Photochemical reactions of the carbonyl group

The ketonic products can be expected to occur in trace amounts in pure polymers and are also components of the complex photo-oxidation products. Photolytic reactions of these ketones are important both in the very early stages of photo-oxidative degradation and in the continuation of an established photo-degradation reaction. The ketones may affect the source of the photodegradation by contributing to free radical processes, chain scission and energy transfer. 

The carbonyl group shows an absorption maximum in the range 2700—3300 A. The n- 7r transition involving the C=0 group probably 

Norrish reactions, type I and type II, are the photochemical reactions of the excited carbonyl group, and in the case of polymers these give rise to degradation of the macromolecule. Type I is the primary process in which the bond between the carbonyl carbon and an a carbon is homolytically cleaved it is also commonly called the a cleavage  

The type I photolysis of ketones, generates free radicals and also causes chain scission. Acyl radicals, reactions (63a) and (66a), are produced by UV irradiation from carbonyl groups situated at chain ends or at some points along the chain. The formation of acyl radicals was observed by ESR measurements [262]. Light of wavelength below 6500 A can convert 

Alkyl radicals (c) may either combine or disproportionate, and so cause no further immediate damage, or, during irradiation in air, may combine with dissolved oxygen in the polymer to give peroxy radicals R02 [88, 191,193]. 

---

The photochemical processes of excited benzophenone are listed in Table 1.8 to illustrate typical photochemical reactions of the carbonyl group. 

---