Matthew Edward Gande

Stabilization mechanisms of hindered amines

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.

Hindered Amine Mechanisms: Part III—Investigations using isotopic labelling☆

Abstract The literature ‘peroxide’ mechanism and our proposed mechanism for the regeneration of hindered amine nitroxyls have been tested with isotopically labelled reactants. Both mechanisms deal with the reaction of alkylperoxy radicals with N-alkyloxy hindered amines, which are products of the reaction of nitroxyl radicals with alkyl radicals. The ‘peroxide’ mechanism predicts the regeneration of nitroxyl and formation of dialkyl peroxide. Our mechanism predicts the regeneration of nitroxyl and formation of ketones, alcohols and carboxylic acids, depending on the oxidizing substrate. The results of this work confirm the new proposed regeneration mechanism but not the previous ‘peroxide’ mechanism. The features of the confirmed mechanism are This basic mechanism is also proposed for acylperoxy radicals

Flash photolysis and time-resolved electron spin resonance studies of triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers as electron and hydrogen atom donors

Abstract Transient absorption spectroscopy and time-resolved electron spin resonance (TR-ESR) were employed to understand the electron and hydrogen atom transfer quenching of triplet benzophenone ( 3 BP) in acetonitrile and benzene by amine and aminoether derivatives of 2,2,6,6-tetramethylpiperidine (hindered amine light stabilizers, HALS). The amines ( I and II ) are efficient quenchers of 3 BP. The observed quenching rate constants were found to be in the range 6 × 10 8 –6 × 10 9 M −1 s −1 and depend on the structure of the amine and the solvent. On the other hand, the aminoethers ( III and IV ) do not exhibit any quenching at all. Reactions of 3 BP with amines I and II in benzene yield ketyl radicals (detected by transient absorption spectroscopy) and the corresponding aminyl or aminomethyl radicals (detected by TR-ESR). Neither experimental method provided any evidence for products of the reactions of 3 BP with aminoethers III and IV . The experimental results are discussed from the viewpoint of the mechanism of polymer stabilization by HALS.