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

Abstract

The termination mechanism in the radical polymerization of acrylonitrile (AN) was determined by the reaction of structurally well-defined polyacryronitrile (PAN) chain-end radical <b>1a</b> and PAN-end mimetic small model radical <b>1b</b>. The contributions of three mechanisms, i.e., the disproportionation (<i>Disp</i>), the combination by carbon-carbon formation (<i>CC-Comb</i>), and the combination by carbon-nitrogen bond formation (<i>CN-Comb</i>), were unambiguously determined. The <i>CN-Comb</i> pathway was experimentally proved for the first time. The selectivity between <i>Disp </i>and<i> CC-Comb</i> showed a good correlation with the viscosity and temperature, and the <i>Disp</i> selectivity increased as the viscosity increased, as previously reported for the termination of other monomers. In contrast, <i>CN-Comb</i> 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. <i>CN-Comb</i> product<b> 5</b> was stable under the polymerization conditions but decomposed to <i>Disp</i> and <i>CC-Comb</i> 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 <i>Disp</i> and <i>CC-Comb</i> compositions.