High melt strength polylactic acid polymers

Abstract

The invention relates to a process for producing a lactic acid-based polymer, which process comprises copolymerizing lactide with a cyclic dimer of another alpha-hydroxy acid as a copolymerized monomer of opposite alpha-carbon chirality.

Description

Technical Field

[0001] This invention relates to improving the melt strength of polylactic acid (PLA) polymers by randomly incorporating small amounts of hydroxy acid comonomers with opposite α-carbon chirality, and the uses of said polymers. Background Technology

[0002] Due to growing environmental concerns, the development of sustainable and renewable materials has become an important part of the plastics industry. Therefore, bio-based and biodegradable polymers, such as poly(lactic acid), have become important.

[0003] Despite other promising features, PLA lacks suitable melt properties, namely low melt strength, melt viscosity, and melt elasticity, resulting in processing limitations and challenges that must be addressed to produce commercially competitive bio-based materials that are similar to or superior to existing petroleum-derived plastics.

[0004] Therefore, there is a need in the art for improved PLA polymers. Invention Overview

[0005] Due to growing environmental concerns, the development of sustainable and renewable materials has become an important part of the plastics industry. Therefore, bio-based and biodegradable polymers, such as poly(lactic acid), have become important. Despite other promising characteristics, PLA lacks suitable melt properties—namely, its low melt strength, melt viscosity, and melt elasticity—leading to processing limitations and challenges that must be addressed to produce commercially competitive bio-based materials that are similar to or superior to existing petroleum-derived plastics.

[0006] This invention demonstrates that random copolymerization of lactide with a small amount (0.4-10 mol%) of diethyl glycolide of opposite chirality significantly increases the melt shear viscosity and elasticity of PLA, as well as the tensile viscosity and melt strength. Both monomers can be synthesized in high yield and with enantiomeric purity via a one-step zeolite-catalyzed method. Improved melt strength and efficient, enantiomeric monomer synthesis lead to more cost-competitive and sustainable biomaterials, while expanding their applicability.

[0007] This invention addresses a problem in the field by unexpectedly improving the melt viscosity and elasticity of PLA by copolymerizing classical monomers with small amounts of comonomers exhibiting inverse stereochemistry. Both enantiomerically pure monomers can be readily synthesized using a one-step zeolite-catalyzed method. PLA's poor melt strength limits its processability when strong tensile forces are applied to the polymer, such as in bottle blow molding, or in film blow molding, fiber spinning, and foaming. A stronger melt can translate to less plastic per article, thus reducing its overall footprint. Therefore, the improved elasticity and melt strength achieved by adding small amounts of comonomers pave the way for more eco-friendly and cost-competitive use of PLA bioplastics with a wider processing and application window. Both the improved melt strength and the use of enantiomerically pure monomers result in more cost-competitive and sustainable PLA-based biomaterials with a broader application window.

[0008] According to the objectives of this invention, as embodied and extensively described herein, the invention broadly relates to the random incorporation of small amounts of α-hydroxy acid comonomers with opposite α-carbon chirality to improve the melt strength of polylactic acid (PLA) polymers, and the uses of said polymers. This effect is successfully reproducible when the reaction is scaled up. The simple manner of introducing the comonomer depends on the ring-opening polymerization scheme of the cyclic dimer of said hydroxy acid. Furthermore, tensile viscosity measurements confirm the presumed melt strength improvement effect of comonomerizing suitable diethyl glycolide. Compared to L-PLA and commercial-grade PLA (Ingeo 7001D) for injection stretch blow molding applications, the large-scale copolymer of (L,L)-lactide and (D,D)-diethyl glycolide (1.2 mol%) exhibits both higher tensile viscosity and higher tensile strength.

[0009] In one aspect of the invention, the one-step conversion of (2- or) α-hydroxybutyric acid to diethyl glycolide in the presence of zeolite H-BEA results in high yields and enantiomeric purity. This enantiomeric pure cyclic ester, when combined with lactide, yields a series of previously unexplored high-molecular-weight random copolymers exhibiting attractive new melt properties. Compared to lactide, small amounts (0.4-10 mol%) of diethyl glycolide with its opposite chiral nature produce polymeric materials with significantly increased melt shear viscosity and melt elasticity compared to L-PLA.

[0010] Another aspect of the present invention is a method for improving the effect of PLA in the absence of chain extenders, blending, compounding or stereocompositing.

[0011] Another aspect of this invention is a novel route for preparing high melt strength materials through copolymerization.

[0012] Some aspects of the present invention are set forth directly in a descriptive format as follows:

[0013] 1. A method for preparing a lactic acid-based polymer, comprising a cyclic dimer of copolylactide and another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0014] 2. The method according to statement 1 is characterized in that the copolymerization of lactide and α-hydroxy acid is a random copolymerization.

[0015] 3. The method according to statement 1, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethyllactide, or a copolymer of (D,D)-lactide and (L,L)-diethyllactide.

[0016] 4. The method according to statement 1, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethylglycol or a copolymer of (D,D)-lactide and (L,L)-diethylglycol to form a poly(lactic acid-co-α-hydroxybutyric acid).

[0017] 5. The method according to any one of statements 1-4, wherein lactide is copolymerized with 0.2-10 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0018] 6. The method according to any one of statements 1-4, wherein lactide is copolymerized with 0.4-5 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0019] 7. The method according to any one of statements 1-4, wherein lactide is copolymerized with 0.7-2 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0020] 8. A copolymer that can be obtained by any one of statements 1-7.

[0021] 9. A copolymer that can be obtained by any one of statements 1-7, characterized in that it exhibits a tensile strength at least 1.5 times that of pure PLA obtained by lactide polymerization.

[0022] 10. A copolymer that can be obtained by any one of statements 1-7, characterized in that it exhibits at least twice the tensile strength of pure PLA obtained by polymerization of lactide.

[0023] 11. A copolymer obtainable by any one of statements 1-7, characterized in that it exhibits at least 1.2 times higher zero-shear viscosity (at 185°C, with an applied strain amplitude of 1-10% and a range of 0.1-100 rad / s) compared to a cyclic dimer of copolylactide and another α-hydroxy acid as a comonomer of the same α-carbon chirality. -1The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0024] 12. A copolymer obtainable by any one of statements 1-7, characterized in that it exhibits a tensile strength of at least 17 N Newtons (N) (this is achieved by melting the melt at 185°C at a rate of 0.05 mm / s). -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by the force of the rotating wheel.

[0025] 13. A copolymer obtainable by any one of statements 1-7, characterized in that it exhibits a zero-shear viscosity of at least 15 kPa s (this is at 185 °C, with an applied strain amplitude of 1-10% and a range of 0.1-100 rad s). -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0026] 14. A copolymer obtainable by any one of statements 1-7, characterized in that it exhibits a tensile strength of 15-41 Newtons (N), preferably 23-25.5 N (this is achieved by the melt at 185°C at a velocity of 0.05 mm / s). -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s -1 The pull-out speed and 0.12mm s -1 (Measurement of the force of linear acceleration rotating on a rotating wheel).

[0027] 15. A copolymer obtainable by any one of statements 1-7, characterized in that it exhibits a zero-shear viscosity of 15-200 kPa s (this is at 185 °C, with an applied strain amplitude of 1-10% and a range of 0.1-100 rad s). -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0028] 16. A copolymer obtained by random copolymerization of lactide and a cyclic dimer of another α-hydroxy acid as a comonomer having opposite α-carbon chirality, characterized in that it contains 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% hydroxy acid comonomer, and has a melt strength of at least 15, and preferably at least 20 Newtons (N) (this is achieved by melting at 185°C with a melt flow rate of 0.05 mm / s). -1 The piston speed pushes through the 2mm diameter die head at 100mm / s. -1 The pull-out speed is 0.12 mm / s -1The linear acceleration is measured by the force of rotation on a rotating wheel, or it is characterized by containing 90-99.6 mol% lactic acid and 10-0.4 mol% hydroxy acid comonomer, with a melt strength of 15-40 N, preferably 20-35 N, more preferably 23-25.5 N (this is achieved by measuring the force of the melt at 0.05 mm / s at 185 °C). -1 The piston speed pushes through the 2mm diameter die head at 100mm / s. -1 The pull-out speed is 0.12 mm / s -1 (Measurement of the force of linear acceleration rotating on a rotating wheel).

[0029] Other aspects of the present invention are set forth directly in statement format as follows:

[0030] 1. A poly(lactic acid-co-2-hydroxybutyric acid) obtained by random copolymerization of (L,L)-lactide with (D,D)-diethylglycol and / or (D,D)-lactide with (L,L)-diethylglycol, characterized in that it contains 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% diethylglycol, and has a melt strength of at least 15 N, and preferably at least 20 N (measured at 185 °C).

[0031] 2. The poly(lactic acid-co-2-hydroxybutyric acid) according to statement 1, characterized in that lactide is randomly copolymerized with 0.2-10 mol% of diethyl glycolide.

[0032] 3. The poly(lactic acid-co-2-hydroxybutyric acid) according to statement 1, characterized in that it contains 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% diethyl glycolide, and has a melt strength of 15 to 40 N, preferably 20 to 35 N, more preferably 23 to 25.5 N (this is measured by force of the melt at 185°C).

[0033] 4. The poly(lactic acid-co-2-hydroxybutyric acid) according to statement 1, characterized in that it exhibits at least 1.5 times the force of polylactic acid obtained by polymerization of lactide or poly(lactic acid-co-2-hydroxybutyric acid) obtained by copolymerization of lactide with diethyl glycolide of the same α-carbon chirality.

[0034] 5. The poly(lactic acid-co-2-hydroxybutyric acid) according to statement 1, characterized in that it exhibits at least twice the force of polylactic acid obtained by polymerization of lactide or poly(lactic acid-co-2-hydroxybutyric acid) obtained by copolymerization of lactide with diethyl glycolide of the same α-carbon chirality.

[0035] 6. The poly(lactic acid-co-2-hydroxybutyric acid) according to any one of statements 1-5 above, characterized in that the poly(lactic acid-co-2-hydroxybutyric acid) exhibits a force of at least 15 N (this is achieved by the melt at 185°C at a velocity of 0.05 mm / s).-1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 (Measurement of the force of linear acceleration rotating on a rotating wheel).

[0036] 7. The poly(lactic acid-co-2-hydroxybutyric acid) according to any one of statements 1-5 above, characterized in that it exhibits a zero-shear viscosity of at least 15 kPa s (which at 185 °C, with a strain amplitude of 1-10% and a viscosity of 0.1-100 rad s) -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0037] 8. The poly(lactic acid-co-2-hydroxybutyric acid) according to any one of statements 1-5 above, characterized in that it exhibits a force of 17-35 Newtons (N), preferably 23-25.5 N (this is achieved by the melt moving at 0.05 mm / s at 185°C). -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 (Measurement of the force of linear acceleration rotating on a rotating wheel).

[0038] 9. The poly(lactic acid-co-2-hydroxybutyric acid) according to any one of statements 1-5 above, characterized in that it exhibits a zero-shear viscosity of 15-200 kPa s (which at 185 °C, with a strain amplitude of 1-10% and a viscosity of 0.1-100 rad s) -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0039] Further embodiments of the present invention are described directly in a descriptive format as follows:

[0040] 1. A copolymer obtainable by means of a cyclic dimer of copolylactide and another α-hydroxy acid (=2-hydroxy acid) as a comonomer with opposite α-carbon chirality.

[0041] 2. The copolymer according to statement 1 can be obtained by a cyclic dimer of copolylactide and 0.2-10 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0042] 3. The copolymer according to statement 1 can be obtained by polymerizing lactide with 0.4-5 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0043] 4. The copolymer according to statement 1 can be obtained by a cyclic dimer of copolylactide and 0.7-2 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0044] 5. The copolymer according to statement 1, characterized in that the copolymerization of lactide and α-hydroxy acid is a random copolymerization.

[0045] 6. The copolymer according to statement 1, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethyllactic acid, or a copolymer of (D,D)-lactide and (L,L)-diethyllactic acid.

[0046] 7. The copolymer according to statement 1, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethylglycol to form poly(lactic-co-2-hydroxybutyric acid), or a copolymer of (D,D)-lactide and (L,L)-diethylglycol to form poly(lactic-co-2-hydroxybutyric acid).

[0047] 8. The copolymer according to any one of the preceding statements 1-7, characterized in that it exhibits at least 1.5 times the force of a copolymer made of lactide and another α-hydroxy acid as a comonomer of the same α-carbon chirality.

[0048] 9. The copolymer according to any one of the preceding statements 1-7, characterized in that it exhibits at least twice the force of a copolymer made from a comonomer of lactide and another α-hydroxy acid as comonomers with the same α-carbon chirality.

[0049] 10. The copolymer according to any one of statements 1-7 above, characterized in that it exhibits a force of at least 17 N (which, at 185°C, is measured at 0.05 mm / s). -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by rotating on a rotating wheel, and it is characterized by exhibiting zero shear viscosity of at least 15 kPa·s (this is at 185 °C, with an applied strain amplitude of 1-10% and a viscosity of 0.1-100 rad·s). -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0050] Further embodiments of the present invention are described directly in a descriptive format as follows:

[0051] 1. A lactic acid-based polymer, characterized in that the polymer comprises a copolymer of lactide and 0.2-10 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0052] 2. The polymer according to embodiment 1, characterized in that the polymer comprises a copolymerization of lactide with 0.4-5 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0053] 3. The polymer according to embodiment 1, characterized in that the polymer comprises a copolymerization of lactide with 0.7-2 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0054] 4. The polymer according to embodiment 1, characterized in that the polymer is substantially composed of a copolymer of lactide and 0.2-10 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0055] 5. The polymer according to embodiment 1, characterized in that the polymer is substantially composed of a copolymer of lactide and 0.4-5 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0056] 6. The polymer according to embodiment 1, characterized in that the polymer is substantially composed of a copolymer of lactide and 0.7-2 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0057] 7. The polymer according to embodiment 1, characterized in that the polymer is composed of a copolymer of lactide and 0.2-10 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0058] 8. The polymer according to embodiment 1, characterized in that the polymer is composed of a copolymer of lactide and 0.4-5 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0059] 9. The polymer according to embodiment 1, characterized in that the polymer is composed of a copolymer of lactide and 0.7-2 mol% of a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

[0060] 10. The polymer according to any one of embodiments 1 to 9, characterized in that the copolymerization of lactide and α-hydroxy acid is a random copolymerization.

[0061] 11. The polymer according to any one of embodiments 1 to 9, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethylglycol, or a copolymer of (D,D)-lactide and (L,L)-diethylglycol.

[0062] 12. The polymer according to any one of embodiments 1 to 9, characterized in that the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethylglycol to form a polymer (lactic acid-co-α-hydroxybutyric acid), or a copolymer of (D,D)-lactide and (L,L)-diethylglycol to form a polymer (lactic acid-co-α-hydroxybutyric acid).

[0063] 13. The copolymer according to any one of the preceding embodiments 1 to 12 is characterized in that it exhibits a copolymerization force of at least 1.5 times that of a cyclic dimer of lactide and another α-hydroxy acid as a comonomer with the same α-carbon chirality.

[0064] 14. The copolymer according to any one of the preceding embodiments 1 to 12 is characterized in that it exhibits a copolymerization force of at least 2 times that of a cyclic dimer of lactide and another α-hydroxy acid as a comonomer with the same α-carbon chirality.

[0065] 15. The copolymer according to any one of embodiments 1 to 12 above, characterized in that it exhibits a force of at least 20 N (which, at 185°C, is measured at 0.05 mm / s). -1 The piston speed pushes through the 2mm diameter die head at 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by rotating the wheel.

[0066] 16. The copolymer according to any one of embodiments 1 to 12 above, characterized in that it exhibits a zero-shear viscosity of at least 15 kPas (which, at 185°C, with an applied strain amplitude of 1 to 10% and a viscosity of 0.1 to 100 rad / s). -1 The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0067] 17. The copolymer according to any one of embodiments 1 to 12 above, characterized in that it exhibits a force of 20 to 35 Newtons (N), preferably 23 to 25.5 N (at 185°C, at 0.05 mm / s). -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by rotating the wheel.

[0068] 18. The copolymer according to any one of embodiments 1 to 12 above, characterized in that it exhibits a zero-shear viscosity of 15 to 200 kPas (at 185°C, with an applied strain amplitude of 1 to 10% and a viscosity of 0.1 to 100 rad / s). -1The dynamic frequency was measured and calculated using a Carreau-Yasuda model.

[0069] 19. A copolymer comprising a random copolymer of lactide and another α-hydroxy acid as a comonomer having opposite α-carbon chirality, characterized in that it comprises 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% hydroxy acid comonomer, having a melt strength of 15 to 40 N, preferably 20 to 35 N, more preferably 23 to 25.5 N (at 185°C, at 0.05 mm / s) -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by rotating the wheel.

[0070] 20. A copolymer comprising a random copolymer of lactide and another α-hydroxy acid as a comonomer having opposite α-carbon chirality, characterized in that it comprises 90-99.6 mol% lactic acid and 10-0.4 mol% hydroxy acid comonomer, having a melt strength of at least 15, and preferably at least 20 N (at 185°C, at 0.05 mm / s) -1 The piston speed pushes through the die head with a diameter of 2mm at 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by rotating the wheel.

[0071] 21. A poly(lactic acid-co-α-hydroxybutyric acid), characterized in that the copolymer is a random copolymerization of (L,L)-lactide and (D,D)-diethylglycolic acid, or a random copolymerization of (D,D)-lactide and (L,L)-diethylglycolic acid, and further characterized in that it contains 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% diethylglycolic acid, and has a melt strength of 15 to 40 N, preferably 20 to 35 N, more preferably 23 to 25.5 N (measured at 185°C).

[0072] 22. A poly(lactic acid-co-α-hydroxybutyric acid), characterized in that the copolymer is a random copolymer of (L,L)-lactide and (D,D)-diethylglycolic acid, or a random copolymer of (D,D)-lactide and (L,L)-diethylglycolic acid, and further characterized in that it contains 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% diethylglycolic acid, and has a melt strength of at least 15 N and preferably at least 20 N (measured at 185°C).

[0073] The further scope of the invention will become apparent from the detailed description given below. However, it should be understood that while the detailed description and specific examples indicate preferred embodiments of the invention, they are given by way of illustration only, as various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art based on this detailed description. It should be understood that the foregoing general description and the following detailed description are exemplary and illustrative only, and not intended to limit the claimed invention. Detailed description Brief description of the attached diagram

[0075] The invention will be more fully understood from the detailed description and accompanying drawings given below. The drawings are given by way of illustration only and are not intended to limit the invention.

[0076] Figure 1 The following are shown: (A) Catalytic one-step synthesis of diethyl glycolide (EG) and random copolymerization with lactide (LD) to obtain PLA with high melt strength. (B) 1 H-NMR (400 MHz, DMSO-d6) results from the catalytic cyclization of L-α-HBA and racemic-α-HBA resulted in EG yields of 94.0% (ee = 99.2%) and 56.0% (ee = 2.6%), respectively. (C) Chiral GC results from the catalytic cyclization of L-α-HBA and racemic-α-HBA, showing the absence of racemic cyclization and pure (L,L)-EG, as well as statistical mixtures of (L,L), (D,D), and meso-EG. (D) Monomer conversion (%) of (D,D)-LD [symbol forming the upper curve] and 1 mol% (L,L)-EG [symbol forming the lower curve] within 60 min (bulk, 170 °C, Sn(Oct)2 (monomer:catalyst = 489), n-dodecanool (initiator:catalyst = 0.7)).

[0077] Figure 2 The following are shown: For copolymers with 9-10 mol% comonomers, (A) complex shear viscosity (|η*|), (B) elastic modulus (G′), and (C) viscous modulus (G”) are shown as functions of angular frequency (ω). For polymers with 0.6-0.7 mol% comonomers, (D) complex shear viscosity (|η*|), (E) elastic modulus (G′), and (F) viscous modulus (G”) are shown as functions of angular frequency (ω). In (A) and (D), at 0.1 rad s -1 Below, the ◇ and error bars represent the mean and standard deviation of all different synthetic P(L-LD) and P(D-LD) combinations.

[0078] Figure 3The following are shown: For copolymers of D-LD with different molar percentages of L-EG, (A) complex shear viscosity (|η*|), (B) elastic modulus (G′), and (C) viscous modulus (G”) as functions of angular frequency (ω). For copolymers of L-LD with different molar percentages of D-EG, (D) complex shear viscosity (|η*|), (E) elastic modulus (G′), and (F) viscous modulus (G”) as functions of angular frequency (ω).

[0079] Figure 4 The following is shown: large-scale tensile viscosity measurements (A) on P(L-LD), P(L-LD-co-D-EG)(1.2 mol% D-EG) and PLA Ingeo 7001D as tensile time (time) e The function, in 3.5s -1 (a) Tensile viscosity (ηe) measured by a tensile viscometer (solid line) at the Hencky strain rate. ■ indicates complex shear viscosity (|η*|) measured by multiplying by a factor of 3 using SAOS according to the Trouton ratio. (b) Average tensile force applied to the polymer material, measured by a traction tensile rheometer.

[0080] The following detailed description of the invention is provided with reference to the accompanying drawings. Identical reference numerals in different drawings identify the same or similar elements. Furthermore, the following detailed description does not limit the invention. Rather, the scope of the invention is defined by the appended claims and their equivalents.

[0081] Bio-based and biodegradable polymers are attractive alternatives to petroleum-based plastics, which are associated with environmental pollution and have thus attracted global public attention.

[0082] Polylactic acid (PLA), an aliphatic polyester composed of lactic acid (LA) monomers, is currently one of the most promising bioplastics on the market [Rafael Auras et al., Poly(Lactic Acid): Synthes, Structures, Properties, Processing, and Applications (John Wiley & Sons, Inc., 2010). This carbohydrate-based thermoplastic is biodegradable under controlled conditions and exhibits excellent mechanical and physical properties comparable to commercial plastics such as polystyrene and polyethylene terephthalate [Garlotta J. Polym. Environ. 9, 63-84 (2001)].

[0083] Several factors have limited the adoption of PLA. Currently, high molecular weight PLA is synthesized via ring-opening polymerization (ROP) of cyclic dienolactone (LD), which is prepared by reverse depolymerization of condensation poly(LA) [Rafael Auras et al., Poly(Lactic Acid): Synthes, Structures, Properties, Processing, and Applications (John Wiley & Sons, Inc., 2010)]. Although dienolactone and ROP are indispensable, the manufacture of LD is a two-step time-consuming and energy-intensive process that requires additional purification, thus increasing production costs. Furthermore, LD yields are mediocre and the method is subject to racemization, resulting in the loss of enantiomeric purity of (L,L)-lactone (L-LD) and therefore some control over PLA [Gruber et al., Continuous process for manufacture of lactidepolymers with controlled led optical purity (1992)].

[0084] Recently, our lab developed a zeolite-based catalytic method that enables the one-step conversion of LA to LD. The highly selective route is superior to the two-step process, potentially in terms of cost and, of course, atomic efficiency (few byproducts) and preservation of stereochemistry (no racemization) [Dusselier et al., Science 349, 79-81 (2015)].

[0085] The more inherent disadvantages of PLA are related to its processing performance and therefore its applicability. PLA has poor melt strength, melt viscosity and melt elasticity, which are key properties of stretch flow-dominated processes such as film blowing, blow molding, fiber spinning, foaming and so on [Lim et al., Prog. Polyrn. Sci. 33, 820-852 (2008); Dorgan, Poly (Lactic Acid): Synthes, Structure, Properties, Processing and Applications, edited by Rafael Auras et al. (John Wiley & Sons, 2010), pp. 125-139].

[0086] Different strategies for improving the melt strength properties of PLA have been described. Some efforts have focused on using chain extenders to produce long-chain, branched, cross-linked, or star-shaped structures in PLA to enhance chain entanglement [Dorgan et al., J. Rheol. (NYNY). 43, 1141-1155 (1999); Michalski et al., Prog. Polym. Sci. 89, 159-212 (2019); Corre et al., Rheol. Acta 50, 613-629 (2011)]. Although chain extenders can improve many properties of PLA, the final polymer structure is often difficult to control, and finding the optimal chain extender concentration remains challenging. Furthermore, chain-extended polymer structures can exhibit reduced degradability [ -Franco et al., Polym. Test. 67, 190-196 (2018); Limsukon et al., Polym. Test. 80, 106-108 (2019)], while the extenders themselves are usually not biodegradable or biocompatible, and in some cases even toxic (diisocyanates).

[0087] The second strategy involves blending PLA with other polymers, as well as compounding it with fillers of varying micron or nano-sized dimensions (e.g., 5 wt%) to enhance its melt behavior [Nofar et al., Polym. Rev. 59, 465-509 (2019)]. However, it is not easy to obtain homogeneous blends or equal particulate dispersions. Furthermore, blending can lead to limitations in recycling or (bio)degradation.

[0088] Finally, stereocomplex microcrystals induced by mixing PLA with opposite chiral properties (L-PLA with 5 wt% D-PLA) can act as crosslinking sites, resulting in melt behavior similar to chain-extended PLA structures [Yamane et al., J. Rheol. (NYNY). 48, 599-609 (2004)]. This would require large-scale availability of pure D-LA, which is not trivial.

[0089] In contrast to these strategies, few studies have correlated the composition of the main chain (linear lactide copolymers) with rheological properties, despite the apparent value of these polyesters in microstructure-viscoelastic relationships. One group has conducted some rheological studies on PLAs rich in syndiotactic and heterogeneous stereotactic structures [Chi le et al., Macromolecules 49, 909-919 (2016)]. The effect of incorporating D-LD into L-PLA has been a subject of study, but the influence of the D / L ratio has not been clearly elucidated or shown to result in negligible changes in polymer melt viscosity [Dorgan et al., J. Rheol. (NYNY). 49, 607-619 (2005); Othman et al., J. Rheol. (NYNY). 55, 987-1005 (2011)].

[0090] This invention relates to the unexpected and significant improvement of the melt strength behavior of PLA by ring-opening copolymerization of LD with (a small amount) diethyl glycolide (EG) of opposite chirality. Figure 1 A).

[0091] Racemic diethyl glycolide (a mixture of D, D+L, L) has been polymerized previously, and its conventional synthesis is homogeneous catalytic, inefficient, and laborious, resulting in a mixture of diastereomers [Yin & Baker, Macromolecules 32, 7711-7718 (1999); Trimaille et al., J. Polym. Sci. Part A Polym. Chem. 42, 4379-4391 (2004)]. To date, enantiomeric homopolymers and copolymers have been prepared solely through condensation polymerization of α-HBA or a combination of α-HBA and LA. This method yields low molecular weight compounds (e.g., 10,000 g / mol) and varying polydispersities, effectively resulting in diverse materials and correspondingly different material properties [Tsuji et al., Biomacromolecules 11, 252-258 (2010); Tsuji et al., Polymer (Guildf). 52, 1318-1325 (2011); Tsuji & Sobue Polymer (Guildf). 72, 202-211 (2015)]. Here, zeolite-based α-hydroxy acid cyclization not only provides efficiency and high yields but also preserves the stereochemistry of the substrate. Enantiomeric pure cyclic ester monomers offer a broad range of unexplored high molecular weight (co)polymers that may possess attractive new properties. Rheological studies of enantiomeric pure LD and EG copolymers showed a strong increase in zero-shear viscosity, elastic modulus, and tensile viscosity compared to conventional PLA, with a clear difference related to the chiral properties of the monomers. While improvements in melt properties are typically achieved through chain extension, blending, or compounding, simple copolymerization is sufficient even for small EG incorporations. Surprisingly, a promising strategy for large-scale implementation has emerged, given the ease with which 1% D-EG (=(D,D)-diethyl glycolide) can be added to lactide batches used in the production of classic L-PLA. The increased melt strength can translate to less plastic weight required per functional article (e.g., blow-molding thinner bottles) and expand the application range of PLA.

[0092] Now referring specifically to the accompanying drawings, according to an embodiment of the present invention, in... Figure 1 A (Showing Monomer Synthesis and Copolymerization) illustrates and demonstrates the improvement of PLA melt strength behavior by ring-opening copolymerization of LD with a small amount of diethyl glycolide (EG) with opposite chiral properties. Example

[0093] Example 1 Catalytic Monomer Synthesis and Copolymerization

[0094] As shown in (Reference 4) and the corresponding patent, the one-step catalytic conversion of lactic acid (LA) to lactide (LD) in the presence of Brønsted acidic zeolite β (H-BEA) was applied to the cyclization of α-hydroxybutyric acid (α-HBA) to form diethyl glycolide (EG).

[0095] The application of L-α-HBA under the same reaction conditions as Dusselier et al. (130 °C, 3 h, toluene, H-BEA (SiO2 / Al2O3 = 25)) yielded only 5% L-EG, compared to 78% for L-LD. This could be overcome by increasing the reaction temperature to 170 °C (xylene, yield = 76%). The reaction in the absence of a catalyst showed—second only to the expected low cyclic ester selectivity—a decrease in conversion with increasing steric hindrance of the alkyl chain at the α-position. In this respect, one could expect α-HBA to tend to form smaller products and potentially favor ring closure. Nevertheless, the reaction under complete monomer conversion using a soluble acid catalyst (sulfuric acid) yielded less than 5% EG, indicating a need to confine the acid functional groups to shape-selective catalysts such as H-BEA. The optimized zeolite reaction (using H-BEA, SiO2 / Al2O3 = 150) proceeded even faster and was more selective for (L,L)-diethyl glycolide (L-EG) (yield = 88%). Distillation of α-HBA prior to the reaction in a single step further increased the L-EG yield to 94%. 1 ¹H-NMR showed excellent enantiomeric purity of 99.6% L-EG (ee = 99.2%), with only 0.4% meso-EG ((L,D)-EG) formed. In contrast to enantiomeric pure α-HBA, the conversion of racemic α-HBA (rac-α-HBA) to cyclic esters was slower and less selective (yield = 56%), but still yielded a statistically significant mixture of L,L;D,D- and L,D- diastereomers, thus demonstrating the absence of racemization in the direct zeolite catalytic route. Figure 1 (B) and (C)). After the reaction, the heterogeneous catalyst was easily removed, and the cyclic ester was separated from the linear oligomer and unreacted monomer phases by phase extraction. Subsequent crystallization yielded a total EG purity of 99.5% (ee≈100%) and a total separation yield of 48%. Obtaining higher separation yields should be possible, but that is not the focus here, where the purity of the ROP is the primary concern.

[0096] Ring-opening polymerization based on Kaihara et al., Nat. Protoc. 2, 2767-2772 (2007) was carried out using stannous octate (Sn(Oct)2) catalyst and 1-dodecanool initiator. This catalytic system ensures the preservation of the stereochemistry, high molecular weight, and low polydispersity of the monomers within the polymer chain. Different incorporations of EG ranging from 0.3 to 10 mol% were employed. Figure 1 (A) n / (m+n)) synthesizes different lactide-based polyesters. The polymers differ in the stereochemistry of the comonomers (L-EG or D-EG versus L-LD or D-LD). 1 H-NMR confirmed that the comonomer was successfully incorporated into the polylactide and that the weight-average molecular weight (M) was [missing information]. w The concentrations varied between 101 and 149 kg / mol (Table 1). Although ring-opening polymerization was employed and high molecular weight was considered necessary, the possibility that a polycondensation route to obtain very similar copolymers with sufficient molecular weight from α-hydroxy acids of opposite chirality could also work is not excluded.

[0097] Synthesize or purchase various benchmarks, namely copolymers of L-LD and 10 mol% D-LD, as well as pure P(L-LD) and P(D-LD). The latter is commercial PLA (Ingeo 7001D, NatureWorks), which has specifications for use in injection stretch blow molding bottle applications.

[0098] The chain structure of a copolymer depends on the relative polymerization rates of the different monomers. If the growth rates are similar, a random chain structure is obtained, while significant rate differences result in block copolymers. As the steric hindrance of the substituents increases, the polymerization rate of alkyl-substituted lactide decreases [Trimaille et al., J. Polym. Sci. Part A Polym. Chem. 42, 4379-4391 (2004); Saiyasombat et al., Polymer (Guildf). 39, 5581-5585 (1998); Vogel et al., Proceedings of the American Chemical Society, (2007), p. 2; Jing et al., Macromolecules 40, 9304-9312 (2007)]. However, the polymerization rate difference between racemic-LD and racemic-EG has been described as being quite small, indicating a high degree of randomness within their copolymer chains [Vogel et al., Proceedings of the American Chemical Society, (2007), p.2.; Yin & Baker Macromolecules 32, 7711-7718 (1999)].

[0099] Here, the randomness of incorporation is explored and quasi-identified by determining the temporal evolution of the comonomer during ROP. Figure 1 (D) Only a non-substantial difference in the relative progress of conversion was detected. The polymerization progress of L-EG (relative to its 1% starting amount) was only slightly slower than that of D-LD (99%), which is most likely to produce strongly randomized copolymers.

[0100] Synthesis of diethyl glycolide

[0101] In a typical reaction, 0.01 mol of (L)- or (D)-α-hydroxybutyric acid and 0.5 g of H-β zeolite are added to a round-bottom flask containing 20 mL of toluene or o-xylene. A custom-made Dean-Stark water separator filled with solvent is attached to the top of the flask. The apparatus is connected to a condenser and heated in an oil bath at 130 or 170 °C. The mixture is stirred for 3 hours. After the reaction, the mixture is homogenized by adding 15 mL of acetonitrile. After homogenization, the zeolite is removed by filtration through a glass frit filter under vacuum. After the first filtration, the catalyst is washed with another 10 mL of acetonitrile.

[0102] Ring-opening polymerization

[0103] Solvent-free ring-opening polymerization (ROP) was carried out in a dry, custom-made round-bottom flask. In a typical experiment, the required amount of monomer was added to the flask in an oxygen- and moisture-free environment. A toluene solution of stannous octoate as a catalyst (monomer:catalyst = 2500:1) and 1-dodecanoic acid as an initiator (70 mol% of catalyst) were added to the monomer. The solvent was removed under vacuum, the flask was filled with argon, and immersed in an oil bath at 170°C for 70 minutes. After polymerization, the flask was cooled, and the polymer was dissolved in chloroform. The synthesized polymer was separated from the remaining monomer and oligomer phases by precipitation in methanol, filtered, and dried under reduced pressure.

[0104] Example 2 Melt Rheology

[0105] Shear rheology

[0106] The melt rheological properties of polymers determine their flow behavior during processing in various applications. Processing PLA remains challenging due to its low melt strength, low melt viscosity, and low melt elasticity, thus limiting its applicability. Melt strength can be indirectly determined by measuring the zero-shear viscosity (η0) using strain-controlled small-amplitude oscillatory shear (SAOS) rheometry. Measurements were performed on copolymers described in Fout! Verwijzingsbron niet gevonden at 185°C and under nitrogen atmosphere, at a constant strain of 1%, using values ​​from 0.1 to 100 rad.s. -1The complex viscosity modulus (|η*|) and the elastic (G′) and viscous (G″) moduli are determined by varying the angular frequency (ω). η0, the plateau value of the complex viscosity at an infinitesimal ω, is determined by applying the Carreau-Yasuda model. Figure 2 Results for copolymers containing 9-10% comonomers are described in (AC), while the same data for copolymers with 0.6-0.7 mol% EG can be found in [reference needed]. Figure 2 (DF).

[0107] All polymer melts tend to plateau at low frequencies, while shear thinning occurs at higher frequencies. Figure 2 (A) and (D)). Compare copolymers with P(L-LD) (with similar M) w Several observations can be made regarding the viscoelasticity of a homemade control (and commercially available PLA (Ingeo) designed for injection stretch blow molding). The addition of 10 mol% D-LD to P(L-LD) appears to decrease both the viscosity and elasticity of pure P(L-LD) to a small extent, consistent with earlier studies that noted only subtle differences in the viscoelastic properties of polylactide with varying D-LD contents [Dorgan et al., J. Rheol. (NYNY). 49, 607-619 (2005); Othman et al., J. Rheol. (NYNY). 55, 987-1005 (2011); Palade et al., Macromolecules 34, 1384-1390 (2001)]. While the viscoelasticity of P(L-LD) incorporated with 10 mol% L-EG appears to have little change, copolymers of 10 mol% EG and LD with opposite stereochemistry show a significant increase in both the complex viscosity and elasticity of the polymer melt. Figure 2 (AC)). Although the shear viscosity (η0) of P(L-LD) and P(D-LD) was determined to be 9.7 and 12.1 kPa·s, the values ​​of P(L-LD-co-D-EG) and P(D-LD-co-L-EG) were estimated to be 15.2 and 71.8 kPa·s, respectively (Carreau-Yasuda model). Since η0 is strongly dependent on chain length [Cooper-whi te & Mackay J. Polym. Sci. Part B Polym. Phys. 37, 1803–1814 (1999); Fox & Loshaek J. Appl. Phys. 26, 28-31 (1955)], one can assume that the increase in viscosity of P(L-LD-co-D-EG) is due to its relatively low M wThe viscosity of P(D-LD-co-L-EG) is limited to 101 kg / mol. However, P(D-LD-co-L-EG) has a viscosity 6.5 times greater than that of P(L-LD), thus exhibiting a uniquely positive effect on the viscoelasticity of polylactide. Comparable results were obtained when much lower amounts of EG (0.6-0.7 mol%) were incorporated into polylactide. Figure 1 (DF)). Copolymers of EG and LD with the same stereochemistry (P(L-LD-co-L-EG and P(D-LD-co-D-EG)) showed limited increases in viscosity and elasticity, while more significant increases were observed for copolymers of L-LD and D-EG or D-LD and L-EG.

[0108] To investigate the effect of the comonomer ratio (EG:LD) on the viscoelasticity of the material, various copolymers with different EG percentages (P(L-LD-co-D-EG) and P(D-LD-co-L-EG)) were analyzed (Table 1 and...). Figure 3 It can be noted that for both P(L-LD-co-D-EG) and P(D-LD-co-L-EG), there is no significant correlation between viscosity or elasticity and the comonomer ratio, except that the effect relative to L or D-PLA always begins to become apparent for incorporations above 0.3 mol%. For P(D-LD-co-L-EG), fairly low η0 and high ω are observed when 0.3 or 2.0 mol% EG is incorporated. c The addition of 0.7%, 4.1%, or 10.5% EG showed a significant improvement in viscoelasticity. For P(L-LD-co-D-EG), very high viscosity and elasticity were observed when using 1.2% or 0.4% D-EG. However, the copolymer with 10.3 mol% D-EG showed a much smaller increase, which may be attributed to its relatively low M compared to other polymers. w Despite M w There are some variations, but these variations are limited and unlikely to cause such a large effect. Furthermore, these effects appear to be reproducible in different copolymers of LD and EG.

[0109] Tensile Rheology

[0110] While shear rheology and zero-shear viscosity provide good indications of the melt properties of copolymers, tensile rheology can be used as a direct measure of melt strength. To confirm the existence of an unusually high zero-shear viscosity, reduce some variability in small-scale experiments, and allow for tensile rheology measurements (sample size), the bulk ROP reaction was scaled up to 100 g. Pure P(L-LD) polymers and copolymers of L-LD with 1.2% D-EG were synthesized and compared with commercial-grade PLA for injection stretch blow molding applications (e.g., bottles).

[0111] At different Hencky strain rates (0.1, 0.5, 1, and 3.5 s⁻¹) -1 Tensile viscosity was measured using a fixture under [a certain condition] to determine the tensile viscosity (η). e ) as stretching time (time) e The function () Figure 4 (A)). The tensile viscosity is related to η0 via a factor of 3, as explained by the Trouton ratio (η0). e =3η0). Comparable tensile viscosities were observed for commercially available PLA grades and synthetic P(L-LD) samples, but P(L-LD-co-D-EG) confirmed its higher viscosity. Despite the strong difference in viscosity, no strain hardening behavior was observed at these Hencky strain rates, consistent with the literature on the tensile viscosity of linear polylactide [Hedegaard et al., J. Rheol. (NYNY). 59, 1397-1417 (2015); Gu et al., J. Rheol. (NYNY). 61, 785-796 (2017)]. Finally, to directly measure melt strength, the polymer melt strands were rotated on a wheel at linear acceleration speeds (100-5000 mm / s), and the tensile force applied by the material was determined using a traction tensile rheometer. P(L-LD-co-D-EG) showed an average force of approximately 24.4 N ( Figure 4 B), which is about twice as high as the force (12.4 N) of P(L-LD). P(L-LD) and commercially available PLA show comparable values.

[0112] In Table 1, and in the accompanying figures, especially Figure 2-4 The technical effects of the present invention have already been demonstrated.

[0113] Differences in the thermal properties of various copolymers were observed. Generally, as the amount of EG comonomer increases, the glass transition temperature (Tg) increases. g ), melting temperature (T) m ) and crystallinity (X) c The downward trend of T (Table 1). If comonomers with opposite stereochemistry are used, then T m and X c It shows a stronger reduction.

[0114] Copolymerization of LD and EG with an inverse stereoconfiguration appears to have a uniquely positive impact on the viscoelasticity of PLA. While improvements in melt properties are primarily the result of chain extension, stereocompositing, blending, and compounding, achieving these effects through copolymerization is unexpected and potentially very promising. Higher melt strength can facilitate certain stretch-dominated processes (extrusion blow molding, thermoforming, film blow molding, foaming, etc.) by broadening the application window of PLA bioplastics simply by adding small amounts of comonomers to existing ROP processes. Furthermore, if thinner stretches can be achieved without compromising the stronger melt, less polymer can be used for a given application, resulting in a smaller eco-footprint for plastic applications.

[0115] Small amplitude oscillation shear

[0116] Small amplitude oscillating shear (SAOS) measurements were performed on an AresMel TS rheometer using a parallel plate apparatus (8 mm diameter) surrounded by a convection oven and purged with N2 gas. Prior to the rheological measurements, polymer samples were compressed into disks with an 8 mm diameter and 1 mm thickness. The polymer disks were vacuum-dried overnight at 80 °C before the SAOS measurements. First, dynamic time-scan measurements were performed under the applied test method to verify the thermal stability of the polymer samples. Changes in G′ and G″ were determined over a period of 300 s at an angular frequency of 10 rad / s, a strain amplitude of 1%, and a temperature of 185 °C. Second, strain scan tests were performed by varying the strain amplitude between 0.1% and 10% to determine the linear viscoelastic state of the material. Frequency scan measurements were performed at 185 °C and a dynamic frequency ranging from 0.1 to 100 rad / s, with strain amplitudes ranging from 1% to 10%. The application range was 0.01 to 1 s. -1 A continuous rate scan test was performed between the shear rates. At each shear rate, a 30-second waiting time was set to ensure a steady state, while a 10-second measurement time was applied.

[0117] Stretching viscosity fixture

[0118] The tensile flow properties of the polymer were determined using a tensile viscosity fixture (EVF) on an AresMeltS rheometer. Tensile viscosity was measured in a strain-controlled tensile test at 185 °C under a nitrogen atmosphere with a Hencky strain of 3.4. The polymer sample was compressed into a rectangular plate with a length of 18.0 mm, a width of 10.0 mm (±0.10 mm), and a thickness of 0.80 mm (±0.05 mm). The plate was dried overnight at 80 °C prior to the tensile measurement. The experimental protocol consisted of three steps. During the first step, the polymer was stretched at a rate of 0.0075 s⁻¹. -1Pre-stretching was performed to compensate for thermal expansion during heating. A 50.0 s delay was applied before pre-stretching to ensure complete melting of the polymer sample. Following pre-stretching, a 5.0 s relaxation step was immediately applied to remove residual stress from the sample. Finally, the sample was stretched at constant Hencky strain rates (0.05, 0.1, 0.5, 1, 3.5 s). -1 Tensile measurements were performed.

[0119] Traction

[0120] To determine the tensile properties of polymer melts, The 2002 capillary rheometer was used in combination with a traction device. Polymer material was added to the cylinder of the capillary rheometer at 185°C. To produce molten polymer strands, the material was passed through a 12mm diameter piston and a 2mm die at a speed of 0.05mm / s. -1 The piston speed pushes the melt out of the cylinder. The molten strand is connected to a traction device, which rotates the molten strand on a wheel at a speed of 100 mm / s. -1 The traction speed is rotating. The speed is 0.12 mm / s. -1 The acceleration increases linearly until the polymer melt breaks.

[0121] Table 1: Copolymerization results of L(D)-LD with D-LD or L(D)-EG via ROP, with weight-average molecular weight (Mb) determined by GPC. w (kg mol) -1 ) and polydispersity (D), T as determined by DSC g and T m (°C), and at G′ and G"ω c (rad s -1 Zero-shear viscosity η0 (Pa s) and angular frequency as determined by SAOS at the crossover point:

[0122]

[0123] a pass 1 Mean insertion percentage of methyl and methine protons as determined by H-NMR (400MHz, CDCl3) (when both are visible).

[0124] b The weight-average molecular weight (M) was calibrated using polystyrene standards. w Considering the Mark-Houwink parameters of PLLA, the experimental molecular weight was corrected [Garlotta J. Polym. Environ. 9, 63-84 (2001)].

[0125] cCommercial-grade PLA from NatureWorks

[0126] d Large-scale polymerization reaction (50-100g)

[0127] *At the amount added before the reaction, L- and D-LD in 1 They cannot be distinguished from each other in H-NMR.

Claims

1. A method for producing a lactic acid-based polymer, comprising the step of copolymerizing lactide with a cyclic dimer of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

2. The method according to claim 1, wherein the copolymerization of lactide with the cyclic dimer of the other α-hydroxy acid is a random copolymerization.

3. The method according to claim 1, wherein the copolymer is a copolymer of (L,L)-lactide and (D,D)-diethyllactide, or a copolymer of (D,D)-lactide and (L,L)-diethyllactide.

4. The method according to claim 3, wherein the copolymerization is a copolymerization of lactic acid-co-α-hydroxybutyric acid.

5. The method according to any one of claims 1 to 4, wherein the copolylactide is a cyclic dimer of 0.2-10 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

6. The method according to any one of claims 1 to 4, wherein the copolylactide is a cyclic dimer of 0.4-5 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

7. The method according to any one of claims 1 to 4, wherein the copolylactide is a cyclic dimer of 0.7-2 mol% of another α-hydroxy acid as a comonomer with opposite α-carbon chirality.

8. A copolymer which can be obtained by the method according to any one of claims 1 to 7.

9. The copolymer according to claim 8, exhibiting a tensile strength at least 1.5 times that of pure PLA obtained by lactide polymerization.

10. The copolymer of claim 8, wherein the zero-shear viscosity exhibited is at least 1.2 times that of the cyclic dimer of copolylactide and another α-hydroxy acid as a comonomer of the same α-carbon chirality, wherein the zero-shear viscosity is obtained by applying a strain amplitude of 1 to 10% and a range of 0.1 to 100 rad / s at 185°C. -1 The dynamic frequency was measured and calculated using the Carreau-Yasuda model.

11. The copolymer of claim 9, wherein the zero-shear viscosity exhibited is at least 1.2 times that of the cyclic dimer of copolylactide and another α-hydroxy acid as a comonomer of the same α-carbon chirality, wherein the zero-shear viscosity is obtained by applying a strain amplitude of 1 to 10% at 185°C in the range of 0.1 to 100 rad / s. -1 The dynamic frequency was measured and calculated using the Carreau-Yasuda model.

12. The copolymer according to any one of claims 8 to 11, exhibiting a tensile strength of at least 17 N Newtons (N), which is achieved by the melt at 185°C with a tensile strength of 0.05 mm / s. -1 The piston speed pushes through a die head with a diameter of 2mm at 100mm / s -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by the force of rotation on the rotating wheel.

13. The copolymer according to any one of claims 8 to 11, exhibiting a zero-shear viscosity of at least 15 kPa s, which, at 185 °C, is subjected to a strain amplitude of 1 to 10% and a range of 0.1 to 100 rad s. -1 The dynamic frequency is measured and calculated by fitting using the Carreau-Yasuda model.

14. The copolymer according to any one of claims 8 to 11, exhibiting a tensile strength of 15 to 41 Newtons (N), preferably 23 to 25.5 N, achieved by the melt moving at 0.05 mm / s at 185°C. -1 The piston speed pushes through a die head with a diameter of 2mm at 100mm / s -1 The pull-out speed and 0.12mm s -1 The linear acceleration is measured by the force of rotation on the rotating wheel.

15. The copolymer according to any one of claims 8 to 11, exhibiting a zero-shear viscosity of 15 to 200 kPa·s, which, at 185°C, with a strain amplitude of 1 to 10% and a viscosity of 0.1 to 100 rad·s... -1 The dynamic frequency is measured and calculated by fitting using the Carreau-Yasuda model.

16. A copolymer obtained by random copolymerization of lactide and a cyclic dimer of another α-hydroxy acid as a comonomer having opposite α-carbon chirality, characterized in that... Its melt strength is at least 15 Newtons (N), containing 90 to 99.6 mol% lactic acid and 10 to 0.4 mol% hydroxy acid comonomer, or characterized in that its melt strength is 15 to 40 N, containing 90 to 99.6 mol% lactic acid and 10 mol% to 0.4 mol% hydroxy acid comonomer.

17. The copolymer according to claim 16, characterized in that... Its melt strength is at least 20 Newtons (N).

18. The copolymer according to claim 16, characterized in that... Its melt strength is 20 to 35 N.

19. The copolymer according to claim 16, characterized in that... Its melt strength is 23-25.5 N.

20. The copolymer according to any one of claims 16 to 19, wherein the melt is accelerated at 185°C at a rate of 0.05 mm / s. -1 The piston speed pushes through a die head with a diameter of 2mm at a speed of 100mm / s. -1 The pull-out speed and 0.12mm s -1 The linear acceleration of the force of the rotating wheel is used to measure the melt strength.

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