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The compositions of the volatile products liberated from polymers in the course of mechanical destruction and thermal degradation have been studied and compared by means of mass spectrometry. For those polymers in which in both cases breakdown of the macromolecules begins with the basic chain, the volatile products are the same. For polymers in which breakdown of the molecules in thermal degradation begins with the stripping of side groups, the volatile products of thermal and mechanical degradation are different. This confirms and extends the ideas of the kinetic theory of fracture concerning the relationship between the processes of mechanical fracture and thermal degradation of polymers.

On the basis of a comparison of the activation energies of the processes of thermal degradation and mechanical fracture of PMM with various additives it is concluded that in the presence of a "pole shift effect" it is more correct to calculate the activation energy U2 of the mechanical fracture process on the assumption that τ0 = const and γ=f(σ, T). The energy U02 is then correlated with the weak-bond rupture energy. In the general case, the activation energy U01, calculated on the assumption that τ0=f(σ, T) ≠ const, i. e., from the slopes of the lg τ -1/T graphs, cannot be identified with the interatomic-bond rupture energy.