Method for producing polylactic acid polymer

Abstract

The present invention relates to a method for producing a polylactic acid polymer, and more particularly to a method for producing a polylactic acid polymer having a high molecular weight, good color characteristics, and a fast polymerization rate by using a specific combination of catalysts in the ring-opening polymerization reaction of a lactic acid oligomer.

Description

Technical Field

[0001] Cross - reference to related application(s) This application claims the benefit of priority based on Korean Patent Application No. 10 - 2021 - 0080925, filed on June 22, 2021, and all the contents disclosed in the document of the Korean Patent Application are included as part of this specification.

[0002] The present invention relates to a method for producing a polylactic acid polymer. Specifically, it relates to a method for producing a polylactic acid polymer having a high molecular weight and good color characteristics by using a specific combination of catalysts in the ring - opening polymerization reaction of lactic acid oligomers.

Background Art

[0003] Polylactic acid (PLA) is a plant - derived resin obtained from plants such as corn and has attracted attention as an eco - friendly material with biodegradable properties.

[0004] Unlike conventionally used petroleum - based resins such as polystyrene resin, polyvinyl chloride resin, and polyethylene, polylactic acid has effects such as preventing the depletion of petroleum resources and suppressing carbon dioxide emissions, so it can reduce environmental pollution, which is a demerit of petroleum - based plastic products. Therefore, as environmental pollution problems caused by waste plastics and the like have emerged as social issues, efforts have been made to expand the scope of application to product fields where general plastics (petroleum - based resins) such as food packaging materials and containers, and electronic product cases are used.

[0005] On the one hand, polylactic acid is produced by polymerizing lactic acid produced by microbial fermentation. However, only polymers with low molecular weights are produced by direct polymerization of lactic acid. To synthesize high molecular weight polylactic acid, there is a method of polymerizing to a higher molecular weight polylactic acid from the low molecular weight polylactic acid obtained by direct polymerization of lactic acid using a chain coupling agent. However, this method has the disadvantages that the process is complex, both the coupling agent and the organic solvent are used, and it is not easy to remove them.

[0006] Currently, the production process of commercially available high molecular weight polylactic acid uses a chemical synthesis method that converts lactic acid into lactide and synthesizes polylactic acid through a ring-opening reaction of the lactide ring. However, even in this case, there is a problem that it is difficult to achieve high molecular weight. In particular, when increasing the amount of the catalyst Sn(Oct)2 used to achieve high molecular weight, the conversion rate per unit time increases, but the actually increased molecular weight is not large. Instead, there is a problem that it has an adverse effect on the color of the resin produced. Also, when reducing the amount of catalyst used due to concerns about resin discoloration, a long polymerization time is required because the conversion rate per unit time is low, and there is a problem that the molecular weight decreases slightly.

[0007] Therefore, there is a need to develop a method for producing a polylactic acid polymer that has excellent physical properties, can be easily polymerized to a high molecular weight so as to be applicable to various industries.

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention provides a method for producing a polylactic acid polymer having a high molecular weight, good color characteristics, and a sufficiently high polymerization rate by using a specific combination of catalysts in the ring-opening polymerization reaction of lactic acid oligomers.

Means for Solving the Problems

[0009] In order to solve the above problems, the present invention provides a method for producing a polylactic acid polymer, which includes a step of ring-opening polymerization of lactide in the presence of a tin-based catalyst and a phosphinite-based cocatalyst to produce a polylactic acid polymer.

[0010] Throughout this specification, unless otherwise specified, "comprising" or "containing" means including a certain component (or constituent) without any special limitation, and shall not be construed as excluding the addition of other components (or constituents).

[0011] (Method for producing a polylactic acid polymer) Currently, in the production process of commercially available high-molecular-weight polylactic acid, a chemical synthesis method is used, which converts lactic acid into lactide and synthesizes polylactic acid through the ring-opening reaction of the lactide ring. However, there is a problem that it is difficult to achieve a high molecular weight suitable for commercialization. When the amount of the catalyst Sn(Oct)2 used for achieving high molecular weight is increased, the conversion rate per unit time increases, but the actually increased molecular weight is not large. On the contrary, it has a problem of adversely affecting the color of the produced resin. Also, when the amount of the catalyst is reduced due to concerns about resin discoloration, the conversion rate per unit time is low, so a long time is required for polymerization, and there is a problem that the molecular weight slightly decreases.

[0012] Therefore, the present inventors have confirmed that by using a specific combination of catalysts in the ring-opening polymerization reaction of lactic acid oligomers, a polylactic acid polymer having a high molecular weight, good color characteristics, and improved polymerization rate can be easily produced, and thus completed the present invention.

[0013] Conventionally, by using a phosphinite-based cocatalyst together with a tin-based catalyst used in the ring-opening polymerization reaction, it has been confirmed that the reaction conversion rate is significantly improved and it is easy to achieve high molecular weight. In particular, it has been confirmed that the weight average molecular weight of the finally produced polymer can be increased by about 15% or more compared to other phosphorus-based cocatalyst compounds such as phosphine, and it is easy to embody a high molecular weight polymer.

[0014] Hereinafter, the present invention will be described in detail.

[0015] According to an embodiment of the present invention, it includes a step of producing a polylactic acid polymer by ring-opening polymerization (ROP) of lactide.

[0016] The ring-opening polymerization is carried out in the presence of a combination catalyst of a tin-based catalyst and a phosphinite-based cocatalyst. Compared with the case of using a tin-based catalyst alone in the past, the polymerization rate is sufficiently fast, the reaction conversion rate is significantly improved, and it is easy to achieve high molecular weight. Also, the amount of the tin-based catalyst used during the reaction can be reduced, which is economical. The finally produced polylactic acid polymer maintains environmental friendliness and biodegradability, and has no problem of color change, so it is easy to apply to various fields. In particular, by using the combination catalyst, a polylactic acid polymer having a very large molecular weight of about 400,000 or more in terms of weight average molecular weight can be easily produced through the ring-opening polymerization reaction.

[0017] In the ring-opening polymerization, the tin-based catalyst may be Sn(Oct)2.

[0018] Also, in the ring-opening polymerization, the phosphinite-based cocatalyst may be one or more selected from the compounds represented by the following Chemical Formula 1.

[0019] [Chemical Formula 1] P(OR 1 )(R 2 )(R 3 ) In the formula, R 1 is a substituted or unsubstituted C 1-30 alkyl, or a substituted or unsubstituted C 6-30 aryl, R 2 and R 3 are each independently a substituted or unsubstituted C 1-30 alkyl, or a substituted or unsubstituted C 6-30It is aryl.

[0020] Preferably, said R 1 is methyl, ethyl, propyl, isopropyl, phenyl, biphenylyl or naphthyl, and more preferably ethyl, propyl, isopropyl or phenyl.

[0021] Preferably, said R 2 and R 3 are each independently methyl, ethyl, propyl, isopropyl, phenyl, biphenylyl or naphthyl, and these may be substituted or unsubstituted with methoxy, ethoxy, propoxy or isopropoxy. More preferably, they are ethyl, propyl, isopropyl or phenyl, and said phenyl may be substituted or unsubstituted with methoxy, ethoxy, propoxy or isopropoxy.

[0022] Preferably, the phosphinite-based cocatalyst may be one or more selected from the compounds represented by the following Chemical Formulas 1-1 to 1-3.

Chemical formula

[0023] The tin-based catalyst and the phosphinite-based cocatalyst may be contained in a molar ratio of 1:0.1 to 1:5. In this molar ratio range, the reaction conversion rate is significantly improved and it is easy to achieve high molecular weight. More preferably, the tin-based catalyst and the phosphinite-based cocatalyst may be used in a molar ratio of 1:0.5 to 1:3, 1:0.8 to 1:2 or 1:0.9 to 1:1.5.

[0024] The tin-based catalyst may be contained in an amount of 0.001 mol% to 0.05 mol% based on the lactide content. Within the above range, it can promote ring-opening polymerization with high reactivity, can be used in a relatively small amount, is excellent in economy, and is preferable because it does not cause color change in the final polymer. More preferably, the tin-based catalyst may be contained in an amount of 0.0015 mol% to 0.03 mol%, 0.0015 mol% to 0.01 mol% or 0.002 mol% to 0.01 mol% based on the lactide content. Also, by satisfying the above content range and at the same time satisfying the molar ratio range with the above-mentioned phosphinite-based cocatalyst, excellent catalytic activity can be realized.

[0025] The phosphinite-based cocatalyst may be contained in an amount of 0.001 mol% to 0.05 mol% based on the lactide content. Since it can promote condensation polymerization with high reactivity within the above range, it is preferable. More preferably, the phosphinite-based cocatalyst may be contained in an amount of 0.0015 mol% to 0.03 mol%, 0.0015 mol% to 0.01 mol% or 0.002 mol% to 0.01 mol% based on the lactide content. Also, it can satisfy the molar ratio range with the above-mentioned tin-based catalyst while satisfying the above content range.

[0026] The ring-opening polymerization reaction may be carried out by bulk polymerization that substantially does not use a solvent. At this time, the fact that substantially no solvent is used can include the case of using a small amount of solvent for dissolving the catalyst, for example, up to less than 10 ml of solvent per 1 kg of the total lactide used. By carrying out the ring-opening polymerization by bulk polymerization, it becomes possible to omit steps such as solvent removal after polymerization, and it is also possible to suppress decomposition or loss of the resin in such a solvent removal step. Also, the polylactic acid polymer can be obtained with a high conversion rate and yield by the bulk polymerization.

[0027] The ring-opening polymerization may be carried out at 150°C to 200°C and 0.5 bar to 2 bar for 30 minutes to 6 hours. Preferably, the reaction may be carried out at 170°C to 190°C and 0.7 bar to 1.5 bar for 1 hour to 3 hours. When polymerizing under the above conditions, a high conversion rate per hour can efficiently produce the target high molecular weight polylactic acid polymer, and the produced polymer has less discoloration.

[0028] On the other hand, the lactide may be produced by a method commonly used in the art, or a commercially available product may be used.

[0029] As commercially available products, L-lactide from FORUSORB, L-lactide from TCI, L-lactide from Sigma-aldrich, or L-lactide from Acrosorganics may be used, but it is not limited thereto.

[0030] Also, the lactide produced through the following steps can be used.

[0031] Specifically, it can include Step 1-1 of subjecting lactic acid to condensation polymerization to produce a lactic acid prepolymer, and Step 1-2 of depolymerizing the low molecular weight lactic acid prepolymer produced through the condensation polymerization reaction to produce lactide.

[0032] Here, the "lactic acid" refers to L-lactic acid, D-lactic acid, or a mixture thereof.

[0033] The reaction conditions for each step are not particularly limited. For example, the 1-1 step may be carried out in the presence of a sulfonic acid catalyst and a tin catalyst. Preferably, the sulfonic acid catalyst is p-toluenesulfonic acid, m-xylene-4-sulfonic acid, 2,4,6-trimethylbenzenesulfonic acid, or p-xylene-2-sulfonic acid. The tin catalyst is SnCl2.

[0034] Also, in the 1-1 step, after reacting at 150°C to 200°C for 110 minutes to 130 minutes, the reaction may be carried out at 150°C to 200°C and 10 mbar to 30 mbar for 1 hour to 5 hours.

[0035] The 1-2 step may be carried out in the presence of a tin catalyst. As the tin catalyst, Sn(Oct)2 is preferred.

[0036] Also, the 1-2 step may be carried out at 200°C to 250°C and 8 mbar to 12 mbar for 1 hour to 5 hours. Preferably, it may be carried out at 200°C to 230°C and 9 mbar to 11 mbar for 2 hours to 4 hours.

[0037] According to an embodiment of the invention, the manufacturing method has an excellent conversion rate by using two kinds of combined catalysts. By using the aforementioned combined catalyst, a high molecular weight polylactic acid polymer can be produced with an excellent conversion rate even when using a relatively short reaction time and / or a small content of the catalyst.

[0038] Preferably, the manufacturing method shows a conversion rate of 75% or more, preferably a high conversion rate of 75% to 99%, 75% to 97%, 80% or more, 90% or more, or 95% or more.

[0039] The conversion rate means the percentage of the reaction amount (number of moles) with respect to the supply amount (number of moles) of lactide. The specific measurement method of the conversion rate will be described in detail in the content of the experimental examples described later.

[0040] (Polylactic acid polymer) According to one embodiment of the invention, a polylactic acid polymer produced by the above-described production method is provided.

[0041] The weight average molecular weight (Mw) of the polylactic acid polymer may be 50,000 to 600,000, preferably 100,000 to 500,000. By satisfying the above range, it is suitable for realizing the physical properties required for commercial applications such as packaging materials, films, fibers, and durable materials.

[0042] The number average molecular weight (Mn) of the polylactic acid polymer may be 25,000 to 300,000, preferably 50,000 to 250,000.

[0043] The polydispersity index (PDI) of the polylactic acid polymer may be 1.0 to 3.0, preferably 1.5 to 2.5.

[0044] The measurement methods of the weight average molecular weight, number average molecular weight, and polydispersity index will be described in detail in the content of the experimental examples described later.

[0045] The polylactic acid polymer exhibits excellent color characteristics and has a value of 2.4 or less, preferably 2.35 or less, 2.15 or less, 2.0 or less, 1.7 or less, 1.5 or less, 1.0 to 2.4, or 1.0 to 2.35, 1.0 to 2.15, 1.0 to 2.0, 1.0 to 1.7, or 1.0 to 1.5 when measuring Y.I. (yellow index) in transmission mode in the solution phase, which is the method presented in this application.

[0046] (Article) Also, according to another embodiment of the invention, an article containing the polylactic acid polymer is provided. Examples of the article include packaging materials, films, non-woven fabrics, fibers, coffee capsules, durable materials, tobacco filters, etc. It can be used to produce environmentally friendly products that are applied to the article, have excellent biodegradation properties, and reduce CO2 generation.

Advantages of the Invention

[0047] As described above, the polylactic acid polymer according to the present invention provides a method for producing a polylactic acid polymer with a high conversion rate per unit time and a high molecular weight by using a catalyst with a specific combination in the ring-opening polymerization reaction of lactic acid oligomers.

[0048] In addition, the polylactic acid polymer maintains environmental friendliness and biodegradability and has excellent color characteristics.

Embodiments for Carrying out the Invention

[0049] Hereinafter, embodiments of the present invention will be described in more detail with the following examples. However, the following examples are only illustrative of the embodiments of the present invention, and the detailed description of the present invention is not limited by the following examples.

Examples

[0050] <Examples and Comparative Examples>

[0051] Example 1 A 20 mL Schlenk Flask was charged with a magnetic stirrer and 1 g of L-lactide (FORUSORB). The catalyst Sn(Oct)2 and the co-catalyst ethylenediphenylphosphinite (EDP) were added to the lactide at 50 ppmol each (molar ratio of Sn(Oct)2:EDP = 1:1). The inside of the flask was evacuated and maintained at 60 °C in an oil bath for 1 hour to remove the solvent and impurities. Then, under an Ar atmosphere, the temperature was raised to 180 °C at normal pressure (1 bar) and a ring-opening polymerization reaction was carried out for 1 hour to produce a polylactic acid polymer.

[0052] Example 2 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the content of the cocatalyst ethylenediphenylphosphinite (EDP) was used at 10 ppmol (molar ratio of Sn(Oct)2:EDP = 1:0.2).

[0053] Example 3 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that isopropoxy(4-methoxyphenyl)(phenyl)phosphine was used at the same content instead of the cocatalyst ethylenediphenylphosphinite (EDP) (molar ratio of Sn(Oct)2:ipr - mppp = 1:1).

[0054] Example 4 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the reaction time was increased from 1 hour to 2 hours.

[0055] Example 5 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the reaction time was increased from 1 hour to 3 hours.

[0056] Comparative Example 1 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the cocatalyst EDP was not used.

[0057] Comparative Example 2 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the cocatalyst EDP was not used and the reaction time was increased to 3 hours.

[0058] Comparative Example 3 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that the cocatalyst EDP was not used, the content of Sn(Oct)2 was increased to 100 ppmol, and the reaction time was maintained at 3 hours.

[0059] Comparative Example 4 In Example 1, a polylactic acid polymer was produced in the same manner as in Example 1, except that triphenylphosphine was used in the same content instead of ethylene diphenyl phosphinite (EDP) as a cocatalyst.

[0060] <Experimental Example> The properties of the polymers produced in the above Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 1.

[0061] 1) Molecular weight property evaluation For the polymers produced in the above Examples and Comparative Examples, the weight average molecular weight, number average molecular weight, and polydispersity index were measured by gel permeation chromatography (GPC: gel permeation chromatography, Tosoh ECO SEC Elite), and the results are shown in Table 1. Solvent: Tetrahydrofuran (THF) (eluent) Flow rate: 1.0 ml / min Column temperature: 40 °C Standard: Polystyrene (corrected by a third-order function)

[0062] 2) Conversion rate measurement For the polymers produced in the above Examples and Comparative Examples, the conversion rate defined as the percentage of the reaction amount (moles) with respect to the supply amount (moles) of lactide was measured.

[0063] Specifically, nuclear magnetic resonance spectroscopy (NMR: nuclear magnetic resonance, Bruker 500 MHz solution NMR) was used, CDCl3 was used as a solvent, 1H NMR was measured, and the conversion rate was measured through a method of comparing the peak area integration of PLA and lactide appearing between 4.9 ppm and 5.2 ppm.

[0064] 3) Color property evaluation For the polymers produced in the above Examples and Comparative Examples, in order to remove residual lactide and moisture and evaluate the color change characteristics, they were dried in a vacuum oven at 135 °C for 24 hours. Next, 2 g of the polymer sample was dissolved in 40 ml of CHCl3, and the Y.I. (yellow index) of the resin was measured using the transmission mode of Ultrascan VIS (HunterLab).

Table 1

[0065] As can be confirmed in Table 1 above, when ring-opening polymerization of lactide was carried out under two kinds of combined catalysts according to the present invention, a high conversion rate of about 75% or more was realized, and it was confirmed that a polymer having a weight average molecular weight of 400,000 or more could be easily produced. In particular, it was confirmed that their color characteristics were improved.

[0066] In the case of Comparative Examples 1 to 3 using only conventional Sn(Oct)2 instead of the two kinds of combined catalysts, it was confirmed that it was difficult to produce a polymer having a weight average molecular weight of 400,000 or more. In particular, in the case of Comparative Example 1, although there was no problem with the color characteristics using Sn(Oct)2 with the same content as in Example 1, it was confirmed that the conversion rate and molecular weight characteristics were significantly lower than those of the Example.

[0067] In the case of Comparative Examples 2 and 3, by increasing the reaction time or increasing the amount of catalyst used, the conversion rate could be made equivalent to that of the Example. However, even in this case, it was confirmed that the degree of increase in molecular weight was low. In particular, in the case of Comparative Example 3 where the amount of Sn(Oct)2 catalyst used was increased, it was confirmed that the Y.I. increased significantly and the color characteristics deteriorated.

[0068] On the other hand, in the case of Comparative Example 4, a phosphorus-based cocatalyst was used together with Sn(Oct)2. By using triphenylphosphine which is not the phosphinite-based catalyst of the present invention, the conversion rate was lower than that of the Example. In particular, it was confirmed that it was difficult to obtain a polymer having a high molecular weight to the desired degree.

Claims

1. comprising a step of producing a polylactic acid polymer by ring-opening polymerization of lactide in the presence of a tin-based catalyst and a phosphinite-based cocatalyst, wherein the tin-based catalyst is Sn(Oct)₂, wherein the phosphinite-based cocatalyst is one or more selected from compounds represented by the following Chemical Formula 1, [Chemical Formula 1] P(OR₁)(R₂)(R₃) (wherein, R₁ is a substituted or unsubstituted C₁₋₃₀ alkyl or a substituted or unsubstituted C₆₋₃₀ aryl, R₂ and R₃ are each independently a substituted or unsubstituted C₁₋₃₀ alkyl or a substituted or unsubstituted C₆₋₃₀ aryl) A method for producing a polylactic acid polymer.

2. The above-mentioned R １ is a method for producing the polylactic acid polymer according to claim 1, wherein R is methyl, ethyl, propyl, isopropyl, phenyl, biphenylyl or naphthyl.

3. Said R ２ and R ３ are each independently methyl, ethyl, propyl, isopropyl, phenyl, biphenylyl or naphthyl, which are substituted or unsubstituted with methoxy, ethoxy, propoxy or isopropoxy. The method for producing a polylactic acid polymer according to claim 1.

4. The method for producing a polylactic acid polymer according to Claim 1, wherein the phosphinite-based cocatalyst is one or more selected from compounds represented by the following Chemical Formulas 1-1 to 1-3. 【Chemical 1】

5. The method for producing a polylactic acid polymer according to Claim 1, wherein the tin-based catalyst and the phosphinite-based cocatalyst are included in a molar ratio of 1:0.1 to 1:

5.

6. The method for producing a polylactic acid polymer according to Claim 1, wherein the tin-based catalyst is included in an amount of 0.001 mol% to 0.05 mol% based on the lactide content.

7. The method for producing a polylactic acid polymer according to Claim 1, wherein the phosphinite-based cocatalyst is included in an amount of 0.001 mol% to 0.05 mol% based on the lactide content.

8. The method for producing a polylactic acid polymer according to Claim 1, wherein the ring-opening polymerization is carried out at 150°C to 200°C and 0.5 bar to 2 bar for 30 minutes to 6 hours.

9. The method for producing a polylactic acid polymer according to Claim 1, wherein the weight average molecular weight of the polylactic acid polymer is 50,000 to 600,000.

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