Elena N. Konyushenko

Chemical Oxidative Polymerization of Aminodiphenylamines

The course of oxidation of 4-aminodiphenylamine with ammonium peroxydisulfate in an acidic aqueous ethanol solution as well as the properties of the oxidation products were compared with those of 2-aminodiphenylamine. Semiconducting oligomers of 4-aminodiphenylamine and nonconducting oligomers of 2-aminodiphenylamine of weight-average molecular weights 3700 and 1900, respectively, were prepared by using an oxidant to monomer molar ratio of 1.25. When this ratio was changed from 0.5 to 2.5, the highest conductivity of oxidation products of 4-aminodiphenylamine, 2.5 x 10 (-4) S cm (-1), was reached at the molar ratio [oxidant]/[monomer] = 1.5. The mechanism of the oxidative polymerization of aminodiphenylamines has been theoretically studied by the AM1 and MNDO-PM3 semiempirical quantum chemical methods combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. Molecular orbital calculations revealed the prevalence of N prim-C10 coupling reaction of 4-aminodiphenylamine, while N prim-C5 is the main coupling mode between 2-aminodiphenylamine units. FTIR and Raman spectroscopic studies confirm the prevalent formation of linear N prim-C10 coupled oligomers of 4-aminodiphenylamine and suggest branching and formation of phenazine structural units in the oligomers of 2-aminodiphenylamine. The results are discussed with respect to the oxidation of aniline with ammonium peroxydisulfate, leading to polyaniline, in which 4-aminodiphenylamine is the major dimer and 2-aminodiphenylamine is the most important dimeric intermediate byproduct.

The role of acidity profile in the nanotubular growth of polyaniline

Conditions of polyaniline (PANI) nanotubes preparation were analyzed. Aniline was oxidized with ammonium peroxydisulfate in 0.4 M acetic acid. There are two subsequent oxidation steps and the products were collected after each of them. At pH > 3, neutral aniline molecules are oxidized to non-conducting aniline oligomers. These produce templates for the subsequent growth of PANI nanotubes, which takes place preferably at pH 2–3. At pH < 2, granular morphology of the conducting PANI is obtained. High final acidity of the medium should be avoided in the preparation of nanotubes, e.g., by reducing the amount of sulfuric acid which is a by-product. Reduction of the peroxydisulfate-to-aniline mole ratio was tested for this purpose in the present study. Lowering of the reaction temperature from 20°C to −4°C had a positive effect on the formation of nanotubes.

NMR Investigation of Aniline Oligomers Produced in the Early Stages of Oxidative Polymerization of Aniline

The products obtained within early stages of the oxidative polymerization of aniline in solutions of various weak organic acids or in water, and aniline oligomers produced by the oxidation of aniline and aniline-(15)N in acetic acid (0.4 M) with a limited amount of oxidant were analyzed using 1H, 13C, and 15N 1D and 2D NMR spectroscopy and 1H PFG NMR. Such products are virtually identical in all cases, according to 1H NMR. They are always a mixture of products, among which one of them is prominent. Both native and neutralized forms of the products were examined. As shown by a combination of 1H DQF COSY, 1H NOESY, 1H-(13)C and 1H-(15)N HSQC, and 1H-(13)C and 1H-(15)N HMBC spectra, both forms of this product contain an oligoaniline moiety ended mostly by phenylamino groups. In a significant amount, the chains contain--either as an inner or terminal group--an unexpected six-member ring with an oxygen-containing substituted quinoneimine structure. The most probable structure of the major product is given. The difference between the native and neutralized forms of the product was examined. It is shown that the oligomeric chains, in particular quinoneimine units of the former one, are protonated. Both forms of the product exhibit a slight paramagnetism, and contain about 2x10(-9) mol g(-1) of unpaired electron spins.