Chemical synthesis, polymers electropolymerization

Although most metal-containing polythiophenes have been synthesized by electropolymerization on an electrode surface, there are many reasons to chemically synthesize these polymers. Chemical synthesis may allow isolation of soluble polymers, enabling complete solution characterization (GPC, light scattering, NMR, etc.) and facilitating conductivity studies. Moreover, it can enable improved thin-film preparation and film deposition onto nonconducting substrates. Finally, monomers that are unsuitable for electropolymerization may be polymerized by chemical methods. 

Type III polymers are of significant interest, since in these materials the metal group may participate in intrachain charge transport. Many examples of this type of polymer have been prepared by chemical polymerization [99], but fewer examples of their synthesis by electropolymerization are known. 

Poly[l,2-bis(3-alkyl-2-thienyl)ethylene] is obtained by either chemical or electrochemical polymerization of l,2-bis(3-alkyl-2-thienyl)ethylene. The sample morphology of chemically and electrochemically prepared polymers is quite different. A bulk powder is obtained by the chemical route, while homogeneous films are produced by electropolymerization. Chemical synthesis would seem to be more convenient to prepare polymers because the oxidation with FeCl3 gives standard quality polymers in good yield. Electropolymerization is more sensitive to the synthesis parameters. Electrochemically prepared films are more sensitive to photooxidation [147]. 

Method of synthesis polymer In final form cannot be processed, therefore precursor polymer Is synthesized first and then converted into film or the final forms. Precursor polymer can be obtained by one of the following methods Wessling route, ring opening polymerization, chemical vapor deposition, electropolymerization, condensation, phase transfer catalysis, or anionic polymerization Rnk, J K, High Performance Polymers, William Andrew, 2008. 

Conducting polymers can be prepared by chemical or electrochemical techniques. Electrochemical synthesis provides easier routes when compared with chemical synthesis and allows control over film formation, especially relevant if polymers are required as thin films deposited on the surface of metallic substrates. However, electrochemically synthesized polymers are usually more porous, a feature that requires consideration when a barrier effect is necessary. Another important aspect in the corrosion field is that the application of potential/current necessary to promote electropolymerization may accelerate dissolution (corrosion) of the metal. In some cases, an oxide pre-layer is deposited between the metal and the polymer to promote adhesion and hinder metal dissolution during the electropolymerization process (Tallman et al., 2002 Spinks et al., 2002). Alternatively, the application of layered coatings based on different conducting polymers can be a strategy to overcome the problem of metal dissolution. In the work of Lacroix et al. (2000), a layer of PPy was firstly deposited on zinc and mild steel in neutral conditions, followed by deposition of PANi in an acidic medium, because the direct deposition of PANi on those metallic substrates was not possible in an acidic medium, causing dissolution of the metal. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

Polypyrroles (PPy s) are formed by the oxidation of pyrrole or substituted pyrrole monomers. In the vast majority of cases, these oxidations have been carried out by either (1) electropolymerization at a conductive substrate (electrode) through the application of an external potential or (2) chemical polymerization in solution by the use of a chemical oxidant. Photochemically initiated and enzyme-catalyzed polymerization routes have also been described but are less developed. These various approaches produce polypyrrole (PPy) materials with different forms—chemical oxidations generally produce powders, whereas electrochemical synthesis leads to films deposited on the working electrode, and enzymatic polymerization gives aqueous dispersions. The conducting polymer products also possess different chemical/electrical properties. These alternative routes to PPy s are therefore discussed separately in this chapter. 

There are four main routes to the synthesis of PAVs [9] polymerizations via quinodimethane intermediates, polycondensations, transition-metal-mediated polycouplings, and metathesis polymerizations. Other methods such as chemical vapor deposition and electropolymerization have also been used on occasion but generally give poorer quality polymers. 

Vinylic monomers such as acrylonitrile (AN) or methacrylonitrile (MAN) undergo an electropolymerization when submitted to electroreduction at metallic cathodes in an anhydrous organic medium [1,2], This synthesis leads to two different kinds of products (i) a physisorbed polymer which can be removed by rinsing with an appropriate solvent and (ii) a so-called grafted polymer, which is not removed with a solvent, even under sonication, and bearing carbon/metal interface chemical bonds which have been identified by X-ray Photoelectron Spectroscopy [2] and EXES [3], The former can be up to several micrometers thick, whereas the latter has a thickness which never exceeds a few hundreds Angstroms. 

On the other hand, a lot of work has been spent on the synthesis of new electropolymerizable monomers, because not all polymers are chemically accessible, and for getting some special structures the electropolymerization was again the privileged path. 

As discussed, the electropolymerized PEDOT-PSS (o- = 80 S cm , 1 s/t ratio = 0.68) has a completely different composition than the chemically polymerized PEDOT-PSS (Electrochemical quartz crystal microbalance (EQCM) analyses have shown that the composition of the electropolymerized PEDOT-PSS is not dependent on the anion concentration [135]. This indicates that the mechanism of synthesis of the polymer strongly influences its composition. During electropolymerization, the first formed (and doped) oligomers are very close to the metal electrode as charge transfer occurs at a tunnel distance. Consequently, those doped oligomers interact electrostatically with the closest sulfonate anions of a PSS chain at the vicinity of the electrode. This mechanism leads to a high concentration of PEDOT in the PEDOT-PSS film, independent on the anion concentration. 

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