Anodic polymerization

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

The anodic polymerization of aniline can occur by a radical cation coupling mechanism analogous to that shown in Scheme 1, with coupling occurring between the N of one molecule and the para-position of another (Structure 4)21,22 However, a variety of other mechanisms have also been proposed,21 and it is likely that their relative rates depend upon the conditions (solvent, potential, pH, etc.) employed. The links between monomers are therefore not exclusively between the N and para-position (head-to-tail coupling). Head-head (-N=N-) and tail-tail (para-para) coupling occur more often as the pH is increased.71... 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

Although thiophenol, like other thiols, normally forms diphenyldisulfide upon oxidation [19], in the presence of a strong proton-donor (CH3NO2/CF3COOH) it can undergo anodic polymerization to give poly-/ -phenylene sulfide (Eq. 5) [26]. 

Electrochemical synthesis utilizes the ability of a monomer to be self-coupled upon irreversible oxidation (anodic polymerization) or reduction (cathodic polymerization). While this method does not always produce materials with well-defined structures (as do the three other polymerization methods to be discussed), electropolymerization, nonetheless, is a rather convenient alternative, avoiding the need for polymer isolation and purification. Of these two routes, anodic polymerization is the most widely explored as monomers such as pyrrole and thiophene are relatively electron-rich and prone to oxidation. For this reason the anodic route will be the focus of the remainder of this presentation. 

These polymers, particularly poly(pyrrole), are most conveniently prepared from the parent molecule via electrolysis. So far, furan, pyrrole, thiophene, and various methylated derivatives have been polymerized by this procedure (10). The anodic polymerization apparently also works for relatively electron rich aromatic compounds such as aniline and azulene (11). 

Since the ionization potential of thiophene is relatively high, the electric fields required for its anodic polymerization are rather steep (= 20V vs SCE). In addition, the simplest supporting electrolyte for this operation is Li BE- and deposition of Li at the cathode (usually Pt) is also energetically unfavorable. Recently, Druy (13) reported that substitution of 2,2 -bithiophene for thiophene gave better quality films, probably due to the lower ionization potential of the dimer relative to thiophene. An additional improvement consisted in replacing the Pt counter electrode by A1 (9). Spectroscopy revealed that dedoped PT films produced with the above improvements were indistinguishable in quality from the chemically coupled PT. 

Electrolytically initiated polymerization may either depend on a direct electron transfer between electrode and monomer, or on the formation of an intermediate which interacts with a monomer molecule in a fast chemical step, thus creating a chain initiator. As an example of the former type of process, the formation of a living polymer from the cathodic polymerization of a -methylstyrene by electrolysis in sodium tetraethylaluminate - tetrahydrofuran may be cited 639 whereas a typical case of the latter type is the anodic polymerization of vinyl monomers by electrolyzing them together with sodium acetate in aqueous solution 63 7,640) Here it is assumed that acetate ion is discharged to form an acetoxy or methyl radical which attacks the monomer molecule in a fast chemical step. 

Another anodic polymerization process which has been the subject of much study, partly because of its industrial importance, is the oxidation of sulfuric acid to persulfuric acid. The eflSciency of this process has been found to run almost parallel with the proportion of HS07 ions present in solutions of different concentration. This fact has been regarded as supporting the view that the first stage in the anodic process is the discharge of IlSOr ions, thus... 

A third category comprises conducting polymers. The film-forming anodic polymerization of monomers, e.g., pyrrole, leads in the majority of cases to porous or even biporous [28] polymer layers, adhering to the substrate. The porosity can be improv at higher current densities, but the overoxidation limit must be considered. Another improvement is possible in terms of the application of graphite felt as a substrate [28, 58, 455]. Last but not least, the co-deposition of dispersed c.b.s in the electrolyte leads to composites with up to 65 wt% c.b. in the polymer layer for... 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

Important processing methods Langmuir-Blodgen teclinique of monolayer production, solution polymerization over the substrate, electrochemical anodic polymerization, chemical oxidation of pyrrole in carbon black suspension... 

In suspensions of carbon black in pyrrole, anodic polymerization takes advantage of the fact that carbon black particles are negatively charged on their surface which makes it possible for them to migrate to a positively charged anode where they become embedded within a growing polypyrrole matrix This production method is suitable for production of materials for sensors, supercapacitors, fuel cells, etc. The effect of carbon black on the chemical oxidation of pyrrole in carbon black suspensions is shown in Figure 6.26. ° ... 

In a recent publication by Miry and coworkers [93], the cobalt(II) complex of dibenzotetraaza[14]annulene was anodically polymerized from a benzonitrile solution onto carbon and platinum electrodes. 

PREPARATIVE TECHNIQUES Various aryl coupling reactions, pyrolysis of the polymer precursors, anodic polymerization. 

If the total polarization is q, then the anodic polymerization is aq, and the cathodic polarization is (1 - aq) thus. 

In EMST, electrochemical reactions based on Faradaic reactions such as metal deposition, anodic dissolution, various oxide formation, and anodic polymerization are common. Ion transfer reactions (ITR) from the electrolyte to the solid or solid to electrolyte can be used for formation of positive or negative structure by deposition or dissolution. ITR can also be performed by electron transfer reaction (ETR) in chemical reactions in the bulk electrolyte. Pure ETR cannot be utilized for microstructuring. Local field distributions at the interface and inside the microstmcrnre play an important role during vertical structure formation by depositions or removal. It also depends on the ionic conductivity of the materials to be deposited or dissolved such as metal, semiconductor, and oxides. 

Among the conjugated polymers, polypyrrole (PPy) is the most representative one for its easy polymerization and wide application in gas sensors, electrochromic devices and batteries. Polypyrrole can be produced in the form of powders, coatings, or films. It is intrinsically conductive, stable and can be quite easily produced also continuously. The preparation of polypyrrole by oxidation of pyrrole dates back to 1888 and by electrochemical polymerization to 1957. However, this organic p>-system attracted general interest and was foimd to be electrically conductive in 1963. Polypyrrole has a high mechanical and chemical stability and can be produced continuously as flexible film (thickness 80 mm trade name Lutamer, BASF) by electrochemical techniques. Conductive polypyrrole films are obtained directly by anodic polymerization of pyrrole in aqueous or organic electrolytes. 

XPS elemental analysis can also be used to detect the nitrogen concentration in the nanocomposite paper. Figure 4.31 shows the nitrogen concentration the graphene/PANI nanocomposites prepared by in-situ anodic polymerization increases with increasing the electropolymerization time [133]. 

Aeiyach, S., B. Zaid, and P.C. Lacaze. 1999. A one-step electrosynthesis of PPy films on zinc substrates by anodic polymerization of pyrrole in aqueous solution. Electrochim Acta 44 2889. 

Tsakova, V., S. Winkels, and J.W. Schultze. 2001. Anodic polymerization of 3,4-ethylenedioxythio-phene from aqueous microemulsions. Electrochim Acta 46 (5) 759-768. 

Irvin et al. [75] reported forming a poly(3,4-difluoro thiophene) by anodic polymerization ... 

Chen, G. Zhuang, G. V Richardson, T. J. Liu, G. Ross, P. N. J. Anodic polymerization of vinyl ethylene carbonate in Li-ion battery electrolyte, Electrochem. Solid-State Lett, 2005, 8, A344-A347. 

Synthesis anodic polymerization of carbazole [643,651,653] or chemical polymerization of A/ -vinylcarbazole [644,646],... 

Several trimers of thienyl-5, S -dioxide with 3,4-ethylenedioxythienyl- and 3,4-ethylenedioxythienyl-5,5-dioxide have been synthesized and the corresponding polymers prepared by anodic polymerization [19]. 

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