Polypyrrole formation

The electropolymerisation of the electrically conducting polymers thiophene (mentioned briefly aready in Chapter 5) and polypyrolle are thought to be produced by a scheme to that given in Fig. 6.22. (The scheme shows polypyrrole formation. Polythiophene is similar in that NH is replaced by S.)... 

Polypyrrole readily forms acceptable films under a wide variety of conditions [86] though there are subtle distinctions in behaviour as a result of exact preparation procedure [87]. Ultrasound at 20 kHz at sufficient intensity impedes polypyrrole formation and removes the polymer coating from the electrode [88]. At higher ultrasonic frequencies (e.g. 800 kHz) a free-standing film is produced which can be peeled from the electrode. This film has the interesting feature that the imprint of the wave-... 

Although some mechanistic details are still controversial, it has been established that the oxidative polymerization (chemically or electrochemically) of pyrrole and pyrrole derivatives proceeds via an E(CE) mechanism which involves cation-radical propagating species. The most commonly accepted mechanism of polypyrrole formation is illustrated in Fig. 57 [237,242]. The polymerization begins with the one-electron oxidation of pyrrole to produce cation radical 399. This cation radical has been... 

Figure 57 Mechanism of polypyrrole formation involving the coupling reaction of two cation radicals. (From Refs. 237 and 242.)...

Several mechanisms have been proposed for both electrochemical and chemical oxidative synthesis of poly thiophenes. The proposed mechanisms are similar to those proposed for polypyrrole formation. The first step of the polymerization is the oxidation of the thiophene monomer to a cation radical. The subsequent steps are controversial. There are several possibilities. The cation radical can couple with another cation radical or with a neutral species. Alternatively, the cation radical can deprotonate to form a neutral radical. This radical can then couple with another radical or with a neutral species. Several of these possibilities are discussed below. 

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

Funkhouser GP, Arevalo MP, Glatzhofer DT, O Rear EA. Solubilization and adsolu-bilization of pyrrole by sodium dodecyl sulfate polypyrrole formation on alumina surfaces. Langmuir 1995 11 1443-7. 

Figure 2.2 A more detailed look at polypyrrole formation.

In 1979, the formation of conductive polypyrrole films by the electrochemical oxidation of pyrrole was reported for the first time This work has stimulated intense and fruitful research in the field of organic conducting polymers. Further important conductive polymers are polythiophene, polyaniline and polyparaphenylene. The development and technological aspects of this expanding research area is covered... 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

Thus, it appears that the transition represented by the anodic peak in the cyclic voltammogram of polypyrrole is due to a changeover in the dominant carrier type and is accompanied by a dramatic contraction of the film. The authors strongly suspected that this contraction was due to electro-striction associated with bipolaron formation. As a further test they also carried out experiments intended to test if proton expulsion from the film occurred on oxidation. They found that it did indeed occur but monotonically at alt potentials > -0.6 F, in agreement with the extremely elegant work of Tsai et at. (1987), and so could not be responsible for the relatively sudden contraction at potentials > —0.2 V. 

Several groups have recently shown (36,42,43,44) that photoanode materials can be protected from pRotoano3ic corrosion by an anodically formed film of "polypyrrole".(45) The work has been extended (46) to photoanode surfaces first"Treated with reagent that covalently anchors initiation sites for the formation of polypyrrole. The result is a more adherent polypyrrole film that better protects n-type Si from photocorrosion. Unlike the material derived from polymerization of I, the anodically formed polypyrrole 1s an electronic conductor.(45) This may prove ultimately important in that the rate of ionTransport of redox polymers may prove to be too slow... 

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

Similar approach has also been taken by Ferain and Legras [133,137,138] and De Pra et al. [139] to produce nanostructured materials based on the template of the membrane with etched pores. Polycarbonate film was also of use as the base membrane of the template, and micro- and nanopores were formed by precise control of the etching procedure. Their most resent report showed the successful formation of ultrasmall pores and electrodeposited materials of which sizes were as much as 20 nm [139]. Another attractive point of these studies is the deposited materials in the etched pores. Electrochemical polymerization of conjugated polymer materials was demonstrated in these studies, and the nanowires based on polypyrrole or polyaniline were formed with a fairly cylindrical shape reflecting the side wall structure of the etched pores. Figure 10 indicates the shape of the polypyrrole microwires with their dimension changes by the limitation of the thickness of the template. 

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