| Abstract
| - The cationic zirconocene trihydrides [Cp‘4Zr2H(μ-H)2]+X-, generated from [Cp‘2ZrH2]2 withvarious 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 thequantification of the effects of the counteranion and of trace moisture on IB/IP copolymers and the natureof the initiating species. Polymer molecular weights increase with decreasing anion nucleophilicity inthe 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 (Mw ≈ 5 × 105 g/mol at −35 °C). There is little reduction ineither rate or molecular weight on addition of isoprene. Polymer molecular weights are substantiallyhigher than with the Et2AlCl/tBuCl initiator system under identical conditions. Water was shown to bean important chain-transfer agent; substoichiometric quantities of water reduced activity, and copolymermolecular 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 hydrogenationstep. Tentative structures for these reactive intermediates are suggested.
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