The inhibited autoxidation of styrene. Part VII. Inhibition by nitroxides and hydroxylamines

Abstract

The inhibition of the oxidation of styrene by the stable aliphatic nitroxide radical, 2,2,6,6-tetramethyl-4-piperidone nitroxide (I), and two stable aromatic nitroxide radicals, 4,4′-dimethoxydiphenyl nitroxide (II) and 4,4′-dinitrodiphenyl nitroxide (III), has been studied. The aromatic nitroxides react with both alkyl (R^·) and peroxy (RO₂^·) radicals, but the aliphatic nitroxide only reacts with alkyl radicals. The hydroxylamines derived from I and II react only with peroxy radicals. They are quite effective oxidation inhibitors, exhibiting a deuterium kinetic isotope effect k_H/k_D ~ 2−3. The hydroxylamine from III inhibits the oxidation of styrene for a short time and then becomes an extremely powerful oxidation catalyst. It is suggested that a product from this hydroxylamine undergoes a rapid chain-branching reaction with styrene.