Oxidative polymerization 2,6-dimethylphenol

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). 

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...

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. 

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. 

Dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-1,4-phenyl-ene ether) with a copper-amine complex by a laccaselike reaction. The activated phenols are coupled to form a dimer. The dimer is activated by a mechanism similar to that by which the polymerization proceeds. The effects of the amine ligands are to improve the solubility and the stability of the copper complex as well as the phenol-coordinated complex and to control the redox potential of the copper complex. 

The ability to polymerize readily via selective oxidation utilizing the abundant and cheap oxidant 02 often represents a desirable low-cost method for upgrading the value of a raw material. The most successful example is the oxidative polymerization of 2,6-dimethylphenol to yield poly(2,6-dimethyl-l,4-phenylene ether) with copper-amine catalysts under an 02 atmosphere at room temperature. Thiophenol also has a labile hydrogen but is rapidly oxidized to yield thermodynamically stable diphenyl disulfide. This formation is based on the more facilitated formation of S—S bond through radical coupling [82] in comparison with the formation of C—S—C bond through the coupling with the other molecules in the para position (Eq. 9). 

Ikeda R, Sugihara J, Uyama H et al (1996) Enzymatic oxidative polymerization of 2, 6-dimethylphenol. Macromolecules 29 8702-8705... 

Also related to their redox activity, a series of copper and manganese bis(dithiolene) complexes has been reported to catalyze the oxidative polymerization of 2,6-dimethylphenol to poly[oxy(2,6-dimethyl-1,4-phenylene)]... 

COPPER IMIDAZOLE COMPLEXES CATALYZE THE OXIDATIVE POLYMERIZATION OF 2,6-DIMETHYLPHENOL WITH DIOXYGEN... 

The oxidative dehydrogenation polymerization of 2,6-dialkylphenoles has been known for many years to be catalyzed by copper amine complexes [1] see figure for 2,6-dimethylphenol oxidation. The mechanism of action of this industrially very important reaction has been studied for some time by several groups. 

The oxidative polymerization of 2,6-dimethylphenol proceeds near room temperature. Moreover, the polymerization reaction does not create any leaving groups. From the aspect of green chemistry, water as a solvent for... 

Specifically, oxidative polymerization is initiated in the presence of no more than 10% of 2,6-dime-thylphenol. At least 95% of the 2,6-dimethylphenol is added to the reaction mixture after the initiation of oxidative polymerization and over the course of at least 50 min. 

C atalytic site of peroxidase is a heme, which is rapidly oxidized in its free form to hematin. p-Ethylphenol was polymerized using hematin as catalyst in an aqueous DMF (325). Iron-)VA -ethylenebis(salicyhdeneamine) (Fe-salen) also can be regarded as model complex of peroxidase. Fe-salen catalyzed an oxidative polymerization of various phenols such as 2,6-dimethylphenol, bisphenol A, cardanol, and urushiol analogues (251,293,326-329). The polymerization of 2,6-difluorophenol by Fe-salen produced a crystalline fluorinated PPO derivative (330). 

The oxidative polymerization of substituted phenols to poly(phenylene oxides is another example where a polymeric chain is formed by carbon-heteroatom coupling. " Thus, the reaction of 2,6-dimethylphenol with oxygen, in the presence of a copper-amine complex, yields high molecular weight poly(2,6-dimethyl-l,4-phenylene oxide). 

PPO is one of the most important engineering plastics first synthesized by Hay et al in 1959 by the oxidative polymerization of 2,6-dimethylphenol using copper(I) chloride/pyridine catalyst under oxygen (1 ). Poly(phenylene oxide) (PPO) is a thermoplastic, linear, noncrystalline polyether commercially produced by the oxidative polymerization of 2,6-dimethylphenol in the presence of a copper-amine catalyst. PPO has become one of the most important engineering plastics widely used for a broad range of applications due to its unique combination of mechanical properties, low moisture absorption, excellent electrical insulation property, dimension stability and inherent flame resistance. PPO finds applications in automotive instrument panels, internal decoration and exterior decoration parts. Typical applications include wheel... 

PPO is generally prepared by oxidative polymerization of 2,6-dimethylphenol in toluene solution in the presence of oxygen over an catalyst. Copper-amine catalysts are undoubtedly the most studied systems whereas other metal systems including manganese chloride (12), cobalt complex (13) have also shown to be effective in bench scale. Various amines including mono-dentate and biden-tate amines have been used as ligand for copper (I) complex which shows a wide range of activity as indicated in Table 2.4 (2,14-16). 

Poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) is most frequently synthesized by the oxidative polymerization of 2,6-dimethylphenol which should result in a polymer containing one phenyl and one phenolic chain end (PPO-OH) (see also Chapter 28 of Volume 5). However, in all cases, a disproportionation reaction occurs to form the side product 3,3, 5,5 -tetramethyl-4,4 -diphenylquinone, which reacts with PPO-OH according to equation (65) in Scheme 62 to yield a certain amount of PPO containing two phenol chain ends (PP0-20H). More recently, Heitz and co-workers and Nava and Percec have developed procedures to prepare PP0-20H according to equations (64) and (66) in Scheme 62. 

The oxidative polymerization of 2,6-dimethylphenol in the presence of a,o)-bis(hydroxyphenol) tetramethyl bisphenol-A polysulfone (PSUT) leads to an ABA triblock copolymer containing PPO segments as A blocks and PSUT as B blocks. The oxidative polymerization of 2,6-dimethyl-phenol takes place through a single electron transfer polymerization mechanism which differs from both step and chain polymerization reactions since the reactivity of the growing chains is molecular weight dependent. Heitz and co-workers labelled this type of polymerization process reactive intermediate polycondensation . ... 

The oxidative polymerization of 2,6-dimethylphenol (invented by Hay et al. [91], see also Chap. 8) and the polymerization of potassium 4-bromo-2,6-dim-ethylphenoxide (see Formula 16.6) have both the character of CCPs. The former CCP needs oxygen as reaction partner and a Cu amine complex as catalyst. Heitz et al. [92] observed that contrary to a normal polycondensation high oligomers were formed at low conversions, and he formulated a speculative radical-cation mechanism. The CCP of 4-bromophenoxide salts needs Cu " or other oxidizing metal ions as reaction partners and also involves a radical-cation mechanism [93]. When 4-methyl- or 4-tert.butylphenol are added as initiators, linear chains having one OH end group are formed, whereas addition of tetramethyl bisphenol yields telechelic polyethers (see Formula 15.6). 

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