Preparation method of low-molecular-weight acrylate polymer

Abstract

The invention belongs to the technical field of high polymer materials, and particularly relates to a preparation method of a low-molecular-weight acrylate polymer. According to the preparation method, macromolecules prepared from terminal alkenyl polyethylene glycol and hydrogen bromide through anti-Markov addition reaction are taken as the initiator, so that the prepared acrylate polymer is relatively low in molecular weight and narrow in molecular weight distribution, and the prepared acrylate polymer can improve the uniformity of PVC foaming; the molecular weight and molecular weight distribution of the prepared acrylate polymer are controlled by adjusting the molar ratio of the reaction monomer to the initiator to the catalyst to the pentamethyldiethylenetriamine; by controlling process parameters such as reaction temperature and reaction time, the initiation efficiency is effectively improved, the molecular weight distribution width is reduced, the impurity removal step is performed after the reaction is finished, and the purity of the product is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and particularly relates to a method for preparing a low-molecular-weight acrylic ester polymer. Background Art

[0002] Foamed polyvinyl chloride (PVC) plastic has characteristics similar to natural wood, and has the characteristics of low density, good weather resistance, good chemical stability, high impact strength, and high toughness. Acrylic polymers, as foaming regulating agents that adjust the foaming properties of foamed polyvinyl chloride, have an important impact on the application of foamed polyvinyl chloride.

[0003] Polyacrylate polymers are a class of high-molecular-weight polymers synthesized primarily from acrylic or methacrylate esters. These polymers exhibit excellent electrical insulation, mechanical properties, weather resistance, and acid and alkali resistance. Copolymers of acrylic monomers also exhibit excellent transparency, plasticity, weather resistance, and good water and chemical resistance. The molecular weight of acrylic polymers is a key performance indicator. Low-molecular-weight acrylic polymers exhibit low viscosity and fluidity, high reactivity, and good compatibility, film-forming properties, and adhesion. Low-molecular-weight PVC foaming-regulating acrylic polymer chains can migrate between PVC molecular chains, reducing the van der Waals forces between PVC molecules, improving heat and shear force transfer efficiency, accelerating the plasticization process of PVC resin, reducing plasticization time, and achieving more uniform plasticization.

[0004] However, the following problems still exist in the preparation process of low molecular weight acrylic polymers: 1. Errors in chain transfer agent dosage or unstable initiator decomposition result in high or low molecular weight of acrylic polymers, and the molecular weight distribution is too wide, making it difficult to accurately control; 2. Residual inhibitors in the polymerization process inhibit the polymerization reaction, or insufficient inert gas protection causes oxygen to combine with free radicals to form peroxyl radicals, resulting in low monomer conversion rate.

[0005] Chinese patent application publication number CN117551228A discloses a method for preparing a low-molecular-weight acrylic ester polymer, comprising the following steps: a) mixing an acrylic ester monomer, an initiator, and a solvent under an inert atmosphere at a temperature of -10°C to 5°C to obtain a mixed solution; b) adding the same solvent as in step a) to a reaction flask under an inert atmosphere and heating to reflux; and c) adding the mixed solution from step a) dropwise to the reaction flask from step b) to react to obtain a low-molecular-weight acrylic ester polymer. This method transforms a one-pot reaction into a process where the reaction solution is added dropwise to a reaction flask without changing the concentration and ratio of the various substances in the mother liquor, the reaction temperature, or the addition of any chain transfer agents. The molecular weight of the acrylate polymer can be controlled by varying the addition rate. However, when the low-temperature initiator in step a) is added dropwise to the reflux reaction in step b), it undergoes explosive decomposition due to the sudden temperature rise, instantly generating a large number of free radicals. This excessively high local initiator concentration results in a broad and unstable molecular weight distribution of the synthesized acrylate polymer. Simultaneously, the solvent continues to evaporate during reflux, but the low-boiling-point acrylate monomers tend to co-evaporate with the solvent, resulting in a low monomer conversion rate. Furthermore, no chain transfer agent is added during the polymerization reaction, relying solely on solvent chain transfer at high temperatures. This results in a high molecular weight for the synthesized acrylate polymer, making it impossible to precisely adjust. Summary of the Invention

[0006] In order to solve the technical problems of wide molecular weight distribution and low monomer conversion rate existing in the above-mentioned related technologies, the present invention provides a method for preparing a low molecular weight acrylic ester polymer.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows: A method for preparing a low molecular weight acrylic polymer comprises the following steps: S1: adding anhydrous ethanol, reaction monomers and initiators into a reactor, stirring evenly, and deoxygenating to obtain a mixed solution A; S2: Under the protection of inert gas, continue to add the catalyst and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 70-80°C, and stir to react for 2-4 hours to obtain a primary product; S3: introducing oxygen into the primary product obtained in step S2, washing, separating the liquids, dripping the organic phase into methanol, standing for 1-2 hours, filtering, and drying to obtain a low molecular weight acrylic acid ester polymer; The initiator is prepared by reacting terminal olefin polyethylene glycol with hydrogen bromide through anti-Markovnikov addition reaction.

[0008] The present invention can obtain an acrylic polymer with low molecular weight and narrow molecular weight distribution through the above-mentioned technical solution. The macromolecule prepared by the anti-Markovnikov addition reaction of terminal olefin polyethylene glycol and hydrogen bromide is used as an initiator, which can achieve accurate control of the polymerization process, controllable molecular weight, narrow molecular weight distribution, and the obtained acrylic polymer chain segments have polar end groups, which are more evenly dispersed in PVC, and can improve the uniformity of PVC foaming. In addition, the reaction temperature and reaction time during the reaction are matched with the active system of the initiator, which can not only ensure the initiation efficiency and avoid initiation hysteresis, but also suppress the side reaction caused by chain transfer, and avoid the widening of the distribution of acrylic polymers caused by high conversion rate. After the reaction is completed, washing and liquid separation are used to remove water-soluble impurities, and then unreacted monomers and small molecules are further removed through methanol precipitation to improve the purity of the low molecular weight acrylic polymer.

[0009] Furthermore, the preparation method of the initiator is as follows: under the protection of inert gas, allyl polyethylene glycol and dichloromethane are mixed, stirred and dissolved, azobisisobutyronitrile is added, stirred and dissolved, the temperature is raised to 50-60°C in a water bath, an aqueous hydrogen bromide solution is added dropwise while stirring, the reaction is kept warm for 4-6 hours, cooled, a saturated sodium bicarbonate solution is added to a pH value of 6.8-7.5, stirred for 10-15 minutes, allowed to stand and separate, the organic phase is collected, dried with anhydrous magnesium sulfate, added dropwise to ether, stirred to produce a white precipitate, filtered, and dried to obtain the initiator.

[0010] The present invention uses allyl polyethylene glycol and hydrogen bromide to undergo an anti-Markovnikov addition reaction under the initiation of azobisisobutyronitrile to prepare a bromine-terminated polyethylene glycol as an initiator. The bromine atom is directly connected to the methylene group, and the terminal group has high activity, which can effectively initiate the chain growth of acrylic ester monomers and has high initiation efficiency. The molecular backbone of the initiator is highly hydrophilic, and the terminal bromine atom and the polyacrylate chain segment formed after initiation are hydrophobic. Therefore, the initiator is amphiphilic and has good compatibility with acrylic ester monomers, which can avoid the problem of localized initiation unevenness caused by phase separation during the polymerization process.

[0011] Furthermore, in the preparation method of the initiator, the molar ratio of allyl polyethylene glycol, azobisisobutyronitrile, and hydrogen bromide is 17-19:1-2:20-25.

[0012] In the present invention, excess hydrogen bromide can ensure complete reaction of double bonds in allyl polyethylene glycol. Excessive use of azobisisobutyronitrile may induce free radical polymerization side reactions during the reaction, resulting in reduced purity of the initiator.

[0013] Furthermore, the mass percentage of the hydrogen bromide aqueous solution in the initiator preparation method is 40%-45%.

[0014] Furthermore, the catalyst in step S2 is cuprous bromide or cuprous chloride.

[0015] Furthermore, the deoxygenation in step S1 is specifically as follows: freezing the mixed solution with liquid nitrogen to a solid state, evacuating the mixture for 10-15 minutes, introducing nitrogen to thaw, and repeating the process 3-5 times.

[0016] In the present invention, oxygen in the mixed solution can be removed by freezing a mixed solution of anhydrous ethanol, a reaction monomer and an initiator with liquid nitrogen and then thawing it with nitrogen. In the repeated process, oxygen in the mixed solution can be effectively removed, thereby preventing the catalyst from being oxidized during the reaction, resulting in catalyst deactivation and affecting the molecular weight distribution of the reaction product.

[0017] Furthermore, the reaction monomers in step S1 are composed of methyl acrylate, methyl methacrylate and methacrylic acid in a mass ratio of 3-7:5-9:1-3.

[0018] Furthermore, the molar ratio of the reaction monomer and the initiator in step S1 to the catalyst and pentamethyldiethylenetriamine in step S2 is 48-52:5-7:1:1.

[0019] The molecular weight of the resulting acrylic polymer is controlled by adjusting the molar ratio of the reaction monomer, initiator, catalyst, and pentamethyldiethylenetriamine. The molar ratio of the reaction monomer to the initiator is crucial for determining the molecular weight of the acrylic polymer. Excessive amounts of reaction monomers and sufficient chain growth lead to increased product molecular weight, increased system viscosity in the later stages of the reaction, and uneven reaction, resulting in a broadened molecular weight distribution. Excessive amounts of initiator lead to insufficient chain growth, decreased initiation efficiency, and increased probability of diradical termination between short-chain free radicals, resulting in a broadened molecular weight distribution. The ratio of catalyst to initiator and pentamethyldiethylenetriamine is crucial for influencing the catalytic performance of the catalyst. Excessive amounts of the catalyst and pentamethyldiethylenetriamine reduce the polymerization rate and the catalytic activity of the catalyst. Insufficient amounts of the catalyst and pentamethyldiethylenetriamine lead to decreased monomer conversion, aggravated side reactions such as diradical termination and chain transfer, and a broadened molecular weight distribution of the product.

[0020] Furthermore, the molar ratio of the reaction monomer and the initiator in step S1 to the catalyst and pentamethyldiethylenetriamine in step S2 is 51:6:1:1.

[0021] Furthermore, the washing in step S3 is washing with dilute hydrochloric acid, the washing times are 3-5 times, the concentration of dilute hydrochloric acid is 0.8-1.2 mol / L; the drying temperature is 40-45° C., and the drying time is 24-30 h.

[0022] Compared with the prior art, the preparation method of the low molecular weight acrylic polymer provided by the present invention has the following technical advantages: (1) The present invention uses a macromolecule prepared by the anti-Markovnikov addition reaction of terminal olefin polyethylene glycol and hydrogen bromide as an initiator, so that the molecular weight of the prepared acrylic polymer is low and the molecular weight distribution is narrow. In addition, the prepared acrylic polymer can improve the uniformity of PVC foaming; (2) In the present invention, the molecular weight and molecular weight distribution of the prepared acrylic polymer are controlled by adjusting the molar ratio of the reaction monomer, initiator, catalyst and pentamethyldiethylenetriamine; (3) The present invention effectively improves the initiation efficiency and reduces the molecular weight distribution width by controlling process parameters such as reaction temperature and reaction time. After the reaction is completed, an impurity removal step is performed to improve the purity of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 SEC spectrum of the acrylic acid ester polymer prepared in Example 1; Figure 2 SEC spectrum of the acrylic acid ester polymer prepared in Example 2; Figure 3 SEC spectrum of the acrylic acid ester polymer prepared in Example 3; Figure 4 SEC spectrum of the acrylic acid ester polymer prepared in Example 4; Figure 5 SEC spectrum of the acrylic acid ester polymer prepared in Comparative Example 1; Figure 6 SEC spectrum of the acrylic acid ester polymer prepared in Comparative Example 2; Figure 7 SEC spectrum of the acrylic acid ester polymer prepared in Comparative Example 3; Figure 8 This is the SEC spectrum of the acrylic acid ester polymer prepared in Comparative Example 4; Figure 9 This is the NMR spectrum of the initiator prepared in Preparation Example 4. DETAILED DESCRIPTION

[0024] The following will be further described in conjunction with specific embodiments, but the present invention is not limited to the following embodiments. Those skilled in the art may make various modifications based on the basic concept of the present invention, but as long as they do not depart from the basic concept of the present invention, they are all within the scope of the present invention.

[0025] In this embodiment, before the initiator is prepared, allyl polyethylene glycol is precipitated with ether for 2.5 hours and then dried in a vacuum drying oven at 40° C. for 24 hours. The hydrogen bromide aqueous solution and dichloromethane are dried with anhydrous sodium sulfate and then deoxygenated with nitrogen for 30 minutes.

[0026] Preparation Example 1 The preparation method of the initiator is as follows: under nitrogen protection, 17 mmol of allyl polyethylene glycol and 100 mL of dichloromethane are added to a three-necked flask, and the mixture is stirred until completely dissolved. 1 mmol of azobisisobutyronitrile is added and the mixture is stirred until completely dissolved. The water bath is heated to 50° C., and a 40% by mass aqueous solution of hydrogen bromide (20 mmol of hydrogen bromide) is added dropwise while stirring at a speed of 300 rpm. The mixture is kept warm for 4 hours and then cooled to room temperature. A saturated sodium bicarbonate solution is slowly added to a pH value of 6.8, stirred for 10 minutes, allowed to stand for stratification, the organic phase is collected, the aqueous phase is extracted twice with 30 mL of dichloromethane, the organic phase is combined with the extracted organic phase, dried with anhydrous magnesium sulfate for 2 hours, and then dropped into excess ether and stirred to produce a white precipitate. The solid is filtered, collected, and placed in a vacuum desiccant for drying at 40° C. for 24 hours to obtain the initiator.

[0027] Preparation Example 2 The preparation method of the initiator is as follows: under nitrogen protection, 19 mmol of allyl polyethylene glycol and 100 mL of dichloromethane are added to a three-necked flask, and stirred until completely dissolved. 2 mmol of azobisisobutyronitrile is added and stirred until completely dissolved. The water bath is heated to 60° C., and a 45% by mass aqueous solution of hydrogen bromide (25 mmol of hydrogen bromide) is added dropwise while stirring at a speed of 500 rpm. The reaction is kept warm for 6 hours, cooled to room temperature, and saturated sodium bicarbonate solution is slowly added to a pH value of 7.5. The mixture is stirred for 15 minutes, allowed to stand for stratification, and the organic phase is collected. The aqueous phase is extracted twice with 40 mL of dichloromethane, and the organic phase and the extracted organic phase are combined, dried with anhydrous magnesium sulfate for 3 hours, and then dropped into excess ether. A white precipitate is generated by stirring, and the solid is collected by filtration and placed in a vacuum desiccant at 45° C. for 24 hours to obtain the initiator.

[0028] Preparation Example 3 The preparation method of the initiator is as follows: under nitrogen protection, 18 mmol of allyl polyethylene glycol and 100 mL of dichloromethane are added to a three-necked flask, and the mixture is stirred until completely dissolved. 1.4 mmol of azobisisobutyronitrile is added and the mixture is stirred until completely dissolved. The water bath is heated to 55° C., and a 43% by mass aqueous solution of hydrogen bromide (22 mmol of hydrogen bromide) is added dropwise while stirring at 400 rpm. The mixture is kept warm for 5 hours and then cooled to room temperature. A saturated sodium bicarbonate solution is slowly added to a pH value of 7.0, stirred for 12 minutes, allowed to stand for stratification, the organic phase is collected, the aqueous phase is extracted twice with 35 mL of dichloromethane, the organic phase is combined with the extracted organic phase, dried over anhydrous magnesium sulfate for 2.5 hours, and then dropped into excess ether and stirred to produce a white precipitate. The solid is filtered, collected, and placed in a vacuum desiccant for drying at 43° C. for 24 hours to obtain the initiator.

[0029] Preparation Example 4 The preparation method of the initiator is as follows: under nitrogen protection, 18.2 mmol of allyl polyethylene glycol and 100 mL of dichloromethane are added to a three-necked flask, and the mixture is stirred until completely dissolved. 1.6 mmol of azobisisobutyronitrile is added and the mixture is stirred until completely dissolved. The water bath is heated to 58° C., and a 42% by mass aqueous solution of hydrogen bromide (23 mmol of hydrogen bromide) is added dropwise while stirring at a speed of 400 rpm. The mixture is kept warm for 5.2 hours, cooled to room temperature, and a saturated sodium bicarbonate solution is slowly added to a pH value of 7.0. The mixture is stirred for 13 minutes, allowed to stand for stratification, and the organic phase is collected. The aqueous phase is extracted twice with 35 mL of dichloromethane, and the organic phase is combined with the extracted organic phase, dried over anhydrous magnesium sulfate for 2.6 hours, and dropped into excess ether. The mixture is stirred to produce a white precipitate, filtered, the solid is collected, and placed in a vacuum desiccant for drying at 43° C. for 24 hours to obtain the initiator.

[0030] Example 1 A method for preparing a low molecular weight acrylic polymer comprises the following steps: S1: Add anhydrous ethanol, reaction monomers and initiator to a reactor, stir evenly, freeze to solid with liquid nitrogen, evacuate for 10 minutes, thaw with nitrogen, and repeat three times to obtain mixed solution A; S2: Under nitrogen protection, add cuprous chloride and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 70°C, and stir at 300 rpm for 2 h to obtain a primary product; S3: The primary product obtained in step S2 was passed through oxygen for 5 minutes, washed three times with 0.8 mol / L dilute hydrochloric acid, separated, the organic phase was dropped into excess methanol for precipitation and allowed to stand for 1 hour, filtered, and the solid was placed in a vacuum drying oven at 40°C for 24 hours to obtain a low molecular weight acrylic ester polymer.

[0031] The initiator described in this embodiment was prepared according to Preparation Example 1. The reaction monomers described in step S1 were composed of methyl acrylate, methyl methacrylate, and methacrylic acid in a mass ratio of 3:5:1; the molar ratio of the reaction monomers and initiator described in step S1 and the catalyst and pentamethyldiethylenetriamine described in step S2 was 48:5:1:1.

[0032] Example 2 A method for preparing a low molecular weight acrylic polymer comprises the following steps: S1: Add anhydrous ethanol, reaction monomers and initiator to a reactor, stir evenly, freeze to solid with liquid nitrogen, evacuate for 15 minutes, thaw with nitrogen, and repeat 5 times to obtain mixed solution A; S2: Under nitrogen protection, add cuprous bromide and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 80° C., and stir at 500 rpm for 4 h to obtain a primary product; S3: The primary product obtained in step S2 was passed through oxygen for 10 minutes, washed five times with 1.2 mol / L dilute hydrochloric acid, separated, the organic phase was dropped into excess methanol for precipitation and allowed to stand for 2 hours, filtered, and the solid was placed in a vacuum drying oven at 45°C for 30 hours to obtain a low molecular weight acrylic ester polymer.

[0033] The initiator described in this embodiment was prepared according to Preparation Example 2. The reaction monomers described in step S1 were composed of methyl acrylate, methyl methacrylate, and methacrylic acid in a mass ratio of 7:9:3; the molar ratio of the reaction monomers and initiator described in step S1 and the catalyst and pentamethyldiethylenetriamine described in step S2 was 52:7:1:1.

[0034] Example 3 A method for preparing a low molecular weight acrylic polymer comprises the following steps: S1: Add anhydrous ethanol, reaction monomers and initiator to a reactor, stir evenly, freeze to solid with liquid nitrogen, evacuate for 13 minutes, thaw with nitrogen, and repeat 4 times to obtain mixed solution A; S2: Under nitrogen protection, add cuprous bromide and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 75°C, and stir at 400 rpm for 3.2 hours to obtain a primary product; S3: The primary product obtained in step S2 was passed through oxygen for 8 minutes, washed four times with 1.0 mol / L dilute hydrochloric acid, separated, the organic phase was dropped into excess methanol for precipitation and allowed to stand for 1.5 hours, filtered, and the solid was placed in a vacuum drying oven at 43°C for 28 hours to obtain a low molecular weight acrylic ester polymer.

[0035] The initiator described in this embodiment was prepared according to Preparation Example 3. The reaction monomers described in step S1 were composed of methyl acrylate, methyl methacrylate, and methacrylic acid in a mass ratio of 5:8:2; the molar ratio of the reaction monomers and initiator described in step S1 and the catalyst and pentamethyldiethylenetriamine described in step S2 was 49:6:1:1.

[0036] Example 4 A method for preparing a low molecular weight acrylic polymer comprises the following steps: S1: Add anhydrous ethanol, reaction monomers and initiator to a reactor, stir evenly, freeze to solid with liquid nitrogen, evacuate for 13 minutes, thaw with nitrogen, and repeat 4 times to obtain mixed solution A; S2: Under nitrogen protection, add cuprous bromide and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 75°C, and stir at 400 rpm for 3.2 hours to obtain a primary product; S3: The primary product obtained in step S2 was passed through oxygen for 8 minutes, washed four times with 1.0 mol / L dilute hydrochloric acid, separated, the organic phase was dropped into excess methanol for precipitation and allowed to stand for 1.5 hours, filtered, and the solid was placed in a vacuum drying oven at 43°C for 28 hours to obtain a low molecular weight acrylic ester polymer.

[0037] The initiator described in this embodiment was prepared according to Preparation Example 4. The reaction monomers described in step S1 were composed of methyl acrylate, methyl methacrylate, and methacrylic acid in a mass ratio of 5:8:2; the molar ratio of the reaction monomers and initiator described in step S1 and the catalyst and pentamethyldiethylenetriamine described in step S2 was 51:6:1:1.

[0038] Comparative Example 1 In this comparative example, a low molecular weight acrylic ester polymer was prepared by the preparation method of a low molecular weight acrylic ester polymer disclosed in the Chinese patent publication document with publication number CN17551228A. The reaction monomers consisted of methyl acrylate, methyl methacrylate and methacrylic acid in a mass ratio of 5:8:2.

[0039] Comparative Example 2 The preparation method of the acrylic ester polymer in this comparative example is similar to that in Example 4. The difference between this comparative example and Example 4 is that in the preparation method of the initiator in this comparative example, the molar ratio of allyl polyethylene glycol, azobisisobutyronitrile, and hydrogen bromide is 13:10:15.

[0040] Comparative Example 3 The preparation method of the acrylic acid ester polymer in this comparative example is similar to that in Example 4. The difference between this comparative example and Example 4 is that the molar ratio of the reaction monomer and the initiator in step S1 of this comparative example and the catalyst and pentamethyldiethylenetriamine in step S2 is 35:12:1:1.

[0041] Comparative Example 4 The preparation method of the acrylic acid ester polymer in this comparative example is similar to that in Example 4. The difference between this comparative example and Example 4 is that the molar ratio of the reaction monomer and the initiator in step S1 of this comparative example and the catalyst and pentamethyldiethylenetriamine in step S2 is 67:5:1:1.

[0042] Test example Monomer conversion rate: The low molecular weight acrylic acid ester polymers prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were tested by gas chromatography; Molecular weight (Mn) and molecular weight distribution (PDI): The relative molecular mass (Mn), weight average molecular weight (Mw) and molecular weight distribution (Mw / Mn) of the low molecular weight acrylic polymers prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were tested by size exclusion chromatography (SEC).

[0043] The test results are shown in Table 1 and Figures 1-8 .

[0044] Table 1 Monomer conversion, Mn and PDI test results

[0045] From Table 1 and Figures 1-4 It can be seen that the monomer conversion rate of the preparation method of the low molecular weight acrylic ester polymer provided by the present invention is greater than 99%, the relative molecular mass of the prepared acrylic ester polymer is about 5000 g / mol, and the SEC curve is a single peak, which basically conforms to the law of normal distribution. The molecular weight distribution is ≤1.13, and the molecular weight distribution is relatively narrow.

[0046] Compared with Example 4, in Comparative Example 1, a small molecule initiator was used to prepare an acrylic polymer, but the conversion rate of the monomer was reduced, and the relative molecular mass and molecular weight distribution increased. This shows that the macromolecular initiator used in the present invention can better control the molecular mass and molecular weight distribution of the polymer product than the small molecule initiator. In Comparative Example 2, the molar ratio of allyl polyethylene glycol, azobisisobutyronitrile, and hydrogen bromide in the preparation method of the initiator was changed, but the relative molecular mass was reduced and the molecular weight distribution increased. This shows that in the preparation process of the initiator, the molar ratio of allyl polyethylene glycol, azobisisobutyronitrile, and hydrogen bromide was changed. The ratio has been optimized; Comparative Example 3 changed the molar ratio of the reaction monomer, initiator, catalyst, and pentamethyldiethylenetriamine, but the monomer conversion rate and the relative molecular mass decreased, and the molecular weight distribution increased. Comparative Example 4 changed the molar ratio of the reaction monomer, initiator, catalyst, and pentamethyldiethylenetriamine, but the monomer conversion rate decreased, and the relative molecular mass and molecular weight distribution increased. This shows that in the synthesis process of low molecular weight acrylate polymers, the molar ratio of the reaction monomer, initiator, catalyst, and pentamethyldiethylenetriamine is the key factor in controlling the molecular weight and molecular weight distribution of the polymerization product.

[0047] In addition, the present invention also carried out nuclear magnetic characterization on the initiator prepared in Preparation Example 4, and the test results are shown in Figure 9 .Depend on Figure 9It can be seen that the characteristic peak of alkenyl (δ5.0-5.2 ppm, -CH=CH2) disappears, the chemical shift δ of 5.4 ppm corresponds to the characteristic peak of -OH, the chemical shift δ of 4.2 ppm corresponds to the characteristic peak of -CH2-, the chemical shift δ of 3.5-3.70 ppm corresponds to the characteristic peak of -CH2-CH2-, and the chemical shift δ of 1.88 ppm corresponds to the characteristic peak of the methylene in -CH2-CH2-Br, which proves that the anti-Markovnikov addition reaction is successful.

[0048] The above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Persons skilled in the art should not modify the above embodiments without departing from the spirit and scope of the present invention. Any equivalent modifications or alterations made by persons skilled in the art without departing from the technical spirit of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A method for preparing a low molecular weight acrylic polymer, characterized in that: The following steps are involved: S1: adding anhydrous ethanol, reaction monomers and initiators into a reactor, stirring evenly, and deoxygenating to obtain a mixed solution A; S2: Under the protection of inert gas, continue to add the catalyst and pentamethyldiethylenetriamine to the mixed solution A prepared in step S1, stir evenly, heat in a water bath to 70-80°C, and stir to react for 2-4 hours to obtain a primary product; S3: introducing oxygen into the primary product obtained in step S2, washing, separating the liquids, dripping the organic phase into methanol, standing for 1-2 hours, filtering, and drying to obtain a low molecular weight acrylic acid ester polymer; The initiator is prepared by reacting terminal olefin polyethylene glycol with hydrogen bromide through anti-Markovnikov addition reaction.

2. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The preparation method of the initiator comprises the following steps: under the protection of inert gas, mixing allyl polyethylene glycol and dichloromethane, stirring and dissolving, adding azobisisobutyronitrile, stirring and dissolving, heating in a water bath to 50-60° C., dropping hydrogen bromide aqueous solution while stirring, keeping the temperature for reaction for 4-6 hours, cooling, adding saturated sodium bicarbonate solution until the pH value is 6.8-7.5, stirring for 10-15 minutes, standing and stratifying, collecting the organic phase, drying with anhydrous magnesium sulfate, dropping into ether, stirring to generate a white precipitate, filtering, and drying to obtain the initiator.

3. The method for preparing a low molecular weight acrylic polymer according to claim 2, wherein: The molar ratio of the allyl polyethylene glycol, azobisisobutyronitrile and hydrogen bromide is 17-19:1-2:20-25.

4. The method for preparing a low molecular weight acrylic polymer according to claim 2, wherein: The mass percentage of the hydrogen bromide aqueous solution is 40%-45%.

5. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The catalyst in step S2 is cuprous bromide or cuprous chloride.

6. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The deoxygenation in step S1 is specifically as follows: freezing the mixed solution with liquid nitrogen to a solid state, evacuating the mixture for 10-15 minutes, introducing nitrogen to thaw, and repeating this process 3-5 times.

7. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The reaction monomers in step S1 are composed of methyl acrylate, methyl methacrylate and methacrylic acid in a mass ratio of 3-7:5-9:1-3.

8. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The molar ratio of the reaction monomer and the initiator in step S1 to the catalyst and pentamethyldiethylenetriamine in step S2 is 48-52:5-7:1:

1.

9. The method for preparing a low molecular weight acrylic polymer according to claim 8, wherein: The molar ratio of the reaction monomer and the initiator in step S1 to the catalyst and pentamethyldiethylenetriamine in step S2 is 51:6:1:

1.

10. The method for preparing a low molecular weight acrylic polymer according to claim 1, wherein: The washing in step S3 is washing with dilute hydrochloric acid, the washing times are 3-5 times, the concentration of dilute hydrochloric acid is 0.8-1.2 mol / L; the drying temperature is 40-45° C., and the drying time is 24-30 hours.

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