Oxidative polymerization of phenols and

The oxidative polymerization of phenols and anilines by enzymatic and chemical methods is an important method for synthesizing polyphenols50 and polyanilines51 in material research. Such polymerizations are often carried out in aqueous conditions. 

The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

In living cells, various oxidoreductases play an important role in maintaining the metabolism of living systems. Most of oxidoreductases contain low-valent metals as the catalytic center. In vitro enzymatic oxidoreductions have afforded functional organic materials.1 Recently, some oxidoreductases such as peroxidase, laccase, and bilirubin oxidase have received much attention as catalyst for the oxidative polymerizations of phenol and aniline derivatives to produce novel polyaromatics. 

Phenolic polymers and phenol-formaldehyde resins are of great commercial interest for a number of electronic and industrial applications (7). However, there have been serious concerns regarding their use due to various toxic effects of formaldehyde and harsh synthesis environments (2). Peroxidase-catalyzed oxidative polymerization of phenol and substituted phenols provides an alternate route for the synthesis of phenolic polymers (3,4), The increased interest in this type of enzyme-based polymerization is mostly due to its environmental compatibility and potential for producing industrial polymers in high yield (5). 

The first chapter by Reihmann and Ritter reviews the recent developments of peroxidase-catalyzed oxidative polymerization of phenol and derivatives with a phenolic OH group. The importance of enzymatic polymerization in general is emphasized. Properties of product polyphenols, characteristics of the enzyme catalysis, and significance of the process and the product are discussed. The second chapter by Uyama and Kobayashi is concerned with the oxidative polymerization of polyphenols, which are compounds containing more than two phenolic OH groups. These compounds include catechols and flavonoids... 

R often Me) formed by oxidative polymerization of phenols using oxygen with copper and an amine (pyridine) as catalysts. The products are thermoplastics used in engineering applications and in electrical equipment. 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

Oxidation peak potentials of phenol derivatives were measured with cyclic voltammetry 0.53, 0.47, 0.47, 0.28, and 0.77 V vs. Ag/ AgCl for phenol, 2,6-dimethyl-, 2,6-diphenyl-, 2,6-dimethoxy-, and 2,6-dichlorophenol respectively. The oxidation potential of phenol and 2,6-dichlorophenol are relatively high and this high potential is one of the reasons why phenol and dichlorophenol could not he polymerized by the oxidation with copper catalyst or lead dioxide. On the other hand, for the electro-oxidative polymerization the potential can he kept slightly higher than the oxidation potential of phenols and the polymerization proceeds. 

The second example is the electro-oxidative polymerization of phenols bearing functional substituents. It is known that salicylic acid forms a stable chelate with copper ion, thus the copper catalyst is deactivated and the polymerization does not occur. On the other hand, salicylic acid was electro-oxidatively polymerized to produce the poly(phenyleneoxide) bearing carboxylic group. 

In the Cu-complex-catalyzed oxidative polymerization of phenols, the substrate (phenol) coordinates to the Cu(II) complex, is then activated, and the... 

The catalytic activity of the metal complex on the oxidative reaction in solution is much influenced not only by the species and the structure of the complexes but also by the chemical environment around them. For instance, in the oxidative polymerization of phenols catalyzed by a Cu complex, the reaction rate varied about 102 times with changes in the composition of the solvent, and the highest rate was observed for polymerization in a benzene solvent162. Thus, we used the copolymer of styrene and 4-vinylpyridine(PSP) as the polymer ligand and studied the effect on the catalysis of the non-polar field formed by the polymer ligand163. ... 

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. 

In the oxidative polymerization of phenols catalyzed by Cu complexes, the substrate coordinates to the Cu(II) complex and is then activated. The activated phenol couples in the next step. The Cu complex acts effectively as a catalyst at concentrations of 0.2-2 mol% compared to the substrate. The oxidation proceeds rapidly at room temperature under an air atmosphere to give poly(phenylene ether) in a quantitative yield. The polymerization follows Michaelis-Menten-type kinetics [55]. Enzymatic oxidation of phenols is an important pathway in the biosynthesis of lignin in plants [56] catalyzed by a metalloenzyme. 

Activation of the dimer in the oxidative polymerization of phenols takes place after the consumption of most of the monomer by the coupling, because the formation constant of the complex between the monomer and the copper... 

Recently, Chen et al. [70-73] and Baesjou et al. [74] reported that the oxidative polymerization of phenol proceeds through a one-step two-electron transfer. The catalysis follows the mechanism shown in Figure 6. 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

The polymerization of compounds having active methyne groups has also been reported [81] (Eq. 8). The oxidative coupling polymerization of these monomers follows a mechanism similar to that of phenols. The catalytic cycle observed in the polymerization of / -phcnylcncdiaminc with Fe(edta) as the catalyst in an aqueous solution differs from that in the polymerization of phenols as follows The activation of monomers usually involves either electron transfer from the anion or elimination of a hydrogen atom from the monomer. The oxidative polymerization of phenols uses the former mechanism of the electron transfer. In contrast, in the case of the polymerization of aromatic diamines as monomers, the neutral amines are coordinated to the catalyst, followed by the subsequent electron transfer and dehydronation. The dehydronation proceeds by the reaction with 02. Another mechanism has also been proposed where dehydrogenation... 

Many amine-copper complexes, as well as a few amine complexes of other metals, and certain metal oxides have since been shown to induce similar reactions (17, 18, 22, 23, 30). This chapter is concerned largely with the mechanism of oxidative polymerization of phenols to linear polyarylene ethers most of the work reported has dealt with the copper-amine catalyzed oxidation of 2,6-xylenol, which is the basis for the commercial production of the polymer marketed under the trade name PPO, but the principal features of the reaction are common to the oxidative polymerization of other 2,6-disubstituted phenols. 

Catalysts are also required in many stepwise polymerizations. For example, reaction of polycarboxylic acids and polyols (Reaction 7) is catalyzed by acids ester interchange, by metal compounds such as titanium alkoxides. On the other hand, polyurea synthesis (Reaction 6) generally does not require a catalyst. Metallic compounds are also useful in oxidative polymerization of phenols to give poly(phenylene oxides), illustrated in Reaction 14. 

A bienzymatic system was developed as catalyst for the oxidative polymerization of phenols.18 The HRP-catalyzed polymerization of phenol in the presence of glucose oxidase and glucose gave the polymer in a moderate yield, in which hydrogen peroxide was formed in situ by the oxidative reaction of glucose catalyzed by glucose oxidase. 

Oxidative polymerization of phenols has been carried out extensively by using horseradish peroxidase and other enzymes. However, these interesting topics lie far beyond the scope of this chapter. 

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