Polymer synthesis electrochemical polymerization

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. 

Nanocomposites of conducting polymers exhibit improved physicochemical and biological properties as compared to their individual counterparts. The integration of secondary component within conducting polymer leads to dramatic increase in different properties that are useful from an application point of view. Size, shape and controlled distribution of the dispersed phase are the critical factors to control the desired properties of a nanocomposite. Different approaches such as in situ synthesis, one-pot synthesis, electrochemical polymerization and vapor-phase polymerization have been employed to synthesize the nanocomposites of conducting polymers with metal or metal oxide nanoparticles, carbon-based materials, ternary nanocomposites, etc. All of these methods have certain advantages and drawbacks. Functional nanocomposites synthesized by these methods display many... 

After the report of poly[2.2]metacyclophane 3 described above, there were no reports on the synthesis of cyclophane-based ju-stacked polymers for over 15 years. In 2001 and 2002, polymerizations of thiophene-substituted [2.2]paracyclophanes were reported independently by two research groups [13-16]. Both groups synthesized jc-stacked polymers by electrochemical polymerization using cyclic voltammetry. For example, Audebert and coworkers reported that the electrochemical polymerization of monomer 5 deposited jr-stacked polymer 6 on a glassy carbon or indium-tin oxide electrode (Scheme 2) [13, 14]. 

Here we introduce a personal point of view about the interactions between conducting polymers and electrochemistry their synthesis, electrochemical properties, and electrochemical applications. Conducting polymers are new materials that were developed in the late 1970s as intrinsically electronic conductors at the molecular level. Ideal monodimensional chains of poly acetylene, polypyrrole, polythiophene, etc. can be seen in Fig. 1. One of the most fascinating aspects of these polymeric... 

Horseradish peroxidase (HRP) is an extracellular plant enzyme that acts in regulation of cell growth and differentiation, polymerization of cell wall components, and the oxidation of secondary metabolites essential for important pathogenic defense reactions. Because of these essential functions, and also because of its stability and ready availability, HRP has attracted considerable attention.13 It has been involved in a number of applications, such as diagnostic assays,14 biosensors,15 bioremediation,16 polymer synthesis,17 and other biotechnological processes.18 More applications in which HRP catalysis is translated into an electrochemical signal are likely to be developed in the near future. 

In addition to the chain reaction, like vinyl polymerizations, electro-lytically step-by-step reactions are also rationally to be applied to polymer synthesis. However, electrochemical reaction is not favorable for such a step-by-step reaction, since a growing polymer chain end must be affected at the electrode at each step of the reaction. Hence, only few peculiar attempts have been found successful. 

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. 

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). 

Electrochemical polymerization is preferred to chemical polymerization, especially if the polymeric product is intended to be used as a polymer film electrode, thin layer sensor, in microtechnology etc., because the potential control is a precondition of the production of good-quality material and the polymer film is formed at the desirable spot that serves as an anode during the synthesis. 

The discovery that doped forms of polypyrroles conduct electrical current has spurred a great deal of synthetic activity related to polypyrroles [216-218], Reviews are available on various aspects of the synthesis and properties of polypyrroles [219,220]. In addition, summaries of important aspects of polypyrroles are included in several reviews on electrically conducting polymers [221-226]. Polypyrrole has been synthesized by chemical polymerization in solution [227-231], chemical vapor deposition (CVD) [232,233], and electrochemical polymerization [234-240]. The polymer structure consists primarily of units derived from the coupling of the pyrrole monomer at the 2,5-positions [Eq. (84)]. However, up to a third of the pyrrole rings in electrochemically prepared polypyrrole are not coupled in this manner [241]. 

Synthesis of conductive polymers can be realized either by addition of oxidizing agents or by electrochemical oxidation at anodic potentials. To distinguish from electrochemical polymerization, polymerization by external oxidizer is often marked in literature as a chemical polymerization. Many types of conductive polymers formed by this way have a strong trend to adsorb on the surfaces... 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

Two major applications have emerged between 1983 and 1993 the first is the use of tellurophene derivatives for the synthesis of electoconducting polymers, and the second is the thermolysis of organotellurium compounds for the production of thin films of cationic tellurides. Semiconductors derived from 3,4-disubstituted tellurophenes have been patented for photosensors <90JAP(K)0241317>. Electrochemical polymerization of benzo[c]tellurophene derivatives provides useful semiconductors <90JAP(K)02263823>, applicable among other uses, for solar cells <88eup273643>. 

Completely different monomers were called for. Before long, three of today s workhorses had been identified pyrrole, aniline and thiophene. In Japan, Yamamoto [38] and in Germany, Kossmehl [39] synthesized polythiophene doped with pentafluoroarsenate. At the same time, the possibilities of electrochemical polymerization were recognized. At the IBM Lab in San Jose, Diaz used oxidative electrochemical polymerization to prepare polypyrrole [40] and polyaniline. [41] Electrochemical synthesis forms the polymer in its doped state, with the counter-ion (usually an anion) incorporated from the electrolyte. This mechanism permits the selection of a wider range of anions, including those which are not amenable to vapor-phase processes, such as perchlorate and tetra-fluoroborate. Electrochemical doping also overcomes an issue associated with dopants... 

The synthesis of conjugated polymers is highly dependent on the effective carbon-carbon single bond generation between unsaturated carbons in aromatic molecules. Aromatic units in conjugated polymers can be benzene, aniline, pyrrole, thiophene, carbazole, naphthalenediimide, perylenediimide (PDI), or their derivatives, etc. Although monomers are various, their synthetic methods can be mainly classified into chemical and electrochemical polymerizations. Chemical polymerization includes chemical oxidative polymerization and metal-catalyzed coupling condensation. 

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