Thermal degradation of para-substituted polystyrenes

The thermal degradation of a series of para-substituted polystyrenes with electron-donating (CH3, NH2) and electron-attracting (NO2,Cl,Br) substituents has been studied in isothermal conditions at 600°C using pyrolysis chromatography/mass spectrometry and in dynamic conditions using thermogravimetry. The pyrolysis of these substituted polystyrenes gives monomer as the main product for all polymers (60-80 wt.%). The other distribution of products supports the view that thermodestruction of these polymers starts from a random chain scission. The main process for all substituted polystyrenes is depolymerization, similar to unsubstituted polystyrene. In the case of para-substituted polystyrenes a good linear dependence was found between Tmax on the thermogravimetry curve and the Hammett constants of the substituents (Tmax =403.5-67.486 ?x). The results indicate that the Hammett relationship can describe quantitatively the trends in Tmax and thus thermostability of substituted polystyrenes and that thermostability of these polymers depends only on electronic nature of substituent and their possibility to stabilise macroradicals forming on chain scission.