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-CH<sub>2</sub>CH<sub>2</sub>-3,3‘){(η<sup>5</sup>-indenyl )[1-Me<sub>2</sub>Si(<i><sup>t</sup></i><sup></sup>BuN)](ZrMe<sub>2</sub>)}<sub>2</sub> [EBICGC(ZrMe<sub>2</sub>)<sub>2</sub>; <b>Zr</b><b><sub>2</sub></b>] and the trityl bisborate dianion (Ph<sub>3</sub>C<sup>+</sup>)<sub>2</sub>[1,4-(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>BC<sub>6</sub>F<sub>4</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>2-</sup> (<b>B</b><b><sub>2</sub></b>) have been synthesized to serve as new types of multicenter homogeneous olefin polymerization catalysts and cocatalysts, respectively. Additionally, the complex [1-Me<sub>2</sub>Si(3-ethylindenyl)(<i><sup>t</sup></i><sup></sup>BuN)]ZrMe<sub>2</sub> (<b>Zr</b><b><sub>1</sub></b>) 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 <b>Zr</b><b><sub>1</sub></b> and monofunctional Ph<sub>3</sub>C<sup>+</sup>B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub><sup>-</sup> (<b>B</b><b><sub>1</sub></b>), the active catalyst derived from bimetallic <b>Zr</b><b><sub>2</sub></b> and bifunctional <b>B</b><b><sub>2</sub></b><b></b>produces ∼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 <b>Zr</b><b><sub>2</sub></b> + <b>B</b><b><sub>2</sub></b> versus <b>Zr</b><b><sub>1</sub></b> + <b>B</b><b><sub>1</sub></b>.