Termination kinetics of free-radical polymerization of styrene over an extended temperature and pressure range

The termination rate coefficient k(t) of the free radical bulk polymerization of styrene is determined between 30 and 90 degrees C up to a maximum pressure of 2 800 bar. The majority of polymerization experiments has been carried out at monomer conversions up to 20 per cent. In this range a single value of k(t) is sufficient to describe termination rate at constant pressure and temperature. Toward higher conversion, significant changes in k(t) are observed. The data are measured by a pulsed laser polymerization technique and partly by conventional chemically initiated experiments, both with 2,2'-azoisobutyronitrile (AIBN) as the initiator. Online spectroscopy is applied toward measurement of styrene conversion. The experimental termination rate coefficients up to 20 per cent monomer conversion are adequately represented by the expression: ln[k(t)(p, T)/(L . mol(-1) . s(-1))] = 20.785 - 1.050 . 10(-3) p/bar + 5.2 . 10(-8) p(2)/bar(2) - 753/T/K + 0.1060/TK . p/bar Activation volume and activation energy of k(t) are very close to the corresponding activation parameters that characterize the pressure and temperature dependence of the inverse of styrene monomer viscosity. Varying laser pulse repetition rate has been used to investigate a potential chain-length dependence of k(t) at low conversion. It turns out that effects of this kind are not sufficiently pronounced to be safely established in view of the experimental precision of +/-25 per cent that is reached in the k(t) determinations.