Thermolysis of polyethylene hydroperoxides in the melt 6. Mechanisms and formal kinetics of product formation in the presence of phenolic antioxidants

The presence of phenols does not affect the nature of thermolysis products. However, the relative amount of the various oxidation products can change significantly. These changes are important from a fundamental point of view. They lead to better understanding of the thermolysis mechanisms. The reduced yield of alcohol groups in the presence of phenols suggests transformation of part of the alkoxy radicals into products other than alcohols. It is attributed to disproportionation between macro-alkoxy and phenoxy radicals yielding a ketone and the original phenol. There is considerable increase of trans-vinylene group formation in the presence of phenols. The explanation is based on another disproportionation reaction involving macro-alkyl and phenoxy radicals to give a double bond and the original phenol molecule. The vinyl groups that can be measured in the presence of phenols are formed on intramolecular hydroperoxide decomposition. The reaction seems to proceed with the tertiary hydroperoxides formed in low-density polyethylene rather than with the normal secondary hydroperoxides. The formation of gamma-lactones results from direct decomposition of alpha,gamma-keto-hydroperoxides in 4-position to alcohol groups initially present and also from decomposition of such structures formed on thermolysis. These data allow for an estimation of intramolecular hydrogen abstraction by peroxy radicals in polyethylene. It is found that the last can reach close to 30%. (C) 2002 Elsevier Science Ltd. All rights reserved.