CN110903452A

CN110903452A - Preparation method of lactic acid copolymer with high relative molecular mass

Info

Publication number: CN110903452A
Application number: CN201911157113.1A
Authority: CN
Inventors: 邹洁; 李峥嵘
Original assignee: Guangzhou Ruite New Material Technology Co Ltd
Current assignee: Guangdong Jusheng New Material Technology Co.,Ltd.
Priority date: 2019-11-22
Filing date: 2019-11-22
Publication date: 2020-03-24

Abstract

The invention relates to the field of high polymer materials, and discloses a preparation method of a lactic acid copolymer with high relative molecular mass, which comprises the steps of preparing a PLA prepolymer, preparing a PCL prepolymer and extruding the PLA copolymer by double screw rods, synthesizing polylactic acid with certain relative molecular mass by a melt polycondensation method, then copolymerizing the polylactic acid with a polyurethane prepolymer of polycaprolactone polyol, extruding the lactic acid copolymer by adopting a double screw rod reactor to prepare the lactic acid copolymer with higher relative molecular mass, wherein the lactic acid copolymer prepared by extruding the polylactic acid copolymer by the double screw rods is thermoplastic and has better mixing effect than kettle type reaction; through reasonable configuration, the extrusion product has better mechanical property through double-screw reaction, the toughness of the reaction extrusion product is increased due to the increase of the PCL content, and the tensile strength and the tensile modulus are reduced.

Description

Preparation method of lactic acid copolymer with high relative molecular mass

Technical Field

The invention relates to the field of high molecular materials, in particular to a preparation method of a lactic acid copolymer with high relative molecular mass.

Background

In the process of preparing the copolymer by using a tank reactor, the viscosity of a reaction system is higher and higher along with the progress of a polymerization reaction, which seriously hinders the continuation of the reaction, generally, the synthesized PLLA has high brittleness, a toughening agent needs to be added in the melt processing process or the PLLA is blended with other materials, the viscosity is increased in the later period of the reaction, the mixing effect of the two components is not ideal, the PLLA with high relative molecular mass is synthesized, the blending step is increased, the cost is increased, the compatibility is poor, and the product performance is poor.

The inventor establishes a brand new process route for preparing the lactic acid copolymer with high relative molecular mass, namely, firstly synthesizing polylactic acid with certain relative molecular mass by adopting a melt polycondensation method, and then carrying out chain extension by using a polyurethane prepolymer of polyester polyol. The route has the advantages that the PLLA with high relative molecular mass is prepared without lactide ring-opening polymerization, so that the cost is reduced; and the toughness of PLLA can be improved by the copolymerization method.

Disclosure of Invention

The invention aims to provide a preparation method of a lactic acid copolymer with high relative molecular mass, so as to improve the toughness of the lactic acid copolymer (PLLA), reduce the production cost and increase the toughness of the PLLA.

In order to achieve the technical purpose and achieve the technical effect, the invention discloses a preparation method of a lactic acid copolymer with high relative molecular mass, which comprises the steps of preparing a PLA prepolymer, preparing a PCL prepolymer and preparing the PLA copolymer by double-screw reaction extrusion, and specifically comprises the following steps:

preparing a PLA prepolymer: carrying out melt polycondensation reaction on lactic acid;

preparing a PCL prepolymer: adding 1, 6-hexamethylene diisocyanate into the PCL subjected to vacuum dehydration under the protection of inert gas for reaction and polymerization;

preparation of PLA copolymer: and respectively adding the PLA and the PCL prepolymer synthesized in advance into a double-screw reactor, processing by using double screws, and directly cooling the melt into strips through water bath after reaction extrusion to obtain the PLA copolymer.

The preparation method of the PLA prepolymer is direct melt polycondensation of lactic acid, and comprises the following specific steps: the preparation method comprises the steps of adding lactic acid into a reaction system, stirring and heating under a vacuum environment, adding a catalyst, and carrying out vacuum dehydration reaction.

Further, the catalyst is stannous chloride.

The preparation method of the PCL prepolymer comprises the following specific steps:

s1: dehydrating the PCL accurately measured at 120 ℃ for 1-2 h in vacuum;

s2: cooling to 45-55 ℃, removing vacuum, adding into 1, 6-hexamethylene diisocyanate, and protecting with nitrogen;

s3: the reaction releases heat, the system is naturally heated for 30-40 min, then slowly heated to 80-90 ℃, and the reaction is carried out for 2-3 h under the condition of heat preservation;

s4: sampling and analyzing the-NCO content, and when the-NCO content is basically consistent with a design value, defoaming to obtain the PCL prepolymer.

The double-screw reactor controls the feeding amount of PLA and PCL prepolymer through a feeder and a metering pump, the PLA is added from a feeding port through the feeder, and the PCL prepolymer enters from the middle part of one area of the screw through the metering pump.

Preferably, the temperature of the four zones of the screw in the double-screw reactor is respectively 155-.

The invention has the following beneficial effects:

1. the invention synthesizes polylactic acid with certain relative molecular mass by a melt polycondensation method, then carries out copolymerization with polyurethane prepolymer of polycaprolactone polyol, adopts a double-screw reactor to extrude lactic acid copolymer to prepare the lactic acid copolymer with higher relative molecular mass, and the lactic acid copolymer prepared by the double-screw reaction extrusion is thermoplastic and has better mixing effect than kettle type reaction.

2. Through reasonable configuration, the extrusion product has better mechanical property through double-screw reaction, the toughness of the reaction extrusion product is increased due to the increase of the PCL content, and the tensile strength and the tensile modulus are reduced.

Drawings

FIG. 1 is a schematic diagram of a process for preparing a PLA-PCL copolymer by a twin-screw reactor method of the present invention.

FIG. 2 is an infrared spectrum of a PLLA and PCL prepolymer at different temperatures during the temperature raising process of the present invention, wherein the curves a-h are specifically the following temperatures: (a)20 ℃, (b)60 ℃, (c)90 ℃, (d)105 ℃, (e)120 ℃, (f)135 ℃, (g)145 ℃, (h)160 ℃.

FIG. 3 is an infrared spectrum of a prepolymer of PLLA and PCL with different reaction times at 160 ℃ according to the present invention, wherein the curves are from top to bottom the following reaction times: 0min,1min,2min,3min,3.5min,4min,4.5min,5min,5.5min,6 min.

FIG. 4 is an IR spectrum of a prepolymer of PLLA and PCL with different reaction times at 170 ℃ according to the present invention, wherein the curves are from top to bottom the following reaction times: 0min,1min,2min,3min,3.5min,4min,4.5min,5min,5.5min,6 min.

FIG. 5 is an IR spectrum of a prepolymer of PLLA and PCL with different reaction times at 180 ℃ according to the present invention, wherein the curves are from top to bottom the following reaction times: 0min,1min,2min, 2.5min,3min,3.5min,4 min.

FIG. 6 is an IR spectrum of various products of the invention, wherein curves a-d are: (a) is PCL, (b) is PLA, (c) is PLA/PCL blend, and (d) is PLA-PCL copolymer.

FIG. 7 is an SEM picture of a PLA-PCL copolymer of the present invention, wherein the picture (a), the picture (c) are electron micrographs of a twin-screw reaction extrusion product, and the picture (b), the picture (d) are electron micrographs of a copolymerization product obtained by a tank reaction.

FIG. 8 is a DSC curve of two sets of PLA and PLA-PCL of the present invention.

FIG. 9 is a polarization microscope photograph of the reaction extrusion product of the present invention, FIGS. a1-a5 are polarization microscope photographs of a crystal growth process at 90 ℃, FIGS. b1-b5 are polarization microscope photographs of a crystal growth process at 105 ℃, and FIGS. c1-c5 are polarization microscope photographs of a crystal growth process at 120 ℃.

FIG. 10 is a stress-strain curve of various PLA-PCL copolymers of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments.

Example 1:

as shown in figure 1, the invention discloses a preparation method of a lactic acid copolymer with high relative molecular mass, which comprises the steps of preparing a PLA prepolymer, preparing a PCL prepolymer and preparing the PLA copolymer by double-screw reaction and extrusion.

The preparation method of the PLA prepolymer is direct melt polycondensation reaction of lactic acid, and comprises the following specific steps: the method is characterized by adding lactic acid into a reaction system, stirring and heating under a vacuum environment, and adding stannous chloride for vacuum dehydration reaction to obtain the product.

s1: dehydrating the PCL accurately measured at 120 ℃ for 1-2 h in vacuum;

s2: cooling to 50 ℃, removing vacuum, adding into 1, 6-hexamethylene diisocyanate, and protecting with nitrogen;

s3: the reaction releases heat, the system is naturally heated for 30-40 min, then slowly heated to 85 ℃, and the reaction is carried out for 2-3 h under the condition of heat preservation;

The preparation method of the PLA copolymer by twin-screw reactive extrusion comprises the following specific steps: adding pre-synthesized PLA and PCL prepolymer into a double-screw reactor respectively, controlling the feeding amount of the PLA and the PCL prepolymer by a feeder and a metering pump in the double-screw reactor, adding the PLA from a feeding port through the feeder, feeding the PCL prepolymer into the middle of one zone of a screw through the metering pump, processing through double screws, and directly cooling melts into strips through a water bath after reaction extrusion, wherein the temperatures of four zones of the screw in the double-screw reactor are respectively 155 plus-material 165 ℃, 160 plus-material 170 ℃, 155 plus-material 165 ℃ and 40r/min of screw rotation speed, and thus obtaining the PLA copolymer.

Example 2:

tracking the reaction process of the PLLA and the PCL prepolymer by heating infrared:

firstly, respectively dissolving a certain amount of PLLA and PCL in anhydrous chloroform according to a certain proportion, then uniformly mixing and stirring the two solutions, coating a film on a glass slide, drying in a vacuum dryer for 4-5 h, and finally testing the obtained blend film sheet by a transmission method through a Nicoletnex-670 FTIR spectrometer under different temperature rising conditions.

To examine the reactivity of PLLA with PCL prepolymer (OCN-PCL-NCO), we tracked the reaction process of the two with increasing temperature in the infrared. See fig. 2, 3, 4, 5, respectively.

From the values (a)20 ℃, (b)60 ℃, (c)90 ℃, (d)105 ℃, (e)120 ℃, (f)135 ℃, (g)145 ℃, (h)160 ℃ marked in FIG. 2, it can be seen that the blend of PLLA and PCL prepolymer was at 2270cm^-1Has a characteristic absorption peak of-NCO end group. In the temperature rise process, due to the rapid temperature rise speed, the reaction degree of the PLLA and the PCL prepolymer is very small, 2270cm^-1The peak variation is small. Therefore, the reaction time must be prolonged to complete the reaction of PLLA with the PCL prepolymer.

FIG. 3 is an IR spectrum of a sample of PLLA and PCL prepolymer at 160 ℃ for different reaction times, and it can be seen that the characteristic absorption peak of-NCO at 2270cm-1 gradually becomes smaller and the characteristic absorption peak of-NCO substantially disappears at 5min, indicating that the PLLA and PCL prepolymer have completely reacted at this time.

FIGS. 4 and 5 are the IR spectra of the PLLA prepolymer and PCL prepolymer at 170 ℃ and 180 ℃ respectively, showing that the characteristic absorption peak of-NCO at 170 ℃ is almost completely disappeared at 4.5min in FIG. 4; from FIG. 5, it can be found that the characteristic absorption peak of-NCO at 180 ℃ disappears substantially completely at 3min, after which the spectrum does not change significantly.

From the above analysis, it is known that the reaction rate of the prepolymer of PLLA and PCL gradually increases and the reaction time becomes shorter as the reaction temperature increases. We can control the reaction time and reaction speed of PLLA and PCL pre-polymer by controlling the reaction temperature.

Example 3:

influence of the copolymer proportion on the relative molecular masses of the copolymerization products:

the magnitude of the relative molecular mass of a polymer directly affects many properties of the polymer. We first examined the effect on the relative molecular mass of the product by preparing PLA copolymers of different PCL ratios. The relative molecular mass of PLLA and its copolymers was determined by viscometry using a copolymerization temperature of 180 ℃ for a copolymerization time of 9min, and the results are shown in Table 1.

As can be seen from the data in Table 1, the relative molecular mass of the PLLA and the PCL prepolymer is greatly improved after the chain extension reaction, because the terminal-NCO group of the PCL prepolymer has high reactivity and is easy to react with the terminal-OH group of the PLLA, and the terminal-NCO group of the PCL prepolymer can also react with-COOH at high temperature, so that the PLLA molecular chain is extended and the relative molecular mass is increased. It can be seen that the relative molecular mass of the PLLA/PCL copolymer increases with the increase of the PCL prepolymer content, and when the PCL prepolymer content reaches a certain value, the relative molecular mass of the PLLA/PCL copolymer reaches a peak value, and the PCL content continues to increase, but the relative molecular mass of the PLLA/PCL copolymer decreases rather, and when the relative molecular mass of the PLLA/PCL copolymer reaches a maximum value, which is not the theoretical [ -NCO ]/[ -OH ] ═ 1:1, it is because the polymerization system contains water and small-molecular lactic acid, etc., and they consume a certain amount of [ -NCO ]. The relative molecular mass of the copolymer reaches a maximum only when the amounts of [ -NCO ] and water, small molecules and [ -OH ] reach a certain equilibrium. The content of PCL prepolymer is continuously increased, side reaction is increased, and the relative molecular mass is reduced.

TABLE 1 relative molecular masses of lactic acid copolymers in different proportions

PLLA/PCL(w/w)	M _w
		100/0	27630
90/10	35750
		80/20	75570
70/30	120280
		60/40	166820

Example 4:

infrared spectroscopic analysis of the twin-screw reaction extrusion product:

FIG. 6 is an infrared spectrum of various products. FIG. 6(a) is an infrared spectrum of a PCL prepolymer. FIG. 6(b) is an infrared spectrum of PLA. Figure 6(c) is an infrared spectrum of a blend of PLA and PCL. FIG. 6(d) is an infrared spectrum of the PLA-PCL copolymer obtained by twin-screw reaction extrusion. As can be seen from FIG. 6(b), PLA was at 1757cm^-1Has a strong C ═ O stretching vibration peak at 1091cm^-1，1132cm^-1，1184cm^-1The stretching vibration peak of C-O-C is shown, which indicates the existence of ester group; at 2996cm^-1Has C-H stretching vibration peak at 1453cm^-1has-CH₃A bending peak. 3441cm^-1Characteristic absorption of terminal-OH groups. As can be seen from FIG. 6(a), the PCL prepolymer was present at 2271cm^-1Has a characteristic absorption peak of-NCO end group. Whereas the characteristic absorption peak of the-NCO end group on the graph of FIG. 6(d) disappeared, indicating that the-NCO group at the chain end of the PCL prepolymer had reacted with the-OH (or-COOH) end group of PLA. To further verify that the obtained product is a copolymerization product, a blending sample of a PLA prepolymer and a PCL prepolymer was prepared and subjected to infrared analysis, and the result is shown in fig. 6 (c). As can be seen from the figure, at 2271cm^-1Characteristic absorption peaks for the-NCO end groups also appear. These show that the PLA prepolymer and the PCL prepolymer react in the twin-screw extrusion process, and the copolymer of the PLA and the PCL can be obtained by the twin-screw reaction extrusion method.

Example 5:

morphological structure analysis of the twin-screw reaction extrusion product:

FIG. 7 is a scanning electron micrograph of the product. Wherein (a) and (c) are electron micrographs of the twin-screw reaction extrusion product, and (b) and (d) are electron micrographs of the copolymerization product obtained by the pot reaction. The method for preparing the PLA copolymer by the kettle type reaction comprises the following steps:

adding the PLA and the PCL prepolymer into a 5L reaction kettle according to a certain proportion, introducing nitrogen, and reacting for 2-30 min at the temperature of 150-190 ℃. After the reaction is completed, the copolymer is discharged to allow it to solidify naturally, and then the resultant product is pulverized for use. During the preparation process, the product is obtained by vacuum-pumping and defoaming, and nitrogen is introduced for protection, so that oxidative discoloration of the polymerization product and influence on the performance of the product are prevented.

The comparison shows that the product obtained by the twin-screw reaction extrusion has better mixing effect, better product compatibility, no phase separation and homogeneous phase. The product obtained in the kettle reactor has a two-phase structure due to poor mixing effect, and a part of components become dispersed phases and are distributed in the system. One phase is PLA hard segment, and the other phase is PCL soft segment.

When a sample is prepared by a scanning electron microscope, the sample needs to be broken at low temperature (broken in liquid nitrogen). From the figure, when the double-screw reaction extrusion product is broken at low temperature, a plurality of cracks are generated on the section under the action of stress, and the copolymerization product obtained in the kettle type reactor basically has no large cracks, which indicates that the product obtained through the double-screw reaction extrusion is uniform copolymerization product due to better uniformity, the toughness of the material is better, the product can bear the action of larger stress, and the mechanical property of the material is more excellent. The product obtained by kettle polymerization is brittle and has poor mechanical properties due to poor mixing effect and the mixing of unreacted prepolymer in the product.

Example 6:

relative molecular mass, material properties, solubility of the reaction extrusion product:

two kinds of PLA with different relative molecular masses and PCL prepolymer with different proportions are respectively subjected to double-screw reaction extrusion to obtain corresponding reaction extrusion products. The relative molecular masses of PLA and its copolymers were determined by viscometry and their solubility was investigated. The specific operation steps are as follows:

the intrinsic viscosity of the polymer was measured using a black-type viscosity measuring agent having a tetrahydrofuran solvent and a constant temperature bath of 30. + -. 0.1 ℃ and an inner diameter of 0.35mm, and the results of calculating the weight average relative molecular mass Mw. of the copolymer using the formula [ η ] ═ 1.25X 10-4Mw0.717 are shown in Table 2:

TABLE 2 relative molecular masses, Material Properties and solubilities of PLA and PLA-PCL

As can be seen from the data in Table 2, after the PLA and the PCL prepolymer are reacted and extruded, the relative molecular mass is greatly improved, and generally can be improved by 4-7 times. This indicates that the PLA and PCL prepolymer did react during the twin-screw reactive extrusion process. When the content of the PCL prepolymer is continuously increased to exceed a certain proportion, allophanate may be generated by reaction to form a crosslinking structure, and the solubility of the product is deteriorated.

Example 7:

thermal analysis of the twin-screw reaction extruded product:

the DSC spectra of polylactic acid and its reaction extrusion copolymerization product are shown in FIG. 8, and the crystallization temperature (Tc) and melting point (Tm) of the extrusion product of the polylactic acid and PCL prepolymer with two different phases and relative molecular masses are measured by DSC, and the related data are shown in Table 3.

From fig. 8, it can be seen that the twin-screw reaction extrusion product of PLA and PCL has only one melting peak, indicating that the copolymer of PLA and PCL has only one crystallization region, probably because the PCL segment is much shorter than the PLA segment, and the chemical bond between the segments limits the motion of the PCL segment, making the PCL segment difficult to crystallize. Melting doublets generally appear in the DSC curve of PLA, probably due to racemization of a portion of the PLA segment during melt polycondensation.

On the one hand, the melting point of the copolymer tends to decrease with the increase of the content of the PCL prepolymer, because the addition of the PCL prepolymer can reduce the crystallization degree of PLA and further reduce the melting point. On the other hand, as the molecular mass of the product increases, the melting point may tend to increase.

DSC analysis of the samples also revealed the effect of melt chain extension on the crystallinity of the product. The crystallization enthalpy of the copolymer is far lower than that of PLA, on one hand, the introduction of PCL chain segment reduces the stereoregularity of PLA molecule, so that the crystallinity of the product is greatly reduced; on the other hand, the crystallinity of the product is reduced because the PCL segment is difficult to crystallize in the copolymer.

TABLE 3 thermal Properties of PLA and PLA-PCL

Sample number	Tm/℃	ΔHm(J/g)	Tc/℃	ΔHc(J/g)	Xc(％)
						PLA1	138.52	29.83	105.72	31.41	33.3
PLA1-PCL	147.21	21.42	93.96	10.15	10.7
						PLA2	165.92	39.66	117.32	47.07	42.2
PLA2-PCL*	147.31	23.75	98.24	20.66	22.4

Example 8:

crystalline morphology observation of the reaction extruded product:

the polymer samples were observed for crystal morphology and crystallization process at a certain temperature by means of a BX 51-hot stage polarization microscope of Olympus corporation, Japan, and photographed for comparison.

The method comprises the following steps: placing a small amount of sample between two cover slips on a hot table, heating to 180 ℃ at the speed of 130 ℃/min, preserving heat for 1min to completely melt the sample, pressing to form a film, then cooling to the crystallization temperature at the speed of 130 ℃/min, observing the crystallization form of the sample, and automatically shooting the isothermal crystallization growth condition of the sample every 20 seconds until the sample is completely crystallized.

Since the nature of the polymer is closely related to its morphology. For highly crystalline polymers such as PLLA, spherulites can exhibit a characteristic black cross-extinction pattern between crossed polarizers in a polarizing microscope, which can be observed with the aid of a common polarizing microscope. A polarization micrograph of the twin-screw reaction extrusion product is shown in FIG. 9, which records the crystal growth process of the copolymerization product at different crystallization temperatures.

FIG. 9 is a typical photograph of the isothermal crystal growth process of the reaction extrusion at 90 deg.C, 105 deg.C, 120 deg.C, etc. As can be seen from the figure, the reaction extrusion product shows a typical spherulite shape under a polarization microscope, and the spherulite growth condition is good, and shows a good black cross extinction phenomenon. And the process of increasing spherulite size with the time of crystallization is clearly seen.

As can be seen from the figure, the spherulite radius of the reaction extrusion product increases and the number of spherulites decreases with the increase of the crystallization temperature. It can be found that the radius of spherulites of the reaction extruded product is the largest and the number is small at 120 ℃. That is because the homogeneous nucleation rate is higher and the spherulite growth rate is higher at lower temperatures, and as the temperature is increased to 120 ℃, the nucleation rate begins to slow, the growth rate also slows, and the crystallization rate also begins to decrease. When the temperature is raised to 135 ℃, crystal nuclei are hardly formed, and the spherulite morphology of the reaction extrusion product is hardly observed.

Example 9:

mechanical properties of the reaction extrusion product:

the tensile properties of the test specimens were measured at room temperature using an AGS-500ND model general purpose material testing machine, manufactured by Shimadzu corporation of Japan, at a tensile speed of 250mm/min and a length between chucks of 100 mm.

FIG. 10 is a graph of the stress-strain curves obtained for two different reaction extrusion products at two different sampling instances, with the relevant data set forth in Table 4.

TABLE 4 tensile Properties of PLA-PCL copolymer

It can be seen from fig. 10 that a distinct yield point appears on the stress-strain curves for both copolymerization products. However, PLA1-PCL has a low PCL content, a small elongation at break, but a large tensile strength and a large tensile modulus, and is a brittle material. And the tensile strength and the tensile modulus of the product are reduced due to the higher content of the PCL in the PLA2-PCL, but the elongation at break is increased and can reach 22.9 percent at most, which shows that the toughness of the reaction extrusion product is increased due to the increase of the content of the PCL.

To summarize:

(1) the polylactic acid with certain relative molecular mass is synthesized by adopting a melt polycondensation method, and then is copolymerized with the polyurethane prepolymer of polycaprolactone polyol to prepare the lactic acid copolymer with higher relative molecular mass.

(2) The feasibility of preparing the lactic acid copolymer by the twin-screw reactive extrusion is proved, the reaction extrusion product is thermoplastic, and the effect of preparing the lactic acid copolymer by the twin-screw reactive extrusion is better than that of kettle type reaction mixing.

(3) The reaction extrusion product presents a typical spherulite shape under a polarizing microscope, presents a good black cross extinction phenomenon, and the radius of the obtained spherulite is maximum when the reaction extrusion product is at 120 ℃.

(4) The reaction extrusion product has better mechanical property, the toughness of the reaction extrusion product is increased by increasing the content of PCL, and the tensile strength and the tensile modulus are reduced.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.

Claims

1. A preparation method of a lactic acid copolymer with high relative molecular mass is characterized by comprising the steps of preparing a PLA prepolymer, preparing a PCL prepolymer and preparing the PLA copolymer by double-screw reaction extrusion, and specifically comprises the following steps:

2. The method of claim 1 for preparing a lactic acid copolymer of high relative molecular mass, wherein: the preparation method of the PLA prepolymer is direct melt polycondensation of lactic acid, and comprises the following specific steps: the preparation method comprises the steps of adding lactic acid into a reaction system, stirring and heating under a vacuum environment, adding a catalyst, and carrying out vacuum dehydration reaction.

3. The method of claim 2 for preparing a lactic acid copolymer of high relative molecular mass, wherein: the catalyst is stannous chloride.

4. The method of claim 3 for preparing a lactic acid copolymer of high relative molecular mass, wherein: the preparation method of the PCL prepolymer comprises the following specific steps:

s1: dehydrating the PCL accurately measured at 120 ℃ for 1-2 h in vacuum;

5. The method of claim 1 for preparing a lactic acid copolymer of high relative molecular mass, wherein: the double-screw reactor controls the feeding amount of PLA and PCL prepolymer through a feeder and a metering pump, the PLA is added from a feeding port through the feeder, and the PCL prepolymer enters from the middle of one area of the screw through the metering pump.

6. The method of claim 5 for preparing a lactic acid copolymer of high relative molecular mass, wherein: the temperature of the four zones of the screw in the double-screw reactor is respectively 155-.