We report the design, synthesis, and characterization of a series of thieno[3,4-b]thiophene acceptor blocks with octyl (T8), phenyl (TP), perfluorooctyl (TF8), and perfluorophenyl (TFP) side groups. Their subsequent copolymerization with dithienylbenzodithiophene by direct arylation polymerization afforded novel low bandgap poly(thienothiophene-alt-dithienylbenzodithiophene) (PTB) polymers. The strongly electron with-drawing TF8 and TFP groups were shown to significantly lower both E-HOMO and E-LUMO levels and gave computed copolymer ground-to-excited state dipole changes (triangle mu(ge)) that were relatively higher than for the nonfluorinated analogues. These materials show favorably aligned energy levels relative to conventional fullerene-type acceptors, which should allow them to perform well in organic photovoltaics.