Study On The Synthesis Of Lactide With Catalyst

The application of PLA and its copolymers is increasing in the fields of biomaterial. Lactide, an intermediate for the preparation of PLA is also catching more attention in this field. The disadvantage of the present procedure for the preparation of lactide is high reaction temperature with low product yield. This leads to the high production cost and prohibited the more general applications of the polylactic acid in the material filed. The present research interest is mainly focuses on the improvement of reaction technique and the development of new catalyst systems for the production of lactide.The mechanism of depolymerization reaction of the polylactic acid (oligomer) is in fact an intramolecular ester exchange reaction and we based our work on this understand. This thesis investigates the preparation and improvement of catalysts for the preparation of lactide from aqueous lactic acid. The thesis consists of the following sections:Part I : ReviewsPolylactic acid is one of the most promising biodegradable polymers and lactide is the intermediate for the preparation of polylactic acid from lactic acid. The recent report for the preparation of lactide has been reviewed. The main methods for the preparation of lactide from lactic acid, the process and the factors that influence the yield of lactide are classified and discussed.Part II: Synthesis of lactide catalysted by rare earth compoundsThe reactions for the preparation of lactide were carried out in the presence of rare earth metal compounds PrCl₃, NdCl₃, SmCl₃, YCl₃ and Nd(O_X)₂Cl·2H₂O. The effects of the amount of catalyst used, the dehydration temperature, the dehydration time, the depolymerization temperature and the depolymerization time on the yield of lactide were investigated in detail. The optimum reaction conditions are dehydration temperature: 110℃, dehydration time:3h, dehydration pressure: 4.0×10⁴Pa, the amount of catalyst: 0.30%, depolymerization temperature:240℃, depolymerization time: 4h, depolymerization pressure:1.0×10⁴Pa, Lactide with moderate yield and high purity was obtained and characterized by IR, ¹H NMR and element analysis. The results showed that these rare earth metal compounds possess moderate catalytic activity for the reaction. Because rare earth metal oxides are abundant in deposit in our country, the new catalytic system has the potential applications in the lactide production process.Part III: Synthesis of lactide catalyzed by palygorskite and palygorskite-supported rare earth complexesA series of acid treated palygorskite and palygorskite-supported rare earth complexes PL-NdCl₃, PL-PrCl₃, PL-YCl₃, and PL-SmCl₃ (where PL= palygorskite) were prepared and used as catalysts for the preparation of lactide from lactic acid. The effects of the amount of catalyst used, the dehydration temperature, the dehydration time, the depolymerization temperature and the depolymerization time on the yield of lactide were investigated in detail. Lactide with high yield and high purity was obtained and characterized by IR, ¹H NMR and element analysis. The highest yield of pure lactide is 66.6% in the optimum reaction conditions. ( Dehydration temperature:100℃, dehydration time:2.5h, dehydration pressure:4.0×10⁴Pa, the amount of catalyst:0.25%, depolymerization temperature:230℃depolymerization time:3h, depolymerization pressure: 1.0×10⁴Pa ) . The new system has the advantages over the reported ones in that the starting material is cheap aqueous lactide, the raw-materials for the preparation of the catalysts are abundant in deposit in our country, the reaction conditions are mild and the product yield is higher.Part IV: Synthesis of lactide catalyzed by montmorillonite and montmorillonite-supported rare earth complexesD,L-lactide was prepared from aqueous D,L-lactic acid catalyzed by acid treated montmorillonite and montmorillonite-supported rare earth complexes MMT-NdCl₃, MMT-PrCl₃, MMT-YCl₃, and MMT-SmCl₃ (where MMT= montmorillonite) respectively. The effects of the amount of catalyst used, the dehydration temperature, the dehydration time, the depolymerization temperature and the depolymerization time on the yield of lactide were investigated in detail. Lactide with high yield and high purity was obtained and characterized by IR, ¹H NMR and element analysis. The highest yield of pure lactide is 68.3% in the optimum reaction conditions. ( Dehydration temperature: 100℃.dehydration time:2.5h, dehydration pressure:4.0×10⁴Pa, the amount of catalyst:0.35%, depolymerization temperature: 220℃, depolymerization time: 3.5h, depolymerization pressure: 1.0×10⁴Pa) . Compared to those of the literature methods, this new system has the advantages in that the starting material is cheap aqueous lactide, the raw-materials for the preparation of the catalysts are abundant in deposit in our country, the reaction conditions are mild and the product yield is higher.