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Liu C, Feng W, Liu S, Kan Z, Li Z. Development of Group IV Metal Complexes Bearing Thioether-Amido Ligands with Enhanced High-Temperature Catalytic Performance toward Olefin Copolymerization. Inorg Chem 2024; 63:19676-19686. [PMID: 39365980 DOI: 10.1021/acs.inorgchem.4c02857] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/06/2024]
Abstract
The development of homogeneous metal catalysts with both high activity and exceptional thermal stability is crucial for the efficient synthesis of polyolefin elastomers (POEs) through solution-phase olefin polymerization. In this study, a series of Hf (Hf1-Hf5), Zr (Zr1), and Ti (Ti1) complexes featuring thioether-amido ligands were synthesized and carefully characterized using advanced techniques such as 1H and 13C NMR spectroscopy as well as single-crystal X-ray diffraction analysis for Hf4 and Hf5. The results revealed that the catalytic activity and 1-octene incorporation efficiency of these metal complexes followed the trend Hf > Zr > Ti, underscoring the significant impact of the metal center on catalytic performance. Furthermore, the choice of ligands was found to play a critical role in dictating the catalytic properties, with ligands bearing less steric hindrance on the sulfur atom proving to be more favorable for copolymerization reactions. Notably, the Hf complex Hf1, featuring a methyl group on the sulfur atom, displayed exceptional catalytic activity as high as 21,060 kg(polymer)·mol-1(Hf)·h-1 toward ethylene/1-octene copolymerization at 120 °C and produced POE with a high molecular weight (Mw = 6.3 × 104 g·mol-1), relatively narrow distribution (Đ = 2.4), and high incorporation of 1-octene (34.1 mol %). This study demonstrates the potential of tailored ligand design in developing efficient metal catalysts for the production of high-value-added POEs.
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Affiliation(s)
- Chuanhui Liu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Wenzheng Feng
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Shaofeng Liu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Ze Kan
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Zhibo Li
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
- College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
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2
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Gao Z, Tian J, Han Y, Liu S, Li Z. Zirconium and Hafnium Complexes Bearing Tridentate ONN-Ligands: Extremely High Activity toward Ethylene (Co)Polymerization. Inorg Chem 2024; 63:18137-18145. [PMID: 39287224 DOI: 10.1021/acs.inorgchem.4c02906] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/19/2024]
Abstract
The pursuit of high-performance catalysts in the realm of polyolefins is a constant goal. In this study, a range of zirconium (1-ZrCl3, 2-ZrCl3, 3-ZrCl4, 12-Zr) and hafnium (1-HfCl3, 12-Hf) complexes featuring phenoxy-imine-amine ONN-ligands (2,6-R2-C6H3-NH-C6H4-N═CH-C6H2-3,5-tBu2-OH; 1-L: R = H; 2-L: R = F; 3-L: R = iPr) were synthesized and characterized using NMR spectroscopy, as well as single-crystal X-ray diffraction for 2-ZrCl3, 3-ZrCl4, and 12-Zr. These Zr and Hf complexes exhibited remarkable efficiency for ethylene homopolymerization and copolymerization with 1-octene when paired with MAO as the cocatalyst. Notably, the Zr complexes outperformed the Hf complexes with the same ligand, underscoring the substantial impact of the metal center on catalytic performance. The substituents and coordination modes of the ligands also exerted significant influence on the catalytic behavior, affecting both the activity and properties of the resulting polymers. Particularly noteworthy was the exceptional activity of 1-ZrCl3, achieving activity as high as 6.30 × 108 g(PE)·mol-1(Zr)·h-1 for ethylene homopolymerization and generating bi- or multimodal distribution polyethylene. The activation of 1-ZrCl3 by 5 or 20 equiv of d-MAO afforded a dinuclear Zr complex bridged by two chlorides (μ-Cl2-(1-ZrCl2)2), which was analyzed and confirmed by in situ 1H NMR spectroscopy and single-crystal X-ray diffraction.
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Affiliation(s)
- Zhihao Gao
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Jiliang Tian
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Yingxia Han
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Shaofeng Liu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Zhibo Li
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
- College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
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Kulyabin PS, Goryunov GP, Iashin AN, Mladentsev DY, Uborsky DV, Ehm C, Canich JAM, Hagadorn JR, Voskoboynikov AZ. Reversible C-C bond formation in group 4 metal complexes: nitrile extrusion via β-aryl elimination. Chem Sci 2024:d4sc02173h. [PMID: 39268207 PMCID: PMC11388100 DOI: 10.1039/d4sc02173h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2024] [Accepted: 08/16/2024] [Indexed: 09/15/2024] Open
Abstract
Pyridylamides of zirconium and hafnium with [C,N,N]-ligands reversibly insert nitriles into M-CAr bonds leading to an observable equilibrium between the starting [C,N,N]-complexes and newly formed [N,N,N]-complexes with a ketimide moiety in a 7-membered metallacycle. The discovered reversible insertion of nitriles into M-CAr bonds represents an unprecedented example of β-aryl elimination from a ketimide ligand in early transition metal complexes. Experimental and computational studies suggest thermodynamic and electronic reasons for this reactivity. Weak orbital overlap between the ketimide nitrogen and the metal, and an unfavorable 7-membered metallacycle destabilize the product of insertion into the M-CAr bond, while the pyridylamide moiety acts as a directing group making the reverse process viable. The influence of non-chelate spectator ligands on the metal center and substituents in nitrile on the thermodynamic stability of the [N,N,N]-complexes was also studied. Exploiting β-carbon elimination in complexes of early transition metals may extend the range of catalysts that are accessible for C-C activation processes in the future.
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Affiliation(s)
- Pavel S Kulyabin
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
| | - Georgy P Goryunov
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
| | - Andrei N Iashin
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
| | - Dmitry Y Mladentsev
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
| | - Dmitry V Uborsky
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
| | - Christian Ehm
- Dipartimento di Scienze Chimiche, Università di Napoli Federico II Via Cintia Napoli 80126 Italy
| | - Jo Ann M Canich
- Baytown Technology and Engineering Complex, ExxonMobil Technology and Engineering Company Baytown Texas 77520 USA
| | - John R Hagadorn
- Baytown Technology and Engineering Complex, ExxonMobil Technology and Engineering Company Baytown Texas 77520 USA
| | - Alexander Z Voskoboynikov
- Department of Chemistry, M. V. Lomonosov Moscow State University Leninskie Gory, 1/3 Moscow 119991 Russian Federation
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Wen Z, Wu C, Chen J, Qu S, Li X, Wang W. Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization. Polymers (Basel) 2024; 16:406. [PMID: 38337295 DOI: 10.3390/polym16030406] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2024] [Revised: 01/19/2024] [Accepted: 01/26/2024] [Indexed: 02/12/2024] Open
Abstract
The development of catalysts has significantly advanced the progress of polyolefin materials. In particular, group 4 (Ti, Zr, Hf) non-metallocene catalysts ligated with [N,N] bidentate ligand(s) have garnered increasing attention in the field of olefin polymerization due to their structurally stability and exceptional polymerization behaviors. Ligands containing nitrogen donors are diverse and at the core of many highly active catalysts. They mainly include amidine, guanidinato, diamine, and various N-heterocyclic ligands, which can be used to obtain a series of new polyolefin materials, such as ultrahigh molecular weight polyethylene (UHWMPE), olefin copolymers (ethylene/norbornene and ethylene/α-olefin) with high incorporations, and high isotactic or syndiotactic polypropylene after coordination with group 4 metals and activation by cocatalysts. Herein, we focus on the advancements and applications of this field over the past two decades, and introduce the catalyst precursors with [N,N] ligand(s), involving the effects of ligand structure, cocatalyst selection, and polymerization conditions on the catalytic activity and polymer properties.
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Affiliation(s)
- Zhao Wen
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
| | - Changjiang Wu
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
| | - Jian Chen
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
| | - Shuzhang Qu
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
| | - Xinwei Li
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
| | - Wei Wang
- SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
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Fang XY, Qin L, Liu J, Shi H, Sun XL, Kuang X, Gao Y, Tang Y. Synthesis and characterization of oxazoline-amine zirconium complexes for ethylene homo- and co-polymerization catalysis. MOLECULAR CATALYSIS 2023. [DOI: 10.1016/j.mcat.2023.113108] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/31/2023]
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Nifant'ev IE, Vinogradov AA, Vinogradov AA, Minyaev ME, Bagrov VV, Salakhov II, Shaidullin NM, Chalykh AE, Shapagin AV, Ivchenko PV. Heterocene-catalyzed ethylene/oct-1-ene copolymerization under MAO-free and low-MAO conditions: The synthesis of highly statistical copolymers and their use in blending with HDPE. POLYMER 2023. [DOI: 10.1016/j.polymer.2023.125836] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/15/2023]
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7
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Xing Y, Xu L, Liu S, Li Z. Dinuclear Group 4 Metal Complexes Bearing Anthracene-Bridged Bifunctional Amido-Ether Ligands: Remarkable Metal Effect and Cooperativity toward Ethylene/1-Octene Copolymerization. Inorg Chem 2023; 62:2859-2869. [PMID: 36719090 DOI: 10.1021/acs.inorgchem.2c04211] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Two types of bifunctional amido-ether ligands (syn-L and anti-L) with the rigid anthracene skeleton were designed to support dinuclear group 4 metal complexes. All organic ligands and organometallic complexes (syn-M2 and anti-M2; M = Hf, Zr, and Ti) were fully characterized by 1H and 13C NMR spectroscopies and elemental analyses. The anti-Hf2 complex showed two confirmations at room temperature with C2-symmetry or S2-symmetry that can inter-exchange, as indicated by VT NMR, while only a C2-symmetric isomer was observed for syn-Hf2 complex at room temperature. However, for Zr and Ti analogues, both syn and anti complexes exhibited only one conformation at room temperature. The molecular structures of complexes syn-Hf2, anti-Hf2, and syn-Ti2 in the solid state were further determined by single-crystal X-ray diffraction, revealing the distances between two metal centers in syn-M2 from 7.138 Å (syn-Ti2) to 7.321 Å (syn-Hf2) but a much farther separation in anti-M2 (8.807 Å in C2-symmetric anti-Hf2). The mononuclear complex (2-CH3O-C6H4-N-C14H9)Zr(NMe2)3 (mono-Zr1) was also prepared for control experiments. In the presence of alkyl aluminum (AlEt3) as the alkylating agent and trityl borate ([Ph3C][B(C6F5)4]) as the co-catalyst, all metal complexes were tested for copolymerization of ethylene with 1-octene at high temperature (130 °C). The preliminary polymerization results revealed that the activity was highly dependent upon the nature of metal centers, and syn-Zr2 showed the highest activity of 9600 kg(PE)·mol-1 (Zr)·h-1, which was about 17- and 2.2-fold higher than those of syn-Hf2 and syn-Ti2, respectively. Benefitting from both steric proximity and electronical interaction of two metal centers, syn-Zr2 exhibited significant cooperativity in comparison to anti-Zr2 and mono-Zr1, with regard to activity and molecular weight and 1-octene incorporation of resultant copolymers.
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Affiliation(s)
- Yanhong Xing
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Lingling Xu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Shaofeng Liu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Zhibo Li
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.,College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
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8
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Chen Y, Zhou S, Yang W, Liu S. Hafnium and Zirconium Complexes Bearing SNN-Ligands Enhancing Catalytic Performances toward Ethylene/1-Octene Copolymerization. Organometallics 2022. [DOI: 10.1021/acs.organomet.2c00485] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Affiliation(s)
- Yanjun Chen
- Zhejiang Collaborative Innovation Center for High Value Utilization of Byproducts from Ethylene Project, Ningbo Polytechnic, Ningbo 315800, China
- College of Chemical Engineering, Ningbo Polytechnic, Ningbo 315800, China
| | - Shengmei Zhou
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
| | - Weiqun Yang
- Zhejiang Collaborative Innovation Center for High Value Utilization of Byproducts from Ethylene Project, Ningbo Polytechnic, Ningbo 315800, China
- College of Chemical Engineering, Ningbo Polytechnic, Ningbo 315800, China
| | - Shaofeng Liu
- Zhejiang Collaborative Innovation Center for High Value Utilization of Byproducts from Ethylene Project, Ningbo Polytechnic, Ningbo 315800, China
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
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Tian J, Feng W, Liu S, Li Z. Titanium Complexes Bearing
NNO‐Tridentate
Ligands: Highly Active Olefin Polymerization Catalysts with Great Control on Molecular Weight and Distribution. CHINESE J CHEM 2022. [DOI: 10.1002/cjoc.202200449] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Affiliation(s)
- Jiliang Tian
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering Qingdao University of Science and Technology Qingdao 266042 China
| | - Wenzheng Feng
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering Qingdao University of Science and Technology Qingdao 266042 China
| | - Shaofeng Liu
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering Qingdao University of Science and Technology Qingdao 266042 China
| | - Zhibo Li
- Key Laboratory of Biobased Polymer Materials, College of Polymer Science and Engineering Qingdao University of Science and Technology Qingdao 266042 China
- College of Chemical Engineering Qingdao University of Science and Technology Qingdao 266042 China
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Tian J, Zhang X, Liu S, Li Z. Chromium Complexes Supported by NNO-Tridentate Ligands: An Unprecedent Activity with the Low Requirement of MAO. Polym Chem 2022. [DOI: 10.1039/d2py00125j] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The development of metal catalysts with high activity and thermal stability but low requirement of MAO as cocatalyst is highly desired for polyolefin industrial application. In this contribution, a series...
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Huo Y, Hu X, Wang J, Hu H, Shi X. Amido-trihydroquinoline ligated rare-earth metal complexes for polymerization of isoprene. NEW J CHEM 2022. [DOI: 10.1039/d2nj00707j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
In combination with a borate, the amido-trihydroquinoline ligated rare-earth metal complexes (Ln = Y, Lu) showed moderate catalytic activities for isoprene polymerization to generate 1,4-enriched polyisoprenes.
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Affiliation(s)
- Yanchen Huo
- Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University, Materials Building, Nanchen Street 333, Shanghai 200444, China
| | - Xiang Hu
- Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University, Materials Building, Nanchen Street 333, Shanghai 200444, China
| | - Jixing Wang
- Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University, Materials Building, Nanchen Street 333, Shanghai 200444, China
| | - Hongfan Hu
- Petrochemical Research Institute, PetroChina, Block A42, Science Base Petro China, Shahe Town, Changping District, Beijing 102206, China
| | - Xiaochao Shi
- Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University, Materials Building, Nanchen Street 333, Shanghai 200444, China
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Copolymerization of Ethylene with Selected Vinyl Monomers Catalyzed by Group 4 Metal and Vanadium Complexes with Multidentate Ligands: A Short Review. Polymers (Basel) 2021; 13:polym13244456. [PMID: 34961007 PMCID: PMC8708287 DOI: 10.3390/polym13244456] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2021] [Revised: 12/13/2021] [Accepted: 12/16/2021] [Indexed: 11/17/2022] Open
Abstract
This paper gives a short overview of homogeneous post-metallocene catalysts based on group 4 metal and vanadium complexes bearing multidentate ligands. It summarizes the catalytic behavior of those catalysts in copolymerization of ethylene with 1-olefins, with styrenic monomers and with α,ω-alkenols. The review is focused on finding correlations between the structure of a complex, its catalyst activity and comonomer incorporation ability, as well as the microstructure of the copolymer chains.
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Synthesis of Ethylene/1-Octene Copolymers with Ultrahigh Molecular Weights by Zr and Hf Complexes Bearing Bidentate NN Ligands with the Camphyl Linker. Catalysts 2021. [DOI: 10.3390/catal11020276] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Ultrahigh molecular weight polyethylene (UHMWPE) is a class of high-performance engineering plastics, exhibiting a unique set of properties and applications. Although many advances have been achieved in recent years, the synthesis of UHMWPE is still a great challenge. In this contribution, a series of zirconium and hafnium complexes, [2,6-(R1)2-4-R2-C6H2-N-C(camphyl)=C(camphyl)-N-2,6-(R1)2-4-R2-C6H2]MMe2(THF) (1-Zr: R1 = Me, R2 = H, M = Zr; 2-Zr: R1 = Me, R2 = Me, M = Zr; 1-Hf: R1 = Me, R2 = H, M = Hf; 2-Hf: R1 = Me, R2 = Me, M = Hf), bearing bidentate NN ligands with the bulky camphyl backbone were synthesized by the stoichiometric reactions of α-diimine ligands with MMe4 (M = Hf or Zr). All Zr and Hf metal complexes were analyzed using 1H and 13C NMR spectroscopy, and the molecular structures of complexes 1-Zr and 1-Hf were determined by single-crystal X-ray diffraction, revealing that the original α-diimine ligand was selectively reduced into the ene-diamido form and generated an 1,3-diaza-2-metallocyclopentene ring in the metal complexes. Zr complexes 1-Zr and 2-Zr showed moderate activity (up to 388 kg(PE)·mol−1(M)·h−1), poor copolymerization ability, but unprecedented molecular weight capability toward ethylene/1-octene copolymerization. Therefore, copolymers with ultrahigh molecular weights (>600 or 337 × 104 g∙mol−1) were successfully synthesized by 1-Zr or 2-Zr, respectively, with the borate cocatalyst [Ph3C][B(C6F5)4]. Surprisingly, Hf complexes 1-Hf and 2-Hf showed negligible activity under otherwise identical conditions, revealing the great influence of metal centers on catalytic performances.
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