Cationic photopolymerization with the aid of pyridinium-type salts

Pyridinium-type salts containing an N-ethoxy group belong to the family of onium salts and are photoinitiators appropriate for the polymerization of monomers such as oxiranes and vinyl ethers which are not polymerizable by a free radical mechanism. The initiation is accomplished by direct or indirect (sensitized) photolysis of the onium ion, with the former being restricted to the wavelength range of self absorption, the latter being applicable at wavelengths of visible light, An additionally useful tool, namely free radical-mediated generation of initiating species enlarges the versatility of pyridinium salts as photoinitiators. In this connection, the oxidation of free radicals by pyridinium-type ions and the free radical-induced fragmentation of alkoxy pyridinium ions are addressed in this article. Moreover, an interesting application is noted concerning the synthesis of novel block copolymers with the aid of the onium salt-based photopolymerization technique.