The Studies On Synthesis, Properties And Application Of Alkyl Aluminum-activated Metallocene Chromium Catalysts For Olefin Polymerization

Since Kaminsky and Sinn's initial discovery that extremely active catalysts for olefin polymerization can be obtained when titanium and zirconium metallocenes are combined with a hydrated form of trimethylaluminum-methylaluminoxane (MAO), intensive research works have been focused on developing new metallocene catalysts for improving catalytic activities and polymer properties. Metallocene catalysts and related other single-site catalysts have allowed the preparation of a wide array of high performance polyolefin products, including highly isotactic, syndiotactic, and atactic polypropylene, linear high-density polyethylene, linear low-density polyethylene, syndiotactic polystyrene, ethylene/styrene copolymers and cyclo-olefin copolymers. However, metallocene catalysts have to be activated with large amounts of MAO or expensive fluorinated borates, such as Ph3C+B(C6F5)4- and Ph3C+B[C6H3(CF3)2]4-,to achieve high catalytic activities. For some olefin polymerization systems, the cost of the cocatalyst is more than that of the catalyst. In fact, the high cost of MAO and the fluorinated borate cocatalysts has in some extent limited the industrial application of metallocene and some other single-site catalysts. Thus, there is a great need to develop new metallocene and other single-site catalyst systems which can provide equivalent catalytic activity and polymer properties with no need of large amounts of expensive cocatalyst. Chromium-based olefin polymerization catalysts hold a quite interesting position among transition metal olefin polymerization catalysts. In recent years, homogeneous single-site Cr-based catalyst systems both for ethylene oligomerization and polymerization, as well as for ethylene trimerization and tetramerization have been reported. We are interested in developing alkylaluminum activated homogeneous Cr-based olefin polymerization catalyst systems with high catalytic activity.A series of half-metallocene chromium(III) complexes bearing a salicylaldiminato ligand, Cp*[2-R1-4-R2-6-(CH=NR3)C6H2O]CrCl [R1='Pr (1,4),'Bu (2,3,5), Ad (6); R2=H (1,2,3), 'Bu (4,5,6); R3='Pr (1,2,5,6),'Bu (3.4)], were synthesized. All complexes were characterized by elemental analyses and the structures of complexes 1-4.6 were determined by X-ray diffraction analysis. These complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. Upon activation with a small amount of AIR3, complexes 1-6 all exhibit good to high catalytic activity for ethylene polymerization and produce ultra-high molecular weight polyethylene (PE) in quasi living fashion under mild conditions. The catalytic activity of these complexes and the molecular weight of the produced PE can be tuned in a broad range by changing the R1, R2 and R3 groups as well as the AlR3 cocatalyst. The highest catalytic activity of 6048 kg PE mol Cr-1 h-1 and the highest molecular weight (Mη) PE of 2.26×106 g mol-1 were obtained with complex 6. The PE products were obtained with these catalyst systems as spherical particles with a diameter of 1-6 mm.Schiff base salicyladiminato ligands derived from salicylaldehydes bearing varied ortho-substituents, i.e., isopropyl. phenyl. or adamantanyl. reacted with CpCrCl2(THF), afforded a series of half-metallocene chromium(III) complexes 1a-j,2a-b,3a-d and 7-8 which showed high activities for ethylene polymerization upon activation with a small amount of triethylaluminium. All complexes were characterized by elemental analyses and the structures of complexes 1b,1c,1d, 1f,1i,3a,8 were determined by X-ray diffraction analysis. Both the catalytic activity of these catalyst systems and the molecular weight of the produced PE can be tuned in a broad range by changing the ligands and the AIR3 cocatalyst.A series of half-metallocene chromium(III) complexes bearing a salicylaldiminato ligand, Cp'[2-R1-4-R2-6-(CH=NR3)C6H2O]CrCl were synthesized. The structures of complexes 1l and 9-12 were determined by single crystal X-ray diffraction analysis. The X-ray crystallographic analysis indicates that these complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. Upon activation with a small amount of AlEts, complexes 1k-l and 9-12 exhibit good to high catalytic activity for ethylene polymerization and produce ultra-high molecular weight polyethylene (PE) under mild conditions. The catalytic activity of these complexes is relatively low when activated with AlMe3 and Al'Bu3. Both the catalytic activity of these catalyst systems and the molecular weight of the produced PE can be tuned in a broad range by changing the ligands and the AIR3 cocatalyst.A number of new chloride-bridged binuclear half-metallocene chromium(Ⅲ) aryloxide complexes [Cp'Cr(OAr)Cl]2 (Cp.'=C5H5 (5a,5b). C5Me5 (5c,5d); Ar=2,6-lPr2C6H3 (5a,5c), 2.6-'Bu2C6H3 (5b.5d)) have been synthesized from the reaction of CpCrCl2(THF) with the lithium salt of corresponding aryloxide ligand in THF. All complexes were characterized by elemental analyses,1H NMR, IR and UV-vis spectroscopy. The molecular structures of complexes 5a and 5c were determined by X-ray crystallography. Upon activation with a small amount of AIR3. all complexes 5a-5d show high catalytic activity for ethylene polymerization, producing high molecular weight polyethylene. The effects of polymerization conditions such as polymerization temperature, alkyl aluminum cocatalyst. and Al/Cr molar ratio on the catalytic activity and properties of the produced polyethylene were investigated.