| Polyolefins are produced catalytically on a vast scale, and the manufactured polymers find use in everything from artificial limbs and food/medical packaging to automotive components and lubricants. New developments in multimetalllic olefin polymerization catalysis are discussed and how modifications in the architecture of the organic ligands surrounding metal centers, the metal···metal proximity, and activators (cocatalysts) can dramatically modify product polyolefin molecular weight, branch structure, and the selectively for incorporating other olefinic comonmers into the polymer chains is shown.;At room temperature, a neutrally charged electron-deficient nickel(II) phenoxyiminato catalyst with an intramolecular hydrogen-bond directed toward the active catalytic site (1b) exhibits 2.5x greater ethylene polymerization activity and 2x greater polyethylene product branching than an analogous catalyst without the hydrogen-bond (2b). Furthermore, catalyst 1b produces substantially greater polyethylene yields in the presence of polar additives such as ethyl ether, acetone, and water, than does 2b, suggesting that the hydrogen-bonding proximate to the metal center significantly modifies the relative rates of competing enchainment and chain transfer processes.;Complex FI-SO2-Ni features a -SO2 - group embedded in the ligand skeleton while control FI-CH2-Ni has the -SO2- replaced by a -CH 2- functionality. In comparison to FI-CH2-Ni, at 25°C FI-SO2-Ni is 18x more active, produces polyethylene with 3.2x greater MW and 1.5x branch content, and is significantly more thermally stable. The FI-SO2-Ni-derived polymer is a hyperbranched polyethylene versus that from FI-CH2-Ni. DFT calculations argue that the distinctive FI-SO2-Ni catalytic behavior versus that of FI-CH2-Ni is associated with non-negligible OSO···Ni interactions involving the activated catalyst.;Two diastereomers (rac-3 and meso-3) of a bimetallic titanium constrained geometry catalyst are isolated, and detailed characterization reveals that interconversion is not possible upon heating or during polymerization. Activation studies with [Ph3C][B(C6F5)4] indicate that the two diastereomers participate in unique activation mechanisms, both resulting in the formation of mu-CH2, mu-CH3 binuclear titanium monocationic complexes. Ethylene homopolymerizations and ethylene/1-octene copolymerizations indicate that distinct cooperative effects, in terms of product molecular weights and comonomer enchainment selectivity, are achieved by bimetallic rac-3 and meso-3 as compared to a monometallic catalyst control. |
