Radical reaction kinetics during homolysis of N-alkoxyamines: verification of the persistent radical effect

Abstract

N-Alkoxyamines cleave into transient alkyl and persistent aminoxyl radicals which then combine and regenerate the parent compounds. Simultaneously, the alkyl species self-terminate, and this causes a continuous build-up of excess aminoxyl. Hence, the back-reaction to the alkoxyamine accelerates and the self-termination slows down in time. An analysis of this self-regulating Persistent Radical Effect shows that in the absence of scavengers the alkoxyamines decay and the aminoxyls appear according to unusual t^1/3 rate laws governed by a rate constant combination. This is verified by studying the aminoxyl release from 2-phenyl-2-(2′,2′,6′,6′-tetramethylpiperidine-1′-oxyl)propane (cumyl–TEMPO). The rate constants of the individual reactions are also determined under isolation conditions and explain the behaviour of the combined reaction system. Mechanistic details of the reactions are given, and their relevance for ‘living’ radical polymerizations is pointed out.