The initiation kinetics of the free-radical polymerization of methyl methacrylate at very high monomer conversions was investigated at 80, 100, and 120°C. Using a HPLC (high performance liquid chromatography) analytical technique with UV detection, the consumption of both monomer and initiator as well as the formation of reaction products of the latter could be followed. It was found that (besides very small amounts of biphenyl and benzoic acid) phenyl benzoate is the main decomposition product. The experimental results were analyzed quantitatively with the help of a reaction scheme which takes into account the most important pathways of that complex chemical reaction system. Thus, it was found that, due to the very strong cage effect in the present polymerization system, the experimentally accessible (apparent) first order rate constant of initiator decomposition is substantially lower than that for the primary cleavage of a benzoyl peroxide molecule. Efficiencies of chain initiation were calculated to be in the range of 0,l to 0,2 in the (nearly) glassy polymer matrix.
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