The rôle of chain ends in the thermal degradation of anionic polystyrene

Direct evidence is given of the initiating rôle played in the thermal degradation of anionic polystyrene by chain ends, either present originally or formed during the degradation. In the early stages of degradation, the most likely bond scission in polystyrenes with benzylic type units (CH₂(C₆H₅)) at both chain ends involves the formation of toluene and an unsaturated terminal unit (CH₂C(C₆H₅)CH₂). The depolymerisation of polystyrene to a mixture of monomer and dimeric, trimeric, etc., fragments is then initiated by further scission at such unsaturated chain ends, giving α-methyl styrene and a depolymerising macroradical.After the early stage of degradation, a further overwhelming contribution to the formation of unsaturated chain ends is derived from chain transfer which occurs during depolymerisation. The concentration of unsaturated chain ends increases throughout the degradation process, thus accelerating the formation of the volatile products of depolymerisation. According to this mechanism of initiation, a constant ratio is found between rates of weight loss and of α-methyl styrene evolution throughout the degradation, independently of the original molecular weight of the polymer.