Free radical promoted cationic polymerization by using biscylphosphine oxide photoinitiators:substituent effect on the reactivity of phosphinoyl radicals

摘要

The cationic polymerization of cyclohexene oxide (CHO)was achieved upon UV irradiation (gamma=380nm)of methylene chloride solutions containing a series of bisacylphosphine oxides and onium salts,such as diphenyliodomium hexafluorphosphate (Ph_2I~+PF_6~-)or N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP~+PF_6~-).A feasible initiation mechanism involves the photogeneration of phosphinoyl radicals and benzoyl radicals in the first step.Subseqsuent oxidation of phosphimoyl radicals by onium salts yields phosphomium ions capable of initiating the polymerizatin of CHO.In agreement with the proposed mechanism,the polymerizatin efficient was directly related to the reduction potential of hte onium salts,i.e.Ph_2I~+PF_6~-(E_red~1/2=-0.2CV)was found to be more efficient than EMP~+PF_6~-(E_red~1/2=-0.7V).The variation in reactivity of the different phosphorous radicals was correlated with p-character of the phosphinoyl radical as reflected by the ~31P hyperfine coupling constant.The results were compared to a monoacylphosphine oxide,(2,4,6-trimethylbenzoly)diphenylphosphine oxide,which showed only a lwo efficiency in promoting cationic polymerization.In addition to CHO monomer,butyl vinyl ehter and N-vinyl carbazole were polymerized in hte presence of bisacylphosphine oxide and onium salts with high efficiency.