Molecular Weight Distributions and Chain-Stopping Events in the Free-Radical Polymerization of Methyl Methacrylate

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Abstract

The chain-stopping and radical-loss events in the seeded emulsion polymerization of methyl methacrylate are determined using information from the complete molecular weight distributions (MWDs) and rate behavior with initiation by γ radiolysis followed by removal from the radiation source (“γ relaxation”). The system follows “pseudo-bulk” kinetics, implying that the chain-stopping and radical-loss events are kinetically the same as in corresponding bulk and solution free-radical polymerizations. It was essential to take SEC (size-exclusion chromatography) band broadening into account when interpreting the MWDs. The MWDs are interpreted by plotting the log(instantaneous number MWD), which is expected to be linear from theory, but it is shown that broadening leads to an upward curvature, consistent with experimental observation in this and many other systems. It is proven that the slope of the true (unbroadened) log(instantaneous number MWD) can be found from that of the experimental (broadened) one by taking the slope at the peak of the SEC distribution. The experimental MWDs are found to be dominated by transfer to monomer. The γ-initiated rate data also show that radical loss is predominantly caused by the rapid diffusion of short radicals generated by transfer to monomer (i.e., the rate coefficient for termination is a function of those for transfer and primary radical termination). The transfer constant inferred from all these data (ktr/kp = 2.3 × 10-5) is in agreement with that obtained by alternative methods (Stickler, M.; Meyerhoff, G. Makromol. Chem. 1978, 179, 2729). All data can be acceptably modeled using diffusion theory to predict the rate coefficients of chain length dependent termination, with the few parameters adjusted to give the fit having values that are in good accord with the expected range.

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