Abstract:
This disclosure provides a method of making polylactic acid using carbon dioxide adducts of carbenes, wherein the adducts of carbenes have a structure represented by formula (I) as follows:

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of International Application No. PCT/CN2009/073677, filed Sep. 2, 2009, which claims the benefit of Chinese Patent Application No. 200810146617.9, filed Sep. 1, 2008. 
    
    
     TECHNICAL FIELD 
     This disclosure is in the technical field of polymeric material, specifically involving the use of carbene derivatives, e.g., CO 2  adducts of carbenes, as catalysts in the preparation of polylactic acid. 
     BACKGROUND 
     Polylactic acid, also known as polylactide, as a completely biodegradable material that derives from renewable resources of agricultural crops, have attracted world-wide attention and research efforts. New methods for manufacturing polylactic acid are needed. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The disclosure herein provides a method that uses a new type of carbene derivatives as the catalyst in the preparation of polylactic acid. 
     In these carbene derivatives, the temperature decarboxylation at the second position (CO 2  removal) varies according to different substituents on ring of N-heterocyclic carbene. Consequently, one can design carbene derivatives according to the temperature of in situ generation of carbene in the polymerization system, therefore controlling the decarboxylation temperature, and control the polymerization process by controlling the temperate. Furthermore, carbon dioxide adducts of carbenes are heated and experience decarboxylation in the polymerization conditions to form the corresponding N-heterocyclic carbenes. CO 2  is vented from the reaction system. N-heterocyclic carbenes catalyzes the lactide ring-opening polymerization to obtain polylactic acid in the presence of alcohol initiators. 
     This disclosure also provides that different substituents on the CO 2  adducts of heterocyclic carbenes will affect the decarboxylation temperature on the 2-position of heterocyclic ring. While the appropriate decarboxylation temperature and its range are determined according to the property of the polymer and the process condition of the polymerization equipment. All CO 2  adducts of heterocyclic carbenes do not have the appropriate decarboxylation temperature for lactide ring-opening polymerization. To ensure that the decarboxylation of CO 2  adducts of carbenes occur in the same temperature range of the ring-opening polymerization, so that to produce active catalyst to promote the catalytic reaction, thermal gravimetric analysis were conducted on CO 2  adducts of carbenes having different substituents on the structure of carbenes. The temperature and rate of decarboxylation of CO 2  adducts of carbenes of various structure were obtained. Among them, a group of suitable catalysts were chosen so that the decarboxylation temperature and the reaction temperature of lactide ring-opening polymerization can properly match. Under the suitable reaction temperature the catalytic ring-opening polymerization can be effectively carried out. Therefore, through controlling its reaction temperature, lactide ring-opening polymerization catalyzed by CO 2  adducts of carbenes becomes controllable. 
     The terminal structure and molecular weight of polylactic acid can be controlled. For example, using active hydrogen containing compounds (ROH) as the initiator in the ring-opening polymerization reaction results in polylactic acid with terminal structure of RO— and —OH. While the ratio of the lactide and initiator determines the target molecular weight of polylactic acid. In the presence of the initiator, N-heterocyclic carbene catalyzed ring-opening polymerization of lactide is living polymerization, while reactive extrusion system also has a good dispersion. The polylactic acid so obtained may have a relatively narrow molecular weight distribution. 
     This disclosure further provides a method of making polylactic acid using CO 2  adducts of carbenes, wherein the adducts has a structure represented by formula (I): 
     
       
                 
         
             
             
         
      
         
         
           
             wherein the dotted line together with the solid line parallel to it represents a single bond or a double bond, X 1  is chosen from S and N; X 2  is chosen from C and N; R 1  and R 2  can be the same or different and are chosen from
           hydrogen,   alkyl groups having 1 to 10 carbon atoms,   alkyl groups having 1 to 10 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   cycloalkyl groups having 3 to 6 carbon atoms,   a halogen atom,   an adamantane group,   a phenyl group, and   a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group;
 
R 3  and R 4  can be the same or different and are chosen from
   hydrogen,   a halogen atom,   a cyano group,   a hydroxyl group,   alkyl groups having 1 to 4 carbon atoms,   alkyl groups having 1 to 4 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   a phenyl group, and   a substituted phenyl group.   Alternatively, R 3  and R 4  are connected to form cycloalkyl or cycloalkenyl rings having 3 to 8 carbon atoms fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said cycloalkyl or cycloalkenyl rings); or R 3  and R 4  are connected to form a benzene ring fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said benzene ring); or R 2  and R 3  are connected to form 5-membered or 6-membered N-heterocyclic rings having no substituent, which are fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 1  and the carbon atom in the 5-membered ring connected to R 3  are also part of said 5-membered or 6-membered N-heterocyclic rings).   
         
           
         
       
    
     Specific examples of CO 2  adducts of carbenes of formula (I) can be represented as in formula (II), formula (III), formula (IV), or formula (V): 
     
       
                 
         
             
             
         
      
     
     In formula (II), R 1  and R 2  can be the same or different and are chosen from
         hydrogen,   alkyl groups having 1 to 10 carbon atoms,   alkyl groups having 1 to 10 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   cycloalkyl groups having 3 to 6 carbon atoms,   a halogen atom,   an adamantane group,   a phenyl group, and   a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group;
 
R 3  and R 4  can be the same or different and are chosen from
   hydrogen,   a halogen atom,   a cyano group,   a hydroxyl group,   alkyl groups having 1 to 4 carbon atoms,   alkyl groups having 1 to 4 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   a phenyl group, and   a substituted phenyl group.   Alternatively, R 3  and R 4  are connected to form cycloalkyl or cycloalkenyl rings having 3 to 8 carbon atoms fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said cycloalkyl or cycloalkenyl rings), or R 3  and R 4  are connected to form a benzene ring fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said benzene ring).       

     In formula (III), R 1  and R 2  can be the same or different and are chosen from
         hydrogen,   alkyl groups having 1 to 10 carbon atoms,   alkyl groups having 1 to 10 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   cycloalkyl groups having 3 to 6 carbon atoms,   a halogen atom,   an adamantane group,   a phenyl group, and   a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group;
 
R 3  and R 4  can be the same or different and are chosen from
   hydrogen,   a halogen atom,   a cyano group,   a hydroxyl group,   alkyl groups having 1 to 4 carbon atoms,   alkyl groups having 1 to 4 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group, t   a phenyl group, and   a substituted phenyl group.   Alternatively, R 3  and R 4  are connected to form cycloalkyl or cycloalkenyl rings having 3 to 8 carbon atoms fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said cycloalkyl or cycloalkenyl rings).       

     In formula (IV), R 1  is chosen from
         hydrogen,   alkyl groups having 1 to 10 carbon atoms,   alkyl groups having 1 to 10 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   cycloalkyl groups having 3 to 6 carbon atoms,   a halogen atom,   an adamantane group,   a phenyl group, and   a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group;
 
R 3  and R 4  can be the same or different and are chosen from
   hydrogen,   a halogen atom,   a cyano group,   a hydroxyl group,   alkyl groups having 1 to 4 carbon atoms,   alkyl groups having 1 to 4 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   a phenyl group, and   a substituted phenyl group.   Alternatively, R 3  and R 4  are connected to form cycloalkyl or cycloalkenyl rings having 3 to 8 carbon atoms fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said cycloalkyl or cycloalkenyl rings) or R 3  and R 4  are connected to form a benzene ring fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2  and the carbon atom in the 5-membered ring next to X 2  are also part of said benzene ring). In formula (V) in, R 1  and R 2  can be the same or different and are chosen from   hydrogen,   alkyl groups having 1 to 10 carbon atoms,   alkyl groups having 1 to 10 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   cycloalkyl groups having 3 to 6 carbon atoms,   a halogen atom,   an adamantane group,   a phenyl group, and   a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group;
 
R 3  is chosen from
   hydrogen,   a halogen atom,   a cyano group,   a hydroxyl group,   alkyl groups having 1 to 4 carbon atoms,   alkyl groups having 1 to 4 carbon atoms and also having one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group,   a phenyl group, and   a substituted phenyl group.   Alternatively, R 2  and R 3  are connected to form 5-membered or 6-membered N-heterocyclic rings having no substituent, which are fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 1  and the carbon atom in the 5-membered ring connected to R 3  are also part of said 5-membered or 6-membered N-heterocyclic rings).       

     The above-described substituted phenyl group can have one, two, or three substituents. The substituents can be the same or different, chosen from halogens, a hydroxyl group, an alkoxy group, and a cyano group, alkyl groups having 1-5 carbon atoms, alkyl groups having 1-5 carbon atoms with one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group 
     The above-described methods of making polylactic acid can be solution polymerization, involving mixing CO 2  adducts of carbenes, the initiator, and lactide monomer in an organic solvent according to a molar ratio of 1:0.2 to 5:2 to 10000, preferably of 1:0.5 to 3:100 to 500, more preferably of 1:0.5 to 2:100 to 300. The reaction temperature is −50 to 250° C., preferably 50° C. to 180° C., more preferably 60° C. to 150° C. The reaction time ranges from 3 seconds to 120 hours, preferably 0.2 to 24 hours, more preferably 0.2 to 15 hours. 
     The above-described method of preparing polylactic acid can also be used bulk polymerization, specifically comprising mixing CO 2  adducts of carbenes, the initiator, and lactide monomer according to a molar ratio of 1:0.2 to 5:2 to 10000, preferably of 1:0.5 to 3:100 to 500, more preferably of 1:0.5 to 2:100 to 300. The reaction temperature is −50 to 250° C., preferably 50° C. to 180° C., more preferably 60° C. to 150° C. The reaction time ranges from 3 seconds to 120 hours, preferably 0.2 to 24 hours, more preferably 0.2 to 15 hours. 
     In the solution polymerization and bulk polymerization, the above-described initiator can be alcohol compounds, such as methanol, ethanol, propanol, isopropanol, n-butanol, tert-butyl alcohol, benzyl alcohol, phenethyl alcohol, ethyl glycol, diethylene glycol, pentaerythritol, preferably n-butanol, benzyl alcohol, and phenethyl alcohol. The above-described lactide can be L-lactide, D-lactide, meso-lactide, or racemic lactide, or a mixture of any proportion between L-lactide and D-lactide. The step to terminate the reaction is to add weak acid (such as formic acid, acetic acid, hydrochloric acid, etc.), oxygen, water, CS 2  or CO 2 , preferably formic acid, acetic acid, water, or CS 2 . 
     In solution polymerization, the organic solvent used can be cyclohexane, hexane, ethyl ether, acetone, cyclohexanone, dioxane, tetrahydrofuran, benzene, toluene, xylene, methylene chloride, chloroform, N,N-dimethylformamide, N,N-dimethylacetamide, or dimethyl sulfoxide, preferably acetone, tetrahydrofuran, toluene, xylene, methylene chloride, chloroform, more preferably tetrahydrofuran and toluene. The reaction can be carried out in an inert gas. The inert gas is preferably argon or nitrogen. The reaction product is preferred to be purified in an antisolvent, such as methanol, ethanol, or water. 
     In bulk polymerization, the reaction can be carried out in inert gas or under vacuum. The inert gas is preferably argon or nitrogen. The vacuum means the absolute pressure in the reactor is 4 to 20 mmHg. The reaction product is dissolved in a good solvent and then precipitated by adding an antisolvent, such as methanol, ethanol, or water purification. The good solvent can be dichloromethane, chloroform, toluene, benzene, acetone or tetrahydrofuran, preferably dichloromethane, chloroform, tetrahydrofuran. 
     The disclosure also provides the utility of CO 2  adducts of carbenes in the preparation of catalyst used in polylactic acid preparation. 
     In the method of using carbene derivatives to catalyze polylactic acid preparation provided in this disclosure, CO 2  adducts of carbenes have different substituents, which are used as the catalyst, have different decarboxylation temperatures. Further, the rate of decarboxylation differs at different temperature. Therefore, one may select a CO 2  adduct of carbene that has a suitable decarboxylation temperature as the catalyst for ring-opening polymerization, so that to control the lactide ring-opening polymerization through controlling the decarboxylation temperature. The removed CO 2  is vented, leaving no pollutant in the polymerization system. This method if effective and environmentally friendly. CO 2  adducts of carbenes are structurally stable, can be stored under normal conditions, keeping its property intact for a long period of time. It is also easy to use, without requiring operation under exacting reaction conditions. The catalytic reactivity is high in both a solution polymerization system and a bulk polymerization system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of this disclosure are described in details with reference to the drawings in the following. 
         FIG. 1 : A TGA graph of a salt of 1,3-bis(2,6-diisopropyl-phenyl)imidazole-2-carboxylic; 
         FIG. 2 : A  1 H NMR spectrum of polylactic acid prepared using a salt of 1,3-bis(2,6-diisopropyl-phenyl)imidazole-2-carboxylic as the catalyst; 
         FIG. 3 : A chromatogram of size-exclusion chromatography of polylactic acid prepared using a salt of 1,3-bis(2,6-diisopropyl-phenyl)imidazole-2-carboxylic acid as the catalyst. 
     
    
    
     DETAILED DESCRIPTION 
     The following examples are used to further illustrate the present disclosure and in no way limit the disclosure. One of ordinary skill in the art would appreciate that these examples do not limit the present disclosure in any aspects, and that appropriate modification and adjustment of the parameters can be made without violating the essence of this disclosure and deviation from the scope of the present disclosure. 
     CO 2  adducts of carbenes used in the examples in this disclosure are shown in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Carbon dioxide adducts of carbenes used in the examples 
               
             
          
           
               
                   
                 No. 
                 Structure 
               
               
                   
                   
               
               
                   
                 A 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 B 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 C 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 D 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 E 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 F 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 G 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 H 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 I 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 J 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 K 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 L 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 M 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 N 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 O 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 P 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 Q 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 R 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 S 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 T 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
             
          
         
       
     
     The range of decarboxylation temperature in this disclosure was obtained using TGA analysis. The equipment and test conditions were: TG/DTA (NETZSCHSTA449C), N 2  protection, heating rate 5° C./min, temperature range 50 to 350° C. Using the salt of 1,3-bis(2,6-diisopropyl-phenyl)imidazole-2-carboxylic acid as an example, its TG/DTG graph is shown in  FIG. 1 . 
     The reaction conversion was measured using  1 H NMR. The equipment and test conditions were: NMR (Bruker DRX500), solvents were d-CHCl 3 , d-DMSO or d-CH 3 COCH 3 . The average molecular weight of polylactic acid and the dispersion were determined using size-exclusion chromatography (SEC), the test conditions were: column temperature: 25° C., solvent: THF (HPLC), flow rate: 1 mL/min, HPLC Pump: Waters 515, Detector: RI (Wyatt Optilab rEX), column: HR3, HR4, and HR5 Styragel connected in series, standard sample: polystyrene (PS) M w =900 to 1.74×10 6  g/mol, PDI&lt;1.1. 
     EXAMPLE 1 
     1,3-bis(2,6-diisopropyl-phenyl)imidazole-2-carboxylic acid salt (A) (29 mg, 75 μmol), benzyl alcohol (5.4 mg, 50 μmol), L-lactide (1.44 g, 10 mmol) were dissolved in 10 mL of xylene. The solution was heated under the protection of the N 2  to 140° C., and reacted for 30 minutes. The reaction was terminated by adding formic acid. The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight, obtaining 1.20 g of white solid. The conversion was 86%. The number average molecular weight of the polylactic acid M n  was 18,460. The polydispersity index PDI was 1.05. The  1 H NMR spectrum and SEC chromatogram of the obtained polylactic acid are shown in  FIGS. 2 and 3  respectively. 
     EXAMPLE 2 
     1,3-bis(o-methylphenyl)imidazoline-2-carboxylic acid salt (J) (73.54 mg, 250 μmol), benzyl alcohol (5.4 mg, 50 μmol), L-lactide (0.72 g, 12.5 mmol) were added into a reaction vessel and heated under the protection of Ar to 130° C. The reaction in the molten mixture was carried out for 1 hour. The reaction was terminated by adding water. The reaction mixture was dissolved in chloroform and then added into ethanol. The precipitate was filtered and dried to a constant weight, obtaining 0.67 g of white solid. The conversion was 95%. The number average molecular weight of the polylactic acid M n  was 32,780. The polydispersity index PDI was 1.16. 
     EXAMPLE 3 
     1-butyl-3-methylimidazolium-2-carboxylic acid salt (O) (18.21 mg, 100 μmol), n-butanol (7.41 mg, 100 μmol), D-lactide (5.76 g, 40 mmol) were added into a reaction vessel. The reaction vessel was under vacuum at a pressure of 7 mmHg. The reaction was carried out at −50° C. for 60 hours. The reaction was terminated by adding hydrochloric acid. The reaction mixture was dissolved with dichloromethane then added into ethanol. The precipitate was filtered and dried to a constant weight, obtaining 2.30 g of white solid. The conversion was 54% conversion. The number average molecular weight of the polylactic acid M n  was 24,850. The polydispersity index PDI was 1.19. 
     EXAMPLE 4 
     1,3-bis(2,6-diisopropyl-phenyl)imidazoline-2-carboxylic acid salt (D) (32.6 g, 75 mmol), methanol (1.6 g, 50 mmol), L-lactide (21.6 g, 150 mmol) was added into the reaction vessel and dissolved using 500 mL of THF. The solution was heated under the protection of Ar to 40° C., stirred and let react for 48 hours. The reaction was terminated by adding benzoic acid. The reaction mixture was condensed and added into methanol. The precipitate was filtered and dried to a constant weight to obtain 13.58 g of white solid. The conversion was 63%. The number average molecular weight of the polylactic M n  was 432. The polydispersity index was PDI 1.00. 
     EXAMPLE 5 
     1-phenyl-3-methylimidazolium-2-carboxylic acid salt (Q) (16.2 mg, 75 μmol), benzyl alcohol (10.8 mg, 100 μmol), D-lactide (2.88 g, 20 mmol) was added into a reaction vessel. The reaction vessel was under vacuum at a pressure of 7 mmHg. The reaction was carried out at 0° C. for 5 days. The reaction was terminated by adding water. The reaction mixture was dissolved in toluene. The solution was then added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 1.3 g of white solid. The conversion rate was 49%. The number average molecular weight of polylactic acid M n  was 18,750. The polydispersity index PDI was 1.08. 
     EXAMPLE 6 
     1,3-dimethyl imidazole-2-carboxylic acid salt (N) (14.01 mg, 75 μmol), ethylene glycol (1.86 mg, 30 μmol), D, L-lactide (2.16 g, 15 mmol) were added into a reaction vessel. The mixture was heated under the protection of Ar to 96° C. The reaction in the molten mixture was carried out for 2 hours. The reaction was terminated by adding CS 2 . The reaction mixture was dissolved in chloroform. The solution was then added into methanol. The precipitate was filtered and dried to a constant weight to obtain 2.10 g of white solid. The conversion was 99% conversion. The number average molecular weight of polylactic acid M n  as 34,900. The polydispersity index PDI was 1.28. 
     EXAMPLE 7 
     1,3-bis(2,4,6-trimethyl-phenyl)imidazoline-2-carboxylic acid salt (E) (32.6 mg, 75 μmol), benzene, ethanol (9.16 mg, 75 μmol), D-lactide (1.44 g, 10 mmol) were added into a reaction vessel. The reaction vessel was under vacuum of at a pressure of 4 mm Hg. The reaction was carried out at 130° C. for 1 hour. The reaction was terminated by adding oxygen. The reaction mixture was dissolved in chloroform and then added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 1.2 g of white solid. The conversion was 97%. The number average molecular weight M n  of polylactic acid as 18,600. The polydispersity index PDI was 1.35. 
     EXAMPLE 8 
     1,3-bis(2,4,-trimethyl-phenyl) imidazole-2-carboxylic acid salt (B) (32.6 mg, 75 μmol), isopropanol (9.01 mg, 150 μmol), D-lactide (1.44 g, 10 mmol) were dissolved in 5 mL of N,N-dimethylformamide. The solution was heated under the protection of Ar to 96° C. and reacted for 12 hours. The reaction was terminated by adding water. The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight to obtain 1.32 g of white solid. The conversion was 92%. The number average molecular weight of polylactic acid M n  was 8,580. The polydispersity index PDI was 1.11. 
     EXAMPLE 9 
     1-butyl-3-methyl benzimidazol-2-carboxylic acid salt (M) (17.41 mg, 75 μmol), ethanol (1.73 mg, 37.5 μmol), D-lactide (8.64 g, 60 mmol) were dissolved in 100 mL of tetrahydrofuran. The solution was heated under the protection of Ar to 50° C. and reacted for 30 hours. The reaction was terminated by adding hydrochloric acid. The reaction mixture was condensed and added into methanol. The precipitate was filtered and dried to a constant weight to obtain 6.20 g of white solid. The conversion was 76%. The number average molecular weight of polylactic acid M n  was 188,750. The polydispersity index PDI was 1.38. 
     EXAMPLE 10 
     1, 3-2-butyl imidazole-2-carboxylic acid salt (I) (16.81 mg, 75 μmol), benzyl alcohol (10.8 mg, 100 μmol), D-lactide (1.44 g, 10 mmol) was added into a reaction vessel. The mixture was heated under the protection of Ar to 120° C. The reaction was carried out in the molten mixture for 2 hours. The reaction was terminated by adding CS 2 . The reaction mixture was dissolved in chloroform and the solution was then added into methanol. The precipitate was filtered and dried to a constant weight to obtain 1.15 g of white solid. The conversion was 97%. The number average molecular weight of the polylactic acid M n  was 14,210. The polydispersity index PDI was 1.32. 
     EXAMPLE 11 
     1,3-bis(o-methylphenyl)imidazole-2-carboxylic acid salt (G) (21.9 mg, 75 μmol), benzene, ethanol (12.21 mg, 100 μmol), L-lactide (1.44 g, 10 mmol) were added into a reaction vessel and heated under the protection of Ar to 250° C. The reaction was carried out in the molten mixture for 20 min. The reaction was terminated by adding hydrochloric acid. The reaction mixture was dissolved in chloroform and added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 1.1 g of white solid. The conversion rate was 89%. The number average molecular weight of the polylactic acid M n  as 11,960. The polydispersity index was PDI 1.48. 
     EXAMPLE 12 
     1,3-dicyclohexyl imidazole-2-carboxylic acid salts (C) (27.62 mg, 100 μmol), tert-butanol (14.81 mg, 200 μmol), D, L-lactide (7.2 g, 50 mmol) were dissolved in 50 mL of toluene. The solution was heated under the protection of Ar to 100° C. and reacted for 30 minutes. The reaction was terminated by adding acetic acid. The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight to obtain 6.02 g of white solid. The conversion rate was 85%. The number average molecular weight of polylactic acid M n  was 26,320. The polydispersity index PDI was 1.06. 
     EXAMPLE 13 
     1,3-di-tert-butyl-imidazole-2-carboxylic acid salt (P) (6.72 mg, 30 μmol), benzyl alcohol (16.21 mg, 150 μmol), D, L-lactide (3.46 g, 24 mmol) were added into a reaction vessel and heated under the protection of Ar to 200° C. The reaction was carried out in the molten mixture for 3 seconds. The reaction was terminated by adding oxygen. The reaction mixture was cooled and dissolved in toluene and added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 2.5 g of white solid. The conversion rate was 76%. The number average molecular weight of the polylactic acid weight M n  was 18,850. The polydispersity index PDI was 1.08. 
     EXAMPLE 14 
     1,3-bis(methylphenyl)imidazole-2-carboxylic acid salt (H) (29.21 mg, 100 μmol), propanol (3.00 mg, 50 μmol), D-lactide (43.2 g, 0.3 mol) were dissolved in 1000 mL of DMSO. The solution was heated under the protection of Ar to 80° C. and reacted for 30 minutes. The reaction was terminated reaction by added CO 2 . The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight to obtain 39.2 g of white solid. The conversion rate was 94%. The number average molecular weight of PLA M n  was 820. The polydispersity index PDI was 1.00. 
     EXAMPLE 15 
     1-ethyl-3-methylimidazolium-2-carboxylic acid salt (F) (15.4 mg, 100 μmol), propanol (3.00 mg, 50 μmol), D-lactide (1.44 g, 10 mmol) were dissolved in 1 mL of DMSO. The solution was heated under the protection of Ar to 20° C. and reacted for 24 hours. The reaction was terminated by adding CO 2 . The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight to obtain 0.26 g of white solid. The conversion rate was 18%. The number average molecular weight of the polylactic acid weight Mn was 8,750. The polydispersity index PDI was 1.08. 
     EXAMPLE 16 
     1,3-bis(methylphenyl)imidazoline-2-carboxylic acid salt (K) (22.06 mg, 75 μmol), methanol (12 mg, 375 μmol), L-lactide (0.72 g, 5 mmol) was added into a reaction vessel and dissolved in 30 mL of tetrahydrofuran. The solution was heated under the protection of Ar to 40° C., stirred and let react for 48 hours. The reaction was terminated by adding benzoic acid. The reaction mixture was condensed and added into methanol. The precipitate was filtered and dried to a constant weight. 0.39 g of white solid. The conversion rate was 54% conversion rate. The number average molecular weight of the polylactic acid M n  was 1,250. The polydispersity index PDI was 1.16. 
     EXAMPLE 17 
     1,3-butyl-3,4-dichloro-imidazole-2-carboxylic acid salt (L) (29.21 mg, 100 μmol), n-butanol (7.41 mg, 100 μmol), D-lactide (5.76 g, 40 mmol) were added into a reaction vessel. The reaction vessel was under vacuum at a pressure of 7 mmHg. The reaction was carried out at 250° C. for 5 hours. The reaction was terminated by adding hydrochloric acid. The reaction mixture was dissolved in dichloromethane and then added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 5.5 g of white solid. The conversion rate was 99%. The number average molecular weight of the polylactic acid weight Mn was 48,780, The polydispersity index PDI was 1.48. 
     EXAMPLE 18 
     1-tert-butyl-4,5-dichloro-imidazole-2-carboxylate (R) (7.6 mg, 30 μmol), benzyl alcohol (16.21 mg, 150 μmol), D, L-lactide (3.46 g, 24 mmol) were added into a reaction vessel. The mixture was heated under the protection of Ar to 200° C. The reaction was carried out in the molten mixture for 3 seconds. The reaction was terminated by adding oxygen. The reaction mixture was cooled and dissolved in toluene. The solution was added into ethanol. The precipitate was filtered and dried to a constant weight to obtain 2.5 g of white solid. The conversion rate was 80%. The number average molecular weight of the polylactic acid weight Mn was 18,800, The polydispersity index PDI was 1.28. 
     EXAMPLE 19 
     1-(1-phenylethyl)thiazole-2-carboxylic acid salt (S) (17.5 mg, 75 μmol), benzyl alcohol (10.8 mg, 100 μmol), D-lactide (1.44 g, 10 mmol) was added into a reaction vessel and heated under the protection Ar to 120° C. The reaction was carried out in the molten mixture for 2 hours. The reaction was terminated by adding CS 2 . The reaction mixture was dissolved in chloroform and then added into methanol. The precipitate was filtered and dried to a constant weight to obtain 1.15 g of white solid. The conversion rate was 97%. The number average molecular weight of the polylactic acid weight Mn was 13,210. The polydispersity index PDI was 1.32. 
     EXAMPLE 20 
     2,4,5-phenyl-1,2,4-triazole-3-carboxylic acid salt (T) (25.6 mg, 75 μmol), isopropanol (9.01 mg, 150 μmol), D-lactide (1.44 g, 10 mmol) was dissolved in 5 mL of N,N-dimethylformamide. The solution was heated under the protection of Ar to 96° C. and reacted for 12 hours. The reaction was terminated by adding water. The reaction mixture was added into methanol. The precipitate was filtered and dried to a constant weight to obtain 1.32 g of white solid. The conversion rate was 92%. The number average molecular weight of polylactic acid M n  was 8,560. The polydispersity index PDI was 1.11.