Aniline monomer

The monomer of the conducting polymer polyaniline is the compound aniline (aminobenzene). (a) Draw the structural formula of the aniline monomer. What is the hybridization of the N atom in (b) aniline (c) polyaniline (d) Indicate the locations of lone pairs, if any, in polyaniline, (e) Do the N atoms help to carry the current Explain your reasoning. See Box 19.1. 

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. 

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... 

Chemical polymerizations of aniline with (NH4)2S208 were initially performed at room temperature. However, later studies showed that the PAn obtained was of relatively low molecular weight and contained significant defect sites such as undesirable branching due to ortho-coupling.5960 Subsequently, the most widely employed temperature for chemical polymerization of aniline monomers has been ca. 1-5°C, as described by MacDiarmid and coworkers,61 providing a PAn whose emeraldine base (EB) forms have molecular weights (Mw) of 30,000-60,000 g mol-1. 

Achieving Regioselective Coupling with Aniline Monomers... 

Employing a polyelectrolyte to bind to and preferentially align the aniline monomers before polymerization (e.g., by S2082-) has shown promise in facilitating the desired head-to-tail coupling of the aniline substrates. During polymerization, the anionic polyelectrolytes such as poly(styrenesulfonate) and poly(acrylate)86-88 also provide the required counterions for charge compensation in the doped PAn products. This can lead to water-soluble or water-dispersed ES products. 

Serves as a template that aligns the aniline monomers before polymerization so as to promote the desired head-to-tail coupling... 

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... 

The most commercially successful method of producing processable forms of CEPs has been the aqueous colloidal dispersion route pioneered by Vincent and Armes118 120 (see two recent reviews by Armes121 and Wessling122). Unlike the methods discussed earlier, which all require extensive derivatization of the aniline monomer, dispersions are readily formed from unsubstituted aniline and are readily manufactured in bulk quantities. 

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. 

A smaller range of counterions has been incorporated into PAn than into PPy therefore, the range of chemical interactions imparted by the counterions has not been as significant. A number of different functional groups have been added to the aniline monomer although these have predominantly been investigated for their influence on the conductivity or polymerization of PAn, as mentioned previously. However, these may also be used to introduce various chemical interactions to PAn, in a manner similar to that described for PPy. 

The first reported optically active PAn s were the PAn/(+)-HCSA and PAn/(-)-HCSA salts (HCSA = 10-camphorsulfonic acid), prepared in our laboratories by the electropolymerization of aniline in the presence of either (+)- or (-)-HCSA.68 69 The optically active dopant anions are believed to induce a preferred one-handed helical arrangement in the PAn chains in these emeraldine salts 4, giving rise to intense CD bands in the visible region. Recent studies by Li and Wang70 have shown that PAn/(+)-HCSA films of exceptionally high optical activity can be electrochemically deposited by polymerizing the aniline monomer in the presence of small amounts of aniline oligomers. 

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. 

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