Abstract
As 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: 1. 1. Hindered amines have multiple mechanisms of functioning as photo-stabilizers of polymers. 2. 2. Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization. 3. 3. Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures are very likely too slow and too inefficient for these to be important polymer photo-stabilizing reactions. 4. 4. Hydrocarbon polymer photo-oxidation 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. 5. 5. Hindered amine stabilizers are able to participate in inhibiting both photo-oxidation 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. 6. 6. Nitroxyls are regenerated from N-alkyloxy hindered amines in a fast, efficient reaction with acylperoxy radicals and in a slow, inefficient reaction with alkylperoxy radicals. We postulate that 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.
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