Termination kinetics of styrene free-radical polymerization studied by time-resolved pulsed laser experiments

The single pulse (SP)-pulsed-laser polymerization (PLP) technique has been applied to measure k(t)/k(p), the ratio of termination to propagation rate coefficients, for the free-radical bulk polymerization of styrene at temperatures from 60 to 100 degrees C and pressures from 1800 to 2650 bar. k(t)/k(p) is obtained by fitting monomer concentration vs. time traces that are determined via time-resolved (mu s) near infrared monitoring of monomer conversion induced by single excimer laser pulses of about 20 ns width. Styrene is a difficult candidate for this kind of measurements as conversion per pulse is small for this low k(p) and high k(t) monomer. Thus between 160 to 300 SP signals were co-added to yield a concentration vs. time trace of sufficient quality for deducing k(t)/k(p) with an accuracy of better than +/-20 per cent. With k(p) being known from PLP-SEC experiments, chain-length averaged k(t) values are immediately obtained from k(t)/k(p). At given pressure and temperature. k(t) is independent of the degree of overall monomer conversion, which, within the present study, has been as high as 20 %. The k(t) value, however, is found to slightly increase with the amount of free radicals produced by a single pulse in laser-induced decomposition of the photoinitiator DMPA (2,2'-dimethoxy-2-phenyl acetophenone). This remarkable: observation is explained by DMPA decomposition resulting in the formation of two free radicals which significantly differ in reactivity. Extrapolation of SP-PLP k(t) data from experiments at rather different DMPA levels and laser pulse energies toward low primary free-radical concentration, yields very satisfactory agreement of the extrapolated k(t) values with recent literature data front chemically and photochemically induced styrene polymerizations.