"Oxidative Polymerization". In: Encyclopedia of Polymer Science

Oxidative polymerization is, formally, abstraction of two hydrogen atoms from a ..... polyphenol was obtained for the first time by controlling the polymer structure ..... presence of a strong acid by electrolysis (198) and by reaction with Lewis ...
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OXIDATIVE POLYMERIZATION Introduction Oxidative polymerization is, formally, abstraction of two hydrogen atoms from a monomer to give a polymer, and thus may be classified as polycondensation. The applicable monomers are mainly aromatic compounds. Electrical and chemical oxidation methods are often used, in which catalysis with dioxygen or hydrogen peroxide is the most favorable; the reaction temperature is moderate and the by-product is water only. Oxidative polymerization is one of the cleanest and lowest loading methods in polycondensation. However, the reaction mechanism is unclear in many cases and the coupling selectivity is not generally easy to control. This article deals with oxidative polymerization of phenols, anilines, thiophenol derivatives, aromatic hydrocarbons, heterocyclic aromatics, and other monomers. The reaction mechanism, the coupling selectivity, and the characteristics of the resulting polymers are discussed.

2,6-Disubstituted Phenols Until the 1950s, oxidation of 2,6-dimethylphenol (2,6-Me2 P) by an oxidant like benzoyl peroxide (1) or alkaline ferricyanide (2) mainly gave 3,3 ,5,5 tetramethyldiphenoquinone (DPQ) (Fig. 1). In 1959, oxidative polymerization of 2,6-Me2 P catalyzed by CuCl/pyridine (Py) under dioxygen leading to poly(2,6dimethyl-1,4-phenylene oxide) (Poly-2,6-Me2 P) was discovered (3). Various catalysts such as copper/substituted-ethylenediamine complexes were developed and Poly-2,6-Me2 P was found completely miscible with polystyrene (4) (see POLYETHERS, AROMATIC). Oxidative Coupling Mechanism. Poly-2,6-Me2 P is a C O coupling product and DPQ is a product via C C coupling. Control of the C O coupling is a most important question (5). Three possible reaction mechanisms for the C O coupling

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Fig. 1. Oxidation of 2,6-dimethylphenol.

selectivity have been proposed as follows: (1) coupling of free phenoxy radicals resulting from one-electron-oxidation of 2,6-Me2 P, (2) coupling of phenoxy radicals coordinated to a catalyst complex, and (3) coupling through phenoxonium cation formed by two-electron-oxidation of 2,6-Me2 P (Fig. 2). Coupling of Phenoxy Radicals Coordinated to Catalyst Complex. Because the oxidative coupling of 2,6-Me2 P with benzoyl peroxide or alkaline ferricyanide affords DPQ as the main product (1,2), it seems that the free phenoxy radical leads to C C coupling. C O coupling results from the phenoxy radical

Fig. 2. Three possible mechanisms for C O coupling in the oxidative polymerization of 2,6-dimethylphenol.

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coordinated to the copper complex [(ii) in Fig. 2]. In CuCl/Py catalysis, increasing the amount of Py to copper (6,7) favors the C O coupling. Substituents at the 2,6-positions of Py (6) or high reaction temperature (8) makes the C C coupling favorable. The kinetic study of Cu/Py catalysis showed that the oxidative polymerization proceeded by a Michaelis–Menten-type reaction mechanism and the C O coupling is preferred when at least one of the radicals is coordinated to copper (9). ESR measurements using a copper(II) acetate/Py complex (10), where Py/Cu = 20, showed two phenoxo-copper(II) complexes, and the oxidative polymerization gave mainly Poly-2,6-Me2 P. For Py/Cu = 2, none of phenoxo-copper(II) complexes was detected and the major product was DPQ. Thus, these data suggest that coupling via the coordinated phenoxy radicals mainly leads to the C O coupling. Coupling of Phenoxonium Cation with 2,6-Dimethylphenol. No ESR signals of free phenoxy radicals in the oxidation of 2,6-Me2 P with a [CuCl(OCH3 )Py] complex were detected (11). It was assumed that the phenoxy radical was oxidized by the Cu(II) complex (totally two-electron-oxidation from the phenol) to give the phenoxonium cation, which can couple with the phenol leading to C O coupling [(iii) in Fig. 2]. For a Cu/N-methylimidazole (NMI) catalyst, the reaction order in copper changed from 1.22 at NMI/Cu=10 to 1.70 at NMI/Cu=75, and the selectivity for the C O coupling reached a maximum at the NMI/Cu ratio of at least 30 (12). From the data, the key intermediate may be a µ-phenoxo dicopper(II) complex, in which two-electron-transfer from phenoxo moiety to two copper atoms can give a phenoxonium cation. By ab initio calculation, the atomic charges for the phenol, phenolate anion, phenoxy radical, phenoxonium cations of 2,6-Me2 P were determined (13). Since the oxygen atoms in all the species were negative, it was considered that the species susceptible to the C O coupling has positive charge on the para-carbon, and therefore, it should be the phenoxonium cation. The coupling of the phenoxy radicals was excluded, because the treatment of 2,6-Me2 P with benzoyl peroxide yielded DPQ (1). The reaction between the phenoxy radical and phenol was ruled out, because 2,6-dimethylanisole did not react in the oxidative polymerization of 2,6-Me2 P (14). If the oxidative coupling of 2,6-Me2 P proceeds via the phenoxonium cation, in the presence of a nucleophilic reagent in excess to the phenol, the phenoxo cation should react with the nucleophile, and hence, Poly-2,6-Me2 P cannot be obtained. In fact, it was found that the oxidative polymerization of 2,6-Me2 P catalyzed by a Cu(tmed) (tmed: N,N,N ,N -tetramethylethylenediamine) complex with an excess amount of n-pentylamine produced Poly-2,6-Me2 P in 75% yields (5). The nucleophilicity of n-pentylamine toward benzyl chloride was confirmed to be much greater than that of 2,6-Me2 P. These facts strongly indicate that such electrophilic intermediates as a phenoxonium cation are not involved in the oxidative polymerization. Coupling of Free Phenoxy Radicals. The above two mechanisms [(ii) and (iii) in Fig. 2] are based on the assumption that the coupling via free phenoxy radicals of 2,6-Me2 P leads to the C C coupling mainly. With alkaline ferricyanide (2), DPQ was produced in 45–50% but the other half of the oxidized material was a yellow nonketonic polymer, probably Poly-2,6-Me2 P. On the other hand, the oxidation with benzoyl peroxide afforded the C C coupling products in total 60% yield (1).

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However, detailed analysis for this oxidation reaction showed that the reaction mechanism did not involve radical intermediates but rather a benzoyl perester intermediate (15). It was reported that the thermal decomposition of benzoyl 2,6dimethylphenyl carbonate, which should generate the phenoxy radical, produced Poly-2,6-Me2 P and DPQ in 35–38% and 10% yield, respectively (16). From these studies, it seems that some complicated experimental results and different understandings were involved in the above assumption of the C C coupling selectivity in the free-radical coupling, which was pointed out before (17). In addition, many studies on the oxidative coupling of 2,6-Me2 P with inorganic oxidants were performed, and in some cases the free radical was observed. The oxidation with MnO2 , PbO2 , and Ag2 O produced Poly-2,6-Me2 P in 60–95% yields. For MnO2 (18) and Ag2 O (19), the free radicals of 2,6-Me2 P and Poly-2,6Me2 P were detected by ESR measurements. By the addition of increasing amount of triethylamine in oxidative polymerization with PbO2 , the formation of DPQ was suppressed and the yield of Poly-2,6-Me2 P was increased (20). On the other hand, the oxidation by hexachloroiridate(IV) anion in an acidic aqueous solution afforded DPQ in 55–65% (21). It was considered from the above studies (20,21) that the selectivity for the C O or C C coupling of 2,6-Me2 P was much affected by a coexisting base or acid. Recently, oxidative coupling of 2,6-Me2 P with Ag2 CO3 was examined in the presence of excess n-pentylamine (nPA) or acetic acid (AcOH) (5). The ratio of the products Poly-2,6-Me2 P and DPQ was 50/50 without these additives, >99/∼0 with nPA, and 0/100 with AcOH. These data indicate that generally in the oxidative coupling of 2,6-Me2 P 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 the following proposal (22): in basic reaction media, a free phenoxy radical is formed leading to C O coupling [(i) in Fig. 2], and in acidic reaction media, a phenoxonium cation could be generated to give the C C coupling. In the electrochemical oxidation of 2,6-Me2 P, the oxidation above pH 5.2 takes place via two monoelectronic steps, the first of which forms the free phenoxy radical; the oxidation below pH 5.2 undergoes one dielectronic step, which might generate the phenoxonium cation (23). It is still unclear whether or not the phenoxonium cation is really involved as the intermediate in the oxidative coupling; however, at least the phenoxonium cation should be easier to form under acidic conditions than under basic conditions. To summarize on the above observations about the reaction mechanisms, the C O/C C coupling selectivity in the oxidative polymerization of 2,6-Me2 P appears to be as follows (5). (1) The coupling via coordinated phenoxy radicals and the coupling of free phenoxy radicals under basic conditions would mainly lead to the C O coupling. (2) The coupling of free phenoxy radicals under acidic conditions or the coupling of phenoxonium cations from the two electron oxidation with phenol would favor the C C coupling.

Chain Extension Mechanism. The chain extension mechanism in the oxidative polymerization of 2,6-Me2 P has been almost established (Fig. 3). It has

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Fig. 3. Chain extension in the oxidative polymerization of 2,6-dimethylphenol.

been widely accepted that two dimeric phenoxy radicals couple with each other to give a quinone-ketal intermediate, which has not been detected. No reactivity of the tail phenoxy group (marked B in Fig. 3) was observed, because the π-conjugation is cut off by the ether bond (25); ie, the head-tail coupling mechanism, in which the oxygen atom of head phenol unit couples at the para-carbon atom of tail phenoxy group (24), is excluded. 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 a quinone-ketal intermediate (B-A-A-B) (26). Two reaction routes from a quinone-ketal intermediate to a tetramer were proposed; one is a quinone-ketal redistribution to give a monomer radical and a trimer radical (27), and the other is a quinone-ketal intramolecular rearrangement (28). The oxidative polymerization from 2,6-Me2 P dimer produced the oligomers of even numbers such as 2,6-Me2 P itself and the trimer (27,29). From these data, the extension mechanism for the oxidative polymerization of 2,6-Me2 P definitely involves quinone-ketal redistribution and it is unclear whether or not quinone-ketal rearrangement actually occurs. For 2-methyl substituted and 2,6-unsubstituted 4-phenoxyphenols, only quinone-ketal rearrangement took place at a low temperature; however, at higher reaction temperatures, quinone-ketal redistribution also occurred (26). Application of the redistribution mechanism resulted in the modification (30–32) and depolymerization (33) of Poly-2,6-Me2 P. In general, the oxidative polymerization of phenols undergoes a stepwise growth mechanism, although the electrochemical oxidative polymerization has been suggested to be a chain reaction mechanism (34). However, phenolic dimers

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and higher oligomers have electron-donating phenoxy groups at p-positions and become easier to oxidize (more reactive) than phenolic monomers. Therefore, until the monomer is almost consumed, the reaction mixture consists mainly of the monomer and polymer, and so it often seems to proceed formally via chain reaction mechanism, but it is actually reactive intermediate polycondensation (35). Other 2,6-Disubstituted Monomers. Various poly(2,6-substituted-1,4phenylene oxide)s possessing alkyl, aryl, alkoxyl, and halogen groups have been produced (see POLYETHERS, AROMATIC). Recently, some functional polymers (36– 39) were synthesized, one of which was converted to a heterocyclic ladder polymer (39). Poly(2,6-difluoro-1,4-phenylene oxide)s with crystallinity (40) and no crystallinity (41) were synthesized. By enzyme catalysis, oxidative polymerization of 3,5-disubstituted-4-hydroxybenzoic acids, with liberation of carbon dioxide, produced poly(2,6-disubstituted-1,4-phenylene oxide)s (42,43).

2- and/or 6-Unsubstituted Phenols A phenoxy radical intermediate has four reactive positions: the oxygen and paracarbon as well as two ortho-carbons. Therefore, for o-unsubstituted phenols, it is difficult to regulate the coupling selectivity. Enzyme catalysts and enzyme model catalysts have been studied for the control of the polymerization of 2- and/or 6unsubstituted phenols. Enzyme Catalyst. Oxidative coupling of phenols is involved in some biological reactions; for example, formation of lignin (qv) or melanin is catalyzed by oxidoreductase enzymes such as peroxidase, oxidase, or oxygenase (44,45). In 1983, horse radish peroxidase (HRP) catalyst was used to remove phenols from waste water, affording water-insoluble low molecular weight phenolic polymers (46). Since HRP was employed as the polymerization catalyst (47–49), the polymerization of various phenols by using HRP with hydrogen peroxide or laccase (an oxidase) with dioxygen has been extensively investigated (50,51). These enzymes show high catalytic activity for generating free radicals from phenols, but are unable to control the coupling selectivity (52). In oxidative polymerization of phenol by HRP catalyst (53,54), soluble polyphenol was obtained for the first time by controlling the polymer structure and molecular weight (55). Regio-controlled poly(hydroxyphenylene) was synthesized by using poly(ethylene glycol) (PEG) as template (eq. 1) (56). The enzymecatalyzed oxidative polymerizations of alkylphenols (57–61), biphenols (62–64), and various functional phenols (65–70) were also performed. For glucose-β-Dhydroquinone, only C C coupling at o-positions was claimed (eq. 2) (65). Chemoselective polymerization of a phenolic monomer having a methacryloyl group (eq. 3) (71) and an ethynyl group (72) was achieved and gave the corresponding polyphenol. “Artificial urushi” was prepared by laccase-catalyzed cross-linking reactions of urushiol analogues (73,74). (see ENZYMATIC POLYMERIZATION).

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Peroxidase Model Catalyst. An N,N  -bis(salicylidene)ethylenediamino iron [Fe(salen)] complex was found to be a cheap peroxidase model catalyst for oxidative polymerization of phenols with hydrogen peroxide (75–79). Hematin also polymerized ethylphenol in a reaction mechanism similar to that for HRP (80).

Tyrosinase Model Catalyst (“Radical-Controlled” Oxidative Polymerization Catalyst). Conventional copper catalysts were not able to give useful polymers from phenols having at least one o-position unsubstituted (26,81,82). Peroxidase (HRP), oxidase (laccase), and peroxidase-model [Fe(salen)] catalysts also showed a limited ability for controlling the coupling selectivity of such phenols (5,53). New copper catalysts were then studied. Copper(I)/diamine complexes, typical conventional catalysts, reacted with dioxygen to give bis(µ-oxo) dicopper(III) complexes (83). In the reaction of HRP with hydrogen peroxide, Fe(IV) O intermediates were formed (84). These active oxygen complexes were subjected to the reaction with phenols to afford “free” phenoxy radical species (83,85). These data suggest that the regioselective coupling cannot be achieved by the catalysts generating “electrophilic” or “radical” active oxygen species. Then, a working hypothesis was made (Fig. 4) (86,87): if a catalyst generates only a “nucleophilic,” strictly speaking “basic,” µ-η2 :η2 -peroxo dicopper(II)complex 1 (88,89), it will abstract a proton (not a hydrogen atom) from phenol to give phenoxo–copper(II) complex 2, equivalent to phenoxy radical–copper(I) complex 3. Intermediate species 2 and/or 3 are not “free” radicals but “controlled” radicals, and therefore regioselectivity of the subsequent coupling will be entirely regulated. The difficulty for conventional catalysts to control the regioselectivity (90,91) is probably due to generation of free radicals. This new concept is characterized by the exclusive formation of controlled phenoxy radicals, and hence the new concept was termed a “radical-controlled” oxidative polymerization (87).

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Fig. 4. Nucleophilic vs electrophilic active oxygen complexes.

Oxidative Polymerization of 4-Phenoxyphenol by Tyrosinase Model Catalyst. “Radical-controlled” oxidative polymerization of 4-phenoxyphenol (PPL) catalyzed by tyrosinase model complexes has been developed (eq. 4)(86,87,92,93).

(4) This was the first simple synthesis of crystalline poly(1,4-phenylene oxide) (PPO) having a melting point by the catalytic oxidative polymerization method, although other time-consuming synthesis procedures have been reported (94– 96). As the tyrosinase model, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate) copper [Cu(Tpzb)] complex (5) and (1,4,7-R3 -1,4,7-triazacyclononane) copper [Cu(LR ): R = isopropyl (iPr), cyclohexyl (cHex), and n-butyl (nBu)] complexes (6) were employed.

To examine the coupling selectivity, the ratio of oxidative coupling dimers formed at the initial stage of polymerization of PPL was investigated (Table 1) (86,87). CuCl/N,N,N  ,N  -tetraethylethylenediamine (teed), which was the sole catalyst reported for oxidative coupling of PPL (26), was also employed (entry 7). As a model system of free phenoxy radical coupling, an equimolar amount of 2,2 azobisisobutyronitrile (AIBN) was used for the oxidation of PPL (entry 8). In the

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Table 1. Dimer Ratio at Initial Stage of Oxidative Polymerization of 4-Phenoxyphenol Initial dimer ratio, % Entry 1 2 3 4 5 6 7 8

Catalyst

Oxidant

Solvent

7

8

9

10

Cu(Tpzb)Cla Cu(Tpzb)Cla Cu(LiPr )Cl2 a Cu(LiPr )Cl2 a Cu(LcHex )Cl2 a Cu(LnBu )Cl2 a CuCl/teeda

O2 O2 O2 O2 O2 O2 O2 AIBN

Toluene THF Toluene THF Toluene Toluene Toluene Toluene

91 91 93 89 95 90 79 82

9 9 7 7 5 9 6 4

0 0 0 1 0 0 2 2

0 0 0 3 0 1 13 12

b

a Polymerization of PPL (0.60 mmol) with Cu complex (0.030 mmol) and 2,6-diphenylpyridine (0.30 mmol) in solvent (1.2 g) under dioxygen (101.3 kPa) at 40◦ C. CuCl (0.030 mmol) and teed (0.015 mmol) was used as the Cu complex in entry 7. b Oxidized by AIBN (0.60 mmol) under nitrogen at 40◦ C.

case of CuCl/teed, four dimers were detected and the structures of the dimers were identified as 7, 8, 9, and 10. Products 7 and 8 are formed by C O coupling, and formation of 9 and 10 is based on C C coupling.

For the CuCl/teed catalyst, considerable amounts of the two C C coupling dimers of 9 and 10 were detected and 7 selectivity was low (79%). The dimer ratio was very similar to that via free-radical coupling by AIBN oxidation, in which considerable amounts of the C C coupling dimers were observed. However, for Cu(Tpzb) (5) in toluene and in THF, and for the Cu(LiPr ), Cu(LcHex ), and Cu(LnBu ) (6) in toluene, none or very little of the C C coupling dimers were detected, showing high regioselectivity of 7. The order of 7 selectivity was Cu(LnBu ) (90%) < Cu(LiPr ) (93%) < Cu(LcHex ) (95%), in good agreement with that of steric hindrance of the substituents (87). These data show that the regioselectivity of phenoxy radical coupling can be controlled by these catalysts. The dimerization catalyzed by the Cu(LiPr ) in THF gave C C coupling dimers to some extent. The resulting polymer was isolated as a methanol-insoluble part. In the cases with little or no C C dimer formation (entries 1–3,5,6), white powdery polymers

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Fig. 5. Reaction mechanism for copper complex catalysis of oxidative polymerization.

with M w of 700–4700 were obtained. The IR spectrum patterns of the polymers were very similar to that of PPO synthesized by Ullmann condensation (95). From the DSC analysis, the polymers showed melting points (T m ) at 171–194◦ C. In polymerizations giving considerable amounts of the C C dimers (entries 4,7,8), the brownish polymers showed no clear melting points in the DSC traces. Reaction Mechanism of Catalytic Cycle. On the basis of the above data, the reaction mechanism for the copper complex catalysis is postulated as follows (Fig. 5). First, the starting copper(II) chloride complex Cu(Tpzb)Cl (5) 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). Regioselective coupling takes place between two molecules of 2 and/or 3 to produce copper(I) complexes (11) as well as the phenylene oxide products having p-linkage selectively, because the steric hindrance of the catalysts blocks the coupling at o-positions. In case of the Cu(Tpzb) complex (5), formation of µ-η2 :η2 -peroxo dicopper(II) complex (1) from 11 was confirmed under dioxygen (89) in both toluene and THF. For the Cu(LiPr ) complex as well as the Cu(LcHex ) and Cu(LnBu ) complexes (6), it was reported that 11 afforded complex 1 in nonpolar solvents such as toluene (97). 1 reacts with phenols to regenerate 2 (98) and hydrogen peroxide (99). 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(LiPr ) complex under dioxygen in THF, 11 gave bis(µ-oxo) dicopper(III) complex (4) (97). 4 abstracts hydrogen atoms from phenols to give bis(µ-hydroxo) dicopper(II) complex (12) and free phenoxy radical (13). Therefore,

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Fig. 6. Reaction mechanism for oxidative coupling and chain extension of 4phenoxyphenol.

this catalytic cycle involves the free-radical coupling with the formation of C C linkages, although production of 2 from complex 12 also takes place (100). The CuCl/teed complex also reacts with dioxygen to give 4 (83). Two computational studies have been performed; one disagreed with the above reaction mechanism (101), but the other was in good agreement (102). Reaction Mechanism of Oxidative Coupling and Chain Extension. Figure 6 shows a specrulative reaction mechanism for oxidative coupling of PPL to produce dimers (87,92). For simplicity, the mechanism is argued here by expressing intermediate structures in the form of free radical rather than controlled radicals. First, two phenoxy radicals are generated from PPL and couple to each other (radical coupling); then only three reaction routes (a, b, and c) can take place, giving rise to a quinone-ketal intermediate, 8 and 9, respectively. From the quinone-ketal, the redistribution path (quinone-ketal redistribution, d) was

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ruled out, and therefore the rearrangement path (quinone-ketal rearrangement) is proposed in the oxidative coupling of PPL. On cleavage of the ketal C O bond, synchronous bond formation of route e to 7, that of f to 10, and that of h to 9 can occur, but that of g regenerates the quinone-ketal. In the “radical-controlled” oxidative coupling of PPL, the radical coupling takes place from controlled phenoxy radical–copper(I) intermediate. Therefore, the steric effect of the catalyst would suppress 8 formation (route b) and inhibit 9 formation (route c), mainly giving quinone-ketal intermediate (route a). Moreover, almost no detection of 10 as well as 9 shows that the catalyst must be kept interacting with the quinone-ketal intermediate during the rearrangement. Probably, the carbonyl group of the quinone-ketal coordinates to the copper(I) atom of the catalyst. Thereby, bond formation at the o-position to 10 and 9 via routes f and h, respectively, would be protected and give 7 predominantly via route e. Further chain extension follows in a similar way to that of dimerization.

Oxidative Polymerization of Other Phenols by Tyrosinase Model Catalyst. The substituent effect of phenol monomers on the reaction rates has been investigated (103). For the Cu(tmed) catalyst, the reaction rates were governed by the O H homolytic bond dissociation energies of the monomers, which are closely related to the electronic effect of substituents. On the other hand, for the Cu(LiPr ) catalyst, the steric effect of substituents at the o-positions depressed the reaction rates. In the oxidative polymerization of phenol, the Cu(LiPr ) catalyst showed high selectivity for C O coupling; however, it did not exclude the formation of C C coupling (104). The resulting polymer consisted mainly of a 1,4-phenylene oxide unit but contained a considerable amount of C C coupling structures, showing no crystallinity. The oxidative polymerization of 2- and 3-methylphenol regioselectively produced soluble poly(phenylene oxide)s, showing good thermal stability (105,106). The polymers obtained from 2,5-dialkylphenols [2,5-R2 P: R = methyl (Me), ethyl (Et), n-propyl (nPr)] are noteworthy (107) (eq. 5). The oxidative polymerization of 2,5-Me2 P catalyzed by Cu(LiPr )Cl2 in toluene under dioxygen produced a white polymer. The M w was 19,300 and the structure was composed exclusively of a 2,5-dimethyl-1,4-phenylene oxide unit. The melting temperatures in the first and second scan (T m1 and T m2 ) were detected at 308 and 303◦ C, respectively. Poly2,5-Me2 P (eq. 5) showing heat-reversible crystallinity was synthesized for the first time. The isomeric polymer Poly-2,6-Me2 P (Fig. 1) showed a melting point at 237◦ C (T m1 ), but once the crystaline part had been totally melted, recrystallization never occurred (T m2 not detected) by slow cooling or after annealing (108). Since thermoplastic polymers are mainly used as melt-moldings, Poly-2,6-Me2 P is generally accepted as an amorphous polymer; however, Poly-2,5-Me2 P is considered as a crystalline one.

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In the oxidative polymerization of 2,5-Et2 P and 2,5-nPr2 P (107), white polymers with M w of 23,100 and 32,200, respectively, possessed only the 1,4-phenylene oxide units. The latter showed heat-reversible crystallinity with T m2 at 276◦ C, however, the former did not show a detectable T m2 . The recrystallization for poly(alkylated phenylene oxide)s after melting seems to be governed by both the position and nature of alkyl substituents. Oxidative Polymerization of Naphthol Derivatives. Oxidative polymerization of 2-naphtol (109) and 1,5-dihydroxynaphthalene (110) has been done using enzyme catalysts. Solid-state polycondensation of 2,6dihydroxynaphthalene with FeCl3 catalyst (111) has been accomplished. Asymmetric oxidative coupling polymerization of 2,3-dihydroxynaphthalenes and their derivatives was achieved by chiral copper catalysts (112–115).

Oxidative Polymerization of Anilines Oxidative polymerization of aniline produces polyaniline (PAN) (eq. 6). This polymer was obtained as aniline black about a century ago (116,117) and has been revived as an electrically conducting polymer (see ELECTRICALLY ACTIVE POLYMERS).

(6) Polyaniline has been synthesized electrochemically (118,119) and with chemical oxidizing agents (120–123); a typical oxidizing agent is ammonium persulfate (120,121). Catalytic oxidative polymerization of aniline using iron salt catalyst (124) or HRP enzyme catalyst (125–129) with hydrogen peroxide, iron salt catalyst with ozone (130), and copper salt catalyst with dioxygen (131) has been done. A photo-induced catalytic system (132,133) and a gas-phase plasma method (134) have also been reported. Strongly acidic reaction conditions in the polymerization are normally selected, because polymer structure and electrical conductivity of PAN depend on the pH of the polymerization reaction (135,136). The reaction mechanism of oxidative polymerization of aniline has been a big controversy (Fig. 7). The dimerization step is generally proposed as (i), in which aniline is one-electron-oxidized to a cation radical, followed by coupling of two molecules of the cation radical to a dimer. The subsequent steps of chain extension are under discussion; routes involving coupling of cation radicals such as (ii)–(iv) (137–140) and routes via electrophilic attack of a two-electron-oxidized quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The addition of electron-rich arenes does not inhibit the polymerization, and therefore the route through the nitrenium ion (vi) seems to be rejected (137). Protonic doping is necessary to convert PAN from an insulator to a conductor (136), and so various dopants such as acid-substituted polymers (143–146) and chiral sulfonic acids (147–149) have been employed. Emulsion polymerization of aniline-afforded PAN particles (150–156) and PAN–silica particles (157,158). Polyaniline nanofibers have been synthesized by template-guided polymerization

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Fig. 7. Reaction mechanism for oxidative polymerization of aniline.

(159,160) and aqueous/organic interfacial polymerization (161–163). Intercalated PANs in V2 O5 (164), VOPO4 (165), MoO3 (166), and FeOCl (167) have been obtained, and included PANs in zeolite molecular sieves (168) and mesoporous channel hosts (169). Many substituted polyanilines were also synthesized (170). Typical examples are self-doping polymers, such as sulfonic acid ring-substituted PANs (171–177), mercaptopropanesulfonic acid substituted PANs (178), N-alkylsulfonic acid substituted PANs (179–181), and N-phenylsulfonic acid substituted PANs (182,183). Polyanilines having boronic acid for detecting sugar and dopamine (184,185) and for controlling self-doped states (186) have been produced. Long-chain-substituted anilines were polymerized at an air–water interface by the Langmuir–Blodgett technique (138,187,188). (see LANGMUIR-BLODGETT FILMS). Polyanilines with liquid crystalline substituents (189,190) and redox-active disulfide unit (191) were synthesized. Oxidative polymerization of 1-aminonaphthlenes gave PAN-like polymers (192,193).

Oxidative Polymerization of Thiophenols and Their Derivatives Thiophenol is oxidatively coupled to give diphenyldisulfide (eq. 7), because formation of an S S bond, resulting from the coupling of thiophenoxyl radicals, happens more readily than formation of a C S bond (194). Oxidative polymerization of 1,3dimercaptobenzene afforded poly(1,3-phenylenedisulfide) (195–197).

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Poly(phenylene sulfide) (PPS) has been synthesized by oxidative polymerization of diphenyl disulfide, which was obtained via oxidation of thiophenol (eq. 8) (198–208). PPS is an engineering plastic with a high melting point, which is manufactured by condensation polymerization eliminating a salt.

(8)

The oxidative polymerization of diphenyl disulfide was carried out in the presence of a strong acid by electrolysis (198) and by reaction with Lewis acids such as SbCl5 (199–201) or quinones such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (202–205). VO catalysts such as vanadyl acetylacetonate with O2 have also been used for the oxidative polymerization (206–208), and the catalytic reaction mechanism involving four-electron reduction of O2 has been discussed (209–215). The chain extension mechanism is shown in Figure 8 (201). Diphenyl disulfide is oxidized to a cation radical, which reacts with diphenyl disulfide to give phenylbis(phenylthio)sulfonium cation, followed by electophilic attack of diphenyl sulfide on the cation. The use of various diaryl disulfides as the monomer produced PPSs substituted with methyl groups at the 2- or 3-, 2,3-, 2,5-, or 2,6-positions, methoxy at the 2-position, etc. (204,207). Reactive functionalized oligomers (216) and block polymers containing alkylene and perfluoroalkylene groups (217,218) have been obtained. Cyclic hexakis(1,4-phenylene sulfide) was synthesized (219), and ringopening polymerization of such cyclic oligomers was also found (220). The synthesis of PPS via poly(sulfonium cation) as a soluble precursor (221) was developed through oxidative polymerization of methyl 4-(phenylthio)phenyl sulfide (222), followed by dealkylation (eq. 9) (223–225).

Fig. 8. Reaction mechanism for oxidative polymerization of diphenyl disulfide.

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(9)

Oxidative Polymerization of Aromatic Hydrocarbons In the 1960s, oxidative polymerization of benzene by CuCl2 -AlCl3 was discovered (eq. 10) (226). Other oxidants such as AlCl3 -MnO2 , FeCl3 , and MoCl5 were employed. These systems allowed the polymerization of various monomers such as toluene, chlorobenzene, diphenyl, naphthalene, and phenanthrene (227).

(10) Oxidative polymerization of bis(1-naphthoxy) monomers is known as the Sholl Reaction (eq. 11) (228). 1,4-Dialkoxybenzenes have been polymerized using FeCl3 (229,230) and oxovanadium catalyst with dioxygen (231) to give poly(2,5dialkoxy-1,4-phenylene)s (eq. 12). Poly(4,6-di-n-butyl-1,3-phenylene) was also obtained (232).

(11)

(12) Three reaction mechanisms of C C coupling in oxidative polymerization of benzenes have been reported (Fig. 9). The first step of each mechanism is one electron oxidation of a monomer to give a cation radical. The next step may be (i) coupling of two cation radicals (233,234), (ii) coupling of one cation radical with one neutral molecule (226,228), or (iii) coupling of one cation radical with many neutral molecules (stair-step mechanism) (235).

Oxidative Polymerization of Heterocyclic Aromatics Oxidative polymerization of pyrrole (236), thiophene (237), furan (238), and selenophene (239) was performed with electrical methods around two decades ago

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Fig. 9. Reaction mechanism for oxidative polymerization of benzene.

(eq. 13). More recently, numerous studies on the resulting polymers, particularly polypyrroles and polythiophenes, have been performed in terms of electrical conductivity (see ELECTRICALLY ACTIVE POLYMERS).

(13) For the oxidative polymerization of heterocyclic aromatics, two reaction mechanisms, similar to that of benzenes, have been proposed (Fig. 10). One is coupling of two cation radicals [(i) in Fig. 10] (240,241), and the other is coupling of one cation radical with one neutral molecule [(ii) in Fig. 10] (242,243). Oxidative Polymerization of Pyrroles. The synthesis of pyrrole blacks was performed by chemical oxidative polymerization with a variety of oxidizing agents such as hydrogen peroxide, lead dioxide, quinines, ferric chloride, and persulfates (244) before the electrical method was employed (236). The catalytic oxidative polymerization under dioxygen to give polypyrrole (PPY) has also been developed (245–247).

Fig. 10. Reaction mechanism for oxidative polymerization of heterocyclic aromatics.

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Fig. 11. Regioisomers of 3-substituted polythiophenes.

Various PPYs having 1-substituted (248), 3-substituted (249,250), and 3,4disubstituted (251–253) groups, and transition metal complex moieties (254–256) have been obtained. PPY particles were produced by colloidal dispersion method (257–259). Monolayer and multilayer PPYs were obtained by thiol–Au interaction (260,261), by deposition on YBa2 Cu3 O7−δ (262,263), by intercalation in FeOCl (264), and by monomer amphiphilicity (265). PPY wires (266–268), PPY tubes (269–271), and PPY microcontainers (272,273) have also been synthesized. Oxidative Polymerization of Thiophenes. Thiophene is oxidatively polymerized to give polythiophene (PTH) by electrochemical oxidation (237); by chemical oxidation with AsF5 (274), NO salts (275), and FeCl3 (276); and by catalytic oxidation with dioxygen (277). PTH can be substituted at the 3- and/or 4-positions (278). 3-Substituted PTHs have regioisomers (Fig. 11), and the regioregularity greatly affects conjugation length and electronic properties. For poly(3-alkylthiophene)s, high regioselectivity of 91–98% HT has been obtained by organometalic reaction (279,280); however, a maximum 89% HT (Fig. 11) has been obtained by oxidative polymerization (281,282). In oxidative polymerization of 3-arylthiophenes (283–285) and 3-alkoxy-4-methylthiophenes (286), 94–96% HT contents were achieved. This regioselectivity has been discussed as arising from the spin density of the radical cations (287,288). Poly(3,4-ethylenedioxythiophene) was developed around 1990, and its combination with poly(styrene sulfonic acid) became practical because of its high filmformability, conductivity, transparency, and stability (289). Many functional PTHs were prepared possessing crown ether (290,291), tetrathiafluvalene-like structures (292,293), C60 -attachment (294,295), probes for affinity chromism (296–298), and various metal complexes (299–306). PTH film obtained by electrochemical deposition was stronger than aluminum film (307). PTH fibers were produced by using capillary flow cell (308) and with electrically independent connections (309). PTHs were included in zeolite (310,311) and such PTH wires (312) were obtained. PTH catenanes and rotaxanes have also been synthesized (313,314).

Oxidative Polymerization of Other Monomers Treatment of m-diethynylbenzene with a copper catalyst and dioxygen gave a poly(phenylene butadiynylene) (315). Use of Cu-incorporated mesoporous materials as the oxidative polymerization catalyst for 1,4-diethynylbenzene produced highly conjugated poly(1,4-phenylene-1,4-butadiynylene) (316).

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Bifunctional p-tolylcyanoacetic esters (317) and monofunctional phenylcyanoacetic esters (318) were oxidatively polymerized using a copper catalyst with dioxygen.

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HIDEYUKI HIGASHIMURA Sumitomo Chemical Company Ltd. SHIRO KOBAYASHI Kyoto University