Oxidative coupling of 2,6-disubstituted phenols

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

Various 2,6-di8ubstituted p-benzoquinones have been prepared by oxidation of the corresponding 2,6-disubstituted phenols with potassium nitrosodisulfonate or lead dioxide in formic acid. Oxidative coupling of 2,6-disubstituted phenols to poly-2,6-disubstituted phenylene ethers followed by treatment of the polymers in acetic acid with lead dioxide is reported to give low yields of the corresponding 2,6-disubstituted p-benzoquinones. 

We were interested in the behaviour of polymeric catalysts in order to confirm that typical polymer effects may occur. Oxidative coupling of 2,6-disubstituted phenols, as developped by Hay (7), was chosen as a model reaction and the catalytic activities of coordination complexes of copper with several polymeric tertiary amines were compared with the activities of their low molecular weight analogs. The overall reaction scheme is presented in scheme 1. 

Table I Kinetic results on oxidative coupling of 2,6-disubstituted phenols at 25°C in the solvent mixture 1,2-dichlorobenzene/methanol (13 2 v/v).

Block copolymers may also be made by condensation polymerization. Elastomer fibers are produced in a three-step operation. A primary block of a polyether or polyester of a molecular weight of 1000-3000 is prepared, capped with an aromatic diisocyanate, and then expanded with a diamine or dihydroxy compound to a multiblock copolymer of a molecular weight of 20,000. The oxidative coupling of 2,6-disubstituted phenols to PPO is also a condensation polymerization. G. D. Cooper and coworkers report the manufacture of a block copolymer of 2,6-dimethyl-phenol with 2,6-diphenylphenol. In the first step, a homopolymer of diphenylphenol is preformed by copper-amine catalyst oxidation. In the second step, oxidation of dimethylphenol in the presence of the first polymer yields the block copolymer. 

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. 

It should be noted that the composition of the product of oxidative coupling of 2,6-disubstituted phenols depends on the type of substrate [131]. For instance, the... 

Poly(phenylene oxides) are produced by the oxidative coupling of 2,6-disubstituted phenols. The polymers are also known as poly(oxyphenylenes) or poly(phenyl ethers), and, in the case of dimethyl compounds, also as poly(xylenols). Copper (I) salts in the form of their complexes with amines catalyze the reaction. Primary and secondary aliphatic amines must be used at low temperatures, since otherwise they are oxidized. Primary aromatic amines are oxidized to azo compounds, and secondary aromatic compounds probably to hydrazo compounds. Pyridine is very suitable. 

The oxidative coupling of 2,6-disubstituted phenols occurs in the presence of an oxidizing agents like alkaline ferrocyanide [75,76], MnO [77], PbO [78], CrO [79], strong base [80], etc., but also takes... 

Oxidative coupling of 2,6-disubstituted phenols is catalyzed by copper-amine complexes [66]. Endres et al. [70] have shown that the ratio of the nitrogen-containing ligand and copper can be used to control C-0 vs. C-C coupling. 

The catalytic activity of copper complexes in oxidative coupling of 2,6-DMP to PPO is significantly improved when polymer-bound 4-aminopyridine is used as ligand [92]. Basic copper-amine complexes also catalyze the oxidative coupling of 2,6-disubstituted phenols [82]. Depending on the size of substituents and the conditions, polymerization or diphenoquinone formation may predominate [83-86]. Small substituents like methyl favor the PPO product. 

Poly(phenylene oxide) is produced by oxidative coupling of 2,6-disubstituted phenols (for example, Ar = 2,6-dimethylphenyl) ... 

Organic compounds having labile hydrogens, such as phenols [41,42], phenylene-diamines [43], and acetylenes [44], can be oxidatively coupled in the presence of specific metal complexes to form polymeric compounds. The oxidative polymerization of 2,6-disubstituted phenols with a copper-amine complex produces poly(2,6-disubstituted phenylene ether) [45-51], Poly(2,6-dimethylphenylene ether) and poly(2,6-diphenylphenylene ether) are commercially produced from 2,6-dimethyl phenol and 2,6-diphenylphenol, respectively (Figure 5). These polymers exhibit excellent performance as engineering plastics. 

Discuss and illustrate with chemical equations the formation of poly(phenylene oxide) by the oxidative coupling reaction of 2,6-disubstituted phenols. 

A large number of other 2,6-disubstituted phenols have been oxidatively coupled. A representative list of the results is presented below. 

The experiments discussed above deal with the copper-amine catalyzed oxidation of 2,6-xylenol, but oxidation of xylenol by metal oxides takes place by the same mechanism (22). The oxidative coupling of other 2,6-disubstituted phenols has the same characteristics as the oxida-... 

Preferred monomers for PPE are shown in Table 4.1 and in Figure 4.1. Alkylphenols are oxidized by air and must be stored under nitrogen to prevent oxidation reactions. The oxidative coupling reaction is a general reaction, suitable for 2,6-disubstituted phenols [10]. However, with bulky substituents only the C—C coupled product, the diphenoquinone, is formed. The oxidation potential of phenols is reduced by alkyl substitution and increased by electron-withdrawing substituents. 

Fig. 14. Dependence of the starting rate of oxygen absorption (/(g) on the degree of functionalization a in the reaction of oxidation coupling of disubstituted 2,6-phenols 1 - DMP 2 - DBTP 3 - DPP [131]...

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