Propagation kinetics in free-radical polymerizations

The combination of pulsed laser initiated polymerizations (PLP) with analysis of the generated polymer by size-exclusion chromatography (SEC) yields reliable individual rate coefficients for polymerizations of a large number of monomers in bulk and in solution. PLP-SEC experiments carried out in the presence of scCO(2) as a solvent show no unambiguous trend: while a significant reduction of k(p) is seen for some monomers, e.g. acrylates, k(p) for monomers such as vinyl acetate and styrene is not affected. It is suggested that the influence of CO2 on acrylate k(p) is not a true kinetic effect and that the experimental findings may be understood in terms of the occurrence of local monomer concentrations in the vicinity of the propagating radical. It is discussed that such local monomer concentrations may also contribute to a better understanding of why k(p) increases with ester size within the acrylate or within the methacrylate family, and why k(p) frequently is influenced by the initiating laser pulse repetition rate.