Controlling Polyethylene Molecular Weights and Distributions Using Chromium Complexes Supported by SNN-Tridentate Ligands

The
diverse properties and applications of polyethylenes depend
on their molecular weights, molecular weight distributions, and chain
topology. Considering the importance of chromium complexes in catalytic
ethylene polymerization and oligomerization, we have synthesized a
series of Cr complexes (Cr1–Cr6)
bearing a SNN-tridentate ligand. In the presence of MAO as cocatalyst,
complexes Cr1–Cr6 exhibited moderate
to extremely high activity (up to 2.4 × 107 g­(PE)
mol–1(Cr) h–1) toward ethylene
polymerization. The influences of the ligand and of various reaction
parameters, including the nature and amount of the cocatalyst and
the reaction temperature and pressure, were systematically investigated
with Cr1. It was found that lower reaction temperatures
(50 °C) and ethylene pressure (5 atm) and larger MAO/Cr ratios
(1500) favored bimodal distributions with the dominant high-Mw fraction. In contrast, higher reaction temperatures
(≥80 °C) and ethylene pressure (40 atm) and lower MAO/Cr
ratios (≤500) almost exclusively led to the production of low-Mw polyethylene waxes with monomodal and narrow
distributions. Based on DFT calculations and UV–vis–NIR
spectroscopy, two types of active species generated by Cr1 and MAO were proposed to be responsible for the production of bimodal
polyethylene. By tuning the structures of the Cr complexes in the Cr1–Cr6/MAO systems and the reaction conditions,
polyethylenes with molecular weights ranging from low-Mw waxes to UHMWPE and monomodal or bimodal distributions
were readily synthesized.