Electrochemical oxidation of pyrroles

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... 

Others have shown that molecular templates, such as a polyacrylate film predeposited on a carbon electrode, can be used effectively to create PPy nanowires via electrochemical oxidation of pyrrole.190191 Electropolymerization in the presence of... 

PPy may be prepared by either chemical or electrochemical oxidation of pyrrole monomer. The most widely accepted polymerization mechanism of PPy is the coupling between radical cations (Figure 8.4)... 

Otero, T.E, and H. Grande. 1996. Termally enhanced conformational relaxation during electrochemical oxidation of pyrrole. J Electroanal Chem 414 171. 

Unfortunately, the thermodynamic conditions for the electropolymerization of monomers sudi as pyrrole or aniline are very unfavorable with common oxidizable metals such as iron and zinc. Indeed, electropolymerization of thee monomers needs a positive potential (0.7 V/ SCE for pyrrole at pH 5 and 0.8 V/ SCE for aniline in IM H2SO4) that is much higher than the metal dissolution potential - 0.7 V/ SCE for iron, - 0.96 V/ SCE for zinc at pH < 8). Consequently, it is obvious fliat attempts to deposit PPy or PAM by electrochemical oxidation of pyrrole or aniline on iron or zinc will be unsuccessful if we consider only the thermodynamic data related to the pure metals and the monomers. 

Conducting polymers are usually synthesized from the appropriate monomers by either chemical or electrochemical oxidative polymerization. Electrochemical polymerization is preferred for better penetration inside the nanopores. Polypyrrole (PPy) is one of the most important and extensively studied conducting polymers (Moreno et al. 1999 Vrkoslav et al. 2006 Lewis et al. 1997 Akundy and Iroh 2001). The deposition of PPy into PSi templates could be achieved by the electrochemical oxidation of pyrrole monomers at constant current or potential in the acetonitrile solution containing tetrabutylammonium perchlorate as supporting electrolyte. Typical potential (E-t) and current transients (i-t) recorded during the deposition of PPy into mesoporous silicon templates are shown in Fig. 3. The mechanism of polymer infiltration into the pores is of major importance in order to obtain the desired structures and control the final morphology. 

In 1979 [10] researchers at IBM in San Jose, led by Diaz, reported that polypyrrole (PP) could be obtained as a freestanding film by electrochemical oxidation of pyrrole in acetonitrile. The polymer was produced at the electrode surface and could be peeled off as a flexible, relatively dense, shiny, blue-black film. Polypyrrole had been already synthesized electrochemically from aqueous sulfuric acid solutions in 1969 by Gardini and co-workers [11], but the poor mechanical and electrical properties of the material obtained discouraged further developments of this synthetic method. 

Kras ko, V. V., Yakovleva, A. A., and Kozlova, N. V., Regularity of polypyrrole synthesis from water medium and mechanism of electrochemical oxidation of pyrrole on a surface of polypyrrole film, Electrochimiya (Russ.), 25, 1056-1062 (1989). 

Polypyrrole has even been synthesized in a molten salt through the electrochemical oxidation of pyrrole. Pickup and Osteryoung [60] reported that a 1 1 mixture of aluminum chloride and n-l-butylpyridium chloride at 40°C was the most favorable combination for producing the best films. A basic (0.8 1) or acidic (1.2 1) melt generated no films. The electrochemical redox kinetic behavior of films prepared from the 1 1 molten salt appeared to be more facile than for films prepared in acetonitrile. Otherwise, the films exhibited very similar properties. 

Polypyrrole and polythiophene, both first described in 1963 as electrically conducting materials [la], experienced a renaissance when Diaz and Street gave new attention to the electrochemical oxidation of pyrrole [21], and Gamier to the polythiphene field transistor. Polyphenylene vinylene, polyaniline, polyphenylene sulfide, polycarbazole, polyindole, polypyrene and polyene fulvene are just a few of the large number of electrically conducting polymers with specific properties and interest [22]. 

Pyrrole was first polymerized in 1916 [1,2] by the oxidation of pyrrole with H2O2 to give an amorphous powdery product known as pyrrole black. However, little further interest was shown in this material until it was electrochemically prepared in the form of continuous films. The electrochemical synthesis of polypyrrole dates to the early work of DalTOllio [3], who also obtained pyrrole blacks by electrochemical oxidation of pyrrole in aqueous sulfuric acid on a platinum electrode. In 1979 [4] electrochemical techniques to synthesize polypyrroles become a useful way to obtain highly conductive free-standing materials. Chemical and electrochemical methods of synthesis have since then been improved in order to optimize the physical and chemical properties of those materials. 

The method of incorporation of ARs into the polymer matrix has been demonstrated by the polymerisation of pyrrole [118]. In the presence of protons, radical IV causes polymerisation of pyrrole to polypyrrole with the incorporated reduced (hydroxylamine) form and oxidised (nitrosonium ion) form of ARs in the polymer matrix. In electrochemical oxidation of pyrrole in the presence of ARs, a coupled electrochemical-chemical synthesis produces polypyrrole films with incorporated nitrosonium ions. They can by reduced to ARs by partial film reduction. 

The formation of a polymer film obtained by electrochemical oxidation of pyrrole was first reported by Diaz and Kanazawa [542, 543]. At least initially, electrooxidation of various substituted pyrroles surprisingly resulted in no polymers. A relationship between the oxidation potential and the dipole moment of the pyrroles was found [544]. Nevertheless, redox active films were obtained from A7-methylpyrrole and N-phenylpyrrole [545]. Figure 58 shows the molecular formula of most pyrroles discussed in the following section. An early review of PPy was provided by Street [546]. 

Derivatives of PPy have also been investigated and some copolymers made. Films of poly-N-methylpyrrole have conductivities much lower than PPy, while film produced from mixtures of pyrrole and N-methylpyrrole have conductivities that vary with composition over five orders of magnitude. The size of any substituent group plays a part in determining the ultimate conductivity of the films, and may even inhibit film formation. For example, poly-N-phenylpyrrole and poly-N-butylpyrrole have reduced conductivities in the range of 0.1-1 S m , while with bulky substituents such as t-butyl and cyclohexyl, brown-black powdery products are obtained instead of films. As with other materials, graft copolymers of PPy have been made by the electrochemical oxidation of pyrrole in the presence of a chain that had been initially modified with a pendant pyrrole unit [43]. 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

In 1968 DairOlio et al. published the first report of analogous electrosyntheses in other systems. They had observed the formation of brittle, filmlike pyrrole black on a Pt-electrode during the anodic oxidation of pyrrole in dilute sulphuric acid. Conductivity measurements carried out on the isolated solid state materials gave a value of 8 Scm . In addition, a strong ESR signal was evidence of a high number of unpaired spins. Earlier, in 1961, H. Lund had reported — in a virtually unobtainable publication — that PPy can be produced by electrochemical polymerization. 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

The Clauson-Kaas pyrrole synthesis was adapted to a soluble polyglycerol (PG) support <060L403>. Electrochemical oxidation of furan 33 in the presence of methanol followed by hydrogenation gave 2,5-dimethoxytetrahydrofuran 34. Cyclocondensation with primary arylamines gave A-arylpyrroles 35. Removal from the PG support was then accomplished by treatment of 35 with LiOH which gave 2-pyrrolepropanoic acids 36. 

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

Anodic oxidation of l,3-diaryl-5-methyl-A2-pyrazoline-5-carboxylic acids in CH3CN-Et4NBF4 proceeded with decarboxylation to the aromatized pyrazoles in high yield.414 Similarly, electrochemical oxidation of N-acetyl-2,3-substituted A4-pyrroline-2-carboxylic acids in water-tetrahydrofuran (3 1) containing KOH forms the corresponding pyrroles (80-98%).415... 

The electrochemical process at the modified electrode can thus be separated into two steps. First is a chemical reaction between PQQ and thiol to produce the reduced form of PQQ, PQQFE. This is followed by electrochemical oxidation of PQQFE to produce PQQ, which completes the cycle. We observed an oxidation potential of PQQEI2 much lower than the 0.5 V vs Ag/AgCl reported earlier for the detection of endo- and exogenous thiols such as cysteine and glutathione17. This difference could be due to changes in the procedure (smaller amount of pyrrole used) as well as different thiol structures. 

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... 

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