Transition metal compounds, processes for their preparation and use, and ring-opening polymerization processes

Abstract

This invention relates to the fields of catalysis and ring-opening polymerization, and discloses a transition metal compound, its preparation method and application, as well as a ring-opening polymerization method. The transition metal compound provided by this invention has the structure shown in formula (1), where n is an integer from 1 to 4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1 to 6 carbon atoms. The transition metal compound of this invention has a simple preparation method and a high monomer conversion rate in ring-opening polymerization.

Description

Technical Field

[0001] This invention relates to the fields of catalysis and ring-opening polymerization, and more specifically, to a transition metal compound, its preparation method and application, and a ring-opening polymerization method. Background Technology

[0002] With the increasing depletion of petroleum-based non-renewable resources, the promotion and application of renewable polymer materials are becoming increasingly crucial for ensuring the sustainable development of society and the economy. Polyester, as a synthetic biodegradable polymer, has wide applications in packaging, fibers, plastics, and coatings. Over the years, with in-depth research, the theories and technologies for synthesizing this type of material have gradually matured. However, the development and application of catalytic systems, as one of the core research directions in this field, still holds extremely high academic research value. Innovative development of catalytic systems holds promise for reducing the cost of existing process technologies.

[0003] Currently, various systems have been explored for the ring-opening polymerization of cyclic ester monomers, including organometallic complexes, alkane (oxy)metals, and non-metallic catalysts. These different catalytic systems differ in their reaction mechanisms for ring-opening polymerization. Based on the different ways in which the cyclic ester monomer is attacked and opened, they can be divided into three polymerization mechanisms: anionic ring-opening polymerization, cationic ring-opening polymerization, and coordination-intercalation ring-opening polymerization. Anionic and cationic ring-opening polymerization reactions typically exhibit drawbacks such as uncontrollable polymerization and difficulty in obtaining high molecular weight polymers. In contrast, coordination-intercalation ring-opening polymerization has advantages such as controllable product molecular weight, fewer side reactions, better controllability, and a narrower molecular weight distribution, resulting in high molecular weight polymers, and is currently the most widely used method in industry. Group IVB transition metal catalysts are typically used for the polymerization of bulk chemical materials such as ethylene and propylene, and are rarely used for ring-opening polymerization. Summary of the Invention

[0004] The purpose of this invention is to provide a new transition metal compound, its preparation method and application, and a ring-opening polymerization method using the new transition metal compound as a catalyst. The preparation method of the transition metal compound of this invention is simple, and the monomer conversion rate of the ring-opening polymerization is high.

[0005] Through in-depth research, the inventors of this invention discovered that the transition metal compound with the specific structure of this invention can be used as a catalyst for the ring-opening polymerization of cyclic ester monomers and can improve the monomer conversion rate of the ring-opening polymerization, thereby completing this invention.

[0006] To achieve the above objectives, a first aspect of the present invention provides a transition metal compound having the structure shown in formula (1).

[0007]

[0008] In formula (1), n ​​is an integer from 1 to 4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1 to 6 carbon atoms.

[0009] Preferably, in formula (1), n ​​is 1 or 2, M is titanium, and X is an alkoxy group with 1-3 carbon atoms.

[0010] Preferably, the transition metal compound is selected from one or more of the following compounds:

[0011] In formula (1), n ​​is 1, M is titanium, and X is a methoxy compound.

[0012] In formula (1), n ​​is 2, M is titanium, and X is a methoxy compound.

[0013] In formula (1), n ​​is 1, M is titanium, and X is an ethoxylated compound.

[0014] In formula (1), n ​​is 2, M is titanium, and X is an ethoxylated compound.

[0015] In formula (1), n ​​is 1, M is titanium, and X is a compound with a propoxy group.

[0016] In formula (1), n ​​is 2, M is titanium, and X is a compound with a propoxy group.

[0017] In formula (1), n ​​is 1, M is titanium, and X is an isopropoxy compound.

[0018] In formula (1), n ​​is 2, M is titanium, and X is an isopropoxy compound.

[0019] According to a second aspect of the present invention, a method for preparing a transition metal compound is provided, wherein the method comprises: a step of reacting a compound with the structure shown in formula (2) with M(X)4 in the presence of an organic solvent.

[0020]

[0021] In formula (2), R is a hydroxyl or amino group;

[0022] In M(X)4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1-6 carbon atoms.

[0023] Preferably, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 1-4.5:1.

[0024] Preferably, the organic solvent is selected from one or more of toluene, n-hexane, and dichloromethane.

[0025] Preferably, the amount of organic solvent used is 5-20 mL, more preferably 5-10 mL, relative to 1 millimole of the compound with the structure shown in formula (2).

[0026] Preferably, the conditions for the contact reaction include: a temperature of 10-35°C and a time of 8-50 hours.

[0027] According to a third aspect of the present invention, a ring-opening polymerization method is provided, wherein the method comprises: ring-opening polymerization of a cyclic ester monomer in the presence of a transition metal compound as described in the first aspect of the present invention.

[0028] Preferably, in the ring-opening polymerization system, the concentration of the transition metal compound is 1-1000 × 10⁻⁶. -6 Moles per milliliter, preferably 1-20 × 10 -6 Moles per milliliter.

[0029] Preferably, the molar ratio of the transition metal compound to the cyclic ester monomer is 1:3000, more preferably 1:400-800.

[0030] Preferably, the cyclic ester monomer is lactide and / or caprolactone.

[0031] Preferably, the conditions for the ring-opening polymerization include: a temperature of 0-200℃ and a time of 0.5-24h.

[0032] According to a fourth aspect of the present invention, the use of the transition metal compound described in the first aspect of the present invention in the ring-opening polymerization of cyclic ester monomers is provided.

[0033] Through the above technical solutions, the present invention can provide a new transition metal compound, its preparation method and application, as well as a ring-opening polymerization method using the new transition metal compound as a catalyst. The preparation method of the transition metal compound of the present invention is simple, and the monomer conversion rate of the ring-opening polymerization is high. Detailed Implementation

[0034] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and 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.

[0035] According to a first aspect of the present invention, a transition metal compound is provided, wherein the transition metal compound has the structure shown in formula (1).

[0036]

[0037] In formula (1), n ​​is an integer from 1 to 4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1 to 6 carbon atoms.

[0038] In this invention, X is an alkoxy group with 1-6 carbon atoms, preferably an alkoxy group with 1-3 carbon atoms.

[0039] Specific examples of alkoxy groups having 1 to 6 carbon atoms include methoxy, ethoxy, propoxy, isopropoxy, butoxy, pentoxy, or hexoxy, among which methoxy, ethoxy, propoxy, or isopropoxy are preferred.

[0040] In this invention, in formula (1), n ​​is an integer from 1 to 4. Examples of n are 1, 2, 3, and 4, among which 1 or 2 is preferred.

[0041] In this invention, in formula (1), M is titanium, zirconium or hafnium, preferably titanium.

[0042] In a preferred embodiment of the present invention, in formula (1), n ​​is 1 or 2, M is titanium, and X is an alkoxy group with 1-3 carbon atoms.

[0043] In a particularly preferred embodiment of the invention, the transition metal compound is selected from one or more of the following compounds:

[0044] In formula (1), n ​​is 1, M is titanium, and X is a methoxy compound.

[0045] In formula (1), n ​​is 2, M is titanium, and X is a methoxy compound.

[0046] In formula (1), n ​​is 1, M is titanium, and X is an ethoxylated compound.

[0047] In formula (1), n ​​is 2, M is titanium, and X is an ethoxylated compound.

[0048] In formula (1), n ​​is 1, M is titanium, and X is a compound with a propoxy group.

[0049] In formula (1), n ​​is 2, M is titanium, and X is a compound with a propoxy group.

[0050] In formula (1), n ​​is 1, M is titanium, and X is an isopropoxy compound.

[0051] In formula (1), n ​​is 2, M is titanium, and X is an isopropoxy compound.

[0052] According to a second aspect of the present invention, a method for preparing the transition metal compound described in the first aspect of the present invention is provided, wherein the method comprises: a step of contacting a compound with the structure shown in formula (2) with M(X)4 in the presence of an organic solvent.

[0053]

[0054] In formula (2), R is a hydroxyl or amino group;

[0055] In M(X)4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1-6 carbon atoms.

[0056] In this invention, in M(X)4, X is an alkoxy group with 1-6 carbon atoms, preferably an alkoxy group with 1-3 carbon atoms.

[0057] Here, specific examples of alkoxy groups having 1 to 6 carbon atoms include methoxy, ethoxy, propoxy, isopropoxy, butoxy, pentoxy, or hexoxy, among which methoxy, ethoxy, propoxy, or isopropoxy are preferred.

[0058] In this invention, M in M(X)4 is titanium, zirconium or hafnium, preferably titanium.

[0059] In a preferred embodiment of the present invention, in M(X)4, M is titanium and X is an alkoxy group with 1-3 carbon atoms.

[0060] In a particularly preferred embodiment of the invention, M(X)4 is selected from one or more of the following compounds.

[0061] In M(X)4, M is a titanium compound and X is a methoxy group.

[0062] In M(X)4, M is a compound in which titanium and X is an ethoxy group.

[0063] In M(X)4, M is a compound in which titanium and X is a propoxy group.

[0064] In M(X)4, M is a compound in which titanium and X is an isopropoxy group.

[0065] According to the present invention, transition metal compounds with different n values ​​in formula (1) can be obtained by controlling the molar ratio of the compound with the structure shown in formula (2) to M(X)4. Specifically, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 can be 1-4.5:1.

[0066] In a preferred embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 1-1.2:1, thereby obtaining a transition metal compound with n=1 in formula (1).

[0067] In another preferred embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 2-2.1:1, thereby obtaining a transition metal compound with n=2 in formula (1).

[0068] In another preferred embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 3-3.1:1, thereby obtaining a transition metal compound with n=3 in formula (1).

[0069] In another preferred embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 4-4.1:1, thereby obtaining a transition metal compound with n=4 in formula (1).

[0070] In a specific embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 1:1, thereby obtaining a transition metal compound with n=1 in formula (1).

[0071] In another specific embodiment of the present invention, the molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 2:1, thereby obtaining a transition metal compound with n=1 in formula (1).

[0072] According to the present invention, a compound with the structure shown in formula (2) is reacted with M(X)4 in the presence of an organic solvent. The organic solvent may be any solvent that is inert to the reactants and capable of dissolving them well; examples include one or more of toluene, n-hexane, and dichloromethane. Toluene is preferred.

[0073] According to the present invention, the amount of organic solvent can be selected according to the amount of the compound with the structure shown in formula (2). Preferably, the amount of organic solvent is 5-20 mL relative to 1 millimole of the compound with the structure shown in formula (2); more preferably, the amount of organic solvent is 5-10 mL relative to 1 millimole of the compound with the structure shown in formula (2).

[0074] According to the present invention, preferably, the contact reaction comprises: dispersing M(X)4 in the organic solvent under an inert atmosphere (e.g., under a nitrogen atmosphere or an argon atmosphere), and then adding the compound with the structure shown in formula (2) dropwise to the solution in which M(X)4 is dispersed to carry out the contact reaction.

[0075] According to the present invention, the conditions for the contact reaction may include: a temperature of 10-35°C and a time of 8-50 hours; preferably, the conditions for the contact reaction include: a temperature of 15-30°C and a time of 10-30 hours.

[0076] According to the present invention, after the contact reaction is completed, the product can be purified using conventional purification methods in the art. In a preferred embodiment of the present invention, after the contact reaction is completed, the contact reaction product is dried to obtain the transition metal compound of the present invention.

[0077] The above drying can be performed using solvent removal methods commonly used in the art. In a preferred embodiment of the invention, the solvent is removed under vacuum at 25-60°C.

[0078] The method for preparing transition metal compounds according to the present invention has a high yield, and the post-processing only requires drying, which is extremely simple.

[0079] According to a third aspect of the present invention, a ring-opening polymerization method is provided, wherein the method comprises: ring-opening polymerization of a cyclic ester monomer in the presence of a transition metal compound as described in the first aspect of the present invention.

[0080] According to the present invention, preferably, in the ring-opening polymerization system, the concentration of the transition metal compound is 1-1000 × 10⁻⁶. -6 More preferably, in the ring-opening polymerization system, the concentration of the transition metal compound is 1-20 × 10⁻⁶ moles per milliliter; -6 Moles per milliliter.

[0081] According to the present invention, preferably, the molar ratio of the transition metal compound to the cyclic ester monomer can be 1:3000; preferably, the molar ratio of the transition metal compound to the cyclic ester monomer is 1:400-800.

[0082] According to the present invention, the cyclic ester monomer can be a cyclic ester monomer commonly used in the art for ring-opening polymerization, for example, one or more of lactide, caprolactone, valproic acid lactide, butyrolactone and methylcaprolactone; preferably, the cyclic ester monomer is lactide and / or caprolactone; more preferably, the cyclic ester monomer is D,L-lactide.

[0083] According to the present invention, when the monomer is in a liquid state, a solvent may or may not be used. When a solvent is used, the solvent may be, for example, toluene and / or n-hexane. There is no particular limitation on the amount of solvent used; it can be the conventional amount used in the art when using solvents.

[0084] According to the present invention, preferably, the conditions for ring-opening polymerization include: a temperature of 0-200°C and a time of 0.5-24h; more preferably, the conditions for ring-opening polymerization include: a temperature of 150-200°C and a time of 0.5-1h.

[0085] In this invention, the transition metal compound undergoes ring-opening polymerization via a coordination-insertion mechanism. For example, when using D,L-lactide for ring-opening polymerization, amorphous polylactic acid can be obtained.

[0086] In this invention, after the ring-opening polymerization is completed, the ring-opening polymer can be obtained by processing it using conventional methods in the art. For example, it can be carried out by adding excess methanol to a solution containing crude polymer product to induce polymer precipitation.

[0087] According to a fourth aspect of the present invention, the use of the transition metal compound described in the first aspect of the present invention in the ring-opening polymerization of cyclic ester monomers is provided.

[0088] Through the above technical solutions, the present invention can provide a new transition metal compound, its preparation method and application, as well as a ring-opening polymerization method using the new transition metal compound as a catalyst. The preparation method of the transition metal compound of the present invention is simple, and the monomer conversion rate of the ring-opening polymerization is high.

[0089] In this invention, the conversion rate can be calculated using... 1 H nuclear magnetic resonance (H nuclear magnetic resonance) 1 The conversion rate is determined by ¹H-NMR experiments and obtained by comparing the integral area of ​​the corresponding chemical shifts of the polymer and monomer in the system. For example, when using the monomer D,L-lactide for ring-opening polymerization, the chemical shift of the hydrogen at the methylene position of amorphous polylactic acid is 5.26-5.12 ppm with an integral area of ​​1, while the chemical shift of the hydrogen at the methylene position of monomer D,L-lactide is 5.09-5.02 ppm with an integral area of ​​0.05. Therefore, the conversion rate is 1 / 1.05 = 95.2%.

[0090] The present invention will be described in detail below through embodiments, but the present invention is not limited to the following embodiments.

[0091] In the following embodiments,

[0092] The conversion rate of the synthesized transition metal compound to the polymer was determined by... 1 H nuclear magnetic resonance (H nuclear magnetic resonance) 1 H-NMR experiments were conducted on a Bruker AVANCE III-400MHz spectrometer with CDCl3 as solvent at 25°C. The conversion rate was calculated by comparing the ratio of the integral area of ​​the corresponding chemical shifts of the polymer and monomer in the system.

[0093] Number-average molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) on a Waters Alliance GPCV2000.

[0094] Example 1

[0095] This example illustrates the synthesis of transition metal compound B-1.

[0096] Transition metal compound B-1: In the compound with the structure shown in formula (1), n ​​is 1, M is titanium, and X is isopropoxy.

[0097] 15 mL of toluene solution was added to a 100 mL Schlenk flask with magnetic stirring at room temperature under a nitrogen atmosphere, followed by 0.89 mL of Ti(O)₂. i After stirring Pr)4 (3 mmol) for 5 minutes to disperse evenly, add 0.45 mL of compound A (R in formula (2) is hydroxyl, 3 mmol), stir for 20 hours, keep the reaction temperature at 25°C, remove the solvent under vacuum at room temperature, and obtain 1.13 g of dark orange oil, with a yield of 97.0%.

[0098] 1 H-NMR (CDCl3): 6.60-6.51ppm (3H), 4.61-4.57ppm (3H), 2.91-2.88ppm (2H), 1.37-1.33ppm (6H), 1.21-1.16ppm (18H).

[0099] Example 2

[0100] This example illustrates the synthesis of transition metal compound B-1.

[0101] Transition metal compound B-2: In the compound with the structure shown in formula (1), n ​​is 2, M is titanium, and X is isopropoxy.

[0102] 15 mL of toluene solution was added to a 100 mL Schlenk flask with magnetic stirring at room temperature under a nitrogen atmosphere, followed by 0.89 mL of Ti(O)₂. i Pr)4 (3 mmol), after stirring for 5 minutes to disperse evenly, add 0.90 mL of compound A (R in formula (2) is hydroxyl, 6 mmol), stir for 24 hours, keep the reaction temperature at 30°C, remove the solvent under vacuum at room temperature, and obtain 1.42 g of dark orange oil, with a yield of 96.1%.

[0103] 1 H-NMR (CDCl3): 6.75-6.69ppm (6H), 5.01-4.51ppm (2H), 3.10-2.85ppm (4H), 1.55-1.35ppm (12H), 1.34-1.10ppm (12H).

[0104] Example 3

[0105] This example illustrates the application of transition metal compound B-1 in the ring-opening polymerization of D,L-lactide.

[0106] A thoroughly dried 100 mL Schlenk flask was rinsed three times with nitrogen. 0.865 g of D,L-lactide, obtained by recrystallization three times from anhydrous ethyl acetate, was weighed and heated to 180 °C. 1 mL of a toluene catalyst solution (containing 10 μmol of transition metal compound B-1) was added. After reacting for 1 hour, the mixture was cooled to room temperature, and 5 mL of deuterated chloroform was added to dissolve the product. A small amount of the solution was taken for conversion determination. Excess methanol was added to the solution containing the crude polymer product to induce polymer precipitation. The resulting product was filtered, dissolved in dichloromethane, and excess methanol was added again to induce polymer precipitation, followed by filtration. The polymer was then vacuum-dried at 50 °C for 24 hours. 1 The conversion rate was 82% as determined by H-NMR. The number-average molecular weight of the polymer, Mn, was 25303, and the molecular weight distribution was 1.561 as determined by GPC.

[0107] Example 4

[0108] This example illustrates the application of transition metal compound B-1 in the ring-opening polymerization of D,L-lactide.

[0109] The method was carried out according to Example 3, except that the reaction time was 3 hours. The same polymer conversion rate was obtained, and GPC analysis showed that the number-average molecular weight Mn = 17766 and the molecular weight distribution = 1.469.

[0110] Example 5

[0111] This example illustrates the application of transition metal compound B-1 in the ring-opening polymerization of D,L-lactide.

[0112] The method described in Example 3 was followed, except that the reaction time was 18 hours. The same result was obtained with a polymer conversion rate of 97%. GPC analysis showed that the polymer number-average molecular weight Mn = 24544 and the molecular weight distribution = 1.544.

[0113] Example 6

[0114] This example illustrates the application of transition metal compound B-1 in the ring-opening polymerization of D,L-lactide.

[0115] The method described in Example 3 was followed, except that 0.58 g of D,L-lactide was added and the reaction time was 0.5 hours. The same result yielded a polymer conversion rate of 92%. GPC analysis showed that the polymer number-average molecular weight Mn = 26021 and the molecular weight distribution = 1.659.

[0116] Example 7

[0117] This example illustrates the application of transition metal compound B-1 in the ring-opening polymerization of D,L-lactide.

[0118] The method described in Example 3 was followed, except that 1.153 g of D,L-lactide was added and the reaction time was 24 hours. The same result was obtained with a polymer conversion rate of 95%. GPC analysis showed that the polymer number-average molecular weight Mn = 23878 and the molecular weight distribution = 1.462.

[0119] Example 8

[0120] This example illustrates the application of transition metal compound B-2 in the ring-opening polymerization of D,L-lactide.

[0121] The method described in Example 3 was followed, except that the same molar amount of transition metal compound B-2 was added to replace transition metal compound B-1. The same result was obtained with a polymer conversion rate of 90%. GPC analysis showed that the polymer number-average molecular weight Mn = 26770 and molecular weight distribution = 1.958.

[0122] Example 9

[0123] The method described in Example 6 was followed, except that the same molar amount of transition metal compound B-2 was added to replace transition metal compound B-1. The same result was obtained with a polymer conversion rate of 94%. GPC analysis showed that the number-average molecular weight of the polymer was Mn = 20113 and the molecular weight distribution was 1.44.

[0124] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A transition metal compound, characterized in that, This transition metal compound has the structure shown in formula (1). In formula (1), n ​​is an integer from 1 to 4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1 to 6 carbon atoms.

2. The transition metal compound according to claim 1, wherein, In formula (1), n ​​is 1 or 2, M is titanium, and X is an alkoxy group with 1-3 carbon atoms.

3. The transition metal compound according to claim 2, wherein, The transition metal compound is selected from one or more of the following compounds. In formula (1), n ​​is 1, M is titanium, and X is a methoxy compound. In formula (1), n ​​is 2, M is titanium, and X is a methoxy compound. In formula (1), n ​​is 1, M is titanium, and X is an ethoxylated compound. In formula (1), n ​​is 2, M is titanium, and X is an ethoxylated compound. In formula (1), n ​​is 1, M is titanium, and X is a compound with a propoxy group. In formula (1), n ​​is 2, M is titanium, and X is a compound with a propoxy group. In formula (1), n ​​is 1, M is titanium, and X is an isopropoxy compound. In formula (1), n ​​is 2, M is titanium, and X is an isopropoxy compound.

4. A method for preparing a transition metal compound, characterized in that, The method includes the step of contacting a compound with the structure shown in formula (2) with M(X)4 in the presence of an organic solvent. In formula (2), R is a hydroxyl or amino group; In M(X)4, M is titanium, zirconium or hafnium, and X is an alkoxy group with 1-6 carbon atoms.

5. The method for preparing the transition metal compound according to claim 4, wherein, The molar ratio of the compound with the structure shown in formula (2) to M(X)4 is 1-4.5:

1.

6. The method for preparing the transition metal compound according to claim 4, wherein, The organic solvent is selected from one or more of toluene, n-hexane, and dichloromethane; Preferably, the amount of organic solvent used is 5-20 mL, more preferably 5-10 mL, relative to 1 millimole of the compound with the structure shown in formula (2).

7. The method for preparing the transition metal compound according to any one of claims 4-6, wherein, The conditions for the contact reaction include: a temperature of 10-35℃ and a time of 8-50h.

8. A ring-opening polymerization method, characterized in that, The method comprises: ring-opening polymerization of a cyclic ester monomer in the presence of a transition metal compound as described in any one of claims 1-3.

9. The ring-opening polymerization method according to claim 8, wherein, In the ring-opening polymerization system, the concentration of the transition metal compound is 1-1000×10⁻⁶. -6 Moles per milliliter, preferably 1-20 × 10 -6 Moles per milliliter.

10. The ring-opening polymerization method according to claim 8, wherein, The molar ratio of the transition metal compound to the cyclic ester monomer is 1:3000, preferably 1:400-1000.

11. The ring-opening polymerization method according to any one of claims 8-10, wherein, The cyclic ester monomer is lactide and / or caprolactone.

12. The ring-opening polymerization method according to claim 11, wherein, The conditions for the ring-opening polymerization include: a temperature of 0-200℃ and a time of 0.5-24h.

13. The use of the transition metal compound according to any one of claims 1-3 in the ring-opening polymerization of cyclic ester monomers.

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