Vinyl monomer electropolymerization

Electropolymerization is typically a method used in the preparation of CPs. Electroinitiated polymerization has been used for special purposes with common vinyl monomers. Electropolymerization is, however, a stoichiometric method in which 2.1-2.7 equivalents of electricity are consumed per monomer unit. Usually electrochemical oxidation is used and the polymer is deposited on the anode. If the end monomers are not taken into account, two equivalents are needed per monomer for the bond formation. The rest is consumed for the oxidation, i.e., doping of the polymer. Schematically, the coupling reaction can be shown as in Fig. 7a. 

Polymer films can also be electropolymerized directly onto the electrode surface. For example, Abruna et al. have shown that vinylpyridine and vinyl-bipyridine complexes of various metal ions can be electropolymerized to yield polymer films on the electrode surface that contain the electroactive metal complex (see Table 13.2) [27]. The electronically conductive polymers (Table 13.2) can also be electrosynthesized from the corresponding monomer. Again, a polymer film that coats the electrode surface is obtained [25]. Electropolymerized films have also been obtained from styrenic, phenolic, and vinyl monomers. 

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. 

Fig. I. Reaction mechanism accounting for the formation of a polymer in solution (the physisorbed polymer) in the course of the electropolymerization of vinylic monomers (here acrylonitrile).

Electropolymerization in situ of Acrylic and Vinylic Monomers 4.1 Pol)rmerizafion of Diacetone Acrylamide... 

These considerations on the intrinsic limits of electropolymerization in situ from water medium seem to indicate that the technique when directly applied to the traditional acrylic and vinyl monomers is unlikely to become advantageous but, a way may be found overcoming these limits once they are identified. 

Most of the vinyl monomers have been electropolymerized on metal electrodes though acrylonitrile has been the monomer given particular attention. As mentioned... 

Whenever vinyl monomers are electropolymerized, two situations must be distinguished depending on whether the vinyl monomer is the precursor of the initiating species or not. Either the polymerization is directly initiated by the activated monomer or the initiation is indirect whenever the active species (radical, anion, or cation) is generated by a compound other than the monomer (conducting salt, solvent, or properly selected additives). Besides these electroinitiated polymerization processes, a very recent report also demonstrated that electrochemistry is a valuable tool to mediate atom transfer radical polymerization (ATRP). Indeed, an externally applied electrochemical potential can activate the copper catalyst by a one-electron reduction of an irutially added air-stable cupric species (Cu /ligand) allowing... 

Figure 6-5 lists most of the pyrrole-, thiophene-, amino- and hydroxy-substituted porphyrins employed for electropolymerization and the references. Monomers 49a - k [136-147] are based on synthetic porphyrins and 50a, b on deuteroporphyrin [148-150]. Surprisingly, the electrooxidation of metal complexes of protoporphyrin-DC dimethyl ester, possibly via the vinyl groups, leads to the deposition of electroactive porphyrin films on the electrode surface [151-153],... 

If a supporting electrolyte is first oxidized to a radical, indirect cationic polymerization can result. The radical in a subsequent step oxidizes the monomer to a first initiating entity, the cation radical. Such an indirect initiation was also suggested for electropolymerization of isobutyl vinyl ether in the presence of BF4 supporting electrolyte ... 

In subsequent years, electropolymerization was definitely the established procedure. Polymerization by cathodic reduction, even exploiting redox mediation at the electrode, has been carried out on a wide series of Fe(II), Ru(II), and Os(II) vinyl-containing complexes based on differently substituted pyridine, and the relevant polymerization mechanism was extensively discussed in the same article [13]. An element of complexity of the RPs electrochemical growth hes in the presence, in the monomer, of additional electroactive groups. 

Polymer-modified electrodes can be prepared either by direct deposition of polymer onto the surface (via drop-, dip- or spin-coating methods) or by polymerization onto the electrode surface (via chemical, electrochemical or photochemical routes). The simplest method to prepare a polymer-based sensor is by drop-coating a small volume of polymer dissolved in a solvent. With time, the solvent evaporates leaving the polymer adsorbed onto the electrode surface. Dip- and spin-coating methods have also been used to obtain more uniform films. These methods are used when polymers are aheady synthesized and need to be immobilized as they are. In situ polymerization is another effective method to prepare polymer-modified electrodes. For electropolymerization, the electrode is immersed in a monomer solution (e.g., pyrrole, thiophene, phenol, aniline...) and a suitable potential (either cathodic or anodic) is applied to allow the formation of the polymer film on the electrode surface. Photopolymerization is rarer in the case of electrochemical sensors. Nevertheless, poly(vinyl alcohol) functionalized with styrylpyridinium and acrylated polyurethane have been used for the development of electrochemical sensors. 

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