Polycyclopentene with controllable molecular weight and polydispersity below 1.1 was obtained by ring-opening metathesis polymerization (ROMP) of cyclopentene at room temperature using a commercial Mo-based ROMP initiator in the presence of trimethylphosphine. Polydispersities were kept low by limiting the degree of cyclopentene conversion. These narrow-distribution polycyclopentenes were catalytically hydrogenated to narrow-distribution linear polyethylenes, providing a facile route to model polyethylenes free from both short- and long-chain branching. The "living'' nature of ROMP also permits the synthesis of well-defined diblock copolymers containing polycyclopentene, including poly(ethylidenenorbornene)-polycyclopentene diblocks. The hydrogenated derivatives of these diblocks represent a new type of crystalline-amorphous diblock, possessing much higher levels of crystallinity than when block copolymers containing anionically polymerized.high-1,4-polybutadiene are hydrogenated.