CN105001403A - Method for preparing high-melting-point stereo-complex polylactic acid material - Google Patents

Abstract

The invention discloses a preparation method of a high-melting-point stereocomposite polylactic acid material. Purified L-lactide and an initiator are subjected to ring-opening polymerization to generate a three-arm branched L-polylactic acid prepolymer, and then a terminal group is used to Activate the terminal hydroxyl group of the prepolymer, and perform ring-opening polymerization of the activated prepolymer and D-lactide. By changing the amount of D-lactide, three-arm branched L-polylactic acid with different molecular weights is synthesized- D-polylactic acid block copolymer, its weight average molecular weight is 5000~45000. DSC results showed that the PLLA-PDLA block copolymer can generate stereocomplex crystals under certain conditions, and the melting temperature of this crystalline region exceeds 220°C. The high melting point stereocomplex polylactic acid material was successfully prepared by the above method.

Description

A kind of preparation method of high melting point stereocomplex polylactic acid material

technical field

The invention relates to a preparation method of a high-melting point stereocomposite polylactic acid material, which belongs to the field of biodegradable polymer materials.

Background technique

Polylactic acid is a renewable and environmentally friendly plastic with good biocompatibility and can be degraded in nature. It is a polymer material that can be plastically processed and has high strength; commercialized polylactic acid PLLA The glass transition temperature T _g of polylactic acid is about 60 ℃, and the melting point is about 155~175 ℃. Therefore, the defects in heat resistance and mechanical properties will limit the application range of polylactic acid materials; based on the thermal properties and mechanical properties of polylactic acid The performance is relatively poor, and many researchers have tried many methods to improve these performances, such as copolymerizing it with other monomers, and the two molecular chains can generate a kind of immediate Stereocomplex --- SC-PLA has completely different properties from ordinary PLLA and its derivatives; stereocomposite material is a mixture of two components with the same composition and different stereo configurations to form a unique property Like polymer materials, the polylactic acid stereocomplex SC crystal has a melting point as high as 230°C, and has excellent heat resistance stability, high mechanical properties and stable degradation characteristics; optical purity, ratio, preparation method, molecular weight, etc. will affect the SC crystal. Formation, optical purity is an important factor affecting the structure and physical properties of polylactic acid and stereocomplexes. At present, scholars at home and abroad have not much research on the influence of optical purity on SC crystals and are not systematic. In addition, for the practical application of SC crystals There are relatively few studies on other aspects.

We can obtain SC crystals through the blend solution of PDLA and PLLA and their molten blends. It is generally believed that the hydrogen bond force between molecular chains is one of the main reasons for the formation of SC crystals; many factors will also affect the stereotype. Complex formation, such as formation conditions, molecular weight and molecular chain structure, when the molecular weight of PDLA and PLLA is relatively high, stereocomplexation will be affected by homopolymer crystallization, in which two molecular chains with opposite configurations will be arranged in parallel and folded , so that stereocomplex crystals can be formed; in the wide-angle X-ray diffraction pattern, it is found that the diffraction peaks of SC are obviously different from those of PLLA and PDLA. 16.7 ^. , 19.4 ^. Diffraction peaks will appear on the surface; Another significant difference between SC and ordinary PLLA is that the melting point of the former is about 230 ° C, while the melting point of ordinary PLLA is only about 180 ° C.

In recent years, the research on the crystallization properties of PLA has become one of the hot issues, and the crystallization rate of PLA can be increased through different approaches, which have high theoretical research value. On the other hand, it can also effectively guide the actual production, and it also plays a very important role in practical applications.

Contents of the invention

Using L-lactic acid as raw material, dehydration, oligomerization, and cracking of lactic acid to obtain crude L-lactide, and then purify the crude L-lactide to generate white needle-like L-lactide; Lactide is used as a monomer, and then trimethylolethane is used as an initiator to carry out ring-opening polymerization to generate a three-arm branched L-polylactic acid prepolymer, and then use terminal group activation to activate the terminal hydroxyl group of the prepolymer. The activated prepolymer and D-lactide were subjected to ring-opening polymerization, and three-arm branched L-polylactic acid-D-polylactic acid block copolymers with different molecular weights were synthesized by changing the amount of D-lactide.

In order to achieve the above object, technical scheme of the present invention is:

A preparation method of a high-melting point stereocomplex polylactic acid material, characterized in that the purified L-lactide and an initiator are subjected to ring-opening polymerization to generate a three-arm branched L-polylactic acid prepolymer, and then activated by end groups The terminal hydroxyl group of the prepolymer is activated, and the activated prepolymer and D-lactide are subjected to ring-opening polymerization to synthesize a three-arm branched L-polylactic acid-D-polylactic acid block copolymer.

The initiator is trimethylolethane.

The lactide preparation method is divided into three parts: dehydration, catalyst oligomerization and cracking.

The catalyst is tetraphenyltin, and the dosage is 0.2%-0.3% of the mass of L-lactic acid.

Purification of lactide: After completely dissolving crude L-lactide in a solvent, filter, stand for crystallization, suction filter and dry to obtain colorless transparent needle crystals.

Described solvent is ethyl acetate.

The active prepolymer is three-arm branched PLLA-SnOct.

Specifically, a high melting point stereocomplex polylactic acid material and a preparation method thereof are carried out according to the following steps:

1. Synthesis of lactide: The experimental process of preparing lactide from lactic acid is mainly divided into three parts: dehydration, oligomerization and cracking.

(1) Dehydration: ① Clean all the instruments first, weigh the dosage of the medicine, and build the experimental device; install a thermometer, a stirring system, a condensation receiving system, a constant temperature water bath device, a vacuum system, and a nitrogen system in a 500mL four-neck flask , electric heating, temperature control system; ② read the atmospheric pressure data, turn on the vacuum pump to check the tightness of the system; ③ add 350mL L-lactic acid, start stirring and adjust to an appropriate speed; ④ gradually increase the temperature to 130°C, normal pressure, tap water condensation, and stir Under dehydration, it takes 1-2h.

(2) Oligomerization: Add the pre-prepared catalyst tetraphenyltin, based on L-lactic acid, the amount of catalyst added is 0.2%~0.3% of the mass of L-lactic acid, open the vacuum and slowly reduce the pressure to a vacuum degree of 700mmHg, slowly Raise the temperature to 140°C, and complete the dehydration condensation of lactic acid in 3-4 hours to obtain oligomeric L-lactic acid.

(3) Cracking: ①Switch the cooling medium of the condenser to circulating hot water at 100°C; ②Replace the receiving bottle; ③Under vigorous stirring, use an oil pump to quickly reduce the pressure to a vacuum of about 700mmHg, and quickly heat up to above 180°C , distill the L-lactide, and end at 220-230°C, continue the reaction for 3-4h, and a light yellow product appears in the receiving bottle, which is the crude L-lactide.

2. Purification of lactide: Since there may be polylactic acid or L-lactic acid in the crude L-lactide, it is necessary to purify the lactide; put the crude L-lactide into a three-necked bottle, and one end of the three-necked bottle Connect the reflux condenser, first add a small amount of ethyl acetate, then gradually add the solvent, heat and boil until all the solids are dissolved, filter while it is hot, cool the filtrate slowly, let it stand for crystallization, wait for all the crystals to precipitate, then suction filter, White crystals were obtained, and then the above steps were repeated to recrystallize four times. The above suction-filtered crystals were dried in a vacuum oven at 40° C. for 12 hours to obtain colorless transparent needle-like crystals.

3. Synthesis of PLLA-PDLA tri-block copolymer: The synthesis diagram of the three-arm branched PLLA-PDLA block copolymer is shown in Figure 1. First, add the purified L-lactide, trimethylolethane and stannous octoate reactants in molar ratio to the reaction flask, fill with N ₂ , then vacuumize, repeat 3 times, and maintain the pressure in the polymerization tube as 1～10kPa, then seal the tube in vacuum; react at 180°C for 2h, then lower the temperature to 130°C, and continue to react for 48h to obtain a light yellow product; after purification and drying of the product, a white powder three-arm branch LiPLLA prepolymer; prepolymer and excess Sn(Oct) ₂ were dissolved in anhydrous toluene, under the protection of N ₂ , the reaction was continued at 60°C for 4h, after the reaction, suction filtered and washed with anhydrous toluene , to remove unreacted stannous octoate and low molecular weight soluble products, after vacuum drying, the three-arm branched PLLA-SnOct activated prepolymer was obtained, and then the three-arm branched PLLA-SnOct activated prepolymer was used as a macroinitiator , DLA is used as a synthetic monomer to continue the above-mentioned ring-opening polymerization process, and finally a block copolymer of three-arm branched PLLA-PDLA is obtained; the pure sample is labeled as PLLA, and the prepared three-arm branched PLLA-PDLA block copolymer is The markers are PLD1/0.5, PLD1/1, PLD1/3.

Beneficial effects: the present invention innovates the research method of the predecessors, and adopts a two-step method to synthesize the three-arm branched block copolymer. The first step: L-lactic acid is used as raw material to synthesize L-lactide first; the second step : Use the synthesized L-lactide as a monomer, tetraphenyltin as a catalyst, and trimethylolethane as an initiator to synthesize L-polylactic acid, and then carry out ring-opening polymerization of D-lactide and L-polylactic acid. Synthesis of three-arm branched PLLA-PDLA block copolymers.

By changing the ratio of L-polylactic acid and D-lactide, block copolymers with different degrees of polymerization can be finally generated; the block copolymers were heat-treated at different temperatures for DSC testing, and the block copolymers and L-lactide Compared with the pure sample of lactic acid, all block copolymers have two melting peaks, the melting peak in the low temperature region is the melting peak of the single component crystal, and the melting peak in the high temperature region is the melting peak of the stereocomplex crystal; At different annealing temperatures, as the annealing temperature increases, the melting temperature of the same block copolymer will gradually increase; this is because the melting temperature is related to the thickness of the wafer, and as the annealing temperature increases, the wafer will be thicker , the melting temperature is higher; compared with other block polymers with different ratios, the block copolymer synthesized by equal amounts of L-lactide and D-lactide, that is, the melting temperature of PLD1/1 is the highest, And it is about 50℃ higher than the pure sample, indicating that it is an ideal heat-resistant material.

Description of drawings

Figure 1 is a schematic diagram of the synthesis of three-arm branched PLLA-PDLA block copolymers.

Figure 2 is a graph of samples measured by DSC.

Detailed ways

Material performance test method described in the present invention is as follows:

It was completed on a differential scanning calorimeter (DSC), and indium was used as a standard sample for calibration; the sample volume was about 5 mg, under the protection of 20 mL/min of N ₂ . First raise the temperature to 240°C at a heating rate of 100°C/min, keep the temperature for 3 minutes, and then quickly drop the temperature at a cooling rate of 100°C/min to 140°C, 150°C, 160°C, 190°C, 195°C and 200°C respectively. ℃, keep the temperature for 1h, quickly drop to room temperature, and then rise from room temperature to 240℃ at 10 ℃/min, and measure the melting temperature (T _m ) and melting enthalpy (ΔH _m ) of the sample.

Example 1:

Recipe: about 20g of L-lactic acid, 10g of D-lactic acid, 0.04g of trimethylolethane, 0.06mL of stannous octoate, and 50mL of anhydrous toluene.

First, add the purified L-lactide, trimethylolethane, and stannous octoate into the three-necked flask according to the above ratio, fill it with N ₂ , and then evacuate it for 3 times to maintain the pressure in the polymerization tube. 1 ~ 10kPa, then seal the tube in vacuum, react at 180°C for 2h, then lower the temperature to 130°C, and continue the reaction for 48h to obtain a light yellow product. After the product is purified and dried, a white powder three-arm branched PLLA prepolymer; dissolve the prepolymer and excess Sn(Oct) ₂ in anhydrous toluene, and continue to react at 60°C for 4h under the protection of N ₂ ; Washing to remove unreacted stannous octoate and low molecular weight soluble products, after vacuum drying, the three-arm branched PLLA-SnOct activated prepolymer was obtained; then the three-arm branched PLLA-SnOct was used as a macromolecular initiator, and DLA was Synthesize the monomer to continue the above-mentioned ring-opening polymerization process, and finally obtain a block copolymer of three-arm branched PLLA-PDLA, which is denoted as PLD1/0.5. Finally, the melting temperature (T _m ) and melting enthalpy of the sample are measured by DSC (ΔH _m ), the test curve is shown in Figure 2, and the results are shown in Table 1.

Example 2:

Recipe: about 20g of L-lactic acid, 20g of D-lactic acid, 0.04g of trimethylolethane, 0.06mL of stannous octoate, and 50mL of anhydrous toluene.

First, add the purified L-lactide, trimethylolethane, and stannous octoate into the three-necked flask according to the above ratio, fill it with N ₂ , and then evacuate it for 3 times to maintain the pressure in the polymerization tube. 1~10kPa, then seal the tube in vacuum; react at 180°C for 2h, then lower the temperature to 130°C, and continue to react for 48h to obtain a light yellow product. After the product is purified and dried, a white powder three-arm Branched PLLA prepolymer; dissolve the prepolymer and excess Sn(Oct) ₂ in anhydrous toluene, and continue to react at about 60°C for 4 hours under the protection of N ₂ . Wash with toluene to remove unreacted stannous octoate and low molecular weight soluble products; after vacuum drying, the three-arm branched PLLA-SnOct activated prepolymer is obtained, and then the three-arm branched PLLA-SnOct is used as the macromolecular initiator, DLA Continue the above-mentioned ring-opening polymerization process for the synthesis of monomers, and finally obtain a block copolymer of three-arm branched PLLA-PDLA, denoted as PLD1/1, and finally, use DSC to measure the melting temperature (T _m ) and heat of fusion of the sample Enthalpy (ΔH _m ), the test curve is shown in Figure 2, and the results are shown in Table 1.

Example 3:

Recipe: about 20g of L-lactic acid, 60g of D-lactic acid, 0.04g of trimethylolethane, 0.06mL of stannous octoate, and 50mL of anhydrous toluene.

Firstly, add purified L-lactide, trimethylolethane, stannous octoate and other reactants into the reaction kettle according to the above ratio, fill it with N ₂ , then vacuumize, repeat 3 times to maintain the pressure in the polymerization tube 1 ~ 10kPa, then seal the tube in vacuum, react at 180°C for 2h, then lower the temperature to 130°C, and continue the reaction for 48h to obtain a light yellow product. After the product is purified and dried, a white powder three-arm Branched PLLA prepolymer; dissolve the prepolymer and excess Sn(Oct) ₂ in anhydrous toluene, and continue to react at about 60°C for 4 hours under the protection of N ₂ . Wash with toluene to remove unreacted stannous octoate and low-molecular-weight soluble products. After vacuum drying, a three-arm branched PLLA-SnOct activated prepolymer is obtained; Continue the above-mentioned ring-opening polymerization process for the synthesis of monomers, and finally obtain a block copolymer of three-arm branched PLLA-PDLA, denoted as PLD1/3; finally, use DSC to measure the melting temperature (T _m ) and heat of fusion of the sample Enthalpy (ΔH _m ), the test curve is shown in Figure 2, and the results are shown in Table 1.

For comparative example 4, see the embodiment disclosed in Patent No. CN200880118429.3.

Put D-lactide (49.0g, 0.34mol) and ethylene glycol (1.0g, 0.016mol) in a 500mL screw-top Teflon container, add 2-ethylhexanoic acid tin-(Ⅱ) solution (142μL , a solution of 1 g of catalyst in 10 mL of toluene) was added to the mixture, the container was placed in an oil bath at 180 ° C for 4 h, the product was poured into an aluminum pot and placed in a vacuum furnace at 110 ° C and 20 mmHg for 16 h, after cooling, the product was polymerized -D-PLA oligomers form opaque white solids; a 250 mL round bottom flask was charged with a solution of poly-L-PLA (10.0 g, 3.1 mmol), CHCl3 (10 mL) and tin-(II)-2-ethylhexanoate (100 μ L, 0.24 mmol), 1,6-hexamethylene diisocyanate (1 mL, 6.24 mmol) was added and the reaction mixture was heated to reflux for 16 h, poly-D-PLA oligomers (10.0 g, 3.1 mol) and 2- Tin ethylhexanoate-(II) solution (100 μL, 0.24 mmol), the reaction was refluxed for 2 h, then the reaction solution was poured into hexane (200 mL), and the reactant was precipitated therein; the product was vacuum filtered to obtain a fluffy white powder, which was After drying in a vacuum oven at 110°C and 20mmHg for 16 hours, the product contains a block copolymer of carbamate groups and segments corresponding to each initial PLA oligomer. Finally, the product is subjected to DSC testing, and the results are shown in Table 1.

Table 1

Claims (7)

1. high-melting-point Stereocomplex poly-lactic acid material and preparation method thereof, it is characterized in that: the L-rac-Lactide after purification and initiator are carried out ring-opening polymerization and generates three arm branching Poly-L-lactic acid prepolymers, adopt end group to activate again and terminal hydroxy group activation is carried out to prepolymer, prepolymer after activation and D-rac-Lactide are carried out ring-opening polymerization, by changing the consumption of D-rac-Lactide, three arm branching Poly-L-lactic acid-dextrorotation polylactic-acid block copolymers of synthesis different molecular weight.

2. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 1, is characterized in that: described initiator is trimethylolethane.

3. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 1, is characterized in that: the preparation method of L-rac-Lactide is divided into dehydration, catalyzer oligomerisation and cracking three part.

4. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 3, is characterized in that: described catalyzer is tin tetraphenyl, and consumption is 0.2% ~ 0.3% of raw materials Pfansteihl quality.

5. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 1, it is characterized in that the purification of described L-rac-Lactide is: after being dissolved completely in a solvent by thick L-rac-Lactide, filtration, standing crystallization, suction filtration, drying obtain water white transparency needle-like crystal.

6. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 5, is characterized in that: described solvent is ethyl acetate.

7. the preparation method of a kind of high-melting-point Stereocomplex poly-lactic acid material according to claim 1, is characterized in that: described active prepolymer is three arm branching PLLA-SnOct.