Systematic Investigations of Ligand Steric Effects on α‑Diimine Nickel Catalyzed Olefin Polymerization and Copolymerization

So far, ligand steric
effects of the α-diimine nickel catalysts
on the polyolefin branching densities are not systematically investigated.
Generally, in contrast to the α-diimine palladium systems, the
branching densities of the polyethylene obtained by the α-diimine
nickel catalysts increased when the more sterically encumbering substituent
was employed. In this contribution, we described the synthesis and
characterization of a series of α-diimine ligands and the corresponding
nickel catalysts bearing the diarylmethyl moiety and varied steric
ligands. In ethylene polymerization, the catalytic activities [(2.82–15.68)
× 106 g/(mol Ni·h)], polymer molecular weights
[Mn: (0.37–131.51) × 104 g mol–1], branching densities [(28–81)/1000
C], and polymer melting temperatures (−4.7–122.9 °C)
can be tuned over a very wide range. To our surprise, the polymer
branching density first rose and then fell when we systematically
increased the steric bulk of α-diimine nickel catalysts, like
a downward parabola, not in line with previous conclusions. In ethylene-methyl
10-undecenoate (E-UA) copolymerization, the catalytic activities [(1.0
× 103) – (104.8 × 104) g/(mol
Ni·h)], copolymer molecular weights [(1.2 × 103) – (242.4 × 103) g mol–1], branching densities [(42–70)/1000 C], and UA incorporation
ratio (0.17–2.12%) can also be controlled over a very wide
range. The tuning in steric ligands enables the tuning of the polymer
microstructures such as molecular weight and branching density. In
this way, the best polyethylene elastomer catalysts are screened out.