Re-examination of the role of hydroperoxides in polyethylene and polypropylene: chemical and physical aspects of hydroperoxides in polyethylene

Hydroperoxides are usually considered the key intermediates in oxidation of polyolefins initiated thermally or photochemically. In this work, the hydroperoxides are generated thermally either on thermooxidation of polyethylene films at temperatures between 80 and 100 °C or on processing polyethylene in a mixer open to air at 150 °C. They are examined essentially by IR spectroscopy. This allows us to distinguish between free hydroperoxide groups with an absorption maximum close to 3550 cm−1 and associated hydroperoxides with an absorption maximum around 3410 cm−1. The free hydroperoxides observed on thermal oxidation of polyethylene films have been attributed previously to isolated hydroperoxides formed in the α-position to vinylidene groups. This is not confirmed in the present study. The free hydroperoxides as well as the associated hydroperoxides are a more or less random distribution of hydroperoxides resulting from the peroxidation of the different sites available in the polymer. Furthermore, the free hydroperoxides are not exclusively isolated hydroperoxides. Associated hydroperoxides contribute to the absorption of the free hydroperoxides at 3550 cm as a consequence of a thermodynamic equilibrium between free and associated hydroperoxide groups close enough spatially to be able to associate. The distribution of the hydroperoxide groups in solid PE depends on the mode of generation of these groups. Thermal oxidation of PE films yields intramolecularly associated hydroperoxide groups in addition to hydroperoxides spread at random according to a Poisson-type distribution. Thermal oxidation in the melt yields a much more uniform distribution of hydroperoxides in polyethylene.