Clarifying the Mechanism of the Denisov Cycle: How do Hindered Amine Light Stabilizers Protect Polymer Coatings from Photo-oxidative Degradation?

Abstract

Hindered amine light stabilizers (HALS) protect polymer coatings against photo-oxidative damage through the formation of nitroxide radicals, which subsequently consume damaging radical species in a process called the Denisov cycle. However, the exact mechanism for this process has been disputed, with a dozen different reaction pathways and over 30 individual reactions previously proposed in the literature. In this work, the full mechanism of the Denisov cycle is elucidated using high-level computational techniques for two different polymer systems. New intermediate species in the cycle have been postulated, and the final products determined. The nitroxide TEMPO can react either with the polymeric radical •R to form an alkoxyamine species >N−OR, with any available alkoxyl radicals •OR to form the oxyaminoether species >N+(O−)−OR, or (more slowly) with the peroxyl radical •OOR to form the trioxide species >N−O−O−O−R. The alkoxyamine goes on to react with the peroxyl radical reforming the nitroxide along with ketone and alcohol products via a caged oxyaminoether and alkoxyl radical intermediate for polyethylene, and via either a caged oxyaminoether and alkoxyl radical intermediate or a caged aminoperoxylether and alkoxyl radical intermediate for polyester. The oxyaminoether undergoes an intramolecular hydrogen transfer reaction to form a hydroxylamine >N−OH and a ketone. The trioxide goes on to react with a secondary alcohol to form a hydroxylamine, a ketone and a hydroperoxide species via a concerted mechanism involving a six-membered ring transition state. The hydroxylamine species is then converted back to the corresponding nitroxide through hydrogen transfer to an alkyl, alkoxyl or peroxyl radical. A possible side reaction, reforming a HALS-type amine occurs through hydrogen donation to the aminyl radical product of the direct decomposition of the trioxide species into aminyl and hydroxyl radicals and dioxygen. This study will assist in the design of new and improved hindered amine light stabilizers for surface coatings.