Gamma irradiation free radical mechanism

Both ionic and free radical mechanisms can be written to explain the foregoing reactions in more detail, but these would be strictly speculative, since no definitive experimental evidence has been obtained. Some sort of benzene ion must form as a result of inelastic collisions between the electrons and benzene molecules in the plasma, but whether these ions or some derivative species are the reactive intermediates is not known. Recently, Potter et al. (11) have shown that styrene, when perfectly dry, does polymerize via an ionic mechanism when irradiated with gamma rays. One can picture a similar mechanism occurring in the electrical discharge. 

Irradiation of air-free solutions of pyrimidines can give products resembling some of those produced by ultraviolet light, but probably by a quite different mechanism. Irradiation of 1,3-dimethyluracil with 200 kV X-rays produces not only the expected 5,6-glycol, but also 6-hydroxyl-l,3-dimethyl-5-hydrouracil, the same isomer as the UV-produced hydrate discussed elsewhere in this chapter. The yield of the hydroxy compound is low, G = 0.35 molecules/100 eV absorbed. Cytosine glycol is formed in gamma-irradiated cytosine solutions, and this deaminates completely to the uracil glycol. These products are probably formed by addition of an OH to the 6-position of the pyrimidine and then dismutation of hydroxy radical. The major products formed in the radiolysis of air-free solutions of pyrimidines or purines have not yet been identified. 

Since radicals arc neither formed nor destroyed during reversible chain transfer, RAFT polymerization must, like conventional radical polymerization, be initiated by a source of free radicals as shown in Scheme 9.38. RAFT polymerization is usually carried out with conventional radical initiators. Most often thermal initiators (e.g. AIBN, ACP, BPO, K2SnOs) are used. S polymerization may be initiated thermally between 100-130°C. Polymerizations initiated with UV irradiation, a gamma source " or a plasma tleld have been reported. In these polymerizations, radicals generated directly from the RAFT agent may be responsible for initiation. It was initially suggested by Pan and coworkers that the mechanism for molecular weight control in and y-... 

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