Controlled polymerization of aniline at sub-zero temperatures

Aniline is polymerized by oxidation with ammonium persulfate in aqueous hydrochloric acid solutions at sub-zero temperatures down to −43°C in the presence of LiCl and ethanol. The polymerization is controlled by concomitant potential, temperature and pH profiling. The concentrations of LiCl and ethanol are so determined to enable complete dissolution of the reactants in solution and to prevent freezing. Six polymerization steps were identified at low temperatures. The reduction of primarily synthesized pernigraniline to emeraldine with FeCl2 is investigated. The as-synthesized and deprotonated emeraldine is purified by extraction with chloroform. The polyaniline is characterized by viscosity, gel permeation chromatography (GPC), UV-Vis-NIR spectroscopy, conductivity, X-ray diffraction and chlorine analysis. High molecular weight polyanilines are synthesized at temperatures lower than −25°C having inherent viscosities within 2.0–2.15 dl/g, Mn = 30 000 g/mol, Mw = 140 000–170 000 g/mol, with Mw/Mn = 2−3. The polymers in the emeraldine base form dissolved in NMP-0.5% LiCl show a more planar conformation as indicated by the wavelength of the exciton peak reaching 680 nm. An enhanced crystallinity in terms of structural perfection is observed by means of X-ray diffraction spectra. The UV-Vis-NIR spectra of polyanilines reduced with FeCl2, protonated with (±)-10-camphorsulfonic acid (CSA), in m-cresol show a smeared polaron peak shifted into the UV. They show a broad convex NIR band peaking far beyond 2700 nm indicating an enhanced polaron delocalization. The conductivity of the films of emeraldine protonated by CSA cast from m-cresol are higher than 300 S/cm for polymers with inherent viscosities exceeding 1.8 dl/g and show an enhanced resistance to ageing. The purification of the emeraldine base by extraction with chloroform increases the molecular weight of the polymers and leads to an enhanced conductivity of the films. Polymers synthesized at low temperatures with FeCl2 reduction have better physical properties probably on account of less branching.