Simple and Sterically Open Nickel Catalyst for Ultrahigh Molecular Weight Polyethylene Synthesis

The
suppression of chain transfer reactions to produce high-molecular-weight
polymers is a long-standing issue in catalytic polymerization, especially
in nickel-mediated olefin polymerization. Installing sterically hindered
ligands into nickel catalysts has become a general principle for producing
high-molecular-weight polyolefins, which are globally important plastics.
Herein, we report a new concept for preparing ultrahigh-molecular-weight
polyethylene (UHMWPE) using an extremely simple nickel catalyst. Bis(o-iminobenzosemiquinonato)nickel precatalysts derived from
sterically open and redox-active noninnocent o-iminobenzoquinone
ligands are easily prepared from commercially available compounds,
which are air-stable and highly soluble. Activation at 20- or 50-fold
of Et2AlCl enables the nickel precatalysts to produce UHMWPEs
(Mn up to 4,211 kDa) under ambient conditions.
In particular, functionalized UHMWPEs are accessible at only 1 bar.
Experimental evidence proposes the formation of a six-coordinate octahedral
nickel active species and rules out the classical four-coordinate
square-planar nickel active species. Density functional theory calculations
further indicate the suppression of chain transfer by this unprecedented
nickel platform with sterically open ligands.