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

Abstract

The single pulse (SP)-pulsed-laser polymerization (PLP) technique has been applied to measure kt/kp, the ratio of termination to propagation rate coefficients, for the free-radical bulk polymerization of styrene at temperatures from 60 to 100°C and pressures from 1800 to 2 650 bar. kt/kp is obtained by fitting monomer concentration vs. time traces that are determined via time-resolved (μs) 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 kp and high kt monomer. Thus between 160 to 300 SP signals were co-added to yield a concentration vs. time trace of sufficient quality for deducing kt/kp with an accuracy of better than ± 20 per cent. With kp being known from PLP–SEC experiments, chain-length averaged kt values are immediately obtained from kt/kp. At given pressure and temperature, kt is independent of the degree of overall monomer conversion, which, within the present study, has been as high as 20%percnt;. The kt 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 kt 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 kt values with recent literature data from chemically and photochemically induced styrene polymerizations.