Pressure and Temperature Dependence of Butyl Acrylate Propagation Rate Coefficients in Fluid CO₂

Propagation kinetics of butyl acrylate (BA) free-radical homopolymerization in fluid CO 2 has been studied by the PLP−SEC (pulsed laser polymerization−size-exclusion chromatography) method at pressures between 200 and 2000 bar and temperatures between −9 and 20 °C. The CO 2 content was close to 45 wt % in all experiments. As a consequence of the poor chain-transfer activity in solution of CO 2, the molecular-weight distributions of the polymer samples show well-resolved PLP structures with at least two, but in most cases three, distinct inflection points. At the p and T conditions of this work, the observed propagation rate coefficients ( k p ) of BA in fluid carbon dioxide at CO 2 contents around 45 wt % are found to be about 40% below the corresponding bulk polymerization values. The activation volume and activation energy of k p are, however, identical for bulk and solution (in CO 2 ) free-radical BA polymerizations within experimental accuracy. The origin of the solvent influence on the propagation reaction is discussed. The observed lower values of k p in solution of CO 2 are assigned to the poor solvent quality of carbon dioxide, presumably resulting in a local monomer concentration at the site of the propagating radical which is smaller than the overall monomer concentration in the system.