Photoinitiation of cationic polymerization. IV. Direct and sensitized photolysis of aryl iodonium and sulfonium salts

Abstract

Aryl iodonium and sulfonium salts are thermally stable photoinitiators for cationic polymerization. Laser flash photolysis of diphenyliodonium and diphenyl-4-thiophenoxyphenylsulfonium hexafluoroarsenates provides direct evidence for homolytic Ar-I and Ar-S bond cleavage to yield phenyliodinium (PhI +.) and, primarily, diphenylsulfinium (Ph 2S +.) ion radicals, respectively. The radical ions were generated independently by flash-induced electron transfer from iodobenzene and diphenylsulfide to a phenanthrolinium salt. The radical ions are highly reactive with nucleophiles, including iodobenzene and cyclohexene oxide, in the case of PhI +.. Apparent secondorder rate constants were determined for the reaction of the transients with several nucleophiles. Quantum yields of acid formation from stationary photolysis of diphenyliodonium and triphenylsulfonium hexafluoroarsenates were found to be significantly higher than yields of iodobenzene and diphenylsulfide, respectively. These results may be explained by facile reaction of PhI +. (or Ph 2S +.) with PhI (or Ph 2S) to yield new onium salts together with a proton. The high reactivity of PhI +. with cyclohexene oxide suggests that the transient may directly initiate cationic polymerization of epoxides. Photoinitiated cationic polymerization by photosensitization of diphenyliodonium and triphenylsulfonium salts is shown to proceed by two distinct electron transfer processes: (1) direct electron transfer from excited-state photosensitizers, and (2) indirect electron transfer from photogenerated radicals. The efficiency of the former process is attributed to instability of the reduction products (from diphenyliodonium and triphenylsulfonium salts) which dissociate in competition with undergoing energy-wastage reverse electron transfer. Amplification of photons in the production of protons (or other reactive cations) is postulated to account for the high quantum yields observed in the latter process. Potential advantages of utilizing the indirect redox process in the design of UV curable hybrid systems, which contain functionality for both radical and cationic polymerization, are noted. The sensitization results also provide evidence against the importance of triplet states of the onium salts in photoinitiator activity.