Oxidative polymerization of phenols revisited

Prog. Polym. Sci. 28 (2003) 1015–1048 www.elsevier.com/locate/ppolysci

Oxidative polymerization of phenols revisited Shiro Kobayashia,*, Hideyuki Higashimurab,* a

Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan b Tsukuba Laboratory, Sumitomo Chemical Co. Ltd, Tsukuba 300-3294, Japan Received 25 December 2002; revised 22 January 2003; accepted 23 January 2003

Abstract This paper presents an overview on recent developments of oxidative polymerizations of phenols, particularly focusing on the coupling selectivity. In 1959, Hay et al. discovered an oxidative polymerization of 2,6-dimethylphenol catalyzed by a copper/amine complex to produce poly(2,6-dimethyl-1,4-phenylene oxide). The reaction mechanism of the selectivity for C– O coupling, however, has not been made thoroughly clear yet. First, the following three reaction mechanisms so far proposed are discussed; (i) coupling of the free phenoxy radicals, (ii) coupling of the phenoxy radicals coordinated to catalyst complexes, and (iii) coupling through the phenoxonium cation. Next, an oxidoreductase enzyme such as peroxidase or oxidase and a peroxidase model complex that have been recently found as the catalyst for oxidative polymerization of phenols are described. The enzyme catalyst allowed chemoselective polymerization of a phenolic monomer having a reactive functional group like methacryloyl group and also induced polymerization of a new phenol monomer such as syringic acid that could not be polymerized by the conventional metal catalysts. Finally, a tyrosinase model complex behaved as a novel regioselective oxidative polymerization catalyst. The catalyst controls the coupling of phenoxy radicals from 2- and/or 6-unsubstituted phenols, which was named as a ‘radical-controlled’ catalyst. By using this catalyst, crystalline poly(1,4-phenylene oxide) was produced from 4-phenoxyphenol for the first time via oxidative polymerization, and a new crystalline polymer with a melting point above 300 8C was synthesized from 2,5-dimethylphenol. Thus, new catalysts and some new polymers have been developed; oxidative polymerization of phenols has now revisited as a clean and low-loading process for synthesis of phenolic polymers. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Oxidative polymerization; Phenols; Poly(phenylene oxide); Coupling selectivity; Enzyme; Peroxidase; Laccase; Tyrosinase model

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1016 2. Reaction mechanism of coupling selectivity in oxidative polymerization of 2,6-dimethylphenol . . . . . .1018 2.1. Coupling of phenoxy radicals coordinated to catalyst complex . . . . . . . . . . . . . . . . . . . . . . . . . .1018 2.2. Coupling of phenoxonium cation with 2,6-dimethylphenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1019 2.3. Coupling of free phenoxy radicals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1021 2.4. Chain extension mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1024 3. Oxidative polymerization of phenols catalyzed by peroxidase, oxidase and peroxidase model complexes1025 3.1. Peroxidase-catalyzed oxidative polymerization of phenol and hydrocarbon-substituted phenols . .1025 * Corresponding authors. E-mail addresses: [email protected] (S. Kobayashi), [email protected] (H. Higashimura). 0079-6700/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0079-6700(03)00014-5

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3.2. Peroxidase-catalyzed oxidative polymerization of phenols having functional groups . . . . . . . . . .1028 3.3. Oxidase-catalyzed oxidative polymerization of phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1030 3.4. Oxidative polymerization of phenols by a peroxidase model complex catalyst . . . . . . . . . . . . . .1031 4. Oxidative polymerization of phenols catalyzed by monooxygenase-model complexes (‘radicalcontrolled’ oxidative polymerization) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1032 4.1. Oxidative polymerization of 4-phenoxyphenol by tyrosinase model complex catalyst . . . . . . . . .1032 4.1.1. Tyrosinase model complex for catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1032 4.1.2. Dimer formation of 4-phenoxyphenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1033 4.1.3. Polymerization of 4-phenoxyphenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1034 4.1.4. Reaction mechanism of catalytic cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1035 4.1.5. Reaction mechanism of oxidative coupling and chain extension . . . . . . . . . . . . . . . . . . .1037 4.1.6. Reaction conditions and other catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1039 4.2. ‘Radical-controlled’ oxidative polymerization of mononucleus phenol derivatives. . . . . . . . . . . .1039 4.2.1. Substituent effect of phenol monomers in ‘radical-controlled’ oxidative polymerization. .1039 4.2.2. ‘Radical-controlled’ oxidative polymerization of phenol, 2-methylphenol, 3-methylphenol, and 2,6-dimethylphenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1041 4.2.3. ‘Radical-controlled’ oxidative polymerization of 2,5-dialkylphenols . . . . . . . . . . . . . . . .1042 4.2.4. ‘Radical-controlled’ oxidative polymerization of other phenol monomers . . . . . . . . . . . .1044 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1044 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1044 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1044

1. Introduction Many studies on oxidation of phenols were reported until 1950s, where only dimeric or oligomeric products were obtained. For example, the oxidation of 2,6-dimethylphenol (2,6-Me2P) by use of benzoyl peroxide [1] or alkaline ferricyanide [2] as oxidant mainly gave 3,30 ,5,50 -tetramethyldiphenoquinone (DPQ) (Scheme 1). In 1959, Hay and co-workers discovered an oxidative polymerization of 2,6-Me2P catalyzed by CuCl/pyridine (Py) under dioxygen leading to poly(2,6-dimethyl-1,4phenylene oxide) (P-2,6-Me2P) [3]. This is the first

Scheme 1.

example for oxidative polymerization to synthesize a phenolic polymer with high molecular weight. After the discovery, various catalysts such as copper/substituted-ethylenediamine complexes were developed and P-2,6-Me2P was found completely miscible with polystyrene (PSt) [4]. Now, P-2,6Me2P/PSt alloy is widely used as an engineering plastics with sales approaching one billion dollar per year. P-2,6-Me2P is a C – O coupling product and DPQ is a product via C – C coupling. How the C – O coupling can be achieved is the most important question in the oxidative polymerization of 2,6Me2P. The reaction mechanism on the coupling selectivity has been studied by many researchers so far, and three possible reaction mechanisms for the C– O coupling selectivity have been proposed as follows (Scheme 2); (i) coupling of free phenoxy radicals resulting from one-electron-oxidation of 2,6-Me2P, (ii) coupling of phenoxy radicals coordinated to catalyst complex, and (iii) coupling through phenoxonium cation formed by two-electron-oxidation of 2,6-Me2P. Although it was recently reported that the reaction mechanism (iii) would be most probable [5], the reaction mechanism for C– O coupling has not been made clarified yet.

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Scheme 2.

Oxidative coupling of phenols is involved in some biological reactions, for example, formation of lignin or melanin is catalyzed by oxidoreductase enzymes such as peroxidase, oxidase, or oxygenase [6,7]. In these two decades, the enzymes and their model complexes have been developed as new oxidative polymerization catalysts and resulted in a remarkable advance to produce new phenolic polymers. Since horse radish peroxidase (HRP) was employed as the catalyst [8,9] in the middle of 1980s, the oxidative polymerization of various phenols by using HRP and soybean peroxidase (SBP) with hydrogen peroxide or laccase (an oxidase) with dioxygen has been extensively investigated in these several years [10 – 14]. These enzymes show high reactivity for generating free radicals from phenols, but are unable to control the coupling selectivity [15]. However, chemoselective polymerization of a phenolic monomer having methacryloyl group was achieved to give the corresponding polyphenol (Scheme 3(a)) [16]. Regioselectivity of coupling was found in the polymerization of some specific phenols; for glucose-b-D -hydroquinone, only C – C coupling at ortho positions occurred (Scheme 3(b)) [17]. Oxidative polymerization of syringic acid with liberating carbon dioxide,

which is not possible for the metal complex catalysts, produced poly(2,6-dimethoxy-1,4-phenylene oxide) (Scheme 3(c)) [18]. Furthermore, a peroxidase model complex possessing almost the same catalytic reactivity as HRP was discussed [19]. A recent work revealed a tyrosinase (one of monooxygenases) model complex as a novel highly regioselective catalyst for oxidative polymerization of 2- and/or 6-unsubstituted phenols leading to poly(1,4phenylene oxide)s [20]. For ortho-unsubstituted phenols, it was extremely difficult to regulate the coupling selectivity by conventional catalysts and even enzyme catalysts, which generate free phenoxy radicals. The tyrosinase model complex controls the nature of phenoxy radicals without formation of free phenoxy radicals, it was therefore, named as ‘radical-controlled’ catalyst. By using this catalyst, crystalline poly(1,4-phenylene oxide) (PPO) was synthesized from 4-phenoxyphenol (PPL) for the first time as catalytic oxidative polymerization (Scheme 4(a)) [21, 22]. Moreover, the oxidative polymerization of 2,5dimethylphenol (2,5-Me 2 P) produced poly(2,5dimethyl-1,4-phenylene oxide) (P-2,5-Me2P), a new crystalline polymer with a melting point beyond 300 8C (Scheme 4(b)) [23].

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Scheme 3.

A typical synthetic method for phenolic polymers other than oxidative polymerization represents nucleofilic substitution of halogenated aromatic compounds by metal phenolates [24]. However, the method needs high reaction temperature and removal process of byproduced salts. Compared with the methods, catalytic oxidative polymerization of phenols has significant advantages such as halogen-free monomers, moderate reaction temperatures, and water as by-product. The catalytic oxidative polymerization, therefore, is to be regarded as a clean and low-loading process for synthesis of phenolic polymers. In this review, the control of coupling selectivity in oxidative polymerization of phenols is mainly focused. First, the reaction mechanism of the coupling selectivity and chain extension in oxidative polymerization of 2,6-Me2P is discussed. Next, oxidative polymerization of phenols by use of enzymes, and finally, the polymerization catalyzed by enzyme-model complexes are described as a new methodology for control of chemoselectivity and regioselectivity.

2. Reaction mechanism of coupling selectivity in oxidative polymerization of 2,6-dimethylphenol 2.1. Coupling of phenoxy radicals coordinated to catalyst complex In 1959, Hay et al. reported that the oxidative polymerization of 2,6-Me2P catalyzed by CuCl and a large excess of Py under dioxygen at room

Scheme 4.

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temperature for about 1 h produced P-2,6-Me2P with the molecular weight of 28,000 in 84% yield (Scheme 1) [3]. Since the oxidative coupling of 2,6-Me2P with benzoyl peroxide or alkaline ferricyanide afforded DPQ as the main product [1,2], it was generally considered that the free phenoxy radical would lead to C – C coupling, assuming that C –O coupling would result from the phenoxy radical coordinated to the copper complex (Scheme 2(ii)). As the factors influencing the C – O/C – C coupling selectivity in the CuCl/Py catalysis, increasing the amount of Py to copper [25] favored C – O coupling, and the substituents at the 2,6-positions of pyridines [25] or the high reaction temperature [26] made C –C coupling favorable. Hay et al. [27] postulated that a dinuclear copper complex formed for a small amount of Py would afford DPQ, and the addition of excess Py would generate a mononuclear copper complex leading to P-2,6-Me2P. On the contrary, Price and Nakaoka [28] speculated the increase of Py would promote dissociation of coordinated radical to free radical, followed by C –O coupling, and remaining coordinated radical would prefer to undergo C – C coupling. In these two proposals, however, there seems to be no adequate evidence. Tsuchida et al. [29] found that, from the kinetic analysis, the oxidative polymerization proceeded via Michaelis– Menten-type reaction mechanism, and as the ratio of Py to copper increased (C –O coupling favored), reaction rate constant (k2) was almost constant but Michaelis constant ðKm Þ became smaller (Eq. (1)). These kinetic results suggested that C –O coupling would be preferred while at least one of the radicals is coordinated to copper.

ð1Þ

The coordinated phenoxy radical can be expressed as the following resonance equation: phenoxo-copper(II)

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complex $ phenoxy radical – copper(I) complex. If the coupling from these species is the ratedetermining-step in the oxidative polymerization, at least either of the two mononuclear species must be possible to be observed. Tsuruya et al. [30] observed free radical species from oxidation of 2,4,6-tri-tbutylphenol (2,4,6-t Bu3P) with a CuCl2/Py/KOH complex by means of ESR, but for that of 2,6-Me2P, its radical species was not detected. Talsi et al. [31] performed the ESR measurement of oxidative polymerization of 2,6-Me2P by use of a copper(II) acetate/Py complex, which showed higher resolution for ESR than a copper halide complex. In the case of Py=Cu ¼ 20; two phenoxo-copper(II) complexes were observed at the beginning of the reaction under anaerobic conditions, and the following oxidative polymerization under dioxygen gave mainly P-2,6Me 2P. For Py=Cu ¼ 2; no phenoxo-copper(II) complex was detected and the major product was DPQ. These data suggested that the coupling via the coordinated phenoxy radical may mainly lead to C – O coupling. 2.2. Coupling of phenoxonium cation with 2,6-dimethylphenol The reaction mechanism of coupling through the phenoxonium cation of 2,6-Me2P (Scheme 2(iii)) was proposed by Kresta et al. [32] and by Challa et al. [5]. Huysmans and Waters [33] observed the primary free phenoxy radicals of 2,6-Me2P and the secondary free phenoxy radical of polymer in oxidation of 2,6-Me2P with Ag2O by the ESR flow technique. Kresta et al. [32] reported that no ESR signals of such free phenoxy radicals in the oxidation with a [CuCl(OCH3)Py] complex were detected by the similar technique. From these data, it was assumed that the phenoxy radical might be oxidized by the Cu(II) complex (totally twoelectron-oxidation from the phenol) to give the phenoxonium cation, which would couple with the phenol leading to C –O coupling. Reedijk et al. [34] performed the kinetic and spectroscopic studies on the oxidative polymerization of 2,6-Me2P catalyzed by a copper(II) nitrate/ N-methylimidazole (NMI)/NaOCH3 in toluene/ methanol. The order of the reaction in copper changed from 1.22 at NMI=Cu ¼ 10 to

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1.70 at NMI=Cu ¼ 75; and the reaction rate and the selectivity for C – O coupling reached their maximum at the NMI/Cu ration of at least 30. The Cu/ NMI complex was obtained as the crystal, possessing mononuclear [Cu(NMI)4(NO3)2] structure from X-ray diffraction analysis. This complex did not react with 2,6-Me2P in the absence of water, but reacted in the presence of water even under nitrogen, and therefore, some water is required for the oxidation. From these data, the reaction mechanism was proposed (Fig. 1) as follows; the key intermediate could be (m-hydroxo)(m-phenoxo)dicopper(II) complex, in which two-electron-transfer from phenoxo moiety to two copper atoms would give phenoxonium cation. It seems to be obscure for this reaction mechanism that excess NMI should depress the formation of this dicopper complex, but as NMI increased, the reaction rate and C – O selectivity became higher. Challa et al. [35] proposed that a similar (m-phenoxo)-dicopper(II) complex would afford phenoxo cation for a copper(II)/N,N,N0 ,N 0 -tetramethylethylenediamine (tmed) complex catalyst. The ab initio calculation on the regioselectivity of oxidative coupling of 2,6-Me2P was performed by

Reedijk et al. [5], and the atomic charges for the phenol, phenolate anion, phenoxy radical, phenoxonium cations (singlet and triplet) of 2,6-Me2P were determined (Fig. 2). These were evaluated as metalfree species, based on the assumption that the coordination to the metal would not drastically influence the atomic charges. Since the oxygen atoms in all the species were negative, it was considered that the species susceptible to C –O coupling would have positive charge on the para carbon, and therefore, it would be the phenoxonium cation (singlet), reacting with the phenol or phenolate anion. Reedijk et al. [5] excluded the coupling of the phenoxy radical with each other leading to C – O coupling, from the reason that the treatment of 2,6-Me2P with benzoyl peroxide yielded DPQ [1]. The reaction between the phenoxy radical and phenol has been generally ruled out, because the oxidative polymerization of 2,6-Me2P in the presence of 2,6-dimethylanisole afforded no products derived from 2,6-dimethylanisole [36]. If the oxidative coupling of 2,6-Me2P proceeds via the phenoxo cation, in the presence of a nucleophilic reagent in excess to phenol or phenolate anion, the phenoxo cation will react with the nucleophile, and hence, P-2,6-Me2P will not be obtained. It has been

Fig. 1. Proposed reaction mechanism for oxidative polymerization of 2,6-Me2P by Cu/NMI/NaOMe catalyst. Symbols L and ArOH stand for NMI and 2,6-Me2P, respectively. Reproduced from J Mol Catal A: Chem 1996;110:195 [34] by permission of Elsevier Science.

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Fig. 2. Atomic charges for (a) neutral phenol, (b) phenolate anion, (c) phenoxy radical, (d) phenoxonium cation (singlet), and (e) phonoxonium cation (triplet) of 2,6-Me2P calculated by ab initio method.

recently found that the oxidative polymerization of 2,6-Me2P catalyzed by [Cu(tmed)(OH)Cl]2 complex in the presence of five-fold molar of n-pentylamine (n PA) toward 2,6-Me2P produced P-2,6-Me2P in 75% yields [37] (Fig. 3(a)). In order to compare the nucleophilicity of n PA with that of 2,6-Me2P, their reaction with benzyl chloride was performed under the similar reaction conditions, giving N-benzyl-npentylamine in a quantitative yield and none of benzyl 2,6-dimethylphenyl ether (Fig. 3(b)). The fact that P2,6-Me2P was mainly produced in the presence of n PA in excess to 2,6-Me2P strongly indicates that such electrophilic intermediates as a phenoxonium cation are not involved in the oxidative polymerization of 2,6-Me2P [37]. 2.3. Coupling of free phenoxy radicals The above two reaction mechanisms are based on the assumption that the coupling via free phenoxy radicals of 2,6-Me2P must lead to C – C coupling

mainly. For supporting this assumption, the experimental results in the oxidation of 2,6-Me2P with alkaline ferricyanide and benzoyl peroxide reported by Waters et al. [1,2] are considered [4,5]. With alkaline ferricyanide [2], DPQ was produced in 45 – 50% and the other half of the oxidized material was a yellow no-ketonic polymer. This polymer was not well characterized but would probably be P-2,6Me2P. On the other hand, the oxidation of 2,6-Me2P with benzoyl peroxide afforded DPQ and the C – C coupling diphenol as the sole oxidative coupling products in total 60% yield [1]. However, the detailed kinetic analysis for this oxidation reaction was performed by Walling and Hodgdon [38], showing that the reaction mechanism would not due to any radical intermediates but to the benzoyl perester intermediate (Scheme 5). Dodonov and Waters [39] reported that the thermal decomposition of benzoyl 2,6-dimethylphenyl carbonate (PhCO3CO2Ar), which should generate the phenoxy radical, produced P-2,6-Me2P and DPQ in 35– 38% and

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Fig. 3. (a) Oxidative polymerization of 2,6-Me2P (1.8 mmol) with 50 wt% catalyzed by [Cu(tmed)(OH)2]Cl2 (0.045 mmol) in the presence of n PA (9 mmol) in toluene (3.6 g) at 40 8C under dioxygen for 24 h. (b) Reaction of 2,6-Me2P (6 mmol) and n PA (30 mmol) with benzyl chloride (0.6 mmol) in toluene (12 g) at 40 8C under argon for 9 days.

10% yield, respectively. From these studies, it seems that some complicated experimental results and different understandings were included in the above assumption of the C –C coupling selectivity due to the free radical coupling, which was also pointed out before [28]. In addition, many studies on the oxidative coupling of 2,6-Me2P by use of inorganic oxidants were performed, and in some cases, the free radical was observed, although phenoxo-metal intermediates were possible to form. McNelis [40] studied

the oxidation with MnO2, PbO2, and Ag2O, producing P-2,6-Me2P with molecular weights of 2000– 20,000 in 60 – 95% yields. For MnO2 [40] and Ag2O [33], the free radicals of 2,6-Me2P and P-2,6-Me2P were detected by ESR measurements. Sodium bismuthate as the oxidant also gave P-2,6-Me2P with molecular weight of 11,000 in 74% yields [41]. Mijs et al. [42] reported that, by the addition of increasing amount of triethylamine in oxidative polymerization with PbO2, the formation of DPQ was suppressed and the yield and molecular weight of P-2,6-Me2P were increased.

Scheme 5.

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On the other hand, Littler and Cecil [43] carried out the oxidation by hexachloroiridate(IV) anion in acidic aqueous solution, affording DPQ in 55– 65% yields and 2,6-dimethyl-p-benzoquinone in 27 – 32% yields. It was considered from the above studies [42,43] that the selectivity for C – O or C – C coupling of 2,6Me2P was much affected by a co-existing base or acid. So, recently oxidative coupling of 2,6-Me2P with Ag2CO3 was examined in the presence of excess n PA or acetic acid (AcOH) (Fig. 4) [37]. It has been found that the amount ratio of P-2,6-Me2P/DPQ as the product was 50/50 in the absence of these additives, and the values showed . 99/ , 0 in the presence of n PA and 0/100 in the presence of AcOH. These data indicate that generally in the oxidative coupling of 2,6-Me2P the addition of a base would lead to the C – O coupling and that of an acid to the C –C coupling. The above observations may be explained by a proposal by Waters [44]; in basic reaction media, a free phenoxy radical would be formed leading to the C – O coupling, and in acidic reaction media, a phenoxonium cation could be generated to give the C – C coupling. Roubaty et al. [45] performed the electrochemical oxidation of 2,6-Me2P and found that the oxidation above pH 5.2 took place via two monoelectronic step, the first of which would form

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the free phenoxy radical, and the oxidation below pH 5.2 underwent dielectronic one step oxidation, which could generate the phenoxonium cation (Fig. 5). It is unclear whether or not the phenoxonium cation is a real intermediate of the oxidative coupling in acidic conditions, however, at least, the phenoxonium cation seems easier to form under acidic conditions than under basic conditions. On the basis of the above studies, the reaction mechanisms for the C –O/C – C coupling selectivity in the oxidative polymerization of 2,6-Me2P are proposed as follows. (1) The coupling of free phenoxy radicals under basic conditions and the coupling via coordinated phenoxy radicals would mainly lead to the C– O coupling. (2) The coupling of free phenoxy radicals under acidic conditions and the coupling of phenoxonium cations from the two electron oxidation with phenol would favor the C – C coupling. In case of the above mechanism (1), it is hard to understand that a measurable level of phenoxy radical species from 2,6-Me2P in the polymerization with [CuCl(OCH3)(Py)2] complex was not detected [32].

Fig. 4. Oxidative polymerization of 2,6-Me2P (0.8 mmol) with 50 wt% Ag2CO3 on celite (2.4 mmol) in the absence and the presence of n PA or AcOH (16 mmol) in toluene (16 g) at 40 8C under argon for 24–48 h.

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Fig. 5. Voltametric study of oxidation of 2,6-Me2P in methanol. Dotted curve: pH ¼ 4:4; solid curve: pH ¼ 6:4; i: intensity of the electronic current; E: electropotential difference. Reproduced from Makromol Chem 1978;179:1151 [45] by permission of Wiley-VCH.

But, another phenoxo-copper(II) complex, which is equivalent to the phenoxy radical – copper(I) complex, was detected with high resolution of ESR [33]. On the other hand, both the atomic charges of the oxygen atom and para carbon atom in the phenoxy radical of 2,6-Me2P are negative (Fig. 2), so C – O coupling seems unfavorable [5]. However, the coupling selectivity should be determined from the consideration of location and molecular orbital interaction of the two phenoxy radical species [46,47]. Furthermore, the findings that the increase of an amine ligand for the Cu/amine catalysts led to C –O coupling mainly and the decrease favored C – C coupling [25,34] can be explained as follows; in a small amounts of the amine, the reaction conditions seems somewhat acidic owing to the Lewis-acidity of copper(II) ion, and an excess amounts of the amine reduces the acidity. 2.4. Chain extension mechanism The chain extension mechanism in the oxidative polymerization of 2,6-Me2P has been almost established, as shown in Scheme 6. Butte and Price [48]

proposed the head – tail coupling mechanism, in which the oxygen atom of head phenol unit couples with the para carbon atom of tail phenoxy group. However, it has been widely accepted that two dimeric phenoxy radicals couple with each other to give a quinoneketal intermediate, which has not been detected in the polymerization of 2,6-Me2P. Bolon [49] showed no reactivity of the tail phenoxy group, because the pconjugation is cut off by the ether bond. Mijs et al. [50] reported that the oxidation of a 4-phenoxyphenol marked with methyl group (let it be A – B as two distinguishable aromatic rings) yielded none of the dimer through a head – tail coupling (A –B – A –B) but the dimer through quinone-ketal intermediate (B – A – A –B). Two reaction routes from a quinone-ketal intermediate to a tetramer have been proposed; the one is a quinone-ketal redistribution to give monomer radical and trimer radical [51] and the other is a quinone-ketal rearrangement via intramolecular rearrangement [52]. Cooper et al. [51] reported that the oxidative polymerization from 2,6Me2P dimer produced the oligomers of even

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Scheme 6.

numbers such as 2,6-Me2P itself and the trimer. From these data, the extension mechanism for the oxidative polymerization of 2,6-Me2P must include quinone-ketal redistribution and whether or not quinone-ketal rearrangement occurs is unclear. For 2-methyl substituted and 2,6-unsubstituted 4-phenoxyphenols, only quinone-ketal rearrangement took place at a low temperature, however, with elevating the reaction temperature, quinone-ketal redistribution also occurred [50]. Then, in the above reaction mechanism via the phenoxonium cation of 2,6-Me2P leading to C – O coupling [5], heterolytic redistribution of tetrameric quinone-ketal can only afford trimer phenoxonium cation and monomer phenolate anion, which cannot produce the linear tetramer and can only revert to the quinone-ketal. Therefore, Reedijk et al. [53] proposed, the rearrangement from the quinoneketal to the tetramer should take place, but could not explain the decrease of the redistribution products at higher temperature. In general, the oxidative polymerization of phenols undergoes via stepwise growth mechanism, although

the electrochemical oxidative polymerization was suggested to be a chain reaction mechanism for the exception [54]. However, phenol dimers and higher oligomers have electro-donating phenoxy groups at ppositions and become easier to oxidize (more reactive) than phenol monomers. Hence, until the monomers are almost consumed, the reaction mixtures consist mainly of the monomers and polymers, so it often seems to be formally chain reaction mechanism. Heitz et al. [55] named this mechanism as a reactive intermediate polycondensation.

3. Oxidative polymerization of phenols catalyzed by peroxidase, oxidase and peroxidase model complexes 3.1. Peroxidase-catalyzed oxidative polymerization of phenol and hydrocarbon-substituted phenols Peroxidases such as HRP and SBP are hemproteins having an iron – porphyrin as the active site (Fig. 6(a)), which catalyze the double

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Fig. 6. Structures of (a) heme, active site of HRP and (b) its model complex, Fe(salen) complex.

one-electron-oxidation of substrates with hydrogen peroxide, liberating two molecules of water. The reaction mechanism seems to be already established [56] (Scheme 7), and the two active oxygen complexes abstract hydrogen atoms from substrates, for example, phenols to give free phenoxy radicals [57]. In 1983, Klibanov et al. [58] developed the removal of phenols from coal-conversion waste water solutions by use of HRP catalyst and hydrogen peroxide, in which the oxidative polymerization of phenols afforded the water-insoluble low-molecularweight polymers from the solutions. Schnitzer et al. [8] performed the enzymatic oxidative polymerization of phenols in phosphate buffer. On the basis of

the findings that enzymes were able to work in organic solvents [59], Dordick et al. [9] found that, in aqueous-organic solvents, the HRP-catalyzed oxidative polymerization of several phenols produced polyphenols with Mn of 400 –26,000. The use of mixtures of buffer with water-miscible organic solvents such as 1,4-dioxane, acetone, N,N-dimethylformamide (DMF), etc. is very important for the synthesis of various phenolic polymers possessing higher molecular weights, because many phenolic polymers are insoluble in water. Structures of these phenol polymers, however, were not well investigated. Akkara et al. [60] performed structural studies of poly(4-phenylphenol) prepared by HRP catalysis, and from FT-IR and CP/MAS 13C NMR studies,

Scheme 7.

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the major linkage is the C –C coupling through the opositions. It was found that the HRP-catalyzed oxidative polymerization of phenol, the simplest and the most important phenolic compound, in 1,4-dioxane/buffer solvent produced a new family of phenol resins (Scheme 8(a)) [61,62]. The resulting polyphenol was partially soluble in DMF and dimethylsulfoxide (DMSO) and had Mn up to 35,000. This structure was confirmed to contain both phenylene and oxyphenylene units by 1H, 13C NMR, and IR spectroscopies. From thermogravimetric analysis (TG), the polyphenol showed high thermal stability up to 350 8C. In this polymerization, the addition of a water-soluble polymer as stabilizer yielded relatively monodisperse polyphenol particles in the sub-micron size (Fig. 7) [63,64]. Solubility of the polyphenol was significantly affected by the reaction conditions, especially solvent composition and buffer pH. By examining these reaction conditions in detail, both structure

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and molecular weight of the polyphenols were controlled, enabling the formation of ‘soluble polyphenol’ [65,66]. The oxidative polymerization of phenol was performed by HRP catalyst with hydrogen peroxide in a mixture of methanol and buffer (pH ¼ 6:9; 50/50 v/v), producing the polymer in 95% yield. The resulting polyphenol was completely soluble in DMF, with Mn of 2800. By acetylation of the polymer, the unit ratio of phenylene and oxyphenylene was determined to be 51:49. Even in free radical coupling system, the reaction solvent affected to control the molecular weight less than several thousands and the structure with phenylene unit content from 41 to 61%, leading to the soluble polyphenol. The polyphenol may provide various applications, especially in replacement of phenol –formaldehyde resins owing to no use of toxic formaldehyde as starting materials. In the HRP-catalyzed oxidative polymerization of p-substituted n-alkylphenol, the polymer yield

Scheme 8.

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Fig. 7. SEM photograph of polymer particles from (a) phenol, (b) 3methylphenol, (c) 4-methylphenol, and (d) 4-phenylphenol. Reproduced from Polym J 1998;30:526 [64] by permission of the Society of Polymer Science, Japan.

increased with an increase of the chain length of nalkyl groups [67]. For the polymerization of msubstituted alkylphenols, the choice of peroxidase greatly influenced the polymer yield. In use of HRP and SBP, the yields of polymers from 3-methylphenol were 97 and 49%, but those of polymers from 3-t-butylphenol became 0 and 99%, respectively, [68]. Tripathy et al. [69] performed the HRPcatalyzed co-polymerization of 4-tetradecylphenol or 4-hexadecylphenol with phenol at the Langmuir through air – water interface, and the resulting polymer had a degree of order not seen in the bulk-synthesized material. Poly(2-naphthol) showing a fluorescence characteristic was obtained via polymerization of 2-naphtol with HRP catalyst [70]. 3.2. Peroxidase-catalyzed oxidative polymerization of phenols having functional groups In case of phenols possessing functional groups, chemoselectivity between the phenolic moiety and the other reactive groups can be achieved by peroxidase catalyst and regioselectivity in the oxidative coupling is occasionally controlled owing to the functional

substituents. It was found that a phenol derivative with a methacryloyl group was chemoselectively polymerized by HRP catalyst [16] (Scheme 3(a)). In this polymerization, only the phenolic moiety was subjected to the oxidative coupling without involving the vinyl polymerization of the methacryloyl group. The resulting polymer with Mn of 1400 was soluble in organic solvents and consisted of phenylene unit and oxyphenylene unit in 70:30. Since the thermal or photochemical crosslinking took place through the remaining methacryloyl group, this polymer can be used as a resist material. The reason for the chemoselectivity seems to be that the free radical species (S· in Scheme 7) do not initiate the radical polymerization of vinyl compounds, for which a mediating initiator such as 2,4-pentanedione is needed [71]. It was also reported that chemoselective oxidative polymerization of 3-ethynylphenol by use of HRP and hydrogen peroxide to produce the polyphenol having the ethynyl group, which was converted to the carbonized polymer in a high yield [72] (Scheme 8(b)). In comparison with this polymerization, the reaction of 3-ethynylphenol with a copper/diamine complex and dioxygen exclusively gave a diacetylene derivative as oxidation product. From biphenols, 4,40 -biphenyldiol [73] and bisphenol-A [74], soluble polyphenol derivatives were obtained by oxidase-catalyzed oxidative polymerization. Both polymers were soluble in polar solvents such as DMF, DMSO, acetone, etc were mainly composed of phenylene and oxyphenylene units. No crosslinking in the oxidative polymerization of these biphenols is characteristic not only with peroxidase catalysts, but also with peroxidase model catalysts as described below. For 4,40 -dihydroxydiphenyl ether as monomer, the HRP-catalyzed oxidative polymerization produced a; v-hydroxyoligo(1,4-phenylene oxide)s, which were formed through a unique pathway involving the elimination of hydroquinone [75]. Dordick et al. [17] synthesized a new redox polymer, poly(hydroquinone), via peroxidase-catalyzed oxidative polymerization of glucose-b-D -hydroquinone (arbutin), a natural product found in various berry plants. HRP or SBP catalyst produced the watersoluble arbutin polymer with Mn of 1600 – 3200 consisting of C – C coupling units at the o-position (Scheme 3(b)). Acidic deglycosylation of the polymer

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gave a poly(hydroquinone) soluble in polar organic solvents, which showed reversible redox properties. On the other hand, a poly(hydroquinone) obtained by electrochemical oxidative polymerization of hydroquinone was insoluble and assigned to have an ortho – meta coupling structure. The peroxidase-catalyzed oxidative polymerization of hydroquinone monobenzoate produced a polymer consisting of a mixture of phenylene and oxyphenylene units, followed by hydrolysis to give the corresponding poly(hydroquinone) [76]. Polymerization of a tyrosine derivative by using a peroxidase, followed by alkaline hydrolysis produced a poly(tyrosine) possessing an amino acid moiety in the side chain (Scheme 8(c)) [77]. The poly(tyrosine) was different from the other poly(tyrosine) with a-peptides structure, which was afforded by use of protease. A polynucleoside having an unnatural backbone was obtained by

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SBP-catalyzed oxidative polymerization of thymidine 50 -(4-hydroxy)phenylacetate (Scheme 9(a)) [78]. The soluble fraction of this polymer had Mn of 21,700; the phenolic peaks in the NMR spectra were significantly broadened. Tripathy et al. [79] reported that the oxidative polymerization of 4phenylazophenol by HRP catalyst produced a polymer with Mn of 3000, which contained mainly C– C coupling structure at o-position with some ones at m-position (Scheme 9(b)). The polymer films exhibited photoinduced absorption dichroism and large photoinduced birefringence with unusual relaxation behavior. Samuelson et al. [80] performed the HRP-catalyzed polymerization of 4sulfonated phenol to yield the polymer, consisting mainly of 1,2-phenylene oxide unit (Scheme 9(c)). The reason of regioselectivity of coupling seems due to the strong substituent effect of azo and sulfonate group at the p-position.

Scheme 9.

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3.3. Oxidase-catalyzed oxidative polymerization of phenols Laccase, one of oxidase enzymes that oxidize substrates by using dioxygen as an oxidant, contains a trinuclear copper active site with an additional mononuclear copper site. This enzyme plays an important role in the polymerization of lignin-related substrates and depolymerization of lignin in nature. As the catalytic system in oxidative polymerization of phenols, laccase with dioxygen shows the similar reactivity as HRP and SBP with hydrogen peroxide. For example, the oxidative polymerization of 2,6Me2P catalyzed by laccase, HRP and SBP were reported to show almost the similar results [81]. The laccase-catalyzed polymerization of 2,6-Me2P in a mixture of acetone and a buffer (pH 5) produced P2,6-Me2P with Mn of 2700 and DPQ in 57% and ca. 40% yield, respectively, showing that the C – C coupling significantly occurs under acidic conditions. 4-Hydroxybenzoic acid derivatives are very difficult to polymerize oxidatively by metal complex catalysts, however, oxidase and peroxidase induced the oxidative polymerization of syringic acid obtained from plants (Scheme 3(c)) [18] and 3,5-dimethyl-4hydroxybenzoic acid [82]. The oxidative polymerization of syringic acid by laccase with dioxygen and HRP or SBP with hydrogen peroxide produced the polymers with Mn higher than 10,000. All analytical data for these polymers obtained from 1H- and 13C

NMR and IR spectroscopies together with MALDITOF MS supported the structure of exclusively 1,4phenylene oxide units with a benzoic acid at the chain end. The postulated reaction mechanism was explained as follows (Scheme 10); the phenoxy radical of syringic acid is formed by the hydrogen abstraction and couples with each other to give the quinoide-type intermediate. Carbon dioxide is liberated from the intermediate to afford a dimer, followed by further coupling to lead to polymer formatin. The enzyme-catalyzed oxidative polymerization of 3,5-dimethyl-4-hydroxybenzoic acid also yielded the P-2,6-Me2P. Furthermore, poly(2,6-dimethoxy-1,4-phenylene oxide) led to poly(2,6-dihydroxy-1,4-phenylene oxide) by demethylation [83] and was used as a macromonomer for a multiblock copolyester [84]. Natural ‘urushi’ is a Japanese lacquer of traditional coatings, which is produced by oxidative coupling of ‘urushiols’ (Fig. 8(a)) catalyzed with laccase in air. Urushiols are phenol derivatives having an alkyl or alkenyl group as a side chain. For modeling of natural urushi, urushiol analogues (Fig. 8(b) and (c)) newly designed and synthesized. Their laccase catalyzed curing in air gave ‘artificial urushi’ whose properties were comparable with those of natural urushi [85 –87]. It is to be mentioned that laccase induced radical polymerization of acrylamide with or without mediator [88] and bilirubin oxidase was employed as an oxidative polymerization catalyst of 1,5-dihydroxynaphthalene [89].

Scheme 10.

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Fig. 8. Structures of natural urushiols (a) and precursors of artificial urushi: (b) and (c) urushiol analogues and (d) cardanol, also an urushiol analogue.

3.4. Oxidative polymerization of phenols by a peroxidase model complex catalyst Oxidoreductase enzymes such as HRP, SBP and laccase are more expensive than conventional copper/amine catalysts, and hence, their cost is a very important factor for the industrial application of these enzymes to the oxidative polymerization catalysts. A N,N0 -bis(salicylidene)ethylenediamino iron complex (Fe(salen), shown in Fig. 6(b)), which can be easily synthesized from cheap materials of salicylaldehyde, ethylenediamine and an iron salt, was found as a peroxidase model catalyst for oxidative polymerization of phenols with hydrogen peroxide [19,90]. Except for that Fe(salen) complex is more readily soluble in organic solvent and less soluble in water, it seems to involve almost no significant difference for

the oxidative polymerization catalyst between Fe(salen) complex and a peroxidase. The oxidative polymerization of 2,6-Me2P catalyzed by Fe(salen) complex catalyst with hydrogen peroxide in dioxane yielded P-2,6-Me2P [91]. The polymerization of bisphenol-A [92] by the Fe(salen) catalyst produced a soluble polyphenol as obtained by peroxidase catalysts [74]. The Fe(salen)-catalyzed polymerization of 2,6-difluorophenol produced crystalline poly(2,6-difluoro-1,4-phenylene oxide) [93]. Furthermore, the polyphenols having crosslinkable groups were prepared by Fe(salen)-catalyzed oxidative polymerization of urusiol analogues (Fig. 8(c)) [94] and cardanol, also an urushiol analogue (Fig. 8(d)) [95] in the similar manner as the enzyme catalysis. Oxidative curing of the polyphenols also gave ‘artificial urushi’ [85 –87,96 – 98]. Fe(salen)-

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catalyzed oxidative coupling of a poly(amino acid) containing phenol moieties yielded a high-molecularweight soluble polymer [99]. Gonsalves et al. [100] reported that hematin, one-electron-oxidation state of hem (Fig. 6(a)), polymerized ethylphenol in a similar reaction mechanism as HRP.

4. Oxidative polymerization of phenols catalyzed by monooxygenase-model complexes (‘radicalcontrolled’ oxidative polymerization) 4.1. Oxidative polymerization of 4-phenoxyphenol by tyrosinase model complex catalyst 4.1.1. Tyrosinase model complex for catalyst Conventional copper catalysts, typically copper/ diamine catalysts are producing P-2,6-Me2P industrially [4], but are not able to give useful polymers from the phenols having at least one o-position unsubstituted [101,102]. Peroxidase (HRP and SBP), oxidase (laccase) and peroxidase-model (Fe(salen)) catalysts changed coupling selectivity by solvent effect to some extent and showed chemoselectivity of reacting only with phenolic moieties. However, these catalysts showed little ability for controlling the coupling selectivity of 2- and/or 6-unsubstituted phenols [15,22], although a few examples of regioselective coupling due to specific substituent groups were reported [17,80]. Then, the followings were noticed as to these problems. Copper(I)/diamine complexes reacted with dioxygen to give bis(m-oxo)

dicopper(III) complexes [103]. In the reaction of HRP with hydrogen peroxide, Fe(IV)yO intermediates were formed [56]. These active oxygen complexes were subjected to the reaction with phenols to afford ‘free’ phenoxy radical intermediates [57,103]. These data suggest that the regioselective coupling cannot be achieved by the catalysts generating ‘electrophilic’ or ‘radical’ active oxygen intermediates. On the basis of this view (Fig. 9), much attention was paid to a ‘nucleophilic’, strictly speaking, ‘basic’ m-h2:h2-peroxo dicopper(II) complex 1, a copper – dioxygen model complex for tyrosinase (a monooxygenase) [104,105]. Then, a working hypothesis was made as follows: complex 1 abstracts protons (not hydrogen atoms) from phenols to give phenoxocopper(II) complex 2, equivalent to phenoxy radical – copper(I) complex 3. Intermediates 2 and/or 3 are not ‘free’ radicals but ‘controlled’ radicals, which is identical to radicals coordinated to copper atoms described above. Thus, if a catalyst generates (and regenerates) only a nucleophilic active oxygen intermediate, followed by the reaction with phenols to give the ‘controlled’ radicals without formation of the ‘free’ radicals, regioselectivity of the subsequent coupling may be entirely regulated. The attempt to avoid coupling at the open o-position of 2-methylphenol with copper/2-alkylpyridine catalysts by postulating formation of ‘controlled’ radicals has been already reported [106,107], However, satisfactory effects were not observed, probably involving generation of ‘free’ radicals. The above concept is characterized by the exclusive formation of

Fig. 9. A working hypothesis for controlling phenoxy radical coupling.

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tyrosinase model, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate) copper (Cu(Tpzb)) complex and (1,4,7-R31,4,7-triazacyclononane) copper (Cu(L R): R ¼ isopropyl (i Pr), cyclohexyl (cHex), and n-butyl (n Bu) complexes were employed (Fig. 10).

Fig. 10. Tirosinase model complexes.

‘controlled’ phenoxy radicals, and hence, the new concept was termed as ‘radical-controlled’ oxidative polymerization [20,22]. ‘Radical-controlled’ oxidative polymerization of PPL catalyzed by tyrosinase model complexes has been developed (Scheme 4(a)) [21,22,108,109]. This was the first synthesis of crystalline PPO having a melting point by the catalytic oxidative polymerization method, although the synthesis by other tedious procedures were reported [110 –112]. As the

4.1.2. Dimer formation of 4-phenoxyphenol The ratio of oxidative coupling dimers formed at the initial stage of polymerization of PPL using various catalysts were investigated (Table 1) [21,22]. In entries 1 – 6, the polymerization catalyzed by Cu(Tpzb)Cl, Cu(Li Pr)Cl 2, Cu(LcHex)Cl 2, and Cu(Ln Bu)Cl2 (5 mol% based on PPL) was performed under dioxygen (1 atm) in toluene or THF at 40 8C. For the comparison with these catalysts, the polymerization catalyzed by CuCl/N; N; N 0 ; N 0 -tetraethylethylenediamine (teed), which was the sole catalyst reported for oxidative coupling of PPL [50], HRP, Fe(salen), and tyrosinase was examined (entries 7, 9, 10, 12). As a model system of free phenoxy radical coupling, the polymerization of PPL oxidized by an

Table 1 Dimer formation at initial stage of oxidative polymerization of PPL Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 a

Catalyst

Cu(Tpzb)Clc,d Cu(Tpzb)Clc,d Cu(Li Pr)Cl2c,d Cu(Li Pr)Cl2c,d Cu(LcHex)Cl2c,d Cu(Ln Bu)Cl2c,d CuCl/teedc,d – d,e HRPd,f Fe(salen)f – d,e Tyrosinased,g – d,e

Oxidant

O2 O2 O2 O2 O2 O2 O2 AIBN H2O2 H2O2 AIBN Air AIBN

Solvent

Toluene THF Toluene THF Toluene Toluene Toluene Toluene Dioxane/buffer(8/2) Dioxane/buffer(8/2) Dioxane/buffer(8/2) Acetone/buffer(5/5) Acetone/buffer(5/5)

Time (h)

0.25 1.7 0.2 7.5 0.2 0.2 0.02 120 0.25 0.25 96 1 96

Conv.a (%)

13 11 9 12 7 12 17 27 11 12 8 14 13

Yieldb (%)

9 7 8 9 7 12 12 15 7 7 6 ,0.1 6

Dimer ratio (%) p-4

o-4

oo-22

oo-13

91 91 93 89 95 90 79 82 37 41 54 – 22

9 9 7 7 5 9 6 4 17 15 12 – 9

0 0 0 1 0 0 2 2 8 7 4 – 10

0 0 0 3 0 1 13 12 38 37 30 – 59

Conversion of PPL (0.60 mmol). Total yield of dimers. c Polymerization catalyzed by Cu complex (0.030 mmol) and 2,6-diphenylpyridine (0.30 mmol) in solvent (1.2 g) under dioxygen (1 atm) at 40 8C. CuCl (0.030 mmol) and teed (0.015 mmol) was used as the Cu complex in entry 7. d Data from Macromolecules 2000;33:1986 [22]. e Oxidized by AIBN (0.60 mmol) under nitrogen at 40 8C (entries 8, 11, and 13) in the same solvents as shown above (entries 1–7, 10–12, respectively). f Polymerization in the presence of HRP (2.4 mg) or Fe(salen) (0.03 mmol) in dioxane (4.8 ml) and buffer of pH ¼ 7 (1.2 ml) with 30% H2O2 (0.030 mmol) at 25 8C. g Reaction with tyrosinase (2.4 mg) in acetone (3 ml) and buffer of pH ¼ 7 (3 ml) under air (1 atm) at 25 8C. b

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Fig. 11. Oxidative coupling dimers of PPL.

equimolar amount of 2,20 -azobisisobutyronitrile (AIBN) was performed in the same solvent (entries 8, 11, and 13). In the case of CuCl/teed, four dimers were detected and the structures of the dimers were identified as p-4, o-4, oo-22, and oo-13 (Fig. 11). Products p-4 and o-4 are formed by the C – O coupling and formation of oo22 and oo-13 is based on the C –C coupling. Almost none of phenol, 4-(4-phenoxyphenoxy)phenol, and o40 were detected. In the early stage of the reaction, the PPL conversion was fairly close to the total yield of these four dimers, and thereby, the dimer ratio can be taken as a good measure of the coupling regioselectivity. For the CuCl/teed catalyst, considerable amounts of the two C – C coupling dimers of oo-22 and oo-13 were detected, and p-4 selectivity was consequently low (79%). These dimer ratios were very similar to those via free radical coupling by AIBN oxidation, in which the formation of the C – C coupling dimers is characteristic. However, for the Cu(Tpzb) in toluene and in THF, and for the Cu(Li Pr), Cu(LcHex), and Cu(Ln Bu) in toluene, none or very little of the C –C coupling dimers were detected, and high regioselectivity of p-4 was achieved (max. 95%). In entries of 3, 5, and 6, the order of p-4 selectivity was Cu(Ln Bu) (90%) , Cu(Li Pr) (93%) , Cu(LcHex) (95%) in good agreement with that of steric hindrance of

the substituents from the X-ray analysis [22]. These data show that the regioselectivity of phenoxy radical coupling can be controlled by these catalysts. On the other hand, the dimerization catalyzed by the Cu(Li Pr) in THF gave the C – C coupling dimers to some extent. In the case of the HRP and Fe(salen) catalyst, the dimer composition was close to that by AIBN in the same solvent, suggesting that HRP and Fe(salen) induced the oxidative coupling in free radical pathways. The oxidation catalyzed by tyrosinase gave almost none of the dimers and insoluble dark-brown powdery materials (yield 3%) at 14% of PPL conversion, whereas AIBN oxidation produced the dimers in 6% yield at 13% of PPL conversion. 4.1.3. Polymerization of 4-phenoxyphenol The resulting polymer was isolated as methanolinsoluble part (Table 2). The polymerization catalyzed by the Cu(LcHex) complex (entry 5) and that oxidized by AIBN (entry 8) were also very slow, and the conversions reached 76 and 69%, respectively, at the final stages. For the Cu(Tpzb) in toluene (entry 1), the consumption was rapid but no more than 55%. In other cases (entries 2-4, 6, 7, 9, 10), the conversions reached 86 –100%, and the polymers were obtained in 40– 89% yields. In the systems with little or no C – C dimer formation (entries 1 – 3, 5, 6), white powdery

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Table 2 Oxidative polymerization of PPL Entry

Catalyst

Oxidant

Solvent

Time (h)

Conv.a(%)

Yieldb(%)

Mn

Mw

1,2,4-unitc (unit%)

Tmd (8C)

1 2 3 4 5 6 7 8 9 10

Cu(Tpzb)Cle Cu(Tpzb)Cle Cu(Li Pr)Cl2e Cu(Li Pr)Cl2e Cu(LcHex)Cl2e Cu(Ln Bu)Cl2e CuCl/teede –e HRPe Fe(salen)

O2 O2 O2 O2 O2 O2 O2 AIBN H2 O2 H 2 O2

Toluene THF Toluene THF Toluene Toluene Toluene Toluene Dioxane/buffer(8/2) Dioxane/buffer(8/2)

14 70 19 81 210 23 24 380 24 24

55 91 98 86 76 87 100 69 99 100

4 54 89 52 28 40 77 8 43 97

700 1500 1200 3800 600 800 5400 1600 –h –h

1100 2900 4700 13700 700 1100 29100 3100 –h –h

1.2 4.8 5.1 (8.0)f 0.8 (2.6)f (7.3)f (6.5)f (8.3)f (7.5)f

186 194 171 nde 186 184 ndg ndg ndg ndg

a b c d f g h i

The reaction conditions are shown in Table 1. Conversion of PPL. Methanol-insoluble part. Estimated from the peak at 970 cm21 of FT-IR spectra. Temperature of the largest endothermic peak in the second scan of DSC measurement. Data from Macromolecules 2000;33:1986 [22]. These values may be inaccurate, because the phenylene units are neglected. Not detected. Insoluble.

polymers were obtained, which were almost soluble in DMF and partially soluble in chloroform. The Mn and Mw were 600– 1500 and 700– 4700, respectively. The IR spectrum patterns of the polymers were very similar to that of PPO synthesized by Ullmann condensation [111]. However, the polymer from PPL showed a small peak at 970 cm21, which is based on the 1,2,4-trioxybenzenes (1,2,4-unit). With almost no formation of the C – C dimers, the 1,2,4-unit ratio of the PPO obtained were 0.8– 5.1 unit%. These data support the structure of the polymers containing 1,4-oxyphenylene linkage as the major unit. From the DSC analysis, the polymers showed melting points ðTm Þ at 171– 194 8C. The Tm values were lower than that of the PPO by Ullmann synthesis (298 8C), which seems to be ascribed to the presence of the ortho C–O linkages in a small amount. In the polymerization giving considerable amounts of the C – C dimers (entries 4, 7 – 10), brownish polymeric materials were formed. The polymers had higher 1,2,4-trioxybenzene unit contents (6.5 –8.3%) and showed no clear melting points in the DSC traces. 4.1.4. Reaction mechanism of catalytic cycle On the basis of the above data, the reaction mechanism for the copper complex catalysis is

postulated as follows (Scheme 11). First, the starting copper(II) chloride complex Cu(Tpzb)Cl or Cu(LR)Cl2 6 reacts with PPL or oligomers of PPL to give phenoxo-copper(II) complex 2, equivalent to phenoxy radical– copper(I) complex 3. Complexes corresponding to 2 from 4-fluorophenol, 2,6dimethylphenol, and 2,6-di-tert-butylphenol were characterized in a separate study [113]. Complex 2 from 4-fluorophenol was stable and its structure was determined by means of X-ray crystallography. For the 2,6-disubstituted phenols, complexes 2 gave the oxidative coupling products at room temperature, indicating that 2 possesses a radical resonance structure 3. These previous data support the structure and reactivity of ‘controlled’ phenoxy radical species 2 and 3. Regioselective coupling takes place between two molecules of 2 and/or 3 to produce copper(I) complexes 7 as well as the phenylene oxide products having p-linkage selectively, because the steric hindrance of the catalysts blocks the coupling at opositions. In case of the Cu(Tpzb) complex, formation of mh2:h2-peroxo dicopper(II) complex 1 from 7 was confirmed under dioxygen [105] in both toluene and THF. For the Cu(Li Pr) complex, it was reported that 7 afforded complex 1 in non-polar solvents such as

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Scheme 11.

toluene [114], and 1 reacted with HBF4 to yield hydrogen peroxide [115]. A similar complex 1 of (hydrotris(3,5-diisopropyl-1-pyrazolyl)borate) copper reacted with 4-fluorophenol to give complex 2 [116] and the complex 7 was proved to react not only with dioxygen, but also with hydrogen peroxide to form the complex 1 [22]. These data strongly indicate that mh2:h2-peroxo dicopper(II) complex 1 is formed and reacts with phenols to regenerate 2 and hydrogen peroxide. Hence, this catalytic system would allow only the regioselective coupling process from 2 and/or 3 and completely exclude free radical coupling reactions; the present reaction is thus recognized as ‘radical-controlled’ oxidative polymerization. For the Cu(Li Pr) complex under dioxygen in THF, 7 gave bis(m-oxo) dicopper(III) complex 4 [114], and 4 did not react with HBF4 [117]. A similar (1,4,7tribenzyl-1,4,7-triazacyclononane) copper complex 4 abstracted hydrogen atoms from its benzyl group to give bis(m-hydroxo) copper(II) complex 8 [118]. These previous data indicate that bis(m-oxo) dicopper(III) complex 4 is formed, followed by abstraction of hydrogen atoms from phenols to give free phenoxy radical 5. Therefore, this catalytic cycle should involve the free radical coupling with the formation

of C –C linkages, although production of 2 from complex 8 also takes place [113]. The Cu(LcHex) complex 7 was ascertained to form m-h2:h2-peroxo dicopper(II) complex 1, which will give 2 similarly to the Cu(Li Pr) complex in toluene. The Cu(Ln Bu) complex would generate not only 1 but also bis(m-oxo) dicopper(III) complex 4, from the fact that (1,4,7-tribenzyl-1,4,7-triazacyclononane)copper complex afforded 4 in dichrolomethane [118]. This seems to be the reason that the dimerization catalyzed by the Cu(Ln Bu) gave a very slight amount of the C – C dimer. For the CuCl/teed complex, it was also reported that copper(I)/peralkylated ethylenediamine complexes reacted with dioxygen to give 4 [103]. The difference of the reaction behavior between tyrosinase and the tyrosinase model complex can be explained below (Scheme 12). In case of tyrosinase, 1 reacts with only one molecule of PPL to give 2 and hydroperoxo-copper(II) complex 9 (each one molecule), followed by ortho oxygenation to produce an o-quinone, which will eventually lead to the darkbrown product. It was reported that the oxidation of psubstituted phenols catalyzed by tyrosinase in the presence of water gave o-quinone intermediates resulting in the subsequent rapid polymerization

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Scheme 12.

[119]. For the model complexes, on the other hand, 1 reacts with two molecules of PPL to give two molecules of 2, leading to PPO via the ‘radicalcontrolled’ coupling. Tyrosinase possesses two functions: the formation of m-h2:h2-peroxo dicopper(II) complex and the oxygenation of phenols. For the present ‘radical-controlled’ polymerization, only the former is required. 4.1.5. Reaction mechanism of oxidative coupling and chain extension The reaction mechanism of oxidative coupling of PPL to produce dimers is speculated as follows (Scheme 13). For simplicity, the mechanism is argued here by expressing intermediate structures in the form of not ‘controlled radical’ but free radical with eliminating the catalyst part. First, two molecules of PPL are oxidized and the two generating radicals are coupled to each other (radical coupling). Since radical coupling does not occur in the 4-phenoxy group of PPL, only three reaction routes (a, b, and c) can take place, giving rise to a quinone-ketal intermediate, o-4, and oo-22, respectively. From the quinone-ketal, the redistribution path (quinone-ketal redistribution, d) was ruled out owing to no detection of phenol and 4-(4-phenoxyphenoxy)phenol, and therefore,

the rearrangement path (quinone-ketal rearrangement) was proposed in the oxidative coupling of PPL (see above). On cleavage of the ketal C – O bond, synchronous bond formation of e to p-4, of f to oo13 and of h to oo-22 occur, although that of g regenerates the quinone-ketal. The formation of o-40 was not observed, showing that C – O bond formation at the o-position (i) is ruled out in quinone-ketal rearrangement. Probably, the rearrangement to o-4 (j) is similarly excluded. In the ‘radical-controlled’ dimerization of PPL, the radical coupling would take place from controlled radical 2 and/or 3 in Scheme 11 [22]. The coupling selectivity of p-4 toward o-4 increased with an increase in the steric hindrance of substituents (R) of Cu(LR) (R ¼ n Bu, i Pr, cHex); this would be explained as follows [108]. The reaction intermediates of two controlled radicals to quinone-ketal (route a in Scheme 13) and o-4 (route b) are assumed as locations A and B (Fig. 12), respectively, in which the two bulky parts of Cu(LR) moieties are to be as remote from each other as possible. As the bulkiness of R increased, situation A is probably more favorable than B, and therefore, the formation of the quinoneketal leading to p-4 is more preferred to that of o-4. The formation of oo-22 (route c) would be hard to

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Scheme 13.

occur inherently, because even the free radical coupling system afforded a slight amount of oo-22. Almost no detection of oo-13 as well as oo-22 in the ‘radical-controlled’ dimerization of PPL suggests that the catalyst must be kept interacting with the quinone-ketal intermediate in the rearrangement. The reaction mechanism is speculated below, in which the rearrangement to p-4 (route e) or oo-13 (route f) is representatively described, because the rearrangement to quinone-ketal (route g) or oo-22 (route h) is understood in the similar way. For Cu(LR) catalyst (case Y), two possible intermediates are assumed (y-1 and y-2 in

Fig. 13). In y-1, two molecules of Cu(L R) complexes are interacting with quinone-ketal even after the radical coupling of two molecules 2 and/ or 3; one molecule of Cu(LR) complex interacts with the oxygen atom of quinone part of quinoneketal, and the other molecule of Cu(LR) complex interacts with the two oxygen atoms of ketal part of quinone-ketal. In y-2, only one molecule of Cu(LR) complex interacts with the oxygen of quinone part of quinone-ketal and the other molecule of Cu(LR) complex is far apart from quinone-ketal. For free radical coupling (case X), a considerable amount of oo-13 was formed,

S. Kobayashi, H. Higashimura / Prog. Polym. Sci. 28 (2003) 1015–1048

Fig. 12. Location of controlled radicals in oxidative coupling. Reproduced from J Polym Sci, Part A: Polym Chem 2000;38:4792 [108] by permission of John Wiley and Sons Inc.

indicating that transition state C to p-4 will be slightly preferred to transition state D to oo-13. From the finding that the formation of oo-13 was completely excluded in case Y, the transition state to p-4 will be much more favored than that to oo13. For y-1, transition state E would be much more preferred than transition state F, in which the steric repulsion between the one phenoxy group and the one copper complex will be unavoidable. Whereas for y-2, the stability of transition state G seems to be nearly equal to that of transition state H. Accordingly, the intermediate in case Y is considered to be y-1. In a similar way of dimerization, oxidative coupling of controlled radicals from PPL and p-4 will give a trimeric quinone-ketal intermediate, from which corresponding rearrangements will afford linear C –O trimer (six oxyphenylene units). Further oxidative couplings between controlled radicals generated from PPL, dimer, trimer, and higher oligomers will lead to linear PPO.

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4.1.6. Reaction conditions and other catalysts The reaction conditions in the ‘radical-controlled’ oxidative polymerization of PPL by Cu(Li Pr) catalyst were examined in detail [108]. The C –O selectivity (, 100%) was little influenced by the catalyst amount, counter halide anion, reaction temperature, and reaction solvent (except THF). The C –O coupling at the o-position increased with a higher temperature or an increase of solvent, whereas the C –C coupling was not involved. However, the C – O selectivity drastically changed from 100 to 24% with different nature of added amines as a base, meaning that it is possible to invert the selectivity from the preference for the C – O coupling to that for the C –C coupling. The effects of reaction temperature and solvent are operated at the stage of the coupling of phenoxy radicals to quinoneketal intermediates, and the effects of the added amines at the stage of the quinone-ketal rearrangement. As the ‘radical-controlled’ oxidative polymerization catalyst, a dicopper complex of a dinucleating ligand (Cu2(L1) in Fig. 14) was employed and produced crystalline PPO [109]. In comparison of the dicopper complex with a copper complex of a mononucleating ligand (Cu(L2)), the coupling selectivity was almost the same, however, the behavior of reaction rate toward the catalyst amount significantly changed. The initial reaction rate for Cu2(L1) was independent of the catalyst amount between 0.5 and 2 mol%, whereas the reaction rate decreased with a decrease in the amount of Cu(L2). These data suggest that the catalyst amount can be reduced by use of the dicopper/dinucleating ligand complex catalyst. 4.2. ‘Radical-controlled’ oxidative polymerization of mononucleus phenol derivatives 4.2.1. Substituent effect of phenol monomers in ‘radical-controlled’ oxidative polymerization The substituent effect of phenol monomers on the reaction rate of the ‘radical-controlled’ oxidative polymerization catalyzed by the Cu(Li Pr) complex was investigated in comparison with that by Cu(tmed) complex [120]. The Cu(tmed) complex was reported to generate bis(m-oxo) dicopper(III) complex 4, which abstracts hydrogen atom from the phenols to give free phenoxy radical 5 [103]. The conversion of each monomer at 3 h was taken as the initial reaction rates

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Fig. 13. Conformations of intermediates in quinone-ketal rearrangement. Reproduced from J Polym Sci, Part A: Polym Chem 2000;38:4792 [108] by permission of John Wiley and Sons Inc.

of the oxidative polymerization (Fig. 15). For the Cu(tmed) catalyst, the order of the reaction rates was phenol , 3-methylphenol (3-MeP) p 2-methylphenol (2-MeP) , 2,5-Me2P , 2,6-Me2P. This order agrees with that of the O – H homolytic bond dissociation energies of the monomers (phenol: 89.9, 3-MeP: 89.4, 2-MeP: 88.2, 2,6-Me2P: 85.5 (kcal/mol) [121]), which are closely related to the electronic factors of the substituents. On the other hand, for the Cu(Li Pr) catalyst, the reaction rate of dimethylphenols was smaller than those of methylphenols. As to dimethylphenols,

the reaction of 2,6-Me2P proceeded slower than that of the 2,5-dimethyl isomer. These data indicate that in the ‘radical-controlled’ oxidative polymerization catalyzed by the Cu(Li Pr) complex, the substituents of the phenol monomers, especially those at the o-position, would hinder the formation of 2. Hence, these data of substituent effect of the phenol monomers, as well as the previous data of that of the catalyst ligands, strongly support the reaction mechanism of ‘radicalcontrolled’ oxidative polymerization, in which the coupling of phenoxy radical species would take place from 2 and/or 3 (Scheme 11).

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Fig. 14. Dicopper complexes as tyrosinase model catalyst.

4.2.2. ‘Radical-controlled’ oxidative polymerization of phenol, 2-methylphenol, 3-methylphenol, and 2, 6-dimethylphenol In the oxidative polymerization of phenol catalyzed by Cu(Li Pr) complex, the ratio of dimers formed at the initial stage was examined in the similar manner as the polymerization of PPL [122]. As the dimers,

two C – O coupling dimers (PPL and 2-phenoxyphenol) and three C –C coupling dimers (4,40 -diphenol, 4,20 -diphenol, and 2,20 -diphenol) were detected. The selectivity for the coupling of the oxygen atom with the p-carbon atom of phenol leading to PPL in the Cu(Li Pr) catalysis (62%) became significantly higher than that in the free radical coupling system with

Fig. 15. Initial reaction rates in oxidative polymerization of five phenols catalyzed by the Cu(Li Pr) or Cu(tmed) complex. The oxidative polymerization of phenols (1.2 mmol) was performed in the presence of the Cu complex (0.0060 mmol based on the copper atom) and 2,6diphenylpyridine (0.060 mmol) in toluene (2.4 g) under dioxygen (1 atm) at 40 8C. Reproduced from Polym Adv Technol 2000;11:733 [120] by permission of John Wiley and Sons Limited.

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Fig. 16. Difference of coupling selectivity in ‘radical-controlled’ oxidative polymerization between phenol and PPL.

AIBN (15%). The Cu(Li Pr) did not exclude the formation of C – C coupling dimers of phenol (37%), because the p-carbon atom in one molecule of controlled radical species reacts with the o- or pcarbon of the other, while such C – C coupling is impossible for the coupling of PPL owing to pphenoxy group (Fig. 16). Hence, the resulting polymer from phenol consisted mainly of a 1,4phenylene oxide unit but contained a considerable amount of C – C coupling structures, showing no crystallinity. However, the temperature of 10% weight loss (Td10) for this poly(phenylene oxide) (, 500 8C) was much higher than that for the polyphenol obtained by peroxidase-catalyzed polymerization (, 400 8C) [63].

The ‘radical-controlled’ oxidative polymerization of 2-MeP and 3-MeP regioselectively produced soluble poly(phenylene oxide)s (Scheme 14). The polymer obtained from 2-MeP consisted mainly of a 2-methyl-1,4-phenylene oxide unit, with Mn of 3800 [123]. The polymer resulting from 3-MeP seemed to contain a 3-methyl-1,6-oxyphenylene units or 3methyl-1,2-oxyphenylene units [124], but the Mn value was relatively high (ca. 40,000). Both the polymers from 2-MeP and 3-MeP showed good thermal stability with Td10 of 437 and 414 8C, respectively. In the oxidative polymerization of 2,6-Me2P catalyzed by Cu(Li Pr)Cl2, DPQ was mainly obtained as the methanol-insoluble part, and a small amount of P-2,6-Me2P was afforded [37]. This result is probably because the steric hindrance of two methyl groups at the 2,6-positions would make the controlled radical species 2 and/or 3 unstable, generating the free phenoxy radical. From the free radical species, the C– C coupling would be favored, because the reaction conditions seem to be somewhat acidic even in the presence of 2,6-diphenylpyridine. This explanation could be supported by the data that the replacement of 2,6-diphenylpyridine with n-pentylamine, in which the reaction conditions are basic, produced only P2,6-Me2P. 4.2.3. ‘Radical-controlled’ oxidative polymerization of 2,5-dialkylphenols In the ‘radical-controlled’ oxidative polymerization of alkyl-substituted phenols, the polymers

Scheme 14.

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Table 3 Oxidative polymerization of 2,5-dialkylphenols (2,5-R2P) Entry

R

Time (h)

Conv.a (%)

Yieldb (%)

Mn ð£1023 Þ

Mw ð£1023 Þ

Tmc1 (8C)

Tc d (8C)

Tmd2 (8C)

1 2 3

Me Et n Pr

24 24 72

99 97 100

78 77 92

3.9 9.9 8.5

19.3 23.1 32.2

308 252 301

242 ndc 247

303 ndc 276

Polymerization of 2,5-R2P (1.2 mmol) in the presence of Cu(tacn)Cl2 (0.06 mmol) and 2,6-diphenylpyridine (0.6 mmol) in toluene (2.4 g) at 40 8C under dioxygen (1 atm). Reproduced from Macromol Rapid Commun 2000;21:1121 [23] by permission of Wiley-VCH. a Conversion of 2,5-R2P. b Yield of methanol-insoluble part. c Not detected. d Temperatures of the highest peaks in the first heating ðTm1 Þ; in the cooling ðTc Þ; and in the second heating ðTm2 Þ of DSC measurement.

obtained from 2,5-dialkylphenols (2,5-R2P: R ¼ methyl (Me), ethyl (Et), n-propyl (n Pr)) are to be noted [23]. The oxidative polymerization of 2,5-Me2P catalyzed by Cu(Li Pr)Cl2 (5 mol% based on monomer) in toluene under dioxygen (1 atm) at 40 8C produced a white polymer in a 78% yield (Table 3). The product was almost insoluble in any organic solvents at room temperature, but completely soluble in o-dichlorobenzene at 100 8C. The Mn and Mw were 3900 and 19,300, respectively. The reason why the Mn value is not so high seems to be due to the precipitation of this polymer in the reaction solvent. From the NMR data, the polymer is composed exclusively of a 2,5-dimethyl-1,4-phenylene oxide unit, indicating no linkage at the 6-position. For the ‘radical-controlled’ oxidative coupling of 2,5-Me2P, the 2,5-dimethyl groups would facilitate the decrease of C – O and C –C couplings at the 6-position, and the 5-methyl group would also depress C – C coupling at the 4-position. In the DSC traces of the polymer, there were not only an endothermic peak in the first heating, but an exothermic peak in the cooling and an endothermic peak in the second heating. The temperatures of the highest peaks in the first and second (Tm1 and Tm2 : melting temperatures) were 308 and 303 8C, respectively, and that in the cooling (Tc : crystallization temperature) was 242 8C. The polymerization of 2,5-Me2P by conventional copper – amine catalysts was reexamined again, however, the resulting polymers contained unregulated structures and the Tc and Tm2 were not detected. Accordingly, P-2,5Me2P showing heat-reversible crystallinity, has been synthesized for the first time [23]. The isomeric polymer P-2,6-Me2P showed a melting point at 237 8C ðTm1 Þ [125]. However, once

the crystal part had been totally melted, recrystallization never occurred during slow cooling (Tc and Tm2 not detected) or after annealing in the temperature range between Tg (209 8C) and Tm1 : Since thermoplastic polymers are mainly used as melt-moldings and the moldings of P-2,6-Me2P show non-crystallinity, P-2,6-Me2P is generally accepted as an amorphous polymer. In the oxidative polymerization of 2,5-Et2P and 2,5-n Pr2P, white polymers were obtained and had Mn of 9900 and 8500, respectively. The polymer resulting from 2,5-n Pr2P possessed only the 1,4phenylene oxide units and showed heat-reversible crystallinity with Tm2 at 276 8C. The polymer resulting from 2,5-Et2P was also identified as poly(2,5-diethyl-1,4-phenylene oxide), however, the polymer did not show detectable Tc and Tm2 : Although it may be possible that the polymer possesses too high molecular weight to recrystallize, another polymer of 2,5-Et2P with Mn of 3600 was synthesized, for which Tc and Tm2 were not detected, too. These data show whether or not recrystallization for poly(alkylated phenylene oxide)s occurs after melting is governed by both the position and group of their alkyl substituents. Particularly, P-2,5-Me2P can be synthesized from industrially available 2,5-Me2P and possesses the highest melting point above 300 8C among the alkyl-substituted polymers. Most of the crystalline plastics with melting points beyond 300 8C are prepared by dehalogenation polycondensation that needs normally high reaction temperature and disposal process for equal molar amounts of halogenated compounds as by-products [24]. P2,5-Me 2P is a candidate of a new superior

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engineering plastics by a clean, low-loading polymerization process, since the catalyst presented has high efficiency and the solvent employed can be recycled. It is further desired to increase the molecular weight of P-2,5-Me2P as well as PPO; copolymerization with other monomers and blockor graft-copolymerization may provide a good approach. 4.2.4. ‘Radical-controlled’ oxidative polymerization of other phenol monomers The ‘radical-controlled’ oxidative polymerization of 2-n-octadecylphenol produced the poly(1,4-phenylene oxide) having a crystallizable side chain [126]. Furthermore, Tsuchida et al. reported that the Cu(Li Pr)-catalyzed oxidative polymerization of 2,6-difluorophenol yielded non-crystalline poly(2,6-difluoro-1,4-phenylene oxide) with a higher molecular weight ðMn ¼ 13; 000Þ [127].

tyrosinase model complexes, however, has been developed and has given a highly probable solution to this important problem. The catalyst generates only controlled radicals without formation of free radicals. Thus, oxidative polymerization of phenols by enzymes and model complexes revisited with bringing about new aspects of catalysis and is currently recognized as an environmentally benign method for synthesis of phenolic polymers.

Acknowledgements Many of studies by the present authors mentioned here were partially supported by NEDO for the project on ‘Precision Polymerization’ and for the ‘Nanostructure Polymer Project’ (AIST).

References 5. Summary Since Hay’s pioneering work on oxidative polymerization of phenols by the copper/amine catalyst system late in 1950s, manufacturing and commercial application of P-2,6-Me2P have been greatly developed. In remarkably short period of time, alloys of P2,6-Me2P with polystyrene became one of the five major engineering plastics. As to the whole reaction mechanisms on the oxidative polymerization of 2,6Me2P, the chain extension mechanism becomes almost understood. However, the coupling mechanism, particularly the selectivity for C – O/C – C coupling, has been unclear and therefore, the reconsideration on this unsolved mechanism seems to be very significant. Although the catalytic oxidative polymerization of phenols is well known as a low-loading and low-cost method, it has been applicable only for 2,6-disubstituted phenols to produce useful polymers. Recently, enzyme catalysts have made it possible to polymerize a variety of phenolic monomers, giving rise to a possible polyphenol material, whose molecular weight, solubility, chemoselectivity in structure were well controlled. Furthermore, regioselective polymerization of 2- and/or 6-unsubstituted phenols has been a challenging problem for these four decades. A new catalyst of

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