Radicals, disproportionation rearrangements

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

Disproportionation mechanisms have been proposed for protonahon reachons and intramolecular rearrangements (see Sects. 4.3.2 and 4.3.3) [54]. The prominent feature is that follow-up processes at the level of the dianion can already take place at potentials corresponding to radical anion formation. In order to evaluate data for disproportionation reactions it is necessary to know the value of the disproporhonahon equilibrium constant. 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

The pathways followed by radical pairs, 42 (disproportionation or coupling with rearrangement) resulting from the type I cleavage of 2-phenylcycloalkanones (41) are influenced by cyclodextrin complexation [164], The product ratio depends both on the size of the cyclic ketone and on... 

In competition, the C(6)-yl and C(5)-yl radicals may disproportionate, possibly via an adduct [reactions (80) and (81)]. This yields the hydrate via an enol [reaction (83)]. The other product is the glycol [reaction (82)]. In the original paper (Al-Sheikhly and von Sonntag 1983), it has been proposed that it maybe formed in an ET reaction. Due the considerable rearrangement energies involved in ET reactions as compared to radical recombination reactions, it is now considered that this ET reaction might occur via an addition/elimination process [reactions (80) and (81)] such as has also been found for other systems. 

The initially formed radical R disproportionates (path a),dimerizes (path b), reacts with active cathodes (path c), rearranges (path d), adds to double bonds (path e) or is reduced to an anion (path f). Products of radical origin (a—e) occur mainly in the reduction of alkyl iodides, benzyl halides and in some cases of alkyl bromides. 

FT-IR results also showed that one new (small) absorption at 1659 cm"1 appeared, which could not be attributed to peroxide decomposition products. This absorption also appeared when the peroxide-curing experiments were carried out using an amorphous EPM, indicating that the absorption did not relate to rearrangement of the third monomer moiety (ENB in this case). It is tentatively concluded that the absorption at 1659 cm 1 is related to EPDM main-chain modifications, resulting from disproportionation reactions of EPDM macroradicals with BHT radical fragments. 

The oxidative coupling of 2,6-disubstituted phenols to poly-(arylene oxides) is a polycondensation reaction, in which polymer molecules couple with other polymer molecules as well as with monomer. Unstable quinone ketals formed by coupling of a polymeric aryloxy radical at the para position of the phenolic ring of a second radical are believed to be intermediates or the reaction. The ketals may be converted to polymeric phenols either by a series of intramolecular rearrangements or by disproportionation to aryloxy radicals, leading to a mobile equilibrium between polymer molecules of varying degree of polymerization. Both processes have been shown to occur, with their relative importance determined by the reaction conditions. 

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