Isobutene Polymerization and Isobutene-Isoprene Copolymerization Catalyzed by Cationic Zirconocene Hydride Complexes

The cationic zirconocene trihydrides [Cp‘ 4 Zr 2 H(μ-H) 2 ] + X -, generated from [Cp‘ 2 ZrH 2 ] 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‘ = C 5 H 4 SiMe 3 ). 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(C 6 F 5 ) 4 ] ≈ [H 2 N{B(C 6 F 5 ) 3 } 2 ] > [CN{B(C 6 F 5 ) 3 } 2 ]. Using [Cp‘ 4 Zr 2 H 3 ] + [CN{B(C 6 F 5 ) 3 } 2 ] -, high copolymer molecular weights are found ( M w ≈ 5 × 10 5 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 Et 2 AlCl/ 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 [H 2 O]. Mechanistic studies suggest that [Cp‘ 4 Zr 2 H(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.