Nickel complexes based on hyperbranched bispyridylamine ligands as catalyst precursors for ethylene oligomerization

Abstract

A series of nickel complexes containing bispyridylamine ligands is synthesized and characterized by UV-Vis, FTIR, ¹H NMR, ESI-MS, and elemental analysis. All nickel complexes activated with methylaluminoxane exhibited moderate activity for ethylene oligomerization (5.30 × 10⁴ to 2.54 × 10⁵ g/(mol·Ni·h)), producing oligomers in the range C₄–C₁₈. The effects of reaction temperature, reaction pressure, Al/Ni molar ratio, and solvent type on the catalytic activity and product selectivity are also investigated. The resulting products are mainly low-carbon olefins (C₄ and C₆). Under the optimized conditions, the complex with R¹ = C₄H₉ formed by the reaction of the corresponding ligand R¹ = C₄H₉ with NiCl₂·6H₂O showed a higher catalytic activity of 2.54 × 10⁵ g/(mol·Ni·h) compared to the complex with R² = C₆H₁₃ and R³ = C₆H₁₁ using toluene as the solvent. Moreover, the selectivity for the C₄ and C₆ oligomers can reach up to 82%.

Introduction

Linear α-olefins (LAOs) are important commodity chemicals with a variety of applications in industrial processes, ranging from co-monomers for the production of polyolefins to intermediates for synthetic lubricants, plasticizers, and surfactants.^1–4 1-Butene is used as a co-monomer in the production of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). Therefore, the production of 1-butene directly influences the market for both HDPE and LLDPE. Several manufactures such as Shell, Idemitsu, and Chevron Phillips Chemical Company use metal-catalyzed multi-step ethylene oligomerization processes to produce LAOs. Most of these processes generate a wide distribution of LAO carbon chain lengths from C₄ (1-butene) to C₂₀ (1-eicosene).^5,6 In this context, high-carbon olefins (C₈–C₁₂) are also particularly valuable, imparting polymers with good tear resistance and other desirable properties. ⁷ For certain applications, high purity of α-olefins is essential because internal olefins are unreactive and thus build up during co-polymerizations. Preventing formation of unwanted polymeric materials is a major task, influencing the profitability of unselective or selective catalytic systems designed to produce LAOs. ⁸

Numerous studies on ethylene polymerizations using nickel catalysts have established that the branching density and distribution strongly depend on the nature of the ligand.^9,10 An effective approach to controlling the branching distribution of polyethylene products is the modification of ligand frameworks. For example, α-diamine nickel catalysts with modified ortho-substituents afforded methyl-branched polyethylene by controlling the chain propagation reaction. ¹¹ In addition, hyperbranched macromolecules are well-defined, highly branched, three-dimensional macromolecules with characteristic globular structures, ¹² imparting the catalyst with good solubility, low viscosity, and high chemical reactivity. In this article, a series of novel hyperbranched nickel complexes with different alkyl groups has been designed for the preparation of LAOs via ethylene oligomerization.^13,14 According to studies, the P and N donors have a strong potential to facilitate the selective ethylene oligomerization.^15–20 At the same time, we investigated the effects of the reaction conditions and the solvent on ethylene oligomerization.

Results and discussion

Synthesis and characterization of the hyperbranched bispyridylamine ligands and nickel complexes

The hyperbranched bispyridylamine ligands 3a–c were readily synthesized by the substitution reaction between the corresponding 1.0 G hyperbranched macromolecules 1a–c and 2-chloropyridine (2) in refluxing tetrahydrofuran (THF; Scheme 1). These ligands were characterized by UV-Vis, Fourier transform infrared (FTIR), and ¹H NMR spectroscopy. Using methanol as the solvent, the UV-Vis spectra shown three absorptions at 209, 260, and 295 nm, respectively (Supplementary Figure S1). The band at 209 nm is assigned to the R band of the n→π* transition for C=O. The bands at 260 and 295 nm are assigned to the B band of π→π* transition and the R band of the n→π* transition for the pyridine rings. In the FTIR spectra of 3a–c (Supplementary Figure S2), a strong and sharp peak at 3297 cm⁻¹ can be ascribed to the stretching vibration of N-H, ²¹ indicating the existence of a secondary amine. The absorption around 3057 cm⁻¹ is due to the C–H stretching vibration of the pyridine ring. Peaks at 1771 and 1592 cm⁻¹ are attributed to the stretching vibrations of C=O and C=N of the pyridine rings, respectively. Moreover, the absorption at 1467 cm⁻¹ is assigned to the pyridine rings. The ¹H NMR spectra of ligands 3a–c (Supplementary Figure S3) in CDCl₃ at room temperature exhibit resonances in the region 7.23–8.39 ppm assigned to the pyridine ring. The signals for the protons of the methyls and methylenes of the alkyl chains can be found at the range from 0.89 to 3.68 ppm, and the protons for the cyclohexylamine occur at 1.13–2.65 ppm. The ¹³C NMR spectra of ligands 3a–c are provided in the Supplementary Figure S4.

Scheme 1.

Synthetic routes of the bispyridylamine ligands and nickel complexes.

The reactions of NiCl₂·6H₂O with ligands 3a–c in methanol at room temperature afford the corresponding nickel complexes 4a–c, which were isolated as green solids in moderate to good yields. Complexes 4a–c were fully characterized by a combination of UV-Vis, FTIR, and ESI-MS spectroscopy. From the UV-Vis spectrum of these complexes (Supplementary Figure S1), the nickel complexes exhibited two absorptions at 208 and 255 nm (4a), 209 and 264 nm (4b), and 204 and 224 nm (4c), respectively. The B band of the π→π* transition is blue shifted from 260 to 254 nm (4b) or 224 nm (4c) and the R band of the n→π* transition disappeared after the nickel atom had coordinated with the ligands. In the FTIR spectra of these complexes, the absorption peaks were broad and weak (Supplementary Figure S2). The C=N peaks are red-shifted from 1592 to 1563 cm⁻¹ due to the fact that the coordination between the nickel ion and nitrogen atom weakens the C=N double bond and enhances the C–H bond bending vibrations of the carbon atom. The results of ESI-MS (Supplementary Figure S5) for 4a–c showed the molecular ion peaks [M + H]⁺ at m/z 870.67, 898.57, and 896.71, respectively. The fragment ion peaks for [3a]⁺, [3b]⁺, and [3c]⁺ occurred at m/z 609.71, 637.31, and 635.38. These data confirmed the proposed structures of the hyperbranched bispyridylamine nickel complexes.

Evaluation of the nickel complexes as catalysts for ethylene oligomerization reaction

These complexes 4a–c were evaluated as catalysts for ethylene oligomerization, using three different solvents (toluene, methylcyclohexane, cyclohexane) with methylaluminoxane (MAO) as the co-catalyst. In all cases, the complexes showed moderate catalytic activities and the resulting distribution of oligomers was in the range C₄–C₁₈. The catalytic activity and product selectivity of the olefins were influenced by the ligand type, the solvent, and reaction conditions.

Effects of solvent and ligand type on the catalytic activity and the product selectivity

Considering the effect of the solvent on the catalytic activity and product selectivity, ²² the catalytic systems were studied with toluene, methylcyclohexane, and cyclohexane and the results are listed in Table 1. The highest activity was observed in toluene. This may be due to the greater polarity of toluene and the poorer solubility of the nickel complexes in methylcyclohexane and cyclohexane. However, the oligomers were mainly low-carbon olefins in this solvent and we chose the toluene as the best solvent.

Table 1.

Effects of the solvent and ligand type on the catalytic activity and the product selectivity. ^a

Catalyst	Solvent	Activity (105 g·(mol Ni·h)−1)	Product selectivity (wt%)
			C4	C6	C8	C10–C18
4a	Toluene	2.54	61.10	21.08	13.68	4.14
	Methylcyclohexane	1.11	71.30	9.47	10.27	8.96
	Cyclohexane	0.53	75.55	10.15	7.63	6.67
4b	Toluene	1.92	50.60	39.52	6.43	3.44
	Methylcyclohexane	1.72	52.75	28.33	9.16	9.76
	Cyclohexane	1.47	66.21	16.84	6.85	10.10
4c	Toluene	1.81	60.43	17.11	14.56	7.42
	Methylcyclohexane	1.63	53.25	27.45	8.07	11.23
	Cyclohexane	1.35	58.76	19.80	13.13	8.31

a

Reaction conditions: catalyst (5 µmol), reaction temperature (55°C), reaction pressure (4 MPa), Al/Ni (700), solvent (total volume: 50 mL), and reaction time (30 min).

From Table 1, we can also see that the ligand type had a great influence on the catalytic activity and product selectivity. Changes in the ligand structure by substituting the butylamine with N-hexylamine and cyclohexylamine led to a decrease of activity (4a > 4b > 4c), which can be explained due to the poorer solubility on increasing the length of the alkyl chain in the ligand backbone in toluene as solvent. Furthermore, the sterics of the nickel complex would hinder the propagation of ethylene and thereby reducing its insertion rate of ethylene. ²³ So, the final oligomers were based on low-carbon olefins mostly and 4a was investigated further as follows.

Effects of the reaction temperature on the catalytic activity and the product selectivity

The effect of the reaction temperature on the catalytic activity and the product selectivity is shown in Figure 1. The results of the oligomerization performed at temperatures between 25°C and 65°C are considered first. On elevating the reaction temperature from 25°C to 65°C, the catalytic activity increases initially and then decreases. The catalytic activity reaches a maximum (2.54 × 10⁵ g/(mol·Ni·h)) at 55°C, but a higher temperature leads to a decrease in the catalytic activity because of the decreasing ethylene solubility in toluene and the catalytic species decomposing at a higher temperature.^24–26 Different reaction temperatures also have an effect on the product distribution. Increasing the reaction temperature from 25°C to 65°C results in a gradual increase in C₄ and C₆ oligomers at the expense of high-carbon olefins. This shows that the rate of chain transfer increases more than the rate of chain propagation.

Figure 1.

Effects of the reaction temperature on the catalytic activity and the product selectivity. Reaction conditions: catalyst (5 μmol), reaction pressure (4 MPa), Al/Ni (700), toluene (50 mL), and reaction time (30 min).

Effects of the reaction pressure on the catalytic activity and the product selectivity

The effect of reaction pressure on the catalytic activity and the product selectivity is shown in Figure 2. From Figure 2, it can be seen that the reaction pressure played a vital role in this catalytic system. When the reaction pressure increased from 1 to 5 MPa, the catalytic activity increases initially and then decreases. The catalytic activity reaches up a maximum (2.54 × 10⁵ g/(mol·Ni·h)) at 4 MPa. This may be due to elevated ethylene pressure resulting in an increase in the ethylene concentration in the solvent, leading to an increase of activity. However, the excess of ethylene prevents the reaction between Ni(II) and MAO which is responsible for formation of the active species. In other words, ethylene acts as a catalyst poison. Moreover, the rate of ethylene diffusion into the active center is slowed down. A combination of the above two factors can account for the observed reaction pressure dependence of the catalytic activity. In addition, the reaction pressure has a significant effect on the product selectivity. The selectivity for high-carbon olefins increases initially and generally decreases. The resulting oligomers are mainly low-carbon olefins.

Figure 2.

Effects of the reaction pressure on the catalytic activity and product selectivity. Reaction conditions: Catalyst (5 μmol), reaction temperature (55°C), Al/Ni (700), toluene (50 mL), and reaction time (30 min).

Effects of the Al/Ni molar ratio on the catalytic activity and the product selectivity

The effect of the Al/Ni molar ratio on the catalytic activity and the product selectivity is shown in Figure 3, with the Al/Ni molar ratio having an important impact on the catalytic performance. As the Al/Ni molar ratio increases from 100 to 900, the catalytic activity increases above and then decreases. The highest activity is 2.54 × 10⁵ g/(mol·Ni·h) at a molar ratio of 700. Meanwhile, the selectivity for C₄ and C₆ was maximized. The increasing Al/Ni molar ratio led to more active sites and hence an increase in the catalytic activity. In contrast, further increases in the MAO concentration can also interfere with the formation of the over-reduced nickel species leading to deactivation of the active species and a decrease in the activity. ²⁷ Furthermore, the oligomer distributions are not obviously changed on increasing the Al/Ni molar ratio, which may due to the non-linear relationship between the ratio of chain transfer and chain propagation.

Figure 3.

Effects of Al/Ni molar ratio on catalytic activity and product selectivity. Reaction conditions: Catalyst (5 μmol), reaction temperature (55°C), reaction pressure (4 MPa), toluene (50 mL), and reaction time (30 min).

Conclusion

A series of novel types of bidentate bispyridylamine ligands 3a–c is easily prepared in one step from the corresponding 1.0 G hyperbranched macromolecule ligands, and are used as bidentate monoanionic ligands to synthesize dichloronickel complexes 4a–c. The hyperbranched bispyridylamine nickel complex 4a activated with MAO shown higher catalytic activity than 4b and 4c in toluene as the solvent under the same conditions. The resulting products were mainly low-carbon olefins. In addition, the effects of reaction temperature, reaction pressure, Al/Ni molar ratio and the solvent on ethylene oligomerization were studied. Under the optimized conditions (T = 55°C, P = 4 MPa, Al/Ni = 700), the catalytic activity of complex 4a in toluene was 2.54 × 10⁵ g/(mol·Ni·h) and the selectivity for C₄ and C₆ oligomers was 82%.

Experimental

Materials and instrumentation

All manipulations involving air- and water-sensitive compounds were carried out under an inert atmosphere in a glovebox or by using standard Schlenk techniques. Solvents were dried from the appropriate drying agents under nitrogen before use. Butylamine, N-hexylamine, cyclohexylamine, NiCl₂·6H₂O and 2-chloropyridine were purchased from Aladdin and used as received. Methyl acrylate, anhydrous ethylenediamine, triethylamine (Et₃N) and toluene were obtained from Tianjin Damao Chemical Reagent. Methanol and THF were provided from Tianjin Kermel Chemical Reagent. MAO (10 wt% in toluene) was bought from Sigma-Aldrich. Polymerization grade ethylene was obtained from Daqing Summit Specially Gases (China). The 1.0 G hyperbranched macromolecules were synthesized according to the literature procedures described in our earlier study. ²⁸

UV-Vis spectra were obtained using on a TU-1901 UV-Vis spectrophotometer with methanol as the solvent. FTIR spectroscopy was performed on a Bruker Vector 22 spectrophotometer from 4000 to 450 cm⁻¹. The ESI-MS was obtained using a micrOTOF-QⅡ mass spectrometer. ¹H NMR measurements were obtained on a Bruker 400 MHz NMR in CDCl₃ solution. The ¹H NMR spectra of all the compounds were recorded at ambient temperature. Gas chromatography (GC) analysis of the oligomers was conducted on a Fuli GC 9720 instrument equipped with an flame ionization detector (FID) and a 50 m (0.2 mm id, 0.5 μm film thickness) HP-PONA column, operating at 50°C for 5 min followed by heating at 10°C min⁻¹ until 140°C, then heating at 5°C min⁻¹ until 240°C, and then for 5 min, injecting the sample.

Synthesis of the hyperbranched bispyridylamine ligands

Synthesis of the hyperbranched bispyridylamine ligand with butylamine as the core, 3a

3,3′-(butylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) (3a): A mixture of Et₃N (8.66 mL, 60.00 mmol) and 2-chloropyridine (5.68 mL, 60.00 mmol) in 25 mL THF was stirred for 15 min at 25°C first and then cooled to 0°C. A solution of 1.0 G hyperbranched macromolecule 1a (3.01 g, 10.00 mmol) in 25 mL of THF was added dropwise. After 30 min of stirring, the mixture was heated to 40°C and stirred for 24 h. The resulting white suspension was filtered to remove Et₃N·HCl. The ligand solution was condensed by rotary distillation and washed with THF. The product 3a was obtained as a yellow viscous ligand: (3.10 g, 5.10 mmol, 51%). UV-Vis (λ_max, nm): 209, 260, and 295. FTIR (KBr pellet, cm⁻¹): ν 3297, 3057, 2963, 2868, 1771, 1592, and 1454. ¹H NMR (400 MHz, CDCl₃, ppm): δ = 0.89 (t, 3H, CH₃, J = 4.84 Hz), 1.26 (m, 2H, CH₂), 1.39 (m, 2H, CH₂), 2.44 (t, 4H, CH₂, J = 7.05 Hz), 2.76 (t, 2H, CH₂, J = 7.59 Hz), 3.32 (t, 4H, CH₂, J = 10.15 Hz), 3.65 (m, 4H, CH₂), 3.68 (t, 4H, CH₂, J = 5.53 Hz), 7.23 (t, 4H, py-H, J = 4.81 Hz), 7.37 (d, 4H, py-H, J = 8.14 Hz), 7.68 (t, 4H, py-H, J = 5.87 Hz), 8.40 (t, 2H, N–H, J = 3.43 Hz), 8.42 (d, 4H, py-H, J = 3.45 Hz). ¹³C NMR (100 MHz, CDCl₃, ppm): δ = 14.00, 20.75, 29.25, 36.03, 38.35, 44.32, 48.87, 56.45, 116.80, 117.17, 137.32, 147.14, 157.30, 170.28. Anal. calcd for C₃₄H₄₃N₉O₂ (609.41): C, 67.00; H, 7.06; N, 20.69. Found: C, 66.89; H, 7.01; N, 20.46.

Synthesis of the hyperbranched bispyridylamine ligand with N-hexylamine as the core, 3b

3, 3′-(hexylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) (3b): A procedure analogous to that used to prepare 3a was used, but starting from N-hexyl substituted bispyridylamine ligand 1b. The product 3b was obtained as a yellow viscous ligand (3.26 g, 5.12 mmol, 51%). UV-Vis (λ_max, nm): 210, 261, 296. FTIR (KBr pellet, cm⁻¹): ν 3307, 3055, 2957, 2867, 1774, 1593, 1467. ¹H NMR (400 MHz, CDCl₃, ppm): δ = 0.91 (t, 3H, CH₃, J = 4.64 Hz), 1.27 (m, 4H, CH₂), 1.38 (m 2H CH₂), 1.40 (m, 2H, CH₂), 2.45 (t, 4H, CH₂, J = 7.01 Hz), 2.76 (t, 2H, CH₂, J = 7.52 Hz), 3.33 (t, 4H, CH₂, J = 9.92 Hz), 3.66 (m, 4H, CH₂), 3.85 (t, 4H, CH₂, J = 8.01 Hz), 7.23 (t, 4H, py-H, J = 4.80 Hz), 7.35 (d, 4H, py-H, J = 8.12 Hz), 7.66 (t, 4H, py-H, J = 5.82 Hz), 8.17 (t, 2H, N–H, J = 3.42 Hz), 8.39 (d, 4H, py-H, J = 3.25 Hz). ¹³C NMR (100 MHz, CDCl₃, ppm): δ = 14.00, 23.16, 27.59, 27.95, 31.73, 36.03, 38.35, 44.32, 48.87, 56.30, 116.80, 117.17, 137.32, 147.14, 157.30, 170.28. Anal. calcd for C₃₆H₄₇N₉O₂ (637.45): C, 67.82; H, 7.38; N, 19.78. Found: C, 67.38; H, 7.05; N, 19.92.

Synthesis of the hyperbranched bispyridylamine ligand with cyclohexylamine as the core, 3c

3, 3′-(cyclohexylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) (3c): A procedure analogous to that used to prepare 3a was used, but starting from cyclohexylamine substituted bispyridylamine ligand 1c. The product 3c was obtained as a yellow viscous ligand (3.29 g, 5.18 mmol, 52%). UV-Vis (λ_max, nm): 210, 260, 297. FTIR (KBr pellet, cm⁻¹): ν 3302, 3061, 2950, 2873, 1772, 1591, 1468. ¹H NMR (400 MHz, CDCl₃, ppm): δ = 1.13 (m, 4H, CH₂), 1.39 (m, 2H, CH₂), 1.63 (m 4H CH₂), 2.44 (t, 4H, CH₂, J = 7.05 Hz), 2.65 (m, 1H, CH), 3.33 (t, 4H, CH₂, J = 8.02 Hz), 3.67 (m, 4H, CH₂), 3.85 (t, 4H, CH₂, J = 7.65 Hz), 7.23 (t, 4H, py-H, J = 4.22 Hz), 7.34 (d, 4H, py-H, J = 8.22 Hz), 7.66 (t, 4H, py-H, J = 5.62 Hz), 8.18 (t, 2H, N–H, J = 3.32 Hz), 8.39 (d, 4H, py-H, J = 3.25 Hz). ¹³C NMR (100 MHz, CDCl₃, ppm): δ = 24.76, 25.92, 30.03, 36.10, 38.35, 44.32, 45.88, 63.38, 116.80, 117.17, 137.32, 147.14, 157.30, 170.28. Anal. calcd for C₃₆H₄₅N₉O₂ (635.43): C, 68.03; H, 7.09; N, 19.84. Found: C, 67.78; H, 6.85; N, 19.76.

Synthesis of the hyperbranched bispyridylamine nickel complexes

Synthesis of the hyperbranched bispyridylamine nickel complex with butylamine as the core, 4a

3, 3′-(butylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) nickel chloride (4a): To a solution of NiCl₂·6H₂O (0.52 g, 4.00 mmol) in methanol (20 mL) was added a solution of 3a (1.22 g, 2.00 mmol) in methanol (20 mL) and the resulting mixture was stirred for 24 h at room temperature. The solvent was removed, and the resulting green solid residue was washed with Et₂O (3 × 15 mL). Complex 4a was obtained as a green solid (1.17 g, 1.35 mmol, 67%). UV-Vis (λ_max, nm): 208, 265. FTIR (KBr pellet, cm⁻¹): ν 3257, 3057, 2963, 2873, 1620, 1564, 1418. ESI-MS (m/z): 870.67 [M + H]⁺. Anal. calcd for C₃₄H₄₃N₉O₂Ni₂Cl₄ (869.00): C, 46.95; H, 4.95; N, 14.50. Found: C, 46.65; H, 4.35; N, 14.69.

Synthesis of the hyperbranched bispyridylamine nickel complex with N-hexylamine as the core, 4b

3, 3′-(hexylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) nickel chloride (4b): This complex was prepared according to the method described for 4a using NiCl₂·6H₂O (0.52 g, 4.00 mmol) and 3b (1.33 g, 2.00 mmol). Complex 4b was obtained as a green solid (1.25 g, 1.39 mmol, 70%). UV-Vis (λ_max, nm): 209, 264. FTIR (KBr pellet, cm⁻¹): ν 3257, 3057, 2963, 2873, 1746, 1547, 1435. ESI-MS (m/z): 898.57 [M + H]⁺. Anal. calcd for C₃₆H₄₇N₉O₂Ni₂Cl₄ (897.00): C, 48.16; H, 5.24; N, 14.05. Found: C, 47.78; H, 5.15; N, 14.25.

Synthesis of the hyperbranched bispyridylamine nickel complex with cyclohexylamine as the core, 4c

3, 3′-(cyclohexylazanediyl)bis(N-(2-(di(pyridin-2-yl)amino)ethyl)propanamide) nickel chloride (4c): This complex was prepared according to the method described for 4a using NiCl_2·6H₂O (0.52 g, 4.00 mmol) and 3c (1.33 g, 2.00 mmol). Complex 4c was obtained as a green solid (1.12 g, 1.25 mmol, 63%; Scheme 1). UV-Vis (λ_max, nm): 204, 224. FTIR (KBr pellet, cm⁻¹): ν 3257, 3057, 2963, 2873, 1771, 1580, 1433. ESI-MS (m/z): 896.71 [M + H]⁺. Anal. calcd for C₃₆H₄₅N₉O₂ Ni₂Cl₄ (895.00): C, 48.27; H, 5.03; N, 14.08. Found: C, 48.18; H, 4.89; N, 14.29.

Ethylene oligomerization

All ethylene oligomerization tests were performed in a 250 mL stainless steel reactor equipped with mechanical stirring, internal temperature control, and a continuous feed of ethylene. The stainless steel reactor was dried in an oven at 140°C for 2 h prior to each run and then placed under vacuum for 30 min. The reactor was evacuated and flushed with N₂ (three times) and ethylene (twice). A typical reaction was performed by introducing the solvent (30 mL), the appropriate amount of co-catalyst, and the catalyst precursor solution under a stream of ethylene. Then, the reactor was immediately pressurized. Ethylene was continuously fed in order to maintain the desired ethylene pressure. After 30 min, the reactor was cooled to 0°C and the reaction was quenched with 10 wt% HCl in ethanol. The distribution of the oligomerization product was analyzed by GC and compared with the standard authentic samples. The catalytic activity was calculated from the increase in product weight.

Supplemental Material

Supplement_Figure – Supplemental material for Nickel complexes based on hyperbranched bispyridylamine ligands as catalyst precursors for ethylene oligomerization

Supplemental material, Supplement_Figure for Nickel complexes based on hyperbranched bispyridylamine ligands as catalyst precursors for ethylene oligomerization by Jun Wang, Jinyi Liu, Liduo Chen, Tianyu Lan and Libo Wang in Journal of Chemical Research