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

Abstract

Free-radical termination kinetics of butyl acrylate (BA) homopolymerization in supercritical CO2 (scCO2) 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, kt/kp, is obtained as a function of monomer conversion. With kp being available from pulsed laser polymerization in combination with size exclusion chromatography (PLP−SEC) experiments in both solution of scCO2 and in bulk, the kt/kp data yield individual (chain-length averaged) kt. Irrespective of polymerization pressure and temperature, kt is larger in scCO2 than in bulk polymerizations. At the highest CO2 content of 46 wt %, this enhancement amounts to about 350%. The activation volumes of kt are similar for polymerizations in scCO2 and in bulk, whereas the activation energies are clearly dissimilar: In solution of scCO2 an apparent activation energy is found which is negative. In bulk polymerization, on the other hand, EA(kt) is positive. The experimental observations indicate that thermodynamic contributions strongly affect termination rate in scCO2 solution.