Electrochemically polymerized polythiophene

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],... 

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. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Quinaxoline-containing monomer 14 was electrochemically polymerized to yield a polythiophene that changed from a yellow to orange color upon the addition of fluoride or pyrophosphate anions (Aldakov and Anzenbacher 2004). Analyte binding could be detected spectroscopically and electrochemically. 

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. 

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). 

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

It is well Renown that organic conducting polymers such as polypyrrole, polythiophene, and polyaniline can be deposited on electrodes by means of electrochemical polymerization, which is successfully carried out through oxidation of monomers in the solution (14). 

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 spin dynamics of ESR was studied in polythiophene doped with C10, prepared by electrochemical polymerization at 300 K [296]. Heavily doped PT is known to show a metallic temperature dependence on ESR linewidth caused by the Elliott mechanism, characteristic of metals, as will be mentioned in section 7 [254,258,287,295,296]. In addition to this, a line broadening due to the spin dynamics is expected. Mizoguchi et al. reported the frequency dependence of the ESR linewidth in PT-CIO4 as shown in Figure 6.51 [296]. The filled circles do not behave simply following the prediction of (6.20) and (6.21) of Q-l-D spin motion, but there is a broadening mechanism other than... 

Polythiophenes usually form disordered layers, in particular if electrochemical deposition is used [55-58], Preparation of plasma-polymerized polythiophene leads to disordered layers with comparable structure to elcctrochemically prepared films [59]. However, if the substrate is put near to the RF source where the concentration of electrons and excited Argon species is high, the concentration of thiophene monomers is low, and the fragmentation probability of the monomer is high, platelet structures with clear grain boundaries could be observed. But these platelets are likely to consist of products other than polythiophene due to the above-mentioned preparation conditions,... 

Cao et al. [133] studied the air stability of re-doped polythiophene which was prepared electrochemically and then compensated by aqueous ammonia as detailed in Table 16.8. Ammonia-compensated polythiophene was found to be quite stable when stored in an ambient atmosphere for 3 months, as neither any weight gain nor any change in infra-red spectmm was observed. Both the chemically re-doped and electrochemically prepared polythiophene showed much better stability as compared to polyacetylene and the air stability of the polymer was found to be dependent on the doping counter-ion as well as the solvent used in electrochemical polymerization. Electrochemically prepared polythiophene from a mixture of CH3CN and CH3NO2 (1 1 by volume) maintained their electrical conductivity, whereas the polymer re-doped chemically by FeCls was observed to be most stable in ambient air. 

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... 

Even though PPy may be used in many application fields, its poor processibility, mechanical, and physical properties have been a large obstacle. To improve the processibility, mechanical and other properties, various kinds of PPy copolymers have been polymerized with many conventional or conducting polymers. PPy was copolymerized electrochemically with polythiophene to improve its disadvantageous properties such as sensitivity to oxygen [47], leading to copolymer films with much less porosity than PPy film (Figure 8.11). 

Early progress in polythiophene chemistry was achieved by the synthesis of mono- and dialkoxy-substituted thiophene derivatives developed by Leclerc [6] and industrial scientists at Hoechst AG [7-9]. However, most polymers of mono- and dialkoxythiophenes exhibited low conductivity in the oxidized, doped state. A breakthrough in this area was the synthesis of polymers of the bicyclic 3,4-ethylenedioxythiophene (EDT or EDOT) and its derivatives—electrochemically polymerized by Heinze et al. and chemically polymerized by Jonas et al. of the Bayer Corporate Research Laboratories [10,11]. In contrast to the nonbicyclic polymers of mono- and dialkoxythiophenes, PEDT has a very stable and highly conductive cationic doped state. The low HOMO-LUMO bandgap of conductive PEDT allowed the formation of a tremendously stable, highly conductive ICP [12]. Technical use and commercialization quickly followed today ICPs based on PEDT are commercially available in multiton quantities. 

The most elegant approach to design polypyrrole, polyaniline or polythiophene-based porphyrin, phthalocyanine or Schiff base matrices involves the electrochemical polymerization of suitably designed substituted N4-macrocyclic monomers. We and others have shown that the electro-oxidative polymerization of such species (see significant examples in Figure 8.3) leads to the formation of films having the electrochemical properties of the monomeric complex" . ... 

In general, electrochemically prepared conducting polymers (e.g., polypyrrole, polythiophene, and polyaniline, etc.) imdergo a. similar polymerization pathway. Figure 13.5 describes the electrochemical polymerization pathway for pyrrole [26,27]. 

Electrochemical polymerization of 3-alkylthiophene has also been studied [60, 67, 68]. The doping potential of poly(3-methylthiophene) is about 0.8 V (vs Ag/AgCl), which is a little lower than that of polythiophene. 

Electrochemical polymerization is a very popular method to prepare conducting heterocyclic polymers. As an example of the mechanism, the electropolymerization of polythiophene starting from the monomer can be schematically represents as follows ... 

Shi and co-workers prepared polythiophene films with different roughnesses by electrochemical polymerization of thiophene in boron trifluoride-diethyl etherate (BFEE) [42]. The highest WCA of 116° on polythiophene film was observed. To further increase the WCA, aligned polythiophene nano-tubes were synthesized using anodized aluminium oxide (AAO) as template. The WCA increased to 134° (Fig. 7). As a comparison, the WCA of polythiophene polymerized in acetonitrile solution was measured to be less than 75°. 

Nicolas et al. also synthesized semi-fluorinated polythiophenes (Scheme 4) [52, 53]. The monomers were chemically polymerized by oxidation with FeCls, or electrochemically polymerized in acetonitrile containing BU4NPF6 as the supporting electrolyte. The electrochemically synthesized films showed rough surfaces. The poly(fluorinated thiophene) films electropolymerized from the monomer with n = 8 and m = 2 showed a WCA of 153°, while the corresponding spin-coated films exhibited a much smaller WCA, due to their smooth surfaces. Their results indicated that the length of the fluorinated chain had weak influence on the surface property of the resulting film. 

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