Mechanism of Polymer Stabilization by Hindered-Amine Light Stabilizers (HALS). Model Investigations of the Interaction of Peroxy Radicals with HALS Amines and Amino Ethers

Peroxy radicals, when generated in the presence of hindered-amine light stabilizers (HALS), oxidized HALS to produce nitroxides. This reaction, in addition to the scavenging of alkyl radicals by nitroxides, forms a cycle describing HALS as a class of excellent polymer stabilizers. As model systems for the mechanism of the oxidation of HALS, both alkyl peroxy and acyl peroxy radicals were produced by photolyzing the photoinitiators, ketones 2,4-diphenylpentan-3-one and 1-benzoyl-1-hydroxycyclohexane, in oxygen-saturated hexane in the presence of HALS amine I and amino ethers 11-V. Significant yields of the nitroxide VI were observed in these reactions. Product analysis shows that the chemical yields of nitroxide VI are dependent on the HALS structure and on the electronic properties of peroxy radicals, being substantially higher for acylperoxy radicals than for alkyl peroxy radicals. ESR analysis shows that the oxidation of HALS amine I to VI was much less temperature dependent than the oxidation of the HALS amino ether 111. A mechanism for the efficient production of VI from HALS with peroxy radicals is proposed.