Kinetic study of thermal dehydrochlorination of poly(vinyl chloride) in the presence of oxygen - III. Statistical thermodynamic interpretation of the oxygen catalytic activity

The mechanism of the hydrogen chloride elimination from poly(vinyl chloride) (PVC) catalyzed by molecular oxygen has been proposed and analyzed. The oxygen catalytic action is ascribed to the excited singlet O-1(2) ((1)Delta (g)) which is generated by an interaction of ground triplet O-3(2) ((3)Sigma (-)(g)) through the charge-transfer and electron exchange mechanisms with either biradical or ionic (zwitterionic) structure of the cyclic: four center polar transition state. An alone oxygen catalytic action is interpreted through Langmuir nondissociative quasi-equilibrium chemisorption process described in terms of the statistical thermodynamics. Assuming the catalyzed thermal dehydrochlorination (DHC) occurring independently from the noncatalyzed one, obtained kinetic parameters, e.g. the activation energy and frequency factor, describe satisfactorily Talamini and Pezzin experimental data. The most important result is that the average decrease of the activation energy in comparison with the noncatalyzed DHC process, determined on the two independent kinetic data sets, is 91 kJ mol(-1), this is just the effective transfer excitation energy to forming the singlet molecular oxygen. Moreover, the inherent role of the singlet oxygen in the oxidative degradation of PVC is suggested. (C) 2001 Elsevier Science Ltd. All rights reserved.