Simply prepared pi-allylnickel complexes were used as external initiators for promoting the polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene in a living/controlled chain growth manner to afford regioregular poly(3-hexylthiophene) with an allyl terminus. The nickel species on the other chain end can initiate the block copolymerization of hexadecyloxylallene and 2-bromo-3-hexyl-S-chloromagnesiothiophene to give a well-defined triblock copolymer containing poly(3-hexylthiophene) and poly(hexadecyloxylallene) segments in one pot via mechanically distinct, sequential living polymerization. Furthermore, such pi-allylnickel(II) complexes can also catalyze the polymerization of a range of vinyl monomers, including styrene, 1-methoxy-4-vinylbenzene, and 1-chloro-4-vinylbenzene as well as tert-butyl acrylate, in living/controlled fashion. The active nickel unit at the growing chain end of these vinyl polymers can also initiate the block copolymerization of 2-bromo-3-hexyl-S-chloromagnesiothiophene to give a series of block copolymers containing vinyl polymer and poly(3-hexylthiophene) segments. The new block copolymerizations have been demonstrated to proceed in living/controlled chain-extension manner. The well-defined conjugated block copolymers are isolated in high yield with controlled molecular weight and tunable compositions.