The cationic zirconocene trihydrides [CP'4Zr2H(mu-H)(2)]X-+(-), generated from [CP'2ZrH2](2) with various trityl salts of weakly coordinating anions, are powerful initiators for the polymerization of isobutene (IB) and its copolymerization with isoprene (IP) (CP' = C5H4SiMe3). This study is concerned with the quantification of the effects of the counteranion and of trace moisture on IB/IP copolymers and the nature of the initiating species. Polymer molecular weights increase with decreasing anion nucleophilicity in the order X = [B(C6F5)(4)] approximate to [H2N{B(C6F5)(3)}(2)] > [CN{B(C6F5)(3)}(2)]. Using [CP'4Zr2H3](+) [CN{B(C6F5)(3)}(2)](-), high copolymer molecular weights are found (M-w approximate to 5 x 10(5) g/mol at -35 degreesC). There is little reduction in either rate or molecular weight on addition of isoprene. Polymer molecular weights are substantially higher than with the Et2AlCl/(BuCl)-Bu-t initiator system under identical conditions. Water was shown to be an important chain-transfer agent; substoichiometric quantities of water reduced activity, and copolymer molecular weights decreased linearly with increasing [H2O]. Mechanistic studies suggest that [CP'4Zr2H-mu-H)(2)](+) does not itself act as a cationic initiator but is transformed into one or more other binuclear (polynuclear?) species, accompanied by alkene insertion into the Zr-H bond and a monomer hydrogenation step. Tentative structures for these reactive intermediates are suggested.