Photochemically and thermally induced radical promoted cationic polymerization using allyl phosphonium salts

Abstract

Three allylic triphenyl phosphonium salts, namely allylphosphonium (AP), methyl allylphosphonium (MAP) and ethoxy carbonyl allylphosphonium (ECAP) salts with a hexafluoroantimonate counter anion were synthesized and characterized. Their capability to act as initiators or coinitiators for the cationic polymerization of oxiranes such as cyclohexene oxide (CHO) and vinyl monomers such as butylvinyl ether and N-vinyl carbazole was examined. These monomers turned out to be polymerizable in the presence of ECAP provided free radicals are generated thermally at 70°C or photochemically at λ>350nm with the aid of appropriate radical sources. The polymerization of CHO was also induced thermally in the absence of radical sources only with ECAP at a very low rate, while all the three types of salts are ineffective in generating reactive species when irradiated at λ=268nm where the salts absorb the light. Regarding sources generating free radicals the mode of action of ECAP concerning promotion of the cationic polymerization seems to be based on the addition–fragmentation mechanism. Accordingly, a free radical adds to the carbon–carbon double bond and fragmentation of the adduct radical results in the formation of a reactive onium radical cation. The possibility of electron transfer mechanism was disregarded since nucleophilic triphenyl methyl radicals were not oxidized with the aid of phosphonium salts.