Cationic photoinitiators: rearrangement reactions from irradiation of halonium salts

Onium salt photoinitiators are increasingly being used for radiation curing of polymers in electronic applications. Irradiation of onium salts in polymeric media generates acid which on further processing can crosslink acid sensitive monomers (e.g. epoxy functional resins), or cleave acid sensitive groups (e.g. poly(p-t-butoxycarbonyloxystyrene)). Irradiation of diarylhalonium salts results in formation of 2-, 3- and 4-halobiaryls by an in-cage fragmentation-recombination reaction, in addition to the escape product, haloarene. The cage and escape products are formed from both homolytic and heterolytic fragmentation of the carbon-halogen bond. Iodonium salts give more in-cage products and favor the homolytic cleavage more than bromonium or chloronium salts. The Rel φ is lower for iodonium salt photolysis than bromonium or chloronium photodecomposition.