Visible light pulsed-OPO-laser polymerization at 450 nm employing a bis(acylphosphine oxide) photoinitiator

The free-radical propagation rate coefficients of both methyl methacrylate (MMA) and styrene (STY) have been measured using a new version of pulsed-laser polymerization (PLP) employing a visible light wavelength (450 nm) generated by a Nd:YAG laser pumped oscillator/power oscillator (OPO). The initiator bis(2,6-dimethoxybenzoyl)(2,4,4-trimethylpentyl)phosphine oxide (BAPO) successfully initiated the polymerizations, and propagation rate coefficients were obtained which are in excellent agreement with values recommended as benchmark values by an IUPAC working party. However the polymer molecular weight distributions were somewhat different from those usually obtained from successful UV-PLP experiments, there being a significant contribution from higher molecular weight overtones, particularly in STY experiments. This was investigated by carrying out simulations of the PLP process, ones aimed in particular at examining an unusual feature of BAPO-initiated polymerizations: that reinitiatable groups are expected to be incorporated as polymer chain ends. Simulations confirmed that the phenomenon of end group reinitiation will have the effect of producing greater amounts of high molecular weight polymer, as observed experimentally. However simulations also revealed that this phenomenon has too small a quantitative effect to explain fully our experimental results. Other simulations suggested that either low radical fluxes or high extents of photoinitiator consumption are more likely to be major factors in explaining our results. It was also confirmed via simulations that observed STY-MMA. differences are primarily due to the different termination mechanisms which are operative: predominantly combination for STY systems; mostly disproportionation for MMA systems