By the use of chemically induced dynamic electron polarisation acylphosphine oxides have been shown to undergo the Type I fission reaction from their triplet states.

The photodecomposition of diphenyl-2,4,Ctrimethylbenzoylphosphine oxide (1) was studied both in solution at 40 "C and in thin films in the presence of 1, I-di-p-tolylethylene as a model substrate for vinyl monomers. Both primary radicals resulting from a-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 fp, in particular fJ2) (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.

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Results and discussionWhen undergoing photo-induced a-cleavage acylphosphine oxides 1 generate acyl radicals 2 and diphenylphosphinyl radicals 3, which should be trapped by TMPO (see Scheme).

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