Thiophene electrochemical polymerization

The group of Martin investigated the use of poly(3,4-ethylenedio3gr-thiophene) electrochemically polymerized onto electrospun poly(L-lactide) deposited on a neural microelectrode. In a series of investigations, it vwis found that poly(pyrrole) generally lowered the impedance of the electrode less than poly(3,4-ethylenedio g4 hiophene). ... 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Wei Y, Chan CC, Tian J, Jang GW, Hsueh KF (1991) Electrochemical polymerization of thiophenes in the presence of bithiophene or terthiophene kinetics and mechanisms of polymerization. Chem Mater 3 888-897... 

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

The electrochemical polymerization of thiophene is apparently rather similar to that of pyrrole and studies have been reported by Tourillon and Gamier133) and by Kaneto et al.134,135). Early studies are reviewed by Tourillon136). The oxidation potential of the monomer is significantly higher (1.6 V v SCE) than that of pyrrole and it might be expected that the more reactive cations would lead to greater structural irregularity in the polymer, which appears to be the case. 

The electrochemical polymerization of pyrrole or thiophene readily lends itself to formation of composites. Polypyrrole-acetylene laminates have been made by using polyacetylene as an electrode 295). The polypyrrole forms as a 5 pm skin on the polyacetylene. If the polyacetylene is first doped, the polypyrrole completely permeates the film. In both cases the conductivity of the composite reached 30-40 S cm-1 and was much less sensitive than that of pure polyacetylene to exposure to moist air or water, so that the polypyrrole protects the polyacetylene even in the case where it permeates the film. In this latter case, treatment with ammonia caused the conductivity to drop by 30 x whereas for the sandwich films the conductivity dropped by 4600 x through the film but only 17 x in the surface layers. 

Polythiophene lends itself to the same routes to composites. A poly(3-methyl-thiophene)-poly(methylmethacrylate) composite has been made by electrochemical polymerization from a solution of thiophene and PMMA in methylene chloride and nitrobenzene. At high current densities the electrode side quickly became highly conducting while the outer side was less so 307). Similar composites have been prepared by chemical routes, using a Grignard reaction, firstly to couple the thiophene units in a step-reaction, then to initiate the polymerization of the methyl methacrylate 315). 

As outlined earlier, three methods of polymerization have been established for the preparation of thiophenes, viz. electrochemical polymerization [189, 190], oxidative chemical polymerization using Lewis acid catalysts such as FeCl3 [191,192], and step-growth condensation polymerization using transition metal-catalyzed coupling reactions [lj]. 

Comparable to thiophene, pyrrole is a five-membered heterocycle, yet the ring nitrogen results in a molecule with distinctly different behavior and a far greater tendency to polymerize oxidatively. The first report of the synthesis of polypyrrole (PPy) 62 that alluded to its electrically conductive nature was published in 1968 [263]. This early material was obtained via electrochemical polymerization and was carried out in 0.1 N sulfuric acid to produce a black film. Since then, a number of improvements, which have resulted from in-depth solvent and electrolyte studies, have made the electrochemical synthesis of PPy the most widely employed method [264-266]. The properties of electrosynthesized PPy are quite sensitive to the electrochemical environment in which it is obtained. The use of various electrolytes yield materials with pronounced differences in conductivity, film morphology, and overall performance [267-270]. Furthermore, the water solubility of pyrrole allows aqueous electrochemistry [271], which is of prime importance for biological applications [272]. 

Recently it has been demonstrated by Guittard et al. that the electrochemical polymerization of semifluorinated thiophenes (226) [407], 3,4-ethylenediox-ythiophenes (227) [408], fluorenes [408], and 3,4-alkylenedioxypyrroles (230) [409] allowed the deposition of semiconducting polymer films with excellent antiwetting properties (superhydrophobic and lipophobic, see Fig. 73). Additional aromatics inserted between fluorinated tail and polymer chain (compounds 228, 231) improve mesogenity and in this way decrease the mobility of the Rp-chains, preorganize the molecules, and thus improve the antiwetting properties [388, 410]. 

Guglielmetti20 reported the synthesis of spironaphthoxazines (compound 24) containing one thiophene entity which are precursors for the preparation of new molecular materials by electrochemical polymerization or copolymerization. 

Another mechanistic possibility is the attack of the thiophene cation radical (420) upon a neutral thiophene monomer (419) to form a cation-radical dimer (421) [247]. The oxidation and loss of two protons leads to formation of the neutral dimer (422). Once again, rapid oxidation of the dimer occurs upon its formation due to its close proximity to the electrode surface and its lower oxidation potential. The cation-radical dimer (423) which is formed then reacts with another monomer molecule in a similar series of steps to produce the trimer 425. A kinetic study of the electrochemical polymerization of thiophene and 3-alkylthiophenes led to the proposal of this mechanism (Fig. 61) [247]. The rate-determining step in this series of reactions is the oxidation of the thiophene monomer. The reaction is first order in monomer concentration. The addition of small amounts of 2,2 -bithiophene or 2,2 5, 2"-terthiophene to the reaction resulted in a significant increase in the rate of polymerization and in a lowering of the applied potential necessary for the polymerization reaction. In this case the reaction was 0.5 order in the concentration of the additive. 

With a view to preparing polymerizable complexes, thiophene-substituted nickel-dithiolene complexes [Ni(L)(L )] have been synthesized and used to prepare films by electrochemical polymerization. The features of the complexes and of the polymers depend on the number of thiophene substituents. In particular, the complex with four thiophene substituents (L = L = thpdt, 12) shows a narrower HOMO-LUMO gap as compared to complexes with two thiophene and two phenyl groups or four phenyl groups [Xmax, nm (e, M- cm- ) 976 (38800) L = L = 12 931 (37700) L = 12, L = 9, R = Ph 866 (30900) L = L = 9, R = Ph] and gave a polymer whose electrochemical features are similar to those of poly[l,2-di(2,5-thienylene)ethane], suggesting that similar extended chains are formed"". 

Polymers in this category are of interest in that they can be easily synthesized, being of very high stability. A couple of these polymers are illustrated in Fig. 16. Poly(p-phenylene) is prepared by the polymerization of benzene with A1C13 and CuCl2 (Kovacic and Kyriakis, 1963 Kovacic and Oziomek, 1964), whereas polypyrrole and polythienylene are prepared by the electrochemical polymerization of each monomer with an appropriate electrolyte (Diaz et al., 1979 Tourillon and Gamier, 1982). Polythienylene is also synthesized by the polycondensation of dihalo-thiophene (Yamamoto et al., 1980). 

The him morphology of electrochemically prepared polythiophene has been shown in numerous studies to be almost identical to that commonly observed for polypyrrole (described in Chapter 2). A nodular surface is observed for both unsubstituted and 3-alkyl substituted thiophenes.92 As with PPy, the electrochemical preparation of PTh at higher current densities produced rougher surface morphologies. The similarity in morphologies suggest a similar growth mechanism for electrochemically polymerized PPy and PTh. 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

In electrochemical polymerization a solution of monomer is oxidized or reduced at the electrode surface to generate reactive radical species which couple together and produce an adherent polymer film at the electrode. The films can be electronically conducting, as in the case of pyrroles, anilines, thiophenes, etc redox conductors in which conduction occurs by self-exchange between discrete redox sites attached to the polymer, as in metal poly(pyridine) complexes or insulating, as in the case of phenols, 1, 2-diaminobenzene, etc. 

Poly thiophene, PTP, and polypyrrole, PPR, blends with PS and PC were prepared by Wang et al. [1990] by thiophene or pyrrole electrochemical polymerization using electrodes coated with PS or PC hlms. The thiophene or pyrrole diffuses into the fihn and polymerizes in-situ in the film. Threshold conductivity occurs at 18 wt% for both conducting polymers in PS. Lower levels exist for PTP (12 wt%) and PPR (7 wt%) in PC. Miscibility of PPR/PC is attributed to the lower threshold limit as phase separated blends would be expected to have higher values. Previous studies with polyacetylene/PS blends reported threshold conductivity at 16 wt% polyacetylene [Aldissi and Bishop, 1985]. 

The heterocycles (13) and (14a) (Table 2) find much application in electrochemical polymerization to prepare electroconductive polymers <89TL1655>. The conductive complexes (239), named 2,6-bis(dicyanomethylene)-2,6-dihydrodithieno[3,2-h 2, 3 -d]thiophene, have been described as potential electron acceptors <89BCJ1547>. 

Electrochemical polymerization is a swift way of obtaining polymeric films of the thiophenes. The surface smoothness of the films depends on the electrode materials used for example smooth polymer... 

From X-ray diffraction, it is known that semicrystalline polythiophene powder consists of completely co-planar molecules [64], in contrast to the oligomers with chain length of and above three. The crystallinity of powders of chemically coupled polythiophene prepared by monomer oxidation with iodine, increases from 35% as synthesized up to 56% after annealing at 753 K for 30 minutes [65]. At the same time the residual iodine content decreased from 3.17% as synthesized to 0.13% after the heat treatment. Whereas annealing at 753 K leads to a first degradation of the polymer, heat treatment at 673 K results in polythiophene with chains of approximately 1200 thiophene units. Electrochemically polymerized polythiophene gives a completely different X-ray diffraction pattern [66],... 

Surprisingly, the electrooxidation of metal complexes of protoporphyrin-IX dimethyl ester, possibly via the vinyl groups, leads to the deposition of electroactive porphyrin films on the electrode surface [107-109], The electrochemical polymerization of pyrrole, thiophene and amine metal-substituted complexes is described in more detail in Section 6.3. 

A linear phthalocyanine containing polyesters 46 has been described [128,129], Recently, a linear polymeric phthalocyanine 47 prepared in a multistep synthesis with an iptycene architecture was briefly described [130]. At first two differently substituted l,3-dihydro-l,3-diiminoisoindolenines were statistically reacted. In the mixture of phthalocyanines obtained, the ethylenedioxythiophene part is introduced and the bis(ethyleneoxythiophene) oppositely substituted phthalocyanine was isolated by flash chromatography. Afterwards the thiophene parts in the phthalocyanine were electrochemically polymerized to obtain 47. 

Completely different monomers were called for. Before long, three of today s workhorses had been identified pyrrole, aniline and thiophene. In Japan, Yamamoto [38] and in Germany, Kossmehl [39] synthesized polythiophene doped with pentafluoroarsenate. At the same time, the possibilities of electrochemical polymerization were recognized. At the IBM Lab in San Jose, Diaz used oxidative electrochemical polymerization to prepare polypyrrole [40] and polyaniline. [41] Electrochemical synthesis forms the polymer in its doped state, with the counter-ion (usually an anion) incorporated from the electrolyte. This mechanism permits the selection of a wider range of anions, including those which are not amenable to vapor-phase processes, such as perchlorate and tetra-fluoroborate. Electrochemical doping also overcomes an issue associated with dopants... 

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