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 CO2 (scCO(2)) and in bulk have been studied by single pulse-pulsed laser polymerization (SP-PLP) at temperatures and pressures up to 120 degrees 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) ht. Irrespective of polymerization pressure and temperature, ht is larger in scCO(2) than in bulk polymerizations. At the highest CO2 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.