Redox Potential as a Predictor of Polyethylene Branching Using Nickel α‑Diimine Catalysts

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.