Microwave Radiation Synthesis Of Poly-l-lactic Acid

Polylactide (PLA) is a biodegradable polymer and produced by chemical synthesis using biorenewable feed stocks as starting materials. It has been extensively processed as variety of biodegradable packaging materials and high-efficient and low-toxic biomedical materials such as controlled-release drug delivery particles, sutures, stents and dialysis media, due to its better degradability and biocompatibility as well as decomposition into carbon dioxide and water in our bodies. PLA has therefore been paid much attention for environment protection and applications in medical field. Polylactide is usually prepared by polymerization of lactic acid or ring-opening polymerization of lactide, of which the ring-opening polymerization process is a general route in industry. However, this route has many shortages such as system complex, high technical specification, low yield and high operating costs, etc. Therefore, researchers around the world hope that a direct polymerization of lactic acid route could be developed as an alternative way, with simple system, low operating costs as well as production of high molecular weight PLA. In this thesis L-polylactides (PLLAs) with linear-chain and star-shape structure was directly synthesized using L-lactic acid as row materials, SnCl₂ as catalyst under microwave irradiation. The as-synthesized polymers were structurally characterized by FTIR and NMR techniques and also analyzed by GPC and DSC. The synthetic conditions were finally optimized.(1) Preparation of PLLA with linear-chain structure by direct polymerizationL-lactide monomer was dehydrated for one hour under vacuum and inert gas atmosphere with microwave irradiation at 120℃, then a certain amount of catalyst SnCl₂ was added and the monomer underwent melting polymerization. The resulted polymer was dissolved in chloroform and grade-precipitated in ethanol, followed by dried at 45℃for more than 24 hrs under vacuum. The polymer was structurally characterized by FTIR and ¹H NMR and verified itself a polylactide with linear-chain structure. The products, synthesized at different reaction temperatures and reaction time, were further analyzed by GPC and DSC techniques and found that the optimized conditions for the polylactide synthesis is at 155℃for 3 hrs with microwave irradiation under vacuum and inert gas protection. The as-synthesized product has high viscosity-average molecular weight and glass transition temperature. And the results showed that the whiteness of the product increased with increasing the flow rate of the inert gas.(2) Preparation of PLLA with star-shape structure by direct polymerization L-lactide monomer was dehydrated for one hour under vacuum and inert gas atmosphere with microwave irradiation at 120℃, followed by addition of a certain amount of initiator pentaerythritol (PET) and catalyst SnCl₂. Then the monomer underwent melting polymerization. The polymer was dissolved in chloroform and grade-precipitated in ethanol, followed by dried at 65℃for more than 24 hrs under vacuum. The polymer was structurally characterized by FTIR and ¹³CNMR. and verified itself a Polylactide (SPLLA) with four-arm star shape. The products, synthesized at different reaction temperatures and reaction time, were further analyzed by GPC and DSC techniques and found that the optimized conditions for the SPLLA synthesis is at 155℃for 7 hrs with microwave irradiation under vacuum and inert gas protection. The as-synthesized SPLLA has higher viscosity-average molecular weight and glass transition temperature. The mechanical properties of PLLA and SPLLA were detected by DMA, the results showed that SPLLA had small stain, rapid recovery and conducive maintain for form factor of products.