Quinone-ketal rearrangement

Similar results were obtained by Cooper and by Mijs et ah (24) from a number of other substituted dimers. With lightly substituted dimers, however, Mijs observed that at low temperatures the initial products consist almost entirely of tetramers the tetramer, moreover, is that corresponding to the quinone ketal rearrangement, rather than to head-to-tail coupling (Reaction 20). 

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

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. 

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 7.3 Schematic representation of the rearrangement of a quinone-ketal moiety that gives rise to polymer extension [16].

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 ... 

The rearrangement process also involves quinone ketals formed by the coupling of two aryloxy radicals. It is suggested that the carbonyl oxygen of a ketal is within bonding distance of the p-position of the next succeeding ring ... 

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