Precise control of molecular weights and their distributions (MWD) in cationic polymerization of cyclopentadiene (CPD) has been achieved with a three-component initiating system: the HCl adduct of CPD or a vinyl ether(initiator), SnCL4 (Lemis acid catalyst), and n-Bu4NCl (additive), in dichloromethane at -78 degreesC. The number-average molecular weights of the polymers increased in direct proportion to monomer conversion as well as to the monomer-to-initiator mole ratio (up to 200), and the MWDs were narrow throughout the reactions (M-w/M-n = 1.2-1.3); those in the additive-free systems were dearly broader (M-w/M-n > 21. Weak bases such as diethyl ether and ethyl acetate were also effective additives, whereas THF inhibited the polymerization. The poly(CPD) was completely soluble in organic solvents, carried the initiator fragment at the a-end (nearly one per chain!, and consisted of nearly equal amounts of 1,2-and 1.4-repeat units, free from other enchainments.