Termination Rate Coefficients of Butyl Acrylate Free-Radical Homopolymerization in Supercritical CO2 and in Bulk

Free-radical termination kinetics of butyl acrylate (BA) homopolymerization in supercritical CO 2 (scCO 2 ) and in bulk have been studied by single pulse−pulsed laser polymerization (SP−PLP) at temperatures and pressures up to 120 °C and 2500 bar, respectively. Polymerization induced by a single laser pulse is monitored via microsecond time-resolved near-infrared (NIR) spectroscopy. From individual SP−PLP experiments performed at several stages during the course of a polymerization, the ratio of termination to propagation rate coefficients, k t / k p, is obtained as a function of monomer conversion. With k p being available from pulsed laser polymerization in combination with size exclusion chromatography (PLP−SEC) experiments in both solution of scCO 2 and in bulk, the k t / k p data yield individual (chain-length averaged) k t . Irrespective of polymerization pressure and temperature, k t is larger in scCO 2 than in bulk polymerizations. At the highest CO 2 content of 46 wt %, this enhancement amounts to about 350%. The activation volumes of k t are similar for polymerizations in scCO 2 and in bulk, whereas the activation energies are clearly dissimilar: In solution of scCO 2 an apparent activation energy is found which is negative. In bulk polymerization, on the other hand, E A ( k t ) is positive. The experimental observations indicate that thermodynamic contributions strongly affect termination rate in scCO 2 solution.