Alkyl macroradicals, recombination

Terminal alkyl macroradicals recombine with NO to produce terminal nitroso groups neighbouring to the terminal double bonds. Similar decomposition and formation of terminal nitroso groups also takes place at room temperature. This supposition is supported by the fact that ARs are synthesised only after long-term exposure of irradiated PTFE to NO. The adjacent terminal double bonds and formed terminal nitroso compounds can enter in a reaction to synthesise ARs ... 

It was noted above that the thermally induced processes occurring during the implantation are coupled with radiation-induced ones that involve the excited polymer units, cation radicals, and trapped electrons. In the case of polyethylene (which is the simplest one of polyolefins), the relaxation of the excited polymer units results in formation of alkyl macroradicals and H radicals the latter can recombine or interact with the polymer yielding molecular hydrogen [21,22] ... 

Only one of the amine groups of this stabiliser is oxidised to ARs during photolysis. The part of monoradicals formed recombines with alkyl macroradicals of polypropylene, and then the second amine group is converted into ARs. 

In the solid phase, the recombination of radicals in the cage is highly efficient, whereas the escape of NO and RO radicals from the cage is quite low. Alkoxy radicals RO generated in reactions (Equation 3.77) decompose or enter into substitution reactions with neighbouring macromolecules to form chain R and end R alkyl macroradicals as well as low-molecular alkyl radicals r. The appearance of the latter radicals in the reaction of PP hydroperoxide with NO was verified in [66] ... 

Under anaerobic conditions, the alkyl macroradicals can recombine by combination and dismutation [Eq. (17)]. 

Recombination (dimerization) reaction of macroradicals may occur parallel to disproportionation in which alkyl radicals decay via transfer of a hydrogen atom from one P-carbon of one of the two radicals to the carbon of the second radical caiying a radical site. 

Another situation is observed during UV photolysis of samples (Figure 4.3b). The induction period in the kinetics of gel formation assumes a peculiar mechanism of AR generation in these conditions. NO3 dissociation via reaction (Equation 4.30) is strongly depressed, and the main path of photo-transformation of nitrate radicals is their decomposition into NO and atomic oxygen. Thus, primary closely located macroradicals R formed in reactions (Equation 4.89) and (Equation 4.90) can recombine with NO. Similarly to nitroso compounds, nitro compormds can add alkyl radicals with the formation of a stable spin adduct. Such an eventuality was shown by ESR in an example of aromatic nitro compounds [60]. For PVP, the mechanism resulting in radical R is ... 

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