Photoinitiators and photoinitiation, 8 The photoinduced α‐cleavage of acylphosphine oxides. Identification of the initiating radicals using a model substrate

Abstract

Abstract The photodecomposition of diphenyl‐2,4,6‐trimethylbenzoylphosphine oxide (1) was studied both in solution at 40°C and in thin films in the presence of 1,1‐di‐ p ‐tolylethylene as a model substrate for vinyl monomers. Both primary radicals resulting from α‐cleavage of 1 were found to add to the olefinic double bond of the model substrate (initiation). The diphenylphosphinoyl radical (3) proved to be twice as effective as the 2,4,6‐trimethylbenzoyl radical (2) under all conditions. Oxygen (air) considerably reduced the initiating efficiencies f p , in particular f p (2) (of radical 2 ). The addition of triethylamine partially restored the overall initiating efficiency in all likelihood by oxygen scavenging. A direct contribution of triethylamine‐derived radicals to the initiation was not observed.