Electropolymerization of pyrrole

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

The permanent inclusion of solution constituents during the electropolymerization of pyrrole was also used to prepare modified electrodes with cobalt phthalocya-nine and glucose oxidase 122423) examples. Another technique of perhaps... 

J.C. Vidal, J. Espuelas, E. Garda-Ruiz, and J.R. Castillo, Amperometric cholesterol biosensors based on the electropolymerization of pyrrole and the electrocatalytic effect of Prussian-Blue layers helped with self-assembled monolayers. Talanta 64, 655 (2004). 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

A PPy/PQQ modified GC electrode was used for amperometric detection of V-type nerve agent decomposition products. The electropolymerization of pyrrole was efficiently used for immobilization of the biocatalyst, PQQ. The introduction of CaCl2 as a supporting electrolyte during electrodeposition significantly improves the response of the sensor to DMAET and DEAET. Amperometric studies targeted to detection of DMAET and DEAET by PPy/PQQ electrode were performed at a constant potential set at 0.25 V, and the electrode characteristics such as sensitivity and the analyte detection limit were determined. 

Several studies have focused on the mode of formation and properties of polypyrrole polymers. Studies of the effects of temperature on polypyrrole conductivity have shown that the polymer formed by electropolymerization of pyrrole and camphor sulfonate as dopant at low temperature has higher conductivity and is stronger than that formed at higher temperatures. X-ray scattering shows that interlayer distance increases with increasing temperature <2002IAS155>. 

A mechanism involving the coupling of cation radicals has also been considered for the electropolymerization of benzene compounds [306,313]. This mechanism occurs by a sequence of events similar to those proposed for the electropolymerization of pyrroles. The first step is the oxidation of benzene to a cation radical (471). Two of these cation radicals combine to form a dication dimer (478). The neutral aromatic dimer (479) is formed upon loss of two protons. This dimer is then reoxidized to a cation radical (480). Chain growth is accomplished by the coupling reaction of this cation radical with other cation radicals followed by deprotonation to form aromatic structures. Polymer growth continues by this sequence of steps until precipitation from solution occurs (Fig. 72). 

An interesting composite material has been prepared by the electropolymerization of pyrrole on polyacetylene electrodes (45). Because oxidized polypyrrole is far more stable in air than polyacetylene, the resulting composite has the stability of polypyrrole. With different synthetic conditions, either a thin layer of polypyrrole was deposited over individual fibrils of the polyacetylene film or a dense polypyrrole film coated the entire film. When the dense polypyrrole film covered the entire polyacetylene film, the interior... 

Composites of polypyrrole and poly(vinyl chloride) have been prepared by several groups (64-67). Polythiophene-poly(vinyl chloride) composites have also been prepared (68). The electropolymerization of pyrrole on poly(vinyl chloride)-coated electrodes yielded composites with mechanical properties (tensile strength, percent elongation at break, percent elongation at yield) similar to poly(vinyl chloride) (65) but with a conductivity of 5-50 S/cm, which is only slightly inferior to polypyrrole (30-60 S/cm) prepared under similar conditions. In addition, the environmental stability was enhanced. Morphological studies (69) showed that the polypyrrole was not uniformly distributed in the film and had polypyrrole-rich layers next to the electrode. Similarly, poly(vinyl alcohol) (70) poly[(vinylidine chloride)-co-(trifluoroethylene)] (69) and brominated poly(vinyl carbazole) (71) have been used as the matrix polymers. The chemical polymerization of pyrrole in a poly(vinyl alcohol) matrix by ferric chloride and potassium ferricyanide also yielded conducting composites with conductivities of 10 S/cm (72-74). 

Cross-linked poly(pyrrole-g-styrene) copolymers were prepared by the electropolymerization of pyrrole in the presence of polystyrene-bound pyrrole (95). Films grown from 1 1 mixtures of pyrrole and the pyrrole-ftmc-... 

Several approaches have been undertaken to construct redox-active polymermodi-fied electrodes containing such rhodium complexes as mediators. Beley [66] and Cosnier [67] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD applied a poly-Rh(terpy-py) (terpy = terpyridine py = pyrrole) modified reticulated... 

PPy films modified by platinum catalyst particles were also considered for electrocatalytic reactions (oxygen reduction and methanol oxidation) by Hepel et al. [41], The incorporation of a PtCl anion was performed during the electropolymerization of pyrrole and monitored by the electrochemical quartz crystal microbalance (EQCM) technique, allowing us to evaluate the amount of platinum obtained after reduction of the PPy/PtCl film. 

Scheme 28. ATRP of pMMA followed by electropolymerization of pyrrole to produce the block copolymers [251]...

The counterion may be catalytic,51 in which case it will have a dramatic effect on the polymerization process. For example, Tiron 4 (shown earlier) has a catalytic effect on the electropolymerization of pyrrole, thereby enabling the process to be carried out at a more rapid rate at lower applied potentials. The ability of the counterion to ion pair with charged oligomers produced as part of the polymerization process will also have an effect. 

An optically active PPy 17 has also been synthesized by the electropolymerization of pyrrole monomer bearing a homochiral sugar covalently attached to the pyrrole nitrogen.132 This chiral polymer discriminated between (+)- and (-)- cam-phorsulfonate ions as potential anionic dopants in cyclic voltammetry studies. 

With transition metal complexes (typically rhodium compounds), it is possible to functionalize pyrrole and to obtain active electrodes for the hydrogenation of ketones. An example showing the behaviour of a film of poly(pyiTole-Rh(lll)bipyridyl) is the reduction of cyclohexanone into cyclohexanol with a chemical yield of 79% [110], This molecular electrode is also suitable for the reduction of water in hydrogen, Electrocatalytic hydrogenation of other ketones, unsaturated ketones or aldehydes, has been studied recently [174-176], These hydrogenation reactions are performed in an aqueous medium and the modified electrodes are obtained after electropolymerization of pyrrole substituted 2,2 -bipy-ridine or 1,10-phenanthroline complexes of Pd(Il) or of Rh(Ill). More precisely, with the Pd(ll) complex... 

The number of anionic transition-metal complexes that may be electrochemically incorporated into PPy is limited and depends primarily on the electrochemical stability of the species. In order to maintain its chemical integrity upon incorporation into the polymer, the complex should not undergo irreversible oxidation (at the anode) or reduction (at the cathode) during the electropolymerization of pyrrole. The transition-metal complexes shown in Figures 12.2 and 12.3 satisfy these criteria. The following consists of a review of the work performed within our laboratories at Monash University on PPy containing transition-metal complexes. 

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