Cold mastication of natural rubber in presence of oxygen leads to scission by a shear mechanism with subsequent stabilization of the free radicals by oxygen, while at high temperatures oxidative scission becomes the dominant mechanism. Studies of the behavior of cis-polybutadiene reveal that similar reactions are effective, but occur at much reduced rates. Shear degradation is particularly difficult to achieve in typical cis-polybutadienes. This is related to the molecular weight distribution of these polymers which usually lack the very high molecular species most subject to shear-induced degradation, in agreement with the theory of F. Bueche. It is shown that when such a high molecular weight fraction is supplied, shear breakdown may be realized. All cis-polybutadienes undergo oxidative breakdown at high temperatures, the scission rate becoming appreciable above 140° C. A number of oxidation catalysts have been found which markedly increase the scission rate without leading to a correspondingly large increase in crosslinking. Significant improvements in processing may be realized in this manner.

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