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/ t BuCl 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.