High Temperature, Living Polymerization of Ethylene by a Sterically-Demanding Nickel(II) α-Diimine Catalyst.

Catalysts that employ late transition-metals, namely Ni and Pd, have been extensively studied for olefin polymerizations, co-polymerizations, and for the synthesis of advanced polymeric structures, such as block co-polymers. Unfortunately, many of these catalysts often exhibit poor thermal stability and/or non-living polymerization behavior that limits their ability to access tailored polymer structures. Due to this, the development of catalysts that display controlled/living behavior at elevated temperatures is vital. In this manuscript, we describe a Ni α-diimine complex that is capable of polymerizing ethylene in a living manner at temperatures as high as 75 °C, which is one of the highest temperatures reported for the living polymerization of ethylene by a late transition metal-based catalyst. Furthermore, we will demonstrate that this catalyst's living behavior is not dependent on the presence of monomer, and that it can be exploited to access polyethylene-based block co-polymers.

Redox-Active Ligands: An Advanced Tool To Modulate Polyethylene Microstructure.

The ability to control catalytic activity and selectivity via in situ changes in catalyst oxidation-state represents an intriguing tool for enhanced polymerization control. Herein, we report foundational evidence that catalysts bearing redox-active moieties may be used to synthesize high molecular weight polyethylene with tailored microstructure. The ability to modulate branching density and identity is facilitated by ligand-based redox chemistry, and is realized via the addition of chemical reductants into the polymerization reactor. Detailed GPC and NMR analyses demonstrate that branching density may be altered by up to ∼ 30% as a function of in situ added reductant.

Modulating Polyolefin Copolymer Composition via Redox-Active Olefin Polymerization Catalysts.

The ability to precisely modulate polymer architecture and composition is a long-standing goal within the field of polymer synthesis. Herein, we demonstrate that redox-active olefin polymerization catalysts may be used to predictably tailor polyolefin comonomer incorporation levels for the copolymerization of ethylene and higher α-olefins. This ability is facilitated via the utilization of a redox-active olefin polymerization catalyst that once reduced via in situ addition of a chemical reductant results in a notable drop in α-olefin incorporation. We attribute this behavior to the reduced catalyst's increased electron density and its concomitant decreased rate of α-olefin consumption. These results are supported by investigations into propylene and 1-hexene homopolymerizations as well as detailed GPC, DSC, GC, and NMR analyses.