Polyglycolic acid with low content of cyclic oligomer and preparation method of polyglycolic acid

Abstract

The invention provides polyglycolic acid with low content of cyclic oligomer and a preparation method of the polyglycolic acid. The content of cyclic oligomer of the polyglycolic acid is less than or equal to 1wt%, and the intrinsic viscosity of the polyglycolic acid is 1.0-2.0 dL / g. The preparation method comprises the following steps: carrying out pre-reaction on glycolide, a catalyst and a molecular weight regulator in a protective gas atmosphere, heating to carry out intermediate reaction, adding a catalyst inert agent, and carrying out final reaction to obtain the polyglycolic acid with low content of cyclic oligomer. The method is carried out at a low reaction temperature, the reaction time is short, side reactions can be reduced as much as possible, meanwhile, the catalyst inert agent is added in the middle reaction, the depolymerization reaction can be inhibited, and the content of cyclic oligomers in the prepared polyglycolic acid product is lower.

Description

Technical Field

[0001] The present invention relates to the field of polyglycolic acid, and more particularly, to a polyglycolic acid with a low content of cyclic oligomers and a method for preparing the same. Background Art

[0002] Polyglycolic acid (PGA) is a linear aliphatic polyester with good biocompatibility and biodegradability, excellent mechanical properties, gas barrier properties and heat resistance. In recent years, it has been widely used in fields such as surgical sutures, tissue engineering, orthopedic fixation and drug release.

[0003] Currently, there are mainly two methods for preparing polyglycolic acid. One is to directly heat and polycondense glycolic acid into polyglycolic acid. The advantage of this method is simplicity and convenience, but the molecular weight of the obtained polyglycolic acid is not high. The other is to ring-opening polymerize glycolide to obtain polyglycolic acid with a relatively high molecular weight, and the molecular weight can reach tens of thousands to hundreds of thousands.

[0004] Cyclic oligomers are a mixture of cyclic molecules formed by the reaction of functional groups during the polyester polymerization reaction, and usually the degree of polymerization is between 3 and 20. During the polymerization and processing of polyester, the main reasons for the generation of cyclic oligomers are that the growing polymer chains backbite or transesterification elimination reactions occur between the chains. During the polyester spinning process, when the melt under high temperature and high pressure is ejected from the spinneret, due to the sudden drop in melt pressure and temperature, the cyclic oligomers therein will not only form microbubbles, hairiness and defects, but also condense on the spinneret and will contaminate the spinneret after long-term high-temperature oxidation. In addition, cyclic oligomers are likely to deposit on the fiber surface during the spinning process, reducing the fiber quality. During the dyeing process of polyester, cyclic oligomers will migrate from the fiber interior and adhere to the fiber surface, reducing the dyeing effect and may enter the dye liquor to form dyeing defects.

[0005] In Patent US9080013B2, stannous chloride and p-toluenesulfonic acid are used as catalysts, and ethylene glycol is used as an initiator to carry out ring-opening polymerization of glycolide. The reaction is carried out at a temperature of 170 °C for 4.5 - 12 h to obtain polyglycolic acid with a relatively high molecular weight. To reduce the reaction time, the method of increasing the reaction temperature is usually adopted. Patent CN115926062A discloses a method for preparing high molecular weight polyglycolic acid, which uses an unsaturated initiator to initiate the ring-opening polymerization of glycolide, and the initiator itself can carry out free radical polymerization. The above two reactions are continuously carried out in an extruder at the same time to in-situ obtain a polyglycolic acid resin with high molecular weight and high melt viscosity. Glycolide (GA), stannous octoate, antioxidant 1010, antioxidant 626, 2-hydroxyethyl methacrylate, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane are mixed evenly in proportion and then reacted and extruded into pellets by a twin-screw extruder. The extruder has 11 sections from the feeding port to the die, and the first section only serves to feed materials and cannot be heated. The temperatures of the 2nd - 11th sections of the extruder are 160 °C, 200 °C, 230 °C, 230 °C, 230 °C, 230 °C, 230 °C, 230 °C, 230 °C, and 230 °C respectively, and the average reaction time of the raw materials is 1.5 - 3.5 minutes. However, increasing the reaction temperature will increase side reactions during the reaction and increase the generation of cyclic oligomers.

[0006] Therefore, it is necessary to study a method for preparing polyglycolic acid, which can be carried out at a lower reaction temperature, has a shorter reaction time, can minimize the occurrence of side reactions, and can inhibit depolymerization reactions, thereby reducing the content of cyclic oligomers. Summary of the Invention

[0007] In order to solve the technical problems existing in the prior art, the present invention provides a polyglycolic acid with a low content of cyclic oligomers and a preparation method thereof.

[0008] The present invention provides a method for preparing polyglycolic acid with a low content of cyclic oligomers. By means of stepwise reaction and the addition of a catalyst inerting agent, the reaction time is shortened, the reaction temperature is reduced, and the depolymerization reaction is inhibited, so as to solve the problem that a large amount of cyclic oligomers are generated during the polymerization process of polyglycolic acid in the prior art.

[0009] During the ring-opening polymerization of glycolide, generally, the reaction conversion rate is increased by increasing the polymerization time and reaction temperature to obtain polyglycolic acid with a relatively high molecular weight and intrinsic viscosity. The prior art is generally a one-step reaction, which generates polyglycolic acid with a relatively high molecular weight in one step at a relatively high temperature. However, the increase in reaction temperature is positively correlated with the increase in side reactions. The present invention adopts a three-step reaction, combining the advantages of reactions at different temperatures. First, a prepolymer with a certain conversion rate is obtained during the pre-polymerization process at a low reaction temperature, thereby reducing the middle-stage reaction and final reaction time at a high reaction temperature. The reaction temperature is reduced, the reaction time is shortened, and there are fewer side reactions.

[0010] One of the objectives of the present invention is to provide a polyglycolic acid with a low content of cyclic oligomers, and the structural formula of the cyclic oligomer is n is from 3 to 20. Based on the total amount of polyglycolic acid being 100 wt%, the content of the cyclic oligomer is less than or equal to 1 wt%, preferably less than or equal to 0.8 wt%; preferably, the weight-average molecular weight of the polyglycolic acid is from 100,000 to 300,000, more preferably from 150,000 to 250,000; the molecular weight distribution is from 1.0 to 3.0, more preferably from 1.0 to 2.0; the intrinsic viscosity of the polyglycolic acid is from 1.0 to 2.0 dL / g, more preferably from 1.2 to 1.6 dL / g.

[0011] Another objective of the present invention is to provide a method for preparing a polyglycolic acid with a low content of cyclic oligomers, comprising the following steps:

[0012] (1) Pre-react glycolide, a catalyst, and a molecular weight regulator under a protective gas atmosphere;

[0013] (2) Heat up the product obtained in step (1) under a protective gas atmosphere to carry out a middle-stage reaction;

[0014] (3) Add a catalyst inerting agent to the product obtained in step (2) and carry out a final reaction under a protective gas atmosphere to obtain the polyglycolic acid with a low content of cyclic oligomers.

[0015] In a preferred embodiment of the present invention,

[0016] For step (1),

[0017] The catalyst is at least one of metal catalysts, preferably a tin metal catalyst, more preferably at least one of stannous octoate, stannous chloride, stannous acetate, tin chloride, tetraphenyltin, and tetrabutyltin oxide;

[0018] The molecular weight regulator is at least one of an alkyl alcohol and water. The alkyl alcohol is preferably at least one of linear monohydric alcohols with 6 to 20 carbon atoms, such as linear monohydric alcohols with 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms.

[0019] In a preferred embodiment of the present invention,

[0020] For step (1),

[0021] The dosage of the catalyst is 10 ppm - 3000 ppm of the weight of glycolide, preferably 50 ppm - 500 ppm, more preferably 80 ppm - 300 ppm;

[0022] The mass ratio of the molecular weight regulator to glycolide is 1:(10 - 3000), preferably 1:(50 - 2500), more preferably 1:(800 - 2200), such as 1:900, 1:1000, 1:1200, 1:1500, 1:1800, 1:2000, 1:2100, etc.

[0023] In a preferred embodiment of the present invention,

[0024] Step (1),

[0025] The protective gas is at least one of nitrogen and inert gas;

[0026] The pre-reaction temperature is 90 - 150 °C, preferably 100 - 130 °C;

[0027] The pre-reaction time is 10 min - 5 h, preferably 30 min - 2 h, more preferably 30 min - 1 h;

[0028] The pre-reaction is carried out in a first reactor.

[0029] In a preferred embodiment of the present invention,

[0030] The protective gas is at least one of nitrogen and inert gas;

[0031] The middle-stage reaction temperature is 150 - 230 °C, preferably 180 - 210 °C;

[0032] The middle-stage reaction time is 5 min - 5 h, preferably 5 min - 2 h;

[0033] The middle-stage reaction is carried out in a second reactor.

[0034] In a preferred embodiment of the present invention,

[0035] Step (3),

[0036] The protective gas is at least one of nitrogen and inert gas;

[0037] The catalyst inerting agent is at least one of phosphate esters, phosphite esters, and phosphates, preferably at least one of trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trimethyl phosphite, triethyl phosphite, tripropyl phosphite, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and potassium dihydrogen phosphate;

[0038] The dosage of the catalyst inert is 10 ppm to 10,000 ppm of the weight of glycolide, preferably 50 ppm to 3,000 ppm, more preferably 400 ppm to 1,200 ppm, such as 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1,000 ppm, 1,100 ppm, etc.

[0039] In the polymerization of glycolide of the present invention, the metal catalyst (taking tin as an example) reacts with the alcohol substance to generate a tin-containing alkoxide as the active center. Glycolide continuously inserts between the Sn-O bonds of the alkoxide to form a polymer. The catalyst inert, as a more stable electron-donating substance than the alkoxide, will hinder the continuous coordination of glycolide and prevent the monomer from breaking the chain and recombining into a cyclic oligomer in the polymer, thereby achieving catalyst inertness.

[0040] The catalyst inert is added after the middle-stage reaction. At this time, the conversion rate of glycolide is 50-90%. This is because the polymerization process of glycolide is a reversible reaction. When the conversion rate is relatively high (>50%), the possibility of the reverse reaction is relatively large. At this time, the catalyst activity can be effectively reduced by the catalyst inert to reduce the generation of reverse reaction products.

[0041] In a preferred embodiment of the present invention,

[0042] Step (3),

[0043] The final reaction temperature is 150-230°C, preferably 180-210°C;

[0044] The final reaction time is 5 min to 5 h, preferably 5 min to 2 h.

[0045] In a preferred embodiment of the present invention,

[0046] The first reactor and the second reactor are each independently selected from at least one of a kettle reactor, a static reactor, a screw extruder, a continuous stirred tank reactor, and an SCP self-cleaning reactor.

[0047] The third object of the present invention is to provide a polyglycolic acid with a low content of cyclic oligomers obtained by the above preparation method.

[0048] Compared with the prior art, the beneficial effects of the present invention:

[0049] The present invention provides a method for preparing polyglycolic acid with a low content of cyclic oligomers. Through three-step reactions: pre-reaction, middle-stage reaction, and final reaction, the advantages of reactions at different temperatures are combined. First, a prepolymer with a certain conversion rate is obtained in the pre-polymerization process at a low reaction temperature, thereby reducing the middle-stage reaction and final reaction times at a high reaction temperature. The reaction temperature is reduced, the reaction time is shortened, and there are fewer side reactions, thereby reducing the content of cyclic oligomers.

[0050] By adding a catalyst inerting agent during the mid-stage reaction, i.e., when the conversion rate of glycolide is 50-90%, as an electron-donating substance more stable than alkoxide, it will hinder the continuous coordination of glycolide, prevent the monomer from breaking the chain and recombining into cyclic oligomers in the polymer, thereby achieving catalyst inertness, inhibiting the depolymerization reaction, and making the polyglycolic acid polymerization process produce fewer cyclic oligomers. Specific embodiments

[0051] The present invention will be specifically described below in conjunction with specific embodiments. It is necessary to point out here that the following embodiments are only for further illustration of the present invention and cannot be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art according to the content of the present invention still fall within the protection scope of the present invention.

[0052] The raw materials used in the examples are all conventional commercially available raw materials.

[0053] Testing methods:

[0054] (1) Determination of the content of cyclic oligomers: Weigh a granular sample of polyglycolic acid, dissolve it in a hot dimethyl sulfoxide solution, dilute it with acetonitrile, inject 1 μL for analysis, and use LC-MS for determination. The chromatographic column is Acquity UPLC BEH C18, the mobile phase is acetonitrile: water, and the mass spectrometry is detected in the ESI positive ion mode.

[0055] (2) Determination of intrinsic viscosity: Using hexafluoroisopropanol as the solvent, use an Ubbelohde viscometer in a constant temperature water bath at 25 °C, and calculate by the "one-point method" formula through measuring the efflux times t 0 and t of the pure solvent and the sample solution.

[0056]

[0057] (3) Determination of the conversion rate of glycolide: Mix an appropriate amount of polyglycolic acid with 50 mL of dry dimethyl sulfoxide, heat to 160 °C until completely dissolved, then cool to room temperature and centrifuge to obtain the supernatant. Take the supernatant and analyze the glycolide content in a gas chromatograph to further calculate the conversion rate of glycolide.

[0058] (4) Determination of the weight-average molecular weight: Dissolve the polyglycolic acid sample in a hexafluoroisopropanol solution containing sodium trifluoroacetate. Using the hexafluoroisopropanol solution dissolved with sodium trifluoroacetate as the eluent, use PMMA standard products for relative molecular weight calibration to obtain the weight-average molecular weight (M w ), number-average molecular weight (M n ) and relative molecular weight distribution PDI.

[0059] Example 1

[0060] 5 kg (43.07 mol) of glycolide, 0.5 g of stannous octoate (100 ppm), and 4.01 g (0.0215 mol) of dodecanol (molar ratio 2000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The pre-reaction temperature was 120 °C and the reaction time was 30 min. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The middle reaction temperature was 180 °C and the reaction was carried out for 1 h. Then, 2.5 g of triethyl phosphate (500 ppm) was added for the final reaction. The reaction temperature was 180 °C and the reaction was carried out for 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.5 dL / g, a weight-average molecular weight of 213,000, a molecular weight distribution of 1.64, and a cyclic oligomer content of 0.7 wt%.

[0061] Example 2

[0062] 5 kg of glycolide, 1.0 g of stannous octoate (200 ppm), and 4.01 g of dodecanol (molar ratio 2000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The pre-reaction temperature was 120 °C and the reaction time was 30 min. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The middle reaction temperature was 190 °C and the reaction was carried out for 1 h. Then, 2.5 g of triethyl phosphate (500 ppm) was added for the final reaction. The reaction temperature was 190 °C and the reaction was carried out for 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.6 dL / g, a weight-average molecular weight of 226,000, a molecular weight distribution of 1.82, and a cyclic oligomer content of 0.8 wt%.

[0063] Example 3

[0064] 5 kg of glycolide, 0.5 g of stannous octoate (100 ppm), and 8.02 g (0.043 mol) of dodecanol (molar ratio 1000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The pre-reaction temperature was 110 °C and the reaction time was 1 h. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The middle reaction temperature was 200 °C and the reaction was carried out for 1 h. Then, 4.0 g of triethyl phosphate (800 ppm) was added for the final reaction. The reaction temperature was 200 °C and the reaction was carried out for 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.4 dL / g, a weight-average molecular weight of 205,000, a molecular weight distribution of 1.68, and a cyclic oligomer content of 0.9 wt%.

[0065] Example 4

[0066] 5 kg of glycolide, 0.5 g of stannous octoate (100 ppm), and 4.01 g of dodecanol (molar ratio 2000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The pre-reaction temperature was 120 °C and the reaction time was 30 min. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The middle-stage reaction temperature was 200 °C and the reaction was carried out for 1 h. Then, 5.0 g of triethyl phosphate (1000 ppm) was added for the final reaction. The reaction temperature was 200 °C and the reaction was carried out for 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.5 dL / g, a weight-average molecular weight of 218,000, a molecular weight distribution of 1.70, and a cyclic oligomer content of 0.8 wt%.

[0067] Example 5

[0068] 5 kg of glycolide, 0.5 g of stannous octoate (100 ppm), and 4.61 g (0.0215 mol) of tetradecanol (molar ratio 2000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The reaction temperature was 120 °C and the reaction time was 30 min. After the pre-reaction, the middle-stage reaction was carried out under a nitrogen atmosphere at a reaction temperature of 180 °C for 2 h. Then, the material was transferred to a twin-screw extruder and reacted under a nitrogen atmosphere. 2.5 g of trimethyl phosphite (500 ppm) was added for the final reaction. The reaction temperature was 200 °C and the reaction was carried out for 10 min. The resulting polyglycolic acid product had an intrinsic viscosity of 1.5 dL / g, a weight-average molecular weight of 217,000, a molecular weight distribution of 1.74, and a cyclic oligomer content of 1.0 wt%.

[0069] Example 6

[0070] 5 kg of glycolide, 0.5 g of stannous chloride (100 ppm), and 4.01 g of dodecanol (molar ratio 2000:1) were added to Reactor 1 under a nitrogen atmosphere for a pre-reaction. The pre-reaction temperature was 120 °C and the reaction time was 30 min. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The middle-stage reaction temperature was 180 °C and the reaction was carried out for 10 min. Then, 2.5 g of trimethyl phosphate (500 ppm) was added for the final reaction. The reaction temperature was 180 °C and the reaction was carried out for 2 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.5 dL / g, a weight-average molecular weight of 207,000, a molecular weight distribution of 1.76, and a cyclic oligomer content of 0.8 wt%.

[0071] Comparative Example 1

[0072] 5 kg of glycolide, 0.5 g of stannous chloride (100 ppm), and 3.41 g (0.0215 mol) of butyric anhydride (molar ratio 2000:1) were added to Reactor 1 for pre-reaction. The pre-reaction temperature was 130 °C and the reaction time was 30 min. After the pre-reaction, the material was transferred to Reactor 2 and reacted under a nitrogen atmosphere. The reaction temperature was 180 °C and the reaction time was 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.3 dL / g, a weight-average molecular weight of 186,000, a molecular weight distribution of 2.07, and a cyclic oligomer content of 1.2 wt%.

[0073] Comparative Example 2

[0074] 5 kg of glycolide, 0.5 g of stannous octoate (100 ppm), and 4.01 g of dodecanol (molar ratio 2000:1) were mixed evenly and then added to a screw extruder. The set temperature of the screw extruder was 230 °C and the residence time of the material in the screw extruder was 10 min. Subsequently, the material was extruded and pelletized. The resulting polyglycolic acid product had an intrinsic viscosity of 1.6 dL / g, a weight-average molecular weight of 236,000, a molecular weight distribution of 1.98, and a cyclic oligomer content of 1.7 wt%.

[0075] Comparative Example 3

[0076] 5 kg of glycolide, 0.5 g of stannous octoate (100 ppm), and 8.02 g of dodecanol (molar ratio 1000:1) were added to a reactor for pre-reaction under a nitrogen atmosphere. The pre-reaction temperature was 110 °C and the reaction time was 1 h. After the pre-reaction, the material was transferred to the reactor and reacted under a nitrogen atmosphere for 1 h at a reaction temperature of 200 °C. After the reaction was completed, 4.0 g of triethyl phosphate (800 ppm) was added to the above material at 230 °C and the reaction continued for 1 h. The resulting polyglycolic acid product had an intrinsic viscosity of 1.4 dL / g, a weight-average molecular weight of 201,000, a molecular weight distribution of 1.89, and a cyclic oligomer content of 1.4 wt%.

[0077] Comparative Example 1, compared with Examples 1, 4 to 6, is a two-stage reaction. No catalyst inert was added, the catalyst was replaced with tin chloride, and the molecular weight regulator was replaced with butyric anhydride. The intrinsic viscosity of the products in Examples 1, 4 to 6 was 1.4 to 1.5 dL / g, the weight average molecular weight was 213,000 to 218,000, the molecular weight distribution was 1.64 to 1.70, and the cyclic oligomer content was 0.7 to 1.0 wt%. The polyglycolic acid product obtained in Comparative Example 1 had an intrinsic viscosity of 1.3 dL / g, a weight average molecular weight of 186,000, a molecular weight distribution of 2.07, and a cyclic oligomer content of 1.2 wt%. It can be seen that, compared with Comparative Example 1, the cyclic oligomer content in Examples 1 to 2, 4 was lower, the intrinsic viscosity of the product was higher, and the weight average molecular weight was higher. This is because no catalyst inert was added and there was no final reaction stage in Comparative Example 1, resulting in incomplete reaction.

[0078] Comparative Example 2, compared with Examples 1, 4, no catalyst inert was added. It was melt-mixed and extruded at a high temperature of 230 °C in a screw extruder. The polyglycolic acid product obtained had an intrinsic viscosity of 1.6 dL / g, a weight average molecular weight of 236,000, a molecular weight distribution of 2.13, and a cyclic oligomer content of 1.7 wt%. It can be seen that, compared with Comparative Example 2, the cyclic oligomer content in Examples 1, 4 was lower. This is because no catalyst inert was added in Comparative Example 2 and the polymerization reaction was carried out in one step at a high temperature of 230 °C, resulting in more side reactions.

[0079] Comparative Example 3, compared with Example 3, the reaction temperature was higher during the final reaction. The cyclic oligomer content of the polyglycolic acid product obtained in Example 3 was 0.9 wt%, while the cyclic oligomer content of the polyglycolic acid product obtained in Comparative Example 3 was 1.4 wt%. It was proved that when polymerizing at a higher temperature, the cyclic oligomer content in the product would increase significantly.

[0080] Examples 1 to 6 were carried out at a lower reaction temperature and a shorter reaction time, which could minimize the occurrence of side reactions. At the same time, by adding a catalyst inert during the middle-stage reaction, the depolymerization reaction could be inhibited. The cyclic oligomer content of the prepared polyglycolic acid product was lower, below 1 wt%.

Claims

1. A polyglycolic acid with a low content of cyclic oligomers, based on 100 wt% of the total amount of polyglycolic acid, the content of cyclic oligomers is less than or equal to 1 wt%, preferably less than or equal to 0.8 wt%; preferably, the weight-average molecular weight of the polyglycolic acid is 100,000 - 300,000, more preferably 150,000 - 250,000; the molecular weight distribution is 1.0 - 3.0, more preferably 1.0 - 2.0; the intrinsic viscosity of the polyglycolic acid is 1.0 - 2.0 dL / g, more preferably 1.2 - 1.6 dL / g.

2. A method for preparing polyglycolic acid with a low content of cyclic oligomers as described in claim 1, comprising the following steps: (1) Pre-react glycolide, a catalyst, and a molecular weight regulator in an atmosphere of a protective gas; (2) Heat up the product obtained in step (1) in an atmosphere of a protective gas to carry out a middle-stage reaction; (3) Add a catalyst inerting agent to the product obtained in step (2), and carry out a final reaction in an atmosphere of a protective gas to obtain the polyglycolic acid with a low content of cyclic oligomers.

3. The method for preparing polyglycolic acid with a low content of cyclic oligomers as described in claim 2, characterized in that: In step (1), the catalyst is at least one of metal catalysts, preferably a tin metal catalyst, more preferably at least one of stannous octanoate, stannous chloride, stannous acetate, tin chloride, tetraphenyltin, tetrabutyltin oxide; and / or, the molecular weight regulator is at least one of an alkyl alcohol and water, and the alkyl alcohol is preferably at least one of straight-chain monohydric alcohols with 6 - 20 carbon atoms.

4. The method for preparing polyglycolic acid with a low content of cyclic oligomers as described in claim 2, characterized in that: In step (1), the dosage of the catalyst is 10 ppm - 3000 ppm of the weight of glycolide, preferably 50 ppm - 500 ppm, more preferably 80 ppm - 300 ppm; and / or, the molar ratio of the molecular weight regulator to glycolide is 1:(10 - 3000), preferably 1:(50 - 2500), more preferably 1:(800 - 2200).

5. The method for preparing polyglycolic acid with a low content of cyclic oligomers as described in claim 2, characterized in that: In step (1), the protective gas is at least one of nitrogen and inert gas; and / or, the pre-reaction temperature is 90 - 150 °C, preferably 100 - 130 °C; and / or, the pre-reaction time is 10 min - 5 h, preferably 30 min - 2 h, more preferably 30 min - 1 h; and / or, the pre-reaction is carried out in a first reactor.

6. The method for preparing polyglycolic acid with a low content of cyclic oligomers as described in claim 2, characterized in that: In step (2), the protective gas is at least one of nitrogen and inert gas; and / or, the middle-stage reaction temperature is 150 - 230 °C, preferably 180 - 210 °C; and / or, the middle-stage reaction time is 5 min - 5 h, preferably 5 min - 2 h; and / or, the middle-stage reaction is carried out in a second reactor.

7. The preparation method of polyglycolic acid with low content of cyclic oligomers as described in claim 2, characterized in that: Step (3), the protective gas is at least one of nitrogen and inert gas; and / or, the catalyst inert is at least one of phosphoric esters, phosphorous esters, and phosphates, preferably at least one of trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trimethyl phosphite, triethyl phosphite, tripropyl phosphite, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and potassium dihydrogen phosphate; and / or, the dosage of the catalyst inert is 10 ppm to 10,000 ppm of the weight of glycolide, preferably 50 ppm to 3,000 ppm, more preferably 400 ppm to 1,200 ppm.

8. The preparation method of polyglycolic acid with low content of cyclic oligomers as described in claim 2, characterized in that: Step (3), the final reaction temperature is 150 to 230 °C, preferably 180 to 210 °C; and / or, the final reaction time is 5 min to 5 h, preferably 5 min to 2 h.

9. The preparation method of polyglycolic acid with low content of cyclic oligomers as described in any one of claims 2 to 8, characterized in that: the first reactor and the second reactor are each independently selected from at least one of a kettle reactor, a static reactor, a screw extruder, a continuous stirred tank reactor, and an SCP self-cleaning reactor.

10. A polyglycolic acid with low content of cyclic oligomers obtained by the preparation method as described in any one of claims 2 to 9.

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