Photochlorination of Polymers

In terms of tons of chemicals per year, this is by far the most important industrial process in which a light-induced reaction plays a key role. The photochemical reaction itself is extremely simple, being the homolytic dissociation of molecular chlorine, Cl2, into Cl atoms, 

These Cl atoms are in fact free radicals and react very efficiently with hydrocarbons through substitution and insertion processes. 

Some of these processes lead to chain reactions in which a new Cl atom is formed in a (dark) free radical reaction. A single Cl atom formed in the photochemical reaction can therefore lead to overall quantum yields of several thousands. 

The purpose of photochlorination of polymers is to improve their stability with respect to fire hazards. Polymers made of C and H atoms only are quite dangerous in this respect, since they undergo combustion reactions in the presence of oxygen to form C02 and H20. Replacement of H by Cl reduces the flammability of such polymers. 

Why should the photochemical formation of chlorine be preferred to the similar, thermal (dark) dissociation There are two distinct reasons for this. 

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