Trithianes as coinitiators in benzophenone-induced photopolymerizations

A series of 1,3,5-trithiane derivatives, including alpha- and beta-isomers of the methyl and phenyl derivatives, was investigated for use as coinitiat ors in benzophenone-induced photopolymerizations. To study the mechanism of the photoinduced production of initiating radicals, nanosecond laser flash photolysis was used. The rate constants for quenching the benzophenone tri pler state by various trithianes and the associated quantum yields of ketyl -radical formation were determined. Photopolymerizations were carried out w ith a multifunctional methacrylate model monomer, and the progress of the p olymerizations was followed by differential scanning calorimetry. The resul ting plots showing the progress of the rates of polymerization and the conv ersions of double bonds were compared to the photochemical information for the various trithianes. It was found that the polymerization efficiency of the coinitiators did not follow the efficiency of photoinduced formation of the initiating radicals. The reasons for this lack of correlation were dis cussed in terms of how the hydrogen-donating ability of coinitiators and th e reactivity of the radicals formed affected the various stages of the poly merization. Further studies with a cleavage-type photoinitiator in the pres ence of the trithianes lent supporting evidence for the role that the trith iane radicals play in the polymerization of the benzophenone/trithiane syst ems investigated in this work.