Method for preparing glycolic acid oligomer, obtained glycolic acid oligomer and application

Abstract

The invention discloses a method for preparing a glycolic acid oligomer, the obtained glycolic acid oligomer and application, and the method comprises the following steps: by taking a glycolic acid aqueous solution as a raw material, sequentially carrying out normal-pressure polycondensation reaction and two-stage reduced-pressure polycondensation reaction to obtain the glycolic acid oligomer, the obtained glycolic acid oligomer is low in terminal carboxyl group content, high in weight-average molecular weight, narrow in relative molecular mass distribution and good in quality.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis technology, and particularly relates to a method for preparing glycolic acid oligomers, the obtained glycolic acid oligomers, and their applications. Background Technology

[0002] Polyglycolic acid (PEG), an aliphatic polyester, possesses high biodegradability and good biocompatibility. It can be hydrolyzed in organisms and metabolized by microorganisms in natural environments, ultimately decomposing into water and carbon dioxide. Furthermore, PEG exhibits good mechanical properties such as heat resistance and tensile strength, and demonstrates excellent gas barrier properties when used in films and sheets. Therefore, PEG can replace commonly used biodegradable polymers and is expected to be used in medical polymer materials, agricultural resource materials, and various packaging or container materials. Currently, PEG has been applied in surgical sutures, artificial skin and blood vessels, bone fixation and repair, controlled drug release, and tissue engineering.

[0003] Glycolide is not only a monomer for synthesizing polyglycolide, but it can also be copolymerized with other cyclic monomers through ring-opening to obtain random copolymers or block copolymers. Typically, the synthesis of glycolide monomers involves two steps: first, low molecular weight glycolic acid ester polymers are synthesized via transesterification of methyl glycolate, or via condensation polymerization of glycolate; second, the glycolate (ester) oligomers are depolymerized by high-temperature distillation to extract the gaseous crude glycolide product from the reaction system.

[0004] The glycolic acid oligomers prepared from methyl glycolate (ME) result in trace amounts of ME in the crude glycolide produced during the subsequent depolymerization reaction. Even after purification of the crude glycolide, trace amounts of ME remain, negatively impacting the preparation of high-purity refined glycolide for the synthesis of high-molecular-weight polyethylene glycol (PEG). However, the trace amounts of ME in the crude glycolide produced from glycolic acid oligomers, after purification with polar solvents, are reduced to less than 5 mmol / kg. This is highly advantageous for preparing high-purity refined glycolide for the synthesis of PEG.

[0005] In the preparation of glycolic acid oligomers from glycolic acid, the terminal carboxyl groups and weight-average molecular weight distribution of the oligomers affect the yield, purity, and acid value of crude glycolide produced by depolymerization, and also have a certain impact on subsequent purification to prepare high-purity refined glycolide. Therefore, how to obtain glycolic acid oligomers with low terminal carboxyl group content, high weight-average molecular weight, and narrow relative molecular mass distribution is a technical problem that needs to be solved in this field, especially how to obtain glycolic acid oligomers with low terminal carboxyl group content. Summary of the Invention

[0006] To overcome the problems existing in the prior art, the present invention provides a method for preparing glycolic acid oligomers and the obtained glycolic acid oligomers. The obtained glycolic acid oligomers have low end carboxyl group content, high weight-average molecular weight, and narrow relative molecular mass distribution, and have good quality.

[0007] One of the objectives of this invention is to provide a method for preparing glycolic acid oligomers, comprising: using an aqueous solution of glycolic acid as a raw material, sequentially undergoing a normal compression polymerization reaction and a two-stage decompression polymerization reaction to obtain the glycolic acid oligomers.

[0008] In a preferred embodiment, the aqueous solution of glycolic acid is a 50wt% to 70wt% aqueous solution of glycolic acid, preferably a 55wt% to 65wt% aqueous solution of glycolic acid, for example, 50wt%, 55wt%, 60wt%, 65wt%, or 70wt%.

[0009] In a preferred embodiment, the gas phase generated by the atmospheric compression polymerization reaction is fractionated and condensed to form an atmospheric pressure liquid distillate and a non-condensable gas, and the liquid phase material generated by the atmospheric compression polymerization reaction enters the two-stage decompression polymerization reaction.

[0010] In a preferred embodiment, in the sequentially performed normal compression polymerization and two-stage decompression polymerization, the reaction temperature is gradually increased and the reaction pressure is gradually decreased.

[0011] In a preferred embodiment, the conditions for the constant compression polymerization reaction include a temperature of 100–160°C.

[0012] In a preferred embodiment, the conditions for the constant compression polymerization reaction include: a temperature of 110–150°C and a pressure of 101–105 kPa (A).

[0013] For example, the conditions for the constant compression polymerization reaction include: a temperature of 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C or 150°C, and a reaction pressure of 101 kPa(A), 102 kPa(A), 103 kPa(A), 104 kPa(A) or 105 kPa(A).

[0014] In a further preferred embodiment, the terminal carboxyl group content of the liquid phase material obtained by the constant compression polymerization reaction is 4000-6000 μmol / g, preferably 4500-5500 μmol / g, for example 4500 μmol / g, 4600 μmol / g, 4700 μmol / g, 4800 μmol / g, 4900 μmol / g, 5000 μmol / g, 5100 μmol / g, 5200 μmol / g, 5300 μmol / g, 5400 μmol / g or 5500 μmol / g.

[0015] In a preferred embodiment, the two-stage decompression polymerization reaction includes a first-stage decompression polymerization reaction and a second-stage decompression polymerization reaction, wherein the liquid phase output of the first-stage decompression polymerization reaction enters the second-stage decompression polymerization reaction.

[0016] In a further preferred embodiment, the gas phases of the first-stage decompression polymerization reaction and the second-stage decompression polymerization reaction are independently fractionated and condensed to form the first-stage decompression liquid distillate, the second-stage decompression liquid distillate, and their respective non-condensable gases.

[0017] Specifically, the gas phase of the first-stage decompression polymerization reaction is fractionated and condensed to form the first-stage decompression liquid phase distillate and its non-condensable gas, and the gas phase of the second-stage decompression polymerization reaction is fractionated and condensed to form the second-stage decompression liquid phase distillate and its non-condensable gas.

[0018] In a preferred embodiment, the first-stage compression polymerization reaction is carried out in the presence of a catalyst selected from one or more of stannous octoate, stannous chloride dihydrate, antimony trioxide, zinc oxide, zinc acetate dihydrate, and zinc acetylacetonate.

[0019] In a further preferred embodiment, in the first stage of compression reduction polymerization, the amount of catalyst used is 0.1 to 0.8 wt% based on the mass of glycolic acid, preferably 0.2 to 0.5 wt%, for example 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, or 0.8 wt%.

[0020] In a preferred embodiment, the conditions for the first stage decompression polymerization reaction include: a temperature 20-50°C higher than that of the normal compression polymerization reaction, and a pressure 60-90 kPa (A) lower than that of the normal compression polymerization reaction.

[0021] In a further preferred embodiment, the conditions for the first stage decompression polymerization reaction include: a temperature 30-40°C higher than that of the normal compression polymerization reaction, and a pressure 70-80 kPa (A) lower than that of the normal compression polymerization reaction.

[0022] For example, the conditions for the first stage of decompression polymerization include: a temperature 30°C, 32°C, 34°C, 36°C, 38°C, or 40°C higher than the temperature of the normal compression polymerization, and a pressure 70 kPa(A), 71 kPa(A), 72 kPa(A), 73 kPa(A), 74 kPa(A), 75 kPa(A), 76 kPa(A), 77 kPa(A), 78 kPa(A), 79 kPa(A), or 80 kPa(A) lower than the pressure of the normal compression polymerization.

[0023] In a preferred embodiment, the terminal carboxyl group content of the liquid phase output (hydroxyacetic acid oligomer) from the first stage compression polymerization reaction is 1500–4000 μmol / g, preferably 2000–3000 μmol / g, for example 2000 μmol / g, 2100 μmol / g, 2200 μmol / g, 2300 μmol / g, 2400 μmol / g, 2500 μmol / g, 2600 μmol / g, 2700 μmol / g, 2800 μmol / g, 2900 μmol / g, or 3000 μmol / g.

[0024] In a preferred embodiment, the conditions for the second-stage compression reduction polymerization reaction include: a temperature 15–35°C higher than that of the first-stage compression reduction polymerization reaction, and a pressure 10–20 kPa (A) lower than that of the first-stage compression reduction polymerization reaction.

[0025] In a further preferred embodiment, the conditions for the second-stage compression reduction polymerization reaction include: a temperature 20-30°C higher than that of the first-stage compression reduction polymerization reaction, and a pressure 15-20 kPa (A) lower than that of the first-stage compression reduction polymerization reaction.

[0026] For example, the conditions for the second-stage compression reduction polymerization reaction include: a temperature 20°C, 22°C, 24°C, 26°C, 28°C, or 30°C higher than the temperature of the first-stage compression reduction polymerization reaction, and a pressure 15 kPa(A), 16 kPa(A), 17 kPa(A), 18 kPa(A), 19 kPa(A), or 20 kPa(A) lower than the pressure of the first-stage compression reduction polymerization reaction.

[0027] In a preferred embodiment, the liquid phase output (hydroxyacetic acid oligomer) of the second-stage compression polymerization reaction has a terminal carboxyl group content ≤300μmol / g, a weight-average molecular weight of 15000-18000Da, and a relative molecular mass distribution PDI ≤1.4.

[0028] In a preferred embodiment, the method includes:

[0029] Step 1: The aqueous solution of glycolic acid is dehydrated under the conditions of normal compression polymerization to obtain a liquid phase material and a gas phase material of glycolic acid. The liquid phase material of glycolic acid is used as the feed for the first stage of reduced compression polymerization. The gas phase material contains free water and glycolic acid. The gas phase material is passed through a fractionation and condensation system to obtain an atmospheric pressure liquid distillate and discharge non-condensable gas.

[0030] Step 2: The liquid-phase material of glycolic acid generated by the normal compression polymerization reaction is subjected to a first-stage compression polymerization reaction under the conditions and catalyst of the first-stage compression polymerization reaction to obtain a liquid-phase glycolic acid oligomer I comprising glycolic acid, water, glycolic acid linear dimer, and glycolic acid linear trimer, and a gaseous material. The liquid-phase glycolic acid oligomer I is used as the feed for the second-stage compression polymerization reaction. The gaseous material contains reaction water and glycolic acid. After passing through a fractionation and condensation system, the gaseous material is used to obtain the distillate of the first compression polymerization reactor and the non-condensable gas is discharged.

[0031] Step 3: The liquid-phase glycolic acid oligomer I generated in the first stage decompression polymerization reaction is subjected to a second stage decompression polymerization reaction under the conditions of the second stage decompression polymerization reaction to obtain liquid-phase glycolic acid oligomer II and gaseous material. The gaseous material contains reaction water, glycolic acid and glycolic acid linear oligomers. The gaseous material is passed through a fractionation and condensation system to obtain the second stage decompression distillate and discharge non-condensable gas.

[0032] A second objective of this invention is to provide a system for preparing glycolic acid oligomers, preferably for carrying out the method described in one objective of this invention. The system includes a normal compression polymerization unit, a first decompression polymerization unit, and a second decompression polymerization unit connected in sequence. A glycolic acid aqueous solution feed line is provided at the feed end of the normal compression polymerization unit, and a catalyst feed line is provided at the feed end of the first decompression polymerization unit.

[0033] In a preferred embodiment, the atmospheric compression polypolymerization unit includes an atmospheric compression polypolymerization reactor and a fractionation and condensation system, wherein the fractionation and condensation system is used to fractionate and condense atmospheric pressure gaseous materials, collect atmospheric pressure distillate, and discharge non-condensable gases.

[0034] In a further preferred embodiment, the normal compression polypolymer reactor includes a gas phase outlet and a liquid phase outlet. The gas phase outlet is connected to the fractionation and condensation system via a pipeline, and the liquid phase outlet is connected to the feed end of the first reduced compression polypolymer reactor via a pipeline.

[0035] In a further preferred embodiment, the fractionation and condensation system includes a fractionation tower, a condenser, and a collection tank. The feed end of the fractionation tower is connected to the gas phase outlet of the atmospheric compression reactor via a pipeline. The fractionation tower, the condenser, and the collection tank are connected in sequence via pipelines. A non-condensable gas discharge pipeline is provided on the collection tank.

[0036] In a preferred embodiment, the first compression polymerization reaction unit includes a first compression polymerization reactor and a fractionation and condensation system two. The fractionation and condensation system two is used to fractionate and condense the gaseous material of the first compression polymerization reactor, collect the distillate of the first compression polymerization reactor, and discharge non-condensable gas.

[0037] In a further preferred embodiment, the first reduced-pressure polymerization reactor includes a gas phase outlet and a liquid phase outlet. Its gas phase outlet is connected to the second fractionation and condensation system via a pipeline, and its liquid phase outlet is connected to the feed end of the second reduced-pressure polymerization reaction unit via a pipeline.

[0038] In a further preferred embodiment, the fractionation and condensation system two includes a fractionation tower two, a condenser two, and a collection tank two. The feed end of the fractionation tower two is connected to the gas phase outlet of the first compression polymerization reactor via a pipeline. The fractionation tower two, the condenser two, and the collection tank two are connected in sequence via pipelines. A non-condensable gas discharge pipeline two is provided on the collection tank two, and the non-condensable gas discharge pipeline two is further connected to a vacuum system.

[0039] In a preferred embodiment, the second compression polymerization reaction unit includes a second compression polymerization reactor and a fractionation and condensation system three. The fractionation and condensation system three is used to fractionate and condense the gaseous material of the second compression polymerization reactor, collect the distillate of the second compression polymerization reactor, and discharge non-condensable gas.

[0040] In a further preferred embodiment, the second reduced-pressure polymerization reactor includes a gas phase outlet and a liquid phase outlet. Its gas phase outlet is connected to the fractionation and condensation system via a pipeline, and its liquid phase outlet is provided with a product delivery pipeline.

[0041] In a further preferred embodiment, the fractionation and condensation system three includes a fractionation tower three, a condenser three, and a collection tank three, wherein the feed end of the fractionation tower three is connected to the gas phase outlet of the second compression polymerization reactor via a pipeline, the fractionation tower three, the condenser three, and the collection tank three are connected in sequence via pipelines, and a non-condensable gas discharge pipeline three is provided on the collection tank three, which is further connected to a vacuum system.

[0042] In a preferred embodiment, the normal compression polymerization reactor, the first decompression polymerization reactor, and the second decompression polymerization reactor are each independently a stirred tank, for example, a vertical stirred tank.

[0043] In a preferred embodiment, each of the three fractionation towers (first, second, and third) is an independently packed tower, and the packing includes one or more of rectangular saddle rings, Pall rings, and Raschig rings.

[0044] In a preferred embodiment, the condenser one, condenser two, and condenser three are vertical or horizontal heat exchangers.

[0045] A third objective of this invention is to provide glycolic acid oligomers obtained using the method described in one objective of this invention or the system described in another objective of this invention.

[0046] In a preferred embodiment, the terminal carboxyl group content of the glycolic acid oligomer is ≤300 μmol / g, the weight-average molecular weight is 15000-18000 Da, and the relative molecular mass distribution (PDI) is ≤1.4.

[0047] The fourth objective of this invention is to provide the application of the method described in the first objective of this invention, or the system described in the second objective of this invention, or the glycolic acid oligomer described in the third objective of this invention in the preparation of glycolide.

[0048] The glycolic acid oligomers prepared by the method or system described in this invention are advantageous for preparing crude glycolide with lower acid value and fewer impurities in the depolymerization reaction. The crude glycolide prepared is more conducive to obtaining high-purity glycolide products with extremely low acid value after purification, and can be used in the production application of synthesizing high molecular weight polyglycolic acid.

[0049] The fifth objective of this invention is to provide a method for preparing glycolide, comprising: preparing a glycolic acid oligomer by the method described in the first objective of this invention, and subjecting the glycolic acid oligomer to a depolymerization reaction to obtain glycolide.

[0050] In a preferred embodiment, the conditions for the depolymerization reaction include a temperature of 250–350°C and a pressure of 1–10 kPa (A).

[0051] In a further preferred embodiment, the conditions for the depolymerization reaction include: a temperature of 260–320°C and a pressure of 2–8 kPa (A).

[0052] In a preferred embodiment, the glycolic acid oligomer is optionally cooled and pulverized prior to the depolymerization reaction.

[0053] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) The method or system described in this invention can reduce the end carboxyl group content of glycolic acid oligomers, increase the weight-average molecular weight, and reduce the loss of raw glycolic acid.

[0056] (2) The present invention prepares glycolic acid oligomers by normal compression polymerization and graded compression reduction polymerization. The glycolic acid oligomers obtained have a terminal carboxyl group content ≤300μmol / g, a weight average molecular weight of 15000-18000Da, and a relative molecular mass distribution PDI ≤1.4, which have good quality.

[0057] (3) The glycolic acid oligomers prepared by the method or system described in this invention are beneficial for preparing crude glycolide with lower acid value and fewer impurities in the depolymerization reaction. The crude glycolide prepared is more conducive to obtaining high-purity glycolide products with extremely low acid value after purification and can be used in the production application of synthesizing high molecular weight polyglycolide. Attached Figure Description

[0058] Figure 1 A schematic diagram of the system described in this invention is shown.

[0059] exist Figure 1 In the diagram, 1-Ambient compression polymerization reactor, 2-Fracturing tower one, 3-Condenser one, 4-Collection tank one, 5-First reduced compression polymerization reactor, 6-Fracturing tower two, 7-Condenser two, 8-Collection tank two, 9-Second reduced compression polymerization reactor, 10-Fracturing tower three, 11-Condenser three, 12-Collection tank three. Detailed Implementation

[0060] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0061] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0062] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0063] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0064] The analytical method for determining glycolic acid oligomers in this invention:

[0065] (1) Determination of end carboxyl group content in glycolic acid oligomers

[0066] The content of terminal carboxyl groups in glycolic acid oligomers was determined by acid-base titration. The sample was heated to 150°C and dissolved in 40 mL of dry dimethyl sulfoxide. After dissolution, the solution was cooled to room temperature, and a few drops of bromophenol blue indicator solution were added; the solution turned yellow. Titration was then performed using a standard concentration of sodium hydroxide in benzyl alcohol; the endpoint was reached when the solution color changed from yellow to green. The content of terminal carboxyl groups in the sample was calculated by calculating the volume of sodium hydroxide solution used to reach the titration endpoint.

[0067] (2) Determination of weight-average molecular weight of glycolic acid oligomers

[0068] An Agilent PL50 gel permeation chromatography (GPC) system was used. The glycolic acid oligomer sample was dissolved in a hexafluoroisopropanol solution containing 5 mM sodium trifluoroacetate. The column temperature was set at 40 °C; the hexafluoroisopropanol solution containing 5 mM sodium trifluoroacetate was used as the eluent; the flow rate was set at 1 mL / min; and the relative molecular mass was corrected using five different standard polymethyl methacrylates to obtain the weight-average molecular weight (M). w Number-average molecular weight (M) n The ratio of weight-average molecular weight to number-average molecular weight is the relative molecular mass distribution (PDI).

[0069] Example 1

[0070] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 103 kPa(A), the temperature was gradually increased from room temperature to 140 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 4880 μmol / g.

[0071] Before feeding into the first reduced-pressure polypolymerization reactor, the temperature was raised to 140°C. The liquid phase of glycolic acid obtained from atmospheric compression polypolymerization was introduced into the reactor via gravity. 146.3 g of stannous octoate (0.3 wt%, based on glycolic acid) was added and mixed thoroughly. The temperature was gradually raised to 170°C, while a vacuum was applied to reduce the system pressure to 23 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the first-stage reduced-pressure polypolymer distillate, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimer, and glycolic acid linear trimer, was used as the feed for the second reduced-pressure polypolymerization reactor. After the vacuum was removed from the first reduced-pressure polypolymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2420 μmol / g.

[0072] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 170°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 6 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 268 μmol / g, the weight-average molecular weight was 16600 Da, and the relative molecular mass distribution (PDI) was 1.31.

[0073] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously reducing the system pressure to 5 kPa(A) under vacuum. The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 96.1%. The purity of the crude glycolide was measured to be 95.5%, and the acid value was 240 μmol / g.

[0074] Example 2

[0075] 80 kg of a 60% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 102 kPa(A), the temperature was gradually increased from room temperature to 130 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 4760 μmol / g.

[0076] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 130°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 96.0 g of stannous chloride dihydrate (0.2 wt%, based on glycolic acid) was added and mixed thoroughly. The temperature was gradually raised to 160°C, while a vacuum was applied to reduce the system pressure to 22 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimers, and glycolic acid linear trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2386 μmol / g.

[0077] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 160°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 190°C, and the system pressure was reduced to 5 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 243 μmol / g, the weight-average molecular weight was 17400 Da, and the relative molecular mass distribution (PDI) was 1.25.

[0078] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 96.4%. The purity of the crude glycolide was measured to be 95.8%, and the acid value was 224 μmol / g.

[0079] Example 3

[0080] 90 kg of a 55% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 105 kPa(A), the temperature was gradually increased from room temperature to 110 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 5230 μmol / g.

[0081] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 110°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 247.5 g of antimony trioxide (0.5 wt%, based on glycolic acid) was added and mixed thoroughly. The temperature was gradually raised to 150°C, while a vacuum was applied to reduce the system pressure to 21 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimers, and glycolic acid linear trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2846 μmol / g.

[0082] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 150°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where it was gradually heated to 180°C. Simultaneously, a vacuum was applied to reduce the system pressure to 1 kPa(A). The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank, and the non-condensable gas was discharged. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 256 μmol / g, the weight-average molecular weight was 16300 Da, and the relative molecular mass distribution (PDI) was 1.29.

[0083] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 96.3%. The purity of the crude glycolide was measured to be 95.6%, and the acid value was 234 μmol / g.

[0084] Example 4

[0085] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 101 kPa(A), the temperature was gradually increased from room temperature to 150 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 4620 μmol / g.

[0086] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 150°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity, along with 48.8 g of zinc oxide (0.1 wt%, based on glycolic acid). The mixture was thoroughly mixed, and the temperature was gradually raised to 180°C while simultaneously evacuating the system to reduce the pressure to 31 kPa(A). The gaseous phase containing reacting water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimers, and glycolic acid linear trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2440 μmol / g.

[0087] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 180°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 11 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 288 μmol / g, the weight-average molecular weight was 15800 Da, and the relative molecular mass distribution (PDI) was 1.35.

[0088] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously reducing the system pressure to 5 kPa(A) under vacuum. The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 95.8%. The purity of the crude glycolide was measured to be 95.2%, and the acid value was 264 μmol / g.

[0089] Example 5

[0090] 80 kg of a 60% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 104 kPa(A), the temperature was gradually increased from room temperature to 150 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 5060 μmol / g.

[0091] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 150°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 288.0 g of zinc acetate dihydrate (0.6 wt%, based on glycolic acid) was added and mixed thoroughly. The temperature was gradually raised to 180°C, while a vacuum was applied to reduce the system pressure to 24 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimers, and glycolic acid linear trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2530 μmol / g.

[0092] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 180°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 8 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 230 μmol / g, the weight-average molecular weight was 16200 Da, and the relative molecular mass distribution (PDI) was 1.23.

[0093] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 96.5%. The purity of the crude glycolide was measured to be 96.1%, and the acid value was 214 μmol / g.

[0094] Example 6

[0095] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L atmospheric compression polypolymerization reactor. Under a pressure of 101 kPa(A), the temperature was gradually increased from room temperature to 140 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polypolymerization reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 4840 μmol / g.

[0096] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 140°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 390.0 g of zinc acetylacetone (0.8 wt%, based on glycolic acid) was added, and the mixture was thoroughly mixed. The temperature was gradually raised to 180°C, while a vacuum was applied to reduce the system pressure to 21 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimer, and glycolic acid linear trimer, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 2280 μmol / g.

[0097] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 180°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 210°C, and the system pressure was reduced to 6 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 212 μmol / g, the weight-average molecular weight was 17800 Da, and the relative molecular mass distribution (PDI) was 1.19.

[0098] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously reducing the system pressure to 5 kPa(A) under vacuum. The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 96.8%. The purity of the crude glycolide was measured to be 96.4%, and the acid value was 208 μmol / g.

[0099] Comparative Example 1

[0100] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L polycondensation reactor. Under a pressure of 0.1 MPa (200 kPa(A)), the temperature was gradually increased from room temperature to 160 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polycondensation reactor. After the pressure was restored to atmospheric pressure, the end carboxyl group content of the glycolic acid liquid material was measured to be 7620 μmol / g.

[0101] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 160°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 146.3 g of stannous octoate (0.3 wt%, based on glycolic acid) was added and mixed thoroughly. The temperature was gradually raised to 170°C, while a vacuum was applied to reduce the system pressure to 23 kPa(A). The gaseous phase containing reacting water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimers, and glycolic acid linear trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 5530 μmol / g.

[0102] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 170°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 6 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 816 μmol / g, the weight-average molecular weight was 11000 Da, and the relative molecular mass distribution (PDI) was 2.48.

[0103] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 80.1%. The purity of the crude glycolide was measured to be 85.3%, and the acid value was 706 μmol / g.

[0104] Comparative Example 2

[0105] 80 kg of a 60% aqueous solution of glycolic acid was added to a 100 L polycondensation reactor. Under pressure of 0.2 MPa (G) (300 kPa (A)), the temperature was gradually increased from room temperature to 180 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polycondensation reactor. After the pressure was restored to atmospheric pressure, the end carboxyl group content of the glycolic acid liquid material was measured to be 7860 μmol / g.

[0106] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 180°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity, and 288.0 g of zinc acetate dihydrate (0.6 wt%, based on glycolic acid) was added. The mixture was thoroughly mixed, and the temperature was maintained at 180°C while a vacuum was applied to reduce the system pressure to 24 kPa(A). The gaseous phase containing reacting water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, linear glycolic acid dimers, and linear glycolic acid trimers, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of the liquid phase glycolic acid oligomer I was measured to be 5030 μmol / g.

[0107] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 180°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 8 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 831 μmol / g, the weight-average molecular weight was 10800 Da, and the relative molecular mass distribution (PDI) was 2.56.

[0108] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 78.2%. The purity of the crude glycolide was measured to be 83.1%, and the acid value was 733 μmol / g.

[0109] Comparative Example 3

[0110] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L polycondensation reactor. Under pressure conditions of 0.3 MPa (G) (400 kPa (A)), the temperature was gradually increased from room temperature to 210 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first reduced-pressure polycondensation reactor. After the pressure was restored to atmospheric pressure, the end carboxyl group content of the glycolic acid liquid material was measured to be 8640 μmol / g.

[0111] Before feeding into the first reduced-pressure polymerization reactor, the temperature was raised to 210°C. The liquid phase of glycolic acid obtained from atmospheric compression polymerization entered the reactor via gravity. 390.0 g of zinc acetylacetone (0.8 wt%, based on glycolic acid) was added, and the mixture was thoroughly mixed. The temperature was maintained at 210°C, and a vacuum was applied to reduce the system pressure to 21 kPa(A). The gaseous phase containing reaction water and glycolic acid was fractionated and condensed to obtain the distillate from the first reduced-pressure polymerization reactor, which was collected in a collection tank, and non-condensable gases were discharged. The liquid phase glycolic acid oligomer I, containing glycolic acid, water, glycolic acid linear dimer, and glycolic acid linear trimer, was used as the feed for the second reduced-pressure polymerization reactor. After the vacuum was removed from the first reduced-pressure polymerization reactor, the terminal carboxyl group content of glycolic acid oligomer I was measured to be 6110 μmol / g.

[0112] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 210°C. The glycolic acid oligomer obtained from the first vacuum polymerization reactor entered the second reactor via gravity, maintaining the temperature at 210°C while simultaneously evacuating to reduce the system pressure to 6 kPa(A). The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the final product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 844 μmol / g, the weight-average molecular weight was 10500 Da, and the relative molecular mass distribution (PDI) was 2.68.

[0113] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while a vacuum was applied to reduce the system pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 75.3%. The purity of the crude glycolide was measured to be 81.4%, and the acid value was 758 μmol / g.

[0114] Comparative Example 4

[0115] 75 kg of a 65% aqueous solution of glycolic acid was added to a 100 L polycondensation reactor. Under a pressure of 103 kPa(A), the temperature was gradually increased from room temperature to 140 °C. The gaseous material containing free water and glycolic acid was fractionated and condensed to obtain an atmospheric pressure liquid distillate, which was collected in a collection tank and the non-condensable gas was discharged. The glycolic acid liquid material in the reactor was used as the feed for the first positive compression polycondensation reactor. The end carboxyl group content of the glycolic acid liquid material was measured to be 4920 μmol / g.

[0116] Before feeding into the first positive compression polypolymer reactor, the temperature was raised to 140°C. The liquid glycolate obtained from atmospheric compression polypolymerization entered the reactor via gravity, and 146.3 g of stannous octoate (0.3 wt%, based on glycolate) was added and mixed thoroughly. The temperature was gradually raised to 170°C, while the system pressure was increased to 0.1 MPa(G). The gaseous material containing reaction water and glycolate was fractionated and condensed to obtain the first-stage positive compression polypolymer distillate, which was collected in a collection tank, and non-condensable gases were discharged. The liquid glycolate oligomer I, containing glycolate, water, glycolate linear dimer, and glycolate linear trimer, was used as the feed for the second reduced-pressure polypolymer reactor. After the first positive compression polypolymer reactor was reduced to atmospheric pressure, the terminal carboxyl group content of glycolate oligomer I was measured to be 8340 μmol / g.

[0117] Before feeding into the second vacuum polymerization reactor, the temperature was raised to 170°C. The glycolic acid oligomer obtained from the first positive compression polymerization reactor entered the second vacuum polymerization reactor via gravity, where the temperature was gradually raised to 200°C, and the system pressure was reduced to 6 kPa(A) under vacuum. The gaseous material containing reaction water, glycolic acid, and linear glycolic acid oligomers was fractionated and condensed to obtain the second-stage vacuum distillate, which was collected in a collection tank to remove non-condensable gases. The liquid glycolic acid oligomer in the reactor was the obtained product. The terminal carboxyl group content of the glycolic acid oligomer was measured to be 1140 μmol / g, the weight-average molecular weight was 9600 Da, and the relative molecular mass distribution (PDI) was 2.89.

[0118] After being discharged and cooled, the glycolic acid oligomers were crushed into powder and added to a depolymerization reactor. The reactor was preheated to 220°C before feeding and gradually increased to 290°C after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa(A). The gaseous crude glycolide generated by depolymerization was collected after condensation, with a yield of 68.4%. The purity of the crude glycolide was measured to be 77.1%, and the acid value was 963 μmol / g.

[0119] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing glycolic acid oligomers, comprising: The glycolic acid oligomer was obtained by sequentially performing normal compression polymerization and two-stage decompression polymerization using an aqueous solution of glycolic acid as raw material.

2. The method according to claim 1, characterized in that, The conditions for the normal compression polymerization reaction include: a temperature of 100–160°C, preferably 110–150°C; and / or a pressure of 101–105 kPa(A); and / or the terminal carboxyl group content of the liquid phase material obtained by the normal compression polymerization reaction is 4000–6000 μmol / g, preferably 4500–5500 μmol / g.

3. The method according to claim 1, characterized in that, The two-stage compression reduction polymerization reaction includes a first-stage compression reduction polymerization reaction and a second-stage compression reduction polymerization reaction. The liquid phase output of the first-stage compression reduction polymerization reaction enters the second-stage compression reduction polymerization reaction. Preferably, in the sequentially performed normal compression polymerization and two-stage decompression polymerization, the reaction temperature is increased step by step, and the reaction pressure is decreased step by step.

4. The method according to claim 3, characterized in that, The conditions for the first-stage compression reduction polymerization reaction include: a temperature 20–50°C higher than that of the normal compression polymerization reaction, preferably 30–40°C higher; and / or a pressure 60–90 kPa(A) lower than that of the normal compression polymerization reaction, preferably 70–80 kPa(A) lower; and / or the terminal carboxyl group content of the liquid phase output of the first-stage compression reduction polymerization reaction is 1500–4000 μmol / g, preferably 2000–3000 μmol / g.

5. The method according to claim 3, characterized in that, The conditions for the second-stage compression reduction polymerization reaction include: a temperature 15–35°C higher than that of the first-stage compression reduction polymerization reaction, preferably 20–30°C higher; and / or a pressure 10–20 kPa(A) lower than that of the first-stage compression reduction polymerization reaction, preferably 15–20 kPa(A) lower; and / or the terminal carboxyl group content of the liquid phase output (hydroxyacetic acid oligomer) of the second-stage compression reduction polymerization reaction is ≤300 μmol / g, the weight-average molecular weight is 15000–18000 Da, and the relative molecular mass distribution (PDI) is ≤1.

4.

6. The method according to any one of claims 3 to 5, characterized in that, The first stage of compression reduction polymerization is carried out in the presence of a catalyst, which is selected from one or more of stannous octoate, stannous chloride dihydrate, antimony trioxide, zinc oxide, zinc acetate dihydrate, and zinc acetylacetonate. Preferably, in the first stage of compression reduction polymerization, the amount of catalyst used is 0.1 to 0.8 wt% based on the mass of glycolic acid, and more preferably 0.2 to 0.5 wt%.

7. The method according to claim 6, characterized in that, The aqueous solution of glycolic acid is a 50wt% to 70wt% aqueous solution of glycolic acid, preferably a 55wt% to 65wt% aqueous solution of glycolic acid; and / or, The gas phase produced by the atmospheric compression polymerization reaction is fractionated and condensed to form an atmospheric pressure liquid distillate and non-condensable gas. The liquid phase material produced by the atmospheric compression polymerization reaction enters the two-stage decompression polymerization reaction; and / or, The gas phases of the first-stage decompression polymerization reaction and the second-stage decompression polymerization reaction are independently fractionated and condensed to form the first-stage decompression liquid distillate, the second-stage decompression liquid distillate, and their respective non-condensable gases.

8. A system for preparing glycolic acid oligomers, preferably used for the method according to any one of claims 1 to 7, the system comprising a normal compression polymerization unit, a first decompression polymerization unit and a second decompression polymerization unit connected in sequence, wherein a glycolic acid aqueous solution feed line is provided at the feed end of the normal compression polymerization unit and a catalyst feed line is provided at the feed end of the first decompression polymerization unit.

9. The system according to claim 8, characterized in that, The atmospheric compression polymerization unit includes an atmospheric compression polymerization reactor and a fractionation and condensation system one. The fractionation and condensation system one is used to fractionate and condense atmospheric pressure gaseous materials, collect atmospheric pressure distillate, and discharge non-condensable gases. Preferably, the normal compression polypolymer reactor includes a gas phase outlet and a liquid phase outlet, wherein the gas phase outlet is connected to the fractionation and condensation system via a pipeline, and the liquid phase outlet is connected to the feed end of the first reduced compression polypolymer reactor via a pipeline. More preferably, the fractionation and condensation system includes a fractionation tower, a condenser, and a collection tank. The feed end of the fractionation tower is connected to the gas phase outlet of the atmospheric compression reactor via a pipeline. The fractionation tower, the condenser, and the collection tank are connected in sequence via pipelines. A non-condensable gas discharge pipeline is provided on the collection tank.

10. The system according to claim 8, characterized in that, The first compression polymerization reaction unit includes a first compression polymerization reactor and a fractionation and condensation system two. The fractionation and condensation system two is used to fractionate and condense the gaseous material of the first compression polymerization reactor, collect the distillate of the first compression polymerization reactor, and discharge non-condensable gas. Preferably, the first compression-reduced polymerization reactor includes a gas phase outlet and a liquid phase outlet, the gas phase outlet being connected to the second fractionation and condensation system via a pipeline, and the liquid phase outlet being connected to the feed end of the second compression-reduced polymerization reactor via a pipeline. More preferably, the second fractionation and condensation system includes a second fractionation tower, a second condenser, and a second collection tank. The feed end of the second fractionation tower is connected to the gas phase outlet of the first compression polymerization reactor via a pipeline. The second fractionation tower, the second condenser, and the second collection tank are connected in sequence via pipelines. A second non-condensable gas discharge pipeline is provided on the second collection tank, and the second non-condensable gas discharge pipeline is further connected to a vacuum system.

11. The system according to claim 8, characterized in that, The second compression reduction polymerization reaction unit includes a second compression reduction polymerization reactor and a fractionation and condensation system three. The fractionation and condensation system three is used to fractionate and condense the gaseous material of the second compression reduction polymerization reactor, collect the distillate of the second compression reduction polymerization reactor, and discharge non-condensable gas. Preferably, the second compression polymerization reactor includes a gas phase outlet and a liquid phase outlet. Its gas phase outlet is connected to the fractionation and condensation system via a pipeline, and its liquid phase outlet is provided with a product delivery pipeline. More preferably, the fractionation and condensation system three includes a fractionation tower three, a condenser three, and a collection tank three, wherein the feed end of the fractionation tower three is connected to the gas phase outlet of the second compression polymerization reactor via a pipeline, the fractionation tower three, the condenser three, and the collection tank three are connected in sequence via pipelines, and a non-condensable gas discharge pipeline three is provided on the collection tank three, which is further connected to a vacuum system.

12. A glycolic acid oligomer obtained by the method of any one of claims 1 to 7 or by the system of any one of claims 8 to 11; preferably, the glycolic acid oligomer has a terminal carboxyl group content ≤300 μmol / g, a weight-average molecular weight of 15000-18000 Da, and a relative molecular mass distribution PDI ≤1.

4.

13. The use of the method according to any one of claims 1 to 7, or the system according to any one of claims 8 to 11, or the glycolic acid oligomer according to claim 12 in the preparation of glycolide.

14. A method for preparing glycolide, comprising: A glycolic acid oligomer is prepared by the method according to any one of claims 1 to 7, and the glycolic acid oligomer is subjected to a depolymerization reaction to obtain glycolide; preferably, the conditions for the depolymerization reaction include: a temperature of 250 to 350°C and a pressure of 1 to 10 kPa (A); more preferably, a temperature of 260 to 320°C and a pressure of 2 to 8 kPa (A).

Images