Cyclic mechanisms of chain termination on nitroxyl radicals in oxidized polymers and hydrocarbons

A kinetic analysis of published schemes for cyclic chain termination on piperidineoxyl radicals in oxidized polymers is carried out. None of the earlier proposed schemes explains the experimentally observed fast regeneration of the nitroxyl radical from hydroxamic ester that ensures the efficient chain termination in the experiment. A new scheme is proposed for the cyclic mechanism of chain termination on nitroxyl radicals in oxidized hydrocarbons and polymers. The scheme is consistent with experimental data. The exothermic hydrogen atom abstraction reaction between the peroxyl radical and hydroxamic ester with the concerted fragmentation of the molecule occupies the central place in this cyclic mechanism. An algorithm is proposed for the calculation of enthalpies, activation energies, and rate constants of these reactions.