A mechanistic study of esters of thiodipropionic acid as antioxidants

Abstract

The mechanism of the thermal decompositon of the dialkyl sulphinyldipropionates alone and in the presence of 2,6-di-tert- butyl-1-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene) - p-tolyloxy (Galvinoxyl)has been investigated. Results show that radical intermediates capable of initiating oxidations are not produced under the conditions of study. This result is also substantiated by studying the effect of dimethy¥sulphinyldipropionate, on the thermal and-azoisobutyronitrile (AZBN)initiated polymer- isation of styrene which in fact shows that sulphoxide has free- radical chain interrupting activity. The effect of the sulphoxide on the polymerisation of styrene initiated by cumene hydroperoxide has, however, shown that free radicals are produced by a reaction between the hydroperoxide and the sulphoxide. This reaction is probably responsible for the pro-oxidant effect which can occur when using diaThyl thiodipropionates as antioxidants and certain possible reactions have been postulated. The means by which dialkyl.sulphinyldipropionates function as antioxidants has been studied by oxygen absorption techniques. Results show that these sulphoxides are capable of both interrupting the free radical chain mechanism of autoxidation and deactivating the hydroperoxide. The free-radical interrupting activity is consistent with the effect of the sulphoxide on the polymerisation of styrene and a common mechanism has been postulated to explain these effects. The deactivation is not due to complex formation between hydroperoxide and sulphoxide. In order to study the effect of the sulphoxide on the hydroperoxide the kinetics and products of the reaction of dimethyl sulphinyldipropionate with cumene hydroperoxide have been studied. Results indicate that a species capable of catalytically decomposing the hydroperoxide is produced. A series of reactions which may explain these results has been proposed but an unambiguous identification of the peroxide decomposing species has not been made.