Living polytetrahydrofuran (PTHF) was terminated with sodium thiophene methonate to yield a polymer with a thiophene group at one end. Copolymerizations of PTHF with pyrrole and thiophene were achieved in water-p-toluene sulfonic acid and acetonitrile-tetrabutylammonium tetrafluoroborate (TBAFB) solvent-electrolyte couples via constant potential electrolyses. Characterizations of the samples were performed by NMR, cyclic voltammetry, FT-IR, thermal analyses, and scanning electron microscopy. Electrical conductivities were measured by the four-probe technique. PTHF/PTh film that was deposited on ITO-glass in a dichloromethane-TBAFB solvent-electrolyte couple was found to exhibit electrochromic behavior and it electrochemically switches between blue oxidized and red reduced states. Optical analyses were carried out to investigate the electronic structure of PTHF/PTh electrochromic copolymer. (C) 2005 Wiley Periodicals, Inc.