Cationic polymerization — Iodonium and sulfonium salt photoinitiators

Abstract

Diaryliodonium, triarylsulfonium, dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts are versatile new classes of photoinitiators of cationic polymerization which have been reported in the last few years. These photoinitiators fall into two basic types based on the mechanisms of their photolysis. The first class comprises diaryliodonium and triarylsulfonium salts which undergo photoinduced fragmentations to generate aryl radicals and either aryliodinium or diarylsulfinium cation-radicals. On the other hand, dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts generate ylides and Brønsted acids by a reversible photolysis. Both classes of onium salts can be photosensitized to respond to long wavelength UV and visible light. Photosensitization occurs chiefly by an electron transfer process. Photogenerated free radicals can also be employed to induce the decomposition of diaryliodonium and certain triarylsulfonium salts by a chain reaction. Modifications within the cations and anions of the above onium salts have a marked influence on the course of the cationic polymerizations which are initiated using these photoinitiators. The structure of the cations exerts a dominant role in the photochemistry and determines the rate at which initiator fragments are produced. The reactivity and propensity of the initiator fragments to undergo termination are controlled by the nature of the anion. Photoinitiated cationic polymerizations using iodonium and sulfonium salts are currently finding use in a number of commercial applications including coatings, adhesives, inks and novel photoresists.