A preparation method of polyglycolic acid

Abstract

The present invention provides a method for preparing polyglycolic acid. The method first uses glycolic acid to prepare a macromolecular polyglycolic acid of 1000-10000, which is then used as an initiator for the subsequent polymerization of glycolic acid / glycolate monomers to prepare the target product, polyglycolic acid with a weight-average molecular weight of 10000-20000. The obtained product is pure white and has a hard texture, overcoming the defect of traditional catalysts that the polymer is easily degraded and discolored due to low stability.

Description

Technical Field

[0001] The present invention relates to the field of polymer synthesis, and in particular to a method for preparing polyglycolic acid. Background Art

[0002] Polyglycolic acid, also known as polyglycolide (PGA), is an aliphatic polyester polymer with the lowest carbon number per unit and the fastest degradation rate, completely hydrolyzing into carbon dioxide and water under the catalysis of microorganisms and enzymes in organisms. Polyglycolic acid also exhibits excellent gas barrier properties, high mechanical strength, good biocompatibility, and reasonable processability. Through post-processing methods such as extrusion, injection molding, spinning, and blow molding, it is widely used in medical devices, oil production, bottles, membranes, and other fields. Its application prospects are of great significance in helping achieve the "dual carbon" goals.

[0003] Currently, there are two main methods for producing polyglycolic acid: one is to first produce a low-molecular-weight polymer through the condensation polymerization of glycolic acid monomers and / or glycolate monomers, then heat and crack the low-molecular-weight polymer into rings to form glycolide, and then produce polyglycolic acid through ring-opening polymerization of glycolide; the other is to produce polyglycolic acid through direct esterification and condensation polymerization of glycolic acid monomers and / or glycolate monomers. The direct esterification method has a simple process route, but the molecular weight of the polyglycolic acid obtained is low, and the processing performance is inferior to that of the product obtained by the ring-opening method.

[0004] The direct esterification of glycolic acid monomers and / or glycolate ester monomers often utilizes tin catalysts, with stannous chloride dihydrate being the most commonly used. Using stannous chloride dihydrate as a polycondensation catalyst presents the following challenges: During polymer production, the catalyst often remains in the polymer system and cannot be removed. The presence of residual groups such as chloride ions makes hydrolysis more likely, increasing acidity and catalyzing the hydrolysis of numerous ester bonds in polyglycolic acid, broadening degradation conditions and shortening degradation time. Furthermore, chloride ions are hydrophilic, making it easier for water molecules from the environment to enter the polymer system, accelerating the hydrolysis process and reducing molecular weight. This significantly limits the polymer's gas barrier properties, strength, and durability, limiting its application in many technical areas.

[0005] Another method uses small organic molecule tin catalysts such as monobutyltin chloride, dibutyltin, or dioctyltin as polycondensation catalysts. However, these catalysts also have the following problems: the thermal stability of the organic tin catalysts is poor, and they are easily decomposed into tin oxides and other impurities, which reduces the catalytic performance. In addition, the thermal decomposition products of the organic tin catalysts are mixed in the polymer system as small molecules, affecting the processing performance and color of the polymer. The polymer usually appears yellow or brown, which affects the processing and use of the polymer.

[0006] CN107177032 discloses a method for preparing high molecular weight polyglycolic acid from glycolic acid or methyl glycolate, comprising the following steps: (1) subjecting monomer glycolic acid or methyl glycolate to a precondensation reaction under the action of a catalyst to obtain a glycolic acid or methyl glycolate prepolymer having an intrinsic viscosity of 0.3-0.7 dl / g; (2) crushing the glycolic acid or methyl glycolate prepolymer obtained in step (1) to a size of 10-300 mesh using a pulverizer to obtain glycolic acid or methyl glycolate prepolymer powder; and (3) sending the glycolic acid or methyl glycolate prepolymer powder obtained in step (2) to a boiling dryer for solid phase polycondensation to obtain polyglycolic acid having an intrinsic viscosity of 1.0 dl / g or more. The catalyst used is a composite of a metal oxide catalyst and a metal salt catalyst in a mass ratio of 0.2:1-3:1; the metal oxide catalyst is selected from antimony trioxide, germanium dioxide, titanium dioxide, tin oxide, or zinc oxide; the metal salt catalyst is selected from zinc acetate, stannous octoate, calcium acetate, zinc acetate dihydrate, stannous chloride, or stannous chloride dihydrate. However, these catalysts are not very stable and remain in the polymer system, accelerating polymer degradation. During the polymerization process, polyglycolic acid is easily decomposed into small molecules, causing discoloration of the polyglycolic acid. Furthermore, the preparation process of these catalysts is complex and prone to contamination by other impurities.

[0007] Therefore, developing a method for preparing polyglycolic acid that is efficient, safe, and has a pure polymerization system is an urgent problem to be solved. Summary of the Invention

[0008] In order to solve the problems of the existing technology and overcome the defects of tin catalysts in preparing polyglycolic acid, the present invention provides a method for preparing polyglycolic acid. The method first uses glycolic acid to prepare a macromolecular polyglycolic acid of 1000-10000, which is then used as an initiator for the subsequent polymerization of glycolic acid / glycolate monomers to prepare the target product, polyglycolic acid with a weight-average molecular weight of 10000-20000.

[0009] The technical solutions of the present invention are as follows:

[0010] The present invention provides a method for preparing polyglycolic acid, which comprises the following steps:

[0011] Step 1: preparing a macromolecular initiator by heating glycolic acid, elemental tin, and a promoter under an oxidizing gas atmosphere to obtain a macromolecular polyglycolic acid with a weight average molecular weight of 1,000 to 10,000, which serves as an initiator for subsequent polymerization;

[0012] Step 2: Carry out a polycondensation reaction of glycolic acid / glycolate monomers and the macroinitiator obtained in step 1 under heating conditions and carrier gas protection to obtain polyglycolic acid with a weight average molecular weight of 10,000-20,000.

[0013] Furthermore, in step 1, the molar ratio of the glycolic acid to the elemental tin is 1:1 to 2:1.

[0014] Furthermore, in step 1, the mass ratio of the accelerator to the elemental tin is 0.1%-5%:1.

[0015] Furthermore, in step 1, the glycolic acid, elemental tin and promoter are reacted at normal pressure, at a reaction temperature of 100-140° C., at a stirring speed of 100-1000 r / min, and an oxidizing gas such as air is introduced. Under these conditions, the reaction is carried out for 2-4 hours, and then the macromolecular initiator and tin slag are separated.

[0016] Furthermore, considering factors such as the melting point of glycolic acid, the heating condition in step 1 is a temperature of 100-140° C., preferably 120° C., to reduce the thermal decomposition and yellowing degree of polyglycolic acid.

[0017] Furthermore, in step 1, the elemental tin can be any one of elemental tin such as tin powder, tin particles, and tin blocks.

[0018] Furthermore, in step 1, the accelerator is any one of 2,5-di-tert-butylhydroquinone and p-tert-butylcatechol. The accelerator can reduce the decomposition reaction of short-chain polyglycolic acid and enhance the processing performance and product performance of the polymer.

[0019] Furthermore, in step 1, the oxidizing gas may be any one of air and oxygen.

[0020] Furthermore, in step 2, the glycolic acid / glycolate monomers and the macroinitiator are heated from 60-100° C. to 150-190° C. under normal pressure to allow most of the monomers to react; then the temperature is raised to 180-210° C., a protective gas is introduced, and the reaction is continued under these conditions for 2-5 hours to further remove small molecules (methanol or oxidative decomposition products) to obtain a polyglycolic acid product with a weight average molecular weight of 10,000-20,000.

[0021] Furthermore, in step 2, considering the yield and color of polyglycolic acid, the first temperature increase of the polycondensation reaction is between the boiling point of glycolic acid / glycolate monomer and the melting point of polyglycolic acid, 150-190°C, preferably 180°C, to reduce the thermal decomposition and yellowing degree of polyglycolic acid.

[0022] Furthermore, in step 2, the ratio of the macroinitiator is 1%-10%, based on the mass of the glycolic acid / glycolate monomer.

[0023] Furthermore, in step 2, the protective gas may be any one of nitrogen, helium or carbon dioxide.

[0024] In the preparation method of the present invention, in step 1, the macromolecular initiator of the present invention is prepared from glycolic acid and elemental tin as raw materials, and 4-tert-butylcatechol or 2,5-di-tert-butylhydroquinone as a promoter, to prepare a macromolecular polyglycolic acid containing a tin active center as an initiator, with a molecular weight ranging from about 1,000 to 10,000. The reaction mechanism is as follows:

[0025]

[0026] Step 1: Elemental tin reacts with phenols to form tin ions;

[0027] Step 2: The tin ions on the phenolate tin exchange protons with the carboxyl groups of glycolic acid, the phenols are restored, and the tin active center exists on glycolic acid;

[0028] Step 3: The glycolic acid with an active center quickly triggers the polymerization reaction of the surrounding glycolic acids to generate a macromolecular glycolic acid initiator.

[0029] The present invention has the beneficial effects:

[0030] (1) The present invention finds that the activity of a macromolecular initiator prepared from elemental tin and glycolic acid in catalyzing the polymerization of glycolic acid or glycolate monomers is similar to that of common industrial polyester catalysts such as stannous chloride dihydrate and organotin catalysts. The flexibility of the organic group makes the catalyst more soluble and the polymerization reaction rate faster. At the same time, the structure of this macromolecular initiator is more complex than that of stannous chloride dihydrate and organotin catalysts, and the presence of crystalline regions increases its own stability, ensures its stability during the polymerization process, and does not easily pyrolyze, thereby reducing the discoloration of polyglycolic acid products due to pyrolysis products.

[0031] (2) The macromolecular initiator does not contain organic acid, chloride ion and other groups. During the storage and use of the polymer in the later stage, hydrolysis phenomenon is less, which prolongs the degradation time and is more suitable for daily use and storage needs.

[0032] (3) Compared with various traditional catalysts used previously, the products obtained with macromolecular initiators have molecular weights similar to those of other catalysts, reducing the yellowing of the products caused by the decomposition of traditional catalysts and eliminating the greening of the products caused by the use of inorganic tin catalysts (stannous chloride dihydrate). The products of the present invention are whiter in color and the overall process cost is reduced. DETAILED DESCRIPTION

[0033] Hereinafter, only certain exemplary embodiments are briefly described. As will be appreciated by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit or scope of the present invention. Therefore, the examples described are only some, not all, embodiments of the present invention.

[0034] Example

[0035] Example 1

[0036] Step 1: Weigh 57.2 g of glycolic acid, 59.6 g of tin powder and 0.6 g of 4-tert-butylcatechol, add them to a four-necked flask, raise the temperature to 120°C under normal pressure to start the reaction, introduce air for oxidation, and react under this condition for 4 hours to obtain the macromolecular initiator polyglycolic acid.

[0037] Step 2: Weigh 50.2 g of glycolic acid and 2.6 g of macromolecular initiator, add them to a 250 mL four-necked flask, mix well, and heat to 190°C under normal pressure to initiate the reaction. Nitrogen is introduced to remove small molecules, and finally a polyglycolic acid product with a weight-average molecular weight of 15,984 is obtained. The product is pure white in color and has a hard texture.

[0038] Example 2

[0039] Step 1: Weigh 57.6 g of glycolic acid, 59.0 g of tin powder and 0.6 g of 2,5-di-tert-butylhydroquinone, add them into a four-necked flask, raise the temperature to 120°C under normal pressure to start the reaction, introduce air for oxidation, and react under this condition for 4 hours to obtain the macromolecular initiator polyglycolic acid.

[0040] Step 2: Weigh 50.6 g of glycolic acid and 2.6 g of a macromolecular initiator, add them to a 250 mL four-necked flask, mix well, and heat to 190°C under normal pressure to initiate the reaction. Then, introduce nitrogen to remove small molecules, ultimately producing a polyglycolic acid product with a weight-average molecular weight of 14876. The product is pure white and has a hard texture.

[0041] Example 3

[0042] Step 1: Weigh 57.0 g of glycolic acid, 60.0 g of tin powder and 0.6 g of 4-tert-butylcatechol, add them into a four-necked flask, raise the temperature to 120°C under normal pressure to start the reaction, introduce air for oxidation, and react under this condition for 3 hours to obtain the macromolecular initiator polyglycolic acid.

[0043] Step 2: Weigh 51.0 g of glycolic acid and 2.5 g of macromolecular initiator, add them to a 250 mL four-necked flask, mix well, and heat to 190°C under normal pressure to start the reaction. Nitrogen is introduced to remove small molecules, and finally a polyglycolic acid product with a weight-average molecular weight of 15062 is obtained. The product is pure white in color and has a hard texture.

[0044] Comparative Example 1

[0045] Weigh 50 g of glycolic acid and 0.2 g of stannous chloride dihydrate, add them to a 250 mL four-necked flask, mix well, and heat to 190°C under normal pressure to initiate the reaction. Nitrogen is introduced to remove small molecules, ultimately producing a polyglycolic acid product with a weight-average molecular weight of 18933 and a light green color.

[0046] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent variation based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing polyglycolic acid, characterized in that: The method consists of the following steps: Step 1: preparing a macromolecular initiator by heating glycolic acid, elemental tin, and a promoter under an oxidizing gas atmosphere to obtain a macromolecular polyglycolic acid with a weight average molecular weight of 1,000 to 10,000, which serves as an initiator for subsequent polymerization; the promoter is 2,5-di-tert-butylhydroquinone or p-tert-butylcatechol; Step 2: Carry out a polycondensation reaction of glycolic acid / glycolate monomers and the macroinitiator obtained in step 1 under heating conditions and protective gas protection to obtain polyglycolic acid with a weight average molecular weight of 10,000 to 20,000.

2. The preparation method according to claim 1, characterized in that In step 1, the molar ratio of the glycolic acid to the elemental tin is 1:1 to 2:

1.

3. The preparation method according to claim 2, characterized in that In step 1, the mass ratio of the accelerator to the elemental tin is 0.1% to 5%:

1.

4. The preparation method according to claim 1 or 2, characterized in that In step 1, the oxidizing gas is air or oxygen.

5. The preparation method according to claim 1 or 2, characterized in that In step 1, the elemental tin is tin powder, tin particles or tin blocks.

6. The preparation method according to claim 1 or 2, characterized in that In step 1, the glycolic acid, elemental tin and accelerator are reacted at normal pressure, a reaction temperature of 100-140° C., a stirring speed of 100-1000 r / min, and air is introduced. Under these conditions, the reaction is carried out for 2-4 hours, and then the macromolecular initiator and tin slag are separated.

7. The preparation method according to claim 1 or 2, characterized in that In step 2, the glycolic acid / glycolate monomers and the macroinitiator are heated at room temperature from 60 to 100° C. to 150 to 190° C. to allow most of the monomers to react. The temperature is then raised to 180 to 210° C., a protective gas is introduced, and the reaction is continued under these conditions for 2 to 5 hours to further remove methanol or small molecules of oxidative decomposition products to obtain a polyglycolic acid product with a weight average molecular weight of 10,000 to 20,000.

8. The preparation method according to claim 1 or 2, wherein in step 2, the ratio of the macroinitiator is 1% to 10%, based on the mass of the glycolic acid / glycolate monomer.

9. The preparation method according to claim 1 or 2, wherein in step 2, the protective gas is nitrogen, helium or carbon dioxide.

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