Ultra-low degree of polymerization modified PVA and preparation method thereof

Abstract

This invention discloses an ultra-low degree of polymerization modified PVA and its preparation method, belonging to the technical field of ultra-low degree of polymerization polyvinyl alcohol preparation. The preparation method includes: S1, mixing vinyl acetate monomer, solvent, and unsaturated carboxylate monomer, carrying out a polymerization reaction under the action of an initiator, and adding a terminator to obtain polyvinyl acetate resin; the unsaturated carboxylate monomer is added all at once or in two batches during the polymerization reaction; S2, subjecting the polyvinyl acetate resin to alcoholysis under alkaline conditions, and drying to obtain ultra-low degree of polymerization modified polyvinyl alcohol. This invention introduces unsaturated carboxylate monomers into the copolymerization process of vinyl acetate, utilizing the electronegativity and steric hindrance effect of the carboxylate monomers to induce chain transfer, simultaneously achieving copolymerization modification and controllable reduction of the degree of polymerization of PVA without relying on toxic chain transfer agents and excess solvents, obtaining ultra-low degree of polymerization modified PVA with a degree of polymerization ≤300 that is completely soluble in cold water at 10℃ for 4–7 minutes.

Description

Technical Field

[0001] This invention relates to the field of ultra-low degree of polymerization polyvinyl alcohol preparation, and particularly to an ultra-low degree of polymerization modified PVA and its preparation method. Background Technology

[0002] Polyvinyl alcohol (PVA) is a water-soluble polymer prepared by polymerizing vinyl acetate (VAC) into polyvinyl acetate (PVAC) and then performing alcoholysis under acid or alkali catalysis. Currently, there are many types of PVA on the market. Among them, ultra-low degree of polymerization (degree of polymerization ≤300) PVA has unique advantages such as low molecular weight, low viscosity, good cold water solubility, and biodegradability, and is widely used in chemical and pharmaceutical fields. For example, it is used in combination with high degree of polymerization PVA as a film-forming material, and as a dispersant in the suspension polymerization preparation of polyvinyl chloride (PVC). Furthermore, modified PVA materials can further improve some properties compared to unmodified PVA. Therefore, ultra-low degree of polymerization modified PVA has attracted widespread attention.

[0003] Copolymerization modification, specifically copolymerizing unsaturated carboxylic acids / carboxylic esters with VAC, can effectively regulate the molecular chain composition and properties of PVA. When the carboxyl groups are converted to hydrophilic sodium carboxylate during alcoholysis, the cold water solubility of the modified PVA is effectively improved. Although copolymerization modification with unsaturated carboxylic acids / carboxylic esters can enhance PVA performance, simply using carboxylic acids / carboxylic esters for copolymerization modification cannot control the degree of polymerization, making it difficult to prepare modified PVA with ultra-low degrees of polymerization.

[0004] Currently, there are two main methods for preparing ultra-low degree polyvinyl alcohol (PVA): changing the composition and ratio of the solvent, or adding chain transfer agents. Changing the solvent composition or excessively adjusting the solvent ratio reduces production efficiency, significantly increases production costs, and burdens solvent recovery equipment. In the process of reducing the degree of polymerization of PVA by adding chain transfer agents, commonly used chain transfer agents include thiols (such as dodecyl mercaptan), mercaptoethanol (such as dimercaptoethanol), and halogenated alkanes (such as chloroform). However, these chain transfer agents can have significant adverse effects on the environment and human health during storage, transportation, and use. For example, chloroform gradually oxidizes in air under light to produce highly toxic phosgene; mercaptoethanol chain transfer agents have a strong odor, are highly toxic, and are difficult to remove; furthermore, the potential residual toxic chain transfer agents in the finished PVA resin also limit its application. These drawbacks severely restrict the large-scale preparation and application of ultra-low degree polyvinyl alcohol. Summary of the Invention

[0005] This invention provides an ultra-low degree of polymerization modified PVA and its preparation method, which solves the technical problem in the prior art that it is difficult to prepare ultra-low degree of polymerization PVA without relying on toxic chain transfer agents and excessive solvents while achieving copolymerization modification.

[0006] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a method for preparing ultra-low degree of polymerization modified PVA, comprising the following steps: S1. Mix vinyl acetate monomer, solvent and unsaturated carboxylate monomer, carry out polymerization reaction under the action of initiator, add terminator to obtain polyvinyl acetate resin. The unsaturated carboxylate monomer is added all at once before the polymerization reaction, or added in two batches during the polymerization reaction; S2. Polyvinyl acetate resin is subjected to alcoholysis under alkaline conditions and dried to obtain ultra-low degree of polymerization modified PVA.

[0007] By introducing unsaturated carboxylate monomers into the polymerization step through the above technical solution, chain transfer is induced by the electronegativity and steric hindrance effect of the carboxylate monomers. This avoids the environmental hazards and residual toxicity caused by the use of toxic chain transfer agents and excessive solvents in traditional processes. At the same time, it eliminates the need for complex post-processing detoxification procedures, shortens the production cycle, and reduces equipment investment and operating costs. This makes the preparation process of ultra-low degree of polymerization modified PVA more environmentally friendly, more economical, and more suitable for large-scale industrial production.

[0008] Further, in step S1, the unsaturated carboxylate monomer is any one or more of sodium acrylate, sodium methacrylate, sodium crotonate, sodium maleate, sodium fumarate, and sodium itaconic acid. All of the above-mentioned unsaturated carboxylate monomers contain polymerizable double bonds and sodium carboxylate groups, which can be incorporated into the PVA molecular chain through copolymerization during polymerization, and can also induce chain transfer using their electronegativity and steric hindrance effects, providing multiple alternatives for monomer selection in different application scenarios.

[0009] Further, in step S1, the solvent is methanol. Based on 400 parts by mass of vinyl acetate monomer, 80-200 parts of methanol are added before the polymerization reaction. If the amount of methanol is less than 80 parts, the system viscosity is too high and the monomer concentration is too high, which is not conducive to the full progress of the chain transfer reaction, resulting in a high degree of polymerization of the obtained PVA. If it is more than 200 parts, although the degree of polymerization can be further reduced, a large amount of methanol is required, increasing the recycling burden and production costs. This invention controls the amount of methanol to 80-200 parts, minimizing the amount of solvent while ensuring a low degree of polymerization, thus balancing the effect of reducing the degree of polymerization and production economy.

[0010] Further, in step S1, when the unsaturated carboxylate monomer is added all at once, the amount added is 2.0 to 2.5 parts based on 400 parts by mass of vinyl acetate monomer. If the amount added all at once is less than 2.0 parts, the concentration of carboxylate monomer is insufficient, the chain transfer effect is weak, and the resulting PVA has a high degree of polymerization, failing to reach the ultra-low degree of polymerization level; if it is more than 2.5 parts, although the degree of polymerization can be further reduced, excessive carboxylate monomer may aggravate side reactions and affect the uniformity of the product; the present invention controls the amount added all at once to 2.0 to 2.5 parts, maintaining good product uniformity while ensuring a low degree of polymerization.

[0011] Furthermore, in step S1, the unsaturated carboxylate monomer is added in two batches as follows: The first batch of unsaturated carboxylate monomers is added before the polymerization reaction. The second batch of unsaturated carboxylate monomers is dissolved in 20 parts of methanol to form a mixed solution and is added dropwise after the initiator is added. Based on 400 parts by mass of vinyl acetate monomer, the amount of the first batch of unsaturated carboxylate monomers added is 0.1 to 0.7 parts, the amount of the second batch of unsaturated carboxylate monomers added is 0.5 to 3 parts, and the dropwise addition time is 2 to 6 hours.

[0012] The above steps involve adding unsaturated carboxylate monomers in two batches. The first batch of monomers establishes the basic chain transfer environment in the early stage of polymerization, while the second batch of monomers is added dropwise continuously during polymerization to precisely maintain the chain transfer strength. This allows for precise control of the polymerization process without changing the total amount of carboxylate monomers, ensuring that the degree of polymerization of the modified PVA is stably controlled in an ultra-low range while maintaining good product homogeneity.

[0013] Further, in step S1, the initiator is a methanol solution of tert-butyl peroxypentanoate with a concentration of 0.2% to 2.0%, and an addition amount of 1 to 15 parts. The half-life of tert-butyl peroxypentanoate matches the polymerization reaction temperature, which can stably and continuously provide free radicals during the reaction process, avoiding the situation where the polymerization rate is too slow and the monomer conversion rate is insufficient due to too low an initiator concentration, or the reaction is too violent and the molecular weight distribution becomes too broad due to too high a concentration; at the same time, the decomposition products of this initiator are small molecule gases, which will not introduce additional impurities into the product, thus ensuring the purity of the modified PVA.

[0014] Furthermore, in step S1, the polymerization reaction temperature is 55–65°C. This temperature range matches the initiator decomposition rate, ensuring the polymerization reaction proceeds fully while preventing overheating that could lead to a broadened molecular weight distribution or explosive polymerization.

[0015] Further, in step S1, the terminator is any one of hydroquinone, 1,4-benzoquinone, and sorbic acid. The terminator is dissolved in methanol to prepare a terminator methanol solution, the concentration of which is 0.2%–2.0%, and the amount added is 1–5 parts. The above-mentioned terminators are all commonly used and highly efficient terminators for free radical polymerization reactions. They can quickly capture residual free radical active centers after the polymerization reaction is completed, thus terminating the chain growth reaction in a timely manner and preventing further increases in the degree of polymerization or the occurrence of secondary crosslinking. This dosage range can ensure complete termination while reducing the adverse effects of terminator residue on subsequent alcoholysis and the water solubility of the product. Furthermore, all three are low-toxicity or non-toxic reagents, meeting the requirements of green production.

[0016] Further, in step S2, the specific steps of alcoholysis under alkaline conditions are as follows: Polyvinyl acetate resin is added to methanol to prepare a resin solution with a solid content of 20%–40%. Then, 3–6 parts of a NaOH methanol solution with a concentration of 20–60 g / L are added for alcoholysis. The alcoholysis temperature is 40–60°C, and the alcoholysis time is 20–60 min. If the alcoholysis temperature is below 40°C and the alcoholysis time is less than 20 min, the reaction rate is too slow, resulting in a low degree of alcoholysis and poor water solubility of the product. If the alcoholysis temperature is above 60°C and the alcoholysis time exceeds 60 min, the alcoholysis is too high, and crystallization occurs due to the formation of hydrogen bonds between a large number of free hydroxyl groups between molecular chains, which also reduces cold water solubility. If the amount of NaOH is less than 3 parts, the degree of alcoholysis is too low; if it is more than 6 parts, the reaction is too fast and difficult to control, and the product color deepens. This invention controls the alcoholysis parameters within the above range, maintaining the degree of alcoholysis in the optimal range, ensuring that the product has both excellent cold water solubility and good material homogeneity.

[0017] Secondly, the present invention provides an ultra-low degree of polymerization modified PVA, which is prepared by the preparation method of ultra-low degree of polymerization modified PVA described in any one of the above contents.

[0018] The beneficial effects of this invention are: 1. This invention utilizes the electronegativity and steric hindrance effect of the carboxylate monomer and vinyl acetate copolymerization reaction in step S1 to induce chain transfer, achieving a controllable reduction in the degree of polymerization without the addition of toxic chain transfer agents or excessive solvents. Simultaneously, hydrophilic sodium carboxylate groups are chemically bonded to the side groups of the PVA molecular chain, completing the copolymerization modification of PVA. Then, through alkaline alcoholysis in step S2, the degree of alcoholysis of the copolymer is controlled within the range of 70%–92%, avoiding complete alcoholysis which would lead to the formation of a crystalline structure due to numerous hydrogen bonds between molecular chains, reducing cold water solubility. Furthermore, a suitable amount of hydrophobic acetate ions is retained in the product to maintain the uniformity of the degree of alcoholysis. The synergistic effect of these two steps ultimately yields ultra-low degree-of-polymerization modified PVA with a degree of polymerization ≤300 that can be completely dissolved in cold water at 10°C for 4–7 minutes. This solves the technical problem of existing technologies that struggle to simultaneously achieve ultra-low degree of polymerization and excellent water solubility of PVA without relying on toxic additives and excessive solvents.

[0019] 2. This invention selects unsaturated carboxylates as comonomers, allowing sodium carboxylate groups to be chemically bonded to the side groups of the PVA molecular chain. The resulting modified PVA molecular structure contains both hydroxyl groups and strongly hydrophilic sodium carboxylate groups, which significantly improves the water solubility of the polymer under low-temperature conditions.

[0020] 3. Compared with existing technologies that use unsaturated carboxylic acids (such as itaconic acid and maleic acid) in the form of free acids for copolymerization modification, the present invention uses carboxylates, which has a fundamental technical advantage. Free acids mainly exist in the molecular state in the polymerization system, making it difficult to effectively induce chain transfer; while carboxylates can be partially ionized in methanol solvent. The carboxylate ions exert a significant electrostatic repulsion effect on the growing chain radicals, while sodium ions increase the steric hindrance of the side groups. The two synergistically enhance the chain transfer efficiency, resulting in a more significant reduction in the degree of polymerization with the same monomer dosage.

[0021] 4. This invention achieves precise dynamic control of chain transfer strength by adding unsaturated carboxylate monomers in two batches, ensuring that the degree of polymerization is stably controlled in an ultra-low range and the molecular weight distribution is narrow. At the same time, by controlling the alcoholysis process, the degree of alcoholysis is controlled in the optimal range, and an appropriate amount of hydrophobic acetate groups are retained on the molecular chain, which effectively inhibits the crystallization tendency caused by a large number of hydrogen bonds between molecular chains. Finally, ultra-low degree of polymerization modified PVA can be completely dissolved in cold water at 10°C for 4 to 7 minutes. Attached Figure Description

[0022] Figure 1 The above is the 1H NMR spectrum of the ultra-low degree of polymerization modified PVA obtained in Example 1 of this invention; Figure 2 The graph shows the cold water solubility test results of ultra-low degree of polymerization modified PVA obtained in Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0023] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0024] It should be noted that the methanol dosage of 80-200 parts by mass in this invention refers to the amount of methanol added to the reactor before the polymerization reaction. When the unsaturated carboxylate monomer is added in two batches, the 20 parts of methanol used to dissolve the second batch of monomer and added dropwise after the initiator is an additional solvent and is not included in the aforementioned 80-200 parts by mass range. In Example 5, the additional 20 parts of methanol solvent does not contain unsaturated carboxylate monomer, and its purpose is to maintain the same total solvent dosage as in the example with two batches added, so as to eliminate the interference of the difference in total solvent dosage on the comparison of the degree of polymerization.

[0025] Example 1 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.52 parts of sodium acrylate were added to a reactor, and the reactor was heated to 62°C. Seven parts of a 0.6% methanol solution of tert-butyl peroxypentanoate initiator were added to the reactor in one step, while another mixture of 1.68 parts of sodium acrylate dissolved in 20 parts of methanol solvent was continuously added dropwise. The reactor temperature was maintained at 62°C, and polymerization was carried out for 5 hours. After the reaction was complete, two parts of a 0.2% methanol solution of sorbic acid were added to terminate the reaction. After the monomer removal was complete, methanol was added again to prepare a resin solution with a solid content of 30%. Five parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 60 minutes, followed by drying at 70°C to obtain the modified PVA resin. The degree of alcoholysis of this polymer was measured to be 89.6%, the degree of polymerization to be 170, and the molecular weight distribution to be 1.39. The polymer was completely dissolved by stirring at a low temperature of 10°C for 4 minutes.

[0026] The modified PVA resin obtained in Example 1 was analyzed by ¹H NMR, and its ¹H NMR spectrum is as follows: Figure 1 As shown. By Figure 1 It can be seen that the peak at 1.9–2.1 ppm represents the proton peak of the un-hydrolyzed methyl hydrogen (integrated area denoted as A), and the peak at 1.2–1.9 ppm represents the proton peak of the methylene group in the polymer backbone (integrated area denoted as 200). The degree of hydrolysis is calculated using the following formula: Degree of hydrolysis (%) = [1 - (A / 3) / 100] × 100%. The degree of hydrolysis of the modified PVA obtained in Example 1 is calculated to be 89.6%. The degree of hydrolysis of the remaining examples and comparative examples was determined using the same method, and the results are shown in Table 3.

[0027] Example 2 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.33 parts of sodium acrylate were added to a reactor, and the reactor was heated to 62°C. Eight parts of a 0.6% methanol solution of tert-butyl peroxypentanoate initiator were added simultaneously, followed by the continuous dropwise addition of a mixed solution of 1.87 parts of sodium acrylate dissolved in 20 parts of methanol solvent. The reactor temperature was maintained at 62°C, and polymerization was carried out for 5.5 hours. After the reaction was complete, three parts of a 0.2% methanol solution of sorbic acid were added to terminate the reaction. After monomer removal, methanol was added again to prepare a resin solution with a solid content of 30%. Four and a half parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 50 minutes, followed by drying at 70°C to obtain the modified PVA resin. The polymer was found to have a degree of alcoholysis of 87.2%, a degree of polymerization of 220, and a molecular weight distribution of 1.40. The polymer was completely dissolved after stirring at a low temperature of 10°C for 6 minutes.

[0028] Example 3 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.37 parts of sodium acrylate were added to a reactor, and the reactor was heated to 62°C. Three parts of a 1.2% methanol solution of tert-butyl peroxypentanoate initiator were added simultaneously, followed by the continuous dropwise addition of a mixed solution of 1.83 parts of sodium acrylate dissolved in 20 parts of methanol solvent. The reactor temperature was maintained at 62°C, and polymerization was carried out for 5.5 hours. After the reaction was complete, two parts of a 0.4% methanol solution of sorbic acid were added to terminate the reaction. After monomer removal, methanol was added again to prepare a resin solution with a solid content of 30%. Six parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 30 minutes, followed by drying at 70°C to obtain the modified PVA resin. The polymer was found to have a degree of alcoholysis of 86.8%, a degree of polymerization of 180, and a molecular weight distribution of 1.68. The polymer was completely dissolved after stirring at a low temperature of 10°C for 7 minutes.

[0029] Example 4 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.46 parts of sodium acrylate were added to a reactor, and the reactor was heated to 62°C. Three parts of a 1.5% methanol solution of tert-butyl peroxypentanoate initiator were added to the reactor in one step, while another mixture of 1.74 parts of sodium acrylate dissolved in 20 parts of methanol solvent was continuously added dropwise. The reactor temperature was maintained at 62°C, and polymerization was carried out for 5 hours. After the reaction was complete, two parts of a 0.4% methanol solution of sorbic acid were added to terminate the reaction. After the monomer removal was complete, methanol was added again to prepare a resin solution with a solid content of 30%. 5.5 parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 45 minutes, followed by drying at 65°C to obtain the modified PVA resin. The degree of alcoholysis of this polymer was measured to be 86.9%, the degree of polymerization to be 170, and the molecular weight distribution to be 1.81. The polymer was completely dissolved after stirring at a low temperature of 10°C for 7 minutes.

[0030] Example 5 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 2.25 parts of sodium acrylate were added to a reactor, and the reactor was heated to 62°C. Five parts of a 0.9% methanol solution of tert-butyl peroxypentanoate initiator were added to the reactor in one go, and 20 parts of methanol solvent were added to maintain a total solvent ratio comparable to that in the batch addition example. The reactor temperature was maintained at 62°C, and polymerization was carried out for 6 hours. After the reaction was complete, one part of a 0.4% methanol solution of hydroquinone was added to terminate the reaction. After the monomer removal was complete, methanol was added again to prepare a resin solution with a solid content of 30%. 5.5 parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 35 minutes, followed by drying at 80°C to obtain the modified PVA resin. The degree of alcoholysis of this polymer was measured to be 89.2%, the degree of polymerization to be 160, and the molecular weight distribution to be 1.76. The polymer was completely dissolved after stirring at a low temperature of 10°C for 4 minutes.

[0031] Example 6 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.65 parts of sodium maleate were added to a reactor, and the reactor was heated to 62°C. Three parts of a 1.2% methanol solution of tert-butyl peroxypentanoate initiator were added to the reactor in one step, followed by the continuous dropwise addition of a mixed solution containing 2.59 parts of sodium maleate dissolved in 20 parts of methanol solvent. The reactor temperature was maintained at 62°C, and polymerization was carried out for 6 hours. After the reaction was complete, four parts of a 0.2% methanol solution of hydroquinone were added to terminate the reaction. After monomer removal, methanol was added again to prepare a resin solution with a solid content of 30%. Five parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 50 minutes, followed by drying at 70°C to obtain the modified PVA resin. The degree of alcoholysis of this polymer was measured to be 88.4%, the degree of polymerization to be 240, and the molecular weight distribution to be 1.30. The polymer was completely dissolved by stirring at a low temperature of 10°C for 4 minutes.

[0032] Example 7 400 parts of vinyl acetate monomer, 160 parts of methanol solvent, and 0.23 parts of sodium itaconic acid were added to a reactor, and the reactor was heated to 62°C. Three parts of a 1.8% methanol solution of tert-butyl peroxypentanoate initiator were added simultaneously, followed by the continuous dropwise addition of a mixed solution of 1.11 parts of sodium itaconic acid dissolved in 20 parts of methanol solvent. The reactor temperature was maintained at 62°C, and polymerization was carried out for 5 hours. After the reaction was complete, five parts of a 0.2% methanol solution of hydroquinone were added to terminate the reaction. After monomer removal, methanol was added again to prepare a resin solution with a solid content of 30%. Three parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 60 minutes, followed by drying at 75°C to obtain the modified PVA resin. The polymer was found to have a degree of alcoholysis of 87.6%, a degree of polymerization of 240, and a molecular weight distribution of 1.54. The polymer was completely dissolved after stirring at a low temperature of 10°C for 5 minutes.

[0033] Comparative Example 1 400 parts of vinyl acetate monomer and 1600 parts of methanol solvent were added to a reactor, and the reactor was heated to 62°C. Then, 12 parts of a 0.6% methanol solution of tert-butyl peroxypentanoate initiator were added simultaneously, and the reactor temperature was maintained at 62°C for 6 hours of polymerization. After the reaction was complete, 3 parts of a 0.2% methanol solution of sorbic acid were added to terminate the reaction. After monomer removal was complete, methanol was added again to prepare a resin solution with a solid content of 30%. 3 parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 50 minutes, followed by drying at 60°C to obtain the modified PVA resin. The polymer was found to have a degree of alcoholysis of 88.6%, a degree of polymerization of 350, and a molecular weight distribution of 2.15. The polymer was essentially dissolved after stirring at a low temperature of 10°C for 12 minutes.

[0034] Comparative Example 2 400 parts of vinyl acetate monomer and 160 parts of methanol solvent were added to a reactor, and the reactor was heated to 62°C. Seven parts of a 0.6% methanol solution of tert-butyl peroxypentanoate initiator were added simultaneously, followed by a continuous dropwise addition of another 20 parts of methanol solvent, maintaining the reactor temperature at 62°C for 5 hours of polymerization. After the reaction was complete, two parts of a 0.2% methanol solution of sorbic acid were added to terminate the reaction. After monomer removal, methanol was added again to prepare a resin solution with a solid content of 30%. Five parts of a 40 g / L NaOH methanol solution were added to the resin solution, and the reaction was carried out at 50°C for 30 minutes, followed by drying at 65°C to obtain the modified PVA resin. The polymer was found to have a degree of alcoholysis of 88.2%, a degree of polymerization of 1760, and a molecular weight distribution of 1.98. It basically dissolved at a low temperature of 10°C for 25 minutes.

[0035] The polymerization process parameters and alcoholysis process parameters of Examples 1-7 and Comparative Examples 1-2 are summarized in Table 1 and Table 2, respectively.

[0036] Table 1. Polymerization parameters in the preparation process of ultra-low degree of polymerization modified PVA

[0037] Table 2. Alcohololysis parameters during the preparation of ultra-low degree of polymerization modified PVA

[0038] The modified PVA obtained from Examples 1 to 7 and Comparative Examples 1 and 2 were subjected to performance tests, and the test items are as follows: (1) Determination of degree of alcoholysis: The degree of alcoholysis of the modified PVA resin was analyzed using a Bruker AVⅡ-400MHz nuclear magnetic resonance spectrometer; (2) Degree of polymerization determination: The degree of polymerization test of the preparation method of low degree of polymerization modified polyvinyl alcohol provided by the present invention refers to GB / T 12010.2-2010 Plastics Polyvinyl Alcohol Materials (PVAL) Part 2: Test methods for performance determination.

[0039] (3) Determination of molecular weight distribution: The molecular weight distribution of the modified PVA resin was determined by gel permeation chromatography.

[0040] (4) Test method for water solubility of particles: The modified PVA particles were separated by a 40-mesh sieve. One part of the particles that passed through the sieve was mixed with 25 parts of water and dissolved at a constant temperature of 10℃ and a speed of 200 rpm.

[0041] The test results are shown in Table 3. Figure 2 As shown: Table 3

[0042] The solubility test results of the modified PVA obtained in Example 1 and Comparative Example 1 in cold water at 10°C are as follows: Figure 2 As shown. By Figure 2 As can be seen, the modified PVA obtained in Example 1 completely dissolved within 4 minutes, and the solution was transparent and homogeneous; while Comparative Example 1 only partially dissolved within 12 minutes, and a small number of undissolved particles were still visible in the solution. These results clearly demonstrate that the ultra-low degree of polymerization modified PVA prepared in this invention exhibits excellent rapid dissolution performance under low-temperature conditions, while Comparative Example 1, due to its higher degree of polymerization, longer molecular chains, more severe interchain entanglement, and lack of modification, showed a significantly reduced dissolution rate in cold water. Figure 2 The solubility test results corroborate the water solubility data in Table 3, further demonstrating the synergistic effect of the present invention in reducing the degree of polymerization and improving water solubility through unsaturated carboxylate copolymerization modification.

[0043] As shown in Table 3, the modified PVA prepared in Examples 1-7 all had a degree of polymerization ranging from 160 to 240, which is considered ultra-low degree of polymerization (≤300). Their degree of alcoholysis ranged from 86.8% to 89.6%, and their molecular weight distribution was narrow, ranging from 1.30 to 1.81, indicating good product uniformity. Regarding solubility in cold water at 10°C, Examples 1-7 all dissolved completely within 4-7 minutes, demonstrating excellent low-temperature rapid water solubility.

[0044] The main difference between Comparative Example 1 and Example 1 is that no unsaturated carboxylate monomers were added, and the amount of methanol was increased significantly to 10 times the conventional amount, while the other conditions remained the same as in the Example. The results of Comparative Example 1 show that simply increasing the amount of methanol has extremely low efficiency in controlling the degree of polymerization. Even with 10 times the amount of methanol as in the Example, the degree of polymerization could not be reduced to an ultra-low range, and the product had a wide molecular weight distribution, poor product uniformity, and poor solubility in cold water, completely failing to achieve the comprehensive performance level of the Examples of the Invention.

[0045] Comparative Example 2, without the addition of any unsaturated carboxylate monomers, exhibited a degree of polymerization as high as 1760 and a cold water dissolution time of up to 25 minutes. This further demonstrates that even with the same methanol dosage and polymerization process as in the examples, it is impossible to effectively reduce the degree of polymerization without the addition of unsaturated carboxylate monomers.

[0046] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for preparing ultra-low degree of polymerization modified PVA, characterized in that, Includes the following steps: S1. Mix vinyl acetate monomer, solvent and unsaturated carboxylate monomer, carry out polymerization reaction under the action of initiator, add terminator to obtain polyvinyl acetate resin. The unsaturated carboxylate monomer is added all at once before the polymerization reaction, or added in two batches during the polymerization reaction; S2. Polyvinyl acetate resin is subjected to alcoholysis under alkaline conditions and dried to obtain ultra-low degree of polymerization modified PVA.

2. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the unsaturated carboxylate monomer is any one or more of sodium acrylate, sodium methacrylate, sodium crotonate, sodium maleate, sodium fumarate, and sodium itaconic acid.

3. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the solvent is methanol, and based on 400 parts by mass of vinyl acetate monomer, 80 to 200 parts of methanol are added before the polymerization reaction.

4. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, when the unsaturated carboxylate monomer is added all at once, the amount added is 2.0 to 2.5 parts based on 400 parts by mass of vinyl acetate monomer.

5. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the unsaturated carboxylate monomer is added in two batches as follows: The first batch of unsaturated carboxylate monomers is added before the polymerization reaction. The second batch of unsaturated carboxylate monomers is dissolved in 20 parts of methanol to form a mixed solution and is added dropwise after the initiator is added. Based on 400 parts by mass of vinyl acetate monomer, the amount of the first batch of unsaturated carboxylate monomers added is 0.1 to 0.7 parts, the amount of the second batch of unsaturated carboxylate monomers added is 0.5 to 3 parts, and the dropwise addition time is 2 to 6 hours.

6. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the initiator is a methanol solution of tert-butyl peroxypentanoate with a concentration of 0.2% to 2.0% and an addition amount of 1 to 15 parts.

7. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the polymerization reaction is carried out at a temperature of 55–65°C.

8. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S1, the terminator is any one of hydroquinone, 1,4-benzoquinone, and sorbic acid. The terminator is dissolved in methanol to prepare a terminator methanol solution. The concentration of the terminator methanol solution is 0.2% to 2.0%, and the amount added is 1 to 5 parts.

9. The method for preparing ultra-low degree of polymerization modified PVA as described in claim 1, characterized in that, In step S2, the specific steps for alcoholysis under alkaline conditions are as follows: polyvinyl acetate resin is added to methanol to prepare a resin solution with a solid content of 20% to 40%, and then 3 to 6 parts of NaOH methanol solution with a concentration of 20 to 60 g / L are added for alcoholysis. The alcoholysis temperature is 40 to 60°C and the alcoholysis time is 20 to 60 min.

10. A modified PVA with ultra-low degree of polymerization, characterized in that, It is prepared by the method for preparing ultra-low degree of polymerization modified PVA according to any one of claims 1-9.

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