Simultaneous cross-linking and main-chain cleavage of linear polymers

If dioxygen is present, additional phenoxyl radicals are formed by reaction according to Eq. (7-7), i.e. by the reaction of triplet excited phenolic groups with O2. 

Therefore, the cross-linking quantum yield is significantly increased if the irradiation is performed in the presence of dioxygen. 

Simultaneous cross-linking and main-chain cleavage of linear polymers 

As has been pointed out in Section 7.1.2, polymers commonly undergo different kinds of bond ruptures simultaneously upon exposure to light, i.e. bond cleavage processes occur both in side chains and in the main chain of linear polymers. Bond rupture in side chains results in the formation of lateral macroradi- 

Interestingly, polyacrylonitrile, poly (methyl acrylate), and polystyrene behave differently in the rigid state and in dilute solution. This may be explained in terms of lateral macroradicals being generated upon the release of side groups in a primary step. The combination of these radicals competes with decomposition through main-chain rupture. In dilute solution, where radical encounters are much less probable than in the rigid state, main-chain rupture predomi- 

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Simultaneous cross-linking and main-chain cleavage of linear polymers 195 Table 7.3 Predominant effects upon UV irradiation of polymers in the absence of oxygen [27]. 

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