Evidence for Polarity- and Viscosity-Controlled Domains in the Termination Reaction in the Radical Polymerization of Acrylonitrile

The termination mechanism in the radical polymerization of acrylonitrile (AN) was determined by the reaction of structurally well-defined polyacryronitrile (PAN) chain-end radical 1a and PAN-end mimetic small model radical 1b. The contributions of three mechanisms, i.e., the disproportionation (Disp), the combination by carbon-carbon formation (CC-Comb), and the combination by carbon-nitrogen bond formation (CN-Comb), were unambiguously determined. The CN-Comb pathway was experimentally proved for the first time. The selectivity between Disp and CC-Comb showed a good correlation with the viscosity and temperature, and the Disp selectivity increased as the viscosity increased, as previously reported for the termination of other monomers. In contrast, CN-Comb is insensitive to viscosity but sensitive to polarity; the selectivity decreases as the polarity of the media increases. The results strongly suggest the presence of two domains in the termination reaction, namely, the polarity- and viscosity-controlled domains. CN-Comb product 5 was stable under the polymerization conditions but decomposed to Disp and CC-Comb products at high temperatures. Therefore, care must be taken in the processing step, such as the molding process, because the physical properties could be altered due to changes in the Disp and CC-Comb compositions.