Thienoisoindigo (TIG) moiety has been paid numerous attentions as an excellent acceptor building block in low-band-gap polymers. Herein, a new TIG-dithiophene alternating copolymer (PTIG2T) was successfully synthesized from an asymmetric TIG-based donor-acceptor (D-A) monomer via the self-condensation-type direct arylation polymerization. PTIG2T exhibited the light absorption over 1000 nm owing to the intramolecular charge transfer in the thin film state, which corresponded to an optical band gap of 1.24 eV. The HOMO and LUMO levels of PTIG2T were determined to be -5.08 and -3.60 eV, respectively. Furthermore, the organic photovoltaic (OPV) with a PTIG2T/PC61BM active layer achieved a power conversion efficiency (PCE) of 3.19%, which is one of the highest PEC achieved by OPVs with TIG-based materials. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 430-436