Deuterium isotope effect in inhibited oxidation of butadiene–styrene rubber

Abstract

The importance of hydrogen abstraction by peroxy radicals as a mechanism of antioxidant action in rubber has been studied by investigation of a deuterium isotope effect with amine and phenolic inhibitors in the oxidation of butadiene‐styrene rubber. Deuterium was substituted for the active hydrogen of N‐phenyl‐2‐naphthylamine and 2,6‐di‐tert‐butyl‐4‐methylphenol by reacting the antioxidants with methylmagnesium iodide and then with deuterium oxide. The oxidation rates of butadiene–styrene polymer inhibited with 3 parts per hundred of these compounds were measured and compared with the oxidation rates using normal antioxidants. A kinetic isotope effect in the rate of oxidation was observed with both the phenolic inhibitor (kD/kH = 1.3 with not less than 77% substitution) and the amine inhibitor (kD/kH = 1.8 with not less than 60% substitution). The existence of an isotope effect leads to the conclusion that hydrogen donation to a peroxy radical is a controlling step in the mechanism by which these antioxidants function to retard the oxidation of rubber.

Inhibition of the Autoxidation of Organic Substances in the Liquid Phase.

“…Completely different isotopic results, ranging from normal to inverse effects, have been reported for the oxidation of butadiene-styrene rubber inhibited by 2,6-di-íerf-butyl-4-methylphenol (BMP) (286), Nphenyl-/8-naphthy lamine (PBN) (286, 287), and diphenylamine (DPA) (287). The following isotopic effects, measured by the rate of absorption of oxygen (*d/&h)> were observed at 90°C.…”

Hydrogen-Halogen Exchange between Silanes and Triphenylmethyl Halides

“…The rate of autoxidation of hydrocarbons in the absence of inhibitors is nearly independent of oxygen concentration,2 but the rate of retarded oxidation in the presence of antioxidants increases in a regular fashion as the partial pressure of oxygen is increased. 6 This change reflects a shift in the initiation process from exclusively peroxide initiation to include reactions involving direct attack of oxygen upon the antioxidant, and possibly upon the hydrocarbon. The formation of free radicals as a result of direct oxidation of the inhibitor is also indicated by the observation that antioxidants often become pro-oxidants at concentrations greater than the optimum.…”

Dihydric Phenols as Antioxidants in Isotactic Polypropylene

“…The direct oxidation, shown by Reaction 2, is regarded by some authors (40,41,42) as an important process which initiates new chains in the autoxidation. Within the observed range of concentrations, the reaction ratio causing the deactivation of antioxidant and/or initiation was not large enough with any of the three antioxidants to cause inversion of activity.…”