Radiolysis and radical-induced scission

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Hydroxyl radicals (HO ) represent one of the most reactive species and may be generated in vitro by the Fe ion-induced decomposition of H O [59], by the light-induced scission of HjO or by H2O radiolysis [60]. Water hydrolysis is unequivocally the most specific method of HO radical generation and the radical yield can be easily altered by the applied energy dose [61]. 

David and co-workers (32) confirmed the occurrence of direct main-chain scission and reported that racemization also occurred during radiolysis of syndio-tactic PMMA. In the presence of a radical scavenger, no racemization was found to take place, suggesting that radical processes are involved in the racemization process (33). These results therefore indicate that irradiation must induce some temporary main-chain scission, which is followed hy recombination of the scission radicals within the cage leading to a concomitant change in the stereochemistry of the polymer. The extent of the racemization observed for a y-ray dose of 8 MGy was reported to be approximately 24%. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

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