Photoinitiator fragmentation processes

Fragmentation processes of photoinitiators form a number of volatile products, which may contribute to indoor air pollution. Benzaldehyde and alkyl-substituted benzalde-hydes are usual components, because Norrish-I is the most important reaction for cleavage. Awell known example is l-phenyl-2-hydroxy-2-methyl-propane-l-one (PHMP). a-Cleavage generates two radicals in the first step. The benzoyl radical may recombine to form benzil, reduction of PHMP leads to l-phenyl-2-methy 1-1,2-propane and acetone, and recombination of the 2-hydroxypropyl radical gives 2,3-dimethyl-2,3-butanediol... 

A typical odour-intensive breakdown product generated by a-cleavage (Norrish I reaction) is benzaldehyde. Owing to recombination and reduction processes further secondary products such as benzil and acetone [50] result from the fragmentation. In principle, odourless photoinitiators can therefore certainly cause significant odour problems via their degradation products. 

However, conceptual issues directly related to the radical reaction in the one-pot process partially impede extension of the scope of the radical amine-thiol-ene conjugation. First, some functional groups (e.g., furan [119-123], double and triple bond) introduced via the amine are incompatible with this radical environment. Additionally, UV curing takes place upon decomposition of a photoinitiator (e.g., DMPA), but model studies revealed that some amines (e.g., benzylamine) react with the formed radical fragments, thus limiting the use of a photoinitiator [48]. 

In contrast to the reversibility of the direct photolysis of dialkylphenacylsulfonium salts, photosensitized photolysis results in fragmentation of the photoinitiator (Scheme 11), Thus, in this latter case, the initiating species, the photosensitizer cation-radical is formed by an irreversible process. Photosensitization of dialkylphenacylsulfonium salts would, therefore, be expected to result in more efficient initiation of cationic polymerization. Indeed, the polymerizations of THF, E-caprolactone, and N-vinyl-2-pyrrolidone proceed smoothly under photosensitized conditions. 

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