Evaluation of monomer reactivity ratios of more complex copolymerization schemes, with application to the penultimate unit effect in methyl acrylate–butadiene copolymerization

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Abstract
A generally applicable computational procedure, which permits the accurate evaluation of the kinetic parameters of intricate and extended copolymerization schemes to be made, is described. This method is based on a numerical integration of the differential equation, and, according to the (improved) curve‐fitting I procedure, experimental errors in both measured variables are considered. Furthermore, a description is given of theFtest, in which a statistical comparison between the resulting residual sums of squares of two different schemes offers a possibility of selecting the most probable kinetic scheme for a given copolymerization system. The capability and applicability of the methods developed is demonstrated for the free radical copolymerization kinetics of methyl acrylate (MA) (M1) and butadiene (BD) (M2) with toluene as solvent. Here, the simple copolymer equation is unsatisfactory because a significant penultimate unit effect in BD macroradical reactivity shows up:k222/k221= 0.84, andk122/k121= 0.53, andk11/k12= 0.088. The microstructure of the copolymer samples, determined by infrared (IR) spectroscopy, shows a decreasing fraction of BD units in the vinyl configuration in favor of the fraction of BD units in thecis‐vinylene andtrans‐vinylene configuration at increasing MA (m) content. Statistical considerations indicate a strongly diminished probability of finding BD (b) in the vinyl configuration in ∼mb∼ transitions. Steric hindrance or polar repulsion of the ester side group of the penultimate MA unit probably account for the increased preference for monomer addition to the C4site over the C2site of the BD macroradical.