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

The cationic polymerization of cyclohexene oxide (CHO) was achieved upon UV irradiation (λ=380 nm) of methylene chloride solutions containing a series of bisacylphosphine oxides and onium salts, such as diphenyliodonium hexafluorophosphate (Ph2I+PF6−) or N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP+PF6−). A feasible initiation mechanism involves the photogeneration of phosphinoyl radicals and benzoyl radicals in the first step. Subsequent oxidation of phosphinoyl radicals by onium salts yields phosphonium ions capable of initiating the polymerization of CHO. In agreement with the proposed mechanism, the polymerization efficiency was directly related to the reduction potential of the onium salts, i.e. Ph2I+PF6− (E1/2red=−0.2 V) was found to be more efficient than EMP+PF6− (E1/2red=−0.7 V). 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-trimethylbenzoyl) diphenylphosphine oxide, which showed only a low efficiency in promoting cationic polymerization. In addition to CHO monomer, butyl vinyl ether and N-vinyl carbazole were polymerized in the presence of bisacylphosphine oxides and onium salts with high efficiency.