Quinone ketal redistribution

Quinone Ketal Redistribution. This mechanism suggests that in the coupling of two aryloxy radicals the oxygen atom of one attacks at the para position of the phenolic ring of the second to yield the unstable quinone ketal. This rapidly decomposes either to yield the aryloxy radicals from which it was formed or two different aryloxy radicals, as shown... 

The experiments cited above show that redistribution, presumably via a quinone ketal intermediate, occurs during the oxidative polymerization of 2,6-xylenol and must be responsible at least partially for the polycondensation characteristics of the reaction. Although the conditions under which Mijs and White demonstrated rearrangement are different from those usually employed for oxidative polymerization of xylenol, it appears certain that this process also contributes to the coupling of polymer molecules. Redistribution and rearrangement are complementary reactions. Dissociation into aryloxy radicals can occur at any point... 

Formation of a-hydroxy-o>hydroxyoligo(oxy-1,4-phenylene)s was observed in the HRP-catalyzed oxidative polymerization of 4,4 -oxybisphenol in aqueous methanol.30 During the reaction, the redistribution and/or rearrangement of the quinone—ketal intermediate take place, involving the elimination of hydroquinone to give oligo(oxy-l,4-phenylene)s. 

Figure 7.4 Schematic representation of the quinone-ketal rearrangement and redistribution [26],...

Fonnation of aryloxy radicals as intennediates was established with ESR spectroscopy studies that showed the presence of both monomeric and polymeric radicals in the reaction mixture. Coupling occurs by two paths one of them through rearrangements and the other through redistribution. In the redistribution process, two aryloxy radicals couple to yield an unstable quinone ketal as shown above. This ketal decomposes rapidly either back into the original aryloxy radicals or into two different aryloxy radicals as follows ... 

It was considered that the free phenoxy radical would lead to C-C coupling, and the C-O coupling would result from the phenoxy radical coordinated to the copper complex. A quinone-ketal intermediate, which could be formed by coupling of copper-mediated phenoxy radicals (the radical pathway b) or between phenoxonium cation and phenolate anion (the ionic pathway a), could explain both chain extension and redistribution mechanism. Therefore, the formation of quinone-ketal is proposed as key intermediate although it has never been detected in polymerization of 2,6-DMP. 

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