Half-titanocene complexes bearing bulky dibenzhydryl-substituted aryloxide ligand: Syntheses, characterization, and ethylene (Co-)polymerization behaviors
Graphical abstract
Introduction
Since the discovery of Ziegler-Natta catalysts, the last half-century has witnessed impressive advances in the development of metallocene and non-metallocene catalysts, which produced a variety of high-performance polyolefin materials [1], [2], [3]. Metallocene catalyst is the most promising choices for olefin polymerization due to its high activity, the best understood structures featured as single-site catalysts, facile modification, and the accessibility of producing new polymer architectures [4], [5]. As the field progressed, in order to promote catalytic performance, several different types of ancillary ligands have been explored for the synthesis of polymerization catalysts [6], [7], [8], [9], [10].
Recent research on transition metal catalysts suggested that incorporation of bulky substituents in the ligands offered a potential improvement of their catalytic behaviors including thermal stability and comonomer incorporation ability [11], [12]. For instance, Sun and coworkers reported that half-titanocene complexes ligated by 2-benzimidazolyl-N-phenylquinoline-8-carboxamide exhibited high catalytic activities toward ethylene polymerization, especially at elevated reaction temperatures [13]. Cp'TiCl2(OAr)/MAO system showed highly active for olefin polymerization and displayed high comonomer incorporation ability in both ethylene/α-olefin and ethylene/cyclo-olefin copolymerizations [14], [15], [16]. Half-titanocene complexes bearing anilide ligands, Cp'TiCl2 [N (2,6-R12C6H3)R2] were efficient catalyst precursors toward ethylene polymerization and ethylene/1-hexene copolymerization to produce ultra-high molecular weight polyethylenes and ethylene/1-hexene copolymers with moderate to high comonomer incorporations [17]. Molecular modeling indicated that introducing bulky substituents into the ligand is benefit to obtain more open TiOC angle of half-titanocene complexes [7], [12] and the bulky ligand can shield the metal center to stablize the active species [18], thus giving rise to high catalytic activity. Such beneficial effects were also observed in the cases of late-transition metal-based catalysts. Investigations involving sterically hindered pyridyl-imino, aryliminoacenaphthylene, and α-dimine supported iron [19], [20], nickel [21], [22], and palladium [23] complexes in olefin polymerizations have revealed some interesting features. These complexes could produce high molecular weight polyethylenes even at high temperature and in some cases living polymerization was realized [24]. Previous research has shown that dibenzhydryl-substituted aryloxide ligands were capable of not only accommodating the geometries of metal complexes flexibly but also preventing the metal center from decomposition [25]. Therefore, it is reasonable to speculate that introducing such bulk aryloxide ligands will exert positive effect on both of catalytic activity and the thermal stability of metallocene catalyst.
Inspired by the above-mentioned successful applications of transition-metal catalysts with bulky liagnds in olefin (co)polymerizations, Herein, three half-titanocene complexes bearing bulky dibenzhydryl-substituented aryloxide ligand were synthesized and their ethylene polymerization and ethylene/1-hexene copolymerization behaviors were investigated in detail.
Section snippets
General consideration
All the manipulations were carried out under argon atmosphere by using standard Schlenk techniques or in a glove box. The solvents were refluxed over CaH2 or sodium-benzophenone and distilled prior to use. Polymerization grade ethylene was further purified by passing through columns of 5 Å molecular sieves and MnO. The Trichloro (cyclopentadienyl) titanium (CpTiCl3) and trichloro (pentamethylcyclopentadienyl) titanium (Cp*TiCl3) were purchased from Aldrich. Methylaluminoxane (1.5 mol/L in
Synthesis and characterization of half-titanocene complexes
The bulky aryloxide ligands were synthesized according to the literature [26]. The aryloxide half-titanocene complexes were prepared in a facile manner (shown in Scheme 1). Complexes 2a and 2b were obtained in good yields by the reactions of CpTiCl3 with 2,6-dibenzhydrylphenol (1a and 1b) in dichloromethane at room temperature for 12 h. The synthesis of 3a was conducted in toluene by refluxing. All the complexes were characterized by NMR spectroscopy and elemental analysis. Moreover, the solid
Conclusion
Half-titanocene complexes bearing monodentate anionic aryloxide ligand with bulky dibenzhydryl moieties [Cp'TiOCl2(2,6-Ph2CHC6H2-4-R)] were synthesized by a simple method and fully characterized. In the presence of MAO, all the complexes exhibited high activities for the ethylene polymerization, and good thermal stability were observed at elevated temperature. The affording polyethylene possessed high molecular weight and narrow polydispersity. These results indicated that introducing the bulky
Acknowledgments
This work was supported by National Basic Research Program of China, Grant No. 2015CB654700 (2015CB674702) and the Natural Science Foundation of China (No.U1462124).
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