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

The mechanism of the hydrogen chloride elimination from poly(vinyl chloride ) (PVC) catalyzed by molecular oxygen has been proposed and analyzed. The o xygen 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)Sigm a (-)(g)) through the charge-transfer and electron exchange mechanisms with either biradical or ionic (zwitterionic) structure of the cyclic: four cen ter polar transition state. An alone oxygen catalytic action is interpreted through Langmuir nondissociative quasi-equilibrium chemisorption process d escribed in terms of the statistical thermodynamics. Assuming the catalyzed thermal dehydrochlorination (DHC) occurring independently from the noncata lyzed one, obtained kinetic parameters, e.g. the activation energy and freq uency 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 t wo independent kinetic data sets, is 91 kJ mol(-1), this is just the effect ive transfer excitation energy to forming the singlet molecular oxygen. Mor eover, the inherent role of the singlet oxygen in the oxidative degradation of PVC is suggested. (C) 2001 Elsevier Science Ltd. All rights reserved.