Thermal Degradation of Chlorocarbon and Fluorocarbon Polymers

The thermal instability of poly(vinyl chloride) is a cause for concern commercially and has, therefore, stimulated extensive investigations. The overall process of degradation is complex and still not completely resolved. Some of the questions that remain are  

Is the overall process of degradation ionic or free radical in nature  

In a more recent study, HCl was also found to be essential for the initiation of the unzipping process, but may or may not be essential to the depolymerization reaction itself, once the process has been started [485]. The above shown ionic mechanism was disputed in a study where ESR signals were recorded during thermal decomposition of the polymer at elevated temperatures [486]. This strongly supports a free-radical mechanism. A free-radical mechanism was proposed earlier by Bamford and Feuton [487]. This mechanism illustrates the formation of hydrochloric acid and is based on rupture of carbon-chlorine bonds  

The first step is formation of a dihydropyran ring. This is followed by a retro-Diels-Alder splitting and leads to regeneration of the a,p-unsaturated ketone and to formation of a double bond in the poly (vinyl chloride) molecule. The double bond initiates a subsequent elimination of hydrogen chloride from that molecule [495]. 

An intramolecular initiation process that explains constant rate of dehydrochlorination was also proposed [496]  

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