Autocatalysis by the nitroxyl radical in the cyclic mechanism of chain termination in polymer oxidation

Abstract

The activation energy and rate constant of the reaction between the nitroxyl radical and N-alkoxyamine as a concerted abstraction–fragmentation reaction have been calculated using the intersecting parabolas model. This reaction proceeds fairly rapidly and leads to nitroxyl radical autoregeneration as a result of the following consecutive reactions:AmO^∙ + AmOR → AmOH + >C=O + Am^∙, RO₂^∙+ AmOH → ROOH + AmO^∙, Am^∙+ O₂ → Am₂^∙, and 2AmO₂^∙ → 2AmO^∙ + O₂. Thus, the nitroxyl radical is an effective radical catalyst for its own regeneration from N-alkoxyamine. The rates of regeneration of the nitroxyl radical from its N-alkoxyamine under the action of alkyl, alkoxyl, peroxyl, nitroxyl, and hydroperoxyl radicals under conditions of polypropylene oxidation inhibited by the nitroxyl radical are compared. It is demonstrated that only peroxyl, hydroperoxyl, and nitroxyl radicals are involved in AmO^∙ regeneration from AmOR.