Structure and Reactivity in Oxidation of Elastomers

Oxidation of hydrocarbons is a much studied and much reviewed subject, yet much remains to be understood about complexities of competing reactions that take place as hydrocarbons react with oxygen. The generally accepted free radical mechanism for low temperature reaction (below 200° C) stems from kinetic studies by Semenov, Hinshelwood, Farmer, Gee, Bolland, and Bateman of hydrocarbon and olefin oxidation in the gas and liquid phase: Such a chain mechanism appears to be straightforward, but studies of autoxidations are complicated by sensitivity to trace metals, which decompose hydroperoxides, and to the nature of reaction products, which may function as inhibitors or accelerators. The character of the reaction may vary considerably with time as these reaction products accumulate. Hydroperoxides may decompose by unimolecular or bimolecular pathways to produce alkoxy, peroxy, or hydroxyl radicals. These are known to have various selectivities and hence will produce different propagating species with differing rates of termination. Alternately hydroperoxides may react with sulfur, nitrogen, aldehydic, basic, or acidic functions producing no radicals at all. Such reactions vary drastically from one system to another. The study of autoxidations thus becomes the challenging problem of trying to separate one reaction from another. One method of avoiding such difficult problems is to work under simplifying conditions. One approach is to study oxidations with an external initiator such as azobisisobutyronitrile or benzoyl peroxide under conditions such that the hydroperoxides formed during oxidation are relatively stable products. This has been most successfully applied to kinetic studies on liquid hydrocarbons containing benzylic or allylic hydrogens. This review is concerned with some recent work on initiated oxidations of hydrocarbon polymers in the solid phase and attempts to show how the results may be related to the more complicated autoxidation process.