Ni-catalyzed olefin polymerization has been predominantly guided by ligand design and complex structure, rather than the activation of these complexes, to achieve a wider variety of polyolefin micro- and macrostructures. Early transition metal complexes demonstrate the importance of the counterion in controlling polyolefin microstructures, but the influence and impact of counterion effects in late transition complexes, specifically for [N, N]- chelated Ni^II complexes have been largely unexplored, due to the absence of a suitable thermodynamically stable complexes. We have developed air and moisture-stable, alkyl or aryl-functionalized Ni^II precatalysts and their mode of activation has been investigated.
The presentation will demonstrate that the need for in-situ alkylation is eliminated and that the complex is activated solely through halide abstraction to form the cationic complex required for polymerization. These catalysts represent an emerging class, which has opened the doors for further exploration of different cocatalysts that form either inner- or outer-sphere ion pairs. The activation with an organoboron cocatalyst results in a well-defined ion pair which, in contrast to ill-defined organoaluminum cocatalysts, can activate the complex via halide abstraction, producing higher molecular weight homo- and copolymers. Comparatively broader branching densities and a gradual increase in short chain branches were achieved under high pressure , which circumvents the need for an elaborated ligand design and copolymerization with α-olefins. DFT studies were utilized to elucidate the ion pair interactions as well as the unique chain-walking mechanism of these Ni^II complexes. Along with the variation in steric bulk over the iminopyridyl ligand, a phenyl group on the bridging carbon was introduced which imposes a rotational barrier, producing higher molecular weight polymers compared to the methyl-substituted analogs. The presentation will illustrate how the iminopyridyl Ni^II system can be modulated, leveraging ion pair interactions and ligand architecture to tune polyolefin molecular weights and microstructures.

Eva Harth

Vanderbilt Univ

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