Isobutene polymerization and isobutene-isoprene copolymerization catalyzed by cationic zirconocene hydride complexes

The cationic zirconocene trihydrides [Cp‘₄Zr₂H(µ-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] ˜ [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 ˜ 5 × 10⁵ g/mol at -35 °C). There is little reduction in either rate or molecular weight on addition of isoprene. Polymer molecular weights are substantially higher than with the Et₂AlCl/^tBuCl 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(µ-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.