A range of poly(3-hexylthiophene)-b-poly(quinoxaline-2,3-diyl) rod–rod diblock copolymers was synthesized in one pot via sequential monomer addition with a Ni(II) complex as a single catalyst at room temperature. The block copolymerization was proved to proceed in a living/controlled chain-extension manner, affording well-defined poly(3-hexylthiophene)-b-poly(quinoxaline-2,3-diyl) block copolymers with controlled molecular weights, narrow molecular weight distributions, and tunable compositions. The synthesized block copolymer was found to self-assemble into well-defined vesicle supramolecular structures in mixed solvents of chloroform and methanol. The assembly in solution induced unique light emissions with the light emission color varying from red to purple and blue depending on solvents used.