Essential elements of thermolytic reaction kinetics for distributions of polymers are re®iewed. Bond scission, occurring randomly along a chain backbone or at the chain end, is the major reaction process in polymer thermolysis. Underlying this bond scission are free radical mechanisms, including initiation and termination, hydrogen abstraction, and beta scission. Population-balance equations for the molecular-weight distributions of macromolecules or their radicals describe the dynamics of the reactions. The go®erning integrodifferential equations for continuous distributions can be sol®ed by moment methods, similarity techniques, other analytical procedures, or numerical methods. The approach has been applied to analytical thermolysis and pyrolysis, polymer degradation for stability characterization or plastics recycling, and coal thermolysis to produce fuels and feedstocks. Experiments ha®e demonstrated the usefulness of the continuous distribution kinetics approach for polymer thermolysis with additi®es, including hydrogen donors, peroxides, and other polymers. Molecular-weight distributions and their moments can be measured by size exclusion chromatography. Experimental data can be interpreted by e®aluating rate parameters, which may be composites of rate coefficients for elementary steps in complex reaction mechanisms.
An experimental study showed the enhancement effect of di-tert-butyl peroxide on poly(Rmethylstyrene) (PAMS) thermal degradation in trichlorobenzene solution (12.5 g/L). Peroxide was continuously injected into the reacting polymer solution at low flow rates (0.3-4.0 mL/h). Samples were taken periodically and analyzed by GPC to determine the temporal behavior of the molecular weight distribution. Peroxide addition fundamentally altered the degradation mechanism to one in which random degradation was significant. A distribution kinetics model based on the fundamental free-radical processes of initiation-termination, β-scission, and hydrogen abstraction was developed to describe the irreversible polymer degradation process. Reaction rate coefficients that are linearly dependent on the polymer molecular weight are the model parameters fitted to experimental data. The Arrhenius activation energies for individual steps in the reaction mechanism were calculated on the basis of the temperature dependence of the reaction rate. The different mechanisms for peroxide-enhanced thermolysis of PAMS and polystyrene are clarified by the distribution kinetics approach.
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