The zirconocene complexes Cp2ZrMe2 and Cp-2*ZrMe2 activated with B(C6F5)(3) initiate the carbocationic polymerization of isobutene and isobutene-isoprene copolymerizations to IIR rubbers at temperatures as high as -30 degrees C. Unlike conventional metal halide initiators, these metallocene-based initiator systems produce both homo- and copolymers of broadly similar molecular weights. Copolymers prepared in the presence of ca. 2 mol % isoprene show a diene incorporation rate of 1.4-1.7%, with the typical 1,4-trans structure. Comparison of the effectiveness of zirconocene dialkyls with that of the metallocene hydrolysis products Cp-2*Zr(OH)(2) and (Cp2ZrMe)(2)(mu-O) in the presence of B(C6F5)(3) suggests that initiation by traces of protons is less efficient than initiation by cationic metallocene alkyl species, while oxo-bridged complexes (Cp-2＇ZrMe)(2)(mu-O)/B(C6F5)(3) (Cp＇ = C5H5 or C5H4SiMe3) are inactive.