Chain-length dependence of free-radical termination rate deduced from laser single-pulse experiments

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Termination rate coefficients of free-radical polymerization are accessible from SP-PLP studies where monomer conversion induced by a laser single pulse is measured with a time resolution of microseconds. Previous experiments with 2,2-dimethoxy-2-phenylacetophenone (DMPA) acting as the initiator revealed that upon variation of the DMPA concentration, the resulting monomer conversion vs. time traces intersect. A detailed kinetic analysis of this unexpected type of behavior is presented. It turns out that such crossings occur in situations where kt is chain-length dependent and, at the same time, the primary initiator-derived free-radical species differ in reactivity toward the monomer. As is known from the literature, this difference in radical reactivity is particularly pronounced with DMPA, which photo-decomposes to a propagating and to a non-propagating free radical. Modeling of the crossing behavior opens a novel route for determining chain-length dependent kt. Results for methyl acrylate (MA) and styrene homopolymerizations at low degrees of monomer conversion, to a maximum of 10%, are presented. The decrease of kt with chain length is modeled via an exponential function. The dependence is significantly larger for MA than for styrene. The exponents derived from simulation studies via PREDICI® are in excellent agreement with corresponding data reported by Olaj et al. for styrene and by de Kock for methyl acrylate.