Thermolysis of polyethylene hydroperoxides in the melt 5. Mechanisms and formal kinetics of product formation

Many products are formed on thermolysis of polyethylene hydroperoxides. This is especially so for the carbonyl compounds involving ketones, in-the-chain ketones and metyl-ketones, carboxylic acids, aldehydes, esters and gamma-lactones. Only gamma-lactones can be easily measured as such by IR spectroscopy because they absorb at a wavelength different from that of the bulk of the carbonyl groups. This is also true for the secondary alcohols and the trans-vinylene groups. Formation of all the products observed can be explained by relatively simple mechanisms. Confrontation of the experimental results with formal kinetics based on the assumed mechanisms leads to improved understanding of these mechanisms. It is concluded that hydrogen abstraction by alkoxy radicals is the only significant source for alcohols on thermolysis of polyethylene hydroperoxides. The abstraction of hydroxyl radicals from hydroperoxides, very often envisaged with alkyl radicals, does not contribute to alcohol formation. Trans-vinylene groups seem to result from disproportionation of geminate alkyl radicals. Intramolecular hydrogen abstraction following intramolecular hydroperoxide decomposition might also contribute to double bond formation. Formation of gamma-lactones is best explained by monomolecular decomposition of special structures formed on processing in air. The last result from reactions following intramolecular hydrogen abstraction by the alkoxy radical formed on pseudo-monomolecular hydroperoxide decomposition. The reactions proposed are in agreement with formal kinetics as well as with thermo chemistry. (C) 2002 Elsevier Science Ltd. All rights reserved.