Block copolymers containing polystyrene and polycyclooctene were synthesized with a ring-opening metathesis polymerization/chain-transfer approach. Polystyrene, containing appropriately placed olefins, was prepared by anionic polymerization and served as a macromolecular chain-transfer agent for the ring-opening metathesis polymerization of cyclooctene. These unsaturated polymers were subsequently converted to the corresponding saturated triblock copolymers with a simple heterogeneous catalytic hydrogenation step. The molecular and morphological characterization of the block copolymers was consistent with the absence of significant branching in the central polycyclooctene and polyethylene blocks [high melting temperatures (114-127 degrees C) and levels of crystallinity (17-42%)]. A dramatic improvement in both the long-range order and the mechanical properties of a microphase-separated, symmetric polystyrene-polycyclooctene-polystyrene block copolymer sample was observed after fractionation. (c) 2006 Wiley Periodicals, Inc.