MECHANISM OF COINITIATION OF PHOTOPOLYMERIZATION OF METHYL-METHACRYLATE BY HEXAARYLBIIMIDAZOLE-HYDROGEN-ATOM DONOR COMBINATIONS - THE ROLE OF ELECTRON-TRANSFER VS DIRECT HYDROGEN-ATOM ABSTRACTION

The photoinitiated polymerization of methyl methacrylate by combinations of 2-(omicron-chlorophenyl)-4,5-diphenylimidazolyl dimer (omicron-Cl-HABI) and various hydrogen-atom donor coinitiators (cumene, the ethylene ketal of benzaldehyde, dimethylaniline, p-toluenesulfinic acid potassium salt (studied as the complex with 18-crown-6), triethylamine, the mercaptans 2-mercaptobenzthiazole, 2-mercaptobenzoxazole and 2-mercaptobenzimidazole, and the disulfide of 2-mercaptobenzthiazole) was studied. The coinitiators were chosen such that some (e.g. cumene) can only initiate polymerization by direct hydrogen-atom abstraction from the coinitiator by the photoproduced imidazolyl radical, whereas others (e.g. dimethylaniline) prefer electron transfer and proton loss pathways to form the initiating radicals. Kinetic studies (measurement of the rates of polymerization, molecular weight determination, calculation of kinetic chain length and rates of initiation) were conducted for all the coinitiators. The mercaptans were examined in detail. Analysis of the results and comparison with the parameters for the unambiguous mechanistic cases demonstrated that the mercaptans react with imidazolyl radicals via electron transfer pathways, not via direct hydrogen-atom abstraction.