Bithiophene monomers (5HBTh and 5HBThO) were prepared by the Suzuki coupling reaction of 3-thienylboronic acid with 2-bromo-5-hexylthiophene and 2-bromo-5-hexylthiophene-1,1-dioxide. The oxidation polymerization mediated by vanadium catalyst gave poly(5HBTh) and oligo(5HBThO). 5HBThO bearing thiophene-1,1-dioxide had an absorption maximum at longer wavelength region than 5HBTh due to the intramolecular charge transfer interaction, while peak maxima blue shifted and their difference became small after the polymerization. Terthiophene monomers (5 '' HTTh, 3,5 '' DHTTh, and 4,5 '' DHTTh) were subsequently prepared by the mono-bromination of 5HBTh followed by the Suzuki coupling reaction with boronic acid derivatives. The vanadium-catalyzed oxidation polymerization of 5 '' HTTh and 4,5 '' DHTTh afforded soluble polymers and the absorption maximum wavelengths red shifted after the polymerization in contrast to bithiophene monomers and 3,5 '' DHTTh. The absorption and emission spectra of poly(5 '' DHTTh) red shifted when compared with those of poly(4,5 '' DHTTh). Thus the presence and position of n-hexyl chain influenced the monomer polymerizability and optoelectronic properties of branched polythiophenes. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3034-3044, 2009