To optimize the experimental conditions for the recycling of plastic wastes, we modeled the thermal degradation process of a standard polystyrene, having a low polydispersity index. The number of initial ruptures, N-o in the macromolecular chain during the pyrolysis at 350 degrees C under nitrogen pressure is determined from the gel permeation chromatography results from the experiments with tetralin as the hydrogen donor solvent, assuming that all radicals have been stabilized by this solvent. The calculation shows that there are 23 depropagation reactions, 40 intramolecular transfers, and for n intermolecular transfers, n - 2 recombinations, and N-o - n + 2 dismutations.