α-Iminocarboxamidato Nickel Complexes

Polyolefins constitute a vast class of macromolecules that range from everyday plastics to sophisticated materials used as hip-replacement joints and in bullet-proof vests. By tailoring the molecular structure of the backbone, researchers can tune the bulk properties of these materials for specific product applications. Transition-metal complexes that mediate the polymerization of ethylene, 1-alkenes, and related monomers provide an important tool for industrial preparation. Although commercially viable options exist for large-scale manufacturing, it is not possible to achieve all desirable polymer targets using these methods. For example, the copolymerization of ethylene with co-monomers bearing polar functionalities under living conditions, where there is high activity and minimum chain transfer and termination, is not available at this scale. Also, given the magnitude of polyolefin production, we anticipate a need to more efficiently manage chemical resources. For these reasons, new metal/catalyst combinations and methods for their applications continue to be an active area of research in both academic and industrial laboratories. This Account focuses on the chemistry of alpha-iminocarboxamidato complexes of nickel carried out in our laboratories. Such complexes can serve as precursors to zwitterionic or neutral active species, depending upon the mode of activation. We have successfully applied the zwitterionic systems in the tandem action of multiple active sites to yield branched polyethylene from ethylene alone; a coordinated effort of catalysts that is reminiscent of metabolic pathways. These polymerizations show many of the characteristics of living polymerizations. The neutral initiators allow for the controlled preparation of copolymers containing ethylene and functionalized co-monomers and can be used to access higher order polymer architectures, such as block, tapered, and pseudo-tetrablock copolymers. Many of these polymers are not readily available using other catalytic methods. In this Account, we first provide a historical narrative of developments leading to ligands derived from the alpha-iminocarboxamide framework. Second, we explore how structural variations of this ligand system are readily obtained and how they can be used to provide insight into the mechanism of activation and for controlling polymerization reactions. We conclude by describing emerging applications, particularly in the area of graft copolymers.