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

Abstract

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 × 10 7 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-M w 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-M w 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-M w waxes to UHMWPE and monomodal or bimodal distributions were readily synthesized.

Hafnium and Zirconium Complexes Bearing SNN-Ligands Enhancing Catalytic Performances toward Ethylene/1-Octene Copolymerization

“…During the reaction, one methyl group migrated from Hf to the carbon of imine (CHN), which led to the addition of imino group and formed the amido-coordination. Accordingly, there was no signal for imino group anymore in the low field of 1 H NMR spectrum of Hf1 (Figure S1), which was a sharp singlet at 8.52 ppm in 1 H NMR spectrum of L1 . The resonance at 4.93 ppm was attributed to CH 3 C H N and exhibited a quartet peak due to its coupling with the migrating methyl group (doublet at 1.43 ppm), which was confirmed by 1 H– 1 H COSY NMR (Figure S3).…”

“…The thio-imino-quinoline ligands ( L1 : R = i Pr; L2 : R = 2,6-Me-C 6 H 3 , Scheme ) were prepared using the procedure reported previously . For the synthesis of metal complexes, HfMe 4 was in situ prepared by the reaction of HfCl 4 with 4 equiv.…”

“…Melting points were determined from the second scan at a heating rate of 10 °C/min following a slow cooling rate of 10 °C/min to remove the influence of thermal history. L1 and L2 were synthesized according to our previous method …”

“…Recently, our groups reported chromium complexes bearing SNN-tridentate ligands that exhibited intriguing catalytic properties toward ethylene polymerization and synthesized a series of high value-added polyethylenes, including low-M w polyethylene waxes, ultrahigh-molecular-weight polyethylene, and bimodal polyethylene. 51 In this context, we herein report the synthesis of Hf and Zr complexes containing SNNtridentate thio-amino-quinoline ligands and their catalytic properties toward copolymerization of ethylene and 1-octene at high temperature (130 or 150 °C).…”

Titanium Complexes Bearing NNO‐Tridentate Ligands: Highly Active Olefin Polymerization Catalysts with Great Control on Molecular Weight and Distribution†

“…[ 10‐17 ] These non‐metallocene catalysts can show high activity, great control on molecular weight ( M w ) and distribution and produce distinctive polyolefins that are difficult to prepare by heterogeneous catalysts or conventional metallocene catalysts. [ 18‐26 ]…”