Pergamon0079-6700(94)00029-8

hog. Polym. Sci., Vol. 20, 155- 183, 1995 Copyright 0 1995 Elswier Science Ltd Printed in Great Britain. All rights reserved 0079-6700/95 $29.00

DIRECT

SYNTHESIS

OF CONDUCTING SIMPLE MONOMERS

POLYMERS

FROM

NAOKI TOSHIMA

and SUSUMU HARA

Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Toyko 113, Japan

CONTENTS1. Introduction2. Polyphenylene

2.1. Chemical polymerization 2.2. Evaporated thin films of PPP 2.3. Electrochemical polymerization 2.4. Polymerization of substituted aromatic hydrocarbons 2.5. Polymerization of condensed aromatic hydrocarbons 3. Polypyrrole 4. Polythiophene 5. Polyaniline 6. Miscellaneous 7. Conclusions and Perspectives References

1.55 156 156 160 161 162 163 164 168 173 177 177 178

1. INTRODUCTION

Conducting polymers such as polyphenylene, l-3 polypyrrole,4 polythiophene and polyaniline67 have b,,; applied in the development of batteries,%15 electronic sensors,32-36 functional electrodes37-43 and so on.44245In devices, 1624 displays, view of the synthetic method used to prepare these materials, conducting polymers can be classified into two major groups: chemically polymerized materials and electropolymerized ones. A principal advantage of chemical polymerization concerns the possibility of mass production at a reasonable cost. This is often difficult to achieve with electrochemical methods. On the other hand, an important feature of the electrochemical polymerization technique involves the direct formation of conducting polymer films which are suitable for use in electronic devices. This review mainly describes the chemical polymerization of simple monomers using oxidizing agents and/or catalysts to make possible the mass production of conducting polymers. In most of the chemical polymerization procedures developed thus far, metal salts are consumed stoichiometrically in the reaction. By-products, which must be carefully separated from the polymers after the reaction, are also produced in large quantities, resulting in high cost. For example, more than half of the cost for the industrial synthesis of polyaniline films arises from extensive posttreatment steps. Effective catalytic processes would, therefore, be highly favored for155

156

N. TOSHIMA and S. HARA

13)

Fig. 1. Chemical

structures for polypuruphenylene (PPP): 1,4-phenylene branched structure (2) and polynuclear structure (3).

structure

(l),

mass production. Electropolymerization is also useful in some cases, particularly when film samples are desired. Both of these methods should be considered separately. The process used often differs from application to application. In this review we do not focus on polyacetylene, one of the most important conducting polymers, since its method of preparation and resulting applications are quite different from those of the polymers described below.2. POLYPHENYLENE

Polyphenylene - essentially a long chain of benzene rings - exhibits high heat resistance and electrical conductivity on doping.** The most important polyphenylene is poly( 1,Cphenylene), or polyparaphenylene (PPP). An ideal PPP would possess all-para-linked chains with high molecular weight, probably furnishing excellent properties. However, no synthetic method for ideal PPP has been reported so far. Note that we use the term PPP even though the great majority of PPPs do not have pure 1,4phenylene structures (Fig. 1, (1)) but have branches (2) or even polynuclear structures (3). 2.1. Chemical Polymerization The most successful procedure for benzene polymerization is that reported by Kovacic (eq. l), in which CuC12 acts as an oxidizing agent, and HCl and CuCl are generated as by-products:246 n/ 0\ + 2n CuClz

*lC13 0

+2nCuCl+2nHCl.

(1)

CONDUCIING POLYMERS

157

Although these by-products make this method less environmentally friendly, this reaction has been investigated widely because of its simple chemistry. To our knowledge, the combination of CuC12 (oxidizing agent)-AlCl, (catalyst) is the best2Y47 amongst various combinations, for example, FeC13-H20,48 MoCI~-H~O,~~ CuC12SbCls: Mn02-A1C1347 and Pb02-AlC1 3.47 The yield of PPP tends to rise rapidly with an increasing molar ratio of AlC13 to CuC12, and reaches a maximum (almost quantitative) at a ratio of two.47 Kovacic and Oziomek observed that CuCl generated during the reaction (eq. 1) acts as an inhibitor by associating with AlQ. Here, CuCl forms a double salt with A1C13as AlCuCb that is soluble in benzene.50 One of us developed a different method for PPP preparation using oxygen as an oxidizing agent (eq. 2)? n/ 0 C&l-AlCIs \ +(n/2)02 * an + n H20. (2)

In this case, CuCl or AlCuCb acts as a catalyst, and the reaction is essentially byproduct free because water is the only other product, as shown in eq. 2. Results are shown in Table 1 for benzene polymerization using the CuCl-AlC13-O2 system (the Toshima method) under various conditions. The addition of excess AlCls (Run 5) afforded a smooth polymerization giving a dark brown solid. Here the AlC13 may be deactivated by water (see eq. 2). No polymerization occurred with CuCl-AQ (1: 1) (Run 1) even though various oxidizing agents were added. In this system, oxygen, activated by AlCuCb, directly oxidized benzene since no Cu(I1) ion was detected by ESR experiments during the reaction. In general, the Cu(1) ion in metal salts is not stable toward oxygen and its oxidation or a disproportionation reaction easily occurs. A toluene solution of A1CuC14, however, is very stable toward oxygen and is a well-known selective carbon monoxide absorbant that has been employed as an industrial process (the COSORB process).52-54As the toluene solution of AlCuC1, is sensitive to water vapor - decreasing its activity for the absorption of carbon monoxide - the water content of gas mixtures must be reduced to less than 1 ppm

Table 1. Chemical oxidative polymerization Charged molar ratio Run 1 2 3 4 5 AlCuCb Excess AlCls 1.0 1.0 1.0 1.0 1.0 0 2.0 2.0 2.0 2.0 + 3.q

of benzene using the CuCl-AK&-02

system*5

Yield (%)t Temp (C) 60 40 60 70 60 Insoluble 0 5 119 187 649 Soluble 0 4 46 41 154 Total 0 9 165 228 803

* Benzene: 20.0; under 1 atm of oxygen for 24 h. 7 Based on the charged CuCl, that is, if the molar ratio of the obtained PPP to CuCl is 0.5, the yield is 100%. $ After the reaction with 2.0 of AlC13 for 24 h, the reaction continued with the addition of another 3.8 of AlCls for 24 h.

158 Table 2. Polymerization Catalyst Metal salt* CoCl* NiC12 CuCl CuCl CuBr vc13AK3

N. TOSHIMA and S. HARA

of benzene using various metal salts under oxygen*5 Yield (%)t A1X3/Metal salt 4.0 4.0 2.0 2.0 2.0 2.1$ Insoluble 25 19 43 21 19 170 Soluble 45 20 22 21 20 11

AlCl, AlC13 A1C13 AlBr3 A1Br3 AlC13

* Metal salt: benzene = 1 : 20, 50C under 1 atm of oxygen for 24 h. t Based on the metal salt.* 160C.

for carbon monoxide separation in the COSORB process. More recently, polystyrene protected AlCuQ and AgAlQ complexes were devised for the se aration of carbon monoxide and ethylene from gas mixtures containing water vapor.5 P 59 The oxidative polymerization of benzene can also be achieved when other metal salts are used. However, the combination of CuCl and AlCls exhibited the best activity (Table 2). In contrast to the Kovacic method that involves stirring a heterogeneous mixture of benzene, AlCls and CuC12, the Toshima method utilizes a homogeneous solution because AlCuC& forms a r-complex with benzene (eq. 3), and is completely soluble in this solvent.50 Here, benzene functions as both a solvent and a ligand: + AlC13+ CuCl AlCuCl,.

This homogeneous reaction system was expected to yield PPP with long phenylene chains. Unfortunately, the degree of polymerization (DP) as estimated by the IR spectroscopy51 was slightly lower than that for Kovacics polymers.@ Moreover the Toshima process provided not only an insoluble polymer but also a soluble product. The insoluble polymer was essentially identical to Kovacics PPP according to IR spectra. The soluble portion was determined to possess considerable quantities of non-aromatic structure as shown by NMR and IR techniques. The DP of PPP cannot be determined by standard techniques used for common polymers because of its insolubility. Brown et al. measured the DP of PPP prepared by the Kovacic method using laser desorption/Fourier transform mass spectrometry (LD/FTMS).61 IR spectroscopy is another useful technique. The DP can be estimated by the intensity ratio of bands centered at 800 and 690 cm-. These are characteristic of 1,Cdisubstituted and monosubstituted benzenes, respectively.3f62V63 Electron spin resonance (ESR) spectra of PPP powders prepared by both the Kovacic and the Toshima methods indicated the presence of free electrons. Crude and purified PPP powders prepared by the Kovacic method exhibited a s in concentration of 1.0 x 1021 (8 phenylene units per spin) and 1.5 x 1018 spins g- P (5.3 x lo3 phenylene units per spin , respectively.64 Purified PPP prepared by the Toshima method showed 1.6 x 104 spins g- that corresponded to 5.0 x lo2 phenylene units

CONDUCTING

POLYMERS

159

per spin.65 The spin concentration of PPPs prepared by the Yamamoto, the Kovacic and the Toshima methods were measured as 1.3 x 1015, 6.3 x 1017 and 2.1 x lo* spin g-l, respectively. These values are smaller than those for PPP films obtained electrochemically using CuCI-AlC13 in nitrobenzene (1 .O x 1019spin g-)67 and CuC12-LiAsF6 in nitrobenzene (1.1 x 1019spin g-).68 Each of the chemical oxidation methods using benzene as a starting material afford brown PPP powders with some cross-linking and polynuclear structure. The condensation polymerization of 1,Cdihalobenzene using alkali metals (Wurtz-Fittig reaction, eq. 4)j9and copper powder (Ullman reaction, eq. 5)70both yield low molecular weight products with irregular structures and are not well suited for the synthesis of PPP:

+Na

0 in dioxane

+ NaCl.

(4)

Yamamoto proposed a dehalogenation condensation process under mild conditions using Ni catalysts, NiC12(bpy) (bpy = 2,2-bipyridine), for example eq. 6:71

NWbpy)BeBr + Mg

-

0

+ MgBr2.

(6)

This method furnished a perfect 1,Cphenylene structure. Unfortunately, the monomer 1,4-dibromobenzene is much more expensive than benzene. Recently, a modification of the Ni catalyst (Ni(cod)2 (cod: l,Scyclo-octadiene)), without employing Mg, resulted in an increase in the yield and in the DP of a PPP sample.7273 The Yamamoto method can provide some polymers that cannot be obtained from their corresponding monomers by other chemical or electrochemical oxidative polymerization techniques.74175 In particular, polypyridine (poly(2,5-pyridinediyl)) and its derivatives are promising candidates for n-type conducting polymers.7c78 Polypyridine can easily be reduced to an electroconductive state by chemical or electrochemical procedures since pyridine is a r-deficient compound. Furthermore, this polymer is soluble in formic acid and hydrochloric acid, while the majority of conducting polymers are insoluble. A formic acid solution of this polymer, when measured by the light scattering method, possessed a DP between 16 and 25. The polymer solution afforded a large degree of depolarization. The Yamamoto method can also be applied to various dihaloaromatic compounds. PPP powders prepared chemically are not electrically conductive. For example, the conductivity of the PPP prepared using CuQ-AlC13 (the Kovacic method) is near lo-l2 S cm-. Whenp-type (acceptor) doping with AsFS, AlC13, FcC13, etc., and n-type (donor) dopin with K, Li, etc. are performed, conductivity increases considerably to 500-0.3 S cm- ? .2p79-81

160

N. TOSHIMAand S. HARA

Although not a direct synthesis for PPP, a new PPP preparation through processable precursors was reported (eq. 7):82 adF NiP_ &:aOH

method occurring

&

cue I

(7)

-Cm* PPP obtained via this route had a highly regular structure and exhibited high electrical conductivity (18 S cm-) after SbFs doping. The precursor, poly(2carboxyphenylene1,4-diyl), whose DP was near 100, was soluble in pyridine, quinoline and aqueous NaOH solutions. It should be emphasized that PPP films were obtained using this procedure.

2.2. Evaporated Thin Films of PPP It is difficult to use chemically synthesized PPP powders to prepare f&s, since these polymers are generally insoluble and infusible. PPP powders prepared by the Kovacic (PPP-K), the Toshima (PPP-T) and the Yamamoto (PPP-Y) methods, however, can be evaporated onto substrates, providing thin films. 66y73y83 Figure 2 shows IR spectra for three deposited thin PPP films together with their powders as source materials.66 According to the IR data, all the PPP samples could be used as source materials to give

a

C

;I . 1000 . .

I I

II . . . . I . . .

1500

1500

1000

-1 C/cm

C/cm-

Fig. 2. IR spectra for evaporated thin fihns (solid lines) and their source (dashed lines) for various PPP powders: (a) PPP-T, (b) PPP-K and (c) PPP-Y .66

CONDUCTING POLYMERS

161

a

b

Fig. 3. Photoluminescence spectra (solid lines) and excitation spectra (dashed lines) for evaporated PPP thin films: (a) PPP-T, (b) PPP-K and (c) PPP-Y.66

PPP thin films using a vacuum evaporation technique, even though the DP (ca. 10) of the thin films was slightly smaller than that for the corresponding source materials. This result presumably suggests that only the smaller DP components evaporated under the conditions used to prepare the films. Alternatively, decomposition of the polymer chains might have led to a decrease in the DP as well. The deposited thin films were partially soluble in tetrahydrofuran (THF) and CHCls . As depicted in Fig. 3, the thin films exhibited different photoluminescence spectra. The PPP-T and the PPP-K thin films showed longer-wavelength luminescence than film PPP-Y, suggesting that PPP-T and PPP-K have some polynuclear structure. An asymmetric property in the photoluminescence spectrum for an evaporated PPP-T thin film was observed when the film was prepared at a low substratetemperature.84 The spectrum from the region near the surface of the film had emission peaks in the range of 400-500nm, whereas that observed from the region near the substrate exhibited an emission above 600 nm. This difference might be due to special features concerning the surface morphology of the film. 2.3. Electrochemical Polymerization As mentioned above, the electrochemical polymerization films on electrodes quite readily (eq. 8):3785s6 n/ of benzene yields PPP

0-

\

+

2n H+ + 2n e-.

(8)

162

N. TOSHIMA

and S. HARA

Here, PPP has not been investigated as widely as polypyrrole, polythiophene and polyaniline. One of the reasons for this is presumably related to the relatively high oxidation potential of benzene, relative to the other monomers. Recently, PPP films were obtained at a rather low potential in acidic media: H2S04,87W89 fuming HzSO~,~ HF,95>96 HF-SbF5y7 SbF5,98 BFsO CF3S03H,91-93 CF3S03H-CF3COOH,94 (C2H5)2,99,100 A1C13,91710-103, Some intriguing properties of these materials were etc. A found very recently.104710s relaxation or so-called memory effect, characterized by a logarithmic law between voltammogram parameters and waiting time in the neutral state, was observed in cyclic voltammograms for PPP films synthesized by electrochemical oxidation of benzene in CF3S03H-CF3COOH.104 Characteristic cyclic voltammograms of PPP films, prepared in SO;! + CF3S03H at low temperatures, with sharp, well-separated redox peaks, were progressively transformed into voltammograms with broad redox peaks by scanning in H2S04 (95%). This was due to dopant exchange and cross-linking of the polymer chains.lo5 The electrochemical polymerization of benzene using a CuCl-AlC13 system, where oxidation by oxygen is replaced by electro-oxidation, has been reported.lo6 PPP films obtained by this method possessed unique fibrillar morphologies when nitrobenzene was used as a solvent. Other PPP films generally have granular or smooth surfaces. A possible mechanism of this indirect electropolymerization is proposed as follows. The anode oxidizes Cu(1) or A1CuC14 in the nitrobenzene solution of CuCl-A1C13, and the resulting Cu(I1) or activated A1CuC14 then oxidizes benzene to its cation radical, ultimately yielding the PPP films. Electroreductive polymerization of 1,6dibromobenzene (the Fauvarque method, eq. 9)lo7-11can be considered as an electrochemical modification of the Yamamoto method. BeBr +2ne--?!0 +2nBrV. (9)

In this case, PPP powders and films were obtained when a mercury pool or a conducting glass plate (SnO* or indium-tin oxide (ITO)) and foil or rod of lithium were used as a working electrode and a sacrificial counter-electrode, respectively. This technique is applicable for the preparation of other polymers such as polypyridine,112-114 poly(2,6-naphthylene),115 poly(2,5-thienylene),116 polyfuran,117 polyfluorene* and polycarbazole.119>120

2.4. Polymerization of Substituted Aromatic Hydrocarbons Alkyl substituted benzenes (toluene, xylenes and so on) can be polymerized by chemica1121m125 electrochemical oxidation1261127processes. The polymerization of and toluene with CuC12 and A1C13 (the Kovacic method) produced a polytoluene consisting of a poly(o-phenylene) backbone with a methyl group at the 4-position. The polymers zigzag structure tends to yield a polynuclear structure, owing to a

CONDUCTING

POLYMERS

163

cyclization reaction with CuQ-AlC13 CH3

(eq. 1O):22

(10)

A polytoluene obtained by the Toshima method was found to contain both solid and liquid species (eq. 1I), both of which are soluble in various organic solvents:125

CuCl-AlC1302 (11)

The solid product possessed a complicated polynuclear structure similar to the Kovacic one, while the liquid product had an ideal polytoluene structure. An excess of AIC13 accelerated the cyclization reaction providing an increase in the solid product formed. The electropolymerization of toluene in liquid SO2 with CF3S03HTBACF3S03 at low temperatures127 yielded oligomers with a polynuclear structure that was not the same as that for the chemically oxidized derivatives. Ohsawa et al. obtained poly(p-methylphenylene) and poly(9, IO-anthrylene) films from the toluene and anthracene, respectively, using LiAsF6-CuC12 and LiAsF6-NiC12[P(allyl)2Ph]Z as electrolytes in nitrobenzene.26 2.5. Polymerization of Condensed Aromatic Hydrocarbons Condensed aromatic hydrocarbons such as naphthalene,2228129 anthracene,30131 etc., can be polymerized both chemically and electrochemically. The reaction of naphthalene in o-dichlorobenzene using FeC13-H20128 or CUCI~-AICI~~~ yielded oligomers or oligomers together with polymers, respectively. The polymers, however, did not contain long chains. The anodic coupling of naphthalene was performed using tetrabutylammonium tetrafluoroborate (TBABFJ as a supporting electrolyte, and DP of the resulting product was found to depend on the solvent used.132 The DP was not high even though an appropriate solvent (1,Zdichloroethane) was chosen. The same authors performed an anodic coupling of pyrene in a similar manner and concluded that condensed aromatics were not suitable for preparing conducting polymers.33 We carried out the anodic coupling of naphthalene using CuCl and AIC13 in nitrobenzene and obtained purple poly(l,4naphthylene) films that contained mainly 6-8

164

N. TOSHIMA and S. HARA

monomeric units, as determined by a LD/FTMS technique.34 More recently, the DP in these materials was found to be much higher (up to 40).35136 Note that polynaphthylene cannot be obtained with the CuCl-AIC13-O2 system (the Toshima method).*36t137 The two procedures for the polynaphthylene (PN) preparation mentioned above furnished only poly(l,Cnaphthylene) films (4), while Satoh et al. obtained electrochemically prepared poly(2,6-naphthylene) films (5) on an IT0 electrode using a composite electrolyte containing CuC12 and LiAsF6.

(4)

(5)

These results suggest that altering the polymerization conditions can result in a change in the binding position of the monomeric units. Other PNs such as poly(l$naphthylene) and poly(2,7_naphthylene) have not been synthesized except by the Yamamoto method.74 Some conducting polymers are also heat-stable materials. Polymer films prepared electrochemically from benzene, naphthalene and diphenyl ether using CuCl-AlC13 in nitrobenzene exhibited significant thermal decomposition at 511, 546 and 486C respectively, under N2, whereas PPP powder prepared by the Toshima method decomposed near 585C. As the PN films are thermally stable, they are useful as source materials for preparing evaporated thin films.35 The evaporation of a PN in high vacuum (1.3 x 10m3Pa) was performed at 530C. The obtained thin film was transparent, amber in color with absorption maxima at 451,427 and 299 nm, as shown in Fig. 4. This evaporated thin film dissolved in acetonitrile and THF.3. POLYPYRROLE

Polypyrrole (PPy) has been very widely investigated in the field of conducting polymers and some of its products (e.g. a capacitortg) are currently on the market. One of the advantages of PPy concerns the low oxidation potential of pyrrole. The half-wave oxidation potential of various organic compounds in acetonitrile + NaC104 (0.5 mol dme3) is summarized in the literature.i3 The values for benzene, toluene, naphthalene, pyrrole and thiophene are +2.08, +1.98, +1.34, +0.76 and +1.6OV vs Ag/Ag+ (0.1 mol dme3), respectively. Apparently, pyrrole is one of the most easily oxidized monomers and a variety of oxidizing agents are therefore available for preparing PPy (Table 3).13%i5 For example, pyrrole can be polymerized using ~~~~,,146,147,152-54 one of the most common oxidizing agents as indicated in eq. 12 below: + 2n FeC13H

+ 2n FeC12 + 2n HCI.

(12)

CONDUCTING POLYMERS

165

1

1

1

i

200

400 600 Wavelength/nm spectrum for an evaporated PN thin film.r3

Fig. 4. The absorption

Table 3. Chemical oxidative polymerization

of pyrrole and its derivatives Conductivity (S cm-)

Monomer Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Pyrrole Octylpyrrole N-MethylpyrroleI2 Br2 Cl2

Oxidizing agent

Solvent H20, CH,CN CH3CN CH3CN ;;;; EtOHH2O H2O H20 H20 H2O H20 H20 H20

Ref. 139, 140 140 141 142 143 144 145 146 146 146 146 146 146 146 147 147 147 147 148 149 150,151 148

8,25 0.5 7 x 1o-2 25 1 36 33 44 8 45 110 190, 130 20, 3 3 90 l-10 1.6, 1.1 6 x 1O-2

K2S208

DDQ* FeOClt KsFe(CN)6 FeC13 - 6H2O Fq(SO& - 5&o Fe(N03)3 . 9H20 Fe(C104)3 - 9H20 FeBr3 CuC12 - 2H20 CuBr2 FeC13 FeC13 FeCls FeC13 Fe(BF& Na3(P04 12W03) Cu(ClO& Fe(ClO& Fe(BF&

MeOH, EtOH C6H6, CHC13 CH3CN HzO/CbH14H2Q

CH3CNC6H14

* DDQ: 2,3-dichloro-5,6-dicyano-p-benzoquinone. t FeOCl is an inorganic solid.

166

N. TOSHIMA and S. HAIL4

To obtain highly conductive PPy, Rapi et al. found that the chemical oxidative polymerization should be performed in water at OC, using a short reaction time and a low oxidizing agent to pyrrole ratio, with amides or m-substituted phenols. The highest conductivity of PPy was 83 S cm-, using Fe(ClO& - 9H20 and mhydroxybenzoic acid as an oxidizing agent and an additive, respectively, in water.46 Machida et al. obtained PPy with a conductivity of 190 S cm- using anhydrous FeCls in CH30H.47 Control of the oxidation potential of FeCls in CHsOH by adding FeCl* before the reaction enhanced the conductivity of PPy up to 220 S cm-. Chemical oxidative polymerization of 3-octylpyrrole using CU(C~O~)~or Fe(C104)3 as an oxidizing agent in acetonitrile yielded a soluble conducting polymer that exhibited characteristics different from poly(3-alkylthiophenes).151 However, the system requires more than an equivalent of oxidizing agent and produces a large amount of by-products in analogy with PPP preparation by the Kovacic method. This is a significant disadvantage for mass production. One of us applied the system CuCl-A1C13-O2 to PPy preparation (eq. 13), where a small amount of CuCl-AlCls was added:i55>56CuCl-

AlCl,

+ W2) 02 H

+

n H20.

(13)

The reaction was carried out using various solvents, since neat pyrrole aggregated as soon as CuCl-AlC& was added. Table 4 details the polymerization of pyrrole using the CuCl-AlCls-O2 system. The yields indicated in Table 4 are based on the amount of CuCl and pyrrole used, respectively. The former value is based on the turnover number for CuCl as a catalyst, suggesting that CuCl works effectively. The conductivities of the as-prepared PPy powders were lower than lo- S cm-, probably because of structural defects. Acetonitrile, however, yielded a fairly pure PPy with excess chlorine and a conductivity around lo- S cm-l. The XPS spectra of chlorine in the PPy powders before and after washing by aqueous ammonia revealed that there were two types of chlorine - ionic and covalent - in the as-prepared PPy. Electrical conductivity ( 10B2S cm-) in the PPy increased more than two orders of magnitude when pyridine was added to the solution. The color of the solution during the reaction was also different from that without pyridine. These results stronglyTable 4. Chemical oxidative polymerization Yield (%) Solvent p-Xylene Nitromethane Nitrobenzene Acetonitrile cucq 1300 1200 1300 700 Pyrrolet 130 120 130 70 C 8.25 5.17 6.35 4.54 H 9.85 3.84 4.92 3.86 of pyrrole using the CuCl-AlC13-02 Elemental analysis N 1.oo 1.00 1.00 1.00 Cl 0.02 0.04 0.02 0.22 0 1.76 1.35 0.84 0.36 system*55

* AlCls : CuCl : pyrrole : solvent = 1: 1: 5 : 50; reactions were carried out at room temperature for 24 h. t Based on the charged CuCl. $ Based on the charged pyrrole.

CONDUCTING

POLYMERS

167 system*

Table 5. Chemical oxidative polymerization Molar ratio VO(acac)z 1 1 0 1 1 lf AlC& 0 0 1 2 2 2

of pyrrole using the VO(acac)z-AICl,-Oz Elemental analysis C 4.38 4.72 4.25 4.79 4.34 H 3.46 6.37 3.62 3.99 3.70 N 1.00 1.00 1.00 1.00 1.00 Cl 0.11 0.33 0.42 0.42 0.30 0 _ 1.39 0.73 0.61 0.61 0.42

Temp. (C) 20 60 60 20 60 20

Yield? (%) 1.1 39.0 26.0 49.0 128.3 25.1

Conductivity (S cm) < lo-lo < lo-lo 1.2 x 1o-4 < lo-lo 0.13

* Reaction time: 24 h. t Based on the charged monomer. $ With the addition of pyridine.

suggest that pyridine co-ordinates with the copper ion, forming an active site. Surprisingly, for the case of added pyridine, ESR spectroscopic data revealed that Cu(1) ions turned into Cu(I1) ions, whereas no Cu(I1) ions were generated without adding pyridine. Bis(acetylacetonato)(oxo)vanadium(IV) - VO(acac)2 - which easily changes its oxidation number, was used for PPy preparation instead of CuCl (Table 5).157 As VO(acac)* itself was not soluble in acetonitrile, VO(acac)2 and pyrrole did not yield PPy in acetonitrile even though the solution was exposed to O2 at room temperature. The addition of AlCls to the dispersed solution, however, resulted in a homogeneous solution producing PPy as a brown precipitate under 02. The addition of AQ without VO(acac);! afforded only the addition polymerization product, resulting in a low electrical conductivity (199 When bithiophene, which is more easily oxidized than thiophene, was used as a monomer, a polymer with a mixed structure intermediate between the dihydro and r-conjugated form was obtained. The product obtained using nitromethane seemed to be more like PTh based

170

N. TOSHIMA and S. HARA

Table 8. Chemical oxidative polymerization of thiophene (Th] and 2,2-bithiophene CuCl-AlCls-O2 system* 99 Yield? (%)100

(BiTh) using the

Elemental analysis C 4.25 4.30 4.56 4.42 4.49 4.39 4.43 4.24 4.18 4.37 4.33 4.12 4.42 H 4.25 4.46 4.33 4.64 4.07 4.03 3.90 4.49 _ 2.38 2.25 3.02 3.23 3.91 _ _ S 1.00 1.00 1.00 1.oo 1.00 1.00 1.00 1.00 1.oo 1.00 1.00 1.00 1.00 Cl 0.02 0.07 0.05 1 0.07 0.13 _ _ 0 0.08 0.31 _ 0.36 _ 0.17 _ 0.49 -

Monomer

Solvent CH3N02 CH3N02 CH3N02 CH2C12 CHCls CCl4 cc14n-C7H16

Conductivity (S cm-> < lo@ _ 1.0 x 10-l 3.2 x 1O-2 1.8 x lo- -

WT&i

W Th TM

Thll

WI

WI WI ThllBiTh$ BiThI CiTh$ BiTh BiThI BiThi BiTh$

CH$N C2HSOH CH3N02 CHsN02 C~HSNO, CHCls CCL4 EtOH CHaCN

138 110 138 101 92 104 77 0 0 70 94 87 88 69 0 0

* AlCla : CuCl : thiophene residue : solvent = 3 : 1 : 5 : 50. t Based on the charged monomer. $3OC. p 50C. )I70C.

both on elemental analysis data and the rather high electrical conductivity. The chemical polymerization of 3-dodecylthiophene using the CuCl-AlCls-O2 system in nitrobenzene yielded a brown powder that was only partly soluble in CHC13, suggesting the probability of some undesirable linkage in the product. The weightaverage (M,) and number-average molecular weights (M,) determined by GPC were 6476 and 47 12, respectively,* which are much lower than those of other poly(3alkylthiophene)s. As mentioned above, FeCls has been employed as an oxidizing agent for the chemical oxidative polymerization of thiophene. As thiophene is more difficult to oxidize than pyrrole, thiophene polymerization generally requires catalytic systems with enhanced Lewis acidity and oxidizing power. For FeC13 . nH20, these depend on the degree of hydration (n). With FeC13 - nH20 as an oxidizing agent in various solvents under Nz, anhydrous FeC13 yielded the addition polymerization product similar to that prepared with AlCls. FeC13 - 2H20 and FeCls - 2.5H20 gave PTh powders with rather high electrical conductivities (Table 9).201It should be emphasized that the solvents used were carefully dried and distilled. Ethanol as a stabilizer in CHCls was removed before drying since water and ethanol result in a decrease of Lewis acidity of anhydrous FeCls, and an increase in the electrical conductivity of the PTh prepared. FeC13 .6H2O does not yield polymer because its Lewis acidity is very weak. When anhydrous FeC13 was used

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Table 9. Chemical oxidative polymerization Iron (III) chloride FeCl, FeCls FeCl, FeC& FeCls FeCls FeCls FeCls FeCls Solvent CHCls CH3N02 CHsCN CHCls CH3N02 CHsCN CHC13 CHCls CHsCN 3 h.

of thiophene using FeC& - nH20.*201 Yield (%)t 99 348 21 84 17 trace 64 0 0 Conductivity 4.3 3.3 29 8.2 2.8 (S cm-)

x 1o-4 x 1o-4

. 2H20 . 2H20 - 2H20 - 2.5H20 - 6H2O - 6H2O

x 1O-2 x 10-l 2.4 x 1O-2 _ -

* FeCls : thiophene = 1.3, room temperature; t Based on the charged FeCls.

acetonitrile, the co-ordination of acetonitrile to FeCls weakened the Lewis acidity of the iron salt, affording PTh with altered conductivity. These results suggest that FeCls - nHzO (n = 2 or 2.5) is suitable for the oxidative polymerization of thiophene. Table 10 surveys results for the oxidative polymerization of thiophene using the FeCls-HZ0 system, where the prescribed molar ratio of H20/FeC13 was prepared in CHCls instead of using FeCls - nH20 salts.201 Both the electrical conductivity and IR spectra (Fig. 5) of the obtained polymers were practically identical with similar data for those prepared using FeCls - nH20 salts, suggesting that the addition of Hz0 in CHCls with anhydrous FeCls produced the same species as that obtained by dissolving in FeCls - nH20. As shown in Fig. 5, the peak intensity at 700 cm- decreases and that at 790 cm- increases with increases in the degree of hydration, suggesting that an aromatization reaction involving the thiophene rings proceeded. For the range ofwith Table 10. Chemical oxidative polymerization of thiophene using FeCls-H20 system*201 Molar ratio H20/FeC1s 0.3 1.3 28 2.3 2.8 3.3 4.3 5.3 611

Yield (%)t 99 64 :& 54 l& 0 0

Conductivity (S cm-) 4.3 1.7 8.2 5.2 5.5 6.6 7.3 x 1o-4 x 1o-2 x 1O-2 x 1o-2 x 10-l x 10-l x 1o-2 _

* FeCls : thiophene = 1: 3; reaction was carried out in CHC13 at room temperature for 3 h. t Based on the charged FeC13. $ FeCls - 2H20 salt was used. $j24h. 11 FeCls - 6H2O salt was used.

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N. TOSHIMA and S. HARA

900

0

ikm

Fig. 5. The IR spectra for PTh powders prepared using the FeCl,-H20-O2 system. H20/ FeC13 = (a) 0.3, (b) 1.3, (c) 2.3, (d) 2.8, (e) 3.3 and (f) 4.3.*01

H20/FeC13 from 2.8 to 4.3, Lewis acidity was suitable for effecting aromatization by oxidation. The highest conductivity was obtained when the ratio of H20/FeC13 was set at 2.5-3.5. Morphology of the PTh powders also depended on this ratio, since PTh prepared at a ratio of 2.8 had porous morphology. Therefore, the H20/FeC13 ratio influenced Lewis acidity, which resulted in changes in the conductivity and the morphology of the polymers obtained with this system. The electrochemical polymerization of thiophene was reported in detail in a recent review. The electrochemical synthesis can afford extensively conjugated and

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conductive PTh films, compared with PTh powders prepared chemically by using oxidizing agents or catalysts described above, and by Grignard coupling of 2,5dihalothiophene derivatives.70t202-206Ev en though a large variety of work on electrochemically synthesized PTh films has been carried out so far,5 interest in PTh films, especially poly(3-alkylthiophenes),207-20g remains. Onoda et al. explored the electrochemical polymerization of 3-n-hexylthiophene.207 As the top of the valence band of poly(3-n-hexylthiophene) (PHTh) is located at a higher energy state than that of PTh, p-type doping (BFI) in PHTh was relatively effective compared with that in PTh; whereas n-type doping (Bu4N+) in PHTh was more difficult than for PTh, since the bottom of the conduction band of PHTh is located at a higher energy state than that of PTh. Poly(3-n-pentylthiophene) (PPTh) and PHTh prepared electrochemically possessed bimodal molecular weight distributions.208 A small amount of additives such as 2,2-bithiophene and 2,2 : 52-terthiophene significantly improved the structural regularity of the polymer chains.208 Electrochemical polymerization of thiophene and 3-alkylthiophenes was carried out on an IT0 (indium-tin oxide) electrode at a relatively high applied potential of 8 V vs Ag/AgC1.2W Although the polymer films obtained had all the properties, such as thermo- and solvatochromism, conductivity, and electronic band structure of PTh and its derivatives, these films possessed irregular structures, due probably to crosslinking and branching in the polymer.5. POLYANILINE

Polyaniline @An), which has been used as a heat resistant paint (aniline black), has recently been applied to batteries, displays and anti-charging materials.45 Both the chemical and the electrochemical oxidation processes for PAn have been investigated from fundamental and practical points of view.617>210121 Table 11 shows various chemical oxidative polymerization procedures for aniline 173,21 l-226 where aniline salts such as anilinium chloride were oxidized by various oxidizing agents. The PAn powders - prepared chemically in HCl (2 M) using (IW4)2S2Og, ICI03 and &Cr207 as oxidizing agents - showed that the maximumTable 11. Chemical oxidative polymerization Monomer Aniline Aniline Aniline Aniline Aniline Aniline Aniline o-Methylaniline Alkylaniline N-Methylaniline N-Alkyldiphenylamine Oxidizing agent of aniline and its derivatives Solvent Ref. 211-215 216,217 218 219 220 221,222 175 223 224 225 226

NW$WEFe&, CuC12 Cu(BF,),c@o4)2

HWq)H20

CHsCN/HBF4 CHsCN HCl, H2SWaq) CHsCN various acids HCKaq) HC104 CHsCN

FeOClK&r207 p2w&; m4)2 (NH4)2S20s (m4)2s208 WBF4)2 s208

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conductivity was around 20 Scm- and that electroactivity was very similar to that for PAn prepared electrochemically, indicating that the type of oxidizing agent influenced neither the conductivity nor the electroactivity of these materials.227 Abe et al. reported soluble, high molecular weight PAn species prepared by chemical oxidative polymerization at low temperature (-4C) using (NH4)&0s in acidic aqueous solutions.*14 After undoping by aqueous ammonia, the M, and the M, of the PAn were found to be 1.6 x lo5 and 2.3 x 104, respectively. The solubility of PAn prepared using (NH4)2SzOs was also determined in various solvents.228 PAn with a A4, of 3.7 x lo3 was obtained (yield: 17.7%) using FeC13 in aqueous HCl at 35C.*16In contrast to the preparation of PAn using (NH4)&08 at -4C,214 a reaction did not occur with FeC13 even at 0C. The molecular weight distribution of PAn prepared at various temperatures using FeC13 was bimodal. The number average molecular weights for the higher and the lower molecular weight fractions of PAn prepared at an optimum temperature of 35C were around lo4 and 103, respectively. The lower molecular weight fraction could be obtained by the dimerization of the cation radical of aniline,216v22%231 whereas the higher molecular weight species was prepared by an incorporation-oxidation mechanism, where monomeric aniline is incorporated into an oxidized aniline oligomer and chain growth takes place by the insertion of the neutral monomer at the end of the oligomer.2161232T233 Note that the lower the HCl concentration, the higher the molecular weight and yield. At 0 mol dmm3, the M, and the yield were 2.04 x lo4 and 51%, respectively.*16 When PAn was synthesized using FeC13 in aqueous HCl at 35C for 1 day under air and N2, the M, obtained under air (3700) was higher than that under N2 (750), suggesting that the oxidation of aniline is promoted by O2 present in air.*16 Gospodinova et al. synthesized PAn using (NH4)2S@g in an aqueous dispersion stabilized by poly(viny1

Table 12. Polymerization

of aniline with oxygen using copper salts as catalysts*236 Yield (%)

Cu salt Cu(BF& Cu(BF& CU(CH3COO)2cu(No3)2

Isolated :t: 14 16 5 34 22 26 20@

Calc. from O2 consumed 17 120 17 18 11 33 30 29 23 0

TOFt (% day-) 240 316 280 320 100 680 440 520 400 0

GUI CuBr2 CuBr cuc12 CuClcuso4

* Reaction of 50 mm01 of aniline with O2 and 5 mm01 of Cu salt in 20 cme3 of acetonitrile/water (l/ 1 v/v) at 30C for 24 h. t Turnover frequency of the Cu salt catalyst for the isolated yield of the polymer per day. $ Reaction for 73 h. $ White precipitates were obtained instead of PAn.

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Table 13. Solvent dependence on the polymerization of aniline with oxygen using copper salts as catalysts*236 Ratio of CH3CN/H20 l:o 3:l 1:l 1:3 0:l _ _ _

Solvent Acetonitrile/water Acetonitrile/water Acetonitrile/water Acetonitrile/water Acetonitrile/water Ethanol Methanol Benzonitrile

Yield (g) 0.4 0.6 1.3 0.7 0.9 0.4 0.8 0.2

* Aniline, 50 mmol; CuC12, 5 mmol; solvent, 20 cm3; 30C for 24 h.

alcohol-co-vinylacetate) without adding acids.234 PAn was obtained only when the initial stage of the polymerization resulted in a drop in pH lower than 2. Compared with these methods, the catalytic chemical polymerization of aniline has been investigated only very recently. Although the CuCl-A1C13-O2 system did not work for the polymerization of aniline, CuC12 or CuBrz has been found to be an effective mediator under O2 without acids. In this case, only neutral aniline gives good results when compared with anilinium chloride. Table 12 shows the chemical oxidative polymerization of aniline using various copper salts-O2 systems.235T236 The catalytic activity of CuCl* depends on the solvents used for the reaction as shown in Table 13. The acetonitrile-HZ0 (1 : 1) solvent gave the highest yield, which was more than three times higher than that given by acetonitrile. However, the isolated polymer presumably contains a partially disordered structure as shown in Fig. 6 (8) since aniline monomer was eliminated at 284C (exothermic reaction) yielding a more stable material. The relatively low electrical conductivity observed was probably due to the disordered structure. Based on UV-VIS spectra for the reaction solution at various molar ratios of aniline to Cu(I1) ion, a possible mechanism is provided in Fig. 7. Here, aniline forms a complex with Cu(I1) in the initial stages of the polymerization. Cu(I1) accepts one electron from aniline in the complex, yielding the radical cation of aniline. The radical cation forms a dimer followed by a subsequent oxidation-step by Cu(I1). The resulting Cu(1) is then reoxidized to Cu(I1) by OZ. The succeeding coupling reaction includes some formation of branch structure. This process might also be used for the polymerization of aniline derivatives such as 2,6-xylidine. Poly(2,6-xylidine)

Fig. 6. A plausible scheme for the thermal degradation catalyst.236

of PAn prepared using CuC12 as a

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o-

P 0 /

.. NH2 ---Cu(II)

NH1 + Cu(II)

CutI) +

o-

iH

t t

t

4Branched polyanillneof aniline catalyzed by CU(II).~~~

Normal polyanilineFig. 7. A possible mechanism of polymerization

1

3

synthesized by this method was practically identical with that obtained by electropolymerization. 3,5Xylidine cannot be polymerized by this method (CuClz-02) because of copper complex formation. When a stronger oxidizing agent - Ce(SO& - was applied to the reaction instead of CuC12-02, a polymer (yield: 46%) was obtained at SOC, 8 h. This strong oxidizing agent was also applicable for the 2,6-, 2,5-, and 2,3-xylidine monomers. The chemical polymerization of aniline using FeS04 in acidic aqueous H202 can be achieved under mild conditions.237 The isolated powdery emeraldine base, a form of PAn, was blue-black and exhibited high solubility in organic solvents. The M, of the polymer was 1.3-l .7 x lo4 and the molecular weight distribution was relatively sharp (M,/Mn = 1.6-2.2). The similar method using Fe(II1) also provided the polymer

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177

blend colloids containing PAn or PPy by oxidative polymerization,238 as well as the emeraldine base polymer by oxidation of leucoemeraldine, the reductive form of polyaniline.23g~240 The electrochemical polymerization of aniline provides films exhibiting a welldefined fibrillar morphology in some cases. This has not been reported for PPy and PTh. This morphology strongly depends on preparation conditions such as the type of electrolyte, the electrode and the electrochemical procedures used.* PAn prepared electrochemically and chemically, like other conducting polymers, exhibits electrochromic behavior. The approximate composition corresponding to a given color was studied in detail.* The thickness of PAn films prepared electrochemically, measured by spectroscopic ellipsometry, increased linearly with the anodic charge density of the fihn~.*~~ The concentration of electroactive moieties, as determined by cyclic voltammetry, was found to be constant over the range of film thicknesses studied. Rutherford backscattering spectroscopy analysis showed a uniform distribution of chloride species, electroactive sites, within the PAn film. The thicker films possessed more chloride than thinner films, suggesting some HCl entrapment within the thicker films. Aniline derivatives such as 4-aminobiphenyl, diphenylamine and N-phenyl- lnaphthylamine can also be electropolymerized in acidic and organic media.242>243 The first two monomers afford poly(4-aminobiphenyl) and poly(diphenylamine) (polyDPA), respectively, with similar cyclic voltammograms and IR spectra. The third monomer gave poly(l\r-phenyl-1-naphthylamine) (polyPNA) exhibiting a lower conductivity (10e3 S cm-) and a lower doping level (30%) than those for polyDPA (2 Scm- and 53%, respectively). Both polyDPA and polyPNA showed very interesting electrochromism.6. MISCELLANEOUS

Polyfuran, 17t244-246 polyisothianaphthene247-24g and polyazulene250-252 can be obtained in a similar manner to that described above, but their properties have not been extensively investigated. Although other conducting polymers are also worth noting, we do not describe them here because we have concentrated only on synthetic methods for conducting polymers that are prepared from simple monomers.7. CONCLUSIONS AND PERSPECTIVES

A number of conducting polymers together with new synthetic procedures have been prepared and others can be expected in the future. Chemical polymerization processes are desirable from the viewpoint of mass production but we also need to restrict by-products and waste materials in order to keep our global environment clean and safe. Catalytic polymerization described in this review is a promising candidate for an environment-friendly chemical process. Catalytic activities and the properties of the obtained polymers, however, are not sufficient at the present time. It is therefore necessary to devise newer synthetic methods for conducting polymers based on highly

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efficient catalytic systems. Looking back at the development of polyethylene (PE) and polypropylene (PP), research and development on catalysts has improved both the reaction efficiencies and the properties of these polymers. For the case of conducting polymers, especially PPP, PPy, PTh and PAn cited in this review, it will be necessary to undertake additional studies on catalytic systems. The authors hope that this review will stimulate further development of conducting polymers along with novel catalytic systems for their preparation.

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