Block copolymers of poly(3-hexylthiophene) and a poly(arylisocyanide) were synthesized in a single pot via the addition of 2-bromo-3-hexyl-5-chloromagnesiothiophene followed by n-decyl 4-isocyanobenzoate to a solution of Ni(1,3-bis(diphenylphosphino)propane)Cl-2. The respective mechanistically distinct polymerizations proceeded in a controlled fashion and afforded well-defined block copolymers with tunable molecular weights and compositions. The block copolymers exhibited microphase separation characteristics in the solid state.