Spin trapping initiation mechanism

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

The addition of small amounts of radical scavenger (such as benzoquinone and diphenylpicrylhydrazyl) led to the appearance of induction periods in the kinetic curves. The duration of the induction periods are proportional to the concentration of the radical scavenger. The presence of atmospheric oxygen slightly slowed the polymerization. These observations indicate that the polymerization proceeds by a radical mechanism. The radicals are formed from the y-radiolysis of the monomers. By comparison to the ESR spectrum of the radicals formed by thermal initiation with azobisisobutyroni-trile in the presence of a spin trap, the radical formed is... 

Fig. 6.14 Experimental evidence for the formation of OH radicals generated in photo-initiated AOPs mechanism of the DMPO spin trapping technique.

Photochemical Sensitization. Photolysis of diaryliodonium salts in the presence of benzoin ethers results in efficient reaction of the iodonium salt [96,97]. Scheme 5 illustrates the mechanism of photolysis according to Ledwith [96] and Timpe [92], Accordingly, photocleavage of benzoin ethers yields easily oxidized ketyl radicals (and acyl radicals which can also initiate radical polymerization). That only ketyl radicals participate in photochemical sensitization of onium salt decomposition was confirmed by ESR spin trapping with benzylidene-tcrt-butylamine-AT-oxide [10b]. As the chemistry... 

Sulfamethoxazole failed to produce any trappable radicals with an array of different spin traps, but naproxen afforded the EPR spectrum shown in Figure 2.11 when irradiated with 330 nm UV-R in the instrument cavity in the presence of 2-methyl-2-nitroso-propane (MNP). The spectrum contains contributions from di-t-butyl nitroxide, a known photoproduct of MNP. The H-atom adduct MNP-H also evident can arise by several different mechanisms, including the trapping of an H atom by MNP the reaction of MNP with an electron followed by protonation and the direct reduction of MNP by an excited state species. In view of the flash photolysis results, it was concluded that photoionization was the major precursor of MNP-H. The third radical corresponded to a C-centered radical carrying a single H atom, leading to the postulate of a decarboxylation reaction as the primary photochemical step. Confirmation of the participation of free radical intermediates came from the initiation of the free radical polymerization of acrylamide with rates as shown in Table 2.1. 

Obviously, more work is required to further substantiate the presence of the proposed radical intermediates in the p-hydroxybenzoate hydroxylase reaction, possibly via EPR and spin-trapping studies. Studies by Detmer and Massey 247) on phenol hydroxylase have indicated that the reaction rate constants for the conversion of meta-substituted substrates plotted versus the Hammett parameters yield a straight line of slope equal to 0.5. This is consistent with the mechanism proposed by Anderson, as the negative slope is expected for an electrophilic aromatic substitution reaction, while the small magnitude of the slope may be indicative of a radical mechanism. Furthermore, recent work by Massey and co-workers on p-hydroxybenzoate hydroxylase utilizing 6-hydroxy-FAD as cofactor and p-aminobenzoate as substrate indicated that the absorption spectrum of intermediate 67 exhibited a satellite band at 440 nm 248). Anderson et al. suggest that the satellite band may result from the formation of an aromatic phenoxyl radical at the C-6 position of the isoalloxazine ring of the flavin 244). This species would result from a shift of the initial peroxyl radical center from C(4a) to C-6 via N(5) 245). 

Because of the low reactivity of pure NO in reactions of atom abstraction as well as addition to double bonds, direct interaction of this reactant with most synthetic polymers does not occur. However, the chemical modification of macromolecules in an atmosphere of NO in conditions of the simnltaneous generation of macroradicals takes place due to spin trapping via the formation of nitroso compounds. The structure of stable nitrogen-containing radicals formed from nitroso compounds in the subsequent reactions gives information on the mechanism of proceeding processes in polymers under the action of various radical-initiating factors. 

Nitric oxide is a low-activity free radical and can be used as a counter of radicals in gas and liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitroso compounds, which are spin traps. Thus, the initiation of free-radical reactions in solid polymers in the presence of nitric oxide provides further information on their mechanism. It is well established that at room temperature NO is not able to remove allylic and tertiary hydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinions on the capability of NO to react with low molecular weight (low molar mass) dienes and polyenes. Some authors believe that NO is able to add to dienes and polyenes, for example, to substituted o-quinonedimethane, phorone and P-carotene, with the formation of free radicals [27-29]. Another way of looking at these reactions lies in the fact that they can be initiated by NO2 impurities [25, 26]. 

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