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 (D/C) 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 (D/C = 67/37 and 13/87 at 100 °C for PMMA and PSt, respectively). Thermodynamic parameters were determined as ΔΔG ‡ d/c = (−6.9 ± 0.3) – T × (−14.4 ± 1.0) × 10–3 (kJ mol–1) for PMMA and ΔΔG ‡ d/c = (−2.0 ± 0.5) – T × (−20.8 ± 1.5) × 10–3 (kJ mol–1) for PSt, in which ΔΔG ‡ d/c and T 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 D/C ratio became constant when the molecular weight of polymers was more than about 3000.

Termination Mechanism of the Radical Polymerization of Acrylates

“…These results also suggest that Disp is highly favored enthalpically and disfavored entropically than Comb is. The same temperature effect is observed for the termination of methyl methacrylate and styrene polymerizations …”

“…The amount of the 4a was calculated to be 50% from the peak integral of the signals of olefinic protons and ethyl ester moiety at the polymer α ‐end, consistent with the exclusive Disp reaction. The selective Disp also occurred in the presence of monomer (100 equiv) in the reaction mixture judged by 1 H NMR, while the analysis by GPC was difficult due to the competing propagation . The effect of the molecular weight (chain length) was next examined.…”

“…We have recently developed a new and highly efficient method to clarify the termination mechanism based on the product analysis . Polymer‐end radicals with controlled number average molecular weight ( M n ) and low polydispersity are generated from the corresponding “living” polymer having the organotellurium‐end group and are reacted with each other giving Disp and/or Comb products.…”

On the Termination Mechanism in the Radical Polymerization of Acrylates

“…Nakamura et al have recently described a new technique to determine the mode of termination using a monodisperse, dormant organotellurium terminated polymer which is activated by UV light . By comparison of the molecular weight distribution (MWD) of the polymer before and after irradiation they determined the mode of termination.…”

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

“…[15][16][17] Polymerend radicals, such as 7c,w ith controlled molecular weight are generated from the corresponding "living" polymer having the organotellurium end group 8c. [15][16][17] Polymerend radicals, such as 7c,w ith controlled molecular weight are generated from the corresponding "living" polymer having the organotellurium end group 8c.…”