Oligo(oxyethylene) chains cross-linked 2,2'-bithiophene (BT-E5-BT) has been synthesized successfully. A freestanding copolymer film based on BT-E5-BT and 3,4-ethylenedioxythiophene (P(BT-E5-BT-co-EDOT)) has been synthesized by electrochemical polymerization. The electrical conductivity of P(BT-E5-BT-co-EDOT) copolymer (16 S m(-1)) has improved by four orders of magnitude compared to the homopolymer of BT-E5-BT (P(BT-E5-BT), 5 3 10 23 S m(-1)) at room temperature. Both homopolymer and copolymer films exhibit well-defined redox and satisfied coloration efficiency. Spectroelectrochemistry studies indicate that the P(BT-E5-BT-co-EDOT) has a lower band gap in the range of 1.83-1.90 eV and shows more plentiful electrochromic colours (green, blue, purple and salmon pink) compared with the homopolymer P(BT-E5-BT). The Copolymer P(BT-E5-BT-co-EDOT) shows the moderate optical contrast (26% of 480 nm) and coloration efficiency (205.41 cm(-1) C-2). The copolymer method provides a novel way to fabricate a free-standing organic electrochromic device. (C) 2015 Wiley Periodicals, Inc.