Poly(3-hexylthiophene)-b-poly(3-pentenylthiophene) and poly(3-hexylthiophene)-b-poly(3-undecenylthiophene) diblock copolymers have been synthesized by McCullough method. X-ray diffraction analysis of the diblock copolymers displayed all the reflection peaks specific to regioregular poly(3-hexylthiophene), indicating that the presence of poly(3-alkenylthiophene) block does not affect the packing of the polymer in the solid state. The synthesized diblock copolymers were subjected to hydroboration/oxidation and hydrosilation to demonstrate the reactivity of the alkenyl substituents. Furthermore, poly(3-hexylthiophene)-b-poly(3-pentenylthiophene) was used as a chain transfer agent for the ruthenium-catalyzed ring-opening metathesis polymerization of cyclooctene to generate a polycyclooctene graft copolymer, which was hydrogenated to give poly(3-hexylthiophene)-b-poly(3-pentenylthiophene-g-polyethylene). The opto-electronic properties and the morphology of the synthesized polymers have been investigated. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012