Re-examination of the thermal oxidation reactions of polymers - 2. Thermal oxidation of polyethylene

Thermal oxidation of polyethylene (PE) shows many distinctive features with that of polypropylene (PP). This is largely a consequence of reduced intramolecular hydrogen abstraction by the peroxy radicals in polyethylene compared to polypropylene. Although the data from model compounds differ widely, some data from PE oxidation seem to indicate an upper limit of approximately 30% for intramolecular hydrogen abstraction in PE compared to 90% and more in PP. The reactions of the alkoxy radicals in PE and in PP show much more analogy than the corresponding reactions of the peroxy radicals. The dominant reaction of the alkoxy radicals is intramolecular hydrogen abstraction, in PE as much as in PP. The beta-scission reaction of alkoxy radicals seems even less important in PE than in PP. It cannot even explain the limited amount of chain scissions usually observed in PE. Thermolysis of hydroperoxides in PE shows specific features resulting from the structure of the hydroperoxide group. The intermolecular bimolecular decomposition is concerted and involves both hydroperoxide groups. It proceeds without chain scission and yields essentially ketones. It is expected not only in the polymer melt where it involves locally high hydroperoxide concentration but even more so in solid phase PE. Pseudo-monomolecular hydroperoxide decomposition is important in the polymer melt and can be seen as the main initiation reaction in the absence of shear. The alpha-alkyl-hydroperoxy radical formed on free radical abstraction of the tertiary hydrogen in alpha-position to the hydroperoxide group is very labile. It is decomposing spontaneously into a ketone and a hydroxyl radical. (C) 2002 Elsevier Science Ltd. All rights reserved.