Materials
All manipulations were performed under a purified and inert atmosphere by nitrogen line or glovebox techniques. All chemicals, trimethylaluminum (TMA, 2 M in hexane), modified methylaluminoxane (MMAO by 7% wt Al in toluene) were purchased from Sigma-Aldrich Co. Dichloromethane (DCM) were dried over calcium hydride. Toluene (> 99.5%) was dried by distillation over sodium and benzophenone. Aniline, 2 6-dimethylaniline, and 2 6-diisopropylaniline were distilled under reduced pressure. Nitrogen and ethylene were dried through columns containing active silica gel, anhydrous potassium hydride, and a molecular sieve.
Characterization
FT-IR analysis was carried out using a BRUKER-IFS48 spectrophotometer (Germany). 1HNMR, 13CNMR, H-H COSY data of ligands were determined on Bruker 400 MHz Ultra-Shield (USA) instruments at 30°C, using CDCl3 and tetramethylsilane (TMS). DSC was conducted with a Mettler-Toledo DSC1 (USA). Thermograms were recorded at -100 ºC to 200 ºC under an N2 atmosphere at heating rates of 10 ºC/min. The contents of camphyl-based Ni(II) complexes were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using Thermo electron corporation make, (IRIS Intrepid 11 XDL instrument). Elemental analysis (C, H, N) was performed on a Vario EL series elemental analyzer. EDS was performed in SEM to investigate the chemical composition of the final catalyst.
Synthesis of camphyl-based ligands and corresponding Ni(II) complexes
All ligands synthesized were characterized by FT-IR, 1HNMR, 13CNMR, H-H COSY and elemental analysis.
Synthesis of H1
To a stirred solution of aniline (12 mmol, 1.1 mL) in toluene (20 mL) was added a solution of trimethylaluminum (6 mL, 2.0 M in hexane) at room temperature, under a nitrogen atmosphere and stirred under reflux conditions. After 2 h, the reaction temperature decreases to 25°C, camphorquinone (5 mmol, 0.83 g) was added, and stirred for 6 h under reflux conditions. Then, the reaction mixture was hydrolyzed with NaOH solution (5%) at 0°C. The synthesized ligand was extracted by ethyl acetate, dried over magnesium sulfate, and evaporated toluene. The collected solids were purified by silica gel (column chromatography) using n-hexane/ethyl acetate (15:1) as eluent. The final product with 63.2% yield was obtained as yellow crystals (in ethanol). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.37–6.58 (m, 10H, Ar-H), 2.67 (m, 1H, tertiary hydrogen at camphyl), 2.0 (m, 1H, CH2 at camphyl), 1.91 (m, 1H, CH2 at camphyl), 1.81 (m, 1H, CH2 at camphyl), 1.69 (m, 1H, CH2 at camphyl), 1.22 (s, 3H, CH3 at camphyl), 0.97 (s, 3H, CH3 at camphyl), 1.02 (s, 3H, CH3 at camphyl due to the mixture of camphyl isomer), 0.85 (s, 3H, CH3 at camphyl). 13C NMR (400 MHz, CDCl3), δ (ppm): 173.99, 172.03 (C = N), 151.05, 150.03 (C-N), 128.66, 128.58, 128.02, 124.11, 123.01, 122.73, 120.13, 119.57, 118.54, 118.26 (Ar-C), 56.41, 54.89, 50.61, 49.96, 45.45, 46.19, 32.02, 33.54, 24.39, 21.07, 20.41, 17.69, 17.09, 12.69, 11.18 (camphyl-C). Anal. Calcd for C22H24N2: C, 83.53; H, 7.66; N, 8.81. Found: C, 83.61; H, 7.56; N, 8.83. See 1HNMR, 13CNMR, H-H COSY, and FT-IR of H1 in the Supporting Information (Figure S1-S3 and S7, respectively).
Synthesis of H2
Following the above process, H2 was purified by silica gel (column chromatography) using petroleum ether/ethyl acetate (15:1) as eluent. The final product with 75.2% yield was obtained as orang crystals (in ethanol). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.29–6.79 (m, 6H, Ar-H), 2.05 (s, 12H, CAr-CH3), 1.47 (m, 4H, CH2 at camphyl), 1.50 (m, 1H, tertiary hydrogen at camphyl), 1.34 (s, 3H, CH3 at camphyl), 1.15 (s, 3H, CH3 at camphyl), 1.01 (s, 3H, CH3 at camphyl). 13C NMR (400 MHz, CDCl3), δ (ppm): 168.71 (C = N), 149.44, 148.39 (C-N), 127.53 (CAr-Me), 123.35, 121.98, 55.56, 51.15, 45.43, 32.88, 23.38, 21.70, 18.28, 11.51. Anal. Calcd for C26H32N2: C, 81.75; H, 8.63; N, 7.25. Found: C, 81.75; H, 8.53; N, 7.35. See 1HNMR, 13CNMR, H-H COSY, and FT-IR of H1 in the Supporting Information (Figure S4-S6 and S8, respectively).
Synthesis of H3
Following the above process, H3 was purified by silica gel (column chromatography) using petroleum ether/ethyl acetate (15:1) as eluent. The final product with 35.3% yield was obtained as yellow crystals (in ethanol). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.28–6.90 (m, 6H, Ar-H), 2.89 (m, 4H, CH(CH3)2), 2.37 (m, 1H, tertiary hydrogen at camphyl), 1.88 (m, 4H, CH2 at camphyl), 1.24 (d, 24H, CH(CH3)2), 0.97 (s, 6H, CH3 at camphyl), 0.76 (s, 3H, CH3 at camphyl). 13C NMR (400 MHz, CDCl3), δ (ppm): 169.02 (C = N), 145.16 (C-N), 136.27, 135.14, 134.51, 133.30 (CAr-iPr), 123.70, 122.91, 122.05, 121.75, 56.06, 50.65, 45.58, 32.33, 28.13, 24.17, 22.71, 11.31. Anal. Calcd for C34H48N2: C, 84.70; H, 9.21; N, 6.09. Found: C, 84.68; H, 9.39; N, 5.93.
Synthesis of C1
Ligand H1 (1.0 mmol, 0.44 g) and (DME)-NiBr2 (1.0 mmol, 0.308 g) were added to DCM (25 mL) at 25°C and stirred for 12 h under a nitrogen atmosphere. The reaction mixture was filtered and washed with hexane (10 mL × 3) and dried in a vacuum. The final solid product with 74.2% yield was obtained. See FT-IR of C1 in the Supporting Information (Figure S9).
Synthesis of C2
Following the above process, C2 was obtained in 80.2% yield. See FT-IR of C2 in the Supporting Information (Figure S10).
Synthesis of C3
Following the above process, C3 was obtained in 87.5% yield.
Ethylene Polymerization
All polyethylene was synthesized using a lab-scale polymerization setup. The setup had a 100 mL stainless steel reactor, a catalyst injector, and a magnetic agitator. An oil circulator between the reactor's inner and outer walls controlled the polymerization reaction's temperature. Before all polymerization reactions, the reactor was purged with pure nitrogen at 110°C for 2 hours and vacuumed to guarantee the absence of moisture and oxygen. Then, the reactor was charged with 19 mL of dry toluene and the required amount of the MMAO, and the solution was degassed. The reactor was warmed to a favorable temperature. To initiate ethylene polymerization, 5 µmol of the appropriate α-diimine complex was suspended in 1 mL toluene and injected into the reactor. In order to monitor the rate of ethylene uptake, a mass flow controller rate was conducted, and consumed ethylene was recovered to keep the reactor at constant pressure. After 1hr, gas was vented and the reaction quenched by ethanol-HCl (95:5). The obtained polymer was washed several times with ethanol and dried in a vacuum oven.
Design of Experiment
Recently, surface methodology (RSM) was used by many researchers in order to optimize and develop reaction engineering. The choice of an allowable experimental design is a necessary consideration in experimental optimization. Design Expert (v12.0; Stat-Ease, Inc.) and Box-Behnken Design (BBD) method was applied to obtain the optimum Pi,removal. Here, a three-factor three-level BBD (Table 1) for RSM was used to define the optimum conditions and study the effect of Tpoly., Al/Ni ratio, and Pethylene, on the yield of polymerization and catalyst activity.
Table 1
Experimental Ranges and Levels of the Al/Ni ratio (F1) and pressure (F2) and temperature (F3)
| Range and level |
Independent variables | Low (-1) | Center (0) | High (+ 1) |
Temperature (F1) | 400 | 600 | 800 |
Pressure (F2) | 1 | 3 | 5 |
Al/Ni ratio (F3) | 35 | 55 | 75 |