Photodehydrochlorination mechanism-formation of polyene structures

In poly(vinyl chloride) containing polyene structures photodehydrochlorination is initiated by intramolecular energy transfer from polyene excited singlet states to the allylic C—Cl bond (reaction 3.326) and/or eventually to the allylic C—H bond (reaction 3.327). Triplet states are less likely to be involved because of a very efficient intersystem crosslinking in polyenes [564]. 

Photolysis of a number of low molecular weight model compounds such as allyl chloride [1720], 2,3-dichlorobutane, 2,4-dichloropentane [1511], 4-chloro-2-butane and 2,4-dichloropentane [1269,1270] occurs by the cleavage of the C—Cl bonds. 

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]. 

UV irradiation causes the formation of polyene structures that are concentrated in a thin layer at the surface [559]. As the polyenes strongly absorb, this layer can act as protective skin which prevents subsequent photodegradation of the internal zones. 

The rate of propagation and consequent polyene sequence length are also enhanced by syndiotactic arrangements, since polymers with high syndiotactic content produce abnormally long polyene sequences [1484]. 

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