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

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 the F test, 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) (M 1 ) and butadiene (BD) (M 2 ) with toluene as solvent. Here, the simple copolymer equation is unsatisfactory because a significant penultimate unit effect in BD macroradical reactivity shows up: k 222 / k 221 = 0.84, and k 122 / k 121 = 0.53, and k 11 / k 12 = 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 the cis ‐vinylene and trans ‐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 C 4 site over the C 2 site of the BD macroradical.