Dehydrochlorination in crystalline structures of commerical poly(vinyl chloride)

The polyene formation during degradation of a commercial suspension-polymerized poly(vinyl chloride) (PVC) in the solid state is discussed with respect to the PVC texture. Above the glass transition temperature (T_g), the polyene length distribution is characterized by a high amount of very short polyenes (conjugation length i < 6) due to the dehydrochlorination in the amorphous phase, and a second population of longer polyenes (i about 14) due to dehydrochlorination in micellar (B-type) crystals. According to the melting range of these crystals (120–180°C), the formation of longer polyenes reaches a maximum at 120°C. A third population of polyenes could be observed investigating the thermal and photolytical degradation of PVC below T_g. The absorption spectra of the degraded samples show a maximum centred at 550 nm (i = 21) due to the dehydrochlorination in the highly ordered lamellar (A-type) crystals. This result verifies the existence of highly syndiotactic sequences in commercial PVC formed during the first stage of polymerization, as demonstrated recently. The preferential dehydrochlorination in these crystals during the initial stage of degradation of PVC below T_g should be considered when discussing the long term stability of PVC products.