a hydrogen donating constituent of the polymerization mixture yielding protons Advantages: The decomposition quantum yields of aryldiazonium salts are relatively high, generally between0.3 and 0.6. Disadvantages: -The thermal instability of the salt prevents long term storage and thereforelimits 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.

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

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

20. 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 isevidenced. In direct irradiation of triphenylsulphonium salts, the heterolytic bond cleavagestarting from the excited singlet state is the preferred reaction pathway

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

22. Phosphonium salts Benzyl or pyrenylmethyl groups containing phosphonium salts produce the respectivecarbon centered cations after a heterolytic bond ruptureaccording to eqn (20). Thesecations are assumed to be the initiating species in cationic polymerization.

23. The excellent initiating ability of phosphonium salts containing pyrenylmethyl groups has been demonstrated for epoxides and vinyl monomers

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

25. N-alkoxy pyridinium salts When absorbing UV light in the presence of a cationically polymerizable monomer, pyridiniumtype 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 pyridiniumtype radical cation and an alkoxy radical. In addition to the radical cation, Brønsted acid formed in the presence of hydrogen donors (monomer, solvent) may initiate the polymerization, as illustrated in eqn (23).

26. The absorption of the pyridinium based photoinitiator lies in the far UV region. As can be seen in Table 5, phenyl substituents shift the absorption maximum towards higher wavelengths by ca. 40 nm.

27. As shown above, the spectral response of simple onium salts is only rarely acceptable fortheir practical application. One possible pathway in tackling this dilemma is the chemicalattachment 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 orhexamethylbenzene areable to form charge-transfer (CT) complexes with pyridinium salts. Being formed inthe electronic ground state, these complexes exhibit higher optical absorptions than thepyridinium salt alone. In these circumstances, the incident light is absorbed by the CT complexes.

28. charge-transfer (CT) complexes For example, the complex formed betweenN-ethoxy-4-cyano pyridinium hexafluorophosphate and 1,2,4-trimethoxybenzene possesses an absorption maximum at 420 nm. The absorption maxima of the two constituents are270 nm and 265 nm for the pyridinium salt and trimethoxybenzene, respectively.

29. Sensitization by classical energy transfer This mechanism involves the electronicexcitation of the sensitizer (S), a molecule possessing a suitable absorption spectrum, to itsexcited state. Energy may be transferred from the sensitizer (S*) to the onium salt (I) byeither resonance excitation or exchange energy transfer. Depending on the two componentsinvolved, 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 excitedonium salt species (I*) are formed. The further reactions may also differ from those, takingplace 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 at least as large as the excitation energy of the photoinitiator E*(I). Ep(S) > Ep(I)

30. Oxidation of free radicals Onium salts can only rarely initiate a cationic polymerization by themselves. Instead, they maybe 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 flexibletype of externally stimulated cationic polymerizations Free radicals may be producedby various modes: photochemically,thermally or by irradiating the system with high energy rays.

32. Table 9. continue

33. The efficiency of onium salts as oxidizing agents is related to their electron affinity. The higher theoxidation power of the onium salt, the higher (more positive) is the reduction potential E1/2red (On+).

34. 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 suitablefor the oxidation of free radicals, these salts have been most frequently used for the oxidationof photogenerated free radicals1. On the other hand, triphenylsulphonium saltshave only limited potential for radical induced cationic polymerizations due to their lowreduction 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 theaid of N-ethoxy-2-methyl pyridinium (EMP+) were used to initiate the polymerization ofbutylvinyl ether and cyclohexene oxide.

35. Sensitization via exciplexes Electron transfer via exciplexes. Sensitizers such as anthracene, perylene or phenothiazone form exciplexes with onium salts. Being formed in the consequence of light absorption by the sensitizer, these energy rich complexes consist of non-excited onium salt and electronically excited sensitizer molecules. In the complexation state, electron transfer to the onium salt is observed, giving rise to positively charged sensitizer species. 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 transferredfrom the sensitizer to the onium salt giving rise to the generation of sensitizer radical cations.

36. The electron transfer (right part in eqn (52)) is energetically allowed, if DG calculated byeqn (56) (extended Rehm–Weller equation) is negative. According to eqn (56), the requirementsare low oxidation potentials, Eox½(S), and relatively high excitation energies, E(S*), ofthe sensitizer. Besides that, only onium salts with high (low negative) reduction potentialsEred½(On+ ) (see Table 10), such as diphenyliodonium or alkoxy pyridinium salts are easily reduced by the sensitizer.

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

38. 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 photolabilecompound undergoing homolytic bond rupture may be used as a radical source. The radicalsadd to the double bond of the allylonium salt thus producing a radical in b position to theheteroatom of the onium salt cation. Consequently, the molecule undergoes fragmentation yielding initiating cations.

39. Notably, upon irradiation of monomer solutions containing EPP+ SbF- 6 at 270 nm in theabsence of any additional radical source, cationic polymerizations take place

40. 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 roomtemperature. The reason, why alkylsulphonium salts are more thermally reactive than arylsulphoniumsalts is that the former are stabilized only by hyperconjugation, whereas the latter are stabilized by resonance.

41. Due to the stabilization of the leaving benzyl cation by resonance,alkylbenzylsulphoniumand alkylarylbenzylsulphonium salts possess a high thermal sensitivity. In Table 18, thealkylbenzylsulphonium initiators which have been used so far are compiled Regarding the effect of the substituent in the aromatic ring, R2, it turned out clearly that electron donating substituents enhance the initiation activity by stabilizing the benzyl cationevolved. For example, with the p-OCH3 derivative, for a 50% conversion in styrene polymerization50oC 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)

42. Other highly thermosensitive benzylsulphonium salts are benzyl phenylalkyl sulphonium salts As far as the substituents in the benzyl group, R2, are concerned, electron donating substituents 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.

43. The activity for pyridinium ring p-substituentswas found to decrease in the order p-CN > H > p-CH3 > p-N(CH3) 2. This order indicatesthat electron accepting groups stabilize the cation on the pyridinium N-atom, therebydiminishing the leaving ability of the pyridinium moiety. If the pyridinium ring issubstituted 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 substituentwas attributed to both steric and electronic factors.

44. Other N-containing onium salts Besides N-benzyl pyridinium salts, further saltswith positively charged nitrogen atoms have been utilized as thermolatent cationic initiators.Recently, the thermal initiation by benzyl ammonium salts having the following generalstructure, 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 benzyl radicals by electron donating groups.

45. 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 thep-NO2–C6H4 substituent (R1) is most effective in increasing the thermal sensitivity. Withinitiators containing this substituent, polymerizations were performed at 50oC.

46. Phosphoniumsalts Substituted benzylphosphonium salts of the general structurewith R being NO2, Cl,H,CH3, OCH3, have been synthesized by the reaction of correspondinglysubstituted benzyl chlorides with triphenylphosphine. In a second reaction step, the counter anionhas been exchanged for the low nucleophilic SbF-6These salts have been used for the thermalpolymerization 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 raisewith changing for more electron withdrawing substituents; the order of reactivity observed isOCH3 , CH3 , Cl , NO2. This is in sharp contrast to benzylsulphonium and benzylpyridiniumsalts, where electron withdrawing substituents reduced the thermal sensitivity by destabilizing thebenzyl cation formed upon thermolysis

48. Indirect acting thermolatent systems Regarding thermal activation, the radical sources listed in Table 20 have been successfully applied in cationic polymerization.

50. Thermally induced addition fragmentation reactions