Synthesis of a novel macroinimer comprising poly(epsilon-caprolactone) (PCL) and thiophene (Th) and its use in electrochromic device (ECD) application have been reported. First, a novel Th monomer (5) with miktofuntional initiator groups (primary hydroxyl and tertiary bromide at the third position of the thiophene ring) was synthesized in a four-step reaction sequence. Density functional theory-predicted bond lengths, angles, and vibrations of 5 were in good agreement with available experimental vibrational spectra. Subsequently, ring-opening polymerization of epsilon-caprolactone (epsilon-CL) was carried out in bulk using 5 as the initiator and tin(II) 2-ethylhexanoate (Sn(Oct)(2)) as the catalyst at 115 degrees C, which led to alpha-thiophene end-capped PCL macroinimer (PCL-Th). Furthermore, PCL-Th macroinimer was used in electrochemical copolymerization with pyrrole (Py) and Th. PCL-Th/PTh copolymer film synthesized on indium tin oxide-coated glass slide showed electrochromic behavior. Optical analyses of the PCL-Th/PTh copolymer film indicated that the copolymer film was suitable to be used as an anodically coloring material for ECD applications. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4180-4192,2011