Thermolysis of polyethylene hydroperoxides in the melt: 2. Formal kinetics of hydroperoxide decomposition

Abstract

Thermolysis of polyethylene hydroperoxides in the melt results from different reactions. The first-order rate of hydroperoxide decomposition in the initial stages of thermolysis increases linearly with the initial hydroperoxide concentration. This seems to indicate that in addition to primary hydroperoxide decomposition there is hydroperoxide decomposition induced by the free radicals arising from the primary reaction. The mathematical models deduced from formal kinetics have been compared to these experimental results. It has been found that conventional kinetics based on chain recombination between randomly distributed free radicals could not explain linear increase of the first-order rate with initial hydroperoxide concentration. Conventional kinetics could explain merely linear increase with the square root of this hydroperoxide concentration. The conclusion was the same in the presence and in the absence of phenolic antioxidants. The best explanation of the experimental data involves geminate chain recombination, i.e. termination between two chains originating both from the same initiation reaction yielding a pair of free radicals. In the absence of phenolic antioxidants, geminate chain termination results from the interaction of two alkyl radicals. In the presence of phenols, this geminate chain termination seems to involve essentially an alkyl radical and a phenoxy radical. Besides polymer solutions, polymer melts certainly are the most homogeneous form possible for polymers. Since geminate chain recombination seems to be dominant in polymer melts, it can be assumed that it will be at least as important with polymer states for which diffusion is even more restricted, e.g. polymers in the solid phase.