Catalyst/Cocatalyst Nuclearity Effects in Single-Site Polymerization. Enhanced Polyethylene Branching and α-Olefin Comonomer Enchainment in Polymerizations Mediated by Binuclear Catalysts and Cocatalysts via a New Enchainment Pathway

The binuclear “constrained geometry catalyst” (CGC) (μ-CH₂CH₂-3,3‘){(η⁵-indenyl )[1-Me₂Si(^tBuN)](ZrMe₂)}₂ [EBICGC(ZrMe₂)₂; Zr₂] and the trityl bisborate dianion (Ph₃C⁺)₂[1,4-(C₆F₅)₃BC₆F₄B(C₆F₅)₃]^2- (B₂) have been synthesized to serve as new types of multicenter homogeneous olefin polymerization catalysts and cocatalysts, respectively. Additionally, the complex [1-Me₂Si(3-ethylindenyl)(^tBuN)]ZrMe₂ (Zr₁) was synthesized as a mononuclear control. For the bimetallic catalyst or bisborate cocatalyst, high effective local active site concentrations and catalyst center−catalyst center cooperative effects are evidenced by bringing the catalytic centers together via either covalent or electrostatic bonding. For ethylene homopolymerization at constant conversion, the branch content of the polyolefin products (primarily ethyl branches) is dramatically increased as catalyst or cocatalyst nuclearity is increased. Moreover, catalyst and cocatalyst nuclearity effects are approximately additive. Compared to the catalyst derived from monometallic Zr₁ and monofunctional Ph₃C⁺B(C₆F₅)₄^- (B₁), the active catalyst derived from bimetallic Zr₂ and bifunctional B₂produces ∼11 times more ethyl branches in ethylene homopolymerization via a process which is predominantly intradimer in character. Moreover, ∼3 times more 1-hexene incorporation in ethylene + 1-hexene copolymerization and ∼4 times more 1-pentene incorporation in ethylene + 1-pentene copolymerization are observed for Zr₂ + B₂ versus Zr₁ + B₁.