Ethylene polymerization catalysts containing asymmetric, sterically hindered alpha-diimine nickel (II) and methods for their preparation

Abstract

The present application relates to the field of metal catalysis, olefin polymerization, in particular to a kind of ethylene polymerization catalyst containing asymmetric big steric hindrance α-diimine nickel (II) and preparation method thereof;2,3-butanedione is used as catalyst skeleton, a big steric hindrance imine group and a small steric hindrance imine group are introduced on the two sides of the skeleton respectively, to ensure the activity of the catalyst in ethylene polymerization, simultaneously because of the introduction of big steric hindrance group, the polyethylene obtained by the polymerization of catalyst has super high molecular weight, the introduction of small steric hindrance group makes the polyethylene obtained by the polymerization of catalyst has higher branching degree, the obtained polyethylene material is thermoplastic elastomer, has good tensile and recovery and other mechanical properties;This kind of catalyst has good application potential in the preparation industry of ethylene polymerization.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal catalysis, olefin polymerization, in particular to a kind of ethylene polymerization catalyst containing asymmetric large steric hindrance α-diimine nickel (II) and preparation method thereof. BACKGROUND

[0002] Thermoplastic elastomers are applied to many fields due to their excellent elasticity and outstanding mechanical properties. There are three different strategies for synthetic thermoplastic elastomers, α-olefin copolymerization, propylene polymerization and ethylene homopolymerization. Ethylene homopolymerization method can be considered as a cost-effective alternative to the more complex and multi-step method of producing elastomeric materials, as it only requires one step for polymerization. However, before this method can be used in any practical application, some problems need to be solved. For example, the molecular weight of the polymer is not high enough, and the elastic properties are not suitable at room temperature or below. SUMMARY

[0003] The purpose of the present application is to provide an ethylene polymerization catalyst containing asymmetric large steric hindrance α-diimine nickel (II) and a preparation method thereof. By regulating the ligand steric hindrance, the steric hindrance of the imine groups contained on both sides of the skeleton is greatly different, so as to control the molecular weight and branching degree of the polymer.

[0004] The ethylene polymerization catalyst containing asymmetric large steric hindrance α-diimine nickel (II) designed in the present application has the following structure:

[0005]

[0006] The synthesis reaction formula of the ethylene polymerization catalyst containing asymmetric large steric hindrance α-diimine nickel (II) is as follows:

[0007]

[0008] The specific preparation method comprises the following process steps:

[0009] (1) Preparation of aniline containing substituent: p-methylaniline or aniline and benzhydrol are heated to 120°C at a molar ratio of 1:2 to 1:3. 0.5-1 equivalent of aniline of anhydrous zinc chloride is dissolved in 0.5 ml of concentrated hydrochloric acid and then added to the mixture, and the temperature is raised to 160°C for 30 minutes. After the reaction is completed, the reaction mixture is cooled to room temperature and dissolved in dichloromethane (200 mL). The dichloromethane layer is extracted with water (3 x 200 mL) and dried over anhydrous magnesium sulfate for 30 minutes. The solution is evaporated under reduced pressure to 10 mL, recrystallized with 100 ml of ethanol, washed with ethanol (3 x 100 ml), filtered and dried to obtain aniline powder containing substituent.

[0010] (2) Preparation of catalyst ligand intermediate: aniline containing substituent and 2,3-butanedione were mixed in a molar ratio of 1:1, 20 mg of p-toluenesulfonic acid monohydrate was added, toluene was used as solvent, stirring reflux at 55°C for 15 minutes. During the stirring process, sample every 5 minutes, observe on thin layer chromatography plate until there is a main point on the thin layer chromatography plate. Concentrate the solution to 10 ml under vacuum. Dilute the remaining solution in methanol (100 mL) to form a yellow solid. After filtration and separation, vacuum drying to obtain a light yellow solid (ligand intermediate).

[0011] (3) Preparation of nickel (II) ethylene polymerization catalyst ligand containing asymmetric bulky α-diimine: ligand intermediate, 1,2,3,4,5,6,7-hexahydro-s-indan-4-amine or 8-benzhydryl-1,2,3,4,5,6,7-hexahydro-s-indan-4-amine and p-toluenesulfonic acid toluene solution was stirred at 110°C for 24 hours, sample every 6 hours during the stirring process, observe on thin layer chromatography plate until there is a main point on the thin layer chromatography plate, concentrate the solution to 10 ml under vacuum, dilute the remaining solution in methanol to form a yellow solid; after filtration and separation, vacuum drying to obtain a yellow ligand powder;

[0012] The molar ratio of ligand intermediate and 1,2,3,4,5,6,7-hexahydro-s-indan-4-amine or 8-benzhydryl-1,2,3,4,5,6,7-hexahydro-s-indan-4-amine is 1:1.

[0013] (4) Preparation of nickel (II) ethylene polymerization catalyst ligand containing asymmetric bulky α-diimine: under nitrogen protection, nickel-based ethylene polymerization catalyst ligand containing asymmetric bulky α-diimine nickel (II) was mixed with ethylene glycol dimethyl ether activated (DME) NiBr2 in a molar ratio of 1:1, dichloromethane was used as solvent, after stirring at room temperature for 12 hours, the suspension was filtered, the mother liquor was removed under vacuum, washed with diethyl ether and dried under vacuum to obtain an orange-red powder solid complex.

[0014] The preparation method of ethylene glycol dimethyl ether activated nickel bromide [NiBr2(DME)] is as follows: under nitrogen protection, anhydrous nickel bromide is dissolved in anhydrous ethanol and stirred to reflux until the solution is dark green, then the filtrate is dried, 1,2-dimethoxyethane and triethyl orthoformate solvent (the solvent is stirred well in advance and refluxed) are added, and the mixture is stirred and refluxed overnight. Yellow solid precipitates. Concentrate the reaction solution, remove the solvent under reduced pressure. After the remaining solvent cools down, remove the upper solution, and the obtained product is soaked and washed with dry 10 mL of 1,2-dimethoxyethane, and the solvent is removed to obtain an orange-yellow solid powder [NiBr2(DME)].

[0015] The big steric hindrance α-diimine nickel(II) ethylene polymerization catalyst of the present application has two big volume groups diphenylmethyl introduced at the ortho position of the imine nitrogen atom and one relatively small imine group. The introduction of the big steric hindrance group increases the ratio of chain growth to chain transfer when the α-diimine nickel(II) catalyst is used to polymerize ethylene, thus increasing the molecular weight of the polymer and improving the thermal stability of the catalyst. The introduction of the relatively small imine group increases the ratio of chain walking to chain growth when the α-diimine nickel(II) catalyst is used to polymerize ethylene, thus increasing the branching degree of the polymer. Therefore, the polymer obtained by using the above nickel catalyst to polymerize ethylene is a thermoplastic elastomer with high molecular weight and high branching degree.

[0016] Experiments show that, in the ethylene polymerization reaction, the reaction temperature is controlled at 10-70℃, the reaction time is 10-30 min, and the catalyst dosage is 2 μmol. Under the activation of the cocatalyst diethyl aluminum chloride (Et2AlCl), the catalyst system has high catalytic activity (up to 10 6 g PE / (mol Ni h) in the ethylene polymerization, and the obtained polyethylene elastomer has high branching degree (up to 86 branches / 1000C).

[0017] The molar ratio of the catalyst to the cocatalyst diethyl aluminum chloride is 1:400.

[0018] Advantages

[0019] The big steric hindrance α-diimine nickel(II) ethylene polymerization catalyst of the present application has two big volume groups diphenylmethyl introduced at the ortho position of the imine nitrogen atom and one relatively small imine group. The introduction of the big steric hindrance group increases the ratio of chain growth to chain transfer when the α-diimine nickel(II) catalyst is used to polymerize ethylene, thus increasing the molecular weight of the polymer and improving the thermal stability of the catalyst. The introduction of the relatively small imine group increases the ratio of chain walking to chain growth when the α-diimine nickel(II) catalyst is used to polymerize ethylene, thus increasing the branching degree of the polymer. Therefore, the polymer obtained by using the above nickel catalyst to polymerize ethylene is a thermoplastic elastomer with high molecular weight and high branching degree. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The polyethylene tensile curve images prepared by the catalysts of Example 1 and Example 2.

[0021] Figure 2 The polyethylene recovery curve images prepared by the catalysts of Example 1 and Example 2.

[0022] Figure 3 The polyethylene prepared by the catalyst of Example 2 at 70℃1 H NMR. DETAILED DESCRIPTION

[0023] In order to better understand the technical solutions of the present application, the present application will be further described in detail below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, and are not limitations on the claims of the present application.

[0024] Reagents and materials used in the examples: all the metal organic reactions were carried out under nitrogen protection, and the solvents were all treated by drying and oxygen removal. The anhydrous methanol and ethanol were analytical pure and used directly. The toluene was dehydrated by molecular sieves, and then refluxed with sodium under nitrogen protection. Before use, the toluene was evaporated.

[0025] Example 1

[0026] (1) Synthesis of 2,6-bis(diphenylmethyl)-4-methylaniline

[0027] A mixture of p-toluidine (10 mmol, 1.0 equivalent) and diaryl carbinol (20 mmol, 2 equivalents) was heated to 120°C. After the mixture was melted, a solution of anhydrous zinc chloride (5 mmol, 0.5 equivalent) and concentrated hydrochloric acid (1.0 ml) was added to the mixture, and the temperature was increased to 160°C. After 30 minutes of reaction at 160°C, the reaction mixture was cooled to room temperature, dissolved in dichloromethane (200 mL). The dichloromethane layer was extracted with water (3 x 200 mL) and dried with anhydrous magnesium sulfate for 30 minutes. The solution was evaporated under reduced pressure to 10 mL, recrystallized with 100 ml of ethanol, washed with ethanol (3 x 100 ml), and filtered and dried to obtain white 2,6-bis(diphenylmethyl)-4-methylaniline powder with a yield of 82%. The preparation reaction formula is as follows:

[0028]

[0029] (2) Synthesis of catalyst ligand (L1)

[0030] The preparation reaction formula is as follows:

[0031]

[0032] A solution of 2,6-bis(diphenylmethyl)-4-methylaniline (2 mmol), 2,3-butanedione (2 mmol), and p-toluenesulfonic acid (20 mg) in toluene (20 mL) was stirred at 55°C for 15 minutes. During the stirring, samples were taken every 5 minutes and observed on a thin layer chromatography plate until there was one main spot on the thin layer chromatography plate. The solvent was evaporated under vacuum to 10 ml. The remaining solution was diluted in methanol (100 mL) to form a yellow solid. After separation by filtration, the yellow ligand intermediate (Z1) solid powder was dried under vacuum.

[0033] A solution of Z1(1 mmol), 1,2,3,4,5,6,7-hexahydro-s-indacen-4-amine (1 mmol) and p-toluenesulfonic acid (20 mg) in toluene (20 ml) was stirred at 110 °C for 24 h. During the stirring process, samples were taken every 6 h and observed on thin layer chromatography plate until the main spot appeared on the thin layer chromatography plate. The solvent was evaporated under vacuum to 10 ml. The remaining solution was diluted in methanol (100 mL) to form a yellow solid. After filtration and separation, the yellow ligand powder (L1) was obtained with a yield of 70% after vacuum drying.

[0034] NMR analysis: 1 H NMR (400 MHz, CDC13) δ 7.25 - 7.14 (m, 9H, Ar-H), 7.11 - 7.01 (m, 10H, Ar-H), 6.87 (s, 1H, Ar-H), 6.66 (s, 3H, Ar-H), 5.24 (s, 2H, Ar-CH), 2.97 - 2.86 (q, J = 7.1 Hz, 5H, -CH2-), 2.73 - 2.68 (d, J = 7.5 Hz, 1H, -CH2-), 2.20 - 2.17 (d, J = 3.8 Hz, 3H, -CH3), 2.15 - 2.06 (q, J = 7.5 Hz, 5H, -CH2-), 1.91 - 1.87 (s, 3H, -CH3), 0.82 - 0.75 (s, 3H, -CH3), 0.70 - 0.64 (s, 1H, -CH2-). 13 C NMR (101 MHz, CDC13) δ 170.22 (C=N), 166.65 (C=N), 145.72, 143.61, 143.51, 142.37, 131.63, 131.51, 129.75, 129.58, 129.41, 129.33, 129.12, 128.55, 128.50, 128.43, 128.12, 128.03, 126.42, 126.30, 126.03, 52.31 (Ar-CH), 52.25 (Ar-CH), 33.00 (-CH2-), 32.89 (-CH2-), 29.85 (-CH2-), 25.66 (-CH2-), 25.46 (-CH2-), 21.29 (-CH2-), 15.95 (-CH3), 15.74 (-CH3), 14.47 (-CH3).

[0035] Mass analysis: ESI-MS (m / z): calcd for C49H47N 2 +: 663.3734, Found, 663.3719, [M+H] +

[0036] (3) Synthesis of Ni1 complex

[0037] Ni1 complex was synthesized by the reaction of (DME)NiBr2(DME = 1,2- dimethoxyethane) with ligand in dichloromethane. 0.2 mmol of the corresponding ligand (L1) was dissolved in 10 mL of dichloromethane under nitrogen atmosphere, then (DME)NiBr2(0.2 mmol, 62 mg) was added to the above solution. The resulting mixture was stirred at room temperature overnight. The solvent was evaporated under pressure, washed with 4 x 5 mL of diethyl ether, filtered, and dried under vacuum to give the product (Ni1) as an orange-red solid with 78% yield. The preparation reaction is shown in the following scheme:

[0038]

[0039] The preparation of ethylene glycol dimethyl ether activated nickel bromide [NiBr2(DME)] was as follows: anhydrous nickel bromide (0.5 mmol, 0.11 g) was dissolved in anhydrous ethanol (15 ml) and stirred under reflux until the solution was dark green. After filtration, the filtrate was evaporated to dryness, and 1,2-dimethoxyethane (10 ml) and triethyl orthoformate (10 ml) were added to the solvent (the solvent was previously stirred well and refluxed). The mixture was stirred under reflux overnight, and yellow solid was precipitated. The reaction solution was concentrated, and the solvent was removed under reduced pressure. After the remaining solvent was cooled, the upper solution was removed, and the product was soaked and washed with dry 10 mL of 1,2-dimethoxyethane. The solvent was removed to give an orange-yellow solid powder [NiBr2(DME)] (124 mg) with 80% yield.

[0040] Example 2

[0041] (1) Synthesis of 2,4,6-tris(diphenylmethyl)aniline

[0042] A mixture of aniline (10 mmol, 1.0 equivalent) and diarylmethyl alcohol (30 mmol, 3 equivalents) was heated to 120 °C. After the mixture was melted, anhydrous zinc chloride (7.5 mmol, 0.75 equivalent) was added to the mixture, and the temperature was increased to 160 °C. After 30 minutes of reaction at 160 °C, the reaction mixture was cooled to room temperature and dissolved in dichloromethane (200 mL). The dichloromethane layer was extracted with water (3 x 200 mL) and dried with anhydrous magnesium sulfate for 30 minutes. The solution was evaporated under reduced pressure to 10 mL, recrystallized with 100 ml of ethanol, washed with ethanol (3 x 100 ml), filtered, and dried to give white 2,4,6-tris(diphenylmethyl)aniline powder with 85% yield. The preparation reaction is shown in the following scheme:

[0043]

[0044] (3) Synthesis of catalyst ligand (L2)

[0045] A solution of 2,4,6-tris(benzhydryl)aniline (2 mmol), 2,3-butanedione (2 mmol) and p-toluenesulfonic acid (20 mg) in toluene (20 mL) was stirred at 55 °C for 15 min. Samples were taken every 5 min during stirring and observed on thin layer chromatography plate until a single spot was observed on the thin layer chromatography plate. The solvent was evaporated under vacuum to 10 ml. The remaining solution was diluted in methanol (100 mL) to form a yellow solid. After filtration separation, vacuum drying gave a yellow ligand intermediate (Z2) solid powder.

[0046] A solution of Z2 (1 mmol), 8-benzhydryl-1,2,3,4,5,6,7-hexahydro-s-indacen-4-amine (1 mmol) and p-toluenesulfonic acid (20 mg) in toluene (20 ml) was stirred at 110 °C for 24 h. Samples were taken every 6 h during stirring and observed on thin layer chromatography plate until a single spot was observed on the thin layer chromatography plate. The solvent was evaporated under vacuum to 10 ml. The remaining solution was diluted in methanol (100 mL) to form a yellow solid. After filtration separation, vacuum drying gave a yellow ligand powder (L2) with a yield of 75%.

[0047] NMR analysis:

[0048] Z2: 1 H NMR (400 MHz, CDC13) δ 7.20 - 7.13 (m, 18H, Ar-H), 6.96 - 6.91 (t, J = 6.4 Hz, 12H, Ar-H), 6.64 (s, 2H, Ar-H), 5.30 (s, 1H, Ar-CH), 5.10 (s, 2H, Ar-CH), 2.35 (s, 3H, -CH3), 0.71 (s, 3H, -CH3). 13 C NMR (101 MHz, CDC13) δ 199.33 (C=0), 168.62 (C=N), 144.91, 144.07, 142.87, 141.94, 138.26, 130.83, 129.32, 129.06, 129.00, 128.36, 127.93, 126.26, 126.02, 125.88, 56.04 (Ar-CH), 52.16 (Ar-CH), 24.78 (-CH3), 14.55 (-CH3).

[0049] L2: 1H NMR (400 MHz, CDC13) δ 7.32 - 7.26 (m, 5H, Ar-H), 7.22 - 7.15 (m, J = 11.9, 6.7 Hz, 20H, Ar-H), 7.03 - 6.88 (m, J = 6.6 Hz, 15H, Ar-H), 6.64 (s, 2H, Ar-H), 5.76 (s, 1H, Ar-CH), 5.33 (s, 1H, Ar-CH), 5.21 (s, 2H, Ar-CH), 2.62 - 2.43 (m, J = 7.5 Hz, 8H, -CH2-), 2.04 - 1.94 (m, 4H, -CH2-), 1.94 (s, 3H, -CH3), 0.79 (s, 3H, -CH3). 13 C NMR (101 MHz, CDC13) δ 170.44 (C=N), 166.74 (C=N), 146.39, 144.54, 143.39, 143.10, 142.96, 142.41, 141.65, 137.66, 131.63, 130.77, 130.31, 129.72, 129.61, 129.39, 129.28, 129.17, 128.56, 128.36, 128.20, 128.14, 128.09, 126.39, 126.12, 126.08, 126.05, 125.43, 56.29 (Ar-CH), 53.05 (Ar-CH), 52.47 (Ar-CH), 32.73 (-CH2-), 30.10 (-CH2-), 25.51 (-CH2-), 16.31 (-CH3), 16.05 (-CH3).

[0050] Mass analysis: L2: ESI-MS (m / z): calcd for C 74 H 65 N2 + : 981.5142, Found, 981.5123, [M+H] +

[0051]

[0052] (3) Synthesis of Ni2 complex

[0053] The Ni1 complexes were synthesized by the reaction of (DME)NiBr2(DME = 1,2-dimethoxyethane) with the ligands in dichloromethane. 0.2 mmol of the corresponding ligand (L2) was dissolved in 10 mL of dichloromethane under nitrogen atmosphere, then (DME)NiBr2(0.2 mmol, 62 mg) was added to the above solution. The resulting mixture was stirred at room temperature overnight. The solvent was evaporated under reduced pressure, washed with 4 x 5 mL of diethyl ether, filtered, and dried under vacuum to give the product (Ni2) as an orange-red solid in 82% yield. The preparation reaction is shown in the following scheme:

[0054]

[0055] The preparation of ethylene glycol dimethyl ether activated nickel bromide [NiBr2(DME)] was the same as Example 1.

[0056] Example 3, Application of Ni1, Ni2 in catalyzing ethylene polymerization

[0057] First, a 350 mL thick-walled pressure-resistant flask with a magnetic bar was placed in an oven at 100 °C for 6 h, and then brought into the glove box by replacing nitrogen. Then, 40 mL of toluene was added to the pressure-resistant flask, which was taken out and connected to a high-pressure resistant gas pipeline with ethylene gas, and the temperature was raised to the required temperature. At the same time, the vacuum pump was turned on to vacuum the polymerization gas pipeline, and after 3 min, the vacuum pump was turned off. Ethylene was introduced, and the nickel complex (2.0 μmol) and diethylaluminum chloride (800 μmol) dissolved in 2 mL of dichloromethane were injected into the polymerization pressure-resistant flask with a syringe under an ethylene atmosphere, the pressure relief valve was slowly adjusted to the required polymerization pressure and maintained, and the time was recorded. Finally, the polymerization reaction was completed, a large amount of 5% (by volume) acidified anhydrous ethanol was added, the polymer was precipitated, filtered, washed several times with anhydrous ethanol, and dried in a vacuum drying oven at 60 °C for 24 h.

[0058] The following table shows the experimental conditions for ethylene polymerization provided by the present application; catalyst (Cat.), temperature (T), yield (Yield), catalytic activity (Act.), polymer molecular weight (M n ), polymer molecular weight distribution (PDI), branching degree (brs), and other polymerization result data.

[0059] Table 1 Ethylene polymerization

[0060]

[0061] a Polymerization conditions: catalyst 2.0 μmol, dichloromethane = 2 mL, [Al] / [Ni] = 400, n-heptane = 20 mL, ethylene pressure 6 atm, time = 10 min;

[0062] bThe unit of activity Act. is 10 6 g mol -1 h -1 ;

[0063] c Polymer molecular weight M n and molecular weight distribution PDI were determined by gel permeation chromatography (GPC) at 150°C in trichlorobenzene and polystyrene standards;

[0064] d The branching degree refers to the number of branches per 1000 carbon atoms, calculated by 1 H NMR nuclear magnetic resonance method;

[0065] e Melting point T m Determined by differential scanning calorimetry (DSC).

[0066] As can be seen from Table 1, the Ni1 and Ni2 catalysts prepared by the present application can catalyze the preparation of polyethylene with high activity, and the molecular weight (up to 10 7 g / mol) and branching degree (up to 86 branches / 1000C) of the prepared polymer are high, and the polymer is a thermoplastic elastomer. Through mechanical property testing of the polyethylene, it is found that the polyethylene prepared by the catalyst system has good mechanical properties and recovery performance, and the recovery rate is as high as 79%. It has moderate tensile strength and good recovery performance.

[0067] The above provides a detailed introduction to the compound, complex catalyst, catalyst composition and preparation method of the olefin polymer provided by the present application. Specific examples are applied in this paper to explain the principles and implementation modes of the present application, but the present application is not limited to the specific implementation modes described in this paper. Those skilled in the art understand that other modifications and variations can be made without departing from the scope of the present application, and these modifications and variations also fall within the protection scope of the claims of the present application.

[0068] From Figure 1 It can be seen that the polyethylene elastomers prepared by the Ni1 and Ni2 catalysts have a wide range of breaking stress values (3.7-21.6 MPa), showing low to moderate stress, and the breaking elongation (617-4058%) shows that the polyethylene elastomer material has high strain. Figure 2 The displayed polyethylene hysteresis experiment results show that these materials have excellent elastic recovery performance at 30-70°C, with an SR value of 66-79%. Figure 3 is the polyethylene material obtained by polymerization of the Ni2 catalyst at 70°C 1 H NMR, and its branching degree is calculated to be 86 / 1000C.

Claims

1. A method for catalyzing the polymerization of ethylene using an asymmetric, sterically hindered α-diimine nickel (II) catalyst, characterized in that: In the ethylene polymerization process, a composite catalytic system consisting of a catalyst and a co-catalyst, diethylaluminum chloride, is used for the reaction; the molar ratio of the catalyst to the co-catalyst, diethylaluminum chloride, is 1:400, and the amount of catalyst used is 2 mmol. The structure of the nickel(II) catalyst containing asymmetric, sterically hindered α-diimine is as follows: 。 2. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 1, characterized in that, The polymerization temperature of ethylene is 10–70℃, and the reaction time is 10–30 minutes.

3. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 1, characterized in that, The preparation steps of the catalyst containing asymmetric, sterically hindered α-diimine nickel(II) are as follows: (1) Preparation of aniline substances containing substituents: heating p-methylaniline or aniline and benzyl alcohol to 120°C, dissolving anhydrous zinc chloride in concentrated hydrochloric acid and adding it to the mixture, heating to 160°C and reacting for 30 minutes, cooling the reaction mixture to room temperature after the reaction is completed, and dissolving it in dichloromethane; The dichloromethane layer was extracted with water and dried over anhydrous magnesium sulfate. The solution was evaporated under reduced pressure to 10 mL, recrystallized with ethanol, washed with ethanol, filtered and dried to obtain aniline powder containing substituents. (2) Preparation of catalyst ligand intermediate: The substituted aniline and 2,3-butanedione prepared in step (1) were mixed, and p-toluenesulfonic acid monohydrate was added. The mixture was stirred and refluxed at 55°C for 15 minutes with toluene as solvent. Samples were taken every 5 minutes during stirring and observed on a thin-layer chromatography plate until a principal spot was found on the plate. The solvent was evaporated and concentrated under vacuum. The remaining solution was diluted in methanol to form a yellow solid. After filtration and separation, the solid was dried under vacuum to obtain a pale yellow solid-ligand intermediate. (3) Preparation of ligands for nickel (II) catalysts containing asymmetric sterically hindered α-diimine: The ligand intermediate, 1,2,3,5,6,7-hexahydro-s-indane-4-amine or 8-diphenylmethyl-1,2,3,5,6,7-hexahydro-s-indane-4-amine and p-toluenesulfonic acid in toluene solution were stirred at 110 °C for 24 hours. During the stirring process, samples were taken every 6 hours and observed on a thin-layer chromatography plate until the main spot appeared on the thin-layer chromatography plate. The solvent was evaporated and concentrated to 10 ml under vacuum. The remaining solution was diluted in methanol to form a yellow solid. After filtration and separation, the yellow ligand powder was obtained by vacuum drying. (4) Preparation of a catalyst containing asymmetric steric hindrance α-diimine nickel (II): Under nitrogen protection, using dichloromethane as solvent, the catalyst ligand containing asymmetric steric hindrance α-diimine nickel (II) was mixed with nickel bromide [NiBr2(DME)] activated by ethylene glycol dimethyl ether. After stirring and reacting at room temperature for 12 hours, the suspension was filtered, the mother liquor was removed from the solvent under vacuum, washed with diethyl ether, and dried under vacuum to obtain an orange-red powder solid complex.

4. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 3, characterized in that: In step (1), the molar ratio of p-methylaniline or aniline and benzyl alcohol is 1:2-1:3; the molar ratio of anhydrous zinc chloride to aniline is 0.5:1-0.75:

1.

5. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 3, characterized in that: In step (2), the molar ratio of aniline containing substituents to 2,3-butanedione is 1:

1.

6. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 3, characterized in that: In step (3), the molar ratio of the ligand intermediate to 1,2,3,5,6,7-hexahydro-s-indane-4-amine or 8-diphenylmethyl-1,2,3,5,6,7-hexahydro-s-indane-4-amine is 1:

1.

7. The method for ethylene polymerization catalyzed by an asymmetric, sterically hindered α-diimine nickel (II) catalyst as described in claim 3, characterized in that: In step (4), the preparation method of nickel bromide [NiBr2(DME)] activated by ethylene glycol dimethyl ether is as follows: under nitrogen protection, anhydrous nickel bromide is dissolved in anhydrous ethanol and stirred and refluxed until the solution is dark green. After filtration, the filtrate is dried, and then 1,2-dimethoxyethane and triethyl orthoformate solvent are added and stirred under reflux overnight, and a yellow solid is precipitated. The reaction solution is concentrated, the solvent is removed under reduced pressure, the remaining solvent is cooled, the upper layer solution is removed, and the obtained product is soaked and washed with 1,2-dimethoxyethane to remove the solvent and obtain an orange-yellow solid powder [NiBr2(DME)].

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