Mechanisms of electropolymerization

Taking the electrochemical stoichiometry into account, the complete reaction equation for the polymerization of a species HMH is  

Detailed kinetic studies in connection with digital simulations do confirm the RR coupling mechanism postulated in older publications as well as the oxidation of the resulting dimer D to the dication D. But the surprising drop in the height of the reduction wave for the redox pair as the concentration 

Independently of this, chronoabsorptiometric measurements by Genies et al. have proved that PPy films grow in timer linear to t and not to j/t. In the opinion of the authors this implies that the rate-determining step during film growth is a radical ion coupling and not the diffusion of the uncharged monomer towards the electrode surface. The attested phenomenon that PPy polymerizes 

These apparent contradictions can be resolved if one keeps in mind that the competition between several reaction paths is dependent upon both the reactivities of the anodically oxidized parent species and the polymer film as well as the reactivity of the surrounding solvent-electrolyte medium. 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well 

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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... 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

The electrochemical polymerization of PVK in LiClOVacetonitrile solution on an SWNT electrode was studied by cyclic voltammetry [194,195], The mechanism of the electropolymerization reaction of VK on the SWNT film was characterized by three stages, chemical-electrochemical-chemical [194]. The main difference between the mechanism of electropolymerization of VK on a Pt electrode only, and an electrode covered with a SWNT film consists in the fact that during the first stage, the formation of a charge-transfer complex results in the formation of VK radical cations and the SWNT radical anions. 

Figure 1. Mechanism of electropolymerization of five-membered heterocycles. Ep oxidation peak potential of monomer.

Pol5unerization is initiated by monomer oxidation to yield the radical cation. The mechanism (55) is believed then to involve either radical-cation/radical-cation coupling or reaction of a radical-cation with a neutral monomer. The mechanism of electropolymerization of five-membered heterocycles with radical-cation/radical-cation conpling is shown in Fig. 6. 

Solvents have a very strong influence both on the mechanism of electropolymerization and on polymer properties. In fact, solvents must simultaneously have a... 

The faster polymer generation at rising proton concentrations in aqueous solutions can be explained from the initial protonation of pyrrole in the a position, as a previous step in the general mechanism of electropolymerization [142]. As is well known, pyrrole can be protonated in all the possible positions (a, (i and N) resulting in three kinds of cationic species (Figure 10.14) [21,136], although the more stable position for protonation is the a carbon. A general feature of protonated pyrrole molecules is that they are not aromatic and hence they are more easily oxidable. In this way we can propose an electrochemical mechan-... 

Vorotyntsev MA, Zinovyeva VA, Konev DV (2010) Mechanisms of electropolymerization and redox activity fundamental aspects. In Costlier S, Karyakin A (eds) Electropolymerization. InTech Open, Rijeka, Croatia, pp 27-50... 

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