CONTROLLED POLYMERIZATION OF ANILINE AT SUBZERO TEMPERATURES

Aniline is polymerized by oxidation with ammonium persulfate in aqueou s hydrochloric acid solutions at sub-zero temperatures down to -43 deg rees C in the presence of LiCl and ethanol. The polymerization is cont rolled by concomitant potential, temperature and pH profiling. The con centrations of LiCl and ethanol are so determined to enable complete d issolution of the reactants in solution and to prevent freezing. Six p olymerization steps were identified at low temperatures. The reduction of primarily synthesized pernigraniline to emeraldine with FeCl2 is i nvestigated. The as-synthesized and deprotonated emeraldine is purifie d by extraction with chloroform. The polyaniline is characterized by v iscosity, gel permeation chromatography (GPC), UV-Vis-NIR spectroscopy , conductivity, X-ray diffraction and chlorine analysis. High molecula r weight polyanilines are synthesized at temperatures lower than -25 d egrees C having inherent viscosities within 2.0-2.15 dl/g, M-n = 30 00 0 g/mol, M-w= 140 000-170 000 g/mol, with M-w/M-n = 2-3. The polymers in the emeraldine base form dissolved in NMP-0.5% LiCl show a more pla nar conformation as indicated by the wavelength of the exciton peak re aching 680 nm. An enhanced crystallinity in terms of structural perfec tion is observed by means of X-ray diffraction spectra. The UW-Vis-NIR spectra of polyanilines reduced with FeCl2, protonated with (+/-)-10- camphorsulfonic acid (CSA), in m-cresol show a smeared polaron peak sh ifted into the UV. They show a broad convex NIR band peaking far beyon d 2700 nm indicating an enhanced polaron delocalization. The conductiv ity of the films of emeraldine protonated by CSA cast from m-cresol ar e higher than 300 S/cm for polymers with inherent viscosities exceedin g 1.8 dl/g and show an enhanced resistance to ageing. The purification of the emeraldine base by extraction with chloroform increases the mo lecular weight of the polymers and leads to an enhanced conductivity o f the films. Polymers synthesized at low temperatures with FeCl2 reduc tion have better physical properties probably on account of less branc hing. (C) 1998 Elsevier Science S.A. All rights reserved.