The Stronger the Better: Donor Substituents Push Catalytic Activity of Molecular Chromium Olefin Polymerization Catalysts

Abstract

The donor strength of bifunctional pyridine-cyclopentadienyl ligands was altered systematically by the introduction of donating groups in the para-position of the pyridine. In the resulting chromium complexes an almost linear correlation between donor strength and the nitrogenchromium distance as well as the electronic absorption maximum is experimentally observed. The connection of electron-donating groups in the ligand backbone leads to an efficient transfer of the electronic influences to the catalytically active metal centre without restricting it through steric effects. Therefore, catalytic olefin polymerization activity, which is already very high for the previously studied catalysts, increase considerably by attaching para-amino groups to the chelating pyridine or quinoline, respectively. Combining electron-rich indenyl ligands with para-amino substituted pyridines lead to the highest catalytic activities observed so far for this class of organo chromium olefin polymerisation catalysts. The resulting polymers are of ultra-high molecular weight and the ability of the catalysts to incorporate comonomers is also very high.

Chromium complexes supported by NNO-tridentate ligands: an unprecedented activity with the requirement of a small amount of MAO

“…Remarkably, an extremely high activity of 1.18 × 10 8 g(PE) mol −1 (Cr) h −1 was obtained with an Al(MAO)/Cr ratio of 200 at 80 °C and 40 atm of ethylene, and it is among the highest for Cr-mediated ethylene polymerization although performed under different conditions. 41,42,60,61 The GPC analyses of the polymers obtained with Cr2 (THF)/MAO at different pressures show unimodal and narrow distributions (Fig. 3A, Đ < 2.0), indicating the typical single site catalysis.…”

“…7 Chromium-based metal complexes, both heterogeneous and homogeneous, are among the most important polyolefin catalysts and have received increasing attention because of their wide use in both ethylene polymerization and oligomerization. [41][42][43][44][45][46][47][48][49][50][51][52] However, Cr complexes bearing phenoxy-imine ligands are rare and underdeveloped. Previously, Gibson explored this ligand family for Cr-based polymerization catalysts, but only low to modest activity (usually less than 10 5 g(PE) mol −1 (Cr) h −1 ) was obtained.…”

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

“…Therefore, the development of tunable catalytic systems that could produce different polyethylenes is highly desirable but challenging. , One of the industrially most relevant catalyst is the chromium-based Phillips catalyst (CrO x /SiO 2 ). − Discovered in 1951 by Hogan and Banks at Phillips Petroleum, this heterogeneous catalyst allows the manufacture of one-third of the high-density polyethylene (HDPE) commercialized, which is the most widely used polymer . Although the Phillips catalyst has been studied for over 70 years by both academic and industrial scientists, the nature of the initiation process, the coordination environment and the oxidation state of the metal in the active species, and the catalytic mechanism remain a hot and sometimes controversial topic. − Furthermore, much effort has been dedicated to the development of well-defined homogeneous Cr-based catalysts, ,− that are more amenable to detailed spectroscopic and structural studies than heterogeneous catalysts. In particular, multidentate ligands containing N, O, S, and/or P donor atoms have allowed a better control of the metal coordination sphere and a fine-tuning of its stereochemical features toward improved catalytic properties .…”

Solid-State Conformational Isomerism Lacking a Gas-Phase Energy Barrier: Its Structural, Spectroscopic, and Theoretical Identification in an Organochromium(III) Complex

“…We have recently reported the synthesis and catalytic performance of a series of organochromium complexes of type A (Figure ) where cyclopentadienyl or indenyl ligands were combined with pyridines or quinolines, which themselves carry an electron-donating amino group. These complexes, after activation with methylaluminoxane, serve as highly active catalysts for the formation of ultrahigh-molecular-weight polyethylene (UHMW-PE) . The solid-state molecular structures of 12 derivatives of A were determined by XRD and showed the expected geometries .…”

“…These complexes, after activation with methylaluminoxane, serve as highly active catalysts for the formation of ultrahigh-molecular-weight polyethylene (UHMW-PE). 6 The solid-state molecular structures of 12 derivatives of A were determined by XRD and showed the expected geometries. 7 Here, we report the synthesis, structural, and electronic properties of a new member of this series, complex 1 (Figure 1), which exhibits properties that are peculiar in comparison to the derivatives reported earlier.…”