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... 

In equation 7, ttimer radical (4) is produced when (3) dissociates. Whenever (4) couples with the other product of equation 7, ie, the 2,6-dimethylphenoxy radical, the tetramer is produced as described. These redistribution reactions of oligomers that proceed by ketal formation and subsequent dissociation ultimately generate terminal quinol ethers which enolize to the more stable terminal phenol (eq. 8). 

This process, which predominates at low temperatures, causes migration of internal ketal stmctures along a chain but does not involve the dissociation to separate aryloxy radicals that occurs during the redistribution process. 

Continuation of this process, with monomer produced by redistribution and then removed by coupling, would lead to a random copolymer. Alternatively, if polymer-polymer coupling were to proceed solely by rearrangement, without dissociation at any stage, either of the ketals I or III would produce only block copolymer. 

The formation of random copolymer, even when the starting materials are preformed homopolymer blocks, as was observed with DMP and MPP, is reasonably explained by the monomer-polymer and polymer-polymer redistribution reactions of Reaction 3 and 9. Block copolymers are accounted for most easily by polymer-polymer coupling via the ketal arrangement mechanism (see Reaction 15, p. 256). 

There are, however, other possible routes to block copolymers successive addition of units of the reactive monomer to the polymer already present, Reaction 5 termination reactions between polymer molecules —side reactions of unknown nature lead to loss of reactive hydroxyl groups (18) possible reactions are ortho carbon-carbon coupling followed by dimerization, addition of amine or water to the ketal intermediate, etc. Block copolymers might even be formed by polymer-polymer redistribution assuming that such redistribution in polymers of greatly different reactivities (such as DMP and DPP), takes place almost exclusively in one type of polymer sequence—that is, that bond scission in a "mixed ketal such as IV occurs always in the same direction—to produce the aryloxy radical corresponding to the more reactive monomer. None of these possible sources of block copolymer can be ruled out on the basis of available evidence. All could produce homopolymer in addition to block copolymer. All of the polymers produced in this work, except for those characterized as completely random copolymers, probably contained at least small amount of one or both homopolymers. 

Just as redistribution can occur without polymerization, polymerization apparently can take place with little redistribution at sufficiently low temperatures, presumably by an intramolecular rearrangement of the ketal (19). In general, high temperatures and low amine concentrations favor redistribution. High temperature increases the extent of dissociation... 

An even closer parallel is found in the work of Kreilick (33, 34), who prepared the ketal derived from 2,6-di-tert-butyl-4-acetoxyphenol. From a study of the effect of temperature on line broadening in the NMR spectrum of this compound he concluded that rearrangement occurs both by dissociation to radicals and recombination and by a direct intramolecular process. The former corresponds to the key reaction of the redistribution mechanism, while the latter is entirely analogous to the ketal rearrangement mechanism (Reaction 15). 

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... 

White has obtained evidence for this process by examining the products of redistribution of monomer with high polymer. At low temperatures the products first formed did not consist only of dimer, as would be expected if redistribution occurred solely by Reaction 7 trimer, tetramer, and higher oligomers were initially present in more than their equilibrium ratio to dimer, indicating that several rearrangement reactions preceded the dissociation of the ketal. 

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. 

A mixture of acetone diallyl ketal and acetone dimethyl ketal acidified with p-toluenesulfonic acid, and after 15 min. at 24° made basic by addition of a soln. of Na-methoxide in methanol acetone allyl methyl ketal. Y 96% based on a redistribution ratio of 1 2 1. F. e. s. N. B. Lorette and W. L. Howard, J. Org. Ghem. 25, 521, 525 (1960). 

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