DEGRADATION AND STABILIZATION OF POLY(VINYL CHLORIDE) .1. KINETICS OF THE THERMAL-DEGRADATION OF POLY(VINYL CHLORIDE)

The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution and rate of polyene sequence formation. Monoalkenes are present in commercial PVC and their concentration increases during thermal degradation. Sequences of 2 to 25-30 conjugated double bonds are formed in parallel reactions and their concentrations increase linearly versus time and extrapolate back through the origin. Their rate constants decay exponentially with the number of double bonds. These results can be accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence (Scheme 1). Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation.