CN113512181B - A kind of polylactic acid that can be processed at low temperature and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of degradable polylactic acid materials, and in particular relates to a low-temperature processable polylactic acid and a preparation method thereof. The low-temperature processable polylactic acid of the present invention is prepared by copolymerizing meso-lactide and D,L-lactide. The novel polylactic acid provided by the invention has the performance of low-temperature processing, which greatly reduces the processing difficulty and production cost of the polylactic acid material, improves the safety of processing the polylactic acid, reduces the production cost, and is beneficial to the application and popularization of the polylactic acid material , has high application potential.

Description

A kind of polylactic acid that can be processed at low temperature and preparation method thereof

technical field

The invention belongs to the technical field of degradable polylactic acid materials, in particular to a low-temperature processable polylactic acid and a preparation method thereof.

Background technique

Polylactic acid (PLA) is a degradable polymer material synthesized from lactic acid, which can be completely degraded into carbon dioxide and water in the natural environment. High molecular weight PLA has the advantages of excellent mechanical properties, easy processing, and good sensory properties such as product gloss and transparency. Therefore, PLA is the most prominent fully biodegradable material in the traditional polymer industry. It is often used to replace some currently commonly used petroleum-based plastics, such as polyethylene, polypropylene, PVC, etc. It is a solution to the increasingly serious white pollution problem. The best material. At present, in response to the country's call to reduce the use of non-degradable plastics, the application prospect of PLA will be broader, and its demand will be further expanded.

The synthesis method of high-molecular-weight PLA is mainly a two-step method, that is, firstly, lactide is synthesized from lactic acid, and then the lactide is then ring-opening polymerization (ROP) to form high-molecular-weight PLA. In the conventional process, meso-lactide in lactide used for synthesizing PLA is usually removed as an impurity, which increases the production cost of PLA.

PLA products are often processed by thermal processing. For example, the processing temperature of poly(L-lactic acid) (PLLA) is as high as 200°C, and the processing temperature of poly(D,L-lactic acid) (PDLLA) is also up to 170°C. Due to the hydrolysis sensitivity and thermal degradability of PLA, high-temperature processing must be accompanied by molecular chain scission, which can also accelerate the hydrolytic degradation caused by trace water in the PLA raw material and water vapor in the processing environment. This leads to a decrease in the molecular weight of PLA and a broadening of the molecular weight distribution, with the result that the properties of PLA products are seriously affected. In addition, with the rapid development of 3D printing technology, 3D printing has expanded from industrial product manufacturing, personalized treatment, preoperative planning and other applications to home applications, children's education applications and other products, such as home 3D printers, 3D printing pens, etc. is trending. As a biodegradable biosafety material, PLA has undoubtedly become the main raw material for household and educational 3D printing applications. However, the high-temperature processing characteristics of PLA lead to the risk of scalding in the application of daily environments such as homes, which is a major obstacle to the application of PLA in daily environments such as homes.

Therefore, it is very necessary to develop polylactic acid materials with low temperature processing properties. Kolstad et al. (Journal of applied polymer science, 1996, 62(7): 1079-1091) found that after introducing a small amount of meso-lactide units into PLLA, the resulting The amorphous region increases the disorder of the molecular arrangement of the copolymer, thereby reducing the melting point (T _m ) of the resulting PLA by 20°C to 50°C compared to PLLA, which can be processed at 150°C to 180°C. However, 150°C to 180°C is still a relatively high processing temperature, which is not conducive to the further promotion and application of materials. In addition, this process is only suitable for PLLA, and for PDLLA, another large type of amorphous polymer in PLA-based materials, there is still a lack of relevant research on reducing the processing temperature.

SUMMARY OF THE INVENTION

In view of the defects of the prior art, the present invention provides a low-temperature processable polylactic acid and a preparation method thereof, the purpose of which is to provide a new low-temperature processable polylactic acid material, which is an amorphous material, and the processing temperature Can be reduced to 105 ~ 120 ℃.

A low-temperature processable polylactic acid, the polylactic acid is composed of repeating unit A, repeating unit B, repeating unit C and repeating unit D;

Wherein, the repeating unit A is

The repeating unit B is

The repeating unit C is

The repeating unit D is

The sum of the mole percentages of the repeating unit A and the repeating unit B is 3% to 74%, and the sum of the mole percentages of the repeating unit C and the repeating unit D is 97% to 26%;

The molar ratio of the repeating unit A to the repeating unit B is any ratio, and the molar ratio of the repeating unit C to the repeating unit D is 1:1.

Preferably, the sum of the mole percentages of the repeating unit A and the repeating unit B is 68%, and the sum of the mole percentages of the repeating unit C and the repeating unit D is 32%.

Preferably, the repeating unit A and the repeating unit B are meso-lactide polymerized units, the repeating unit C and the repeating unit D are D,L-lactide polymerized units, and in the polylactic acid , the copolymerization unit content of the meso-lactide polymerization unit and the D,L-lactide polymerization unit is 9% to 12%;

And/or, the molecular weight of the polylactic acid is 10,000-200,000; preferably, 39,000-87,000.

And/or, the processing temperature of the polylactic acid for thermal processing is 105-120°C.

Preferably, in the polylactic acid, the copolymerization unit content of the meso-lactide polymerization unit and the D,L-lactide polymerization unit is 10% to 11%;

Preferably, the polylactic acid is prepared by copolymerizing 3-74 mol% of meso-lactide with 97-26 mol% of D,L-lactide.

Preferably, the polylactic acid is prepared by copolymerizing 68 mol% of meso-lactide with 32 mol% of D,L-lactide.

Preferably, the polylactic acid is prepared by reacting the mixed lactide of meso-lactide and D,L-lactide at 60-90°C for 4-14 hours, and then reacting at 100-180°C for 12 hours. Obtained after ~24 hours.

Preferably, the polylactic acid is obtained by reacting the mixed lactide at 70-90° C. for 7-14 hours, and then reacting at 140° C. for 14 hours.

The present invention also provides a method for preparing the above-mentioned polylactic acid, comprising the following steps: reacting the mixed lactide of meso-lactide and D,L-lactide at 60-90° C. for 4-14 hours, and then at 100° C. Obtained after 12-24 hours of reaction at ~180°C.

Preferably, the polylactic acid is obtained by reacting the mixed lactide at 70-90° C. for 7-14 hours, and then reacting at 140° C. for 14 hours.

Preferably, the reaction is carried out in a deoxygenated environment;

And/or, the reaction is carried out under the action of a catalyst, and the catalyst is selected from stannous octoate.

Preferably, the amount ratio of the catalyst to the mixed lactide is a molar ratio of 1:(2000-7000).

The present invention also provides a degradable plastic product prepared from the above-mentioned polylactic acid.

In the present invention, the "meso-lactide" refers to meso-lactide, which forms repeating unit A and repeating unit B in the process of preparing polylactic acid; "D,L-lactide" refers to Racemic lactide formed from equal amounts of D-lactide and L-lactide, which forms repeating unit C and repeating unit D in the process of preparing polylactic acid.

"The copolymerization unit of meso-lactide polymerization unit and D, L-lactide polymerization unit" refers to the copolymerization unit of meso-lactide and D-lactide (or L-lactide), including the following three Copolymerization units: D-lactide/meso-lactide/D-lactide unit (referred to as RR-RS-RR), L-lactide/meso-lactide/L-lactide unit (abbreviated as SS-RS-SS) or L-lactide/meso-lactide/D-lactide units (abbreviated as SS-RS-RR). The content of this copolymerized unit is calculated as follows:

(1) test the hydrogen nuclear magnetic resonance spectrum of the polylactic acid;

(2) Calculate the fitted peak area S _5.21 at δ=5.21 ppm and the fitted peak total area S _{5.10 to 5.25} in the range of δ=5.10 to 5.25 ppm;

(3) utilize the following formula to calculate the content C _meso-D,L-LA of the copolymerization unit of the meso-lactide polymerization unit and D,L-lactide polymerization unit:

C _meso-D,L-LA =S _5.21 /S _{5.10 to 5.25} × 100%.

The invention prepares the polylactic acid material with low temperature processing performance, which can not only greatly reduce the production cost of the current polylactic acid product, but also greatly broaden the application scene of the polylactic acid material, for example, it can be applied to the application of daily environment such as family, and has Very good application prospect.

The melting point of meso-lactide (meso-LA) is 53°C, while the melting point of D,L-lactide (D,L-LA) is 124°C to 128°C. The inventor found that when the technical solution of the present invention is adopted, after the copolymerization of meso-LA and D,L-LA, due to the low T _m of the low molecular weight meso-LA segment in the copolymer, it can be melted at low temperature, and the melted The meso-LA segment will have similar miscibility and plasticization effect on the PDLLA segment, thereby significantly reducing the processing temperature of the copolymer.

In a preferred solution of the present invention, the polymerization process adopts the reaction conditions of low temperature first and then high temperature, which can avoid the sublimation of meso-LA and improve the polymerization effect of meso-LA and D,L-LA.

Obviously, according to the above-mentioned content of the present invention, according to the common technical knowledge and conventional means in the field, without departing from the above-mentioned basic technical idea of the present invention, other various forms of modification, replacement or change can also be made.

The above content of the present invention will be further described in detail below through the specific implementation in the form of examples. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following examples. All technologies implemented based on the above content of the present invention belong to the scope of the present invention.

Description of drawings

Fig. 1 is meso-co-D, L-PLA copolymer (A-D) and PDLLA (E) H NMR spectra, wherein, the meso-LA molar content in the raw material lactide is: 74% (Example 4 , A), 68% (Example 3, B), 62% (Example 2, C), 52% (Example 1, D);

Fig. 2 is the proton nuclear magnetic resonance spectrum of meso-co-D, L-PLA copolymer, wherein, the temperature of low temperature reaction is 90 ℃ (Example 6, A), 60 ℃ (Example 5, B);

Fig. 3 is the hydrogen nuclear magnetic resonance spectrum of meso-co-D, L-PLA copolymer, wherein, the time of low temperature reaction is 14 hours (Example 8, A), 4 hours (Example 7, B);

FIG. 4 is the 1H NMR spectrum of meso-co-D, L-PLA prepared in Comparative Example 2. FIG.

Detailed ways

The reagents and chemical materials used in the following examples and experimental examples are all commercially available. The mixed lactide is synthesized according to the method in the prior art, using lactic acid as the raw material.

Example 1

Take 10 grams of mixed lactide containing 52% of meso-LA and 48% of D and L-LA, add 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 7 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). The H NMR spectrum is shown in Figure 1D.

For pure poly(D,L-lactic acid) (PDLLA, Figure 1E), the multiplet at δ = 1.50-1.62 ppm is assigned to the _-CH3 proton on the PDLLA chain, and the multiplet at δ = 5.12-5.25 ppm The ratio of -CH protons belonging to the chain is 3:1, which is consistent with the structure of PDLLA. Compared with PDLLA, in the ¹ H NMR spectrum of meso-co-D, L-PLA, in the multiplet at δ=5.12～5.25ppm, there is an additional peak of 5.21ppm, and the peak at 5.21ppm is derived from meso- The copolymerization unit of lactide (meso-LA) and D-lactide (D-LA) or L-lactide (L-LA), namely D-LA/meso-LA/D-LA unit (RR- RS-RR), L-LA/meso-LA/L-LA unit (SS-RS-SS) or L-LA/meso-LA/D-LA unit (SS-RS-RR). Studies have shown that PDLLA will appear peaks at 5.22ppm and 5.23ppm at the same time, and when meso-LA is copolymerized with D,L-LA, the resulting copolymer will appear at 5.21ppm and 5.23ppm. Absorption peaks at the same time. In Figure 1D, peaks at 5.21 and 5.23 both appeared, indicating that meso-LA and D,L-LA had been successfully copolymerized to form meso-co-D,L-PLA.

According to I _5.10-5.25 /I _4.34-4.41 in Fig. 1 (integrated area I _5.21 at δ=5.10-5.25 ppm and integrated area I _4.34-4.41 at δ=4.34-4.41 ppm), the molecular weight of the copolymer can be calculated by formula 1 . According to the peak area (S _5.21 ) fitted by δ=5.21ppm and the total peak area (S _5.10～5.25 ) fitted by δ=5.10～5.25ppm, the meso-co-D in the L-PLA can be calculated by formula 2. The content of -D,L-LA copolymerized units (C _meso-D,L-LA ).

M _n =(I _5.13-5.25 /I _4.3-4.5 )×72+73 (Formula 1)

C _meso-D,L-LA =S _5.21 /S _5.10～5.25 × 100% (Formula 2)

After calculation, the copolymerized units of meso-D and L-LA in the meso-co-D, L-PLA of this example are 12%. The number-average molecular weight of the copolymer obtained by GPC detection was 52,161.

Example 2

Take 10 grams of mixed lactide with a meso-LA content of 62% and D, L-LA content of 38%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 7 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). Among them, the mole percent of meso-lactide polymerized units is 62%, and the mole percent of D,L-lactide polymerized units is 38%. The H NMR spectrum is shown in Figure 1C.

After calculation, the copolymerized units of meso-D and L-LA in meso-co-D, L-PLA in this example are 10%, and the number average molecular weight of the copolymer obtained by GPC detection is 51364.

Example 3

Take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 7 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). Among them, the mole percentage of meso-lactide polymerized units is 68%, and the mole percentage of D,L-lactide polymerized units is 32%. The H NMR spectrum is shown in Figure 1B.

After calculation, the copolymerized units of meso-D and L-LA in meso-co-D, L-PLA in this example are 10%. The number average molecular weight of the copolymer obtained by GPC detection was 48682.

Example 4

Take 10 grams of mixed lactide containing 74% of meso-LA and 26% of D, L-LA, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 7 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). The H NMR spectrum is shown in Figure 1A.

After calculation, the copolymerized units of meso-D and L-LA in meso-co-D, L-PLA in this example are 11%, and the number average molecular weight of the copolymer obtained by GPC detection is 42884.

Example 5

Take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 60°C for 10 hours and at 140°C for 18 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). The H NMR spectrum is shown in Figure 2A.

After calculation, the copolymerized units of meso-D, L-LA in meso-co-D, L-PLA in this example are 9%, and the number average molecular weight of the copolymer obtained by GPC detection is 78402.

Example 6

Take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 90°C for 10 hours and at 140°C for 18 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). The H NMR spectrum is shown in Figure 2B.

After calculation, the meso-co-D, L-PLA in this example has 11% of meso-D, L-LA copolymerized units, and the molecular weight is 86752.

Example 7

Take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 4 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). Among them, the mole percentage of meso-lactide polymerized units is 68%, and the mole percentage of D,L-lactide polymerized units is 32%. The H NMR spectrum is shown in Figure 3B.

After calculation, the meso-co-D, L-PLA in this example has 9% of meso-D, L-LA copolymerized units, and the molecular weight is 39234.

Example 8

Take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, and 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 70°C for 14 hours and at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain a polylactic acid product (meso-co-D, L-PLA). Among them, the mole percentage of meso-lactide polymerized units is 68%, and the mole percentage of D,L-lactide polymerized units is 32%. The H NMR spectrum is shown in Figure 3A.

After calculation, the meso-co-D, L-PLA in this example has 10% of meso-D, L-LA copolymerized units, and the molecular weight is 42314.

Example 9

One-step heating method to synthesize meso-co-D, L-PLA: take 10 grams of mixed lactide with a meso-LA content of 68% and a D, L-LA content of 32%, 0.0056 grams of stannous octoate, and vacuum at room temperature for 2 hours to exclude oxygen. After sealing, the temperature of the polymerization reaction was 140°C, and the time of the polymerization reaction was 24 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40°C and vacuum dried to constant weight. The hydrogen nuclear magnetic resonance spectrum of the obtained polymer is shown in Figure 4. There is no absorption peak at 5.21 ppm in the figure, indicating that there is no meso-LA and no meso-LA in the polymer. D, the copolymerized unit of L-LA. This proves that during the polymerization reaction, the one-step heating method adopted in this example is difficult to form a copolymerization unit of meso-LA and D,L-LA, while the two-step heating method described in Examples 1-8 is in the realization of meso-LA and D,L-LA. The copolymerization of D, L-LA is more efficient.

Comparative Example 1

Synthesis of PDLLA: Take 10 grams of D,L-lactide, add 0.0056 grams of stannous octoate, vacuumize at room temperature for 2 hours, and remove oxygen. After sealing, the reaction was carried out at 140°C for 14 hours. The obtained polymer was purified twice by a dichloromethane-absolute ethanol co-precipitation system, and once again by a dichloromethane-n-hexane co-precipitation system. The purified product was placed in a vacuum drying oven at 40° C. and vacuum-dried to constant weight to obtain PDLLA. The H NMR spectrum is shown in Figure 1E.

Experimental Example 1 Melt Flow Index Test

According to the standard GB/T3682.1-2018, the melt flow index of meso-co-D, L-PLA was detected by HRZ-400B melt flow rate analyzer. The temperature is 100°C, 105°C, 110°C, and 115°C, and the load is 2.16 kg. The test results of the samples of Example 1, Example 6, Example 8, Example 9 and Comparative Example 1 are shown in Table 1.

Table 1 Melt flow index (MFR) of PDLLA and meso-co-D, L-PLA

It can be seen from the data in the table that the MFR values of the samples of each example are significantly improved compared to the PDLLA sample of Comparative Example 1. The above results show that the copolymerization of meso-LA and D, L-LA can significantly increase the melt flow index of the copolymer, that is, the meso-co-D, L-PLA obtained after the copolymerization of meso-LA and D, L-LA is compared. Compared with PDLLA, the processing temperature can be significantly reduced.

Secondly, for meso-co-D, L-PLA-11% (Example 6), meso-co-D, L-PLA-10% (Example 8) and meso-co-D with the same proportion of raw materials, The MFR values of L-PLA (Example 9), meso-co-D, L-PLA-11% and meso-co-D, L-PLA-10% with higher content of co-units were relatively high. This shows that in the polymerization process of mixed lactide, compared with the one-step heating method, the two-step heating method is used to prepare polylactic acid with more copolymerized units, which is a more preferred technical solution.

In addition, when the molar percentages of meso-LA and D,L-LA in the mixed lactide as the raw material were 68% and 32%, respectively, the prepared samples (meso-co-D, L-PLA-11%, L-PLA-11%, meso-co-D, L-PLA-10% and meso-co-D, L-PLA) MFR increased relatively high; while the molar percentages of meso-LA and D, L-LA in mixed lactide were respectively At 52% and 48%, the MFR increase of the prepared samples (meso-co-D, L-PLA-12%) was relatively low. This shows that, in the technical solution of the present invention, the preferred ratios of meso-LA and D,L-LA in the mixed lactide are 68% and 32% by mole.

Experimental Example 2 Low temperature processing example

The meso-co-D, L-PLA samples prepared in Example 1 were processed and tested, and the PDLLA sample was used as a control to process the polymer samples by a combination of twin-screw extruder extrusion and injection molding. The processing steps for:

Put PDLLA, meso-co-D, L-PLA particles (particles with length and width less than 5mm) into the barrel of the twin-screw extruder, circulate in the twin-screw for 3 minutes, and then extrude into the barrel of the injection molding machine for injection molding. Remove the strips and store in a desiccator at room temperature.

Among them, the specific parameters are compared as follows:

Screw temperature: for PDLLA, the screw temperature is 165 °C; for meso-co-D, L-PLA, the screw temperature is 105 °C;

Injection temperature: For PDLLA, the barrel temperature is 165°C; for meso-co-D, L-PLA, the barrel temperature is 105°C.

The injection pressure was all 180 bar, the mold pressure was 10 bar, and the mold temperature was 33°C.

It can be seen from the above processing examples that the meso-co-D,L-PLA sample of Example 1 can be processed at a lower temperature.

It can be seen from the above examples and experimental examples that the novel polylactic acid provided by the present invention has the performance of low temperature processing. The processing temperature can be reduced from 165°C to 105°C for PDLLA material. This greatly reduces the processing difficulty of polylactic acid materials, improves the safety of processing polylactic acid, reduces production costs, is conducive to the application and promotion of polylactic acid materials, and has high application potential.

Claims (9)

1. a polylactic acid that can be processed at low temperature, is characterized in that, described polylactic acid is made up of repeating unit A, repeating unit B, repeating unit C and repeating unit D; Wherein, the repeating unit A is

, The repeating unit B is

, The repeating unit C is

, The repeating unit D is

; The sum of the mole percentages of the repeating unit A and the repeating unit B is 3% to 74%, and the sum of the mole percentages of the repeating unit C and the repeating unit D is 97% to 26%; The mol ratio of the repeating unit A and the repeating unit B is any ratio, and the mol ratio of the repeating unit C and the repeating unit D is 1:1; The polylactic acid is prepared by the following method: the mixed lactide of meso-lactide and D, L-lactide is reacted at 60-90° C. for 4-14 hours, and then reacted at 100-180° C. for 12 hours. Obtained after ~24 hours. 2. polylactic acid according to claim 1, is characterized in that: the mol percent sum of described repeating unit A and described repeating unit B is 68%, and the mol percent of described repeating unit C and described repeating unit D is 68% The sum is 32%. 3. polylactic acid according to claim 1, is characterized in that: described repeating unit A and described repeating unit B are meso-lactide polymerization unit, described repeating unit C and described repeating unit D are D, L-lactide polymerization unit, in the polylactic acid, the copolymerization unit content of meso-lactide polymerization unit and D,L-lactide polymerization unit is 9% to 12%. 4. polylactic acid according to claim 3, is characterized in that: in described polylactic acid, the copolymerization unit content of meso-lactide polymerization unit and D, L-lactide polymerization unit is 10%～11%; And/or, the molecular weight of the polylactic acid is 10,000-200,000. 5. according to the polylactic acid according to claim 1, it is characterized in that: described polylactic acid is by molar percentage 3%～74% meso-lactide and mole percentage 97%～26% D, L-propylene Made of lactide copolymer. 6. the preparation method of the polylactic acid described in any one of claim 1-4, is characterized in that: comprise the steps: by meso-lactide and D, the mixed lactide of L-lactide is at 60～90 After 4~14 hours of reaction at ℃, and then obtained after 12~24 hours of reaction at 100℃~180℃. 7 . The preparation method according to claim 6 , wherein the polylactic acid is obtained by reacting the mixed lactide at 70 to 90° C. for 7 to 14 hours, and then at 140° C. for 14 hours. 8 . 8. preparation method according to claim 6 is characterized in that: described reaction is carried out under deoxygenation environment; And/or, the reaction is carried out under the action of a catalyst, and the catalyst is selected from stannous octoate; And/or, the consumption ratio of described catalyst and described mixed lactide is mol ratio 1:(2000～7000). 9. A degradable plastic product prepared from the polylactic acid according to any one of claims 1-5.

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