Study On Stereoselective Polymerization Of PLA

Poly (lactic acid) (PLA) polymers are one of the most important environmental friendly polymers because of their biodegradability, renewable resources, and excellent properties. PLA polymers can be employed as a promising cost-effective alternative to commodity petro- chemical-based plastics so as to lessen the dependence of plastics on oil and diminish the pollution caused by waste plastic products. PLA has two chiral structures-PLLD and PDLA. Both polycondensation of lactic acid and ring-opening polymerization (ROP) of lactides can be utilized to prepare poly(lactic acid). Properties of PLA are decided by its microstructure. Therefore, the stereoseletive polymerization of PLA is paid more attention to control PLA microstructure. So far, the stereoselective polymerization mainly focuses on the ring-opening polymerization of lactide, while the stereoselective polymerization of lactic acid is less studied.In this paper, PLA is prepared through direct synthesis of melt polycondensation of D,L-lactic acid. Viscometry, polarimetry, FT-IR, differential scaning calorimetry(DSC), X-ray diffraction(XRD), gel filtration chromatography(GPC), and nuclear magnetic resonance (NMR) are employed for the characterization of polymers obtained. The effects of reaction time, temperature, weight of catalyst, molar ratio of two catalysts were detected. All six complex catalyst systems used are stereoselective. The catalyst system of SnCl₂/naphthalene disulphonic acid (B) and Sn(Oct)₂/naphthalene disulphonic acid (E) can be used to prepare PLA with high optical activity, which is mainly composed of L-Lactic acid units.The optical activity and molecular weight of PLA will change with the reaction conditions. High temperature and long reaction time tend to obtain fuscous polymers. PLA with optical activity of 97%, viscosity of 0.24 can be prepared with B as catalyst in following conditions: reaction time 15 hours, temperature 180℃, SnCl₂ 0.5wt%, molar ratio of SnCl₂/naphthalene disulphonic acid 0.5, and vacuum -0.1MPa. PLA with optical activity 98%, viscosity 0.26 can be prepared with E as catalyst in following conditions: reaction time 15 hours, reaction temperature 180℃, Sn(Oct)₂ 0.5wt%, molar ratio of Sn(Oct)₂/naphthalene disulphonic acid 0.5, and vacuum -0.1MPa.PLA with high optical activity can be prepared through D, L-lactic acid catalyzed by Tin salt and proton acid systems, which will deduce PLA cost and lessen the pollution of solvents during its process.