Elucidating Secondary Metal Cation Effects on Nickel Olefin Polymerization Catalysts

Secondary metal cations, such as alkali and transition metal ions, have been shown to enhance the catalytic performance of nickel and palladium olefin polymerization catalysts. Their beneficial effects can manifest in different ways, such as increasing rates of polymerization, altering polymer microstructures, enhancing catalyst thermal stability, or a combination of these effects. We have systematically quantified secondary metal ion influences on nickel phenoxyphosphine polyethylene glycol (PEG) complexes. We demonstrate that cation tuning could readily achieve three-dimensional structures and electronic environments that are not easily accessible through conventional ligand tuning. This study led to the development of extremely active ethylene polymerization catalysts. For example, the nickel–lithium complex gave activity and turnover number as high as 7.0 × 104 kg PE/mol Ni·h and 2.5 × 106 mol ethylene/mol Ni, respectively, and the nickel–cesium complex showed unusual thermal stability up to 90 °C (activity = 2.3 × 104 kg/mol h, turnover number = ∼4.1 × 105 mol ethylene/mol Ni, and Mn = 1.6 × 104 g/mol). We provide both experimental and computational data showing that secondary metals impact the relative stability of cis and trans isomers, which is a phenomenon not shown previously. Unlike in our earlier work, which was limited by poor nuclearity control and/or secondary metals that were too far from the catalyst center, the nickel phenoxyphosphine–PEG complex is an ideal platform for future studies of cation-controlled polymerization.