Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF POLYLACTIC ACID

Abstract

The invention disclosed in this application relates to an improved process for the preparation of polylactic acid by dehydration condensation of a lactic acid in the presence of an inert atmosphere using an inorganic solid acid catalyst.The polylactic acid thus obtained has been characterized and the polymer obtained has molecular weight of(Mn) 39,680 and polydispersity value 1.07 The poly lactic acid prepared by this process of present invention is a very well known biodegradable polymer and it is proving to be a viable alternative to pharmaceutical based plastics for many applications.

Full Text

Field of Invention The invention relates to an improved process for the preparation of polyiactic acid. Polylactic acid (PLA) is highly versatile, biodegradable, aliphatic polyester derived from 100% renewable resources, such as com and sugar beets PLA offers great promise in a wide range of commodity applications-Background of Invention In spile of its excellent balance of properties, the commercial viability has historically been limited by high production costs (greater than $2/Ib). Until now PLA has enjoyed little success in replacing petroleum-based plastics in commodity applications, witfi most initial uses limited to biomedical applications such as sutures, (E. S. Lipinsky, R. G. Sinclair, Chem. Eng. Prog. 1986, 82 (8), 26). The announcement of the formation of a new company, Cargill Dow LLC, in 1997 brought two large companies together to focus on the production and marketing of PLA with the intention of significantly reducing the cost of production and making PLA a large-volume plastic (M Naitove, Plasl. Technol. 1998,44(1), 13). PLA can be prepared by both direct condensation of lactic acid and by the ring-opening polymerization of the cyclic lactide dimer Fig. 1 given below. Because flie direct condensation route is an equilibrium reaction, difficulties removing trace amounts of water in the later stages of polymerization generally limit the ultimate molecular wei^t achievable by this method. Most work has focused on the ring-opening polymerization, although Mitsui Toatsu Chemicals has patented an azeotropic distillation process using a high-boiling solvent to drive the removal of water m the direct esterification process to obtain Cargill LLC has developed a patented, low-cost continuous process for the production of lactic acid based polymers(P. R. Gruber, E. S. Hall, J, J, Kolstad, M. L. Iwen , R. D, Benson, R, L, Borchardt US Patent 5142 023). The process combines the substantial environmental and economic benefits of synthesizing both lactide and PLA in the melt rather than in solution and for the first time, provides a commercially viable biodegradable commodity polymer made from renewable resources. The process starts with a continuous condensation reaction of aqueous lactic acid to produce low molecular weight PLA pre-polymer as shown in Fig.2. Next, the prepolymer is converted into a mixture of lactide stereoisomers using tin catalysts to enhance the rate and selectivity of the intramolecular cyclization reaction. The molten lactide mixture is then purified by vacuum distillation. Finally, high molecular weight PLA is produced using a tin catalyzed, ring-opening lactide polymerization in ttie melt, completely eliminating the use of costly and environmentally un-friendly solvents. After the polymenzation is complete, any remaining monomer is removed under vacuum and recycled to the start of the process. This process is currently in operation at an 8 x 106 lb per year market development facility in Minnesota, The construction of a 300 x 106 lb per year commercial scale PLA plant in North America was recently announced by Cargill Dow LLC for start up in 2002 with plans to construct an additional plant in Europe in the near ftiture(R. Westervelt, Chem. Week 2000, 162 (3), 9). PLA has been extensively studied by many researchers and reviewed in detail in recent publications (P, Dubois, R. Jerome, A Lofgren, A. C. Albertsson, J. Macromol. Set. Rev macromol. Chem. P/(>"S. 1995, C35, 379). Polymeric materials that are biodegradable, and at the same time amenable for ^plications like paper coatings, fibres, films and packaging will have for reaching implications for polymer industry- Polylactic acid (PLA) probably satisfies these conditions and in addition provides some unique features like stereoisomeric polymers, (various architectures like syndiotactic and heterotactic polyliu;tic acids), tunsdile modulus, good ultimate tensile strength (with suitable blends), excellent grease and oil resistance, easy lowtemperature heat sealability and good barrier to flavours and aromas In terms of physical properties, PLA is comparable at least to polystyrene. The rheological characteristics of PLA make it suitable for sheet extrusion, film blowing and fiber spinning but only marginally acceptable for some other types of fabrication. However, the improvement in rheological properties can be achieved by branching, which can alter the molecular weight as welt as viscosity and melt strength. The potential of polylactic acid in controlled release of drug deliveiy especially chlorpromazine was carried out in vitro in an immersion medium and the results show that the micro-sphere dimensions, drug content and polymer molecular weight had an effect on the rate of drug release- When PLA is used in orthopedic and oral surgeries as fixation or augmentation devices, PLA of high molecular weight is needed to produce devices of high mechanical strength. On the contrary, such high molecular wei^ts are not necessary for polylactides (PLA) when they are used as carriers for drug delivery systems. In such pharmaceutical applications, lacfide copolymers of low molecular weights are generally preferred because they are degraded in the body more quickly than PLA of high molecular weights {S. Hyu Hyon, K, Jamshidi, Y- ]kida,Biomaterials 1997,16, 1503-1508). Various catalysts have been used for the preparation of polylactic acid by ring opening polymerization with out using the organic solvents (K. Hiltunen, J, V Seppala, M. Harkonen, Macromolecuies. 1997,30,373). All polymerizations were carried out in the meh, using different catalysts and polymerization temperatures. The products were characterized by DSC, GPC and "T-NMR spectrum. The molecular weights were determined Gel Permeation Chromatography (GPC), The weight-average molecular weights of prepared prepolymers determined by GPC varied from 3,600 to 32,600 g/mol. depending on the catalyst and polycondensation conditions, these results are shown in Table 1. In DSC studies the glass transition temperatures of the resulting polymers varied from 24 to 5 ] °C The effect of the catalysts on the final properties of the polylactide especially the molecular weight of the polymer by direct condensation process has been investigated. The flow chart of the reaction sequence is shown in Scheme 1. A variety of catalysts like SnCl4, coordination catalysts containmg Zn and Al, aluminum isopropoxide, Lewis acid catalysts have been employed for the direct condensation polymerization of the lactic acid and polylactic acid having a high molecular weight has been obtained only when an organic solvent was used (M. Ajioka, K, Enomoto, K. Suzuki, A. Yamaguchi, BullChem- Soc. Jpn , 1995, 68. 2125). A bnef summary of the results is given in Tablel. The Role of PLA in Green revolution Legislation aimed at making plastic waste collection, sorting and treatment meeting mandatory rules is being prepared at both national and intcmationai levels. In the European Union, the triggenng factor of most legislative and standardization work has been the European Directive 94/62/EC dated December 31st, 1994 on packaging waste. This directive clearly specifies composting as part of the recycling scheme, PLA meets or exceeds all the specifications imposed on packagmg materials and is dierefore the best altemative to common plastics when considering the need to dramatically decrease ftie plastic waste sent to municipal dumps because no other adequate way of disposing of it are either economically viable or sufficiently safe for people"s health. Legislation will help to promote PLA as the preferred material to put an end to what may be seen as man"s assault on tfie environment. In the context of India, PLA can be a better substitute for various plastic use and can easily find wide acceptance. The market potential for PLA Recent estimates put the PLA market for films and non-woven/fibers products alone at about 122,000 metric tons/year in 2003-2004, 390,000 metric tons/year in 2008 and reaching 1,184,000 to 1,842,000 metric tons/year by 2010. Such estimates are considered very realistic and probably even on the pessimistic side. Indeed, it only covers a small percentage of the existing market for common plastic materials used for packaging purposes. One of the limiting factor fiar PLA"s market penetration was until now the relafively high price of the product but, with new high capacity production facilities coming on-line, this factor will be less and less an hindrance for the substitution of PLA to common plastics. Also newer and cheaper methods can be developed for extensive cost reduction, A price/market model developed by the PST Group clearly shows that for markets of about 900,000 metric tons/year, PLA selling price will favorably compare with oil-based plastic matenals used by the packaging industry. The importance of PLA: The lactic acid needed to make PLA is currently mainly produced by fermenting carbo hydrates from agricultural origin, namely saccharose and starch hydrolysate (glucose). Other raw materials have been suggested in order to decrease PLA"s production costs, with a particular emphasis on agrochemical wastes like molasses or whey instead of usmg refined materials, but purification costs increase draniatically with a reduction of substrate purity. In Europe, saccharose is produced from sugar beets vv^ereas starch is mainly obtained from wheat and, to a lesser degree, fi"om potatoes or com In the US it is this latter cereal which is the main source of starch. Producing 390,000 metric tons of PLA in 2008 would entail the cultivation of 70,000 ha, 187,000 ha or 121,000 ha of farm land respectively for sugar beets, wheat and com corresponding to 3.3%, 19% or 2.9% of average acreage used for these three commodities in the 15-atates European community. The Indian situation can be much different but it is likely that agricultural options have to be always explored given the conditions and stringent requirements which Indian cultivators are facing tod^ in terms of water shortage and cHmatic changes and failure of monsoons It may be necessary a proper estimate is made on this commodity and the sources for this material in Indian context so as K) evolve strategies for adoption of PLA in Indian plastic industiy. Any way it will considerably contribute to our oil import bill. Lactic acid containing a chiral centre can give rise to optical isomers and hence it can also give rise to variety of arcliitectures as has been mentioned above. It may be possible to exploit this option extensively for generating a variety of products In this connection, it is necessary thai one visualizes these options clearly. The optical isomeric polymers can be generated by both the ring opening polymerization as well as by direct condensation polymerization. US patent no 5,310,865 (May 10, 1994) describes A process for preparing poiyhydroxycarboxylic acid comprising of conducting dehydration condensation of a hydroxycaiboxylic acid or an oligomer thereof in a reaction mixture containing an organic solvent and less than 500 ppm of water to obtain poiyhydroxycarboxylic acid having a weight average molecular weight of at least about 15,000, which is very less when compared to the invented process US patent no 5,428,126 (June 27, 1995) describes a preparation process of aliphatic polyester having a weight average molecular weight of 15,000 or more by conducting a direct polycondensation reaction of an aliphatic polyhydric alcohol or a mixed aliphatic polyhydric alcohol and an aliphatic polybasic acid or a mixed aliphatic polybasic acid, or additionally a hydroxycarboxylic acid or a mixed hydroxycarboxylic acid or an oligomer of the hydroxycarboxylic acid in a reaction mixture containing an organic solvent. The aliphatic polyester thus obtained contains an extremely small amount of impurities having low color and can exhibit satisfactory strength in the form of films, filaments and other shaped articles. Here also the molecular weight obtained is very low. us patent no 5,142,023 (August 25, 1992) discloses a process for the continuous production of polylactide polymers from lactic acid which incorporates removal of water or a solvent carrier to concentrate the lactic acid feed followed by polymerization to a low-molecular-weight prepolymer. This prepolymer is fed to a reactor in which a catalyst is added to facilitate generation of lactide, the depolymerization product of polylactic acid. The lactide generated is continuously fed to a distillation system as a liquid or vapor wherein water and other impurities are removed. The resultant purified liquid lactide is fed directly to a polymerization process. The polylactic acid molecules obtained have an average molecular weight of between about 100 and about 5000. US patent no 5,247,059 (September 21, 1993) describes a process for the continuous production of substantially purified lactide and lactide polymers from an ester of lactic acid including the steps of forming crude lactide in the presence of a catalyst means to form a condensation reaction by¬product and polylactic acid and depolymerizing the polylactic acid molecules followed by subsequent purification of the crude lactide in a distillation system. A purified lactide is then polymerized. Here it needs purifying the crude lactide formed in one of the step lo form a substantially purified lacride by distilling the crude lactide and the ester of lactic acid is first condensed in the presence of said catalyst means to form a condensation reaction byproduct and polylactic acid molecules having an average molecular weight of between about 100 to about 5,000, and tfie polylactic acid molecules are at least partially depolymerized in the presence of catalyst means for catalyzing the depolymerization of the polylactic acid molecules to form lactide molecules. US patent no 5,258,488 (November 2, 1993) descnbes a process for the continuous production of polylactide polymers from lactic acid which mcorporates removal of water or a solvent earner to concentrate the lactic acid feed followed by polymerization to a low-molecular-weight prepolymer. This prepolymer is fed to a reactor in which a catalyst is added to facilitate generation of lactide, the depolymerization product of polylactic acid- The lactide generated is continuously fed to a distillation system as a liquid or vapor wherein water and other impurities ^e removed. The resultant purified liquid lactide is fed directly to a polymerization process. Polymerizing lactic acid in the concentrated lactic acid solution in one of the steps by fiirther ev^oration of the aqueous medium to form polylactic acid molecules having an average molecular weight of between about 100 and about 5000 takes place. us patent no 5.357,035 (October 18, 1994) describes a process for the continuous production of polylaclide polymers from lactic acid which incorporates removal of water or a solvent carrier to concentrate the lactic acid feed followed by polymerization to a low-molecular-weight prepolymer This prepolymer is fed to a reactor in which a catalyst is added to facilitate generation of lactide, the depolymenzation product of polylactic acid. The lactide generated is continuously fed to a distillation system as a liquid or vapor wherein water and other impurities are removed. The resuhant purified liquid lactide is fed directly to a polymerization process. Here the polymerization is carried out in the presence of organic solvents and in one of the steps of generating a crude lactide from a polylactic acid mixture comprising polylactic acid having an average molecular weight of no greater than 5,000. To overcome these problems and to achieve high molecular weight polymer, we have developed an improved process for the preparation of polylactic acid using organic solvents only for precipitarion of the polymer. Specifically when compared with the process that are hitherto developed for the preparation of polylactic acid by using mineral acids as catalysts, the utilization of solid acid catalyst is environmentally benign process. Use of sulphuric acid is not environmentally acceptable, while solid acids are environmentally acceptable. Secondly handling mineral acids is not safe. Objectives oflnvention: Accordingly the main objective of the present invenrion is to provide an improved process for the preparation of polylactic acid which is an environmentally benign process. Another objective of the present invention is to provide an improved process for the preparation of polylactic acid having higher molecular weight of about 39000 by maintaining the process conditions as appropriate and acceptable for commercial e?qiloit3tion. StiLl another objective of the invented process is to provide an improved process for the preparation of polylactic acid by direct condensation polymerization. In the process of the present invention we have taken into consideration the impact of the parameters such as (a) temperature of the reaction (b) the molecular weight of the polymer produced. Accordingly the present invention provides an improved process for the preparation of polylactic acid from lactic acid which comprises; (i) heating a solid acid catalyst at a temperature in the range of I20°C to I80"C to remove any u/ater of hydration. (ii) mixing lactic acid and the catalyst as obtained m step (i) in a reaction vessel equipped with a Dean-Stark trap to remove the condensed water formed during the reaction. (iii) continuously passing an inert gas for maintaining an inert atmosphere over the surface of the mixture obtained in 5tep(ii). (iv) placing the reaction vessel on an oil bath at a temperature inthe range of120^C to 180°C for a penod in the range of 30 mins to 6 hrs. (v) pouring the reaction mixture into an organic solvent to precipitate polylactic acid. (vi) filtering the white precipitate of polylactic acid, (vii) drying the polylactic acid under reduced pressure and (viii) recovering the solid acid catalyst from the filtrate by concentration The amount of the catalyst which may be employed may be in a molar ratio of around 1: 2000 to 1: 8000 with respect to lactic acid. Nitrogen gas or any other inert gas like argon may be used for maintaining the inert atmosphere over the surface of the reaction mixture consisting of lactic acid and the catalyst. The reaction vessel is placed in an oil badi maintained at a temperature in the range of 140°C to 160°C, preferably 150°C for a period in the range of 2 to 4 hrs preferably for3hrs. The reaction mixture is poured into an organic solvent such as methanol, ethanol etc preferably methanol to precipitate potylactic acid. The details of the invention are given in the Examples, provided below, are given for the purposes of illustration only and (herefore cannot be construed to limit the scope of the invention. Example 1 The catalyst H3[P(W301o)4] x H2O (HPA) was heated to die temperature o£150°C for 3 hrs for removing the water content Using a reaction vessel equipped with a Dean-Stark trap, lOg of lactic acid and 40 mg of (0.4 wt%) of H3[P(WjO,10)4] x H2O (HPA) treated as stated above were taken in a reaction vessel equipped with a Dean-Stark trap, A continuous nitrogen gas flow was maintained over the surface of the polymerization mixture to maintain inert atmosphere. Thereafter the reaction mixture was poured into lOOmL methanol The precipitated mixture was filtered and dried under reduced pressure. White polylactic acid was obtained. The yield of the polylactic acid was 44% with the purity of 99%. Recovery of solid acid catalyst: The filtrate obtained after isolating Ihe polylactic acid was concentrated to remove methanol solvent and the solid acid catalyst obtained was recovered. For NMR measurements, the polylactic acid prepared by the process described in example! was dissolved in chlorofbrm-di in 5mm NMR tubes at room temperature. The concentration of poiylactic acid was about 10% by weight Differential scanning calorimetric (DSC) measurements ■ were made at a heating rate of 20""C/min, IR spectral analysis was made by KBr pellet mode. Molecular weights (Mn and M„) and polydispersity (Mn/Mn) were detennined with respect to polystyrene standards by gel permeation chromatography. Characterization The polylactic acid product obtained by the process described in Example- J was characterized by these techniques descnbed below. JR Characterization The IR spectrum of poiylactic acid obtained by using the solid acid catalysts showed the presence of bands at 1769 cm"" and 1092 cm"confimiing the ester group in the polymer. A broad band around 3509 cm" is due to presence of hydroxyl groups in the polymer NMR Characterization ""C NMR of potylatic acid obtained from using solid acid catalysts showed lhe chemical shifts values as shown below. The synthesized polymer showed signals at 5 (ppm) (a) 169.5 (b) 69,0 and (c) 16.6 are in accordance with literature values (M. Ajioka, K. Enomoto, K. Suzuki, A. Yamagiichi, BullChem. Soc. Jpn, 995. 68, 2125). Molecular weight Characterizations The weight average molecular weight obtained for the polymer by using solid acid catalysts was determined by GPC and the value is found to be around 39,680 Thermal characterization The decomposition temperature of synthesized polylactic acid was observed around 322°C. The glass transition temperature of the potylactic acid synthesized by solid acid catalyst was found to be55°C. Example! The catalyst phosphomofybdic acid H3PO4M0O3.24H2O (HPA) was first heated around I60°C for 3hrs for removing the water content. A mixture of 10 g of lactic acid and 40 mg of (0,4 wt %) of the catalyst heated as mentioned above were taken in a reaction vessel equipped with a Dean Stark trap, A continuous nitrogen gas flow was maintained over the surface of the polymerization mixture to maintain an inert atmosphere. The vessel was placed in an oil bath at 160°C over a period of 4 hours. Thereafter the reaction mixture was poured into lOOmL of methanol. The precipitated product was filtered and dried under reduced pressure. White polylactic acid was obtained. The yield of the polylactic acid was 42% with the purity of 99%, The filtrate obtained after isolating polylactic acid was concentrated to remove methanol solvent and the solid acid catalyst obtained was recovered. The polylactic acid obtained was characterized with the help of spectrai, GPC, TGA and DSC data These results indicated that polylactic acid obtained by the above mentioned process showed the same charactenstics as those of the polylactic acid prepared by the process showed the same charac ten sties as tfiose of the polylactic acid prepared by the process described in Example I. Example 3 The catalyst cesium salt of 12- tungstophospheric acid was first heated to around I50°C for removing the water content. iO g of (actfc acid and 40 mg (0.4 w( %) of Cs sait of the catalyst, treated as stated above, were t^en in a reaction vessel equipped with a Dean-Stark trap, A continuous nitrogen gas flow was maintained over the surface of the polymerization mixture The vessel was placed on an oil bath at i50°C over a period of 3 hours and the reaction mixture was poured into lOOmL of methanol. The precipitated product was filtered and dried under reduced pressure. The yield was 42% with purity of 99% The filtrate obtained after isolating polylactic acid was concentrated to remove methanol solvent and the solid acid catalyst obtained was recovered. The polylactic acid obtained was characterized with the help of spectral, GPC, TGA and DSC data. These results indicated that polylactic acid obtained by the above mentioned process showed the same characteristics as those of the polylactic acid prepared by the process descnbed in Example 1, £xample-4 The catalyst silica was first heated to around 140°C for removing the water content. J Og of lactic acid and 40 mg (0,4 wt %) of sihca treated as mentioned above were taken using a reaction vessel equipped with a Dean-Stark trap. A continuous nitrogen gas flow was maintained over the surface of the polymerization mixture. The vessel was heated in an oil bath at 150°C over a period of 3 hours and the reaction mixture was poured into l00mL of methanol. The precipitated product was filtered and dried under reduced pressure. The yield was 22% with the purity of 95% Exampe-5 The catalyst alumina was first heated to around 140C for removing the water content, 10 g of lactic acid and 40 mg (0.4 wt %) of the catalyst alumina, treated as above, were taken in a reaction vessel equipped with a Dean -Stark trap A continuous nitrogen gas flow was maintained over the surface of the polymerization mixture. The vessel was heated in an oil bath anl50°C over a period of 3 hours and the reaction mixture was poured into J OOmL of methanol. The precipitated product was filtered and dried under reduced pressure. The yield was 19% widi the purity of 94%. In ordered to compare the efficiency of the solid acid catalyst used in the process of the present invention with (hat of H2SO4, hitherto a known proems, (he following experiment {Example ,6) was carried out. £xampIe-6 10 g of lactic acid and 0,4 wt% of H2SO4 as the catalyst were taken in a reaction vessel equipped with a Dean-Stark trap. A contionous nitrogrn gas flow was maintained over the surface of the polymerization mixture. The vessel was placed in an oil bath at 150°C over a period of 6 hours. The reaction mixture was poured into I OOmL of methanol. The precipitated product was dried under reduced pressure. White polylactic acid was obtained in 42% yield. The product was white crystalhne solid with hi^ purity of 99%. We have carried out the reaction at the same reaction temperature (150C) We have not obtained any polymer after 3 hrs completion of the reaction. This may be due to the conditions such as temperature, time might not have been sufficient for the reacrion to proceed. So as to overcome this we have carried out the polymerization reaction for 6 h and we characterized the product by NMR.IR, DSCandGPC, The result obtained showed that the molecular weight of the polylactic acid obtained using sulphuric acid catalyst for the preparation was very low of the order of 5000 havmg glass transition temperature at 37,8°C Since the reaction conditions like temperature of 150°C and reaction time of 3 hrs were not • adequate for the process to polymerize, small chain oiigomers only remained in the product. It was not possible to recover the catalyst. Advantages of the invention 1. In the conventional acid catalyzed route the maximum molecular weight obtained for polylactic acid IS around 30000 while this process produces polylactic acid having molecular weight of 39000 and above. 2. The solid acid catalyst used is recoverable while in the conventional process the acid is not recoverable. Hence the process is economical 3 The use of solid acid catalyst is environmentally acceptable 4 The quality of the product obtained is superior in terms of molecular weight, crystallinity and glass transition temperature. We Claim 1. An improved process for the preparation of polylactic acid whih comprises (i) heating a solid acid catalyst at a temperature in the range of 120°C to 180°C to remove any water of hydration- (ii) mixing catalyst and lactic acid (in the ratio 1= 4000 to 1=8000) as obtained in step (i) in a reaction vessel equipped with a Dean-Stark trap to remove the condensed water formed during the reaction, (iii) continuously passing an inert gas for maintaining an inert atmosphere over ihe surface of the mixture obtained in step (ii) (iv) placing the reaction vessel in an oil bath at a temperature in the range of 120°C to 180°C for a period in the range of 30 min to 6 h, (v) pouring the reaction mixture into an organic solvent (typically l00mL for 5g of polymer) to precipitate polylactic acid. The polymer that is precipitated is separated from the solvent by filtration. (vi) filtering the white precipitate of polylactic acid. The precipitate is dried at 50C under reduced pressure of 200mm Hg for removing any of the residual solvent molecules in the polymer. (vii) drying the polylactic acid under reduced pressure (viii) the catalyst that was used for the polymerization reaction is recovered by evaporating the solvent, from the filtrate in step (vii), 2 An improved process as claimed in claim 1 wherein the inorganic solid catalyst which is used is selected fi"om known inorganic solid acid catalysts heteropoly acids such as silicotungstic acid [H4Si[W30io]4 X H2O (HPA)], salts of heterapoly acids like cesium salt of I2-tungstophosphoric acid [CS2.5H05PW12O40], and other heteropoly acids with high Bronsted acidity such as tungstophosphoric acid H3[P(W30io)4] x H2O (HPA) and other solid acids like alumina, silica. 3. An impraved process as claimed in claims 1 & 2 vAierein the inorganic solid catalyst is heated in step (i) at a temperature of 150°C for 3 h, 4. An improved process as claimed in claims 1 to 3 wherein the inert gas employed is selected from nitrogen, argon etc. 5. An improved process as claimed in claims 1 to 4 wherein the reaction vessel is heated at 150°C for 3 hrs 6.. An improved process as claimed in claim 1 wherein the solvent used in step (v) is selected from methanol ethanol and the like. 7.. An improved process for the preparation of polylactic acid substantially as herein described with reference to the Examples 1 to 5 "

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