Dialkyl phenacylsulfonium salt

A somewhat different mechanism is visualized for the photodecompositions of dialkyl phenacylsulfonium salt... 

Kinetie studies show that dialkyl phenacylsulfonium salts compare favorably in their initiating efficiency to diaryliodonium and triarylsulfonium salts in the polymerization of epoxides. An induction period was reported in polymerizations of vinyl and 1-propenyl ethers due to termination reactions with the photo generated ylides. ... 

Based on the above, an initiating composition for cationic photopolymerization, with visible and long-wavelength UV light was described by Crivello et al. The structure of the monomers plays a key role in these photosensitization processes. Useful aromatic ketones are camphoquinone, benzyl, 2-isopropylthioxanthone, or 2-ethylanthraquinone. The monomer-bound radicals reduce diaryliodonium salts or dialkyl phenacylsulfonium salts rapidly to form monomer-centered cations. These cations then initiate the polymerization of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potential, however, such as triarylsulfonium salts, do not undergo this reaction. 

J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkyl-phenacylsulfonium and dialkyl(4-hydroxyphenyl)sulfonium salt photoinitiators. Macromolecules 1981, 14(5), 1141-1147. 

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