Unexpected Precatalyst σ‑Ligand Effects in Phenoxyimine Zr‑Catalyzed Ethylene/1-Octene Copolymerizations

Recent
decades have witnessed intense research efforts aimed at
developing new homogeneous olefin polymerization catalysts, with a
primary focus on metal-Cl or metal-hydrocarbyl precursors. Curiously,
metal-NR2 precursors have received far less attention.
In this contribution, the Zr-amido complex FI2ZrX2 (FI = 2,4-di-tert-butyl-6-((isobutyl­imino)­methyl)­phenolate,
X = NMe2) is found to exhibit high ethylene polymerization
activity and relatively high 1-octene coenchainment selectivity (up
to 7.2 mol%) after sequential activation with trimethyl­aluminum,
then Ph3C+B­(C6F5)4−. In sharp contrast, catalysts with traditional
hydrocarbyl ligands such as benzyl and methyl give low 1-octene incorporation
(0-1.0 mol%). This unexpected selectivity persists under scaled/industrial
operating conditions and was previously inaccessible with traditional
metal-Cl or -hydrocarbyl precursors. NMR, X-ray diffraction, and catalytic
control experiments indicate that in this case an FI ligand is abstracted
from FI2Zr­(NMe2)2 by
trimethylaluminum in the activation process to yield a catalytically
active cationic mono-FIZr species. Heretofore this process
was believed to serve only as a major catalyst deactivation pathway
to be avoided. This work demonstrates the importance of investigating
diverse precatalyst monodentate σ-ligands in developing new
catalyst systems, especially for group 4 olefin polymerization catalysts.