Synthesis Of Biodegradable Poly (Lactic Acid)

Poly(lactic acid) (PLA), a relatively new commodity polymeric material derived from biorenewable feedstocks such as corn starch, is a biodegradable aliphatic polyester which decomposes into carbon dioxide and water in a typical compost environment. This polymer has usually been prepared by ring-opening polymerization of lactide. Unfortunately, PLA obtained by this method usually has high price because the purification of lactide generally need repeated cristallization. PLA with high molecular weight obtained from ring-opening polymerization of lactide is limited. The direct condensation polymerization is theoretically feasible, and the studies to develop a feasible process for the commercial production of PLA are of realistically significant.PLA has been conventionally produced by a multi-step process, including the isolation of an intermediate dilactide (a cyclic dimmer of lactic acid) followed by a ring-opening polymerization. But this method is also expensive and complicated for it includes many time-consuming and funds-consuming processes.In order to approach to the desired process for a high-molecular-weight polylactic acid, melt-polycondensation of aqueous lactic acid was studied. For getting PLA used for biomedicine, catalyst-free direct polycondensation of aqueous lactic acid with step by step heating was studied. The poly (lactic acid) prepared by the simple method has high yield, low cost, no solvent and no catalyst left, and the molecular weight is higher. Then we concentrated on testing a variety of new catalyst systems. The effects of the amount of the catalysts used, the reaction temperatures, the reaction pressure and the reaction time on the polymerization were investigated in details. The monomer we used in the polymerization is an aqueous solution of lactic acid containing about 85-90% pure monomer. The new procedures are much simple, cheap, convenient and afford polymers with high yield and molecular weight and narrow polydispersity. The novel melt polymerization of D, L-LA was firstly conducted with the catalyst system we have used. Furthermore, aqueous lactic acid is cheap and comes from renewable resource. The procedure is environmentally benign without the use of any solvents. The PLA we have prepared is expected to be a broad application and this direct process can be a new route to the synthesis of the high polymer of P (D, L) LA, and an even alternative to the currently adopted lactide polymerization method. The characteristic features of polylactic acid obtained by this direct process were analyzed by NMR, IR, GPC, TG and DTA et al.(1) The synthesis of PLA catalyzed by solid superacid. Replacement of homogeneous liquid acids by heterogeneous solid superacids as catalysts in the chemical industry is expected to bring about an ease of product separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of catalyst with high selectivity. Furthermore, the heterogeneous solid catalysts can offer other advantages, such as no corrosion of the reaction and no environmental problem for disposal of used catalyst. The P(D,L)LA with Mv more than 18000 and yield 60% was prepared through melt polymerization catalyzed by solid superacid SO₄^2-/ TiO₂/Sm³⁺(0.5%) at 180℃for 14 hours under 0.085Mpa.(2) Direct polycondensation of aqueous lactic acid catalyzed by acid anhydrides. Acetic anhydride, cis-butenedioic anhydride, phthalic anhydride, pyromellitie dianhidride were studied separately. As a result, the P(D,L)LA with Mw 17000-23000 Da was prepared through melt polymerization catalyzed by acid anhydrides (1-2wt%) at 180℃for 10-14 hours under 0.085Mpa.The yield is 60-70% and the polydispersity is 1.3-1.6. This kind of polymer might be useful in biomedical applications.(3) Direct polycondensation of aqueous lactic acid co-catalyzed by acid anhydrides and SnCl₂·2H₂O. Acetic anhydride, succinic anhydride, cis-butenedioic anhydride, phthalic anhydride, pyromellitie dianhidride companied with SnCl₂·2H₂O were studied separately. As a result, the P(D,L)LA with Mw 34000-50000 Da was prepared through melt polymerization co-catalyzed by acid anhydrides (0.07-1.4wt%) and SnCl₂·2H₂O at 160-180℃for 10-14 hours under 0.085Mpa.The yield is 60-80% and the polydispersity is 1.4-1.7.(4) Melt polycondensation of aqueous lactic acid to poly (lactic acid) cocatalyzed by phthalic anhydride and rare earth compounds. It has been found that combination of phthalic anhydride with rare earth compounds can be a good catalyst not only for obtaining a high molecular weight(25000-34000) polymer with a narrow polydispersity (1.5-1.7) in bulk at 180℃for 10 hours under 0.085Mpa, but also a high yield(60-70%). The suitable amount of phthalic anhydride is 1-2.5wt% and rare earth compounds are 0.5-2.5wt%.(5) Direct polycondensation of aqueous lactic acid catalyzed by SnCl₂·2H₂O actived by NaOH with different concentration. As a result of this study, the P(D,L)LA with Mw 16000-25000 was prepared through melt polymerization catalyzed by SnCl₂·2H₂O actived by NaOH with different concentration(0.25-0.5wt%) at 180℃for 10 to 14 hours under 0.085Mpa.The yield is over 70% and the polydispersity is 1.3-1.5.Based on the study mentioned above, pilot study for degradation of PLA in different medium was conducted. It is reasonable concluded that the degradation of PLA in acid medium is faster than that in neutral medium, and that the degradation of high molecular weight PLA is slower than that of low molecular weight. At last, the application of PLA was tried, we successfully encapsulated calcium gluconate and prepared chewing-gum matrix with biodegradable PLA. It suggests that PLA is a good basic material for drug delivery system and be useful in biomedical application.