Thermal degradation of vinyl polymers I—Thermal degradation of polystyrene-poly(α-methylstyrene) mixtures

Abstract

Thermal degradation of polystyrene (PS) has been induced at temperatures at which it is normally stable (260° to 287°C) by using poly(α-methylstyrene) (PMS) as a radical producing agent. PMS degrades at these temperatures in a kinetically simple manner. The decompositions were followed by measuring the monomers evolved from the mixtures by gas-liquid chromatography. Experiments where the PMS molecular weight was varied over a wide range established that, whereas the degradation of PMS was unaffected by the presence of PS, the PS degradation varied as an inverse function of PMS molecular weight. These observations were taken to mean that the system was heterogeneous, consisting of micelles of PMS embedded in a PS matrix. The PMS could then only initiate PS by completely unzipping to produce monomer radicals which could diffuse into the PS matrix. The heterogeneity of the system was confirmed kinetically. It was found that the rate constant of primary scission of PMS was given by the expression 5 × 10 18 exp ( −65 000 RT ) and that the degradation of the polymer was unaffected by the type of catalyst used in its synthesis. The depropagation of PS was found to be described by the equation k d k 1 2 t = 65 exp ( −10 500 RT ) where k d and k t are the rate constants of depropagation and termination respectively. This equation was shown to hold from 5 000 up to 100 000 molecular weight of PS.