Electrochemical Polymerizations

Polymerization at a planar electrode has been considered theoretically and, assuming a reversible electrode process, it has been possible to obtain an approximate solution to the full, time-dependent problem. Using this solution in terms of a transient current versus time curve, it is reported that it is possible to determine the specific rate constants for propagation and termination.  

A survey of the literature on electropolymerization covering the past few years indicates a growing use of modem electroanalytical techniques to investigate the nature of the electrode processes involved of particular interest are linear and cyclic voltammetry together with the use of the rotating disc and ring-disc electrodes. This development is most likely associated with the introduction of reliable, stable potentiostats incorporating solid-state electronics. 

Previously Funt and co-workers had used a similar electrode to study stable radical-cations (from polynuclear compounds) electrogenerated at the disc (anode) and collected at the ring (cathode). In the presence of either isobutyl vinyl ether or styrene, the concentration of the carbocations reaching the ring decreased. From studies of the collection efficiency, values of the first-order rate constants showed a similar trend to those obtained by other methods. Mengoli and Vidotto were the first to use such radical-cations in electropolymerization and in a recent paper they extend their work on the reaction of radical-cations 

Collins and Thomas have produced coherent coatings of cross-linked polymer on steel cathodes by the electropolymerization of acrylamide and IVfV -methylene bisacrylamide in aqueous solutions of zinc chloride. They present evidence for the probable intervention of a monomer-metal cation complex as the responsible initiator srwcies. The pH-dependence of the electrocoating indicates the involvement of ZnOH+ and that, together with the mono- 

Although most workers in the field of electropolymoization employ as anodes metals which are stable under conditionsof strong anodic polarization, Koval chuk et at. have studied the polymerization of two vinyl monomers, acrylonitrile and acrylamide, in the presence of soluble anodes such as Cu, Ni, Fe, Co, Mn, Cr, and Ce. In the case of acrylonitrile, aqueous systems were used in conjunction with added peroxide or persulphates. Using a copper anode, for example, genera- 

The electrochemical cell arrangement described for production of PPy (see Chapter 2) is applicable to the electropolymerization of PAn. The polymerization cell design is of equal importance for the preparation of PAn as for PPy. All the requirements of the cell design apply, and the same versatility of the form of assembly can be attained, although in practice PAn has not been investigated as extensively as the latter in this regard. The only additional requirement of a polymerization cell for PAn is that the electrode and construction materials should be stable in acid media. 

Electrochemical polymerization is routinely carried out in an acidic aqueous solution of aniline. This low pH is required to solubilize the monomer and to generate the PAn/HA (HA = acid) emeraldine salt as the only conducting form of PAn. Constant potential (potentiostatic) or potentiodynamic techniques are generally employed because the overoxidation potential for PAn is very close to that required for monomer oxidation. 

Defects due to overoxidation of the polymer have been proposed however, the exact nature of overoxidation is not known. One theory is that crosslinking occurs,5 whereas another proposes the opening of the chain after the formation of a paraqui-none.6 It is reported that a short-term increase of the applied potential to 0.9-1.1 V (versus Ag/AgCl) during potentiostatic deposition gives more adherent films.7 

The growth of PAn has been found to be self-catalyzing11 12 the more polymer deposited, the higher the rate of polymer formation. A mechanism for this has been proposed, involving adsorption of the anilinium ion onto the most oxidized form of PAn, followed by electron transfer to form the radical cation and subsequent reoxidation of the polymer to its most oxidized state.12 

Koval chuk, L. A. Mirkind, and N. S. Tsvetkov, Otkrytiya Izobret, Prom. Obraztsv. Tovamye Znaki, 1980,81. 

The production of thin pure poly(tetramethylene glycol) films has been studied by Dubois et al. The films were deposited electrochemically on Pt anodically polarized in THF/LiC104. If (C4H,)4NC104 is used as the electrolyte, doped films are produced. These same workers have continued their studies on the production of poly(phenylene oxide) films containing reactive groups. Babai and Gottesfeld have used ellipsometric measurements to follow the in situ growth of polymeric films on Pt by the electro-oxidation of phenolic compounds from aqueous solution. 

Homogeneous thin films ( 1000 A) of polyacenaphthalene have been produced on Pt and Au electrodes by electrochemical oxidation of the monomer. Tourillon et alP have produced thin films of poly acetonitrile doped with CIO4 and BF4 ions the films are heat resistant up to 200 °C for 30 min. 

Polymerizations in bulk and solution have been reported for several monomers. Methyl methacrylate has been polymerized at the anode in aqueous solution using H2S04/FeS04 by a free radical mechanism. Sankar and Palit have studied the 

Tourillon, J. E. Dubois, and P. C. Lacaze,/. Chim. Phys., Phys. Chim. Biol., 1979,76, 369. 

Similar reactions have been attempted in [BMIM][Bp4] and [Ci2MIM](BF4] [62], While the reaction in [Ci2MIM][BF4] proceeded smoothly to give PMMA and the ionic liquid was reused after purification it failed to do so in [BMIM][BF4]. This was attributed to the lack of solubility of the PMMA product in this ionic liquid. 

ATRP has also been used for the polymerization of ionic liquid-like, vinylbenzyl-substituted imidazolium salt, monomers to give ionic polymers [63]. The reaction rates and ultimate conversions were dependent upon the solvent used and reaction conditions (concentrations of reagents/catalysts, temperature etc.). Given that polar aprotic solvents, such as acetonitrile and DMF were preferred for the reactions and each led to different rates, it would be interesting to have seen the effect of using a non-polymerizable ionic liquid as the solvent for the reaction. 

Kubisa et al. [64] have been exploring the use of chiral ionic liquids in polymer synthesis. Using ionic liquids with a chiral substituent on the imidazolium ring for the ATRP of methyl acrylate gave a small but definite effect on polymer tacticity, with more isotactic polymer formed than in simple [BMIM][PF6]. They also found that the use of ionic liquids led to fewer side reactions. Ionic liquids have been used as solvents in biphasic ATRP to facilitate the separation of the products from the catalysts [65]. 

Percec [66] has demonstrated the effect of the l-butyl-3-methylimidazolium hex-afluorophosphate ionic liquid on the li ving radical polymerization of MMA initiated with arenesulfonyl chlorides and catalyzed by the self-regulated CuzO jl,2 -bipyridine catalyst. A dramatic acceleration ofthe polymerization vras observed with an initiation efficiency of 100%, giving polymer vrith molecular weight distribution of 1.1 and perfect bifunctional chain-ends. 

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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. 

Conducting polymer composites have also been formed by co-electrodeposition of matrix polymer during electrochemical polymerization. Because both components of the composite are deposited simultaneously, a homogenous film is obtained. This technique has been utilized for both neutral thermoplastics such as poly(vinyl chloride) (159), as well as for a large variety of polyelectrolytes (64—68, 159—165). When the matrix polymer is a polyelectrolyte, it serves as the dopant species for the conducting polymer, so there is an intimate mixing of the polymer chains and the system can be appropriately termed a molecular composite. 

Bipyridyl,4-methyl-4 -vinyl-electrochemical polymerization, 6,25 electropolymerization, 6,16 Bipyridyls bis(ZV-oxide) metal complexes, 2, 496 metai complexes, 2, 89, 90,93 steric effects, 2, 90 2,2 -Biquinolyl... 

Ideal electrochemical polymerization was considered to give ideal linear and conjugated polymeric chains. The real situation is that films electrogenerated from the basic monomers are insoluble and infusible. Only polyaniline films are partially soluble in some solvents. 

The reproducibility of the electrodeposition of conducting polymer films has been a very difficult issue. It has long been realized that each laboratory produces a different material and that results from different laboratories are not directly comparable.82 We have experienced reproducibility problems with almost all of the electrochemically polymerized materials used in our work. 

Electrochemical cells for microwave conductivity measurements, 445 Electrochemical measurements with microwave frequencies, diagrammated, 448, 449 with microwaves, 478 Electrochemical polymerization... 

Fawcett, and the structure of the mercury-ethanol interface, 59 Feldberg and Rubinstein, theory of electrochemical polymerization, 560 Film... 

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. 

The historical development of chemically electrodes is briefly outlined. Following recent trends, the manufacturing of modified electrodes is reviewed with emphasis on the more recent methods of electrochemical polymerization and on new ion exchanging materials. Surface derivatized electrodes are not treated in detail. The catalysis of electrochemical reactions is treated from the view of theory and of practical application. Promising experimental results are given in detail. Finally, recent advances of chemically modified electrodes in sensor techniques and in the construction of molecular electronics are given. 

Scheme 24 Electrochemical Polymerization of wheel-and-axle type phosphorus porphyrins...

The monomer and lower oligomers are soluble in the electrolyte, but with increasing polymerization degree the solubility decreases. After attaining some critical value, an insoluble film is formed on the anode. Lower (soluble) oligomers can also diffuse from the electrode into the bulk of the electrolyte, hence the faradaic yield of electrochemical polymerization is, at least in the primary stages, substantially lower than 100 per cent. 

Pyrrole derivatives substituted in positions 1-, 3-, or 4- have also been electrochemically polymerized (positions 2- and 5- must be free for polymerization). Besides homopolymers, copolymers can also be prepared in this way. Other nitrogen heterocycles that have been polymerized by anodic oxidation include carbazole, pyridazine, indole, and their various substitution derivatives. 

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... 

Tetra(o-aminophenyl)porphyrin, H-Co-Nl TPP, can for the purpose of electrochemical polymerization be simplistically viewed as four aniline molecules with a common porphyrin substituent, and one expects that their oxidation should form a "poly(aniline)" matrix with embedded porphyrin sites. The pattern of cyclic voltammetric oxidative ECP (1) of this functionalized metal complex is shown in Fig. 2A. The growing current-potential envelope represents accumulation of a polymer film that is electroactive and conducts electrons at the potentials needed to continuously oxidize fresh monomer that diffuses in from the bulk solution. If the film were not fully electroactive at this potential, since the film is a dense membrane barrier that prevents monomer from reaching the electrode, film growth would soon cease and the electrode would become passified. This was the case for the phenolically substituted porphyrin in Fig. 1. 

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]. 

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

Fig.9 Electrochemical polymerization of pyrrole on the electrode-bound redox enzyme oxidase...

A platinum disk electrode was electrolytically platinized in a platinum chloride solution to increase the surface area and enhance the adsorption power. The platinized platinum electrode was then immersed in a solution containing 10 mg ml l ADH. 0.75 mM and 6.2 mM NAD. After sufficient adsorption of these molecules on the electrode surface, the electrode was transferred into a solution containing 0.1 M pyrrole and 1 M KC1. Electrochemical polymerization of pyrrole was conducted at +0.7 V vs. Ag/AgCl. The electrolysis was stopped at a total charge of 1 C cm 2. An enzyme-entrapped polypyrrole membrane was deposited on the electrode surface. 

Both the catalytic and electrochemical polymerization reactions appear to result from electron donation from the aromatic species to the surface producing a radical cation that leads to electrophilic addition. 

Although much less so than pyrrole polymers, indole polymers are beginning to be synthesized and studied as new materials. Electropolymerized films of indole-5-carboxylic acid are well-suited for the fabrication of micro pH sensors and they have been used to measure ascorbate and NADH levels. The three novel pyrroloindoles shown have been electrochemically polymerized, and the polymeric pyrrolocarbazole has similar physical properties to polyaniline. 

Parent (unsubstituted) PF was first synthesized electrochemically by anodic oxidation of fluorene in 1985 [266] and electrochemical polymerization of various 9-substituted fluorenes was studied in detail later [220,267]. Cyclic voltammogram of fluorene ( r1ed= 1.33 V, Eox = 1.75 V vs. Ag/Ag+ in acetonitrile [267]) with repetitive scanning between 0 and 1.35 V showed the growth of electroactive PF film on the electrode with an onset of the p-doping process at 0.5 V (vs. Ag/Ag+). The unsubstituted PF was an insoluble and infusible material and was only studied as a possible material for modification of electrochemical electrodes. For this reason, it is of little interest for electronic or optical applications, limiting the discussion below to the chemically prepared 9-substituted PFs. 

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