INITIATION PROCESSES IN POLYMER DEGRADATION

Thermal degradation of vinyl polymers often occurs by a free radical propagation process and thermooxidative degradation almost always does. The nature of the initiation process, particularly in commercial polymers, is not well understood and is normally considered to be be due either to homolysis of the weakest C-C or C-H bond in the molecule or to the direct reaction of oxygen with the polymer to give free radicals. In practice neither of these is likely under conditions to which polymers are subjected during manufacture and use and the evidence suggests that mechanochemical scission of the polymer chain to macroalkyl radicals is the primary initiating process leading to the formation of hydroperoxides which are the cause of subsequent degradation. It is shown that alkyl radical traps effectively remove macroalkyl radicals during processing, thus delaying mechanooxidation. The nitroxyl radicals formed also contribute to subsequent environmental stabilisation of elastomers and thermoplastic polymers.