Thermal dehydrochlorination of poly(vinyl chloride), 2. Transiently and permanently acting structural defects

Abstract

Samples with an increased content of chloroallyl structures were prepared by chemical degradation of poly(vinyl chloride) (PVC) using potassium tert‐butoxide. The samples were thermally dehydrochlorinated in an inert atmosphere, and their dehydrochlorination curves were determined by the continuous potentiometric method. The results show that the chloroallyl structures affect the dehydrochlorination of PVC only transiently. The same conclusion is obtained by an analysis of experimental data of the dehydrochlorination of PVC samples enriched in structures with the chlorine atom bound to the branching site (structures with the chlorine atom at the tertiary carbon atom). A general conclusion is that all structural defects, which facilitate the elimination of hydrogen chloride along the polymer chain and exist in the polymer before its heat stress, play only a transient role in the thermal dehydrochlorination of PVC. These structural defects initiate the splitting‐off of hydrogen chloride during the induction period of dehydrochlorination and form conditions for a gradually increasing effect of the basic initiation mechanism, which in the course of dehydrochlorination gives rise to chloroallyl structures. The formation of chloroallyl structures is brought about by a reaction between regular structural PVC units and s‐cis‐enone, dienone, or polyenone structures. A possible mechanism of this reaction, consisting in an interchain syn‐elimination of hydrogen chloride, is suggested.

Mechanism of Autocatalysis in the Thermal Dehydrochlorination of Poly(vinyl chloride)

“…Such units are generally believed to be much more stable than the structural defects mentioned above. ,, Hence, in the absence of perturbing factors, loss of labile defects in the initial stage of degradation should lead to autodeceleration in the rate of evolution of HCl. This behavior is, indeed, observed under some conditions. , Yet, in other cases, when most or all of the original defects have reacted, dehydrochlorination does not slow appreciably and may even autoaccelerate, sometimes drastically. , The reason for these kinetic effects has been the subject of much debate, and they have been said by some researchers , to result from the presence of α,β-unsaturated ketone groups in the polymer. Supposedly formed by adventitious air oxidation, the cisoid forms of such structures were suggested to serve as true catalysts for the dehydrochlorination of ordinary monomer units, a process that would continually create thermally labile allylic sites and thereby prevent deceleration. , On the other hand, there is no conclusive evidence for the presence of enone groups in commercial PVC, , and their proposed catalytic proficiency could not be confirmed by other workers in model-compound studies. ,, …”