Preparation method and application of modified alkyl aluminoxane

Abstract

The invention provides a preparation method and application of modified alkyl aluminoxane. Comprising the following steps: carrying out first reaction on paraformaldehyde and trialkyl aluminum in a solvent to obtain a precursor solution; performing pyrolysis treatment on the precursor solution to obtain a solution containing alkylaluminoxane; and adding acetone into the solution containing alkylaluminoxane, and carrying out a second reaction at 0-100 DEG C to obtain the modified alkylaluminoxane. The method is beneficial to reducing the content of free alkyl aluminum in the modified alkyl aluminoxane.

Description

Technical Field

[0001] This invention relates to the field of fine chemical synthesis technology, specifically to a method for preparing and applying modified alkylaluminoxanes. Background Technology

[0002] Alkyl aluminum oxanes play a crucial role in olefin polymerization by activating / stabilizing the central metal of the catalyst. They can serve as core co-catalysts in high-end polyolefin catalyst systems, applied in polymerization fields such as cyclic olefin copolymers (COC), polyolefin elastomers (POE), polymethylpentene (PMP), metallocene polyethylene (mPE), and metallocene polypropylene (mPP). Alkyl aluminum oxanes possess a layered structure, and their product solutions typically contain alkyl aluminum oxanes, adsorbed alkyl aluminum, and free alkyl aluminum. Their structure and composition significantly influence the performance indicators of the polymerization products (polymers).

[0003] Currently, the synthesis methods of alkylaluminoxanes are basically divided into two categories: the first category is the hydrolysis method, which includes direct hydrolysis using free water and indirect hydrolysis using water of crystallization. Regardless of whether it is a direct or indirect hydrolysis process, the resulting alkylaluminoxane product solution contains a large amount of unhydrolyzed free alkylaluminum, which is usually above 5%. The second category is the non-hydrolysis method, which mainly uses alkylaluminum to react with carbon dioxide, alcohols, and organic acid compounds to obtain a precursor. The precursor is then pyrolyzed to obtain the aluminoxane product (as described in patents US 5831109 and US 5728855). However, during the pyrolysis process of this method, 1 / 3 equivalent of the alkylaluminum in the raw material is released and exists in a free form in the alkylaluminoxane product solution, resulting in a high content of free alkylaluminum in the alkylaluminoxane product solution. These free alkylaluminum molecules promote chain transfer and, under the action of olefin polymerization catalysts, generate a large number of low molecular weight polymers in the olefin polymerization reaction. These low molecular weight polymers are present in the final polymer product, increasing the soluble content in the polymer product, causing the polymer product to become sticky, and affecting product quality.

[0004] Therefore, how to reduce the content of free alkyl aluminum in alkylaluminoxane products is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a method for preparing modified alkylaluminoxanes and their applications, which can reduce the content of free alkylaluminum in modified alkylaluminoxanes.

[0006] This invention provides a method for preparing modified alkylaluminoxane, comprising the following steps: reacting paraformaldehyde and trialkylaluminum in a solvent to obtain a precursor solution; subjecting the precursor solution to pyrolysis to obtain a solution containing alkylaluminoxane; adding acetone to the solution containing alkylaluminoxane and carrying out a second reaction at 0-100°C to obtain the modified alkylaluminoxane.

[0007] Optionally, the degree of polymerization of the paraformaldehyde is 2 to 300, preferably 5 to 100, and more preferably 10 to 50.

[0008] Optionally, the molar ratio of the trialkylaluminum to the formaldehyde structural unit in the paraformaldehyde is 1:(0.2-10), preferably 1:(0.5-2.0); and / or, the molar ratio of the trialkylaluminum to the acetone is 1:(0.1-50), preferably 1:(0.3-10).

[0009] Optionally, the number of carbon atoms in the alkyl group of the trialkylaluminum is 1 to 10.

[0010] Optionally, the trialkylaluminum includes one or more of trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum, trioctylaluminum, and tridecylaluminum.

[0011] Optionally, the temperature of the first reaction is -30 to 130°C, preferably 10 to 100°C; and / or, the solvent includes one or more of liquid alkanes and aromatic hydrocarbons, preferably one or more of toluene, methylcyclohexane, isooctane or heptane.

[0012] Optionally, the temperature of the pyrolysis treatment is 100–200°C, preferably 120–150°C; and / or, the time of the pyrolysis treatment is 1–100 h.

[0013] This invention provides a polyolefin catalyst comprising the modified alkylaluminoxane obtained by the preparation method described above.

[0014] This invention provides a method for preparing polyolefins, comprising polymerizing olefin monomers under the catalytic action of a polyolefin catalyst as described above, to obtain the polyolefin.

[0015] Optionally, the monomer of the olefin includes ethylene and / or 1-butene.

[0016] This invention provides a method for preparing and applying modified alkylaluminoxanes, which can reduce the content of free alkylaluminum in the modified alkylaluminoxanes, resulting in modified alkylaluminoxanes that contain virtually no free alkylaluminum, with the content of free alkylaluminum being almost zero. This modified alkylaluminoxane can be used in olefin polymerization reactions, exhibiting high co-catalytic activity. The soluble content in the prepared polymer is less than 1%, avoiding the problem of polymer stickiness. The polymer has a lower number of methyl groups per thousand carbons and a lower degree of branching, improving the quality of the polymer product. Furthermore, the reaction conditions are milder, and the production process is safe and controllable. Attached Figure Description

[0017] Figure 1 The image shows the 1H NMR spectrum of the modified alkylaluminoxane from Example 1. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0019] This invention provides a method for preparing modified alkylaluminoxane, comprising the following steps: reacting paraformaldehyde and trialkylaluminum in a solvent to obtain a precursor solution; subjecting the precursor solution to pyrolysis to obtain a solution containing alkylaluminoxane; adding acetone to the solution containing alkylaluminoxane and carrying out a second reaction at 0-100°C to obtain modified alkylaluminoxane.

[0020] According to the inventors' research and analysis, using paraformaldehyde as an oxygen source and acetone as a modifier to synthesize the above-mentioned modified alkylaluminoxane can remove free alkylaluminum from the modified alkylaluminoxane, reducing its content to near zero. This avoids the participation of high levels of free alkylaluminum in the olefin polymerization reaction catalyzed by the modified alkylaluminoxane, reduces the formation of low molecular weight polymers (soluble substances) in the olefin polymerization reaction, and avoids the problem of sticky products in the olefin polymerization reaction. When the above-mentioned modified alkylaluminoxane is used in the olefin polymerization reaction, under the premise of the same main catalyst, the modified alkylaluminoxane has higher co-catalytic activity. In addition, the polymer has a lower number of 1000 carbon methyl groups and a lower degree of branching, which improves the quality of the polymer product, and the reaction conditions are milder, making the production process safer and more controllable.

[0021] The modified alkylaluminoxane prepared in the embodiments of the present invention is specifically a solution containing modified alkylaluminoxane, which can be post-processed, such as concentrated, aged, crystallized, etc., and can be stored and sold or directly used in downstream olefin polymerization reactions.

[0022] The degree of polymerization of the above-mentioned paraformaldehyde can be 2 to 300, preferably 5 to 100, and more preferably 10 to 50.

[0023] The number of carbon atoms in the alkyl group of the above-mentioned trialkylaluminum can be 1 to 10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any combination thereof. That is, the number of carbon atoms in the alkyl group of the modified alkylaluminoxane can be 1 to 10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any combination thereof.

[0024] Trialkylaluminum can be a single trialkylaluminum or a mixture of multiple trialkylaluminums.

[0025] In some embodiments, the above-mentioned trialkylaluminum includes one or more of trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum, trioctylaluminum, and tridecylaluminum. Correspondingly, the modified alkylaluminoxane may include one or more of methylaluminoxane, ethylaluminoxane, isobutylaluminoxane, hexylaluminoxane, octylaluminoxane, and decylaluminoxane.

[0026] The solvents mentioned above may include one or more of liquid alkanes and aromatic hydrocarbons, such as toluene, methylcyclohexane, isooctane, and heptane.

[0027] In some embodiments, the molar ratio of formaldehyde structural units in trialkylaluminum and paraformaldehyde is 1:(0.2 to 10), for example, 1:0.2, 1:1, 1:2, 1:5, 1:10 or any combination thereof, preferably 1:(0.5 to 2.0).

[0028] The temperature of the first reaction can be -30 to 130°C, for example, -30°C, -20°C, 0°C, 10°C, 50°C, 100°C, 130°C or any combination thereof, preferably 10 to 100°C, which helps to reasonably control the rate of the first reaction and keep the molecular weight of the alkylaluminoxane within a suitable range.

[0029] In some embodiments, the process of reacting paraformaldehyde and trialkylaluminum in a solvent to obtain a precursor solution includes: mixing paraformaldehyde and a solvent, then slowly adding trialkylaluminum to carry out the first reaction to obtain the precursor solution. Stirring can be continuously performed during the above process.

[0030] Furthermore, the aforementioned first reaction process can be carried out in a jacketed flask. In this embodiment of the invention, the volume of the jacketed flask is not particularly limited, for example, it can be 800 to 1000 mL.

[0031] The above precursor solution is subjected to pyrolysis treatment, specifically, the precursor in the precursor solution is subjected to pyrolysis treatment to obtain a solution containing alkylaluminoxane.

[0032] The temperature of the above-mentioned pyrolysis treatment can be 100 to 200°C, for example, 100°C, 120°C, 140°C, 160°C, 180°C, 200°C or any combination thereof, preferably 120 to 150°C; the time of the above-mentioned pyrolysis treatment can be 1 to 100 hours, for example, 1 hour, 20 hours, 40 hours, 60 hours, 80 hours, 100 hours or any combination thereof.

[0033] Next, acetone is added to the above-mentioned solution containing alkylaluminoxane, and a second reaction is carried out at 0–100°C, for example, 0°C, 10°C, 20°C, 40°C, 60°C, 80°C, 100°C, or any combination thereof, preferably 20–60°C, to obtain modified alkylaluminoxane. In the above process, acetone can react with free alkylaluminum in the solution containing alkylaluminoxane to generate organoaluminum, thereby reducing the content of free alkylaluminum in the modified alkylaluminoxane; moreover, this organoaluminum does not have catalytic activity for olefin polymerization. In the process of using the above-mentioned modified alkylaluminoxane to catalyze the olefin polymerization reaction to obtain the olefin polymer, a high content of free alkylaluminum is avoided from participating in the olefin polymerization reaction, thereby reducing the content of low molecular weight polymers in the olefin polymerization product.

[0034] The molar ratio of the above-mentioned trialkylaluminum and acetone can be 1:(0.1 to 50), for example 1:0.1, 1:0.3, 1:1, 1:10, 1:20, 1:30, 1:50 or any combination thereof, preferably 1:(0.3 to 10).

[0035] The time for the second reaction can be 0.5 to 10 hours, for example, 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 7 hours, 8 hours, 10 hours or any combination thereof, preferably 0.5 to 4 hours.

[0036] Understandably, after the second reaction is completed, the product of the second reaction can be heated at 30–150°C to remove (evaporate) the unreacted acetone, to obtain modified alkylaluminoxane (a solution containing modified alkylaluminoxane).

[0037] This invention also provides a polyolefin catalyst, comprising a modified alkylaluminoxane obtained according to the above preparation method.

[0038] Understandably, the aforementioned polyolefin catalyst also includes a main catalyst. This invention does not impose any particular limitation on the main catalyst; any polyolefin main catalyst commonly used in the art can be used, such as the main catalyst shown in Formula 1.

[0039]

[0040] In Equation 1, X represents Me.

[0041] The main catalyst shown in Formula 1 above can be synthesized according to the method described in Example 19 of Chinese Patent Document CN111943977A. The materials and reagents used in the synthesis of this main catalyst can be obtained commercially.

[0042] This invention also provides a method for preparing a polyolefin, comprising polymerizing an olefin monomer under the catalytic action of the aforementioned olefin catalyst to obtain the aforementioned polyolefin.

[0043] In some embodiments, the monomers of the olefins described above include ethylene and / or 1-butene.

[0044] Taking the polymerization reaction of ethylene and 1-butene as an example, the above-mentioned main catalyst (e.g., the main catalyst shown in Formula 1) and the modified alkylaluminoxane prepared in the embodiments of the present invention are added to a raw material system including 1-butene, and the temperature is raised to 130-170°C, for example, 130°C, 140°C, 150°C, 160°C, 170°C or any combination thereof, and ethylene at a pressure of 2-4 MPa is introduced to carry out the polymerization reaction. After the polymerization reaction is completed, the product of the polymerization reaction (polymer) is collected from the reaction liquid of the polymerization reaction.

[0045] For example, the pressure of the ethylene can be a range of 2 MPa, 3 MPa, 4 MPa, or any combination thereof.

[0046] The above polymerization reaction can be carried out in a reactor, such as a stainless steel high-pressure reactor. In this embodiment of the invention, the volume of the reactor is not particularly limited, for example, it can be 2 to 3 L.

[0047] To avoid contamination by impurities, the above-mentioned reaction vessel can be cleaned with a solvent (such as dried n-hexane). In practice, the solvent can be added to the reaction vessel, heated to 180-210°C, and stirred continuously for 25-35 minutes. After cooling to room temperature, the solvent can be released, and the temperature can be raised to 150-170°C and vacuum dried for 50-70 minutes, during which nitrogen gas can be used to purge the vessel 10-15 times. Finally, the reaction vessel can be cooled to below 80°C to complete the cleaning of the reaction vessel.

[0048] The above-mentioned raw material system including 1-butene may also include a solvent (e.g., n-hexane), wherein the volume ratio of n-hexane to 1-butene may be (3.5 to 4.5):1, for example, 3.5:1, 4:1, 4.5:1 or any combination thereof.

[0049] Furthermore, the ratio of the main catalyst and 1-butene shown in Formula 1 can be (4.5 to 5.5) μmol:1L, for example, 4.5 μmol:1L, 5 μmol:1L, 5.5 μmol:1L, or any combination thereof; the ratio of the modified alkylaluminoxane and the main catalyst shown in Formula 1 in the embodiments of the present invention can be adjusted according to Al / M (molar ratio) = 290 to 310, for example, 290, 300, 310, or any combination thereof.

[0050] The polymerization reaction time can be 5 to 15 min, for example, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, or any combination thereof.

[0051] The process of collecting the polymerization product (polymer) from the reaction solution of the polymerization reaction includes: venting (exhausting) the ethylene in the reactor, adding (releasing) the polymerization reaction solution to (draining) ethanol, causing the polymer (solid) to precipitate from the reaction solution, collecting the precipitated polymer (solid), and drying the polymer (solid) to constant weight to obtain the polymerization product (polymer). The drying temperature can be 55–65°C, for example, 55°C, 60°C, 65°C, or any combination thereof; the drying process can be carried out in a vacuum drying oven.

[0052] The present invention will now be described in more detail through specific embodiments.

[0053] The raw materials used in each embodiment and comparative example can be purchased through ordinary commercial channels, and no additional purification treatment has been performed unless otherwise specified.

[0054] Paraformaldehyde can be obtained according to the preparation methods described in Examples 1-6 and Comparative Examples 1-2 of Chinese Patent Document CN101845130A. For example, paraformaldehyde is prepared according to the method described in Comparative Example 2. After sealing, the paraformaldehyde is stored for a long time. As the storage time increases, the degree of polymerization of the paraformaldehyde increases. On the 8th day of storage, paraformaldehyde with a degree of polymerization of 78 is obtained; on the 13th day of storage, paraformaldehyde with a degree of polymerization of 149 is obtained; on the 42nd day of storage, paraformaldehyde with a degree of polymerization of 261 is obtained; and on the 47th day of storage, paraformaldehyde with a degree of polymerization of 295 is obtained. The principle of this degree of polymerization increase is well known in the industry, and is described in detail in, for example, in "Research on the Preparation Process and Additives of Paraformaldehyde, Journal of Nanjing University of Technology, 2004, vol26". The preparation process of some of the paraformaldehyde used in the embodiments of the present invention is as follows: Paraformaldehyde with a degree of polymerization of 8 is prepared according to the method described in Example 6 of Chinese Patent Document CN101845130A; Paraformaldehyde with a degree of polymerization of 10 is prepared according to the method described in Example 1 of Chinese Patent Document CN101845130A; Paraformaldehyde with a degree of polymerization of 47 is prepared according to the method described in Comparative Example 2 of Chinese Patent Document CN101845130A; and after preparing paraformaldehyde according to Comparative Example 2 of Chinese Patent Document CN101845130A... After being stored for 8 days, paraformaldehyde with a degree of polymerization of 78 was obtained. After being prepared according to the method of Comparative Example 2 in Chinese Patent Document CN101845130A and stored for 13 days, paraformaldehyde with a degree of polymerization of 149 was obtained. After being prepared according to the method of Comparative Example 2 in Chinese Patent Document CN101845130A and stored for 42 days, paraformaldehyde with a degree of polymerization of 261 was obtained. After being prepared according to the method of Comparative Example 2 in Chinese Patent Document CN101845130A and stored for 47 days, paraformaldehyde with a degree of polymerization of 295 was obtained.

[0055] Example 1

[0056] This embodiment provides a method for preparing modified alkylaluminoxanes, including the following steps:

[0057] The temperature of a mixed solution of 20.8 g paraformaldehyde (degree of polymerization 295) and 500 mL toluene was controlled at -30℃. Under continuous stirring, 10 g trimethylaluminum (the molar ratio of trimethylaluminum to formaldehyde structural units in paraformaldehyde was 0.2) was slowly added and stirred for 18 h to carry out the first reaction, thus obtaining a precursor (methylaluminoxane precursor) solution.

[0058] The above precursor solution was pyrolyzed at 100°C for 100 h, with continuous stirring during the pyrolysis process, to obtain a solution containing methylaluminoxane.

[0059] Add 2.4 g of acetone to the above solution containing methylaluminoxane, and carry out the second reaction at 0 °C for 10 h. Then, heat the reaction solution of the second reaction to 50 °C and evaporate it at atmospheric pressure for 4 h to remove unreacted acetone, and obtain modified alkylaluminoxane (a solution containing modified alkylaluminoxane).

[0060] This embodiment also provides a method for preparing polyolefins, including the following steps:

[0061] After mixing 0.8 L of n-hexane and 0.2 L of liquid 1-butene, the main catalyst shown in Formula 1 was added, and the modified alkylaluminoxane (a solution containing the modified alkylaluminoxane) was added according to an Al / M molar ratio of 300. The mixture was heated to 150 °C, and ethylene at a pressure of 3 MPa was introduced to carry out the polymerization reaction for 10 min. Then, the ethylene in the reactor was vented, and the reaction solution of the polymerization reaction was added to (released to) ethanol to allow the polymer (solid) to precipitate from the reaction solution. The precipitated polymer (solid) was collected and dried at 60 °C to constant weight to obtain the product (polymer) of the polymerization reaction.

[0062] Comparative Example 2

[0063] POE38680 particles manufactured by Dow Chemical Company were used as a control sample.

[0064] Comparative Example 3

[0065] POE38688 particles manufactured by Dow Chemical Company were used as a control sample.

[0066] Following the procedure in Example 1, the modified alkylaluminoxanes and polymers of Examples 2-14 and Comparative Examples 1, 3, and 4 were prepared. The types and masses of trialkylaluminum, the degree of polymerization and mass of paraformaldehyde, the molar ratio of trialkylaluminum to formaldehyde structural units in paraformaldehyde, the temperature and time of the first reaction, the temperature and time of pyrolysis, the mass of acetone, the molar ratio of trialkylaluminum to acetone, the temperature and time of the second reaction, and the temperature at which unreacted acetone was evaporated are summarized in Table 1.

[0067] Table 1

[0068]

[0069]

[0070] Test case

[0071] 1. The following parameters of the above embodiments and comparative examples were tested:

[0072] 1) Content of free alkyl aluminum in modified alkylaluminoxane: Free alkyl aluminum has obvious characteristic peaks in the range of -0.2 to 1.0 ppm in the NMR H spectrum. If the characteristic peaks are not detected, it proves that the content of free alkyl aluminum is 0. The specific results are shown in Table 2.

[0073] 2) Catalytic activity (polymerization activity) of modified alkylaluminoxane: The catalytic activity (g / mol) of modified alkylaluminoxane was calculated by the mass (g) of polymer produced per unit mole (mol) of main catalyst used per 10 min. The specific results are shown in Table 2.

[0074] 3) Weight-average molecular weight, molecular weight distribution (MWD) of the polymer products, and content (wt%) of comonomers (monomers other than ethylene, such as 1-butene): The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the polymers were measured using high-temperature gel permeation chromatography (GPC) coupled with infrared spectroscopy (IR) (high-temperature GPC-IR). The molecular weight distribution (MWD) was calculated by dividing the weight-average molecular weight by the number-average molecular weight. The test temperature was 150℃, polystyrene was used as the standard, and trichlorobenzene was used as the solvent at a flow rate of 1.0 ml / min. The specific results are shown in Table 2.

[0075] 4) Soluble content in polymer: Tested using temperature gradient interaction chromatography (TGIC), the specific results are shown in Table 2;

[0076] 5) SF Composition (CH3 / 1000TC): The number of methyl groups per 1000 carbons in the polymer was determined using temperature gradient interaction chromatography (TGIC) to characterize the degree of branching of the polymer; the specific results are shown in Table 2.

[0077] 6) The 1H NMR spectrum (¹H NMR) of the modified alkylaluminoxane from Example 1 was determined, as follows: Figure 1 As shown.

[0078] 2. Test Results

[0079] Table 2

[0080]

[0081] Data Analysis:

[0082] The modified alkylaluminoxane (a solution containing the modified alkylaluminoxane) prepared in the embodiments of the present invention has high catalytic activity (copolymerization activity or polymerization activity) for ethylene and 1-butene, while the soluble content in the polymerization product (polymer) is less than 1%.

[0083] In Comparative Example 1, acetone was not used, and the polymer's properties deteriorated.

[0084] Compared with Example 9, Comparative Example 4 showed that due to the low temperature of the second reaction, the acetone reaction was incomplete, resulting in a high content of free alkyl aluminum, a slight decrease in the polymerization activity of the modified alkyl aluminum oxane, and a deterioration in polymer performance.

[0085] Compared with Example 9, Comparative Example 5 showed that due to the excessively high temperature of the second reaction, the acetone reacted excessively, which not only eliminated the free alkyl aluminum but also reacted with the modified alkyl aluminum oxane, thereby destroying the structure of the modified alkyl aluminum oxane (active substance), resulting in a decrease in its polymerization activity and an insignificant decrease in polymer performance.

[0086] Compared with Example 9, Comparative Example 6 did not use acetone, and the polymerization activity of the modified alkylaluminoxane was not reduced by much, but the polymer performance deteriorated.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a modified alkylaluminoxane, characterized in that, Includes the following steps: Paraformaldehyde and trialkylaluminum were reacted in a solvent to obtain a precursor solution. The precursor solution was subjected to pyrolysis to obtain a solution containing alkylaluminoxane; Acetone was added to the solution containing the alkylaluminoxane, and a second reaction was carried out at 0–100°C to obtain the modified alkylaluminoxane.

2. The preparation method according to claim 1, characterized in that, The degree of polymerization of the paraformaldehyde is 2 to 300, preferably 5 to 100, and more preferably 10 to 50.

3. The preparation method according to claim 1, characterized in that, The molar ratio of the trialkylaluminum to the formaldehyde structural unit in the paraformaldehyde is 1:(0.2-10), preferably 1:(0.5-2.0); And / or, the molar ratio of the trialkylaluminum to the acetone is 1:(0.1 to 50), preferably 1:(0.3 to 10).

4. The preparation method according to claim 1, characterized in that, The trialkylaluminum has 1 to 10 carbon atoms in its alkyl group.

5. The preparation method according to claim 4, characterized in that, The trialkylaluminum includes one or more of trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum, trioctylaluminum, and tridecylaluminum.

6. The preparation method according to any one of claims 1-5, characterized in that, The temperature of the first reaction is -30 to 130°C, preferably 10 to 100°C; And / or, the solvent includes one or more of liquid alkanes and aromatic hydrocarbons, preferably one or more of toluene, methylcyclohexane, isooctane or heptane.

7. The preparation method according to any one of claims 1-5, characterized in that, The temperature of the pyrolysis treatment is 100-200℃, preferably 120-150℃; And / or, the pyrolysis treatment time is 1 to 100 hours.

8. A polyolefin catalyst, characterized in that, The modified alkylaluminoxane is obtained by the preparation method according to any one of claims 1-7.

9. A method for preparing a polyolefin, characterized in that, The method includes polymerizing olefin monomers under the catalysis of the polyolefin catalyst of claim 8 to obtain the polyolefin.

10. The preparation method according to claim 9, characterized in that, The monomers of the olefin include ethylene and / or 1-butene.

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