Zip dehydrochlorination

Since the UV degraded C-PVC still contains substantial amounts of the initial CHC1-CHC1 structure, one can expect the chlorine radicals evolved to also initiate the zip-dehydrochlorination of these structures. The resulting chlorinated polyenes will then be further destroyed by the laser irradiation, so that finally all the C-PVC polymer is converted into a purely carbon material within a fraction of a second. 

At a temperature above 80 °C, poly (vinyl chloride) eliminates hydrogen chloride and allylic chlorinated structures appear, with 4-chloro-2-hexene being considered as a model. At the processing temperature (180-200°C), the main problem of poly (vinyl chloride) stabilization is preventing the zip dehydrochlorination that induces discoloration and cross-linking of the polymer. 

Zip dehydrochlorination that proceeds efficiently, regardless of the presence or absence of oxygen, and yields polyene structures and HCl (section 3.9.4). 

The zip dehydrochlorination basically proceeds in a cage reaction, which explains why long polyenes are mostly found in poly(vinyl chloride) photo-lyzed in the solid state. In solution, where the chlorine radical (Cl ) can more easily diffuse out of the cage, growth of polyene sequences is less favoured and discoloration is much less pronounced [169]. 

In the photolysis of chlorinated poly(vinyl chloride), the radical evolved by C—Cl photocleavage still possesses chlorine atoms in the P and P positions (—CHCl—CH—CHCl—) it will thus be able to initiate a second zip-dehydrochlorination, as well as the former initiated by the chlorine radical, with formation of twice the amount of HCl as in poly(vinyl chloride). This is illustrated by the overall reaction scheme shown in Fig. 3.42. 

Samples which contain chlorinated polyenes (3.73) can be further completely dehydrochlorinated by laser radiation (488 nm from an argon ion laser) by the zip-dehydrochlorination mechanism induced by chlorine (Cl radicals) [552, 554-557, 1965] ... 

Thus the zip-dehydrochlorination which normally follows allylic activation does not occur. 

---