Chain-length dependent termination in pulsed-laser polymerization, 7. The evaluation of the power-law exponent b from the chain-length distribution in the low frequency (single-pulse) limit for the reference systems styrene and methyl methacrylate in bulk at 25°C

The chain-length distribution (CLD) was examined for polymers prepared by “low frequency pulsed laser polymerization” (LF-PLP), i. e. for very long pulse separations in the so-called “low frequency” or “single pulse limit”. The data were fitted to the theoretical CLD which could be derived in a closed form for a chain-length dependent rate coefficient k_t and the parameter b that characterizes this chain-length dependence was determined by this fitting procedure. b values close to 0.2 were obtained for styrene as well as for MMA, indicating a moderate chain-length dependence of k_t at low conversions. This result, which is in good agreement with data evaluated by other methods in our laboratory, points to the fact that under these conditions end-segment diffusion is the rate-determining step in bimolecular termination. Factors like moderate chain transfer to monomer and uncertainties with respect to the mechanism of termination (combination or disproportionation) appear to have very little influence on this result.