Free-radical polymerization kinetics in extended ranges of temperature, pressure and monomer conversion

Free-radical polymerizations have been studied within extended ranges of pressure, temperature, and monomer conversion, including reactions in supercritical phase, where polymerization rate and polymer properties may be continuously tuned. The methods by which propagation (k(p)) and termination (k(t)) rate coefficients of homopolymerizations may be determined as a function of P and T, partly up to 3000 bar and 250 degrees C, are based on using pulse lasers for inducing polymerization (PLP) either in conjunction with size exclusion chromatography (SEC), for k(p) measurement, or in conjunction with online spectroscopic analysis of monomer conversion, for k(t)/k(p) measurement. With k(p) from PLP-SEC, the k(t)/k(p) data allow to deduce also individual rate coefficients for the termination step. k(p) primarily depends on temperature and pressure with the values being close to each other within a monomer family, such as the acrylic ester or the methacrylic ester family. Termination rate, which refers to a radical-radical process, depends on P, T, monomer conversion, solvent type, and on chain length. The major changes of k(t) with conversion are illustrated using a model that takes segmental diffusion, translational diffusion, and reaction diffusion into account.