The ability to control polyethylene branching density is of great interest as a means by which a polymer’s thermomechanical properties may be tailored. One particularly interesting way in which this can be achieved is by altering the electronic characteristics of Pd- and Ni-based α-diimine catalysts through the inclusion of electron-withdrawing or electron-donating substituents onto the ligand scaffold; however, a few critical fundamental studies are absent from the literature. These include a systematic examination of electronic perturbations of Ni-based α-diimine catalysts, as well as how placement of donating or withdrawing substituents on the backbone versus N-aryl moieties of the α-diimine ligand framework impact polymer topology. In addition, no method currently exists by which the polymer topology may be predicted based on an intrinsic characteristic of the (pre)catalyst or ligand without requiring extensive polymerization studies. Herein, we use both experimental and computational methods to understand how the placement of electron-donating or electron-withdrawing substituents on Ni α-diimine catalysts affects PE branching density, and compare those results to the analogous unsubstituted catalyst. We will show that inclusion of electron-withdrawing substituents decreases resultant PE branching density, whereas electron-donating substituents exhibit little to no change in PE branching density. Finally, we will show that as the placement and identity of donating or withdrawing substituents are varied, so too is the redox half-wave potential (E1/2) of the precatalysts, which can be used to generate a predictive curve by which PE branching density may be estimated for other substituted Ni-based α-diimine catalysts without the need for extensive polymerization studies.
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