Ethylene Polymerization Properties Research Articles

Designing easily accessible tridentate hafnium catalysts for ethylene/1-octene copolymerization

Polyolefin elastomers (POEs) have attracted widespread interest in both academia and industry as an important class of polyolefin material. Catalyst plays a critical role in the production of high-performance POEs. In this study, we design and synthesize a series of easily accessible tridentate pyridine amino hafnium catalysts with flexible ligands and study their properties in ethylene polymerization and copolymerization with 1-octene. These tridentate pyridine amino hafnium catalysts exhibited activity of up to 3.1 × 106 g mol−1 h−1, producing polyethylene with high molecular weight of up to 64.8 × 104 g mol−1 in ethylene polymerization at 150 °C. More importantly, these hafnium catalysts produced ethylene/1-octene copolymers with high comonomer incorporation of up to 12.9 mol% and molecular weight of up to 111.9 × 104 g mol−1(Mw), along with great mechanical properties (stress up to 17 MPa and strain up to 1000 %).

Emulsion Polymerization Strategy for Heterogenization of Olefin Polymerization Catalysts

Heterogenization of homogeneous metal catalysts on solid supports has been widely studied for product morphology control in ethylene polymerization. In this contribution, an emulsion polymerization strategy was introduced to achieve heterogenization of various imino-based nickel olefin polymerization catalysts. A series of polymeric microspheres were prepared through emulsion copolymerization of vinyl-functionalized imino ligands with comonomers such as styrene or methyl methacrylate. The corresponding heterogeneous nickel catalysts demonstrated superior properties in ethylene polymerization and copolymerization with methyl 10-undecylenate. Their catalytic properties can be controlled by tuning the comonomer type, composition, and particle size. Most importantly, this strategy can achieve product morphology control and avoid reactor fouling while generating polyethylene products with minimum inorganic contamination. In addition, the introduction of polymer microspheres can improve the mechanical properties and surface properties of polyolefin products.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization

AbstractIn this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans–cis isomerization of the azobenzene moiety. The initially targeted azobenzene‐functionalized α‐diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine–amine system bearing interrupted conjugation with the metal center, and a sandwich‐type α‐diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light‐induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light‐mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization.

In this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans-cis isomerization of the azobenzene moiety. The initially targeted azobenzene-functionalized α-diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine-amine system bearing interrupted conjugation with the metal center, and a sandwich-type α-diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light-induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light-mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Hydrogen-Bonding-Induced Heterogenization of Nickel and Palladium Catalysts for Copolymerization of Ethylene with Polar Monomers.

The practical synthesis of polar-functionalized polyolefins using transition-metal-catalyzed copolymerization of olefins with polar monomers is a challenge; the use of heterogeneous catalysts is little explored. Herein, we report the synthesis of heterogeneous naphthoquinone-based nickel (Ni/SiO2 ) and palladium (Pd/SiO2 ) catalysts through hydrogen bonding interactions of the ligands with the silica surface. Ni/SiO2 exhibits high activities (up to 2.65×106 g mol-1 h-1 ) during the copolymerization of ethylene with 5-hexene-1-yl-acetate, affording high-molecular-weight (Mn up to 630 000) polar-functionalized semicrystalline polyethylene (comonomer incorporation up to 2.8 mol %), along with great morphology control. The resulting copolymers possess improved surface properties and great mechanical properties. Pd/SiO2 can mediate ethylene copolymerization with polar monomers with moderate activity to produce high-molecular-weight copolymers with tunable comonomer incorporation.

Influence of Pore Structure and Metal-Node Geometry on the Polymerization of Ethylene over Cr-Based Metal-Organic Frameworks.

Metal–organic frameworks (MOFs) have received increasing interest as solid single‐site catalysts, owing to their tunable pore architecture and metal node geometry. The ability to exploit these modulators makes them prominent candidates for producing polyethylene (PE) materials with narrow dispersity index (Ð) values. Here a study is presented in which the ethylene polymerization properties, with Et2AlCl as activator, of three renowned Cr‐based MOFs, MIL‐101(Cr)‐NDC (NDC=2,6‐dicarboxynapthalene), MIL‐53(Cr) and HKUST‐1(Cr), are systematically investigated. Ethylene polymerization reactions revealed varying catalytic activities, with MIL‐101(Cr)‐NDC and MIL‐53(Cr) being significantly more active than HKUST‐1(Cr). Analysis of the PE products revealed large Ð values, demonstrating that polymerization occurs over a multitude of active Cr centers rather than a singular type of Cr site. Spectroscopic experiments, in the form of powder X‐ray diffraction (pXRD), UV/Vis‐NIR diffuse reflectance spectroscopy (DRS) and CO probe molecule Fourier transform infrared (FTIR) spectroscopy corroborated these findings, indicating that indeed for each MOF unique active sites are generated, however without alteration of the original oxidation state. Furthermore, the pXRD experiments indicated that one major prerequisite for catalytic activity was the degree of MOF activation by the Et2AlCl co‐catalyst, with the more active materials portraying a larger degree of activation.

Study of the composition and morphology of new modifications of titanium‐magnesium catalysts with improved properties in ethylene polymerization

ABSTRACTData on new modifications of supported titanium‐magnesium catalysts (TMCs) with improved performance in ethylene polymerization are reported. These catalysts possess a high and stable activity, an enhanced ability to regulate molecular weight of the polymer by hydrogen, a controllable particle size at a narrow particle size distribution, and the ability to produce the polymer with an increased bulk density. Various physicochemical methods were used to obtain data on the chemical composition of novel supports and catalysts, their phase composition and crystal structure as well as the pore structure. The results obtained were used to discuss possible correlations between composition and structure of TMCs and their catalytic properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2545–2558

Ethylene polymerization by salicylaldimine nickel(II) complexes containing a dibenzhydryl moiety.

The synthesis, characterization and ethylene polymerization properties of a series of salicylaldimine Ni(ii) complexes with a dibenzhydryl moiety are described. These Ni complexes are designed to bear systematically varied electron donating and withdrawing substituents. These complexes are among the most active salicylaldimine Ni(ii) catalysts in ethylene polymerization. Moreover, these complexes demonstrate good thermal stability, maintaining high activity at 80 °C. The polyethylene generated possesses a high molecular weight, moderate branching density and high melting temperature. Surprisingly, the electronic perturbations only exert moderate influence on the ethylene polymerization process despite the covalently close proximity of the electron donating or withdrawing substituents to the metal center. Comparing with the traditional iso-propyl substituted nickel analogue, these new complexes show up to an order of magnitude higher activity, generating polyethylene with much higher molecular weight, similar or slightly lower branching density and much higher melting temperature.

1,2-Azaborolyl-Ligated Titanium(IV) Half-Metallocenes: Synthesis, Characterization, and Catalytic Activity in Ethylene Polymerization

The synthesis and characterization of 1,2-azaborolyl (Ab)-ligated titanium(IV) half-metallocene complexes are presented, and their catalytic properties in ethylene polymerization have been evaluated. Mono-Ab TiCl3 complexes 3 and 4 can be conveniently converted to other half-metallocene derivatives (5–8) in moderate to good yields. The molecular structures of titanium complexes 3–8 have been determined by X-ray diffraction analysis. Upon activation with methylaluminoxanes (MAO), mono-Ab Ti half-metallocenes demonstrated high catalytic activity in ethylene polymerization (1.50 × 107 g of PE (mol of Ti)−1 h–1) at a low [MAO] (Al:Ti = 30) .

Ethylene Polymerization by Xanthene‐Bridged Dinuclear α‐Diimine NiII Complexes

AbstractThe synthesis, characterization, and ethylene polymerization properties of a series of xanthene‐bridged dinuclear α‐diimine NiII complexes are described. The dinucleating α‐diimine ligands were prepared from the xanthene‐bridged di‐anilines, which were synthesized and isolated on the multigram scale without using column chromatography. The di‐anilines could potentially be used to prepare various dinucleating imine‐containing olefin polymerization ligands. In ethylene polymerization, the dinuclear α‐diimine NiII complexes showed good thermal stability and high catalytic activity. In addition, the polyethylene generated by these dinuclear α‐diimine NiII complexes showed high molecular weight, narrow polydispersity, and very low branching density. Metal–metal cooperativity effects was invoked to explain the slow β‐hydride elimination process and the correspondingly slow chain‐walking process.

Zirconium and hafnium complexes with new tetra-azane ligands: Synthesis, characterization and catalytic properties for ethylene polymerization

Two new anilido-imine tetra-azane ligands, 1,2-[(2′-(ArNH)C6H4HCN)]2C6H4 (Ar=2,6-Me2C6H3 (L1H2) and 2,6-iPr2C6H3 (L2H2)), were synthesized by the condensation reaction of o-phenylenediamine with the corresponding 2-(arylamino)benzaldehyde, and their zirconium and hafnium complexes, L1MCl2 (M=Zr (1b), Hf (1c)) and L2MCl2 (M=Zr (2b), Hf (2c)), were synthesized in high yields (61–66%) by the reactions of L1Li2 and L2Li2 with MCl4 in toluene. Direct HCl-elimination reactions of L1H2 with MCl4 (M=Ti, Zr, Hf) in toluene at 140°C under vacuum afforded the products L1HMCl3 [M=Ti (1a′), Zr (1b′), Hf (1c′)] with a partially deprotonated tridentate ligand in good to high yields (48–70%). All the new complexes were characterized by 1H and 13C NMR spectroscopy and the molecular structures of 1b and 2c were determined by single crystal X-ray diffraction analysis. The metal centers in both complexes are six-coordinated with a distorted octahedral geometry. Upon activation with MAO or AlR3/Ph3CB(C6F5)4, complexes 1b–1c and 2b–2c all exhibit moderate catalytic activity for ethylene polymerization and produce linear polyethylene with ultra-high molar masses (100–184×104g/mol).

Highly Robust Palladium(II) α-Diimine Catalysts for Slow-Chain-Walking Polymerization of Ethylene and Copolymerization with Methyl Acrylate.

A series of sterically demanding α-diimine ligands bearing electron-donating and electron-withdrawing substituents were synthesized by an improved synthetic procedure in high yield. Subsequently, the corresponding Pd complexes were prepared and isolated by column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, thus generating polyethylene with a high molecular weight and very low branching density. Similar properties were observed for ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer were semicrystalline solids. The (co)polymers had unique microstructures originating from the unique slow-chain-walking activity of these Pd complexes.

Highly Robust Palladium(II) α‐Diimine Catalysts for Slow‐Chain‐Walking Polymerization of Ethylene and Copolymerization with Methyl Acrylate

AbstractA series of sterically demanding α‐diimine ligands bearing electron‐donating and electron‐withdrawing substituents were synthesized by an improved synthetic procedure in high yield. Subsequently, the corresponding Pd complexes were prepared and isolated by column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, thus generating polyethylene with a high molecular weight and very low branching density. Similar properties were observed for ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer were semicrystalline solids. The (co)polymers had unique microstructures originating from the unique slow‐chain‐walking activity of these Pd complexes.

Further studies on the effects of substituents at the Cp and salicyladiminato ligands of half-sandwich chromium(III) complexes on their catalytic properties for ethylene polymerization

A series of new half-sandwich chromium(III) complexes Cp′[2-R1-6-(CHNR2)C6H2O]CrCl [R1 = iPr (1), tBu (2, 3, 4, 5); R2 = iPr (2, 4, 5), tBu (1, 3); Cp′ = C5H5(1, 2, 3), C5H2Ph3 (4), C5Me4Ph(5)] were synthesized from the reactions of Cp′CrCl2(THF) with corresponding salicyladiminato lithium. All complexes were characterized by elemental analyses and the structures of complexes 1, 2, 3 and 5 were determined by X-ray diffraction analysis. These complexes show moderate catalytic activities for ethylene polymerization upon activation with a relatively small amounts of AlMe3 or AlEt3, producing high molecular weight polyethylene in the form of spherical particles. Under similar conditions, the catalytic activity of complexes carrying a same salicylaldiminato ligand changes with the cyclopentadienyl ligand in the order Me5Cp > Me4PhCp > 1,2,4-Ph3Cp > Cp. The catalytic activity of these catalysts is also affected significantly by the AlR3 cocatalyst in the order AlMe3 > AlEt3.

Mechanochemical synthesis of zirconium and hafnium phenoxyimine complexes L2MCl2 (L = N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroanilinate anion) and their catalytic properties in ethylene polymerization

A new method for preparation of zirconium and hafnium phenoxyimine complexes L2MCl2 (L is N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroanilinate anion, M = Zr, Hf) by the solid state interaction of N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroaniline, the corresponding metal chlorides, and sodium hydride under mechanical activation followed by heating of the activated mixture was developed. The obtained complexes have a high catalytic activity in the reaction of ethylene polymerization.

Ni(II) Phenoxyiminato Olefin Polymerization Catalysis: Striking Coordinative Modulation of Hyperbranched Polymer Microstructure and Stability by a Proximate Sulfonyl Group

The synthesis, structural characterization, and ethylene polymerization properties of two neutrally charged Ni(II) phenoxyiminato catalysts are compared and contrasted. Complex FI-SO2-Ni features a...

Efficient Synthesis of Titanium Complexes Bearing Tridentate [N–,N,O–] Anilido-Imine Ligands and Their Catalytic Properties for Ethylene Polymerization

A series of tridentate anilido-imine [N,N,O] ligands 2-(2-(R12-2,6-C6H3NH)C6H4)HC═N(4-tBu-6-R2-C6H2OH) [R1 = Me, R2 = tBu (LaH2); R1 = Et, R2 = tBu (LbH2); R1 = iPr, R2 = tBu (LcH2); R1 = Me, R2 = Ad (LdH2); R1 = Et, R2 = Ad (LeH2); R1 = iPr, R2 = Ad (LfH2)] were synthesized and characterized. Reaction of the free ligand LdH2 with TiCl4 in toluene at low temperature affords the zwitterionic complex Ld+H2Ti–Cl4 (1d). The zwitterionic complex can be dissolved in THF to form THF-solvated complex 1d′. Heating 1d in toluene at 40 °C affords the neutral complex LdHTiCl3 (2d) by losing a HCl. Complex 2d can be fully converted to the final complex LdTiCl2 (3d) at 140 °C under vacuum by losing another HCl. Complexes 3a–3c, 3e, and 3f were also synthesized in high yields in the same one-pot procedure. All complexes were characterized by 1H and 13C NMR spectroscopy, and the molecular structures of 1d′, 3b, 3d, and 3e were determined by single-crystal X-ray diffraction analysis. The titanium centers in complexes 3b, ...

New Titanium(IV) Complexes with 2-Cyclopentadienylbenzylamido Ligands: Synthesis, Characterization, and Catalytic Properties for Ethylene Polymerization and Copolymerization with 1-Hexene

A series of new half-sandwich titanium(IV) complexes chelated with 2-(tetramethylcyclopentadienyl)benzylamido ligands, 2-Me4CpC6H4CH2(R)NTiCl2 (R = iPr (1), Cy (2), nPr (3), 4-MePh (4)), have been ...

Binuclear zirconium complexes with bidentate N-(ortho-dimethylaminobenzyl)anilide ligands: Synthesis, characterization, and catalytic properties for ethylene polymerization and copolymerization with 1-hexene

A series of binuclear zirconium complexes of the type [ortho-(Me2N)C6H4CH2NArZrCl2(μ-Cl)]2 [Ar=2,6-Me2C6H3 (1), 2,6-Et2C6H3 (2), 2,6-iPr2C6H3 (3)] were synthesized by reactions of ZrCl4 with corresponding ortho-(Me2N)C6H4CH2NArLi. All new zirconium complexes were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The molecular structures of complexes 1 and 2 were determined by single crystal X-ray diffraction analysis. Upon activation with AliBu3 and Ph3CB(C6F5)4, complexes 1–3 exhibit moderate catalytic activity for ethylene polymerization and copolymerization with 1-hexene, producing high or ultra high molecular weight polyethylene or relatively high molecular weight poly(ethylene-co-1-hexene) with reasonable 1-hexene incorporation.

Observations and Mechanistic Insights on Unusual Stability of Neutral Nickel Complexes with a Sterically Crowded Metal Center

A series of novel nickel(II) methyl pyridine complexes based on beta-ketiminato and phenoxyiminato ligands, [(2,6-(i)Pr(2)C(6)H(3))N=CH(8-R(1)C(10)H(7))O]Ni(Me)(Py) (5a, R(1) = phenyl; 5b, R(1) = norbornyl), [(2,6-(i)Pr(2)C(6)H(3))N=CH(8-R(1)C(10)H(5))O]Ni(Me)(Py) (6a, R(1) = phenyl; 6b, R(1) = norbornyl), and [(2,6-(i)Pr(2)C(6)H(3))N=CHCHC(2'-R(2)C(6)H(4))O]Ni(Me)(Py) (7a, R(2) = H; 7b, R(2) = phenyl), have been synthesized and characterized. Molecular structures of 5a and 6b were further confirmed by X-ray crystallographic analysis. These complexes showed very different catalytic properties for ethylene polymerization. Design and construction of a special catalyst structure like 5a with a phenyl group directed toward the nickel(II) center proved to be an effective strategy for improving catalyst stability. Remarkably, according to (1)H NMR spectroscopy, bis-ligated deactivation of complex 5a was entirely avoided, while a rapid deactivation rate was observed with regard to the parent complex C without a phenyl group in the R(1) position. As a result, higher activities of complex 5a were obtained relative to complex C under the same polymerization conditions. In addition, the ligand backbone was found to have a great influence on polymerization behaviors. Specifically, the polyethylenes with greatly decreased molecular weights were produced by neutral nickel phenoxyiminato catalysts 6a,b in comparison with values for the corresponding beta-ketiminato catalysts 5a,b. This is best explained by DFT results that the ethylene insertion and chain termination barriers for complex 6a are very different from those of complex 5a.