Photolysis light-induced degradation

In contrast to polyolefins, the majority of engineering plastics do have absorptions at wavelengths present in terrestrial sunlight, so that for these polymers photolysis can also play an important role. For these polymers in principle there are three mechanisms that can describe their light induced degradation ... 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

The degradation can be photochemically induced (a) homolytic or (b) heterolytic cleavage at the weaker bonds. The photolysis of the type (a) may lead to elimination reactions and the type (b) may lead to free radical formation. The point of bond cleavage may not be the seat for light absorption. The energy can migrate from unit to unit until it finds itself at the seat of reaction. 

One way in which most polymers do decay is by the action of oxygen in the air and of light You will be familiar with the way that some polymers go yellow after a time and some become brittle. Coloured plastics, in particular, absorb light and oxygen-induced radical reactions follow. The polymer becomes too cross-linked and loses flexibility. One ingenious application of this natural process helps to degrade the polythene rings that hold cans of beer in packs. These are often discarded and decay quite quickly because some carbon monoxide has been incorporated into the polyethylene to make it more sensitive to photolysis. 

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