Electron spin resonance hindered amine stabilizers

Fully hindered amines are excellent UV stabilizers, but poor thermal antioxidants, for polyolefins. We have found that partially hindered bicyclic amines, such as 3,3-dialkyldecahydroquinoxalin-2-ones, are excellent UV stabilizers and also excellent thermal antioxidants as well. The oxidation of these bicyclic amines with m-chloroperbenzoic acid was studied by electron spin resonance spectroscopy. They form stable, partially hindered nitroxyl radicals (6-line spectra). However, these primary radicals are easily oxidized to a new, fully hindered nitroxyl radical (3-line spectra) which are also very stable. Field desorption mass spectroscopic studies of the oxidation of these amines show that the primary nitroxyl radicals, which form first, lose two hydrogen atoms and add an oxygen. Thus, these bicyclic partially hindered amines are unique in being both stable and at the same time hydrogen atom donors. 

Step EN, Turro J, Gande ME, Klemchuck PP. Flash photolysis and time-resolved electron spin resonance studies triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers and hydrogen atom donors. J Photochem Photobiol A Chem 1993 74 203-10. 

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