CN1326997A - Method for preparing high molecular weight poly lactic acid by direct polycondensation - Google Patents

Abstract

The invention discloses a method for preparing high molecular weight polylactic acid by direct polycondensation. This method uses a dehydrating agent to selectively remove water in a closed environment. Water is continuously removed during the condensation process, but there is no loss of lactide, so polylactic acid with very high molecular weight can be prepared. Since no organic solvents are used, very clean PLA can be obtained at low operating costs.

Description

Method for preparing high molecular weight polylactic acid by direct polycondensation

The invention relates to a method for producing polylactic acid, in particular to a method for preparing high-molecular-weight polylactic acid by a direct method.

Polylactic acid is a synthetic polymer compound with excellent biocompatibility and complete biodegradation. Since the raw material lactic acid can be produced by fermentation of agricultural products, it is considered to be a sustainable and environmentally friendly product. Its synthesis method can be divided into two categories: (1) direct method: directly prepared by condensation of lactic acid: (2) indirect method: prepare lactide from lactic acid through cyclization, and then ring-opening polymerization of lactide to form polylactic acid . Due to the fact that the polycondensation reaction and depolymerization reaction cannot be independently controlled, direct condensation can only produce low molecular weight polylactic acid with small molecular weight and limited use value. The indirect method can prepare high-molecular-weight polylactic acid with a large molecular weight, but in the process of preparing the intermediate product lactide, due to unavoidable side reactions such as polycondensation, a large amount of lactic acid oligomers and other compounds other than lactide are generated. Rings, these by-products not only make the separation and purification of lactide very difficult, but the recycling of by-products also makes the process very complicated. The preparation of lactide not only consumes energy, but also requires a large investment in equipment-this is the main reason for the high production cost of polylactic acid.

解聚反应为： Lactic acid condensation reaction is:

The depolymerization reaction is:

The above reactions are all reversible. In order to increase the molecular weight of polylactic acid, it is necessary to suppress the depolymerization reaction while promoting the condensation reaction. Commonly used small molecule removal measures such as vacuuming, heating, and inert gas purging can effectively remove water molecules and promote the condensation reaction, but it is inevitable that the depolymerization reaction will be accelerated due to the removal of lactide at the same time. This is the root cause of the low yield of direct polycondensation and the difficulty in obtaining high molecular weight polylactic acid. Experiments have shown that different catalysts can affect the relative rates of condensation and depolymerization reactions and determine the molecular weight change during the polymerization reaction. But in principle, if other side reactions are not considered, the final molecular weight of polylactic acid is not determined by the catalyst when the reaction system reaches equilibrium, but by the concentration of small molecule water and lactide in the system.

In order to obtain high molecular weight polylactic acid, the concentration of water molecules in the reaction system must be low enough, while the concentration of lactide must be high enough. The literature (Masanobu Ajioka, Katashi Enomoto, Kazuhiko Suzuki, and Akihiro Yamaguchi, Bull. Chem. Soc, Jpn., 68, 2125-2131 (1995)) describes a method of direct condensation polymerization to high molecular weight polylactic acid. The method is to selectively remove water through azeotropic rectification under high vacuum condition by using a high-boiling-point organic solvent such as diphenyl ether. However, due to the use of organic solvents and high vacuum distillation, the complexity and cost of operation are increased. Moreover, since the solvent in the finished polylactic acid is also difficult to completely remove, the performance of the product is also affected.

The purpose of the present invention is to disclose a method for preparing high-molecular-weight polylactic acid by direct polycondensation using a dehydrating agent to selectively remove water, so as to overcome the complex operation, high operating cost, and difficulty in completely removing the solvent in the finished polylactic acid existing in the prior art. Removal, defects of poor performance of the product.

Technical concept of the present invention is such:

In order to remove water in the reaction system without losing lactide, a dehydrating agent can be used to selectively remove water in a closed environment. During the condensation process, water is continuously removed without any loss of lactide, so polylactic acid with very high molecular weight can be obtained. Adsorption dehydration can be carried out under negative pressure, but high vacuum is not required. This polylactic acid prepared by direct condensation method characterized by selective dehydration by a dehydrating agent in a closed environment is the core of the present invention. Since no organic solvents are used, very clean polylactic acid can be obtained at low operating costs.

Technical scheme of the present invention:

Method of the present invention comprises the steps:

Put lactic acid or low-molecular-weight polylactic acid and dehydrating agent together in a closed polymerization tank, use vacuum or inert gas to remove the oxygen in the polymerization tank, then close the reaction tank, and then carry out the polymerization reaction at a certain temperature .

The reactor can be in a vacuum state, or contain a certain amount of inert gas. React at a temperature of 60-300° C. for 0.1-100 hours and cool to obtain polylactic acid with an average molecular weight of 10,000-500,000.

Said dehydrating agent is a dehydrating agent utilizing chemical reaction or physical adsorption dehydrating agent.

Dehydrating agents that use chemical reactions to dehydrate are more effective because they do not adsorb lactide. Such dehydrating agents include one of Na, K, metal oxides, non-metal oxides, acids, alkalis, inorganic salts capable of forming crystalline hydrates with water, and mixtures thereof;

Preferred metal oxides are CaO, BaO, MgO, _Al2O3 , _Na2O or _{K2O ;}

The preferred non-metal oxide is P ₂ O ₅ ;

Preferred acids are phosphoric acid, pyrophosphoric acid or polyphosphoric acid;

The preferred base is NaOH or KOH;

Preferred inorganic salts are CaCl ₂ , K ₂ CO ₃ , CaSO ₄ , MgCl ₂ , MgSO ₄ , MnSO ₄ , MnCl ₂ , CuSO ₄ , NiSO ₄ or ZnSO ₄ ;

Dehydrating agents utilizing physical adsorption include silica gel and various dehydrating resins such as acrylic resins. In principle, any dehydrating agent that can lower the equilibrium partial pressure of water at the temperature of the polymerization reaction and cannot (or seldom) adsorb lactide or dehydrate or oxidize lactide and polylactic acid can be used. The dehydrating agent can be used once, and can also be used after regeneration.

The theoretical amount of dehydrating agent can be calculated according to the type of dehydrating agent and the water to be removed during the lactic acid condensation reaction process. In order to achieve a better dehydrating effect, the actual amount can be 1.1 to 1.5 times the theoretical amount.

In the preferred solution of the present invention, in order to reduce the amount of dehydrating agent and the cost of regeneration, lactic acid can be pre-condensed by conventional methods to remove part of the water. After reaching a certain degree of polymerization, melt or solid-state polycondensation is carried out under closed conditions.

In order to increase the removal rate of water inside the polymer, on the one hand, the reaction can be carried out at a higher temperature, on the other hand, a smaller particle size can be used during solid-state polycondensation, or the surface can be updated during melt polycondensation. Therefore, it is best to crush the prepolymer to an average particle size of 0.01 mm to 5 mm, and then carry out polycondensation;

According to the present invention, when the dehydrating agent is used for melting or solid-phase polycondensation, the reactor can be pre-evacuated, and then the reactor is sealed and polymerized at a certain temperature. Depending on the polymerization temperature and polymerization time, the pressure in the reactor may be negative pressure or positive pressure. The reactor can also be filled with inert gas, such as N ₂ , He, CO ₂ , so that the polycondensation can be carried out under relatively high pressure.

According to the present invention, the gas in the polymerization reactor can be in a static state, and the water molecules diffuse from the polymer to the polymer melt or the solid surface, then diffuse to the surface of the dehydrating agent, and are finally captured by the dehydrating agent.

In order to increase the diffusion rate of water molecules from the polymer surface to the dehydrating agent surface, the gas in the reactor can be forced to circulate. If solid-phase polycondensation is carried out and forced circulation is adopted, gas-solid reaction equipment such as packed bed, moving bed and fluidized bed can be used for polymerization. (In principle, existing melt polymerization and solid phase polymerization reactors can be used).

It can be seen from the technical solution disclosed above that the operation of the present invention is simple, water is continuously removed during the condensation process, but lactide is not lost, so polylactic acid with a very high molecular weight can be produced. Since no organic solvents are used, very clean polylactic acid can be obtained at low operating costs.

The following examples will further describe the present invention, but the purpose is not to limit the scope of the present invention. The raw material used in the example is L-lactic acid purchased from PURAC company, wherein the lactic acid content is 87.5-88.5%. The molecular weight of polylactic acid was determined by the viscosity-average method, the solvent was tetrahydrofuran, and the temperature was 37°C. The molecular weight is calculated using the following formula:

[η]=1.07×10 ^-4 M _η ^0.75 where [η] is the intrinsic viscosity number, and M _η is the viscosity average molecular weight.

Example 1

Into a 500ml three-neck flask, add 300g of 88% L-lactic acid, and place it in an oil bath at 110°C for vacuum dehydration for 6 hours. Lactic acid weighs 245g after dehydration. _Add catalyst _{SnCl 2 and TSA} (p- _toluene sulfon Acid. React in vacuum (<10mmHg) and 130°C for 5 hours, then react at 150°C for 5 hours and then cool down rapidly. Break the prepolymer with a household mixer to obtain a lactic acid prepolymer with a molecular weight of 12,000 .

Take 50g of lactic acid prepolymer and 20g of CaO and spread them on a petri dish, and place them layer by layer in a stainless steel container with a diameter of 15mm. Replace the air in it with high-purity nitrogen and then evacuate it, repeating this at least three times. After the last vacuuming, close the inlet and outlet valves of all containers, and transfer to a thermostat at 130°C to react for 10 hours to obtain polylactic acid with a molecular weight of 23,000.

Example 2

Get 25g of lactic acid prepolymer and 10g of CaO obtained in Example 1, layer and tile in a stainless steel container, replace the air therein with high-purity nitrogen repeatedly and vacuumize. Finally, the container was sealed and transferred to a thermostat at 135° C. for 20 hours of reaction. A white polylactic acid with a molecular weight of 78,000 was obtained.

Example 3

Take 2g of polylactic acid and 5g of CaO obtained in Example 2, spread them in layers in a stainless steel container, repeatedly replace the air with high-purity nitrogen and vacuumize, and finally seal the container and transfer it to a thermostat at 170°C for 20 hours. . Polylactic acid with a molecular weight of 146,000 was obtained.

Embodiment 4～5

Take 2g of polylactic acid and 5g of CaO obtained in Example 2 and spread them in layers in a stainless steel container, replace the air with high-purity nitrogen repeatedly and vacuumize it, and finally seal it and transfer the container to a thermostat at 144°C or 160°C for reaction 20h. Get 124,000 and 131,000 PLA respectively.

Example 6-9

Take 2g of prepolymers of different particle sizes (20-40 mesh, 40-60 mesh, 60-80 mesh, 80-100 mesh) obtained in Example 1, and layer and spread them in a stainless steel airtight container together with 20g CaO In the chamber, the air in it is repeatedly replaced with high-purity nitrogen, then vacuumized, and finally sealed. The container was transferred to a constant temperature oven at 130°C for polymerization for 9 hours, followed by polymerization at 145°C for 6 hours, and reaction at 166°C for 6 hours. The molecular weight of polylactic acid obtained is shown in Table 1. particle size Molecular weight (×10-4) Example 6 20-40 17 Example 7 40-60 19.2 Example 8 60-80 16.7 Example 9 80-100 17

Table 1

Example 10

Adopt the method identical with embodiment 3, dehydrating agent is the concentrated phosphoric acid of 50g, obtains the polylactic acid of molecular weight 89,000.

Example 11

Adopt the method identical with embodiment 3, dehydrating agent is 50gNaOH, obtain the polylactic acid of molecular weight 64,000.

Example 12

Using the same method as in Example 3, the dehydrating agent was 50g K ₂ CO ₃ , and polylactic acid with a molecular weight of 54,000 was obtained.

Example 13

Using the same method as in Example 3, using 80 g of MnSO ₄ as the dehydrating agent, polylactic acid with a molecular weight of 57,000 was obtained.

Example 14

Adopt the method identical with embodiment 3, dehydrating agent is 60g silica gel, obtain the polylactic acid of molecular weight 68,000.

Example 15

Mix 5g of lactic acid with 0.1g of SnCl ₂ and place it in a petri dish, and spread 10g of CaO in a petri dish. The two petri dishes were placed in a stainless steel container in layers, and the air in them was repeatedly replaced with high-purity nitrogen and vacuumized. Finally, the container was sealed and transferred to a thermostat at 160°C for 8 hours, and then continued to react at 140°C for 12 hours to obtain polylactic acid with a molecular weight of 51,000.

Example 16

Using the same method as in Example 3, the polymerization reactor was equipped with a stirring device, and the stirring speed was 1200 rpm. The result is 162,000 polylactic acid.

Comparative example 1

Take 2 g of the polylactic acid obtained in Example 2 and spread it flat in a stainless steel container, repeatedly replace the air with high-purity nitrogen and vacuumize it, and finally seal the container and transfer it to a thermostat at 170°C for 20 hours to react. Polylactic acid with a molecular weight of 39,000 was obtained.

Claims (10)

1. the method for preparing high molecular weight polylactic acid by direct polycondensation, is characterized in that, the method comprises the steps: Put lactic acid or low-molecular-weight polylactic acid and dehydrating agent together in a closed polymerization tank, remove oxygen in the polymerization tank, and react at a temperature of 60-300°C for 0.1-100 hours to obtain an average molecular weight of 10,000-50 million polylactic acid. 2. The method according to claim 1, characterized in that said dehydrating agent is Na, K, metal oxides, non-metal oxides, acids, alkalis, inorganic salts that can form crystal hydrates with water One and its mixture, or silica gel or dehydrating resin. 3. The method according to claim 2, characterized in that said dehydrating agent is CaO, BaO, MgO, Al ₂ O ₃ , Na ₂ O, K ₂ O, P ₂ O ₅ , phosphoric acid, pyrophosphoric acid, One of polyphosphoric acid, NaOH, KOH, CaCl ₂ , K ₂ CO ₃ , CaSO ₄ , MgCl ₂ , MgSO ₄ , MnSO ₄ , MnCl ₂ , CuSO ₄ , NiSO ₄ or ZnSO ₄ , or an acrylic resin. 4. The method according to claim 1, 2 or 3, characterized in that the actual amount of the dehydrating agent is 1.1 to 1.5 times the theoretical amount. 5. The method according to claim 1, 2 or 3, characterized in that the reactor is in a vacuum state or contains an inert gas. 6. The method according to claim 1, 2 or 3, characterized in that the pressure in the reactor can be negative pressure or positive pressure. 7. The method according to claim 1, 2 or 3, characterized in that the gas in the reactor can be in a static state or a forced circulation state. 8. The method according to claim 1, 2 or 3, characterized in that the prepolymer is pulverized to an average particle size of 0.01 mm to 5 mm, and then polycondensed. 9. The method according to claim 6, characterized in that the prepolymer is pulverized to an average particle size of 0.01 mm to 5 mm, and then polycondensed. 10. The method according to claim 7, characterized in that the prepolymer is pulverized to an average particle size of 0.01 mm to 5 mm, and then polycondensed.