Molecular weights of polymer residues from the thermal degradation in vacuum of two polymethyl methacrylate samples, one polymerized with benzoyl peroxid e and the other without an y initiator, have been obtained over wide extents of degradation at various constant temperatures.A linear relation, dependent upon the temperature but i ndependent of rate of volatilization and rate of change of l /M w , is found when log R is plotted against l /M w • (R = ratio of residue weight to original specimen weight, and .Mw= weight-average molecular weight.) The results are explained on the basis of a free-radical chain-reaction mechanism in which intermolecular chai n transfer (activation e nergy = 29 kilocalories) is the primary, or perhaps the sole, cause of the molecular-weight decrease.
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V.E. Hart (1956) studied this question.