Pulsed-laser polymerization-gel permeation chromatographic determination of the propagation-rate coefficient for the methyl acrylate dimer: A sterically hindered monomer

The methyl acrylate dimer (MAD) is a sterically hindered macromonomer, and the propagating radical can fragment to an unsaturated end group. The propagation-rate coefficient (k(p)) for MAD was obtained by pulsed-laser polymerization (PLP). The Mark-Houwink-Sakaruda parameters required for the analysis of the molecular weight distributions (MWDs) were obtained by multiple-detector gel permeation chromatography (GPC) with on-line viscometry. The small radical created by the fragmentation results in a short-chain polymer that means the MWD may no longer be given by that expected for "ideal" PLP conditions; simulations suggest that the degree of polymerization required for "ideal" PLP conditions can be obtained from the primary point of inflection provided the GPC traces also show a clear secondary inflection point (radicals terminated by the second, rather than the first, pulse subsequent to initiation). Over the temperature range of 40-75 degreesC, the data can be best fitted by k(p)/dm(3) mol(-1) s(-1) = 10(6.1) exp(-29.5 kJ mol (1)), with a moderately large joint confidence interval for the Arrhenius parameters. The data are consistent with an increased activation energy and reduced frequency factor as compared with acrylate or methacrylate; both of these changes can be ascribed to hindrance. (C) 2001 John Wiley & Sons, Inc.