Process for the preparation of ethylene-vinyl acetate copolymers with dynamically controlled molecular weight and its distribution and applications thereof

Abstract

The application discloses a preparation method and application of ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution, and relates to the technical field of ethylene-vinyl acetate copolymer. According to the chain transfer constant, the solvent is divided into three categories through a solvent classification and selection strategy, high and low activity initiators are matched, and different EVA is prepared through dynamic regulation of the adding time of the multi-phase solvent and monomer gradient feeding in three processes, so that the GPC peak type, molecular weight and distribution of the EVA are accurately regulated. The selection of the solvent is changed from a passive reaction medium role to an active 'performance regulator', the performance of the EVA can be systematically optimized, and the performance of customized EVOH material is realized.

Description

Technical Field

[0001] This invention relates to the field of ethylene-vinyl acetate copolymer technology, specifically to a method for preparing and applying ethylene-vinyl acetate copolymers with dynamically controlled molecular weight and distribution. Background Technology

[0002] Ethylene-vinyl alcohol copolymer (EVOH) is currently the highest-performance high-barrier polymer material with excellent molding and processability, and is widely used in packaging materials, automotive materials, oxygen-barrier pipes, textiles, and medical devices. Ethylene-vinyl acetate copolymer (EVA) is the precursor for EVOH synthesis, and its properties are closely related to the final properties of EVOH. EVA is prepared by copolymerization of ethylene and vinyl acetate (VA) under the action of an initiator. Its properties largely depend on two key structural parameters: VA content and molecular weight and its distribution. VA content mainly affects the polymer's polarity, flexibility, transparency, crystallinity, and adhesion. For example, as the VA content increases, the flexibility, transparency, and adhesion to the substrate of EVA significantly improve, while crystallinity and melting point decrease.

[0003] As is well known, the structure of a polymer affects its properties, and its molecular weight (molecular weight) or The molecular weight and its distribution (MWD) are the foundation for determining its macroscopic properties. Generally speaking, a higher molecular weight means longer molecular chains and a more compact entanglement between molecular chains, which macroscopically manifests as higher strength and tear and impact resistance. However, excessively high molecular weight leads to a sharp increase in melting point (or viscous flow temperature) and melt index, resulting in poor processing fluidity of the material and higher requirements for processing equipment and process conditions.

[0004] MWD is typically measured using the Polydispersity Index (PDI) = / The molecular weight distribution (MWD) characterizes the distribution of molecular chains of different lengths within a polymer, and it has a decisive impact on the material's properties. A wide MWD (i.e., a high PDI value) indicates that the material contains both high-molecular-weight and low-molecular-weight components. The high-molecular-weight portion contributes to the material's strength and toughness, while the low-molecular-weight portion acts like a "plasticizer," reducing melt viscosity and partially improving the material's flowability and processing properties. Therefore, EVA with a wide MWD is commonly used in hot melt adhesives, extrusion coatings, and other fields. Conversely, a narrow MWD indicates that the polymer's molecular chain length is more uniform, which represents superior physical properties, such as higher tensile strength, better stress cracking resistance, and better performance in high-end applications (such as precision thin films and photovoltaic encapsulation materials).

[0005] Currently, industrial production methods for EVA mainly include high-pressure bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Molecular weight control primarily relies on polymerization temperature, pressure, and the amount of chain transfer agent used; however, this control has limited precision and makes it difficult to achieve precise design of the MWD peak shape. Furthermore, the introduction of emulsifiers, dispersants, and other additives during the reaction process may affect the purity of the product and the performance of the final product.

[0006] Chinese invention patent CN120535681A discloses high molecular weight EVA with a narrow molecular weight distribution and its preparation method. This method controls the polymer's molecular weight, molecular weight distribution, and structure through controlled-radical polymerization. It primarily alters the free radical concentration, generation rate, initiation efficiency, and auto-acceleration phenomenon in the polymerization system by introducing "dormant species" and "active species," thereby inhibiting chain termination and achieving linear molecular weight growth. This method mainly relies on the relationship between polymerization reaction kinetics and free radical concentration, generation rate, and initiation efficiency, controlling the EVA molecular weight through adjusting the feeding method, pulse heating, and composite initiators. However, this method can only achieve high molecular weight EVA with a narrow molecular weight distribution, making its function relatively limited. Furthermore, it requires high precision in controlling the vinyl acetate monomer, ethylene gas, and system temperature, and the operation is cumbersome, hindering industrial production.

[0007] Chinese invention patent CN115636886A discloses a microreactor system and a method for preparing polymers using it, as well as the application of EVA. This method overcomes the influence of gas-liquid phase mass and heat transfer in traditional batch reactors, decouples the dependence of ethylene content on reaction pressure, and accurately controls the composition and structure of EVA through various delivery units. Each delivery unit enhances the mixing process of ethylene and the solution, achieving rapid dissolution and dispersion of ethylene. By controlling the reaction temperature / pressure of the homogeneous system, the amount of ethylene added, and the effectiveness of the initiator, the copolymerization process of ethylene and vinyl acetate is achieved efficiently, effectively controlling the molecular weight of EVA and the ethylene content. However, this method has high equipment requirements (requiring a microreactor system) and stringent requirements on the ratio of ethylene to vinyl acetate, making large-scale industrial application difficult.

[0008] High-pressure solution polymerization has become the mainstream EVA production method due to its advantages such as mild reaction temperature, uniform heat transfer, and ease of control. In solution polymerization, the solvent not only serves as the reaction medium, but its own properties also profoundly affect the polymerization process. In existing processes, the choice of solvent is largely based on its ability to dissolve monomers and the control of reaction temperature (e.g., controlling the medium temperature through reflux).

[0009] However, research on utilizing the chemical properties (such as chain transfer ability) and physical properties (such as compatibility with monomers and azeotropic behavior) of solvents as a precise and active dynamic control medium to achieve systematic design of EVA molecular weight, distribution, and even GPC peak shape is very limited. Therefore, developing a new method that can utilize multiphase mixed solvents and their dynamic addition to precisely and predictably control the molecular weight, distribution, and even the properties of later products during the EVA polymerization process, in order to meet the customized performance requirements of EVA materials in different application scenarios, has significant theoretical importance and enormous economic value. Summary of the Invention

[0010] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies and provide a method for preparing ethylene-vinyl acetate copolymers with dynamically regulated molecular weight and distribution. This method overcomes the limitations of traditional methods that rely on temperature, pressure, and general-purpose chain transfer agents for coarse-grained control of EVA molecular weight and distribution, and establishes a new, predictable, and refined control technology path based on solvent selection. It systematically utilizes the chain transfer effect and compatibility behavior of multiphase solvents in EVA polymerization to achieve customized design of the GPC peak shape of EVA products. Solvent selection is transformed from a passive reaction medium role to an active "performance regulator," providing a complete and reliable technical solution for producing customized EVA and EVOH materials with specific properties.

[0011] Solvent classification and selection based on chain transfer constant (Cs):

[0012] The core principle is to classify solvents according to their chain transfer capabilities in the free radical polymerization of vinyl acetate. Chain transfer is an important side reaction in free radical polymerization, in which a growing polymer chain radical (P·) abstracts an atom (usually a hydrogen atom) from a solvent molecule (S), deactivating itself to form a stable polymer (PH), while simultaneously generating a new small molecule radical (S·) that can initiate a new polymer chain.

[0013] The rate of this process is determined by the chain transfer constant C. S definition:

[0014] ;

[0015] Where k tr,s It is the chain transfer rate constant to the solvent; k p Cs is the chain growth rate constant. The larger the Cs value, the higher the chain transfer activity of the solvent, the easier it is to terminate existing molecular chains and initiate new chains, thereby leading to a decrease in the average molecular weight of the polymer and a wider distribution.

[0016] Based on the chain transfer activity in the vinyl acetate polymerization system, this invention classifies common solvents into the following three categories:

[0017] Class I solvents: Weak chain transfer solvents (low-activity solvents)

[0018] The Cs of these solvents are very small, typically Cs × 10⁻⁶. 4 <5, it mainly acts as a dispersion medium during polymerization, with almost no active α-H, and has minimal interference with chain growth. The chain transfer reaction of the solvent is effectively suppressed, mainly occurring through bimolecular termination or chain transfer to the monomer, which is beneficial for generating high molecular weight, narrow-distribution EVA products.

[0019] Class II solvents: medium-chain transfer agents (moderately reactive solvents)

[0020] It exhibits some chain transfer activity, with a moderate Cs value, typically 5. <Cs×10 4 <50. During polymerization, due to chain transfer, the molecular weight can be effectively controlled, and the distribution becomes wider, which is beneficial for producing EVA products with medium molecular weight and medium distribution width.

[0021] Class III solvents: Strong chain transfer agents (highly reactive solvents)

[0022] It exhibits high chain transfer activity and a large Cs value, typically Cs × 10. 4 >50. Due to the presence of active α-H, chain transfer reactions become the main means of controlling molecular weight during polymerization, resulting in a significant decrease in molecular weight and a marked broadening of the molecular weight distribution; this is attributed to the increased randomness in the number and length of molecular chain segments due to frequent chain termination and re-initiation processes.

[0023] Based on the above, this invention proposes the following control schemes:

[0024] Option 1: Multiphase dynamic solvent dropping method

[0025] This method is suitable for preparing bimodal ethylene-vinyl acetate copolymers with a narrow molecular weight distribution (PDI < 3). During the polymerization reaction, by controlling the state of the reaction system (such as viscosity, solids content, and molecular weight), and dynamically changing the solvent composition at appropriate times, the chain transfer activity of the polymer chains in the reaction system can be actively influenced, thereby achieving free control over the overall molecular weight and its distribution. For example, after the solids content of the polymerization reaction reaches a certain limit, adding another solvent with different chain transfer activity midway through the reaction can achieve a bimodal distribution of molecular weight at the microscopic level, thus balancing mechanical strength and excellent processing flowability at the macroscopic level. The timing of addition is also a crucial point; adding it too early or too late will have adverse effects on the polymerization system.

[0026] Option 2: Using a low-activity initiator system in conjunction with a multiphase dynamic solvent for synergistic regulation, prepare bimodal ethylene-vinyl acetate copolymers with a wide molecular weight distribution and PDI ≥ 3.

[0027] This method is suitable for preparing bimodal ethylene-vinyl acetate copolymers with a wide molecular weight distribution and a PDI ≥ 3. In the early stage of the reaction, a highly reactive initiator ensures sufficient growth of the high molecular weight component. In the later stage, a low-reactivity initiator combined with a solvent with high chain transfer properties is dynamically introduced to initiate the low molecular weight component and achieve sufficient solvent chain transfer. Through precise selection of the introduction timing and synergistic control of dynamic introduction, the bimodal molecular weight distribution can be precisely controlled, thereby influencing the macroscopic properties of the material.

[0028] Option 3: Coupling and control of monomer feeding method and dynamic solvent

[0029] This method is suitable for preparing trimodal ethylene-vinyl acetate copolymers. The combined effect of three batches of gradient monomer addition and dynamic addition of multiphase solvent, through matching the monomer concentration gradient with the solvent chain transfer activity gradient, enables the preparation of trimodal EVA, while simultaneously improving the overall strength and processability of EVOH.

[0030] The specific technical solution of the present invention is as follows:

[0031] In a first aspect, the present invention provides a method for preparing ethylene-vinyl acetate copolymers with dynamically adjustable molecular weight and distribution, comprising the following steps:

[0032] S1: Add type I solvent and vinyl acetate monomer to the reaction vessel;

[0033] S2: Sealed reactor, with protective gas used to replace the air inside the reactor;

[0034] S3: Start stirring and heat the solution in the reactor to 30-100 ℃;

[0035] S4: Introduce ethylene gas into the reactor and stir, maintaining a constant pressure of 1-10 MPa inside the reactor;

[0036] S5: Preparation of ethylene-vinyl acetate copolymers with different properties

[0037] (1) When preparing a bimodal ethylene-vinyl acetate copolymer with a narrow molecular weight distribution of PDI < 3, in step S1, the mass of vinyl acetate monomer is 100 parts, and the following operations are continued based on step S4:

[0038] (1-1) Dissolve the highly active initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the highly active initiator is 0.1-1 parts and the mass of the type I solvent is 4-6 parts.

[0039] (1-2) When the solid content of the reaction system is detected to be above 30%, the rotational viscosity is >3000 cP, and the peak value of the slice is... When the concentration is >100000 g / mol, inject a type III solvent into the reactor and continue the reaction. The mass of the type III solvent is 24-26 parts.

[0040] (1-3) When the solid content of the reaction system reaches 55-60%, the rotational viscosity is >8000 cP, and the peak value of the slice is... The reaction was stopped when the concentration reached >200,000 g / mol.

[0041] (2) When preparing a bimodal ethylene-vinyl acetate copolymer with a wide molecular weight distribution of PDI≥3, in step S1, the mass of vinyl acetate monomer is 100 parts, and the following operations are continued based on step S4:

[0042] (2-1) Dissolve the highly active initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the highly active initiator is 0.1-0.5 parts, and the mass of the type I solvent is 4-6 parts;

[0043] (2-2) When the solid content of the reaction system is detected to be above 25%, the rotational viscosity is >2000 cP, and the peak value of the slice is... When the concentration is >60000 g / mol, the low-activity initiator is dissolved in a Class III solvent and injected into the reactor to continue the reaction; the mass of the low-activity initiator is 0.1-0.5 parts, and the mass of the Class III solvent is 24-26 parts.

[0044] (2-3) When the solid content of the reaction system reaches 50-55%, the rotational viscosity is >6000 cP, and the peak value of the slice is... The reaction was stopped when the concentration reached >180,000 g / mol.

[0045] (3) When preparing a trimodal ethylene-vinyl acetate copolymer with a wide molecular weight distribution of PDI≥3, in step S1, the mass of vinyl acetate monomer is 20-30 parts, and the following operations are continued based on step S4:

[0046] (3-1) Dissolve the low-activity initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the low-activity initiator is 0.1-0.5 parts and the mass of the type I solvent is 8-10 parts.

[0047] (3-2) When the solid content of the reaction system is detected to be above 20%, the rotational viscosity is >1500 cP, and the peak value of the slice is... When the concentration is >50000 g / mol, the second batch of vinyl acetate monomer is dissolved in a type II solvent and injected into the reactor to continue the reaction; the mass of the second batch of vinyl acetate monomer is 34-36 parts, and the mass of type II solvent is 6-8 parts.

[0048] (3-3) When the solid content of the reaction system is detected to be above 40%, the rotational viscosity is >5000 cP, and the peak value of the slice is... When the concentration is >100,000 g / mol, the third batch of vinyl acetate monomer is dissolved in a Class III solvent and injected into the reactor to continue the reaction; the mass of the third batch of vinyl acetate monomer is 38-40 parts, and the mass of the Class III solvent is 4-6 parts.

[0049] (3-4) When the solid content of the reaction system reaches 55-60%, the rotational viscosity is >8000 cP, and the peak value of the slice is... The reaction was stopped when the concentration was >120,000 g / mol.

[0050] Wherein, the Class I solvent is tert-butylbenzene or cumene, the Class II solvent is N,N-dimethylaniline, and the Class III solvent is carbon tetrabromide; the highly active initiator is dimethyl azobisisobutyrate; and the low-activity initiator is 1,1-di-tert-butylperoxide cyclohexane.

[0051] S6: After the reaction is complete, stop heating, cool the reactor to room temperature, and release the remaining pressure;

[0052] S7: Pour the polymer solution obtained in step S6 into a solvent to precipitate, filter, wash, and vacuum dry the solid product to constant weight to obtain the ethylene-vinyl acetate copolymer product.

[0053] Preferably, in step S1, the mass of the type I solvent is 10-30 parts.

[0054] Preferably, in step S2, the protective gas is nitrogen, argon, or helium.

[0055] Preferably, in step S3, the stirring speed is 100-1000 rpm, and the heating rate is 10-50℃ / min.

[0056] Preferably, in step S4, the stirring speed is 100-1000 rpm and the stirring time is 10-30 min.

[0057] Preferably, in step S7, the solvent is an insoluble solvent that can cause the EVA solution to precipitate, specifically deionized water, ethanol, or n-hexane; the vacuum drying temperature is 40-100℃, and the vacuum drying time is 1-12h.

[0058] Secondly, the present invention provides an application of the above-mentioned method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and its distribution, wherein the ethylene-vinyl acetate copolymer prepared by the above-mentioned method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and its distribution is alcoholyzed and granulated to obtain ethylene-vinyl alcohol copolymer.

[0059] Compared with the prior art, the present invention has the following advantages:

[0060] 1. Through the solvent classification and selection strategy of this invention, precise control of molecular weight distribution is achieved, enabling the design of EVA's molecular weight, PDI value, and GPC peak shape, thus achieving a leap from coarse adjustment to fine control. Furthermore, this invention controls the MWD of EVA by changing the type of solvent, allowing for systematic optimization of EVA performance and achieving customized EVOH material performance. For example, bimodal EVOH prepared by dynamic dropwise addition of a multiphase mixed solvent combines the advantages of high and low molecular weight components and can be used to manufacture special blown films or pipes requiring high melt strength and good ductility. The high molecular weight fraction in EVA with a narrow molecular weight distribution exhibits excellent mechanical strength, toughness, and weather resistance, making it suitable for high-end photovoltaic encapsulation films, durable agricultural films, and other fields; EVA with a wide molecular weight distribution combines good strength and excellent processing flowability, making it ideal as a high-performance hot melt adhesive, wire and cable sheathing material, and foaming material.

[0061] 2. The control method of this invention is simple, cost-effective, and easy to implement industrially. It is mainly achieved by optimizing the selection and combination of solvents, without requiring major modifications to existing hardware equipment such as solution polymerization reactors. The solvents involved are all conventional chemical raw materials, with controllable costs and easy to scale up for production. Attached Figure Description

[0062] Figure 1 This is the GPC spectrum of the EVA product prepared in Example 1 of this invention.

[0063] Figure 2 This is the GPC spectrum of the EVA product prepared in Example 2 of this invention.

[0064] Figure 3 This is the GPC spectrum of the EVA product prepared in Example 3 of this invention.

[0065] Figure 4 This is the GPC spectrum of the EVA product prepared in Comparative Example 1 of this invention.

[0066] Figure 5 This is the GPC spectrum of the EVA product prepared in Comparative Example 2 of this invention.

[0067] Figure 6 This is the GPC spectrum of the EVA product prepared in Comparative Example 3 of this invention. Detailed Implementation

[0068] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0069] Example 1

[0070] This embodiment prepares a bimodal ethylene-vinyl acetate copolymer with a narrow molecular weight distribution and a PDI < 3, specifically including the following steps:

[0071] S1: Add 125g of Class I solvent tert-butylbenzene and 500g of vinyl acetate monomer to a 1L high-pressure reactor.

[0072] S2: Seal the high-pressure reactor and replace the air inside the reactor three times with nitrogen.

[0073] S3: Start stirring at 500 rpm and heat the solution in the high-pressure reactor to the reaction temperature of 70°C at a rate of 30°C / min.

[0074] S4: Use an air compressor to pump ethylene gas into the high-pressure reactor, stir for 20 minutes at a stirring speed of 500 rpm, and maintain the pressure inside the reactor at a constant value of 5 MPa.

[0075] S5: Dissolve 2.5g of dimethyl azobisisobutyrate initiator in 25g of tert-butylbenzene solvent (a type I solvent), and inject the solution into a high-pressure reactor using a high-pressure injection pump to initiate polymerization.

[0076] S6: React at 70℃ and 5MPa. Real-time reaction status monitoring is performed during the reaction. When the overall solid content at the real-time sampling point reaches 30% or more, the rotational viscosity of the system is >3000cP, and the peak value of the slice is [not specified]. When the concentration is >100000 g / mol, 125 g of the Class III solvent carbon tetrabromide is injected into the reaction system using a high-pressure injection pump, and the injection continues for 1 hour.

[0077] S7: After the injection of Class III solvents is completed, continue the reaction until the overall solid content at the real-time sampling point reaches 60%, the rotational viscosity of the system is >8000 cP, and the peak value of the slice reaches [value missing]. When the pressure reaches >200,000 g / mol, stop the reaction and heating, cool the high-pressure reactor to room temperature, and release the remaining pressure.

[0078] S8: Pour the polymer solution into deionized water to precipitate, filter, wash, and dry the resulting solid product in a vacuum oven at 80°C for 8 hours to constant weight to obtain the EVA product.

[0079] Example 2

[0080] This embodiment prepares a bimodal ethylene-vinyl acetate copolymer with a broad molecular weight distribution and a PDI ≥ 3, specifically including the following steps:

[0081] S1: Add 50g of type I solvent cumene and 500g of vinyl acetate monomer to a 1L high-pressure reactor.

[0082] S2: Seal the high-pressure reactor and replace the air inside the reactor three times with argon gas.

[0083] S3: Start stirring at 100 rpm and heat the solution in the high-pressure reactor to the reaction temperature of 30°C at a rate of 10°C / min.

[0084] S4: Inject ethylene gas into the reactor using an air compressor, stir for 30 minutes at a stirring speed of 100 rpm, and maintain the pressure inside the reactor at a constant value of 1 MPa.

[0085] S5: Dissolve 1.25g of dimethyl azobisisobutyrate initiator in 25g of cumene solvent (a type I solvent), and inject the solution into the reactor using a high-pressure injection pump to initiate polymerization.

[0086] S6: React at 30℃ and 1MPa. Real-time reaction status monitoring is performed during the reaction. When the overall solid content at the real-time sampling point reaches 25% or more, the rotational viscosity of the system is >2000cP, and the peak value of the slice is... When the concentration is >60000 g / mol, 1.25 g of 1,1-di-tert-butylperoxycyclohexane initiator is dissolved in 125 g of carbon tetrabromide, a Class III solvent, and injected into the reaction system using a high-pressure injection pump. The injection continues for 2 hours.

[0087] S7: After injection, continue the reaction until the overall solids content at the real-time sampling point reaches 50%, the system rotational viscosity is >6000 cP, and the peak value of the slice reaches 100%. When the pressure reaches >180000 g / mol, stop the reaction and heating, cool the reactor to room temperature, and release the remaining pressure.

[0088] S8: Pour the polymer solution into ethanol to precipitate, filter, wash, and dry the resulting solid product in a vacuum oven at 40°C for 12 hours to constant weight to obtain the EVA product.

[0089] Example 3

[0090] This embodiment prepares a trimodal ethylene-vinyl acetate copolymer, specifically including the following steps:

[0091] S1: Add 62.5g of Class I solvent tert-butylbenzene and 125g of vinyl acetate monomer to a 1L high-pressure reactor.

[0092] S2: Seal the high-pressure reactor and replace the air inside the reactor three times with helium.

[0093] S3: Start stirring at 1000 rpm and heat the solution in the high-pressure reactor to the reaction temperature of 100℃ at a heating rate of 50℃ / min.

[0094] S4: Inject ethylene gas into the reactor using an air compressor, stir for 10 minutes at a stirring speed of 1000 rpm, and maintain the pressure inside the reactor at a constant value of 10 MPa.

[0095] S5: Dissolve 2.5g of 1,1-di-tert-butylperoxide cyclohexane initiator in 50g of tert-butylbenzene solvent (a type I solvent), and inject the solution into the reactor using a high-pressure injection pump to initiate polymerization.

[0096] S6: React at 100℃ and 10MPa, with real-time monitoring of the reaction status during the reaction. The reaction is considered complete when the overall solid content at the real-time sampling point reaches 20% or more, the rotational viscosity of the system is >1500 cP, and the peak value of the slice reaches [value missing]. When the concentration is >50000g / mol, the second batch of 175g vinyl acetate monomer is dissolved in 37.5g of N,N-dimethylaniline, a type II solvent, and injected into the reaction system using a high-pressure injection pump. The injection continues for 1 hour.

[0097] S7: After injection, continue the reaction until the overall solids content at the real-time sampling point reaches 40%, the system rotational viscosity is >5000 cP, and the peak value of the slice reaches 10%. When the concentration is >100000g / mol, the third batch of 200g vinyl acetate monomer is dissolved in 25g of carbon tetrabromide, a Class III solvent, and injected into the reaction system using a high-pressure injection pump. The injection continues for 1 hour.

[0098] S8: After injection, continue the reaction until the overall solids content at the real-time sampling point reaches 60%, the system rotational viscosity is >8000 cP, and the peak value of the slice reaches [value missing]. When the pressure reaches >120000 g / mol, stop the reaction and heating, cool the reactor to room temperature, and release the remaining pressure.

[0099] S9: Pour the polymer solution into n-hexane to precipitate, filter, wash, and dry the resulting solid product in a vacuum oven at 100°C for 1 hour to constant weight to obtain the EVA product.

[0100] Comparative Example 1

[0101] The difference from Example 1 is that in step S6, when the overall solid content of the real-time sampling point reaches 10% and the rotational viscosity of the system is 900-1500 cP, the injection of the Class III solvent carbon tetrabromide begins.

[0102] Comparative Example 2

[0103] The difference from Example 1 is that in step S6, when the overall solid content of the real-time sampling point reaches 40% and the rotational viscosity of the system is 4000-4500 cP, the injection of the Class III solvent carbon tetrabromide begins.

[0104] Comparative Example 3

[0105] The preparation method of the ethylene-vinyl acetate copolymer of Comparative Example 3 includes the following steps:

[0106] S1: Add 250g of methanol and 500g of vinyl acetate monomer to a 1L high-pressure reactor.

[0107] S2: Seal the high-pressure reactor and replace the air inside the reactor three times with nitrogen.

[0108] S3: Start stirring at 500 rpm and heat the solution in the high-pressure reactor to the reaction temperature of 70°C at a rate of 10°C / min.

[0109] S4: Use an air compressor to pump ethylene gas into the high-pressure reactor, stir for 10 minutes at a stirring speed of 500 rpm, and maintain the pressure inside the reactor at a constant value of 5 MPa.

[0110] S5: Dissolve 2.5g of dimethyl azobisisobutyrate initiator in 25g of methanol solvent, and inject it into a high-pressure reactor through a high-pressure injection pump to initiate polymerization.

[0111] S6: React at 70℃ and 5MPa. When the overall solid content reaches 60% and the rotational viscosity of the system is >8000cP, stop the reaction and heating. Cool the high-pressure reactor to room temperature and release the remaining pressure.

[0112] S7: Pour the polymer solution into deionized water to precipitate, filter, wash, and dry the resulting solid product in a vacuum oven at 80°C for 8 hours to constant weight to obtain the EVA product.

[0113] The EVA products prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to alcoholysis and granulation to obtain EVOH particles. The EVA products of each example and comparative example were analyzed by gel permeation chromatography (GPC), and the obtained spectra are shown below. Figure 1-6 As shown in Table 1, the molecular weight and its distribution are as follows. Melt index and mechanical properties of EVOH particles were tested. The melt index was tested according to GB / T3682.1-2018 "Plastics - Determination of melt mass flow rate (MFR) and melt volumetric flow rate (MVR) - Part 1: Standard methods", and the mechanical properties were tested according to ASTM D638-2022 "Plastics - Determination of mechanical properties". The test results are shown in Table 1.

[0114] Table 1. EVA molecular weight and its distribution, EVOH melt index and mechanical property test results for each example and comparative example.

[0115]

[0116] Depend on Figure 1As shown in Table 1, the GPC spectrum of the EVA prepared in Example 1 exhibits a typical bimodal morphology with a narrow molecular weight distribution. A main peak appears in the high molecular weight region (approximately 231 kg / mol), and a distinct shoulder peak or secondary peak appears in the low molecular weight region (approximately 109 kg / mol). The overall PDI value is also low (approximately 2.8). EVOH simultaneously possesses high strength (tensile strength 29 MPa) contributed by the high molecular weight fraction and good processing fluidity (melt index 8.3 g / 10 min) contributed by the low molecular weight fraction. This indicates that Example 1, by controlling the dynamic addition of multiphase solvents, can alter the chain transfer environment of the reaction system, thereby successfully synthesizing EVA with an asymmetric or multimodal molecular weight distribution. This opens up new avenues for the design of functionalized and customized EVA materials.

[0117] Depend on Figure 2 As shown in Table 1, the GPC spectrum of the EVA prepared in Example 2 exhibits a typical bimodal shape with a broad molecular weight distribution. Compared to Example 1, the overall molecular weight is significantly lower and the peak distribution is broader, while the intensity is slightly reduced, resulting in improved processing flow properties. This verifies the feasibility of using a high-activity initiator + low-activity initiator system combined with a multiphase solvent, through precise selection of the introduction timing and synergistic control of dynamic introduction, to prepare EVA with a broad molecular weight bimodal distribution.

[0118] Depend on Figure 3 As can be seen from the data in Table 1, the GPC spectrum of the EVA prepared in Example 3 exhibits a typical three-peak distribution, with a relatively wide overall distribution but uniform peak shape. Compared with Examples 1 and 2, it significantly improves processing fluidity while maintaining mechanical strength. This well demonstrates that by matching the monomer concentration gradient with the multiphase solvent chain transfer activity gradient and adding it in multiple dynamic batches, it is possible to prepare EVA with a uniform three-peak distribution, while simultaneously improving the overall strength and processability (melt index) of EVOH.

[0119] Depend on Figure 4-5As shown in Table 1, the GPC spectra of EVA prepared in Comparative Examples 1-2 exhibit a typical bimodal distribution, but the distribution is asymmetrical. This is due to the different timing of solvent addition, which shifts the position of the molecular weight peak. In Comparative Example 1, when the Class III solvent was added too early, the premature change in the chain transfer environment led to uneven molecular weight distribution, with the peak being narrow at the beginning and wide at the end, resulting in excessively large molecular weights and tailing phenomena in the later stages. In Comparative Example 2, when the Class III solvent was added too late, the polymerization reaction had already progressed to the later stages, and the addition of the new solvent caused the molecular weight peak to be wide at the beginning and narrow at the end, resulting in insignificant overall positive control effect. The differences in later performance also demonstrate that adding Class III solvent too early or too late will seriously affect the mechanical properties and processability of the later products. This fully confirms the necessity of combining reaction stage control with solvent type selection to choose the appropriate addition point in Example 1.

[0120] Depend on Figure 6 As can be seen from the data in Table 1, in Comparative Example 3, EVA prepared using traditional processes exhibits a symmetrical and slightly broadened single-peak distribution in its GPC spectrum. =62.6 kg / mol =130.2 kg / mol, PDI=2.12. The tensile strength of EVOH is 15 MPa, the elongation at break is 190%, and the melt index is 5.2 g / 10 min. Its mechanical properties and processing fluidity are generally poor. This indicates that conventional processes have a poor effect on controlling the molecular weight and distribution of EVA, and the macroscopic performance indicators of EVOH are generally poor. Compared with the embodiments of this invention that use multiphase solvent synergy and chain transfer dynamic control, the control effect and performance are significantly different.

Claims

1. A method for preparing ethylene-vinyl acetate copolymer with dynamically controlled molecular weight and distribution, characterized in that, Includes the following steps: S1: Add type I solvent and vinyl acetate monomer to the reaction vessel; S2: Sealed reactor, with protective gas used to replace the air inside the reactor; S3: Start stirring and heat the solution in the reactor to 30-100 ℃; S4: Introduce ethylene gas into the reactor and stir, maintaining a constant pressure of 1-10 MPa inside the reactor; S5: Preparation of ethylene-vinyl acetate copolymers with different properties (1) When preparing a bimodal ethylene-vinyl acetate copolymer with a narrow molecular weight distribution of PDI < 3, in step S1, the mass of vinyl acetate monomer is 100 parts, and the following operations are continued based on step S4: (1-1) Dissolve the highly active initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the highly active initiator is 0.1-1 parts and the mass of the type I solvent is 4-6 parts. (1-2) When the solid content of the reaction system reaches 30%, the rotational viscosity is >3000 cP, and the peak value of the slice is... When the concentration is >100000 g / mol, inject a type III solvent into the reactor and continue the reaction. The mass of the type III solvent is 24-26 parts. (1-3) When the solid content of the reaction system reaches 55-60%, the rotational viscosity is >8000 cP, and the peak value of the slice is... The reaction was stopped when the concentration reached >200,000 g / mol. (2) When preparing a bimodal ethylene-vinyl acetate copolymer with a wide molecular weight distribution of PDI≥3, in step S1, the mass of vinyl acetate monomer is 100 parts, and the following operations are continued based on step S4: (2-1) Dissolve the highly active initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the highly active initiator is 0.1-0.5 parts, and the mass of the type I solvent is 4-6 parts; (2-2) When the solid content of the reaction system is detected to be above 25%, the rotational viscosity is >2000 cP, and the peak value of the slice is... When the concentration is >60000 g / mol, the low-activity initiator is dissolved in a Class III solvent and injected into the reactor to continue the reaction; the mass of the low-activity initiator is 0.1-0.5 parts, and the mass of the Class III solvent is 24-26 parts. (2-3) When the solid content of the reaction system reaches 50-55%, the rotational viscosity is >6000 cP, and the peak value of the slice is... The reaction was stopped when the concentration reached >180,000 g / mol. (3) When preparing a trimodal ethylene-vinyl acetate copolymer with a wide molecular weight distribution of PDI≥3, in step S1, the mass of vinyl acetate monomer is 20-30 parts, and the following operations are continued based on step S4: (3-1) Dissolve the low-activity initiator in a type I solvent and inject it into the reactor to initiate polymerization; the mass of the low-activity initiator is 0.1-0.5 parts and the mass of the type I solvent is 8-10 parts. (3-2) When the solid content of the reaction system is detected to be above 20%, the rotational viscosity is >1500 cP, and the peak value of the slice is... When the concentration is >50000 g / mol, the second batch of vinyl acetate monomer is dissolved in a type II solvent and injected into the reactor to continue the reaction; the mass of the second batch of vinyl acetate monomer is 34-36 parts, and the mass of type II solvent is 6-8 parts. (3-3) When the solid content of the reaction system is detected to be above 40%, the rotational viscosity is >5000 cP, and the peak value of the slice is... When the concentration is >100,000 g / mol, the third batch of vinyl acetate monomer is dissolved in a Class III solvent and injected into the reactor to continue the reaction; the mass of the third batch of vinyl acetate monomer is 38-40 parts, and the mass of the Class III solvent is 4-6 parts. (3-4) When the solid content of the reaction system reaches 55-60%, the rotational viscosity is >8000 cP, and the peak value of the slice is... The reaction was stopped when the concentration was >120,000 g / mol. Wherein, the Class I solvent is tert-butylbenzene or cumene, the Class II solvent is N,N-dimethylaniline, and the Class III solvent is carbon tetrabromide; the highly active initiator is dimethyl azobisisobutyrate; and the low-activity initiator is 1,1-di-tert-butylperoxide cyclohexane. S6: After the reaction is complete, stop heating, cool the reactor to room temperature, and release the remaining pressure; S7: Pour the polymer solution obtained in step S6 into a solvent to precipitate, filter, wash, and vacuum dry the solid product to constant weight to obtain the ethylene-vinyl acetate copolymer product.

2. The method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution as described in claim 1, characterized in that, In step S1, the mass of type I solvent is 10-30 parts.

3. The method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution as described in claim 1, characterized in that, In step S2, the protective gas is nitrogen, argon, or helium.

4. The method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution as described in claim 1, characterized in that, In step S3, the stirring speed is 100-1000 rpm, and the heating rate is 10-50℃ / min.

5. The method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution as described in claim 1, characterized in that, In step S4, the stirring speed is 100-1000 rpm and the stirring time is 10-30 min.

6. The method for preparing ethylene-vinyl acetate copolymer with dynamically regulated molecular weight and distribution as described in claim 1, characterized in that, In step S7, the solvent is deionized water, ethanol, or n-hexane; the vacuum drying temperature is 40-100℃, and the vacuum drying time is 1-12h.

7. The application of a method for preparing ethylene-vinyl acetate copolymer with dynamically controlled molecular weight and distribution, characterized in that, The ethylene-vinyl acetate copolymer prepared by the method for preparing ethylene-vinyl acetate copolymer with dynamic control of molecular weight and distribution as described in any one of claims 1-6 is alcoholyzed and granulated to obtain an ethylene-vinyl alcohol copolymer.

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