An unsaturated polylactic acid copolyester and a method for preparing the same

Abstract

The application relates to the technical field of polylactic acid copolyesters, and discloses an unsaturated polylactic acid copolyester and a preparation method thereof, a structural formula of the unsaturated polylactic acid copolyester being as follows: wherein m and n are natural numbers greater than 10; R1 is a hydroxyl group, an alkoxy group or an arylalkoxy group; and R2 is hydrogen or an alkyl group. The polylactic acid copolyester is obtained by copolymerization of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione, the two structures are very similar, the copolyester obtained through ring-opening copolymerization has good toughness, good thermodynamic properties, and does not reduce the degradability of the polyester.

Description

Technical Field

[0001] The present invention relates to the technical field of polylactic acid copolyester, in particular to an unsaturated polylactic acid copolyester and a preparation method thereof. Background Art

[0002] Polyester, due to its high molecular weight, high hydrophobicity, and high chemical bond energy, is difficult to degrade by microorganisms, resulting in its long-term persistence and accumulation in the environment. "White pollution" has become a global problem. Consequently, interest in biodegradable polyesters has grown in recent years, with PLA, PPC, PBS, PBAT, PHA, and other products now being industrialized.

[0003] Polylactic acid (PLA) is a popular biodegradable polyester on the market. It has a biomass source and can be chemically synthesized using lactic acid, a product of microbial fermentation, as a monomer. It is biodegradable and absorbable, and has high strength. The degradation products of PLA are lactic acid, carbon dioxide, and water, all of which are harmless natural small molecules. Although PLA has a high melting point and glass transition temperature, it has poor toughness and is brittle, and generally requires modification to meet actual needs. Toughening PLA by adding small molecule toughening agents is a common method. Although it has certain effects, the plasticizer is prone to seepage over time, which reduces the stability and safety of polyester.

[0004] Many studies have been conducted to improve the toughness of polyester while ensuring its stability and safety. For example, patent CN103328547A discloses a polylactic acid resin film that improves the toughness of polyester by copolymerizing polylactic acid and polyurethane polyol. However, this also reduces the heat resistance of the polyester, and the glass transition temperature and melting point of the resulting copolyester are significantly lower than those of polylactic acid. Patent CN105440605A discloses a polylactic acid resin that incorporates aliphatic-aromatic copolyester segments into the polylactic acid resin to enhance its flexibility. However, the introduction of aromatic polyester significantly reduces the degradability of the polyester. Summary of the Invention

[0005] In order to solve the technical problem of modified polylactic acid copolyester having good degradability, toughness and heat resistance, the present invention provides an unsaturated polylactic acid copolyester and a preparation method thereof. The polylactic acid copolyester is obtained by copolymerizing lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione. Because the structures of the two are very similar, the copolyester obtained by ring-opening copolymerization has good toughness and good thermodynamic properties, and does not reduce the degradability of the polyester.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] In a first aspect, the present invention provides an unsaturated polylactic acid copolyester, wherein the structural formula of the unsaturated polylactic acid copolyester is:

[0008]

[0009] Wherein, m and n are natural numbers greater than 10; R1 is hydroxy, alkoxy or arylalkoxy; and R2 is hydrogen or alkyl.

[0010] The structure of the unsaturated polylactic acid copolyester in the present invention is obtained by copolymerization of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione. Because the structures of the two are very similar, the copolyester obtained by ring-opening copolymerization has good toughness and good thermodynamic properties, and does not reduce the degradability of the polyester.

[0011] In a second aspect, the present invention also provides a method for preparing an unsaturated polylactic acid copolyester, comprising the following steps: in the presence of an initiator, a catalyst and a co-catalyst, lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione are subjected to a ring-opening copolymerization reaction in a solvent to obtain an unsaturated polylactic acid copolyester.

[0012] The structural formula of the 3-methylene-6-methyl-1,4-dioxane-2,5-dione is as follows:

[0013] 3-Methylene-6-methyl-1,4-dioxane-2,5-dione has a very similar structure to lactide and can be produced from lactide. The resulting copolyester exhibits a high melting point and glass transition temperature, excellent thermodynamic properties, and good biodegradability. The resulting copolyester is rich in unsaturated double bonds, providing numerous modification sites for the copolyester. This allows for more efficient and convenient subsequent performance tuning of the polyester. For example, by reacting with the double bonds on the polyester, side chains with different structures and functional groups can be introduced, allowing for more convenient and comprehensive customization of PLA properties.

[0014] Preferably, the ring-opening copolymerization reaction is carried out under nitrogen protection at -60 to 0° C. for 30 to 120 minutes.

[0015] Preferably, the total molar concentration of the lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione in the solvent is 0.5 to 3 mol / L; and the molar amount of the lactide accounts for 60 to 90% of the total molar amount of the lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione.

[0016] The ratio of monomer copolymerization will affect the degradability of copolyester fibers, as well as the mechanical properties and spinnability in subsequent processing.

[0017] Preferably, the co-catalyst structural formula is:

[0018]

[0019] Wherein, R3 and R4 are each selected from one of methyl, ethyl, propyl, phenyl, cyclohexyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, 4-chlorophenyl, 2,6-dimethylphenyl, 2,4-dimethoxyphenyl and 2,4,6-trimethoxyphenyl.

[0020] The co-catalyst used is highly efficient and non-toxic, with high selectivity and a monomer conversion rate greater than 99%. It can also completely retain the double bond on 3-methylene-6-methyl-1,4-dioxane-2,5-dione, which is beneficial for providing more modification sites in the subsequent step.

[0021] Preferably, the molar ratio of the co-catalyst to the initiator is 1 to 6:1.

[0022] The co-catalyst urea is used in combination with a strong base. The co-catalyst can reduce the activity of the strong base catalyst to a certain extent, affecting the reaction equilibrium and promoting the reaction in the forward direction. The strong base abstracts one hydrogen of the urea, and the urea anion forms hydrogen bonds with the initiator and monomer, resulting in high selectivity and monomer conversion for catalytic ring-opening polymerization. However, urea alone is not sufficiently active to catalyze ring-opening polymerization. Furthermore, if the urea dosage is too low, the reaction will be too vigorous, with simultaneous chain growth and scission, which is detrimental to molecular weight growth. The intense exothermic reaction can also cause double bond reactions in 3-methylene-6-methyl-1,4-dioxane-2,5-dione, hindering the production of linear copolyesters and impairing processability.

[0023] Preferably, the molar ratio of the catalyst to the initiator is 1 to 10:1.

[0024] Preferably, the initiator is a diol; the initiator is one of terephthalic acid methanol, 1,4-cyclohexanedimethanol, ethylene glycol, 1,4-butanediol, 1,2-propylene glycol and 1,3-propylene glycol; the molar amount of the initiator is 0.3 to 1% of the total molar amount of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione.

[0025] Polyols as initiators will produce branched polyesters, while diols help form linear copolyesters, which can provide better spinnability and processability.

[0026] Ring-opening polymerization can control molecular weight by adjusting the ratio of monomer to initiator. Theoretically, one initiator molecule can initiate polymerization to form one polymer chain. Within a certain range, increasing the monomer to initiator ratio results in a higher molecular weight polyester, with greater variation in molecular weight between polymer chains, leading to a wider molecular weight distribution. Within this range, the resulting copolyester has a higher molecular weight and a narrower molecular weight distribution, resulting in better processability.

[0027] Preferably, the catalyst is one or more of sodium hydride, potassium hydride, aluminum hydride, magnesium hydride, phosphazene base t-BuP4, and phosphazene base t-BuP2; the solvent is one of benzene, toluene, tetrahydrofuran, dimethyl sulfoxide, and N,N-dimethylformamide; and the lactide is one or more of L-lactide, D-lactide, racemic lactide, and meso-lactide.

[0028] Preferably, the ring-opening copolymerization reaction is terminated by using a terminator, which is one of acetic acid, acetic anhydride, benzoic acid, and phosphoric acid; and the molar ratio of the terminator to the catalyst is 1 to 10:1.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) Polylactic acid copolyester is obtained by copolymerizing lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione. Because the structures of the two are very similar, the copolyester obtained by ring-opening copolymerization has good toughness and thermodynamic properties, and does not reduce the degradability of polyester;

[0031] (2) The cocatalyst can make the ring-opening polymerization more efficient. The copolyester is rich in unsaturated double bonds, which provides many modification sites for the copolyester. The subsequent performance of the polyester can be adjusted more conveniently and efficiently.

[0032] (2) By adjusting the raw material ratio and reaction parameters, a linear copolyester is finally obtained, and the molecular weight and molecular weight distribution of the copolyester are both within a reasonable range, with high processability and fluidity, which is conducive to subsequent spinning or film forming. DETAILED DESCRIPTION

[0033] The technical solutions of the present invention are described below with specific embodiments, but the protection scope of the present invention is not limited thereto:

[0034] Overall embodiment

[0035] An unsaturated polylactic acid copolyester has the structural formula:

[0036]

[0037] Wherein, m and n are natural numbers greater than 10; R1 is hydroxy, alkoxy or arylalkoxy; and R2 is hydrogen or alkyl.

[0038] The preparation method of the unsaturated polylactic acid copolyester comprises the following steps:

[0039] (1) Under nitrogen protection, an initiator, a catalyst and a co-catalyst are dissolved in a solvent and stirred at -60 to 0°C for 5 to 10 minutes; the molar amount of the initiator is 0.01 to 1% of the total molar amount of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione, the molar ratio of the catalyst to the initiator is 1 to 10:1, the molar ratio of the co-catalyst to the initiator is 1 to 6:1, and the molar ratio of the terminator to the catalyst is 1 to 10:1.

[0040] (2) Lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione are uniformly mixed and added to the above reaction system, the total molar concentration of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione in the solvent is 0.5-3 mol / L, and the molar ratio of lactide to 3-methylene-6-methyl-1,4-dioxane-2,5-dione is 1-9:1-9; then the reaction is continued at -60-0°C for 30-120 minutes, a terminator is added to terminate the reaction, the reaction mixture is precipitated in methanol, and centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0041] The initiator is a diol, specifically one of terephthalic acid methanol, 1,4-cyclohexanedimethanol, ethylene glycol, 1,4-butanediol, 1,2-propylene glycol and 1,3-propylene glycol.

[0042] The structural formula of the co-catalyst is: R3 and R4 are each selected from one of methyl, ethyl, propyl, phenyl, cyclohexyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, 4-chlorophenyl, 2,6-dimethylphenyl, 2,4-dimethoxyphenyl and 2,4,6-trimethoxyphenyl.

[0043] The catalyst is one or more of sodium hydride, potassium hydride, aluminum hydride, magnesium hydride, phosphazene base t-BuP4, and phosphazene base t-BuP2.

[0044] The solvent is one of benzene, toluene, tetrahydrofuran, dimethyl sulfoxide and N,N-dimethylformamide.

[0045] The terminator is one of acetic acid, acetic anhydride, benzoic acid and phosphoric acid; and the molar ratio of the terminator to the catalyst is 1-10:1.

[0046] The lactide is one or more of L-lactide, D-lactide, racemic lactide and meso-lactide.

[0047] Example 1

[0048] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0049] (1) Under nitrogen protection, phosphazene base t-BuP2 (0.5 mmol, 183.8 mg), 1-(4-methoxyphenyl)-3-(4-chlorophenyl)urea (1.5 mmol, 422.3 mg), and p-phenylenediol (0.5 mmol, 69 mg) were dissolved in 44 mL of tetrahydrofuran and stirred at -30°C for 5 min;

[0050] (2) Lactide (45 mmol, 6.48 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (5 mmol, 0.71 g) were mixed evenly and added to the above reaction system. The reaction was continued at -30°C for 60 min. 0.1 mL of acetic acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0051] Example 2

[0052] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0053] (1) Under nitrogen protection, phosphazene base t-BuP2 (0.5 mmol, 183.8 mg), 1-cyclohexyl-3-(4-chlorophenyl)urea (1.5 mmol, 379.2 mg), and cyclohexanedimethanol (0.5 mmol, 72.1 mg) were dissolved in 44 mL of toluene and stirred at -40°C for 5 min;

[0054] (2) Lactide (40 mmol, 5.76 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (10 mmol, 1.42 g) were mixed evenly and added to the above reaction system. The reaction was continued at -40°C for 60 min. 0.1 mL of acetic acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0055] Example 3

[0056] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0057] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.5 mmol, 317.0 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (1.5 mmol, 429.8 mg), and ethylene glycol (0.5 mmol, 31.0 mg) were dissolved in 44 mL of tetrahydrofuran and stirred at -50°C for 6 min; (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system, and the reaction was continued at -50°C for 70 min. 0.1 mL of acetic anhydride was added to terminate the reaction, and the reaction mixture was precipitated in 200 mL of methanol, centrifuged, and dried to obtain unsaturated polylactic acid copolyester.

[0058] Example 4

[0059] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0060] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.5 mmol, 317.0 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (1.5 mmol, 429.8 mg), and ethylene glycol (0.5 mmol, 31.0 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min; (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system, and the reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction, and the reaction mixture was precipitated in 200 mL of methanol, centrifuged, and dried to obtain unsaturated polylactic acid copolyester.

[0061] Example 5

[0062] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0063] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.125 mmol, 79.3 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.375 mmol, 107.4 mg), and ethylene glycol (0.375 mmol, 23.3 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min;

[0064] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0065] Example 6

[0066] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0067] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.25 mmol, 158.5 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.75 mmol, 214.9 mg), and ethylene glycol (0.25 mmol, 15.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min; (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system, and the reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction, and the reaction mixture was precipitated in 200 mL of methanol, centrifuged, and dried to obtain unsaturated polylactic acid copolyester.

[0068] Example 7

[0069] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0070] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.0625 mmol, 39.6 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.1875 mmol, 53.7 mg), and ethylene glycol (0.1875 mmol, 11.6 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min;

[0071] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 70 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0072] Example 8

[0073] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0074] (1) Under nitrogen protection, KH (0.25 mmol, 10.0 mg), 1-(4-methoxyphenyl)-3-(4-chlorophenyl)urea (0.75 mmol, 189.6 mg), and 1,4-butanediol (0.25 mmol, 22.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min;

[0075] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0076] Example 9

[0077] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0078] (1) Under nitrogen protection, NaH (0.25 mmol, 6.0 mg), 1-(4-methoxyphenyl)-3-(4-chlorophenyl)urea (0.75 mmol, 189.6 mg), and ethylene glycol (0.25 mmol, 15.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min;

[0079] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0080] Comparative Example 1

[0081] The difference from Example 6 is that no co-catalyst is added.

[0082] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0083] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.25 mmol, 158.5 mg) and ethylene glycol (0.25 mmol, 15.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50 °C for 6 min;

[0084] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0085] Comparative Example 2

[0086] The difference from Example 6 is that the reaction temperature is too high.

[0087] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0088] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.25 mmol, 158.5 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.75 mmol, 214.9 mg), and ethylene glycol (0.25 mmol, 15.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min; (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system, and the reaction was continued at 20°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction, and the reaction mixture was precipitated in 200 mL of methanol, centrifuged, and dried to obtain unsaturated polylactic acid copolyester.

[0089] Comparative Example 3

[0090] The difference from Example 6 is that the molar ratio of monomers is 10:90.

[0091] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0092] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.25 mmol, 158.5 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.75 mmol, 214.9 mg), and ethylene glycol (0.25 mmol, 15.5 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min; (2) Lactide (5 mmol, 0.72 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (45 mmol, 6.39 g) were mixed evenly and added to the above reaction system, and the reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction, and the reaction mixture was precipitated in 200 mL of methanol, centrifuged, and dried to obtain unsaturated polylactic acid copolyester.

[0093] Comparative Example 4

[0094] The difference from Example 6 is that the ratio of monomer to initiator is 400:1.

[0095] The preparation method of unsaturated polylactic acid copolyester comprises the following steps:

[0096] (1) Under nitrogen protection, phosphazene base t-BuP4 (0.125 mmol, 79.3 mg), 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea (0.375 mmol, 107.4 mg), and ethylene glycol (0.125 mmol, 7.8 mg) were dissolved in 19 mL of tetrahydrofuran and stirred at -50°C for 6 min;

[0097] (2) Lactide (30 mmol, 4.32 g) and 3-methylene-6-methyl-1,4-dioxane-2,5-dione (20 mmol, 2.84 g) were mixed evenly and added to the above reaction system. The reaction was continued at -50°C for 100 min. 0.1 mL of phosphoric acid was added to terminate the reaction. The reaction mixture was precipitated in 200 mL of methanol, centrifuged and dried to obtain unsaturated polylactic acid copolyester.

[0098] Table 1

[0099] Note: In Table 1, the monomer molar ratio is the molar ratio of lactide to 3-methylene-6-methyl-1,4-dioxane-2,5-dione; the monomer concentration is the total molar concentration of 3-methylene-6-methyl-1,4-dioxane-2,5-dione and lactide in the solvent; and the raw material molar ratio is monomer (3-methylene-6-methyl-1,4-dioxane-2,5-dione and lactide): catalyst: initiator: co-catalyst.

[0100] After the reaction of Examples 1-9 and Comparative Examples 3-4, samples were taken for nuclear magnetic resonance testing to obtain the conversion rates of the two monomers, and the weight average molecular weight of the obtained copolyesters was tested. The results are shown in Table 2.

[0101] Table 2

[0102] Soil Degradation Test: The copolyesters prepared in Example 6 and Comparative Examples 1-4 were hot-pressed into 0.2 mm thick films on a flatbed vulcanizer, cut into 1 cm x 1 cm slices, and buried in soil. The films were removed after 30, 60, and 90 days, washed, dried, and their weight loss was measured. The copolyesters prepared in Example 6 and Comparative Examples 1-4 were tested for tensile strength and elongation at break in accordance with GB / T 1040.2-2006, Determination of Tensile Properties of Plastics. The copolyesters prepared in Example 6 and Comparative Examples 1-4 were also tested by DSC to determine their melting points and glass transition temperatures. The results are shown in Table 3.

[0103] Table 3

[0104] As shown in Table 2-3, Comparative Example 1 did not add urea as a co-catalyst compared to Example 6. As a result, the double bond on 3-methylene-6-methyl-1,4-dioxane-2,5-dione reacted to form a cross-linked product, indicating that adding urea as a co-catalyst can improve the selectivity of the catalytic system, causing 3-methylene-6-methyl-1,4-dioxane-2,5-dione to undergo only a ring-opening reaction. Comparative Example 2 increased the reaction temperature compared to Example 6, and a cross-linked product was also produced in Comparative Example 2, indicating that high reaction temperatures make the double bond on 3-methylene-6-methyl-1,4-dioxane-2,5-dione more reactive, and the reaction should be carried out at a lower temperature. The cross-linked products obtained in Comparative Examples 1 and 2 have a high degree of cross-linking, no clear melting point and glass transition temperature, poorer toughness than the polyester obtained in Example 6, and worse degradation performance.

[0105] Compared with Example 6, the proportion of lactide component in the polyester of Comparative Example 3 is higher, the polyester has better regularity and higher crystallinity, and the melting point and glass transition temperature of the polyester are also higher than those of the polyester obtained in Example 6. The tensile strength is higher, but the toughness is poor and the degradation performance is poor.

[0106] Compared with Example 6, Comparative Example 4 has a higher ratio of monomer to initiator, and the resulting polyester has a larger molecular weight, better strength and toughness, but slightly worse degradation performance.

[0107] The above are only preferred embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the description of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. An unsaturated polylactic acid copolyester, characterized in that: The structural formula of the unsaturated polylactic acid copolyester is: ； Wherein, m and n are natural numbers greater than 10; R1 is hydroxy, alkoxy or arylalkoxy; and R2 is hydrogen or alkyl.

2. A method for preparing the unsaturated polylactic acid copolyester according to claim 1, characterized in that: The method comprises the following steps: in the presence of an initiator, a catalyst and a co-catalyst, lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione are subjected to a ring-opening copolymerization reaction in a solvent under nitrogen protection at -60 to 0°C for 30 to 120 minutes, wherein the molar amount of the initiator is 0.3 to 1% of the total molar amount of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione, and the molar amount of lactide accounts for 60 to 90% of the total molar amount of lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione, to obtain an unsaturated polylactic acid copolyester; The catalyst is one or more of sodium hydride, potassium hydride, aluminum hydride, magnesium hydride, phosphazene base t-BuP4, and phosphazene base t-BuP2; The structural formula of the co-catalyst is: ； Wherein, R3 and R4 are each selected from one of methyl, ethyl, propyl, phenyl, cyclohexyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, 4-chlorophenyl, 2,6-dimethylphenyl, 2,4-dimethoxyphenyl and 2,4,6-trimethoxyphenyl.

3. The preparation method according to claim 2, wherein The total molar concentration of the lactide and 3-methylene-6-methyl-1,4-dioxane-2,5-dione in the solvent is 0.5-3 mol / L.

4. The preparation method according to claim 2, wherein The co-catalyst is 1-(4-methoxyphenyl)-3-(4-chlorophenyl)urea, 1-cyclohexyl-3-(4-chlorophenyl)urea or 1-cyclohexyl-3-(4-trifluoromethylphenyl)urea.

5. The preparation method according to claim 2, wherein: The molar ratio of the co-catalyst to the initiator is 1-6:

1.

6. The preparation method according to claim 2, wherein: The molar ratio of the catalyst to the initiator is 1-10:

1.

7. The preparation method according to claim 2, 5 or 6, characterized in that: The initiator is a diol.

8. The preparation method according to claim 7, wherein: The solvent is one of benzene, toluene, tetrahydrofuran, dimethyl sulfoxide, and N,N-dimethylformamide; the lactide is one or more of L-lactide, D-lactide, racemic lactide, and meso-lactide.

9. The preparation method according to claim 2, wherein: The ring-opening copolymerization reaction is terminated by using a terminator, which is one of acetic acid, acetic anhydride, benzoic acid, and phosphoric acid; and the molar ratio of the terminator to the catalyst is 1-10:1.

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