Abstraction reaction, cross-linking

From the value of hond-dissociation energy, it is predictable which bond, in preference to others, should be more susceptible to chain scission. However in practice, depolymerization is a also combination of other reactions, such as hydrolysis, dehydration, hydrogen abstraction, and cross-linking. 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

Addition reaction of peroxide-generated macroalkyl radicals with the reactive unsaturation in MA is shown in reaction scheme 4. The functionalised maleic-polymer adduct (II, scheme 4) is the product of hydrogen abstraction reaction of the adduct radical (I, scheme 4) with another PP chain. Concomitantly, a new macroalkyl radical is regenerated which feeds back into the cycle. The frequency of this feedback determines the efficiency of the cyclical mechanism, hence the degree of binding. Cross-linking reaction of I occurs by route c ( scheme 4). 

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

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