In our quest to develop regioregular borane acceptor-modified thiophene polymers, we prepared a series of new alkynylphenylborane-functionalized monomers and explored their polymerization. The regioregular polymer rr-P1 and an alternating copolymer with 3-hexylthiopene, rr-P2, were successfully obtained by Stille-type polymerization. An X-ray crystal structure of the bithiophene model system 2-Br revealed coplanar thiophene rings and the formation of pi-stacks, despite the presence of bulky dimesitylborane moieties. For the polymers, rr-P1 showed the most bathochromic absorption (520 nm) and emission bands (620 nm), indicating the importance of high regioregularity and the beneficial effect of alkynyl relative to alkyl pendent groups. A higher energy absorption at ca. 340 nm could be assigned to charge transfer to the borane pendent groups based on DFT calculations on molecular model compounds that mimic the polymer repeat units. This assignment was further supported by fluoride anion binding studies.