There are two main ways to synthesize polylactic acid (PLA): one is to synthesize polylactic acid by direct polycondensation of lactic acid, which is often done by melt polycondensation, melt polycondensation-solid state polymerization or solution polycondensation, which is collectively referred to as PC method. The other is to obtain polylactic acid by ring-opening polymerization of lactide, that is, lactide monomer is first prepared by dehydration and cyclization of lactic acid monomer, and then polylactic acid with high degree of polymerization is obtained by ring-opening polymerization of lactide, which is collectively called ROP method.
The synthesis route of PLA is shown in figure 1.
Fig. 1 Synthetic route of PLA
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| CAS NO. | Product Name | Inquiry |
| 26023-30-3 | Polylactic acid | Inquiry |
| 26100-51-6 | Polylactic Acid, Mw ~60,000 | Inquiry |
| 31587-11-8 | DL-Polylactic acid | Inquiry |
| 26969-66-4 | Poly(dl-lactic acid) | Inquiry |
| 31587-11-8 | DL-Polylactic acid | Inquiry |
| 598-82-3 | Dl-lactic acid(90%) | Inquiry |
| 50-21-5 | Lactic acid | Inquiry |
Direct Polycondensation of Lactic Acid
The direct polycondensation method is the direct dehydration of lactic acid to polylactic acid, the molecular weight of polylactic acid synthesized by this method is generally tens of thousands, the molecular weight is relatively low, and the molecular weight range is also wide. However, the polylactic acid with higher molecular weight can be obtained by removing the water in the polymerization process and controlling the reaction temperature. The methods mainly include solution polycondensation and melt polycondensation. The principle is shown in figure 2.
Fig. 2 Preparation of polylactic acid by direct polycondensation of lactic acid
Under appropriate conditions, the hydroxyl and carboxyl groups between lactic acid molecules are directly dehydrated and condensed, the small molecular water products are removed, and the reaction is carried out in the direction of polymerization, thus PLA is prepared. The preparation of PLA by direct polycondensation can be divided into three main stages: ① reducing free water content; ② oligomer polycondensation; and ③ melt polycondensation to obtain higher molecular weight PLA.
The preparation of PLA by direct polycondensation of lactic acid is a reversible reaction, in which there is a balance of free lactic acid, free water, PLA, and lactide. With the progress of the reaction, the viscosity of the system increases continuously, and it becomes more and more difficult to remove water from the viscous melt in the later stage of polymerization, even under vacuum conditions. Higher temperature (> 200 °C) is beneficial to the removal of small molecular water, but it is also beneficial to the formation of lactide. At the same time, the system is accompanied by a series of side reactions, such as transesterification, which may form cyclic products of different sizes. PLA with low molecular weight is easy to hydrolyze, has poor mechanical properties and has no use value.
To prepare high molecular weight polymers through condensation polymerization, polymer grade lactic acid with high optical purity must be used, side reactions must be suppressed, and water and other by-products must be removed as much as possible. Researchers have tried various methods to increase the molecular weight of PLA obtained by direct polycondensation, including: ① taking away small molecular water by forming an azeotropic mixture with the solvent to prepare high molecular weight PLA, with a molar mass of up to 300,000g/mol; ② in the later stage of the reaction, melt polymerization is used to reduce the viscosity of the system, remove small molecules of water, and increase the molecular weight; ③ a chain extender is used to couple the PLA molecules prepared by the lactic acid condensation polymerization method to increase the molecular weight of PLA.
- Solution Polycondensation Method
The solution polycondensation method generally uses the solvent azeotropic method to remove water and reflux to prepare high molecular weight polylactic acid. The more thoroughly the water is removed from the system, the higher the molecular weight of the polylactic acid obtained. Therefore, this method has high requirements for solvents. On the one hand, it cannot participate in the reaction, and on the other hand, it must be able to dissolve the polymer and azeotrope with water. The reaction temperature of the solution polycondensation system is always at the azeotropic point, so it can avoid the decomposition of the polymer caused by heating and local temperature changes. However, the disadvantage of this method is that the use and recovery of organic solvents will make the operation and equipment more complicated, and the use of organic solvents with high boiling points or odor will increase the difficulty of polymer purification.
- Melt Polycondensation Method
Melt polycondensation is a bulk polymerization method that occurs above the melting point of the polymer without the involvement of any medium. Melt polycondensation is to directly heat the system for polycondensation reaction. During the reaction process, the system must always be in a molten state, and the generated water and other by-products must be removed by vacuum extraction or by inert gas. The molecular weight of the polymer obtained by this method is generally smaller than that of the solution method. However, by treating the prepolymer, increasing the reaction time, lowering the reaction temperature, or using better catalysts for further polycondensation, the molecular weight of the polylactic acid polymer can be increased. Therefore, the selection of catalysts for recondensation polymerization and the control of reaction conditions are the focus of research on melt polycondensation methods, which mainly include reactive extrusion polymerization, melt-solid phase polymerization, chain extension reaction and other methods.
Ring-opening Polymerization of Lactide
High molecular weight PLA can be prepared by ring-opening polymerization of L-lactide, and the molecular weight, molecular chain structure and physicochemical properties of PLA can be regulated. At present, this method is used in the synthesis of industrial PLA resin. The method can be realized by melt polymerization, bulk polymerization, solution polymerization and suspension polymerization. Among them, melt polymerization is considered to be the simplest and repeatable method. Lactide ring-opening polymerization is used to prepare PLA. First, lactide is prepared from lactic acid, and then PLA is obtained from lactide ring-opening polymerization through melt polymerization technology.
PLA with different molecular structures can be obtained by ring-opening polymerization of lactide with different structures. Stannous octanoate is usually used as catalyst and fatty alcohol as initiator to prepare PLA by ring-opening polymerization of lactide. Different ratios of L-lactide and D-lactide copolymers can also be prepared by this method. The reaction process of this method is shown in figure 3. Stannous octanoate is the most commonly used catalyst, which can well control the reaction rate and molecular weight. The reaction mechanism of ring-opening polymerization of lactide to PLA depends on polymerization conditions, type of catalyst, concentration of initiator and solvent.
Fig. 3 Preparation of PLA by ring-opening polymerization of lactide(R: H, alkyl, etc.)
The ring-opening polymerization method can control the synthesis molecular weight of polylactic acid by controlling the reaction time, temperature and selecting different catalysts, but the purification process of polylactic acid is complicated and the cost is high. According to the different catalysts used and the reaction mechanism, it can be divided into ionic ring-opening polymerization, coordination ring-opening polymerization and organic ring-opening polymerization.
- Ionic Ring-opening Polymerization
Ionic ring opening polymerization can be further divided into cationic polymerization and anionic polymerization. Cationic catalysts in cationic polymerization mainly refer to proton acid type. The polymerization mechanism of proton acid type involves the interaction between H+ and oxygen atoms in lactic acid monomers to form oxygen ions, which promote the breaking of alkoxy bonds, monomer ring opening, and the production of acyl cations, leading to chain growth. In anionic polymerization, anionic catalysts mainly refer to alkali metal alkoxy compounds. The reaction mechanism of catalytic polymerization is the nucleophilic reaction between alkoxy anions and carbonyl groups in lactic acid monomers, which promotes the cleavage of alkoxy bonds, generates growth active centers, and undergoes chain growth polymerization reaction. The growth of chains in cationic polymerization occurs on chiral carbon, often accompanied by racemization reactions at temperatures above 50°C, leading to the generation of amorphous polylactic acid. Therefore, this method is rarely used for the preparation of polylactic acid. Similarly, the strong basicity of anions in anionic polymerization will promote the deprotonation of lactic acid chiral carbon, which leads to the racemization or even termination of the reaction, which limits the increase of the molecular weight of polylactic acid and the application of this method.
(1) Cationic Polymerization
The initiation mechanism of cationic ring-opening polymerization is shown in Figure 4. Cationic initiators used for polymerization include: protonic acids (HCl, RSO3H, etc.), Lewis acids (AlCl3, MnCl2, SnCl4, Sn(oct)2, etc.), alkylating reagents (CF3SO3CH3, etc.) and other acidic compounds.
Fig. 4 Mechanism of Cationic Polymerization
(2) Anionic Polymerization
Fig. 5 shows the mechanism of ROK catalytic polymerization. It may be initiated by two ways: the first is the deprotonation of LA, and the second is the opening of the loop by nucleophilic attack. Therefore, the two pathways can be easily distinguished from whether they contain initiator or not by chain end group analysis.
Fig. 5 Mechanism of Anionic Polymerization
- Coordination Ring-opening Polymerization
The commonly used initiators for the coordination ring opening polymerization of LA are tin carboxylate salts, aluminum isopropoxide, aluminum alkoxide, or bimetallic alkoxides. Alkoxyaluminum is an effective catalyst for ring opening polymerization of cyclic esters, such as Al(Oi-Pr)3, which is widely used to study the mechanism of LA polymerization. However, its catalytic activity is not high. Under the conditions of 125-180°C, the bulk polymerization of LA takes several days to obtain lower molecular weight PLA. Due to the time required for the catalytic reaction of Al(Oi-Pr)3 and the inability of Al3+ to metabolize in the human body, its accumulation in the body can easily lead to Alzheimer's disease. Therefore, as a catalyst for the synthesis of biodegradable polyester, Al(Oi-Pr)3 is not widely used.
The most widely used catalyst in the industrial production of PLA is tin carboxylate compounds, especially stannous octanoate, which have much higher activity than Al(Oi-Pr)3. In the presence of alcohol reagents, not only can it catalyze the polymerization of LA quickly, but it can also better control the reaction process. Therefore, in industrial production, it is highly favored by people, but the research on the catalytic polymerization mechanism of stannous octanoate is quite controversial. It is generally believed that there are two possibilities, one is the cation or activated monomer mechanism; Another mechanism is coordination insertion.
- Organic Ring-opening Polymerization
In order to better meet the application of biomedical polylactic acid materials, it is necessary to study metal-free green and safe organic catalysts. Referring to the literature, it can be found that this kind of catalysts mainly include amines, thiourea and other organic catalysts. There will be no metal residues in the polylactic acid synthesized by this kind of catalyst, so it can be better used in medical materials, but the research in this area is still in the exploratory period, and the research on the mechanism is not deep enough, so researchers need to do further research.
