A process for the preparation of a cyclic poly(l-lactide)

Abstract

The application relates to a preparation method of cyclic poly(L-lactide), and belongs to the technical field of polymer preparation. The method uses L-lactide as a polymerization monomer, adopts a Salalen type rare earth metal alkoxy complex as a catalyst, and successfully prepares cyclic poly(L-lactide) with a high molecular weight through a solvent-free ring-opening polymerization reaction at a reaction temperature of 110-180 DEG C. The method has the advantages of high catalytic efficiency, high molecular weight of the obtained polymer, cyclic structure of the polymer, and the like, is green and solvent-free, and the product is a polymer containing only a cyclic topological structure.

Description

Technical Field

[0001] This invention relates to the field of polymer preparation technology, and in particular to a method for preparing cyclic poly(L-lactide). Background Technology

[0002] Compared to linear polymers, cyclic polymers lack chain ends, thus exhibiting many different properties, such as higher glass transition temperatures, smaller hydrodynamic radii, and lower intrinsic viscosities (see: Josse, T.; Winter, JD; Gerbaux, P.; Coulembie, O. Angew. Chem. Int. Ed. 2016, 55, 13944-13958.). Therefore, cyclic polymers can exhibit unique properties distinct from linear polymers, possessing significant application potential, particularly in emerging technologies such as drug delivery, surface modification, and hierarchical assembly (see: Chen, CJ; Weil, TJ Nanoscale Horiz. 2022, 7, 1121-1135.). Currently, the main methods for synthesizing cyclic polymers are ring-closure and ring-expansion methods (see: Haque, FM; Grayson, SM Nat. Chem. 2020, 12, 433-444.). Traditional ring-closure methods for synthesizing cyclic polymers require highly diluted conditions, making the reaction process demanding and difficult to scale up. Limited by unfavorable thermodynamic entropy, the ring-closure method can only synthesize cyclic polymers with low molecular weights (typically M...). n< 25,000 g / mol). Furthermore, the ring-closure method suffers from drawbacks such as low reaction efficiency, long reaction time, and a tendency to generate linear polymers (see: Gao, RT; Xu, L.; Li, SY; Liu, N.; Chen, Z.; Wu, ZQChem. Eur. J. 2023, 29, e202300916.). The ring-expansion method, on the other hand, involves the direct growth of cyclic structures through the repeated insertion of cyclic monomers into a cyclic catalyst or initiator. This strategy fundamentally overcomes the entropy disadvantage of traditional ring-closure routes, allowing the synthesis of cyclic polymers at conventional concentrations. This method enables the large-scale preparation of cyclic polymers from milligrams to grams within a wide reaction time window ranging from seconds to days, and can efficiently obtain high molecular weight cyclic polymers (see: Kricheldorf, HR; Lee, SR Macromolecules 1995, 28, 6718-6725.). However, most existing methods for preparing cyclic poly(L-lactide) by ring expansion have the disadvantage of producing impure products (see: Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H.Chem. Rev. 2001, 101, 3747-3792.).

[0003] In 2007, Culkin, DA, and Waymouth, RM, et al. first reported a method for preparing cyclic (poly-L-lactide) via ring-opening polymerization of L-lactide in tetrahydrofuran solution using N-heterocyclic carbene as an organic catalyst. However, this catalytic system has drawbacks such as low product molecular weight (the highest being only about 26 kg / mol) and insufficient purity of the cyclic topology (see: Culkin, DA; Jeong, W.; Csihony, S.; Gomez, ED; Balsara, NP; Hedrick, JL; Waymouth, RM Angew. Chem., Int. Ed. 2007, 46, 2627-2630.).

[0004] In 2011, Weil et al. reported an aluminum-based catalyst for the preparation of cyclic (poly-L-lactide) polymers via melt bulk or solution ring-expansion polymerization of lactide. This catalyst exhibited high activity and yielded cyclic polymers with high molecular weight and narrow distribution. However, MALDI-TOF mass spectrometry characterization revealed the presence of methoxy-terminated linear polymers in the products, indicating insufficient purity of the cyclic topology (see: Weil, J.; Mathers, RT; Getzler, YDYLMacromolecules 2012, 45, 1118-1121.).

[0005] In 2015, Bonnet, F. catalyzed the bulk polymerization of L-lactide using Ln(BH4)3(THF)3 as a catalyst at 130 °C in a solvent-free environment to obtain cyclic poly(L-lactide). However, the molecular weight of the obtained polymer was only 18 kg / mol, and the polymer contained linear poly(L-lactide) (see: Bonnet, F.; Stoffelbach, F.; Fontaine, G.; Bourbigot, S. RSC Adv. 2015, 5, 31303-31310.).

[0006] In 2017, Kricheldorf, HR discovered that five-membered cyclic dibutyltin (DSTL) can catalyze the bulk melt polymerization of L-lactide at high temperatures to generate cyclic poly(L-lactide). However, the catalyst exhibits low catalytic activity, and linear poly(L-lactide) is formed in the product (see: Kricheldorf, HR; Weidner, SM; Scheliga, F. Polym. Chem. 2017, 8, 1589-1596.).

[0007] Therefore, existing methods for preparing cyclic poly(L-lactide) often result in low molecular weight polymers and the formation of linear polymers. Thus, developing highly active catalytic methods for L-lactide polymerization to prepare high-molecular-weight, pure cyclic poly(L-lactide) is a challenging problem. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for preparing cyclic poly(L-lactide), specifically a method for preparing cyclic poly(L-lactide) under bulk polymerization conditions. This method is green and solvent-free, produces polymers containing only cyclic topologies, and has advantages such as high catalytic efficiency, high molecular weight of the obtained polymers, and all polymers possessing cyclic structures.

[0009] To achieve the above-mentioned objectives, the specific technical solution adopted by the present invention is as follows:

[0010] The purpose of this invention is to provide a method for preparing cyclic poly(L-lactide), comprising the following steps: under an inert atmosphere, L-lactide is mixed and sealed with a Salalen-type rare earth metal alkoxy complex catalyst, and a solvent-free bulk ring-opening polymerization reaction is carried out to obtain cyclic poly(L-lactide).

[0011] In one embodiment of the present invention, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0012] In one embodiment of the present invention, the structural formula of the Salalen-type rare earth metal alkoxy complex is as follows:

[0013] ; where RE is samarium, ytterbium or yttrium, preferably yttrium.

[0014] In one embodiment of the present invention, the molar ratio of the Salalen-type rare earth metal alkoxy complex to the L-lactide is 1:1000~3500.

[0015] In one embodiment of the present invention, the temperature of the ring-opening polymerization reaction is 110°C to 180°C.

[0016] In one embodiment of the present invention, the temperature of the ring-opening polymerization reaction is 130°C to 180°C.

[0017] In one embodiment of the present invention, the ring-opening polymerization reaction takes 10 min to 12 h; preferably 10 min to 10 h.

[0018] In one embodiment of the present invention, the structure of the cyclic poly(L-lactide) is as follows:

[0019] Where n≥195.

[0020] In one embodiment of the present invention, the cyclic poly(L-lactide) has a molecular weight of 28.3 kg / mol to 60.6 kg / mol.

[0021] In one embodiment of the present invention, the ring-opening polymerization reaction further includes the following separation and purification steps: dissolving the polymerization product in an organic solvent, adding a precipitant for precipitation, separating and drying.

[0022] Compared with the prior art, the above-described technical solution of the present invention has the following advantages:

[0023] This invention provides a method for preparing cyclic poly(L-lactide). The method uses L-lactide as the monomer and a Salalen-type rare earth metal alkoxy complex as the catalytic system. Through solvent-free ring-opening polymerization at a reaction temperature of 110 °C to 180 °C, high-molecular-weight cyclic poly(L-lactide) is successfully prepared.

[0024] The Salalen-type rare-earth metal alkoxy complexes used in this invention possess unique ligand structure characteristics. Salalen ligands are a type of "semi-rigid, semi-flexible" heteroligand, whose framework simultaneously contains imine and reduced amine groups, combining the rigid framework of a Schiff base with the flexible side arms of an amino-bridged bisphenol. This structure allows the ligand to form an asymmetric coordination environment around the rare-earth metal center: the imine side is more rigid, which is beneficial for stabilizing the metal center; the amine side is more flexible, which is beneficial for adjusting steric hindrance and electronic effects, providing an ideal coordination platform for subsequent catalytic reactions.

[0025] Rare earth metals have unique advantages as central ions: First, they have strong oxyphilicity, enabling them to form stable coordination with the carbonyl oxygen of lactide monomers, promoting monomer activation and ring-opening; second, they have high coordination numbers and tunable coordination configurations, allowing them to form stable six- or seven-coordinate structures with Salalen ligands, ensuring the structural integrity of the catalytic center during polymerization; furthermore, their moderate Lewis acidity allows them to effectively catalyze ring-opening reactions without triggering severe racemization side reactions, thus maintaining the optical purity of the product.

[0026] Regarding the polymerization mechanism, this catalytic system follows a coordination insertion mechanism. First, the L-lactide monomer coordinates with the rare-earth metal center. Subsequently, the alkoxy initiating group attacks the carbonyl carbon of the monomer, resulting in acyl-oxygen bond cleavage and ring-opening insertion of the monomer, forming a new rare-earth active center at the end of the polymer chain. As the polymerization proceeds, the active chain continues to grow. Under the high-temperature conditions of bulk polymerization, when the polymer chain grows to a critical length, it tends to undergo an intramolecular "bitback" reaction driven by thermodynamics: the oxygen atom at the end of the active chain attacks the ester group in the same molecular chain, forming a cyclic topology.

[0027] Salalen-type rare earth catalysts play a synergistic role in this process: on the one hand, the rigid imine portion of the ligand provides a stable coordination environment, ensuring the smooth progress of the chain growth process; on the other hand, the flexible amine portion of the ligand effectively suppresses side reactions such as transesterification by adjusting steric hindrance, avoiding premature chain closure that leads to excessively low molecular weight, and ultimately achieving the controllable synthesis of high molecular weight cyclic poly(L-lactide). Attached Figure Description

[0028] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0029] Figure 1 This is a matrix-assisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrum of the cyclic poly(L-lactide) prepared in Example 1 of this invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0031] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0032] Example 1

[0033] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:1500) catalyzed by Salalen-type yttrium alkoxy complexes:

[0034] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.08 g of L-lactide (7.5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 140 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (0.94 g, yield 87%). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n The molecular weight was 42.6 kg / mol, and the molecular weight distribution was 1.7. The polymer was characterized by matrix-assisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrometry (e.g., [missing information]). Figure 1 As shown in the figure, the obtained polymers all have a cyclic topology.

[0035] Example 2

[0036] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0037] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 150 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.32 g of polymer (92% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 43.8 kg / mol and a molecular weight distribution of 1.7.

[0038] Example 3

[0039] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2500) catalyzed by Salalen-type yttrium alkoxy complexes:

[0040] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.80 g of L-lactide (12.5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 160 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.60 g of polymer (89% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 45.8 kg / mol and a molecular weight distribution of 1.8.

[0041] Example 4

[0042] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:3000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0043] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 2.16 g of L-lactide (15 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (1.92 g, yield 89%). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 48.3 kg / mol and a molecular weight distribution of 1.8.

[0044] Example 5

[0045] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:3500) catalyzed by Salalen-type yttrium alkoxy complexes:

[0046] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 2.52 g of L-lactide (17.5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 180 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (2.27 g, 90% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 46.3 kg / mol and a molecular weight distribution of 1.8.

[0047] Example 6

[0048] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:1000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0049] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 0.72 g of L-lactide (5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (0.57 g, yield 79%). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 42.1 kg / mol and a molecular weight distribution of 1.8.

[0050] Example 7

[0051] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:1500) catalyzed by Salalen-type yttrium alkoxy complexes:

[0052] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.08 g of L-lactide (7.5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (0.94 g, yield 87%). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 45.3 kg / mol and a molecular weight distribution of 1.9.

[0053] Example 8

[0054] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0055] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.33 g of polymer (92% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 51.2 kg / mol and a molecular weight distribution of 1.8.

[0056] Example 9

[0057] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2500) catalyzed by Salalen-type yttrium alkoxy complexes:

[0058] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.80 g of L-lactide (12.5 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.60 g of polymer (89% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 50.6 kg / mol and a molecular weight distribution of 1.7.

[0059] Example 10

[0060] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0061] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 130 °C and reacted for 10 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.28 g of polymer (89% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 39.3 kg / mol and a molecular weight distribution of 1.6.

[0062] Example 11

[0063] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0064] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 140 °C and reacted for 3 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.25 g of polymer (87% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 40.7 kg / mol and a molecular weight distribution of 1.7.

[0065] Example 12

[0066] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0067] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 160 °C and reacted for 1.5 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.30 g of polymer (90% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 43.4 kg / mol and a molecular weight distribution of 1.8.

[0068] Example 13

[0069] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0070] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 170 °C, and the reaction was carried out for 0.5 h. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.30 g of polymer (90% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC), and the molecular weight M of the polymer was determined. n It has a concentration of 43.4 kg / mol and a molecular weight distribution of 1.8.

[0071] Example 14

[0072] This embodiment provides a method for preparing cyclic poly(L-lactide) by ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes:

[0073] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 180 °C and reacted for 10 min. After cooling to room temperature, chloroform was added to dissolve the product. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.30 g of polymer (90% yield). The obtained polymer was analyzed by gel permeation chromatography (GPC) to determine the molecular weight M. n It has a concentration of 34.8 kg / mol and a molecular weight distribution of 1.7.

[0074] Comparative Example 1

[0075] This comparative example provides a comparative experiment on the ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes, aiming to investigate the polymerization behavior under low-temperature conditions (100℃):

[0076] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 100 °C and reacted for 10 h before cooling to room temperature. Observation of the reaction system showed that the product was viscous but not completely solidified. Adding chloroform to dissolve the product revealed a large amount of unreacted monomer residue. The resulting solution was poured into n-hexane, resulting in only a small amount of precipitate. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 0.07 g of polymer (5% yield). Gel permeation chromatography analysis of the obtained polymer failed to yield a valid GPC spectrum due to the low molecular weight and small sample amount. Matrix-assisted laser desorption / ionization time-of-flight mass spectrometry analysis showed that the product mainly consisted of lactide monomer and a small amount of oligomers (degree of polymerization n < 10), with no high molecular weight polymers detected.

[0077] Results analysis: Compared with Example 10 (130℃, 10 h, yield 87%, M... nCompared to the 39.3 kg / mol polymerization rate, this comparative example lowered the reaction temperature to 100°C, resulting in almost no efficient polymerization, extremely low yield (only 5%), and no high molecular weight polymer obtained. This indicates that the reaction temperature is crucial to this catalytic system: at excessively low temperatures (100°C), the polymerization rate is extremely slow, making it difficult to initiate or hindering chain growth, thus preventing the synthesis of high molecular weight polylactide. This comparative example demonstrates that the reaction temperature range used in this invention is a key process condition for achieving efficient polymerization and high molecular weight products.

[0078] Comparative Example 2

[0079] This comparative example provides a comparative experiment on the ring-opening polymerization of L-lactide (molar ratio 1:2000) catalyzed by Salalen-type yttrium alkoxy complexes, aiming to investigate the effect of excessively high reaction temperature (190℃) on the properties of the polymerization product.

[0080] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 1.44 g of L-lactide (10 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 190 °C and reacted for 10 h. After cooling to room temperature, the reaction system was observed. The product was a dark brown solid with a significantly deepened color. The product was dissolved in chloroform, and the solution turned yellowish-brown. The resulting solution was poured into n-hexane, precipitating the polymer. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding 1.33 g of polymer (92% yield). The obtained polymer was analyzed by gel permeation chromatography, and the molecular weight M of the polymer was determined. n It has a concentration of 33.3 kg / mol and a molecular weight distribution of 1.9.

[0081] Results analysis: Compared with Example 10 (130℃, 10 h, yield 87%, M... n Compared to the original (39.3 kg / mol, molecular weight distribution 1.6), this comparative example increased the reaction temperature to 190℃. Although the yield slightly increased (92%), the product color became significantly darker (dark brown), and the molecular weight distribution broadened significantly (from 1.6 to 1.9), indicating significant thermal degradation or side reactions under high-temperature conditions. This demonstrates that reaction temperature has a crucial impact on this catalytic system: excessively high temperatures (above 180℃) can trigger severe transesterification or thermal degradation side reactions, leading to a darker product color and decreased structural uniformity. This comparative example proves that the reaction temperature range used in this invention is a key process condition for achieving controllable polymerization and high product quality.

[0082] Comparative Example 3

[0083] This comparative example provides a comparative experiment on the ring-opening polymerization of L-lactide (molar ratio 1:4000) catalyzed by Salalen-type yttrium alkoxy complexes, aiming to investigate the effect of an excessively high monomer / catalyst ratio on the polymerization reaction.

[0084] In a glove box under high-purity nitrogen protection, a magnetic stir bar, 0.0035 g of Salalen-type yttrium alkoxy complex (0.005 mmol), and 2.88 g of L-lactide (20 mmol) were added to a 5 mL thick-walled pressure-resistant bottle that had undergone dehydration and deoxygenation treatment. The pressure-resistant bottle was placed in an oil bath at 180℃ and reacted for 2 h. After cooling to room temperature, chloroform was added to dissolve the product. Observation of the reaction system revealed that the viscosity of the system was low, and a large amount of unreacted monomers remained. The resulting solution was poured into n-hexane, and only a small amount of precipitate formed. After filtration, the obtained polymer was dried in a vacuum drying oven to constant weight, yielding a polymer (0.26 g, yield 9%). Gel permeation chromatography analysis of the obtained polymer failed to obtain an effective GPC spectrum due to the low molecular weight and small sample amount. Matrix-assisted laser desorption / ionization time-of-flight mass spectrometry analysis showed that the product mainly consisted of lactide monomers and a small amount of oligomers (degree of polymerization n < 8), and no high molecular weight polymers were detected.

[0085] Results Analysis: Compared with Example 5 (monomer / catalyst ratio 3500:1, 180℃, 2 h, yield 90%, Mn = 46.3 kg / mol), this comparative example increased the monomer / catalyst ratio to 4000:1, while keeping all other reaction conditions identical. The results showed that the polymerization reaction was almost impossible to proceed effectively, with the yield plummeting to 9%, and no high molecular weight polymer was obtained. This indicates that the monomer / catalyst ratio has a crucial impact on this catalytic system: when the catalyst concentration is too low (i.e., the monomer / catalyst ratio is too high), the number of active centers is insufficient to effectively initiate and maintain the chain growth process, leading to inhibited polymerization. This comparative example demonstrates that the monomer / catalyst ratio range used in this invention is a key process condition for achieving efficient polymerization and high molecular weight products.

[0086] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing cyclic poly(L-lactide), characterized in that, Includes the following steps: L-lactide was mixed with a Salalen-type rare earth metal alkoxy complex catalyst under an inert atmosphere and sealed for solvent-free bulk ring-opening polymerization to obtain cyclic poly(L-lactide). The structural formula of the Salalen-type rare earth metal alkoxy complex is: Where RE represents samarium, ytterbium, or yttrium; The molar ratio of the Salalen-type rare earth metal alkoxy complex to the L-lactide is 1:1000~3500; The ring-opening polymerization reaction is carried out at a temperature of 130℃~180℃.

2. The preparation method according to claim 1, characterized in that, The inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

3. The preparation method according to claim 1, characterized in that, The ring-opening polymerization reaction takes 10 min to 12 h.

4. The preparation method according to claim 1, characterized in that, The structure of the cyclic poly(L-lactide) is as follows: Where n≥195.

5. The preparation method according to claim 1, characterized in that, The molecular weight of the cyclic poly(L-lactide) is 28.3 kg / mol to 60.6 kg / mol.

6. The preparation method according to claim 1, characterized in that, The ring-opening polymerization reaction is followed by the following separation and purification steps: dissolving the polymerization product in an organic solvent, adding a precipitant to precipitate, separating and drying.

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