Electrochemical polymerization formation

Tafel plots, during electrode polymerization, 316 Technology of electrochemical polymer formation, 427 Temperature coefficient and the interfacial parameter, 183 and the potential of zero charge, 182 of potential of zero charge as a function of crystal phase, 87... 

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. 

This research rests upon oxidative electrochemical polymerization (ECP) of solutions of the functionalized metallotetraphenylporphyrln monomers shown in Fig. 1. These oxidations lead to formation of thin, cross-linked polymeric films of the metallotetraphenylporphyrins on the electrode surface. The films contain from ca. 4 to 500 monolayer-equivalents of porphyrin sites, which are In high concentration (ca. 1M) since the polymer backbone consists solely of the porphyrins themselves as the backbone units. The polymeric films adhere to the electrode and... 

Due to the rapid development of material science in recent years, much interest has been focused on the investigation of silicon-silicon bond formation. We discovered the electrochemical polymerization of organohalosilanes in the mid seventies as a possible alternative to alkali metal reduction [1,2]. Since then, several papers have been published on this subject [3,4,5 and ref. therein]. 

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. 

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 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. 

In situ polymerization, and electrochemical polymerization in particular [22], is an elegant procedure to form an ultra thin MIP film directly on the transducer surface. Electrochemical polymerization involves redox monomers that can be polymerized under galvanostatic, potentiostatic or potentiodynamic conditions that allow control of the properties of the MIP film being prepared. That is, the polymer thickness and its porosity can easily be adjusted with the amount of charge transferred as well as by selection of solvent and counter ions of suitable sizes, respectively. Except for template removal, this polymerization does not require any further film treatment and, in fact, the film can be applied directly. Formation of an ultrathin film of MIP is one of the attractive ways of chemosensor fabrication that avoids introduction of an excessive diffusion barrier for the analyte, thus improving chemosensor performance. This type of MIP is used to fabricate not only electrochemical [114] but also optical [59] and PZ [28] chemosensors. 

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

The success of the polymerization depends on the solvent used for the process. Most studies of electropolymerized MPc have concentrated on the electrochemical polymerization of MPc(NH2)4 complexes [89-93], The polymerization process of these complexes involves the oxidation of the amino group forming radicals which attack phenyl rings of neighboring molecules [93], The formation of the polymers of (OH)MnPc(NH2)4 and OTiPc(NH2)4 on glassy carbon electrode (GCE) was successfully achieved via electropolymerisation of these complexes in DMF by repetitive scanning at a constant scan rate of 0.1 Vs-1. Simple adsorption of the monomer onto carbon electrodes (using MnPc derivatives) has been reported [94],... 

More recent efforts focused on surface modification of conductive polymers by the SECM, fabrication, and characterization of microstructures. Mandler et al. developed an approach for the formation of a 2D conducting polymer on top of an insulating layer. This approach, based on electrostatically binding a monomer (anilinium ions) to a negatively charged self-assembled monolayer of co-mercaptodecanesulfonate [MDS, HS(CH2)ioS03 ] followed by its electrochemical polymerization. The polyanion monolayer exhibited the properties similar to those of a thin polymer film [167]. 

More detailed studies were devoted to the mechanism of electrochemical polymerization applied to Fe(II), Ru(II) and Os(II) complexes containing 2,2 -dipyridyl and monofunctional monomer (L) such as 4-VP, bis(4-pyridyl)ethylene, trans-4-stilbazole or N-(4-pyridyl)acrylamide [86], The first stage of electrochemical polymerization is shown to be the formation of a radical-anion, e. g. by the following scheme... 

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. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru (bpy)(CO)2Cl]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 

Conductive polymers may be synthesized via either chemical or electrochemical polymerization methods. Electrodeposition of conductive polymers from electrolytes is, thus, feasible provided that the depositing polymer is not soluble in the electrolyte.206 Conductive polymers can be deposited from the electrolytes containing the monomers via either electrooxidation or electroreduction, based on the monomer type used. Similar to that of metals, the electrodeposition of polymers is based on nucleation and growth. The deposition mechanism involves oxidation of monomers adsorbed on the electrode surface, diffusion of the oxidized monomers and oligomerization, formation of clusters, and eventually film growth.213... 

As for PPy s, there has been an explosion of interest in the synthesis of PAn s with nanodimensions, as such materials have been shown to have enhanced electronic and electrochemical properties. Formation of PAn nanoparticles has been achieved via polymerization in micelles, using either sodium dodecyl sulfate (SDS)211 or DBSA212 214 as the surfactant stabilizer. Particle sizes in the range of 10-30 nm with conductivities as high as 24 S cm-1 have been reported. 

One of the interesting things about the redox polymers is their use in the creation of the molecular electronic devices.3-5 Redox polymer films on electrodes have been fabricated using chemical modification, electrochemical polymerization, polymer coating, and so on.88 Recently, stepwise complexation methods have been employed to fabricate multiple complex layers.89,90,91 In this section, the stepwise preparation of bis(tpy)metal polymer chains by combining terpyridine (tpy) ligand self-assembled monolayer (SAM) formation and metal-tpy coordination reactions is described as an example. This method realized the formation of a desired number of polymer units and a desired sequence of Co-Fe heterometal structures in the polymer chain.92... 

In addition to electrochemical polymerization, reactive monomers can be polymerized onto surfaces by using radio frequency (rf) plasma polymerization [194-197]. In this technique an electric discharge through the vapor forms a reactive plasma that chemically modifies the surface. Examples of applications of rf plasma-polymerized surfaces include the formation of (C2F4) films on fiber optic sensors for detection of volatile organics [198] and the formation of alkylamine surfaces on glass fibers by plasma treatment for subsequent chemical modification [199]. 

Poly(dimethylsilane) can be synthesized by electrochemical reduction of dimethyldichlorosilane in 30% yield [87] and poly(methylphenylsilane) has also been synthesized by reduction of PhMeSiCl2 [88]. In this method the problem is the oxidation of the anode by elemental chlorine to form metal chlorides, or the formation of corrosive hydrogen chloride [89]. Therefore the electrochemical polymerization of hydrosilanes like phenylmethylsilane, phenylsilane, or hexylsilane has also been investigated. Relatively low molecular weight polymers could be obtained in 32-70% yield [90] ... 

Electrochemical polymerization of Cgo derivatives is possible. Starting from a dialkynylated methanofullerene, Diederich et al. observed formation of... 

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