A method for preparing polyethylenes having long side chain ethylene end groups

Abstract

This invention provides a method for preparing polyethylene with long-chain ethylene end groups. The method includes: introducing a first material containing 95-99.5 wt% ethylene into a tubular reactor after pressurization and preheating; introducing a second material containing 1 wt%-100 wt% non-conjugated diene into the tubular reactor after pressurization; and introducing a third material containing 5 wt%-30 wt% peroxide into the tubular reactor through a second material inlet. The ethylene, non-conjugated diene, and peroxide undergo a polymerization reaction to produce polyethylene. Further, this invention discharges a fourth material consisting of unreacted material and polyethylene from the outlet of the tubular reactor. After cooling and multi-stage depressurization, molten polyethylene is obtained. The molten polyethylene is then granulated and sent to a degassing system for degassing. This invention improves the crosslinking efficiency, melt strength, and purity of low-density polyethylene by introducing long-chain vinyl groups into the main chain of low-density polyethylene.

Description

Technical Field

[0001] This invention relates to the field of high-pressure ethylene polymerization, and more specifically to a method for preparing polyethylene with long side-chain ethylene end groups. Background Technology

[0002] Low-density polyethylene (LDPE) is widely used in food, medical, and electronic packaging films, extrusion coating, wires and cables, and insulation materials. Cross-linked polyethylene (XLPE), in particular, is valued for its excellent weather resistance, chemical resistance, heat resistance, and superior electrical properties. It is well known in the art that LPE produced by different processes exhibits variations in molecular weight and its distribution, as well as its degree of branching and its distribution. For example, LPE prepared using a batch reactor has a wide molecular weight distribution and abundant long and short branches, resulting in better flexibility and higher melt strength; while LPE prepared using a tubular reactor has a narrow molecular weight distribution and better optical properties. Although LPE prepared using the high-pressure polyethylene (HPPE) process already possesses a greater number of branching sites and carbon-carbon double bonds, in some special applications, it still consumes a relatively large amount of cross-linking agent, which can affect the electrical, mechanical, and other performance properties of the final product.

[0003] By introducing long-chain ethylene end groups into the main chain of low-density polyethylene (LDPE), its reactivity can be enhanced, making it a platform polymer that can be further customized. The long-chain ethylene end groups can improve melt strength during high-temperature processing, making it easier to process (e.g., blown film, extrusion), greatly expanding its application potential in high-end films, cable insulation, polymer alloys, adhesives, and other fields. Therefore, developing LPE with long-chain ethylene end groups is of great significance. Summary of the Invention

[0004] The purpose of this invention is to overcome the limitations of existing technologies in controlling the molecular chain structure of polyethylene, and to provide a method for preparing polyethylene with long side-chain ethylene end groups. This method improves the crosslinking efficiency, melt strength, and purity of low-density polyethylene by introducing long side-chain ethylene into the main chain of low-density polyethylene.

[0005] To achieve the above objectives, the present invention provides a method for preparing polyethylene with long side-chain ethylene end groups, comprising, 1) The first material containing 95-99.5 wt% ethylene is introduced into the first material inlet of the tubular reactor after being pressurized and preheated; 2) A second material containing 1wt%-100wt% of a non-conjugated diene is pressurized and introduced into the tubular reactor; wherein the mass flow ratio of the non-conjugated diene to ethylene is 0.0001 wt% ~ 0.05 wt%. 3) A third material containing 5wt%-30wt% peroxide is introduced into the tubular reactor through 2-6 second material inlets; 4) Under conditions of 200-300 MPa inlet pressure and 270-310℃ maximum temperature in a tubular reactor, ethylene, non-conjugated diene, peroxide and molecular weight regulator react to form polyethylene. 5) Unreacted material and a fourth material composed of polyethylene are continuously discharged from the outlet of the tubular reactor. After cooling and multi-stage depressurization, the final molten polyethylene is obtained. 6) Molten polyethylene is degassed during the extrusion granulation process to remove some of the non-conjugated dienes dissolved in the polyethylene. After granulation, it is sent to the degassing system for deep degassing. The degassing process in the extrusion granulation process is either vacuum degassing or inert gas-assisted degassing, where the inert gas is nitrogen or water vapor.

[0006] In a preferred embodiment of the present invention, the first material is pressurized using a reciprocating compressor, and 0.1wt%-1wt% of an antioxidant is added to the lubricating oil of the compressor.

[0007] Typically, but not limited to, the aforementioned antioxidants may be one or more of phenolic antioxidants, amine antioxidants, nitrogen-oxygen antioxidants, thioester antioxidants, copper passivators, etc., preferably phenolic antioxidants, amine antioxidants, and nitrogen-oxygen antioxidants, such as hydroquinone, 2,6-di-tert-butyl-p-cresol, 4-methoxyphenol, p-tert-butylcatechol, nonyl diphenylamine, octyl / butyl mixed alkylated diphenylamine, 2,2,6,6-tetramethylpiperidin-1-oxy, etc., or a mixture thereof.

[0008] In a preferred embodiment of the present invention, the first material contains less than 3 wt% of a molecular weight regulator, and the molecular weight regulator is controlled (based on the amount of non-conjugated diene added). After the addition of the molecular weight regulator, the melt index increases by at least 50%, and the melt index of the polyethylene is between 0.2-10 g / 10 min (2.16 kg, 190 °C, standard).

[0009] In a preferred embodiment of the present invention, a preheater is provided between the reactor inlet and the reciprocating compressor, so that the temperature of the first material is raised to 150-200°C before entering the tubular reactor. The tubular reactor has 2-6 second material inlets, which are arranged along the tube side of the tubular reactor. The tubular reactor has 2-6 local temperature maximum values, and the local temperature maximum values ​​are between 270-310°C.

[0010] In a preferred embodiment of the present invention, the non-conjugated diene has 6-10 carbon atoms, preferably one or a mixture of 1,5-hexadiene, 1,6-heptadiene, 1,7-octadiene, 1,8-nonadiene, and 1,9-decadiene, and the molecular weight regulator is one or a mixture of alkane, monoolefin, and aldehyde having 2-4 carbon atoms.

[0011] In a preferred embodiment of the present invention, the second material is introduced into the tubular reactor in one or more combinations of the following methods: a) The second material is mixed with the first material before pressurization, and after pressurization and preheating, it is introduced into the first material inlet of the tubular reactor; b) The second material is mixed with the pressurized and preheated first material and introduced into the first material inlet of the tubular reactor; c) The second material is introduced into the tubular reactor from one or more second material inlets; d) The second material and the third material are mixed and then introduced into the tubular reactor through the second material inlet.

[0012] In a preferred embodiment of the present invention, the peroxides in the third material are composed of a variety of different types of peroxides. Let DT be the difference between the maximum local temperature and the inlet temperature of the first material in the tubular reactor. Then the number of peroxide types b satisfies: b×a=DT, where the value of parameter a is in the range of 20-60℃, preferably 30-50℃.

[0013] In a preferred embodiment of the present invention, the peroxide, while satisfying the foregoing limitations, can be selected from known peroxides in the industry (such as TPPPI, TBPPH, TPB, DTBP, etc.) based on the matching relationship of the reaction temperature. The present invention does not limit the specific peroxide. Preferably, the mass flow rate of the peroxide to ethylene is 0.001 wt% ~ 0.01 wt% (10-100 ppmw).

[0014] In a preferred embodiment of the present invention, the second material and the third material are mixed under conditions below 10 MPa, and then pressurized by a feed pump before being introduced into the tubular reactor from the inlet of the second material. The temperature of the mixture of the second material and the third material before pressurization is below 50°C, the temperature of the mixture of the second material and the third material after pressurization is below 50°C, and the residence time from the outlet of the feed pump to the inlet of the second material is less than 20 minutes.

[0015] In a preferred embodiment of the present invention, the second material and the third material are mixed under conditions below 3 MPa, and then pressurized by a feed pump before being introduced into the tubular reactor from the inlet of the second material. The temperature of the mixture of the second material and the third material before pressurization is below 30°C, the temperature of the mixture of the second material and the third material after pressurization is below 30°C, and the residence time from the outlet of the feed pump to the inlet of the second material is less than 10 minutes.

[0016] In a preferred embodiment of the present invention, the second material contains 10-1000 ppmw of antioxidant. The antioxidant may be one or more of N-oxygen compounds, C-center stable free radicals, quinone methylates, nitroso compounds, etc., preferably nitrogen-oxygen antioxidants, such as one or a mixture of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxy, 2,2,6,6-tetramethylpiperidine-1-oxy, 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxy, 2,2,5,5-tetramethylpyrrolidine-1-oxy, etc.

[0017] In a preferred embodiment of the present invention, the tubular reactor is periodically heated and descaled according to the polyethylene production. The liquid level in the first separator is below 50%. The second separator includes two separators connected in parallel, with the other being cleaned offline while one separator is in operation.

[0018] In a preferred embodiment of the present invention, the organic peroxide and the solvent constitute a third material, wherein the third material is one or a mixture of n-alkanes with 8-15 carbon atoms, or one or a mixture of isoalkanes, and the atmospheric boiling point of the solvent and the non-conjugated diene differs by more than 10°C.

[0019] In a preferred embodiment of the present invention, when the polyethylene production of the tubular reactor is less than 90% of the maximum production corresponding to the grade, the online descaling function of the tubular reactor is activated.

[0020] The present invention also provides a polyethylene prepared by the method proposed in the present invention, wherein the polyethylene has a melt index between 0.2-10 g / 10 min (2.16 kg, 190 °C, standard), a density of 0.915-0.930 g / cm3, a melt expansion ratio of 1.5-2.0, and a side chain ethylene content of >4 carbon atoms of 0.001-2 mol.

[0021] Compared with the prior art, the present invention has the following advantages: (1) By controlling the content of molecular weight regulator and non-conjugated diene, the content of long side chain ethylene end group of low density polyethylene can be significantly increased. As a more active functional group, the ethylene end group reacts with other polyethylene molecular chains under the action of crosslinking agent, thereby significantly improving the crosslinking efficiency and melt strength of low density polyethylene. At the same time, by regularly removing scale, the crystal point problem caused by diene crosslinking can be avoided.

[0022] (2) By controlling the concentration of non-conjugated dienes, adding antioxidants to non-conjugated dienes, and controlling the temperature before and after pressurization, the risk of self-polymerization of non-conjugated dienes can be reduced.

[0023] (3) Introducing non-conjugated dienes and peroxides into tubular reactors separately or mixing them before introducing them into tubular reactors can not only reduce the number of ultra-high pressure pumps and lower the investment and maintenance costs of the equipment, resulting in good economic benefits; but also control the concentration gradient of non-conjugated dienes in tubular reactors, thereby preparing various polyethylene products, and the equipment has a high degree of flexible production. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a process flow for preparing polyethylene with long side-chain ethylene end groups, which is an optional feature of this invention. Figure 2 This is a schematic diagram of a process flow for preparing polyethylene with long side-chain ethylene end groups, which is an optional feature of this invention. Figure 3 This is a schematic diagram of the process flow for polyethylene in Comparative Example 1. Figure 4 This is a schematic diagram of the process flow for polyethylene in Comparative Example 2. Detailed Implementation

[0025] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The invention will be described in more detail below with reference to embodiments, but the invention is not limited thereto. For those skilled in the art, several improvements and modifications can be made without departing from the principles of the invention, and these improvements and modifications are also considered to be within the scope of protection of the invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] The key equipment, valves, and material flow streams shown in the attached diagram are as follows: 1- First compressor, used to pressurize the first material to the pressure required at the inlet of the tubular reactor; 2-Preheater, used to preheat the first material to the temperature required at the inlet of the tubular reactor; 3-Tube reactor, used for the production of polyethylene; 4- First separator, used to separate unreacted material and polyethylene under higher pressure conditions; 5- Second separator, used to separate unreacted material and polyethylene under lower pressure conditions; 6-Extrusion granulator, used to extrude and granulate molten polyethylene from the second separator 5; 7-Degassing system for removing ethylene and non-conjugated dienes from particulate polyethylene; 8a / 8b / 8c - Peroxide feed pump, used to convey one or a mixture of a second material containing non-conjugated diene, a third material containing peroxide, or a mixture thereof to tubular reactor 3; 9-Non-conjugated diene conveying system for conveying a second material containing a non-conjugated diene to the inlet of the first compressor 1, or the inlet of the tubular reactor 3, or the inlet of the distribution mixing system 10, or the inlet or outlet of at least one of the peroxide feed pumps 8a / 8b / 8c. 10-Dispensing and mixing system for dispensing and / or mixing a second material containing a non-conjugated diene and a second material containing a peroxide; 11a / 11b - Pressure reducing valve, used to reduce the pressure of materials; 12-First material; 13-Second material; 14-Third material; 15 - Fourth material.

[0028] exist Figure 1 and Figure 2The apparatus shown is used to prepare polyethylene, and the specific steps are as follows: A first material 12 containing 95-99.5 wt% ethylene is continuously introduced into a tubular reactor 3 after being pressurized by a first compressor 1 and preheated by a preheater 2. A second material 13 containing 1 wt%-100 wt% non-conjugated diene is pressurized by a non-conjugated diene conveying system 9 and mixed with the first material 12 before being introduced into the tubular reactor 3, and / or mixed with a third material 14 in a distribution and mixing system 10, and then introduced into the first material inlet and / or the second material inlet of the tubular reactor 3. The distribution and mixing system 10 includes containers, pipes, valves, instruments, etc., and can distribute the second material 13 and the third material 14 separately. In some embodiments, a portion or all of the second material and a portion or all of the third material are introduced into the tubular reactor from different second material inlets; in other embodiments, a portion of the second material and a portion of the third material are mixed and then introduced into the tubular reactor from different second material inlets.

[0029] The first material preferably contains less than 3 wt% of a molecular weight regulator (such as propylene) to adjust the melt index of polyethylene, and the second material preferably contains 10-1000 ppmw of an antioxidant.

[0030] The inlet pressure of the tubular reactor 3 is controlled at 200-300 MPa by controlling the outlet pressure of the first compressor 1, and the maximum temperature of the tubular reactor 3 is controlled at 270-310℃ by controlling the amount of the third material 14 added. In the tubular reactor 3, ethylene, non-conjugated diene, peroxide, and molecular weight regulator undergo a polymerization reaction to continuously produce low-density polyethylene. Unreacted material and a fourth material 15 composed of polyethylene are continuously discharged from the outlet of the tubular reactor 3. After cooling and at least two stages of pressure reduction (via pressure reducing valves 11a and 11b), the fourth material 15 yields the final molten polyethylene, which is then fed into an extrusion granulator 6 to remove some volatiles, granulated, and then sent to the degassing system 7. The degassing system 7 typically consists of a degassing silo, a packaging silo, a blending silo, some necessary fans, heat exchangers, and pipes and valves. By continuously introducing air into the degassing hopper in the degassing system 7, ethylene, molecular weight regulators, non-conjugated dienes, and other small molecules are removed from the polyethylene granules.

[0031] Example 1 exist Figure 2In the process flow shown, the first material has a flow rate of 10 t / h and an ethylene purity of 99 wt%. The second material has a flow rate of 10 kg / h and a non-conjugated diene (1,9-decadiene) concentration of 50 wt%, containing 20 ppm of 2,2,6,6-tetramethylpiperidin-1-oxy. 20 wt% of the second material is introduced before the first compressor, and the remaining 80 wt% is mixed with the third material and introduced through the second material inlet of the tubular reactor (in this embodiment, the tubular reactor has three second material inlets). The third material contains peroxides TPPPI, TBPPH, TPB, and DTBP in a mass ratio of 1:1:1:1, with isododecane as the solvent and a peroxide concentration of 30 wt%. The tubular reactor has three temperature peaks of 300°C, 300°C, and 300°C, with an inlet temperature of 160°C and an inlet pressure of 270 MPa. The molecular weight regulator is propylene, added at a rate of 15 kg / h. The lubricating oil of the first compressor contains 0.6 wt% of the antioxidant nonyldiphenylamine and 0.5 wt% of 2,2,6,6-tetramethylpiperidine-1-oxy. Vacuum devolatilization is used during the extrusion granulation process.

[0032] The resulting low-density polyethylene had a melt index of 2.0 g / 10 min, a vinyl content of 0.5 / 1000C, and an ethylene monomer conversion rate of 27.5%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreased to 26.9%, and the volatile content of non-conjugated dienes was 20 ppmw.

[0033] Example 2 exist Figure 2 In the process flow shown, the first material has a flow rate of 10 t / h and an ethylene purity of 99 wt%. The second material has a flow rate of 5 kg / h and a non-conjugated diene (1,9-decadiene) concentration of 50%, containing 20 ppm of 2,2,6,6-tetramethylpiperidin-1-oxy. 20% of the second material is introduced before the first compressor, and 80% is mixed with the third material and introduced through the second material inlet of the tubular reactor. The second material contains 200 ppmw of antioxidant, specifically 2,2,6,6-tetramethylpiperidin-1-oxy. The third material contains peroxides TPPPI, TBPPH, TPB, and DTBP in a mass ratio of 1:1:1:1, with isododecane as the solvent and a peroxide concentration of 30 wt%. The tubular reactor has three temperature peaks at 300℃, 300℃, and 300℃, with an inlet temperature of 160℃ and an inlet pressure of 270 MPa. The molecular weight regulator is propylene, added at a rate of 20 kg / h. 1 wt% of the antioxidant nonyldiphenylamine is added to the lubricating oil of the first compressor. Vacuum devolatilization is employed during the extrusion granulation process.

[0034] The resulting low-density polyethylene had a melt index of 2.3 g / 10 min, a vinyl content of 0.35 g / 1000°C, and an ethylene monomer conversion rate of 27.8%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreased to 27.3%, and the volatile content of non-conjugated dienes was 28 ppmw.

[0035] Example 3 exist Figure 2 In the process flow shown, the first material has a flow rate of 10 t / h and an ethylene purity of 99 wt%. The second material has a flow rate of 5 kg / h and a non-conjugated diene (1,7-octadiene) concentration of 50%, containing 20 ppm of 2,2,6,6-tetramethylpiperidin-1-oxy. 20% of the second material is introduced before the first compressor, and the remaining 80% is mixed with the third material and introduced through the second material inlet of the tubular reactor. The third material contains peroxides TPPPI, TBPPH, TPB, and DTBP in a mass ratio of 1:1:1:1, with isododecane as the solvent and a peroxide concentration of 30 wt%. The tubular reactor has three temperature peaks at 300℃, 300℃, and 300℃, with an inlet temperature of 180℃ and an inlet pressure of 240 MPa. Propylene is added as a molecular weight regulator at a rate of 18 kg / h. 1% of the antioxidant nonyldiphenylamine is added to the lubricating oil of the first compressor. Vacuum devolatilization is used in the extrusion granulation process.

[0036] The resulting low-density polyethylene had a melt index of 2.1 g / 10 min, a vinyl content of 0.30 g / 1000°C, and an ethylene monomer conversion rate of 26.0%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreased to 25.4%, and the volatile content of non-conjugated dienes was 26 ppmw.

[0037] Example 4 The difference from Example 1 is that the non-conjugated diene was 1,7-octadiene, and the second feed flow rate was 8 kg / h. The resulting low-density polyethylene had a melt index of 1.9 g / 10 min, a vinyl content of 0.48 g / 1000°C, and an ethylene monomer conversion rate of 27.1%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreased to 26.6%, and the volatile matter content of the non-conjugated diene was 15 ppmw.

[0038] Comparative Example 1 The difference from Example 1 is that, in Figure 3In the process flow shown, non-conjugated diene (1,9-decadiene) without antioxidants is directly added to the first material before the first compressor, and after compression, it is introduced into the tubular reactor. The lubricating oil of the first compressor contains no antioxidants. No vacuum devolatilization is performed during extrusion granulation. The melt index of low-density polyethylene is 2.0 g / 10 min, the vinyl content is 0.51 / 1000C, and the ethylene monomer conversion rate is 27.5%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreases to 23.6%, and the volatile matter content of the non-conjugated diene is 50 ppmw. Compared to the previous example, due to the higher risk of self-polymerization of non-conjugated dienes, the self-polymerization of ethylene and 1,9-decadiene under ultra-high pressure conditions leads to decreased operational stability of the first compressor, and the delivery pipeline of the first compressor needs to be cleaned regularly.

[0039] Comparative Example 2 The difference from Example 1 is that, in Figure 4 In the process flow shown, non-conjugated dienes without antioxidants are either distributed via a distribution mixing system or directly added to the tubular reactor after being mixed with a third material. No vacuum devolatilization is performed during extrusion granulation. The lubricating oil in the first compressor contains no antioxidants. The melt index of the low-density polyethylene is 2.0 g / 10min, the vinyl content is 0.48 / 1000C, and the ethylene monomer conversion rate is 27.5%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreases to 25.1%. Compared to Example 1, two additional ultra-high pressure pumps are required, and the volatile content of the non-conjugated dienes is 48 ppmw.

[0040] Comparative Example 3 Unlike Example 1, this reaction did not contain non-conjugated dienes, and the propylene flow rate was 26 kg / h. No vacuum devolatilization was performed during extrusion granulation. The lubricating oil in the first compressor contained no antioxidants. The melt index of the low-density polyethylene was 2.2 g / 10 min, the vinyl content was 0.13 / 1000C, and the ethylene monomer conversion rate was 28.3%. After 72 hours of continuous operation, the ethylene monomer conversion rate decreased to 27.2%. Compared to Example 1, the vinyl content decreased by 74%.

[0041] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing polyethylene with long side-chain ethylene end groups, characterized in that, Includes the following steps: 1) The first material containing 95-99.5 wt% ethylene is introduced into the first material inlet of the tubular reactor after being pressurized and preheated; 2) A second material containing 1wt%-100wt% of a non-conjugated diene is pressurized and introduced into the tubular reactor; wherein the mass flow ratio of the non-conjugated diene to ethylene is 0.0001 wt% ~ 0.05 wt%. 3) A third material containing 5wt%-30wt% peroxide is introduced into the tubular reactor through 2-6 second material inlets; 4) Under conditions of 200-300 MPa inlet pressure and 270-310℃ maximum temperature in a tubular reactor, ethylene, non-conjugated diene, peroxide and molecular weight regulator react to form polyethylene. 5) Unreacted material and a fourth material composed of polyethylene are continuously discharged from the outlet of the tubular reactor. After cooling and multi-stage depressurization, the final molten polyethylene is obtained. 6) Molten polyethylene is degassed during the extrusion granulation process to remove some of the non-conjugated dienes dissolved in the polyethylene. After granulation, it is sent to the degassing system for deep degassing. The degassing process in the extrusion granulation process is either vacuum degassing or inert gas-assisted degassing, where the inert gas is nitrogen or water vapor.

2. The method according to claim 1, characterized in that, The first material is pressurized using a reciprocating compressor, and 0.1wt%-1wt% of antioxidant is added to the lubricating oil of the compressor.

3. The method according to claim 1, characterized in that, The first material contains less than 3 wt% of a molecular weight regulator.

4. The method according to claim 1, characterized in that, In step 1), the temperature of the first material is raised to 150-200℃ and then enters the first material inlet of the tubular reactor. The 2-6 second material inlets are arranged along the tube side of the tubular reactor. The tubular reactor has 2-6 local temperature maximum values, and the local temperature maximum values ​​are between 270-310℃.

5. The method according to claim 1, characterized in that, The non-conjugated diene has 6-10 carbon atoms, preferably one or a mixture of 1,5-hexadiene, 1-6-heptadiene, 1,7-octadiene, 1,8-nonadiene, and 1,9-decadiene, and the molecular weight regulator is one or a mixture of alkane, monoolefin, and aldehyde having 2-4 carbon atoms. The second material is introduced into the tubular reactor in one or more of the following ways: a) The second material is mixed with the first material before pressurization, and after pressurization and preheating, it is introduced into the first material inlet of the tubular reactor; b) The second material is mixed with the pressurized and preheated first material and introduced into the first material inlet of the tubular reactor; c) The second material is introduced into the tubular reactor from one or more second material inlets; d) The second material and the third material are mixed and then introduced into the tubular reactor through the second material inlet.

6. The method according to claim 1, characterized in that, The peroxides in the third material are composed of various types of peroxides. Let DT be the difference between the maximum local temperature and the inlet temperature of the first material in the tubular reactor. Then the number of peroxide types b satisfies: b×a=DT, where the value of parameter a ranges from 20 to 60℃.

7. The method according to claim 5, characterized in that, The second and third materials are mixed under conditions below 10 MPa, and then pressurized by a feed pump before being introduced into the tubular reactor from the inlet of the second material. The temperature of the mixture of the second and third materials before pressurization is below 50°C, the temperature of the mixture of the second and third materials after pressurization is below 50°C, and the residence time from the outlet of the feed pump to the inlet of the second material is less than 20 minutes.

8. The method according to claim 7, characterized in that, The second and third materials are mixed under conditions below 3 MPa, and then pressurized by a feed pump before being introduced into the tubular reactor from the inlet of the second material. The temperature of the mixture of the second and third materials before pressurization is below 30°C, the temperature of the mixture of the second and third materials after pressurization is below 30°C, and the residence time from the outlet of the feed pump to the inlet of the second material is less than 10 minutes.

9. The method according to claim 1, characterized in that, The second material contains 10-1000 ppmw of antioxidants.

10. The method according to claim 1, characterized in that, The tubular reactor is periodically heated and descaled according to the polyethylene production. The liquid level in the first separator is below 50%. The second separator consists of two separators connected in parallel, with the other being cleaned offline while one separator is in operation.

11. The method according to claim 1, characterized in that, The third material further includes a solvent, which is one or a mixture of n-alkanes with 8-15 carbon atoms, or one or a mixture of isoalkanes, and the atmospheric boiling point of the solvent and the non-conjugated diene differs by more than 10°C.

12. A polyethylene prepared by the method according to any one of claims 1-11, characterized in that, The polyethylene has a melt index of 0.2-10 g / 10 min at 190°C and 2.16 kg load, a density of 0.915-0.930 g / cm3, a melt expansion ratio of 1.5-2.0, and a side chain ethylene content of >4 carbon atoms of 0.001-2 mol.

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