Application of spin trapping technique to radical polymerization, 16. Photo‐decomposition of diphenyl disulfide and initiation mechanism: Evaluation of relative reactivities of vinyl monomers toward phenylthio radical

To clarify the initiation process in radical polymerizations with diphenyl disulfide (DPDS) as initiator, spin trapping technique was applied to the reaction of vinyl monomers with the phenylthio radical (1), which was produced by photolysis of DPDS. 2‐Methyl‐2‐nitrosopropane (BNO) was used as a spin trapping agent. When DPDS was photo‐decomposed in benzene in the presence of BNO radical 1 was trapped as the corresponding aminyl oxide radical 2, which is unstable at room temperature. The photolysis of DPDS in vinyl monomers of type H2CCXY, however, gave only aminyl oxide radicals resulting from their propagating radicals C6H5SCH2C˙XY. Contrary to the results obtained with tert‐butoxyl radicals, radical 1 was found to undergo exclusively addition reaction to the double bond of the α‐methyl‐substituted monomers, such as methyl methacrylate or methacrylonitrile, with no hydrogen‐abstraction reaction from their α‐methyl group. Relative rates of the initiation reactions of vinyl monomers with radical 1 were also determined by spin trapping technique. It was found that the reactivities of vinyl monomers toward radical 1 are controlled by both polar and resonance effects.