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Half-titanocene complexes bearing bulky dibenzhydryl-substituted aryloxide ligand: Syntheses, characterization, and ethylene (Co-)polymerization behaviors
Polymer 100 (2016) 188e193
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Half-titanocene complexes bearing bulky dibenzhydryl-substituted aryloxide ligand: Syntheses, characterization, and ethylene (Co-) polymerization behaviors Bo Dong a, b, Hexin Zhang b, Hua Li c, Heng Liu b, Jun Guo b, Chunyu Zhang b, Xu Zhang a, Yanming Hu b, Guangping Sun a, *, Xuequan Zhang b, ** a b c
College of Materials Science and Engineering, Jilin University, Changchun 130022, PR China Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Shangdong Non-Metallic Materials Institute, Jinan 250000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2016 Received in revised form 9 August 2016 Accepted 11 August 2016 Available online 13 August 2016
Three half-titanocene complexes 2a-3a bearing dibenzhydryl-substituted aryloxide ligands CpTiCl2(O2,6-Ph2CHeC6H2-4-Me), 2a; CpTiCl2 (O-2,6-Ph2CHeC6H2-4-OMe), 2b; Cp*TiCl2(O-2,6-Ph2CHeC6H2-4Me), 3a were synthesized. All the complexes were characterized by NMR spectroscopy and elemental analysis. The molecular structures of complex 2a and 2b were further determined by single crystal X-ray diffraction, both of them adopt a three-legged distorted tetrahedral geometry in which the aryl substituents are orthogonally to the aryloxide group. Activated by methylaluminoxane (MAO), these complexes showed high activities (up to 2100 kg mol1 (Ti) h1) for ethylene polymerization. The resultant polyethylenes had high molecular weight and narrow molecular weight distributions. Furthermore, for the ethylene/1-hexene copolymerization, these complexes exhibited moderate activites and moderate to high 1-hexene incorporation ability. The sequence distributions of poly (ethylene-co-1-hexene)s are greatly influenced by the substituent on the cyclopentadineyl group of titanium complexes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bulky aryloxide Half-titanocene complexes Ethylene polymerization Copolymerization
1. Introduction Since the discovery of Ziegler-Natta catalysts, the last halfcentury has witnessed impressive advances in the development of metallocene and non-metallocene catalysts, which produced a variety of high-performance polyolefin materials [1e3]. 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 [6e10]. Recent research on transition metal catalysts suggested that incorporation of bulky substituents in the ligands offered a potential
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Sun), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.polymer.2016.08.045 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
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 2benzimidazolyl-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/a-olefin and ethylene/cyclo-olefin copolymerizations [14e16]. 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 TieOeC 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 latetransition metal-based catalysts. Investigations involving sterically
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hindered pyridyl-imino, aryliminoacenaphthylene, and a-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.
2. Experimental 2.1. 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 toluene) was purchased from Akzo Nobel Chemical Inc. Other reagents and solvents were commercially available. The NMR spectra of ligands and complexes were recorded on a Bruker-300 MHz at ambient temperature with CDCl3 as a solvent (dried by CaH2). The NMR spectra of the polymers were recorded on a Varian Unity-400 NMR spectrometer at 135 C with oeC6D4Cl2 as a solvent. The weight-average molecular weight (Mw) and molecular weight distributions (Mw/Mn) of the polymers were measured at 135 C by a PL-GPC 200 high-temperature gel permeation chromatography equipped with three Plgel 10 mm mixed-BLS type columns. 1,2,4-Trichlorobenzene was used as eluent at a flow rate of 1.0 mL/min. The experiments settings were calibrated by polystyrene standard EasiCalPs-1 (PL Ltd). The melting points of the copolymers were determined by differential scanning calorimetry (DSC) on a TA DSC Q20 instrument at a heating/cooling rate of 10 ºC/min from 50 to 200 C, and the data were obtained by second heating scan.
2.2. X-ray crystallographic details Crystals of 2a and 2b were obtained by laying n-hexane onto dichloromethane solutions. Data collections were performed at 88.5 C on a Bruker SMART APEX diffractometerwith a CCD area detector using graphite monochromated Mo K radiation (l ¼ 0.71073 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to collect the reflection data file. Refinement was performed on F2 anistropically for all non-hydrogen atoms by full-matrix least-squares method. Details of X-ray structure determinations and refinements are summarized in the Supporting Information. CCDC 14481101448111.
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2.3. Synthesis of bulky aryloxide ligands 2.3.1. 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1a) In a 250 mL round-bottom flask was charged diphenylmetanol (15.42 g, 83.7 mmol), 4-methyl-Phenol (6.75 g, 42.0 mmol) and a stirring bar. The reaction flask was heated to 160 C to produce a melt followed by the addition of a solution of HCl/ZnCl2 (2.22 mL, 73.2 mmol HCl; 37 mmol ZnCl2, 5.2 g) dropwise via a glass pipet. The reaction mixture solidified, and the reaction was allowed to proceed for an additional 2 h. After the reaction flask was cooled to room temperature, the crude solid was dissolved in hot dichloromethane (300 mL). The organic layer was washed with water (3 100 mL), and then dried over anhydrous magnesium sulfate. The solvent evaporated, the yellow residue was purified by washing with ethyl acetate to give a white power (10.2 g, 46% yield). 1 H NMR (CDCl3): d 7.29e7.19 (m, 12H, AreH), 7.10e7.08 (m, 8H, AreH), 6.33 (s, 2H, AreH), 5.44 (s, 2H, CH(Ph)2), 2.01 (s, 3H, Me). 13C NMR (CDCl3): d 149.03, 142.67, 130.77, 129.29, 128.40, 126.50, 51.01, 20.91. Anal. Calcd for C33H28O: C, 89.96; H, 6.41. Found: C, 89.69; H, 6.31. 2.3.2. 2,6-Bis(diphenylmethyl)-4-methoxylphenoxide (1b) Synthesis of 1b was performed according to the same procedure as that of 1a (7.2 g, 53% yield). 1H NMR (CDCl3): d 7.29e7.19 (m, 12H, AreH), 7.11e7.09 (m, 8H, AreH), 6.26 (s, 2H, AreH), 5.69 (s, 2H, CH(Ph)2), 4.10 (s, OH), 3.47 (s, 3H, OMe). 13C NMR (CDCl3): d 153.01, 145.31, 142.35, 132.34, 129.18, 128.36, 126.52, 114.10, 55.07, 51.06. Anal. Calcd for C33H28O: C, 86.81; H, 6.18. Found: C, 86.75; H, 5.97. 2.4. Synthesis of the complexes 2a, 2b and 3a 2.4.1. (Cyclopentadienyl)[2,6-Bis(diphenylmethyl)-4-methyl] aryloxide titanium dichloride (2a) To a stirred solution of CpTiCl3 (0.5 g, 2.27 mmol) in dried CH2Cl2 (10 mL), 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1a) (1.0 g, 2.27 mmol) was added slowly at 78 C, then the mixture was warmed to room temperature and stirred for an additional 12 h. The solvent was evaporated under vacuum to get a red residue. The powder was washed twice with diethyl ether (10 mL) and filtered. The filter cake was dissolved in CH2Cl2/hexane at 30 C and afforded the red microcrystal (0.72 g, 48% yield). 1H NMR (CDCl3): d 7.27e7.14 (m, 12H, AreH), 7.07e7.06 (m, 8H, AreH), 6.53 (s, 2H, AreH), 6.14 (s, 2H, CH(Ph)2), 5.57 (s, 5H, Cp), 2.01 (s, 3H, Me). 13C NMR (CDCl3): d 162.63, 144.01, 130.01, 129.76, 128.25, 126.42, 121.25, 48.64, 21.33. Anal. Calcd for C38H32Cl2OTi: C, 73.21; H, 5.17. Found: C, 72.87; H, 5.06. 2.4.2. (Cyclopentadienyl)[2,6-Bis(diphenylmethyl)-4-methoxyl] aryloxide titanium dichloride (2b) The synthesis procedure of complex 2b was same as preparation of 2a (0.91 g, 63% yield). 1H NMR (CDCl3): d 7.34e7.21 (m, 12H, AreH), 7.15e7.13 (m, 8H, AreH), 6.33 (s, 2H, AreH), 6.24 (s, 2H, CH(Ph)2), 5.62 (s, 5H, Cp), 3.51 (s, 3H, OMe). 13C NMR (CDCl3): d 159.33, 154.81, 143.72, 135.24, 129.68, 128.31, 126.54, 121.14, 114.53, 55.06, 48.95. Anal. Calcd for C38H32Cl2O2Ti: C, 71.38; H, 5.04. Found: C, 70.94; H, 5.14. 2.4.3. (Pentamethylcyclopentadienyl)[2,6-Bis(diphenylmethyl)-4methoxyl]aryloxide titanium dichloride (3a) 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1.0 g, 2.27 mmol) (1a) was added dropwise to a stirred toluene solution (10 mL) containing Cp*TiCl3 (0.65 g, 2.27 mmol) at 78 C. The mixture was warmed to room tempature and then refluxed for 24 h. The solvent was evaporated and washed with diethyl ether twice then filtered, the desired product 3a was obtained (0.62 g, 39% yield). 1H NMR (CDCl3): d 7.22e7.18 (m, 12H, AreH), 7.13e7.12 (m, 8H, AreH), 6.66
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(s, 2H, AreH), 6.18 (s, 2H, CH(Ph)2), 2.03 (s, 3H, Me), 1.75 (s, 15H, Cp*). 13C NMR (CDCl3): d 159.60, 144.84, 133.65, 129.85, 129.29, 128.40, 126.50, 51.02, 20.90, 12.76. Anal. Calcd for C43H42Cl2OTi: C, 74.46; H, 6.10. Found: C, 73.66; H, 6.11. HRMS (EI): calcd for C43H42Cl2OTi, 692.2092; Found: 692.2074. 2.5. Preparation of ethylene polymerization and co-polymerzation A 100 mL stainless steel autoclave equipped with a magetic stirrer and a temperature controller was heated in a vacuum at 80 C and recharged with ethylene three times, then cooled to room temperature under an ethylene atmosphere. A toluene solution of the precatalyst (with 1-hexene) was transferred into the reactor. After the desired reaction temperature was reached, the required amount of the cocatalyst was added with maintaining the total volume at 60 mL, and then the autoclave was immediately pressurized to 6 atm. The ethylene pressure was kept constant during the reaction. The reaction mixture was stirred at the desired temperature for 30 min. The polymerization was then quenched by injecting acidified ethanol containing HCl (3 M). The polymer was collected by filtration, washed with water and ethanol, and dried to a constant weight under vacuum. 3. Result and discussion 3.1. 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 structures of complexes 2a and 2b were further confirmed by single crystal X-ray diffraction. Single crystals of complexes 2a and 2b suitable for X-ray diffraction were grown from dichloromethane/n-hexane (1:1) at 30 C. The molecular structures are shown in Figs. 1 and 2, respectively, selected bond lengthes and angles are summarized in Table 2 (in the Supporting Information). Both the structures of 2a and 2b were analogous to the previously reported aryloxide halftitanocene complexes, they adopt a three-legged distorted tetrahedral geometry around the central metal comprised of one Cp ring, two chlorine atoms, and a 2,6-dibenzhydrylphenol ligand [27]. The distances between the centroid of Cp ring and titanium atom have no obvious difference between 2a (2.029 Å) and 2b (2.022 Å). The angle of Cl1eTi1eCl2 in 2a (102.0 ) is about the same as that in 2b (102.2 ). The bond distance of Ti1eO1 in 2a (1.772 Å) is shorter than that in 2b (1.785 Å). The aryl rings of dibenzhydryl are almost
Fig. 1. Molecular structure of complex 2a. Thermal ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.
orthogonally to the phenoxyl plane, not only blocking the metal center on the axial space of Ti1eO1eC1 but also providing sterically force for the larger angle of Ti1eO1eC1 than that of the reported Cp-based complex [14]. Meanwhile, the Ti1eO1eC1 angle in 2a (167.2 ) is obviously larger than that in 2b (162.9 ). As the prevois report, the larger angle leads to more electron donation from oxygen to titanium [28]. The angle of Cpcent-Ti1-O1 in 2a (119.2 ) is also somewhat larger than that in 2b (118.9 ). The relatively large angle indicated that the bulky aryloxide ligand may rotate on the TieOeC axial space flexibly to accommodate the geometries [29].
3.2. Ethylene polymerization Upon activation with MAO, all the complexes were investigated for ethylene polymerization along with CpTiCl3 for comparison, and the results are shown in Table 1. First, in order to optimize the polymerization conditions, the influences of Al/Ti ratio and temperature on the polymerization were examined. It was found that the Al/Ti ratio had a significant influence on the catalytic activity. For complex 2a, the activity increased with an increase of Al/Ti ratio from 500 to 3000 and then decreased slightly with further increase in the Al/Ti ratio. It is worthy to note that 2a bearing bulky dibenzhydryl-substituted aryloxide ligand exhibited high catalytic activity (1920 kg mol1 (Ti) h1), which is much higher than the reported (2,6-disisopropyl) aryloxide-substituted half-titanocene (104 kg mol1 (Ti) h1) [30]. This result may be attributed to that the dibenzhydryl-substituted aryloxide ligand provides sterically force for the more open angle of TieOeC, which is beneficial to the electron donation from oxygen (Pp) to titanium (Pp) [7,28]. In addition, 2a/MAO afforded polyethylenes with high molecular weight (Mw ¼ 752 103 g mol1) and relative narrow molecular
Scheme 1. Synthetic route for titanium complexes.
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Fig. 2. Molecular structure of complex 2b. Thermal ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.
Table 1 Ethylene polymerization with 2a, 2b, 3a and CpTiCl3/MAO.a Entry
Cat.
Al/Ti
T (oC)
Yield (g)
Activityb
Mwc (103)
Mw/Mnc
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2a 2a 2a 2a 2a 2a 2a 2a 2b 3a 3a 3a 3a CpTiCl3
500 1000 2000 3000 5000 3000 3000 3000 3000 3000 5000 3000 3000 3000
25 25 25 25 25 50 60 70 25 25 25 50 70 25
0.30 0.44 0.83 1.92 1.30 1.84 1.32 0.82 1.02 2.10 1.98 1.43 0.86 0.02
300 440 830 1920 1300 1840 1320 820 1020 2100 1980 1430 860 20
1020 788 651 752 442 589 384 489 442 603 576 463 382 n.d.
1.9 3.9 3.2 2.0 2.1 3.3 4.5 2.7 2.2 2.4 2.2 4.1 4.9 n.d.
a Polymerzation: carried out in 60 mL of toluene for 30 min at 25 C with 2 mmol of Ti, under an ethylene pressure of 6.0 atm. b Activity: kg mol1 (Ti) h1. c Determined by high temperature GPC.
weight distribution (Mw/Mn ¼ 2.1e3.2). The unimodal GPC curves and the relatively narrow molecular weight distributions of the resultant polymers indicate the presence of a single active site in the polymerization system. As the reaction temperature elevated, the catalytic activity and the molecular weight of resultant polyethylenes decreased. This behavior may be due to the accelerated polymer chain transfer and
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b-hydride elimination reactions as well as the lower concentration of ethylene in solution at high temperatures [31]. It should be noted that the catalyst system remained high catalytic activity (1840 kg mol1 (Ti) h1), which is in contrast to the traditional metallocene catalysts, whose activity remarkably decreased as the polymerization temperature increased to 50 C [4,32]. Although the activity (820 kg mol1 (Ti) h1) at 70 C was about half that at 50 C, while it is much higher than that of unmodified titallocene catalyst, indicating highly thermal stability. The afore-mentioned polymerization behaviors indicated that the bulky aryloxide ligand plays an important role in stabilizing the active species to suppress chain transfer and termination at high temperatures. Similar result was observed in the case of iminopyridine iron complexes with dibenzhydryl substituents [20]. Based on the optimum condition established, the other complexes were examined to understand the influence of ligand structure on their catalytic performance. All the precatalysts showed high activities ranging from 1020 kg mol1 (Ti) h1 to 2100 kg mol1 (Ti) h1, while CpTiCl3/MAO catalytic system exhibited much lower activity of 20 kg mol1 (Ti) h1. The activity of 2a with methyl at 4-position of aryloxide was about twice that of methoxyl substituented 2b. As the data revealed by single crystal Xray diffraction, the Ti1eO1eC1 angle of 2a (167.2 ) was larger than that of 2b (162.9 ), likely because the larger angle of TieOeC, the more electron donation from oxygen to titanium atom, thus stabilizing the active species and sequencially leading to the high activity [12]. In addition to the effect of bulky aryloxide on the catalytic activity, it has been reported that the cyclopentadienyl ring bearing five methyl substituents can afford more electron donation to the metal center for high activity, thus 3a exhibited the highest activity of 2100 kg mol1 (Ti) h1 [17]. 3.3. The ethylene/1-hexene copolymerization behaviors The ethylene/1-hexene copolymerization with these complexes as precatalysts was further investigated (Table 2). Although these complexes activated by MAO were inactive for 1-hexene homopolymerizaiton, the ethylene/1-hexene copolymerizaiton successfully proceeded. This might be attributed to that ethylene replaces the cocatalyst for coordination with active site, and thus providing enough space for 1-hexene coordination [33]. All the complexes showed moderate catalytic activities in the range of 320e1180 kg mol1 (Ti) h1 in ethylene/1-hexene copolymerization. While, compared with ethylene homopolymerization, the catalytic activities and the molecular weights of the resultant polymers decreased with the increasing 1-hexene concentration, indicating that the polymer chain terminated during the insertion of increasing 1-hexene comonomer [31]. Determined by 13C NMR, the 1-hexene incorporation levels were in the range of 7.3e34.4%,
Table 2 Ethylene/1-hexene copolymerization with 2a, 2b and 3a/MAO systems.a Entry
Cat.
1-Hexene (mol/L)
Yield (g)
Activityb
Mwc (103)
Mw/Mnc
Hexene content (%)d
Tme (ºC)
15 16 17 18 19 20 21
2a 2a 2a 2a 2b 3a 3a
0.16 0.32 0.48 1.00 0.48 0.16 0.48
0.90 1.02 0.74 0.93 0.32 1.18 0.61
900 1020 740 930 320 1180 610
462 103 55 25 117 379 194
4.1 4.2 2.9 2.1 1.7 2.4 2.0
n.d. 10.8 22.3 34.4 17.9 7.3 15.6
122.0 119.5 116.9 None 119.3 93.2,108.6 78.6,120.9
a b c d e
Polymerization: carried out in 60 mL of toluene for 30 min at 50 C with 2 mmol of Ti, under an ethylene pressure of 6.0 atm. Activity: kg mol1 (Ti) h1. Determined by high temperature GPC. Estimated by13C NMR spectra. Determined by DSC.
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Table 3 Monnomer Sequence Distributions ethylene/1-hexene copolymers obtained with 2a, 2b and 3a/MAO system.a. Entry
16 17 19 21 a b c d
Cat.
2a 2a 2b 3a
Content (mol%)b
10.8 22.3 17.9 15.6
Triad sequencec (%)
Dyad sequencec (%)
rErHd
EEE
EEH þ HEE
HEH
EHE
EHH þ HHE
HHH
EE
EH þ HE
HH
72.7 54.8 66.3 53.7
11.1 17.8 11.1 24.5
trace 2.2 Trace 4.6
6.5 8.9 7.0 14.7
9.6 16.2 15.5 2.3
Trace 0.1 trace trace
78.2 63.7 71.9 69.9
21.7 28.1 20.2 28.9
4.8 8.2 7.8 1.1
3.18 2.53 5.62 0.37
Polymerization: see Table 2. 1-hexene content estimated by13C NMR spectra. Calculated by13C NMR spectra. rErH ¼ 4 [EE][HH]/[EH þ HE]2.
and the level increased with the increasing concentration of 1hexene. Complex 2a displayed the highest 1-hexene incorporation ability among the present complexes. As revealed by the X-ray diffraction data, the large Cp(cent)-Ti-O angle in 2a leads to the open space for 1-hexene coordination [34]. The monomer sequence distributions and the rE$rH values were calculated and summarized in Table 3 [35]. The contents of EHH þ HHE and EHE triad sequences in the copolymers obtained with 2a and 3a are quite different. The EHH þ HHE content of the copolymers obtained with 2a were 16.2%, much higher than that obtained with 3a, while the copolymer obtained with 3a had the highest EHE content. Determined by DSC analysis (Fig. 3), the Tm value of the copolymers obtained with 2a decreases slowly from 122.0 C to 119.3 C with increasing incorporation level of 1hexene, and the lower molecular weight and the higher EHH þ HHE content indicated that higher content of continuous hexene units perform in the polymer chain. Furthermore, the incorporation level of 1-hexene increased to 33.4%, the resultant copolymer become amorphous. The copolymer obtained with 3a had two Tm values (Fig. 4), likely because polyethylene chains with different lengthes exist in the copolymer. In the 13C NMR spectrum (Fig. 5), HHH triad sequence (aa) exists in the copolymer with 22.3% comonomer incorporation obtained with 2a, but not observed in that obtained with 3a (Fig. 6). The rE$rH values imply that 1-hexene inserts in the polymer chain is not in a random manner [18]. The monomer sequence distributions and rE$rH values greatly depend on the substituent of cyclopentadienyl group, indicating that the ligand structure has an important influence on selecting next inserted monomer unit during the chain propagation [29].
Fig. 3. The DSC curves of poly (ethylene-1-hexene)s obtained with 2a/MAO under various 1-hexene concentration.
Fig. 4. The DSC curves of poly (ethylene-1-hexene)s obtained with 3a/MAO under various 1-hexene concentration.
4. Conclusion Half-titanocene complexes bearing monodentate anionic aryloxide ligand with bulky dibenzhydryl moieties [Cp'TiOCl2(2,6Ph2CHeC6H2-4-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
Fig. 5. 13C NMR spectrum of entylene/1-hexene copolymer obtained with 2a/MAO system (Table 2, Entry 17).
B. Dong et al. / Polymer 100 (2016) 188e193
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References
Fig. 6. 13C NMR spectrum of entylene/1-hexene copolymer obtained with 3a/MAO system (Table 2, Entry 21).
narrow polydispersity. These results indicated that introducing the bulky dibenzhydryl on the aryloxide ligand could provide large steric force to open the TieOeC angle of the titanium complex for the more electron donation to the metal center, which was benefit to stabilize the active specie for the high activity for the ethylene polymerization. In addition, the present catalytic systems displayed high activity for the ethylene/1-hexene copolymerization and produced the copolymers with relatively high 1-hexene incorporation. Furthermore, higher 1-hexene incorporation and 1-hexene continuous units in the copolymers obtained with Cp-based 2a than that with Cp*-based 3a, indicating that Cp structure play an important role in the composition and sequence distribution of the resulting copolymers.
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).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.08.045.
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Half-titanocene complexes bearing bulky dibenzhydryl-substituted aryloxide ligand: Syntheses, characterization, and ethylene (Co-) polymerization behaviors Bo Dong a, b, Hexin Zhang b, Hua Li c, Heng Liu b, Jun Guo b, Chunyu Zhang b, Xu Zhang a, Yanming Hu b, Guangping Sun a, *, Xuequan Zhang b, ** a b c
College of Materials Science and Engineering, Jilin University, Changchun 130022, PR China Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Shangdong Non-Metallic Materials Institute, Jinan 250000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2016 Received in revised form 9 August 2016 Accepted 11 August 2016 Available online 13 August 2016
Three half-titanocene complexes 2a-3a bearing dibenzhydryl-substituted aryloxide ligands CpTiCl2(O2,6-Ph2CHeC6H2-4-Me), 2a; CpTiCl2 (O-2,6-Ph2CHeC6H2-4-OMe), 2b; Cp*TiCl2(O-2,6-Ph2CHeC6H2-4Me), 3a were synthesized. All the complexes were characterized by NMR spectroscopy and elemental analysis. The molecular structures of complex 2a and 2b were further determined by single crystal X-ray diffraction, both of them adopt a three-legged distorted tetrahedral geometry in which the aryl substituents are orthogonally to the aryloxide group. Activated by methylaluminoxane (MAO), these complexes showed high activities (up to 2100 kg mol1 (Ti) h1) for ethylene polymerization. The resultant polyethylenes had high molecular weight and narrow molecular weight distributions. Furthermore, for the ethylene/1-hexene copolymerization, these complexes exhibited moderate activites and moderate to high 1-hexene incorporation ability. The sequence distributions of poly (ethylene-co-1-hexene)s are greatly influenced by the substituent on the cyclopentadineyl group of titanium complexes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bulky aryloxide Half-titanocene complexes Ethylene polymerization Copolymerization
1. Introduction Since the discovery of Ziegler-Natta catalysts, the last halfcentury has witnessed impressive advances in the development of metallocene and non-metallocene catalysts, which produced a variety of high-performance polyolefin materials [1e3]. 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 [6e10]. Recent research on transition metal catalysts suggested that incorporation of bulky substituents in the ligands offered a potential
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Sun), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.polymer.2016.08.045 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
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 2benzimidazolyl-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/a-olefin and ethylene/cyclo-olefin copolymerizations [14e16]. 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 TieOeC 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 latetransition metal-based catalysts. Investigations involving sterically
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hindered pyridyl-imino, aryliminoacenaphthylene, and a-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.
2. Experimental 2.1. 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 toluene) was purchased from Akzo Nobel Chemical Inc. Other reagents and solvents were commercially available. The NMR spectra of ligands and complexes were recorded on a Bruker-300 MHz at ambient temperature with CDCl3 as a solvent (dried by CaH2). The NMR spectra of the polymers were recorded on a Varian Unity-400 NMR spectrometer at 135 C with oeC6D4Cl2 as a solvent. The weight-average molecular weight (Mw) and molecular weight distributions (Mw/Mn) of the polymers were measured at 135 C by a PL-GPC 200 high-temperature gel permeation chromatography equipped with three Plgel 10 mm mixed-BLS type columns. 1,2,4-Trichlorobenzene was used as eluent at a flow rate of 1.0 mL/min. The experiments settings were calibrated by polystyrene standard EasiCalPs-1 (PL Ltd). The melting points of the copolymers were determined by differential scanning calorimetry (DSC) on a TA DSC Q20 instrument at a heating/cooling rate of 10 ºC/min from 50 to 200 C, and the data were obtained by second heating scan.
2.2. X-ray crystallographic details Crystals of 2a and 2b were obtained by laying n-hexane onto dichloromethane solutions. Data collections were performed at 88.5 C on a Bruker SMART APEX diffractometerwith a CCD area detector using graphite monochromated Mo K radiation (l ¼ 0.71073 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to collect the reflection data file. Refinement was performed on F2 anistropically for all non-hydrogen atoms by full-matrix least-squares method. Details of X-ray structure determinations and refinements are summarized in the Supporting Information. CCDC 14481101448111.
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2.3. Synthesis of bulky aryloxide ligands 2.3.1. 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1a) In a 250 mL round-bottom flask was charged diphenylmetanol (15.42 g, 83.7 mmol), 4-methyl-Phenol (6.75 g, 42.0 mmol) and a stirring bar. The reaction flask was heated to 160 C to produce a melt followed by the addition of a solution of HCl/ZnCl2 (2.22 mL, 73.2 mmol HCl; 37 mmol ZnCl2, 5.2 g) dropwise via a glass pipet. The reaction mixture solidified, and the reaction was allowed to proceed for an additional 2 h. After the reaction flask was cooled to room temperature, the crude solid was dissolved in hot dichloromethane (300 mL). The organic layer was washed with water (3 100 mL), and then dried over anhydrous magnesium sulfate. The solvent evaporated, the yellow residue was purified by washing with ethyl acetate to give a white power (10.2 g, 46% yield). 1 H NMR (CDCl3): d 7.29e7.19 (m, 12H, AreH), 7.10e7.08 (m, 8H, AreH), 6.33 (s, 2H, AreH), 5.44 (s, 2H, CH(Ph)2), 2.01 (s, 3H, Me). 13C NMR (CDCl3): d 149.03, 142.67, 130.77, 129.29, 128.40, 126.50, 51.01, 20.91. Anal. Calcd for C33H28O: C, 89.96; H, 6.41. Found: C, 89.69; H, 6.31. 2.3.2. 2,6-Bis(diphenylmethyl)-4-methoxylphenoxide (1b) Synthesis of 1b was performed according to the same procedure as that of 1a (7.2 g, 53% yield). 1H NMR (CDCl3): d 7.29e7.19 (m, 12H, AreH), 7.11e7.09 (m, 8H, AreH), 6.26 (s, 2H, AreH), 5.69 (s, 2H, CH(Ph)2), 4.10 (s, OH), 3.47 (s, 3H, OMe). 13C NMR (CDCl3): d 153.01, 145.31, 142.35, 132.34, 129.18, 128.36, 126.52, 114.10, 55.07, 51.06. Anal. Calcd for C33H28O: C, 86.81; H, 6.18. Found: C, 86.75; H, 5.97. 2.4. Synthesis of the complexes 2a, 2b and 3a 2.4.1. (Cyclopentadienyl)[2,6-Bis(diphenylmethyl)-4-methyl] aryloxide titanium dichloride (2a) To a stirred solution of CpTiCl3 (0.5 g, 2.27 mmol) in dried CH2Cl2 (10 mL), 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1a) (1.0 g, 2.27 mmol) was added slowly at 78 C, then the mixture was warmed to room temperature and stirred for an additional 12 h. The solvent was evaporated under vacuum to get a red residue. The powder was washed twice with diethyl ether (10 mL) and filtered. The filter cake was dissolved in CH2Cl2/hexane at 30 C and afforded the red microcrystal (0.72 g, 48% yield). 1H NMR (CDCl3): d 7.27e7.14 (m, 12H, AreH), 7.07e7.06 (m, 8H, AreH), 6.53 (s, 2H, AreH), 6.14 (s, 2H, CH(Ph)2), 5.57 (s, 5H, Cp), 2.01 (s, 3H, Me). 13C NMR (CDCl3): d 162.63, 144.01, 130.01, 129.76, 128.25, 126.42, 121.25, 48.64, 21.33. Anal. Calcd for C38H32Cl2OTi: C, 73.21; H, 5.17. Found: C, 72.87; H, 5.06. 2.4.2. (Cyclopentadienyl)[2,6-Bis(diphenylmethyl)-4-methoxyl] aryloxide titanium dichloride (2b) The synthesis procedure of complex 2b was same as preparation of 2a (0.91 g, 63% yield). 1H NMR (CDCl3): d 7.34e7.21 (m, 12H, AreH), 7.15e7.13 (m, 8H, AreH), 6.33 (s, 2H, AreH), 6.24 (s, 2H, CH(Ph)2), 5.62 (s, 5H, Cp), 3.51 (s, 3H, OMe). 13C NMR (CDCl3): d 159.33, 154.81, 143.72, 135.24, 129.68, 128.31, 126.54, 121.14, 114.53, 55.06, 48.95. Anal. Calcd for C38H32Cl2O2Ti: C, 71.38; H, 5.04. Found: C, 70.94; H, 5.14. 2.4.3. (Pentamethylcyclopentadienyl)[2,6-Bis(diphenylmethyl)-4methoxyl]aryloxide titanium dichloride (3a) 2,6-Bis(diphenylmethyl)-4-methylphenoxide (1.0 g, 2.27 mmol) (1a) was added dropwise to a stirred toluene solution (10 mL) containing Cp*TiCl3 (0.65 g, 2.27 mmol) at 78 C. The mixture was warmed to room tempature and then refluxed for 24 h. The solvent was evaporated and washed with diethyl ether twice then filtered, the desired product 3a was obtained (0.62 g, 39% yield). 1H NMR (CDCl3): d 7.22e7.18 (m, 12H, AreH), 7.13e7.12 (m, 8H, AreH), 6.66
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(s, 2H, AreH), 6.18 (s, 2H, CH(Ph)2), 2.03 (s, 3H, Me), 1.75 (s, 15H, Cp*). 13C NMR (CDCl3): d 159.60, 144.84, 133.65, 129.85, 129.29, 128.40, 126.50, 51.02, 20.90, 12.76. Anal. Calcd for C43H42Cl2OTi: C, 74.46; H, 6.10. Found: C, 73.66; H, 6.11. HRMS (EI): calcd for C43H42Cl2OTi, 692.2092; Found: 692.2074. 2.5. Preparation of ethylene polymerization and co-polymerzation A 100 mL stainless steel autoclave equipped with a magetic stirrer and a temperature controller was heated in a vacuum at 80 C and recharged with ethylene three times, then cooled to room temperature under an ethylene atmosphere. A toluene solution of the precatalyst (with 1-hexene) was transferred into the reactor. After the desired reaction temperature was reached, the required amount of the cocatalyst was added with maintaining the total volume at 60 mL, and then the autoclave was immediately pressurized to 6 atm. The ethylene pressure was kept constant during the reaction. The reaction mixture was stirred at the desired temperature for 30 min. The polymerization was then quenched by injecting acidified ethanol containing HCl (3 M). The polymer was collected by filtration, washed with water and ethanol, and dried to a constant weight under vacuum. 3. Result and discussion 3.1. 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 structures of complexes 2a and 2b were further confirmed by single crystal X-ray diffraction. Single crystals of complexes 2a and 2b suitable for X-ray diffraction were grown from dichloromethane/n-hexane (1:1) at 30 C. The molecular structures are shown in Figs. 1 and 2, respectively, selected bond lengthes and angles are summarized in Table 2 (in the Supporting Information). Both the structures of 2a and 2b were analogous to the previously reported aryloxide halftitanocene complexes, they adopt a three-legged distorted tetrahedral geometry around the central metal comprised of one Cp ring, two chlorine atoms, and a 2,6-dibenzhydrylphenol ligand [27]. The distances between the centroid of Cp ring and titanium atom have no obvious difference between 2a (2.029 Å) and 2b (2.022 Å). The angle of Cl1eTi1eCl2 in 2a (102.0 ) is about the same as that in 2b (102.2 ). The bond distance of Ti1eO1 in 2a (1.772 Å) is shorter than that in 2b (1.785 Å). The aryl rings of dibenzhydryl are almost
Fig. 1. Molecular structure of complex 2a. Thermal ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.
orthogonally to the phenoxyl plane, not only blocking the metal center on the axial space of Ti1eO1eC1 but also providing sterically force for the larger angle of Ti1eO1eC1 than that of the reported Cp-based complex [14]. Meanwhile, the Ti1eO1eC1 angle in 2a (167.2 ) is obviously larger than that in 2b (162.9 ). As the prevois report, the larger angle leads to more electron donation from oxygen to titanium [28]. The angle of Cpcent-Ti1-O1 in 2a (119.2 ) is also somewhat larger than that in 2b (118.9 ). The relatively large angle indicated that the bulky aryloxide ligand may rotate on the TieOeC axial space flexibly to accommodate the geometries [29].
3.2. Ethylene polymerization Upon activation with MAO, all the complexes were investigated for ethylene polymerization along with CpTiCl3 for comparison, and the results are shown in Table 1. First, in order to optimize the polymerization conditions, the influences of Al/Ti ratio and temperature on the polymerization were examined. It was found that the Al/Ti ratio had a significant influence on the catalytic activity. For complex 2a, the activity increased with an increase of Al/Ti ratio from 500 to 3000 and then decreased slightly with further increase in the Al/Ti ratio. It is worthy to note that 2a bearing bulky dibenzhydryl-substituted aryloxide ligand exhibited high catalytic activity (1920 kg mol1 (Ti) h1), which is much higher than the reported (2,6-disisopropyl) aryloxide-substituted half-titanocene (104 kg mol1 (Ti) h1) [30]. This result may be attributed to that the dibenzhydryl-substituted aryloxide ligand provides sterically force for the more open angle of TieOeC, which is beneficial to the electron donation from oxygen (Pp) to titanium (Pp) [7,28]. In addition, 2a/MAO afforded polyethylenes with high molecular weight (Mw ¼ 752 103 g mol1) and relative narrow molecular
Scheme 1. Synthetic route for titanium complexes.
B. Dong et al. / Polymer 100 (2016) 188e193
Fig. 2. Molecular structure of complex 2b. Thermal ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.
Table 1 Ethylene polymerization with 2a, 2b, 3a and CpTiCl3/MAO.a Entry
Cat.
Al/Ti
T (oC)
Yield (g)
Activityb
Mwc (103)
Mw/Mnc
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2a 2a 2a 2a 2a 2a 2a 2a 2b 3a 3a 3a 3a CpTiCl3
500 1000 2000 3000 5000 3000 3000 3000 3000 3000 5000 3000 3000 3000
25 25 25 25 25 50 60 70 25 25 25 50 70 25
0.30 0.44 0.83 1.92 1.30 1.84 1.32 0.82 1.02 2.10 1.98 1.43 0.86 0.02
300 440 830 1920 1300 1840 1320 820 1020 2100 1980 1430 860 20
1020 788 651 752 442 589 384 489 442 603 576 463 382 n.d.
1.9 3.9 3.2 2.0 2.1 3.3 4.5 2.7 2.2 2.4 2.2 4.1 4.9 n.d.
a Polymerzation: carried out in 60 mL of toluene for 30 min at 25 C with 2 mmol of Ti, under an ethylene pressure of 6.0 atm. b Activity: kg mol1 (Ti) h1. c Determined by high temperature GPC.
weight distribution (Mw/Mn ¼ 2.1e3.2). The unimodal GPC curves and the relatively narrow molecular weight distributions of the resultant polymers indicate the presence of a single active site in the polymerization system. As the reaction temperature elevated, the catalytic activity and the molecular weight of resultant polyethylenes decreased. This behavior may be due to the accelerated polymer chain transfer and
191
b-hydride elimination reactions as well as the lower concentration of ethylene in solution at high temperatures [31]. It should be noted that the catalyst system remained high catalytic activity (1840 kg mol1 (Ti) h1), which is in contrast to the traditional metallocene catalysts, whose activity remarkably decreased as the polymerization temperature increased to 50 C [4,32]. Although the activity (820 kg mol1 (Ti) h1) at 70 C was about half that at 50 C, while it is much higher than that of unmodified titallocene catalyst, indicating highly thermal stability. The afore-mentioned polymerization behaviors indicated that the bulky aryloxide ligand plays an important role in stabilizing the active species to suppress chain transfer and termination at high temperatures. Similar result was observed in the case of iminopyridine iron complexes with dibenzhydryl substituents [20]. Based on the optimum condition established, the other complexes were examined to understand the influence of ligand structure on their catalytic performance. All the precatalysts showed high activities ranging from 1020 kg mol1 (Ti) h1 to 2100 kg mol1 (Ti) h1, while CpTiCl3/MAO catalytic system exhibited much lower activity of 20 kg mol1 (Ti) h1. The activity of 2a with methyl at 4-position of aryloxide was about twice that of methoxyl substituented 2b. As the data revealed by single crystal Xray diffraction, the Ti1eO1eC1 angle of 2a (167.2 ) was larger than that of 2b (162.9 ), likely because the larger angle of TieOeC, the more electron donation from oxygen to titanium atom, thus stabilizing the active species and sequencially leading to the high activity [12]. In addition to the effect of bulky aryloxide on the catalytic activity, it has been reported that the cyclopentadienyl ring bearing five methyl substituents can afford more electron donation to the metal center for high activity, thus 3a exhibited the highest activity of 2100 kg mol1 (Ti) h1 [17]. 3.3. The ethylene/1-hexene copolymerization behaviors The ethylene/1-hexene copolymerization with these complexes as precatalysts was further investigated (Table 2). Although these complexes activated by MAO were inactive for 1-hexene homopolymerizaiton, the ethylene/1-hexene copolymerizaiton successfully proceeded. This might be attributed to that ethylene replaces the cocatalyst for coordination with active site, and thus providing enough space for 1-hexene coordination [33]. All the complexes showed moderate catalytic activities in the range of 320e1180 kg mol1 (Ti) h1 in ethylene/1-hexene copolymerization. While, compared with ethylene homopolymerization, the catalytic activities and the molecular weights of the resultant polymers decreased with the increasing 1-hexene concentration, indicating that the polymer chain terminated during the insertion of increasing 1-hexene comonomer [31]. Determined by 13C NMR, the 1-hexene incorporation levels were in the range of 7.3e34.4%,
Table 2 Ethylene/1-hexene copolymerization with 2a, 2b and 3a/MAO systems.a Entry
Cat.
1-Hexene (mol/L)
Yield (g)
Activityb
Mwc (103)
Mw/Mnc
Hexene content (%)d
Tme (ºC)
15 16 17 18 19 20 21
2a 2a 2a 2a 2b 3a 3a
0.16 0.32 0.48 1.00 0.48 0.16 0.48
0.90 1.02 0.74 0.93 0.32 1.18 0.61
900 1020 740 930 320 1180 610
462 103 55 25 117 379 194
4.1 4.2 2.9 2.1 1.7 2.4 2.0
n.d. 10.8 22.3 34.4 17.9 7.3 15.6
122.0 119.5 116.9 None 119.3 93.2,108.6 78.6,120.9
a b c d e
Polymerization: carried out in 60 mL of toluene for 30 min at 50 C with 2 mmol of Ti, under an ethylene pressure of 6.0 atm. Activity: kg mol1 (Ti) h1. Determined by high temperature GPC. Estimated by13C NMR spectra. Determined by DSC.
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Table 3 Monnomer Sequence Distributions ethylene/1-hexene copolymers obtained with 2a, 2b and 3a/MAO system.a. Entry
16 17 19 21 a b c d
Cat.
2a 2a 2b 3a
Content (mol%)b
10.8 22.3 17.9 15.6
Triad sequencec (%)
Dyad sequencec (%)
rErHd
EEE
EEH þ HEE
HEH
EHE
EHH þ HHE
HHH
EE
EH þ HE
HH
72.7 54.8 66.3 53.7
11.1 17.8 11.1 24.5
trace 2.2 Trace 4.6
6.5 8.9 7.0 14.7
9.6 16.2 15.5 2.3
Trace 0.1 trace trace
78.2 63.7 71.9 69.9
21.7 28.1 20.2 28.9
4.8 8.2 7.8 1.1
3.18 2.53 5.62 0.37
Polymerization: see Table 2. 1-hexene content estimated by13C NMR spectra. Calculated by13C NMR spectra. rErH ¼ 4 [EE][HH]/[EH þ HE]2.
and the level increased with the increasing concentration of 1hexene. Complex 2a displayed the highest 1-hexene incorporation ability among the present complexes. As revealed by the X-ray diffraction data, the large Cp(cent)-Ti-O angle in 2a leads to the open space for 1-hexene coordination [34]. The monomer sequence distributions and the rE$rH values were calculated and summarized in Table 3 [35]. The contents of EHH þ HHE and EHE triad sequences in the copolymers obtained with 2a and 3a are quite different. The EHH þ HHE content of the copolymers obtained with 2a were 16.2%, much higher than that obtained with 3a, while the copolymer obtained with 3a had the highest EHE content. Determined by DSC analysis (Fig. 3), the Tm value of the copolymers obtained with 2a decreases slowly from 122.0 C to 119.3 C with increasing incorporation level of 1hexene, and the lower molecular weight and the higher EHH þ HHE content indicated that higher content of continuous hexene units perform in the polymer chain. Furthermore, the incorporation level of 1-hexene increased to 33.4%, the resultant copolymer become amorphous. The copolymer obtained with 3a had two Tm values (Fig. 4), likely because polyethylene chains with different lengthes exist in the copolymer. In the 13C NMR spectrum (Fig. 5), HHH triad sequence (aa) exists in the copolymer with 22.3% comonomer incorporation obtained with 2a, but not observed in that obtained with 3a (Fig. 6). The rE$rH values imply that 1-hexene inserts in the polymer chain is not in a random manner [18]. The monomer sequence distributions and rE$rH values greatly depend on the substituent of cyclopentadienyl group, indicating that the ligand structure has an important influence on selecting next inserted monomer unit during the chain propagation [29].
Fig. 3. The DSC curves of poly (ethylene-1-hexene)s obtained with 2a/MAO under various 1-hexene concentration.
Fig. 4. The DSC curves of poly (ethylene-1-hexene)s obtained with 3a/MAO under various 1-hexene concentration.
4. Conclusion Half-titanocene complexes bearing monodentate anionic aryloxide ligand with bulky dibenzhydryl moieties [Cp'TiOCl2(2,6Ph2CHeC6H2-4-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
Fig. 5. 13C NMR spectrum of entylene/1-hexene copolymer obtained with 2a/MAO system (Table 2, Entry 17).
B. Dong et al. / Polymer 100 (2016) 188e193
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References
Fig. 6. 13C NMR spectrum of entylene/1-hexene copolymer obtained with 3a/MAO system (Table 2, Entry 21).
narrow polydispersity. These results indicated that introducing the bulky dibenzhydryl on the aryloxide ligand could provide large steric force to open the TieOeC angle of the titanium complex for the more electron donation to the metal center, which was benefit to stabilize the active specie for the high activity for the ethylene polymerization. In addition, the present catalytic systems displayed high activity for the ethylene/1-hexene copolymerization and produced the copolymers with relatively high 1-hexene incorporation. Furthermore, higher 1-hexene incorporation and 1-hexene continuous units in the copolymers obtained with Cp-based 2a than that with Cp*-based 3a, indicating that Cp structure play an important role in the composition and sequence distribution of the resulting copolymers.
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).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.08.045.
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