Alkoxysubstituted benzo[c][1,2,5]thiadiazole electron accepting units were prepared and copolymerized with various thiophene-based electron donating monomers to produce new low bandgap polymers P1-4. The materials showed broad absorption in the range from 300 to 700 nm with bandgaps below 2 eV in solution. Efficiencies of over 1% were obtained from photovoltaic cells using P4 with PCBM as acceptor. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: PolymChem 49: 4387-4397, 2011