Aniline monomer oxidation

Fig. 2.16. Plot of the current recorded at +0.1 V at a poly(aniline)/poly(vinylsulfonate)-coated glassy carbon electrode (deposition charge ISO mC, geometric area 0.38 cm2) rotated at 9 Hz in 0.1 mol dm-3 citrate/phosphate buffer at pH 7 as a function of the NADH concentration. The currents are corrected for the background current (<0.2 p.A) in each case. Data for three different films prepared under identical conditions are shown. The inset shows the current for oxidation of NADH at +0.1 V at an uncoated glassy carbon electrode treated identically to the coated electrode except that no aniline monomer was added to...

The mechanism of polyaniline formation is an area of active research and controversy. The wide range of reaction conditions used in polyaniline synthesis and the resulting differences in the structure and characteristics of the polymers has probably contributed to the proposal of many different mechanisms. The majority of the proposed mechanisms begin with the oxidation of aniline to a cation radical (445). Two of these cation radicals couple to form /V-phenyl-p-phenylenediamine (443). The oxidation of the aniline monomers to form dimeric species is the slow step in the polymerization [271,285,286]. The subsequent steps of polymer growth are under discussion. 

One proposed mechanism for the electrochemical polymerization of aniline is shown in Fig. 66 [287]. Aniline is oxidized to cation radical 445 which dimerizes to form dication dimer 446. Deprotonation (- 2H +) of 446 gives 443. The oxidation of dimer 443 gives cation radical 447 which is further oxidized to diiminium dication 448. The coupling of 448 with 445 followed by the loss of two protons gives 449. The addition of aniline units to the polymer chain continues in a similar manner by the coupling of the terminal diiminium dication group with monomer cation radical 445. 

PAn s are most commonly prepared through the chemical or electrochemical oxidative polymerization of the respective aniline monomers in acidic solution. However, a range of polymerization techniques has now been developed, including... 

Electrochemical polymerization is routinely carried out in an acidic aqueous solution of aniline. This low pH is required to solubilize the monomer and to generate the PAn/HA (HA = acid) emeraldine salt as the only conducting form of PAn. Constant potential (potentiostatic) or potentiodynamic techniques are generally employed because the overoxidation potential for PAn is very close to that required for monomer oxidation. 

Plasma polymerization of aniline in the absence of a solvent or a chemical oxidant, giving neutral undoped PAn, was first described in 1984.110 This method has been further developed111112 recently with, for example, Cruz and coworkers, describing the deposition of PAn film using radio frequency (RF) glow discharges between stainless steel electrodes and at 0.02-0.08 atm pressures. The aniline monomer reacts with electrons in the plasma, and the polymer deposits on the reactor wall after growth.100... 

A wide range of alkyl- and alkoxy-substituted PAn s of the general types 6 and 7 have been synthesized by the chemical or electrochemical oxidation of appropriately substituted aniline monomers.133 Such substitution imparts markedly improved solubility in organic solvents to the emeraldine salt products compared to the parent (unsubstituted) PAn/HA salts. The poly(2-methoxyaniline) (POMA) species, in particular, has been the subject of extensive studies.134 137 This species has the additional attractive feature of being soluble in water after being wet with acetone. 

Oxidative polymerization of substituted aniline monomers is frequently more difficult than that of aniline. A significant recent advance has therefore been the... 

A significant recent development has been the synthesis of PAn s in which chiral substituents have been covalently attached to the aniline rings. Chiral ethers were bound to the aniline monomer before its chemical oxidation by persulfate,139 producing chiral PAn s possessing strong optical activity as evidenced by their CD spectra. Previous routes to chiral PAn s had employed chiral dopant anions to induce chirality into PAn chains. 

McCarthy and coworkers126 229 reported a template-guided synthesis of water-soluble chiral PAn nanocomposites. The nanoparticles were prepared by the physical adsorption of aniline monomer onto a templating poly(acrylic acid) in the presence of (+)- or (-)-CSA, followed by chemical oxidation. Using this approach, optically active nanocomposites of approximately 100 nm diameter were formed. Earlier work by Sun and Yang230 using polyelectrolytes produced similar nonchiral dispersions in which the PAn chain is interwound with a water-soluble polymer by electrostatic forces.231 Similar work by Samuelson and coworkers utilized DNA as a chiral template for PAn.232... 

An elaboration of this technique, incorporating the concept of template-guided synthesis, is the use of nanoporous matrices such as zeolites and polycarbonates as a template within whole pores to perform the chemical polymerization of aniline monomers. For example, Wu and Bein254 have prepared nanofilaments of conducting PAn in the 3-nm-wide channels (pores) of the aluminosilicate host, MCM-41, through initial adsorption of aniline vapor into the dehydrated host followed by oxidation with (NH)2S208. 

PAn has been integrated into other structures (polyethylene, polyterephthalate, polyester, and polystyrene) by soaking in monomer solutions and then exposing to acidic oxidant (FeCl3) solutions 41 Results suggest that the polymers were swollen by the aniline monomer and that polymerization occurred within these swollen media to produce dispersed PAn granules. Conductivities on the order of 10 1 S cm-1 were obtained. 

Materials studied are polyaniline, poly(3-octylthio-phene)(POT), and mixtures of poly(3-octylthiophene) (POT) and poly(ethylvinylacelate) (EVA). The synthesis of polyaniline was carried out by oxidation of aniline monomer using (NH4)2S20g in HCl (1 M) -I-HBF4 (1 M) or in the eutectic NH4F, 2.35 HF as... 

Emeraldine salt was synthesised according to [47] by oxidation of an aniline monomer using ammonium peroxidisulphate. 

FIGURE 5.18 Template-guided polymerization to make water-soluble polyaniline (PanAquas). Aniline monomer is complexed to a polyacid template and then oxidatively polymerized inacontrolled fashion. (After Nguyen, M. T., Kasai, P., Miller, ). L., and Diaz, A. F. 1994. Macromolecules, 27, 3625.)... 

Unlike the porous membrane, colloidal particles (such as PS or silica particle) are another type of template for the preparation of CPCs (Figure 11.10c). In previous work, core/shell PS/PANI composite particles were prepared by chemical oxidative seeded dispersion polymerization. A conventional coating protocol was employed as follows. The aniline monomer was dissolved in a strongly acidic solution in the presence of the PS seed latex (an alternative method involves using a miscible aniline hydrochloride monomer without external acid). Then polymerization was initiated by the addition of oxidant aqueous solution. The suspended PS particles were coated with PANI by in situ deposition of the formed conducting polymer or oligomer from the aqueous phase. 

Polymerization of aniline Polyaniline was prepared with oxidative polymerization of aniline in aqueous acidic media (IM HCl) with ammonium persulfate as an oxidant by following the method used by Mattes et al. (1,2). The molar ratio of monomer/oxidant used was 4/1. The reaction was carried out at OT for 3 hours, and the precipitate was formed during the reaction. The precipitate was, subsequently, filtered and washed with deionized water until the filtrate was colorless. The as-synthesized polyaniline in its protonated form was treated with IM NH4OH for 15 hours to yield emeraldine base powder, followed by drying under vacuum for over 48 hours at room temperature. 

---