Chain transfer, thermal degradation depropagation

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

For fractionated PMMA polymerised with AIBN as initiator and laurylmercaptan as transfer agent, the rate constants obtained indicated a change in thermal degradation mechanism at approximately 400 °C, where depropagation to the polymer chain end began to compete with the first-order termination process [a.l67]. This led to a kink in the Arrhenius plot of the rate constants, where below 400 °C the activation energy was approximately 130 kJ/mol and above 400 °C it was ca. 250 kj/mol. 

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