The control of regioselectivity in the cationic polymerization of cyclopentadiene (CPD) was examined with initiating systems consisting of the HCl adduct of CPD (1; initiator) and a Lewis acid (activator/catalyst). Among a series of metal halides as catalysts, weak Lewis acids such as ZnX2 (X = Cl, Br, I) gave the highest [1,4]-contents (64, 70, and 76%, respectively). In particular, the 1/ZnBr2 system induced controlled polymerization to give poly(CPD)s with narrow molecular weight distributions (MWDs) (M-w/M-n = 1.3-1.5) and relatively high regioselectivity ([1,4] = 70%). In contrast, SnCl4, TiCl4, and other strong Lewis acids resulted in less controlled microstructures ([1,4] = 45-50%). Other reaction parameters, e.g., solvents, additives, and temperatures, did not dramatically affect regioselectivity, giving 55-60% [1,4]-contents almost invariably.