Condensation copolymerization reactions of carbazole 3,6-diboronate with 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (DTBT) only produce low-molecular-weight donor (D)-pi-acceptor (A) copolymers. High-molecular-weight copolymers for use in optoelectronic devices are necessary for achieving extended pi-conjugation and for controlling the copolymer processibility. To elucidate the cause of the persistently low molecular weight, we synthesized three 3,6-carbazole-based D-A copolymers using copolymerizations of N-9'-heptadecanyl-3,6-carbazole with DTBT, N-9'{2-[2-(2-methoxyethoxy)-ethoxy]-ethyl}-3,-6-carbazole with DTBT, and N-9'-heptadecanyl-3,6-carbazole with alkyl-substituted DTBT. We investigated several parameters for their influence on molecular copolymer weight, including the conformation of the chain during growth, the solubility of the monomers, and the dihedral angles between the donor and acceptor units. Size exclusion chromatography, UV-vis absorption spectroscopy, and computational studies revealed that the low molecular weights of 3,6-carbazole-based D-A copolymers resulted from conjugation breaks and the resulting high coplanarity, which led to strong interactions between polymer chains. These interactions limited formation of high-molecular-weight-copolymers during copolymerization. The strong intermolecular interactions of the 3,6-carbazole moiety were exploited by incorporating 3,6-carbazole units into poly[9',9'-dioctyl-2,7-flourene-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] prepared from 9',9'-dioctyl-2,7-flourene and DTBT. Interestingly, the number average molecular weight increased gradually with increasing 2,7-fluorene monomer content but the number of conjugation breaks was a range of 6-7. The hole mobilities of the copolymers were studied for comparison purposes. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4368-4378, 2011