Stabilization mechanisms of hindered amines

Abstract

AbstractAs a result of studying the interaction of hindered amine stabilizers (2, 2, 6, 6‐tetramethylpiperidines) with simple hydroperoxides, peroxy radicals, and acylperoxy radicals, the last two in AIBN‐initiated oxidation experiments in chlorobenzene, the following conclusions have been reached: Hindered amines have multiple mechanisms of functioning as photostabilizers of polymers. Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization. Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures occur at varying rates with varying effectiveness for stabilization. With favorable alignment among reaction rates for oxidation propagation and termination, reactions between tetramethylpiperidines and alkylperoxy radicals can play a significant role in oxidation inhibition. Hydrocarbon polymer photooxidation proceeds by two major paths ‐ the usually accepted alkyl radical/alkylperoxy radical/hydroperoxide route and the usually neglected aldehyde/acyl radical/acylperoxy radical/peracid route. Hindered amine stabilizers are able to participate in inhibiting both photooxidation reactions ‐ they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitroxyl radicals in the process; the nitroxyl radicals trap alkyl radicals and the hindered amines trap alkylperoxy radicals to inhibit the other oxidation pathway. Nitroxyls are regenerated from N‐alkyloxy hindered amines in a fast, efficient reactions with acylperoxy radicals and in slow reactions with alkylperoxy radicals. We postulate neither reaction yields peroxides: carboxylic acids and oxidized alkyloxy substituents are obtained from the first reaction; alcohols and oxidized alkyloxy substituents are obtained from the second reaction.