Chain-Length Dependence of Termination Rate Coefficients in Acrylate and Methacrylate Homopolymerizations Investigated via the SP−PLP Technique

Termination rate coefficients, k_t, of alkyl acrylate and alkyl methacrylate homopolymerizations at 40 °C and pressures of 1000 and 2000 bar have been measured up to high degrees of monomer conversion using the time-resolved single-pulse−pulsed-laser polymerization (SP−PLP) technique. The chain-length dependence (CLD) of k_t has been deduced from SP−PLP data by adopting the power-law model, k_t = i^-α, where i is the chain length. For methacrylates at low degrees of monomer conversion, α is close to the theoretically predicted value of 0.16. At conversions above 20% the exponent α increases significantly with increasing conversion. This effect becomes particularly pronounced in the gel effect region, where α, e.g. for MMA, reaches values close to unity. In the case of acrylates with small alkyl ester side chain, such as methyl acrylate, α is also close to 0.16 at low conversions and increases toward higher conversions. In the case of acrylates with larger alkyl ester side chain, such as dodecyl acrylate and 2-ethylhexyl acrylate, however, α is close to 0.4 even at low degrees of monomer conversion. The latter effect is strongly indicative of intramolecular chain transfer, which generates significant amounts of midchain radicals in the system. The fact that such transfer processes take place is supported by SP−PLP data on alkyl acrylates polymerized in mixtures with supercritical carbon dioxide.