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

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journal contribution
posted on 07.05.2021, 07:05 by Xiaopei Li, Tasuku Ogihara, Tatsuhisa Kato, Yasuyuki Nakamura, Shigeru Yamago
The termination mechanism in the radical polymerization of acrylonitrile (AN) was determined through the product analysis for the reaction of structurally well-defined polyacrylonitrile (PAN) chain-end radical 1a and PAN-end mimetic small model radical 1b. The contributions of three competing 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, with CN-Comb being identified experimentally for the first time. The selectivity between Disp and CC-Comb showed a good correlation with the viscosity and temperature of the reaction system, 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 the viscosity but sensitive to a polarity of media; CN-Comb is suppressed in polar media. These results strongly suggest the presence of two pathways in the termination reaction, namely, the polarity- and viscosity-controlled pathways. The CN-Comb product 5 was stable during the polymerization but decomposed into Disp and CC-Comb products at high temperatures. Therefore, care must be taken in PAN processing, such as the melt molding, as changes in Disp and CC-Comb composition can affect the physical properties of PAN-based materials.