2, 6-Dimethylphenol, polymerization

In the former case the diphenoquinone is formed exclusively while in the latter case small amounts of low molecular weight polymer have been observed. As would be expected, substituents which raise the oxidation potential of the phenol retard the polymerization. Thus whereas 2.6-dimethylphenol polymerizes readily at room temperature, temperatures in the neighborhood of 60° C are required to polymerize 2-chloro-6-methylphenol at comparable rates and even higher temperatures are necessary to oxidize 2.6-dichlorophenol. 

Although redistribution and coupling can be observed separately, oxidative polymerization under ordinary conditions involves both reactions and redistribution of oligomers to form monomer followed by removal of the monomer by coupling is an important mechanism of polymer growth. Redistribution in dimethylphenol polymerizations is extremely rapid. Addition of monomer to a polymerizing solution causes an immediate drop in the solution viscosity almost to the level of the solvent, as redistribution of polymer with monomer converts the polymer already formed to a mixture of low oligomers. 

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. 

Poly(phenylene ether) Alloys. Poly(phenylene ether) resins (91), composed of phenoHc monomers, have a very high T. The commercial resins are based on 2,6-dimethylphenol. The resin is produced by oxidative polymerization in toluene solution over an amine catalyst (see also PoLYETPiERS, aromatic). 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

Isaacson and Sawhney (60) studied the reactions of a number of phenols and smectite with transition metal (Cu, FeJ+) and nontransition metal exchangeable cations. IR spectra of the clay-phenol complexes showed that all the clays studied transformed the sorbed phenols. The transformation occurred to a much greater extent in clays with transition metal cations than in those with the non-transition metal cations. In a subsequent study, Sawhney et al. (61) studied the polymerization of 2,6-dimethylphenol on air-dried homoionic Na-, Ca-, A1-, and Fe-smectite at 50°C. A portion of the adsorbed 2,6-dimethylphenol was transformed into dimers, trimers, tetramers, and quinone-type compounds. The nature of the exchange cations had an effect on both sorption and transformation and decreased in the order Fe Al > Ca > Na. 

It is well known that 2,6-dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-l,4-phenyleneoxide) with a copper amine complex as catalyst in the presence of oxygen at room temperature (Eq. 1)... 

Electro-oxidative polymerization of 2,6-disubstituted phenols is listed in Table I, with the polymerizations catalyzed by the copper-pyridine complex and oxidized by lead dioxide. 2,6-Dimethylphenol was electro-oxidatively polymerized to yield poly(2,6-dimethylphen-yleneoxide) with a molecular weight of 10000, as was attained by other polymerization methods. The NMR and IR spectra were in complete agreement with those measured for the other polymerization... 

The following questions on the electro-oxidative polymerization arose. First, why various phenol derivatives were smoothly polymerized which could not occur by the oxidation with the copper catalyst or lead dioxide. Secondly, why the activated phenol was reacted preferentially through C-0 coupling to form the poly(phenyleneoxide). The mechanism of the electro-oxidative polymerization is discussed below by using the example of 2,6-dimethylphenol. 

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

Table H. Electro-oxidative Polymerization of 2,6-Dimethylphenol and its Dimer...

A series of eight closely related N,0-based hgands was used to imravel possible steric and electronic influences in the copper-catalyzed polymerization of 2,6-dimethylphenol [248]. Applying a biphasic toluene/water emulsion, with an in situ prepared Cu(Melm)-catalyst (Melm = 1-methylimidazole),... 

The mechanism of the oxidative polymerization of 2,6-dimethylphenol (XOH) with an amine-Cu complex is represented by Eq. (20)145 147. ... 

Table 14. Oxidative polymerization of 2,6-dimethylphenol catalyzed by polymer-Cu complexes... 

The overall reaction rate and the rate constant of the electron-transfer step are summarized in Table 17 for the polymer-Cu-catalyzed oxidation of substrates such as 2,6-dimethylphenol (XOH) and ascorbic acid15 . The ks values for polymer-Cu-catalyzed oxidation are larger than those for monomeric-Cu-catalyzed oxidation. Particularly in the oxidative polymerization of XOH, it is obvious that the electron-transfer step is accelerated by polymer ligands, and the large value of ke is in agreement with the higher rate of polymer-Cu-catalyzed polymerization. Therefore, the... 

The oxidative polymerization reaction is rapid at room temperature. Oxidation of 2.6-dimethylphenol readily gives high polymer with only a minor amount of the diphenoquinone (VIII R=R1=CH3). This polymer is now being produced commercially. In general when the substituents are small (Table 4) the polymer is formed preferentially (35). If one of the substituents is as large as tert-butyl or both as large as isopropyl then the diphenoquinone is preferentially formed. 

Propylene oxide is one of the raw materials used to manufacture rubbery and crystalline polyepoxides. R. J. Herold and R. A. Livigni describe propylene oxide polymerization with hexacyanometalate salt complexes as catalyst. Polyphenylene oxide is made by copper catalyzed oxidative coupling of 2,6-dimethylphenol. G. D. Cooper, J. G. Bennett, and A. Factor discuss the preparation of copolymers of PPO by oxidative coupling of dimethylphenol with methylphenylphenol and with diphenylphenol. 

This reaction has been actively studied since it was first reported by Hay in 1959 (I), but most of the extensive literature, which includes several recent reviews (2-8), deals primarily with the complex polymerization mechanism. Few copolymers have been prepared by oxidative coupling of phenols, and only one copolymer system has been examined in any detail. Copolymers of 2,6-dimethylphenol (DMP) and 2,6-diphenylphenol (DPP) have been prepared and the effect of variations in polymerization procedure on the structure and properties of the copolymers examined (4, 9) this work has now been extended to copolymers of each of these monomers with a third phenol, 2-methyl-6-phenylphenol (MPP). This paper presents a study of the DMP-MPP and MPP-DPP copolymers and a comparison with the DMP-DPP system previously reported. 

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. 

Copolymers of 2,6-dimethylphenol with 2-methyl-6-ter -butylphenol, 2,6-diisopropylphenol, 2-methyl-6-phenylphenol, and 2,6-diphenylphenol have been reported (16) but only the 2,6-dimethylphenol (DMP)-2,6-diphenylphenol (DPP) pair, which is described in this report, has been examined in detail. This system is particularly attractive because high molecular weight homopolymers can be obtained under suitable conditions from both monomers, facilitating the analysis of the copolymers. Both random and block copolymers have been obtained by varying the polymerization conditions (1). 

These redistribution reactions of polymer molecules with other polymer molecules as well as with monomer, continue throughout the polymerization and should result in randomization of the polymer. Inasmuch as dimethylphenol is among the most reactive and diphenylphenol the least reactive of the phenols which have been oxidized successfully to linear high polymers, it appears likely that oxidation of any mixture of phenols will yield random copolymers. 

Oxidation and polymerization of catechol, pyrogallol and 2,6-dimethylphenol (FTIR spectra) SEM coupled with energy dispersive X-ray spectrometry investigation of reaction products on surface of clay minerals, 13C NMR, MALDI MS study of reaction products... 

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