Chain Length Dependent Propagation Rate Coefficient <i>k</i>_p in Pulsed-Laser Polymerization: Variation with Temperature in the Bulk Polymerization of Styrene and Methyl Methacrylate

Abstract

The pulsed-laser polymerization (PLP) with subsequent GPC analysis of the resulting polymers was applied to the bulk polymerization of styrene and methyl methacrylate in the temperature range 25−70 °C. The chain length dependence of the rate coefficient of chain propagation kp previously found in 25 °C experiments was confirmed; its extent increases even slightly with temperature. The effect is interpreted to be caused by a progressive displacement of monomer from the proximity of the radical chain end by the rest of the chain, thus decreasing the local monomer concentration which results in a formal decrease of kp if the average monomer concentration is inserted into its calculation. Aided by computer simulations based on this concept this decrease could be modeled and parametrized; the difference between the extrapolated values of kp for zero and infinite chain length kp(0) and kp(∞) amounts to about 40−60% of kp(0), somewhat depending on temperature, monomer, and modeling function. The variation of the kp data actually observed is markedly less, of course (about 25−35%). The chain length for which the decrease from kp(0) to kp(∞) is 50% complete was calculated to be on the order of 100, somewhat higher for the (stiffer) poly(MMA) chain.