Aging and degradation of polyolefins. II. γ‐initiated oxidations of atactic polypropylene

Abstract

Abstract Oxidations of bulk atactic polypropylene (PP) have been carried out at 22 and 45°C, and the dependence of rate of formation of each product on rate of initiation has been determined. The principal product is PP hydroperoxide, formed in a half‐order reaction. One termination product is polymeric dialkyl peroxide, formed in a first‐order reaction. Other termination and propagation products, alcohols and carbonyl compounds, are formed in reactions that are mostly first‐order in initiation. At 22°C, G is 9–63. G is about three times as great at 45°C as at 22°C. Experiments with 2,6‐di‐ tert ‐butyl‐ p ‐cresol shows that it can inhibit all non‐cage propagation and all formation of PP hydroperoxide, but that it does not affect cage reactions of initiating radicals and their successors. Only about 16% of the initiating PPO 2 · radicals escape the cage at 45°C. Oxidations of PP, n ‐hexane, and their mixture with both peroxide and γ‐ray initiation show that nearly all the initiating radicals escape the cage in solution but that the concentration of PPO 2 · radicals is much less than in bulk because of a much faster chain termination. Both the propagation and termination constants for PP oxidation are much faster in solution, but the changes compensate so that k p /(2 k t ) ½ is about the same in solution as in bulk.