(Butadiene)metallocene/B(C₆F₅)₃ Pathway to Catalyst Systems for Stereoselective Methyl Methacrylate Polymerization:  Evidence for an Anion Dependent Metallocene Catalyzed Polymerization Process

The ansa-zirconocene dichlorides [Me₂Si(C₅H₄)(3-R-C₅H₃)]ZrCl₂ 7a−e (R = H, CH₃, cyclohexyl, −CHMe₂, −CMe₃) were reacted with butadiene−magnesium to yield the respective (η⁴-butadiene)metallocenes 17a−e. The chiral examples give a mixture of two s-cis and two s-trans diastereomers. The strong Lewis acid B(C₆F₅)₃ adds selectively to a terminal butadiene carbon atom to yield the (butadiene)metallocene/B(C₆F₅)₃ betaine complexes 18a−e. Initially, the formation of the Z-18 isomers is preferred. These consecutively rearrange to the thermodynamically favored isomers E-18. The dipolar systems 18 are active single component metallocene catalysts for the stereospecific polymerization of methyl methacrylate. With increasing steric bulk of the attached single alkyl substituent an increasingly isotactic poly(methyl methacrylate) is obtained. A similar trend is observed in the methyl methacrylate polymerization at the [Me₂Si(C₅H₄)(3-R-C₅H₃)]ZrCH₃⁺ catalysts (9a−e) that were conventionally prepared by methyl abstraction from the corresponding ansa-zirconocene dimethyl complexes by treatment with B(C₆F₅)₃. A comparison of the poly(methyl methacrylates) obtained at these two series of catalysts has revealed substantial differences in stereoselectivity that probably originate from an influence of the respective counteranions. An initial reactive intermediate of methyl methacrylate addition to the dipolar single component metallocene catalyst E-18a was experimentally observed and characterized by NMR spectroscopy at 253 K. The subsequently formed series of [PMMA−C₄H_6-B(C₆F₅)₃]^- anion oligomers (at the catalyst 18c) was monitored (after quenching) and characterized by electrospray mass spectrometry.