Termination Mechanism in the Radical Polymerization of Methyl Methacrylate and Styrene Determined by the Reaction of Structurally Well-Defined Polymer End Radicals

Abstract

A novel method to determine the termination mechanism of radical polymerization, i.e., the selectivity between disproportionation (Disp) and combination (Comb), is developed. The method relies on product analyses of the reaction of polymer-end radicals, which are generated from structurally well-controlled living polymers, and the analyses of molecular weight and end-group structure of the product polymers by GPC, mass spectroscopy, and ¹H NMR unambiguously determined the contribution of two competing pathways. The termination mechanism in the polymerization of methyl methacrylate (MMA) and styrene was investigated as a proof of principle of the method by using the corresponding polymers prepared by organotellurium-mediated radical polymerization. The ratios of Disp and Comb (<i>D</i>/<i>C</i>) of poly­(methyl methacrylate) (PMMA) or polystyrene (PSt) end radicals at 25 °C were 73/27 or 15/85, respectively, and the results agreed well with the previous reports. The contribution of the Comb increased at higher temperature in both cases, though the temperature dependence was less pronounced in PSt radicals (<i>D</i>/<i>C</i> = 67/37 and 13/87 at 100 °C for PMMA and PSt, respectively). Thermodynamic parameters were determined as ΔΔ<i>G</i>^‡_d/c = (−6.9 ± 0.3) – <i>T</i> × (−14.4 ± 1.0) × 10^–3 (kJ mol^–1) for PMMA and ΔΔ<i>G</i>^‡_d/c = (−2.0 ± 0.5) – <i>T</i> × (−20.8 ± 1.5) × 10^–3 (kJ mol^–1) for PSt, in which ΔΔ<i>G</i>^‡_d/c and <i>T</i> are difference in Gibbs energy undergoing Disp and Comb, and temperature in Kelvin, respectively, by carrying out the same experiments between −20 to +100 °C. The parameters reveal that Comb is enthalpically less favored but entropically more favored than Disp in both cases. The effects of molecular weight (chain length) were also investigated, and the <i>D</i>/<i>C</i> ratio became constant when the molecular weight of polymers was more than about 3000