Chain length dependent propagation rate coefficient kp in pulsed-laser polymerization:: Variation with temperature in the bulk polymerization of styrene and methyl methacrylate

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 degreesC. The chain length dependence of the rate coefficient of chain propagation k(p) previously found in 25 degreesC 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 k(p) 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 k(p) for zero and infinite chain length k(p)(0) and k(p)(infinity) amounts to about 40-60% of k(p)(0), somewhat depending on temperature, monomer, and modeling function. The variation of the k(p) data actually observed is markedly less, of course (about 25-35%). The chain length for which the decrease from k(p)(0) to k(p)(infinity) is 50%, complete was calculated to be on the order of 100, somewhat higher for the (stiffer) poly(MMA) chain.