Conducting polymers electrochemical polymerization

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. 

As an example of a practical situation where the instrument has been used, the study of an electrically conductive polymer, poly(3-octylthiophene), is representative. This polymer is electrochemically polymerized on a Pt-disc, which then is used as a working electrode in a conventional three-electrode electrochemical cell. In this example glassy-carbon was used as an auxiliary electrode, while the reference electrode was a Ag/AgCl (3M KCl). The supporting electrolyte was 0.1 M LiBF4 in propylene carbonate. 

Sotzing, G.A., J.R. Reynolds, and P.J. Steel. 1996. Electrochromic conducting polymers via electrochemical polymerization of bis(2-(3,4-ethylendioxy)thienyl) monomers. Chem Mater 8 882. 

G. Zotti, S. Zecchin, G. Schiavon, A. Berlin, Low defect neutral, cationic and anionic conducting polymers from electrochemical polymerization of N-substituted bipyrroles. Synthesis, characterization, and EQCM analysis, Chemistry of Materials 2002, 14, 3607. 

Oriented Polymers. In the preparation of an electrically conducting polymer, e.g., PT or PMT, the two ends of a polymer sample, electrochemically polymerized and still containing solvent, are fixed. The polymer is then dried and heated to cause it to contract, so that molecular orientation is produced in the longitudinal direction. This process can give a 2 to 4 fold increase in the electrical conductivity of the polymer [700]. 

M. A. Sato, S. Tanaka, and K. Kaeriyama, Soluble conducting polymers by electrochemical polymerization of thiophenes having long alkyl substituents, Synth. Met. 18 229 0981). 

A dopant with a long aliphatic tail embedded in an electrically conductive polymer on electrochemical polymerization make. the polymer mechanically superior to the one with an inorganic dopant. The aliphatic nature of contracting in H2O holds the dopant in the polymer on reduction and brings a small cation into the polymer to neutralize the freed anion. When a large cation is used instead of small one, the redox reaction goes slow. It seems that redox reaction occurs only when the reactive site is electrically balanced. 

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. 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

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. 

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. 

Apart from the insulating polymeric matrices, conductive polymers such as polypyrrole and polyaniline have been used as nanocomposite electrodes by chemical or electrochemical polymerization [13, 17, 116, 117]. Such materials provide high conductivity and stability. However, the use of insulating polymers can be more advantageous than the conductive polymers when employed in cyclic voltammetry. 

Electrochemical polymerization of heterocycles is useful in the preparation of conducting composite materials. One technique employed involves the electro-polymerization of pyrrole into a swollen polymer previously deposited on the electrode surface (148—153). This method allows variation of the physical properties of the material by control of the amount of conducting polymer incorporated into the matrix film. If the matrix polymer is an ionomer such as Nafion (154—158) it contributes the dopant ion for the oxidized conducting polymer and acts as an effective medium for ion transport during electrochemical switching of the material. 

Because the oxidation potential of the polymer is lower than that of the monomer, the polymer is electrochemically oxidized into a conducting state, kept electrically neutral by incorporation of the electrolyte anion as a counter-ion. This is an essential since precipitation of the unoxidized, insulating polymer would stop the reaction. Both coulometric measurements and elemental analysis show approximately one counter-ion per four repeat units. An important feature is the fact that the polymerization is not reversible whereas the oxidation of the polymer is. If the polymer film is driven cathodic then it is reduced towards the undoped state. At the same time neutrality is maintained by diffusion of the counter-ions out of the film and into the electrolyte. This process is reversible over many cycles provided that the film is not undoped to the point where it becomes too insulating. It is possible to use it to put new counter-ions into the film, allowing the introduction of ions which are too nucleophilic to be used in the synthesis. The conductivity of the film for a given degree of oxidation depends markedly on the counter-ion, varying by a factor of up to 105. 

In conventional polymer synthesis copolymerization is a common strategy for modifying polymer properties. In electrochemical polymerizations of the kind used to make conducting structures, it is expected to be difficult to make good copolymers unless the oxidation potentials of the two monomers are sufficiently close that one is not significantly preferred over the other 190). 

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