JP3352890B2 - Polylactic acid and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a polylactic acid produced from a raw material monomer mainly composed of lactide and a method for producing the same. More specifically, the present invention can produce polylactic acid with a very small catalyst addition amount of 50 ppm or less. Further, the present invention relates to a method for producing polylactic acid without using a solvent for diluting a raw material monomer, and a polylactic acid obtained by this method.

[0002]

2. Description of the Related Art Polylactic acid has been used in the medical field because it is a polymer that can be decomposed in a living body and has excellent mechanical properties. However, polylactic acid is decomposed by microorganisms even in a natural environment. From the viewpoint of environmental protection, development to various industrial uses and consumer uses is expected.

As a method for producing polylactic acid, lactide, which is a cyclic ester compound, is used as a raw material monomer, and is heated to a predetermined temperature condition in the presence of a catalyst such as stannous octylate to carry out ring-opening polymerization of lactide. A method for producing polylactic acid by performing the method is known.

For example, Japanese Patent Application Laid-Open No. 5-93050 discloses that
A method is disclosed in which a mixture containing a lactide monomer is continuously supplied to a reactor to convert 75% or more of the monomer mixture into a polymer. Specifically, a plurality of stirring tanks are connected in series, and A method is shown in which 35 to 85% conversion is performed in the reaction tank, and the conversion is increased to 75 to 95% in the second reaction tank. However, in order to achieve the above conversion rate using this method, the viscosity of the content is increased by improving the conversion rate, but the raw material monomers added one after another are carbonized in order to maintain the stirring efficiency. It is diluted with a viscosity adjusting solvent such as a hydrogen-based solvent or a halogen-based solvent to maintain stirring workability. Therefore, it was necessary to provide a step of removing the solvent after the production of the polymer.

Therefore, Japanese Patent Application Laid-Open No. Hei 7-26001 discloses that
A method for continuously producing a biodegradable polyester polymer having a melt viscosity of 500,000 poise (p) or less and a weight average molecular weight of 10,000 or more by using a static mixer without using a viscosity adjusting solvent as described above. It is shown.

By the way, when the ring-opening polymerization of lactide proceeds and the conversion rate to the polymer increases and the reaction solution thickens, the catalyst is easily taken into the polymer. At this time, if the amount of the catalyst incorporated in the polymer is large, the polymer is deteriorated or decomposed during processing such as molding or spinning, so that the polymer molecular weight is reduced, the polymer melt viscosity is reduced,
Furthermore, coloring of the polymer and the like were caused, and a high-quality and stable product could not be manufactured.

However, in an attempt to minimize the amount of catalyst used in the polymerization, for example, 50
If the addition amount is reduced to not more than m, there arises a problem that the dispersion in the polymerization reaction system becomes non-uniform and ring-opening polymerization proceeds only locally. Therefore, it is customary to add a large amount of the catalyst, and in the method of Japanese Patent Application Laid-Open No. 5-93050,
0.1 to 0.65 wt% (1000 to 6500 ppm)
And the catalyst described in JP-A-7-2600.
In the method of Japanese Patent Publication No. 1, 200 to 400 ppm of a catalyst is added.

As described above, since the catalyst of several hundred ppm or more is added in the conventional method, after pelletizing the polymer, the polymer is charged into a washing vessel and washed with a solvent such as acetone. It was essential to wash out the catalyst to reduce its content, and then to dry the solvent with hot air such as nitrogen gas. Therefore, development of a method for producing polylactic acid capable of reducing the catalyst content to 50 ppm or less without performing such catalyst washing has been demanded.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and has a catalyst addition amount of 50 pp.
It is intended to provide a method for producing polylactic acid in which the washing and drying steps of the polymer pellets can be omitted, and polylactic acid with a small amount of impurities produced by this method.

[0010]

Means for Solving the Problems According to the present invention which has solved the above-mentioned problems, in producing polylactic acid from a starting monomer mainly composed of lactide, the total amount of catalyst added to the starting monomer is set to 50 ppm or less. The reaction is interrupted or added continuously little by little, thereby performing the reaction while delaying the progress of the reaction, and increasing the stirring power stepwise or continuously in response to the increase in the viscosity of the reaction solution due to the progress of the reaction. According to the present invention, the catalyst can be added without being diluted with a solvent. When adding the catalyst, it is desirable to add the catalyst to the raw material monomer in the raw material monomer supply line provided upstream of the reactor, instead of adding the catalyst into the reactor.

In the present invention, the polymerization reaction is carried out in two steps, and in the first polymerization step, the reaction temperature is 170.degree.
C. or lower, the conversion is reduced to 20% or more and 50% or less by retaining for 2 to 15 hours. In the second polymerization step, the reaction temperature is maintained at 180 to 210 ° C. for 1 to 8 hours to increase the conversion. It is recommended to be 40% or more and 80% or less.

Further, a third polymerization step is provided subsequent to the second polymerization step. In the third polymerization step, the reaction temperature is kept at 180 ° C. to 210 ° C. for 1 to 5 hours to further increase the conversion. In this case, it is desirable that the content be 60% or more and 95% or less.

The reactor for performing the first polymerization step includes:
It is recommended to use a vertical stirring tank having large vertical paddle blades of upper and lower stages, as the reactor for performing the second polymerization step, use a vertical stirring tank having double helical blades,
Alternatively, a tubular reactor may be used. As the reactor for performing the third polymerization step, a horizontal reactor or a tubular reactor can be used.

If a degassing step for vaporizing and removing unreacted monomers is provided after the above polymerization step, the problem caused by the mixture of residual monomers can be reduced or eliminated. The above deaeration step is 180 ° C or higher and 260 ° C or lower, 0.001 to 1.1 kg
It may be performed under the condition of / cm ² . According to the production method of the present invention, it is possible to produce polylactic acid having an average molecular weight of 50,000 or more, a catalyst content of 50 ppm or less by weight, and containing no hydrocarbon solvent or halogen solvent. It is.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional method for producing polylactic acid, the addition of a catalyst of several hundred ppm or more is considered to be indispensable, and there has been no report from the viewpoint of minimizing the amount of catalyst. The reason is that if the amount of added catalyst is small, not only the reaction time is prolonged, but also if the conversion rate increases with the progress of the reaction, the catalyst itself is incorporated into the polymer having a high viscosity, and the polymerization is localized. This is because it only progresses.

The present inventors have intensively studied a method for minimizing the total amount of the catalyst to eliminate the washing step of the catalyst. As a result, surprisingly, even if the catalyst addition rate is limited to a very small amount of 50 ppm or less,
By controlling the operating conditions in each reactor, it has been found that a satisfactory conversion can be achieved and the washing step can be omitted.

According to the present invention, in producing polylactic acid from a raw material monomer mainly composed of lactide, an addition rate of a catalyst to the raw material monomer is set to 50 ppm or less,
By continuously adding this little by little, the reaction proceeds for a long time while delaying the progress of the reaction, and is stirred stepwise or continuously in response to the increase in the viscosity of the reaction solution composed of the raw material monomer and its polymer. By increasing the power, a reaction that tends to proceed locally with a very small amount of catalyst can be performed uniformly over the entire reaction vessel.
Therefore, the catalyst can be added without diluting with a solvent, and the starting monomer need not be diluted with a viscosity adjusting solvent.

It is recommended that the reaction temperature be increased stepwise or continuously according to the progress of the reaction. If the residence time for performing the polymerization reaction is 3 to 28 hours (preferably 8 hours or more), the starting monomer Conversion can be 85% or more, and the average molecular weight of the polymerized polylactic acid is 50,0
It can be between 00 and 300,000.

Specifically, the polymerization reaction is carried out in two steps, and in the first polymerization step, the conversion is maintained at a reaction temperature of 170 ° C. or more and 190 ° C. or less for 2 to 15 hours (preferably 6 hours or more) to reduce the conversion. In the second polymerization step, the reaction temperature is set to be higher than that in the first polymerization step, and is retained at 180 ° C. to 210 ° C. for 1 to 8 hours (preferably 2 hours or more) in the second polymerization step to increase the conversion rate. Increase (improve) 40%
It is recommended to be at least 80%.

Further, a third polymerization step is provided subsequent to the second polymerization step. In the third polymerization step, the reaction temperature is kept at 180 ° C. or more and 210 ° C. or less, and the conversion is further enhanced by retaining the mixture for 1 to 5 hours. In this case, it is desirable that the content be 60% or more and 95% or less.

If the reaction temperature in each of the above polymerization steps is too high, the polymer will be undesirably colored, and if the reaction temperature is too low, the reaction rate will be slow and the residence time will be greatly increased in order to secure a predetermined polymer ratio. It is necessary to lengthen it, which is not desirable.

The residence time is preferably controlled according to the reaction temperature. For example, when the reaction temperature is 180 ° C. in the first polymerization step, the residence time is preferably about 9 hours.
Based on this, the residence time may be shortened at a temperature higher than 180 ° C, and the residence time may be increased at a temperature lower than 180 ° C. In the second polymerization step, the residence time is desirably about 3 hours at 200 ° C., which may be shorter on the high temperature side and longer on the low temperature side. If the residence time is too short, a sufficient conversion cannot be obtained, while if the residence time is too long, the conversion will be too high,
There is a risk that the stirring will become difficult and the operation will stop.

In the first polymerization step, it is desirable to carry out the polymerization at a viscosity of 1,000 p or less.
It is more desirable to target the polymerization at a viscosity of around 0p. As the reactor used in the first polymerization step, for example, it is recommended to use a vertical stirrer described in JP-A-5-49890, and the specific configuration is as shown in FIG. The paddle blades 11 are arranged in front of the lower paddle blades 12 vertically adjacent to each other at a crossing angle of 45 to 75 degrees in the rotational direction, while the outer ends of the paddle blades 11 are retracted as the lower paddle blades 12. By using a high-efficiency stirring blade formed on the blade (for example, a full zone blade manufactured by Shinko Pantech Co., Ltd.), it is desirable to form a flow pattern covering the entire stirring tank, and to use a stirring device having improved stirring efficiency.

In the second polymerization step, it is desirable to carry out the polymerization at a viscosity of 10,000 p or less,
It is more desirable to aim to carry out the polymerization at a viscosity of around 000p. As a reactor used in the second polymerization step,
A vertical stirring tank having double helical blades may be used, or a tubular reactor may be used. As the former, for example, a stirring device described in JP-A-6-154573 or a frame 21 instead of a shaft as shown in FIG.
It is recommended to use a stirrer having a built-in high-viscosity stirring blade (for example, a log bone blade manufactured by Shinko Pantech Co., Ltd.) having a helical ribbon blade 22 and a bottom ribbon blade 23.

In order to further enhance the conversion, a third polymerization step is desirably provided, and the reaction temperature is 180 ° C. or higher.
It is desirable to carry out the polymerization so as to achieve a condition of a conversion rate of 60% or more and 95% or less by retaining at 10 ° C. or less for 1 to 5 hours. In this case, when a reaction temperature of 200 ° C. is employed, the residence time is desirably about 3 hours. Based on this, the residence time may be shorter on the high temperature side and longer on the low temperature side.

In the third polymerization step, it is desirable to carry out the polymerization at a viscosity of 30,000 p or less.
It is more preferable to target a viscosity of around 0000p.
As the reactor used in the third polymerization step, a horizontal reactor or a tubular reactor may be used. As the horizontal reactor, a horizontal stirring tank having a single-shaft or twin-shaft stirring device or a scraping device is used, Alternatively, it is preferable to use a single or twin screw kneader or kneader.

Following the above polymerization step, it is recommended to provide a deaeration step for vaporizing and removing unreacted monomers. By providing the degassing step, it is possible to allow the unreacted monomer to remain after the completion of the polymerization reaction, so that the conversion of the polymer in the polymerization step can be reduced. Therefore, the residence time in the polymerization step does not become excessively long, and the size of the polymerization apparatus can be appropriately suppressed. In addition, since the viscosity of the mixture of the polymer and the monomer to be treated becomes small, the power of stirring or transfer required for the polymerization step can be reduced.

As a degassing device used in the degassing step, it is recommended to use a so-called liquid film falling type heat exchanger that simultaneously performs heating and gas-liquid separation. May be used. Degas conditions are 18
In the temperature range of 0 ° C. or more and 260 ° C. or less, 0.001-1.
It is desirable to carry out under a pressure of 1 kg / cm ² .

The present invention is not limited by the type of the catalyst, and any general catalyst may be used as a catalyst for promoting ring-opening polymerization, such as stannous chloride, stannous bromide, or stannous iodide. Tin, stannous sulfate, stannic oxide, tin octylate, tetraphenyltin, titanium tetrachloride, ethyl titanate, butyl titanate, propyl titanate, glycol titanate,
Zinc chloride, zinc oxide, zinc acetate, antimony trioxide, antimony trifluoride, antimony acetate, lead oxide, aluminum oxide, iron oxide, manganese carbonate, manganese acetate and the like can be used.

In the present invention, the amount of the catalyst added is 50 pp.
m or less, but from the viewpoint of the quality of the polylactic acid to be produced, the smaller the better, the more preferable is 30 ppm or less. According to the present invention, even if the catalyst addition amount is 10 ppm or less, polylactic acid is produced. can do. However,
If the added amount of the catalyst is too small, a long residence time is required. Therefore, it is preferable to add at least 3 ppm or more, more preferably 5 ppm or more.

According to the method of the present invention as described above, the average molecular weight is 50,000 or more, and the catalyst content is 50 pp by weight.
m or less can be produced without a washing step. Moreover, for the purpose of adjusting the viscosity of the product during the polymerization step, it is not necessary to add a hydrocarbon solvent such as toluene or xylene or a halogen solvent such as chloroform. It is possible to produce polylactic acid containing no system solvent.

According to the present invention, the residence time in the stirrer is longer than in the conventional method. Therefore, in order to maintain the same production efficiency as in the conventional method, it is necessary to increase the volume of the reactor. become. However, according to the present invention, the washing step for removing the catalyst, the step for removing the solvent, and the distillation step for separating the solvent can be omitted. Since ancillary equipment other than a typical production process is not required, the production efficiency can be dramatically improved from the viewpoint of the entire polylactic acid production system.

In melt-spinning using polylactic acid, if the amount of catalyst in polylactic acid is large, thread breakage occurs frequently in the spinning process. Therefore, in the conventional production method, polylactic acid is pelletized and the catalyst is washed. It was essential. On the other hand, according to the production method of the present invention, the step of washing the solvent after producing the polylactic acid can be omitted, so that it is not necessary to pelletize the polylactic acid as in the conventional method. It is also possible to provide a direct melt spinning step after the production of

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention. It is included in the technical range of.

[0035]

EXAMPLE 1 FIG. 1 is a schematic explanatory view showing a typical example of the production process of the present invention, wherein 1 is a raw material monomer storage tank, 2 is a catalyst container, and 3 is a first
Vertical stirring tank, 4 is a second vertical stirring tank, 5 is a horizontal reactor, 6
Is a kneading extruder for degassing, 7 is an unreacted monomer recovery device, P1
P5 is a pump.

As the first vertical stirring tank, a vertical stirring tank (130 liters) equipped with paddle-type high-efficiency stirring blades (Shinko Pantech full zone blades: 2.2 kW variable motor) was used, and the second vertical stirring tank was used. As the stirring tank, a vertical stirring tank (135 liters) equipped with a double helical type high-viscosity stirring blade (Shinko Pantech log bone blade: 2.2 kW variable motor) was used. Further, as the third horizontal reactor, A horizontal stirring tank (108 liters, manufactured by Shinko Mex Co., Ltd.) in which a stirring blade type stirrer was installed was used.

In the first vertical stirring tank, 10 kL of molten lactide was added.
g / h and tin octylate as a catalyst at 0.1 g / h (10
ppm), and ring-opening polymerization was carried out at an average temperature of 180 ° C. and under a nitrogen gas flow at a pressure of 1.03 kg / cm ² . A mixture of polymer and lactide (polymer ratio: 33%) was taken out by a gear pump so that the average residence time in the first vertical stirring tank was 9 hours, and was fed to the second polymerization apparatus.

In the second vertical stirring tank, the average temperature was 200 ° C.
Under a nitrogen gas flow, ring-opening polymerization was carried out at a pressure of 1.03 kg / cm ² . A mixture of polymer and lactide (polymer ratio: 69%) was taken out by a gear pump so that the average residence time in the second vertical stirring tank was 3.5 hours, and introduced into a horizontal reactor.

In the horizontal reactor, ring-opening polymerization was carried out at an average temperature of 200 ° C. and under a nitrogen gas flow at a pressure of 1.03 kg / cm ² . A mixture of polymer and lactide (polymer ratio: 88%) was taken out with a gear pump so that the average residence time in the horizontal reactor was 4.5 hours, and introduced into a deaerator.

As a deaerator, a twin-screw kneading extruder (KTX manufactured by Kobe Steel) equipped with a vacuum vent is used to remove lactide, which is an unreacted monomer, by deaeration, thereby increasing the polymer ratio. The polylactic acid was taken out of the deaerator in the form of a strand, cooled, cut and pelletized.

In the steady state, unreacted lactide is degassed at about 0.9 kg / h, and 9.1 is discharged.
Polylactic acid pelletized at kg / h (91% yield) could be obtained. The average molecular weight of the obtained polymer was 253,000.

Examples 2 to 10 As shown in Tables 1 and 2, polylactic acid was produced in the same manner as in Example 1 except that the residence time, the operation time, and the amount of catalyst in each polymerization apparatus were changed. In Example 4, polyethylene glycol was used as a raw material in an amount of 4.0.
%, And in Example 5, 23.0%. In Example 6, lauryl alcohol was added as a molecular weight regulator. The catalyst addition amount was 10 pp for Examples 2 to 6.
m, 5 ppm for Example 7, 30 ppm for Example 8, and 50 ppm for Examples 9 and 10.

[0043]

[Table 1]

[0044]

[Table 2]

According to the method of the present invention, even when the catalyst amount is 50 ppm or less, the molecular weight is 100,000 or more and the polymer ratio is 9
It can be seen that 9% or more of polylactic acid can be produced. It can be seen that the addition of a copolymer or lauryl alcohol reduces the molecular weight but facilitates degassing and increases the purity of the polymer.

Examples 11 and 12 Example 1 was repeated in the same manner as in Example 1 except that a tubular reactor was used as the second polymerization apparatus or the third polymerization apparatus.
Performed 1,12.

[0047]

[Table 3]

Also in Examples 11 and 12 using a tubular reactor, polymerization with a catalyst amount of 10 ppm is sufficiently possible, and polylactic acid having a polymer ratio of 99.5% can be obtained by providing a degassing step. did it.

Examples 13 and 14 In Examples 13 and 14, the extruder with a vent was used as a second deaerator, and a liquid film falling-down heat exchanger was provided as a first deaerator in the preceding process. Example 1 except that degassing was performed
Polylactic acid was produced in the same manner as described above.

[0050]

[Table 4]

In Example 13, the polymer ratio was 99.9% even when the molecular weight was 200,000 or more. In Example 14, degassing was performed without using a horizontal reactor, but by using both degassing devices 1 and 2, the polymer ratio was reduced to 99%.
%.

Comparative Examples 1 to 4 Comparative examples as shown in Table 5 were carried out by changing the operating temperature and the like in the production method of Example 1. In addition, Comparative Example 1,
3 was 10 ppm, and the catalyst amounts of Comparative Examples 2 and 4 were 50 ppm.
ppm.

[0053]

[Table 5]

In Comparative Example 1, the reaction temperature did not occur because the operating temperature of the first vertical stirring tank was too low. In Comparative Example 2, the temperature in the first vertical stirring tank was low, and in Comparative Examples 3 and 4, the temperature in the second vertical stirring tank was low, and the viscosity of the content increased due to an increase in the polymer ratio. The operation was stopped due to the inability to stir.

[0055]

As described above, according to the present invention, a polylactic acid having a catalyst content of 50 ppm or less and containing no viscosity adjusting solvent can be produced without providing a washing step. It became. Therefore, in the production line according to the present invention, the washing step for removing the catalyst, the solvent removing step, and further the distillation step for collecting and separating the solvent can be omitted, which has been required so far. Since additional equipment other than the direct production process of polylactic acid is not required, the production system for polylactic acid can be greatly simplified.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a typical production process of the method of the present invention.

FIG. 2 is a schematic explanatory view illustrating a vertical stirring tank suitable for a first vertical stirring tank.

FIG. 3 is a schematic explanatory view illustrating a vertical stirring tank suitable for a second vertical stirring tank.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Raw material monomer storage tank 2 Catalyst container 3 1st vertical stirring tank 4 2nd vertical stirring tank 5 Horizontal reactor 6 Deaeration kneading extruder 7 Unreacted monomer recovery device 11 Upper stage paddle blade 12 Lower stage paddle blade 21 Frame 22 Helical ribbon wing 23 Bottom ribbon wing

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroshi Miyagawa 4-3-1, Bingo-cho, Chuo-ku, Osaka City Kobe Steel, Ltd. Osaka Branch Office (72) Inventor Kazuhisa Fujisawa 1-5-5, Takatsukadai, Nishi-ku, Kobe-shi No. 5 Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Koji Yamamoto 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Kobe Steel Co., Ltd.Kobe Research Institute (72) Inventor Hitomi Ohara Kyoto City No. 1, Nishi-no-Kyowabaracho, Nakagyo-ku, Japan Shimanzu Seisakusho Sanjo Plant (72) Inventor Seiji Sawa 1 Nishi-no-Kyowa-cho, Nakagyo-ku, Kyoto Shimazu Seisakusan Sanjo Plant (72) Inventor Yasuhiro Fujii Yasuhiro Fujii, Nishi-no-Kyowa-cho, Nakagyo-ku, Kyoto Shimadzu Corporation Sanjo Plant (72) Inventor Masahiro Ito 1 Nishinokyo Kuwaharacho, Nakagyo-ku, Kyoto-shi (56) References US Patent 5,136,017 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08G 63/00-63/91

Claims (15)

(57) [Claims] In producing polylactic acid from a starting monomer mainly composed of lactide, the total amount of a catalyst added to the starting monomer is set to 50 ppm or less, and the total amount of the catalyst is intermittently or continuously added little by little. A method for producing polylactic acid, comprising: performing a reaction while delaying the progress, and increasing the stirring power stepwise or continuously in response to an increase in the viscosity of the reaction solution due to the progress of the reaction, thereby performing a homogeneous reaction. 2. A catalyst is added to a raw material monomer supply line provided upstream of a reactor for performing a polymerization reaction.
3. The method for producing polylactic acid according to 1.). 3. The method according to claim 1, wherein the catalyst is added without being diluted with a solvent. 4. The production method according to claim 1, wherein the reaction temperature is increased stepwise or continuously as the reaction proceeds. 5. The production method according to claim 1, wherein the conversion of the raw material monomer is 85% or more. 6. The polymerized polylactic acid has an average molecular weight of 50,0.
The method according to any one of claims 1 to 5, wherein the number is from 00 to 300,000. 7. The residence time for carrying out the polymerization reaction is 3 to 28.
The method according to claim 1, wherein the time is a time. 8. The polymerization reaction is carried out in two steps. In the first polymerization step, the conversion is kept at a reaction temperature of 170 ° C. or more and 190 ° C. or less for 2 to 15 hours so that the conversion is 20%.
In the second polymerization step, the reaction temperature is kept at 180 ° C. or more and 210 ° C. or less for 1 to 8 hours to increase the conversion rate to 40% to 8%.
The method according to claim 1, wherein the content is 0% or less. 9. A third polymerization step is further provided subsequent to the second polymerization step. In the third polymerization step, the conversion rate is further increased by retaining at a reaction temperature of 180 ° C. or more and 210 ° C. or less for 1 to 5 hours. The method according to claim 8, wherein the content is increased to 60% or more and 95% or less. 10. The production method according to claim 8, wherein a vertical stirring tank having upper and lower multistage large paddle blades is used as a reactor for performing the first polymerization step. 11. The production method according to claim 8, wherein a vertical stirring tank having double helical blades or a tubular reactor is used as the reactor for performing the second polymerization step. 12. The production method according to claim 9, wherein a horizontal reactor or a tubular reactor is used as the reactor for performing the third polymerization step. 13. A degassing step for vaporizing and removing unreacted monomers after the polymerization step.
3. The production method according to any one of 2. 14. The degassing step is performed at a temperature of 180 ° C. or more and 260 ° C. or less under a condition of 0.001 to 1.1 kg / cm ^2.
3. The production method according to 3. 15. A polylactic acid having an average molecular weight of 50,000 or more, a catalyst content of 50 ppm or less by weight, and containing no hydrocarbon solvent or halogen solvent.