Hindered radiolysis

The multistep reduction mechanism of multivalent cations is known partially from pulse radiolysis studies [19]. For example, the reduction of Au ClJ into Au Cl and the disproportionation of Au into Au and Au have been directly observed and the rate constants determined [52]. However, the last step of reduction of Au complexed by Cl into Au° is not observed by pulse radiolysis because the Caq scavenging by the precursors Au is more efficient. Moreover, the disproportionation of Au or of other monovalent cations is thermodynamically hindered by the quite negative value of °(MVM°)( °(MV M )< °(M"/M )) (Section 20.4.4). 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

Phenols are stronger AO/AR than HAS. Contrary to the transformation of phenols into quinone methides, discoloration-free products are formed from HAS. Combinations HAS/phenol are, therefore, used to obtain protection at minimum discoloration. The efficient protection of PP or of a block copolymer of propylene with 1.5% ethylene irradiated with a dose of 5 Mrad was reached with 0.1% 28, R = H and 0.05% Irganox 1076. Moreover, it was found that hindered phenolic AO prevent radiolysis of the ester type HAS (e.g. 28) [226]. Combinations of HAS with phenolic AO may, therefore, be recommended for commercial stabilization of y-irradiated hydrocarbon polymers [227]. 

Accordingly, a concept was developed, that involves one of the organic compounds, e.g., toluene as a fullerene-dissolving medium (12,13). The systematic fullerene reduction was then obtained via addition of adequate co-solvents, namely, acetone and 2-propanol. Acetone was chosen as an efficient electron scavenger to hinder a reaction between solvated electrons and toluene. Followed by a fast protonation a radical species with a reducing character is formed. In addition, the (CH3)2 COH species is identical with the main product of the radiolysis of the second co-solvent, 2-propanol. 

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