Transformation of sterically hindered amines (HALS) to nitroxyl radicals: What are the actual stabilizers?

Abstract

By means of pulse radiolysis and laser flash photolysis data, in this paper detailed information is given about transient elementary reactions which are involved in the transformation of sterically hindered amines (2,2,6,6-tetramethyl-substituted piperidines, HALS compounds) into nitroxyl radicals. Hence, ionization of the amines proceeds (i) by electron transfer to parent cations in butyl chloride solution, (ii) by oxidation with one-electron oxidants such as e.g. sulfate radical anion, and (iii) by sensitized electron transfer to carbonyl triplets. By direct time-resolved observation of all intermediates, the amine radical cations were found to react in a sequence via aminyl and aminylperoxy radicals, finally forming nitroxyl radicals. Nitroxyl radicals themselves are persistent, but efficiently scavenge alkyl, alkoxy and alkylperoxy radicals. The results of the liquid state model experiments suggest the conclusion that the HALS compounds primarily have to be transformed into nitroxyl radicals and the latter ones are the actual stabilizers.