Chain-Length-Dependent Termination of Styrene Bulk Polymerization up to High Degrees of Monomer Conversion

Single pulse-pulsed laser polymerization in conjunction with the highly time-resolved detection of radical concentration by electron paramagnetic resonance (SP-PLP-EPR) was applied for the first time toward measuring termination kinetics up to high degrees of monomer conversion with styrene bulk polymerization up to 80% conversion taken as an example. Monomer conversion was mimicked by the addition of polystyrene. The composite model turns out to provide an adequate representation of the termination kinetics. A constant value of the power-law exponent for small chain radicals, alpha(s), holds up to about 50% conversion, in which region the model parameter k(t)(1,1), the rate coefficient for termination of two radicals of size unity, exhibits a minor decay. Above 50% polymer content, both alpha(s) and k(t)(1,1) decrease significantly. Above 80% polymer, the chain-length dependence almost disappears, and termination runs under reaction-diffusion control. As the region of large radicals is not accessible at higher monomer conversion, the knowledge from the composite model has been used to represent the chain-length dependence of large radicals. To check for the quality of the measured and predicted rate parameters, PREDICI estimates on the basis of this data have been compared to experimental results for chemically initiated bulk styrene polymerizations up to almost full conversion. The satisfactory comparison suggests that the data obtained by the SP-PLP-EPR studies are suitable for kinetic simulations of styrene bulk polymerization over the entire conversion range.