Homogeneous Single-Component Betaine Ziegler−Natta Catalysts Derived from (Butadiene)zirconocene Precursors

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Abstract
(Butadiene)zirconocene adds B(C6F5)3 at a terminal diene carbon atom to yield the zirconocene−(μ-hydrocarbyl)−borate betaine Cp2Zr[C4H6-B(C6F5)3] (4). The dipolar complex 4 contains a distorted π-allyl moiety and features an additional stabilizing Zr−F−C(arene) coordination. Under kinetic control, an isomeric betaine system is formed, characterized by an internal Zr+···CH2[B]- ion-pair interaction, that rearranges to 4 upon heating. A great variety of ansa-metallocene(butadiene) complexes and related systems cleanly form analogous metallocene−(μ-conjugated diene)−borate betaines upon treatment with B(C6F5)3 and related Lewis acids. Most of these systems represent very active homogeneous single-component Ziegler−Natta catalysts for α-olefin polymerization and copolymerization. In addition, these betaine catalysts are ideally suited for carrying out mechanistic studies in active Ziegler−Natta catalyst systems. They allow for an experimental observation of the first alkene insertion step at the active single-component catalyst. This feature has been used for studying the mechanism of transfer of the stereochemical information from the bent metallocene backbone and for an experimental characterization of the energy profile of the alkene addition/alkene insertion reaction sequence in active homogeneous Ziegler−Natta systems. The neutral dipolar single-component catalysts (e.g., 4) produce a polyolefin-modified R‘-(CHR-CH2)-C4H6-B(C6F5)3- counterion at the end of the initiation period upon entering into the repetitive active catalytic cycle.

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