Electropolymerization mechanism

All these findings prove that the electropolymerization mechanism of donor-substituted thiophenes and pyrroles does not involve a chain-propagation... 

The final conclusion of this short discussion is that electropolymerization is a fast method (a film of about 5 //mean be obtained by polarization in 1 rnin) that uses a complex mechanism (Fig. 12) in which electropolymerization, cross linking, degradation, and chemical polymerization can coexist to produce a mixed material with a cross-linked and electroactive part and a passive fraction.67-71 However, ifwe control the variables acting on the kinetics of the different simultaneous reactions, the complexity also provides flexibility, allowing us to obtain materials tailored for specific applications. 

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

Chain length is another factor closely related to the structural characterization of conducting polymers. The importance of this parameter lies in its considerable influence on the electric as well as the electrochemical properties of conducting polymers. However, the molecular weight techniques normally used in polymer chemistry cannot be employed on account of the extreme insolubility of the materials. A comparison between spectroscopic findings (XPS, UPS, EES) for PPy and model calculations has led some researchers to conclude that 10 is the minimum number of monomeric units in a PPy chain, with the maximum within one order of magnitude n9- 27,i28) mechanical qualities of the electropolymerized films,... 

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

Electrochemistry is one of the most promising areas in the research of conducting polymers. Thus, the method of choice for preparing conducting polymers, with the exception of PA, is the anodic oxidation of suitable monomeric species such as pyrrole [3], thiophene [4], or aniline [5]. Several aspects of electrosynthesis are of relevance for electrochemists. First, there is the deposition process of the polymers at the electrode surface, which involves nucleation-and-growth steps [6]. Second, to analyze these phenomena correctly, one has to know the mechanism of electropolymerization [7, 8]. And thirdly, there is the problem of the optimization of the mechanical, electrical, and optical material properties produced by the special parameters of electropolymerization. 

The mechanism of electropolymerization is still not fully understood. The one certainty is that in the very first step the neutral monomer is oxidized to a radical cation. It must have an oxidation potential that is accessible via a suitable solvent-electrolyte system and should react more quickly with identical species than with nucleophiles in the electrolyte solution. Therefore, as a general rule, polymerization without defects becomes less successful with increasing oxidation potential of the starting monomer, for example, in... 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

Recently the mechanisms of pyrrole electropolymerization have been reviewed in Ref. [9b]. By the anodic reaction, an electron is withdrawn from the pyrrole monomers and cationic radicals are formed. The cationic radicals undergo a series of chemical-electro-chemical-chemical reactions and, as the result, the polymerization proceeds. If the cationic... 

Electrolytic polymerization or electrolytically initiated polymerization, or shortly electro-initiated polymerization or electropolymerization, generally means initiation by the electron transfer processes which occur at the electrodes of an electrolytic cell containing monomer and electrolyte, in that by controlling the electrolysis current it is possible to control the generation of initiating species. Under appropriate conditions it may proceed by a free radical, anionic or cationic mechanism. In addition to the electrolytic addition polymerization, production of polymers through condensation reaction by electrolytic means should also be covered. Examples of each of these propagation mechanisms have now been reported in the literature. 

The preparation of molecular composites by electropolymerization of heterocycles in solution with polyelectrolytes is an extremely versatile technique, and many polyelectrolyte systems have been studied. The advantages of this method include the use of aqueous systems for the polymerization. Also, the physical and mechanical properties of the overall composite depend on the properties of the polyelectrolyte, so material tailorability is feasible by selection of a polyelectrolyte with desirable properties. 

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