A Study On Synthesis Of Star-shaped Poly(D, L-lactic Acid)

Polylactic acid is biodegradable and the polymer synthesized by renewable raw materials, can be a fundamental solution to the "white pollution" and the increasing depletion of oil resources. Compared with linear polymers, star-shaped polymers have lower crystallinity, melt viscosity, molecular surface have a higher functionality, smaller size and other properties of fluid dynamics. Therefore, we can obtain a higher molecular weight in the lower viscosity at the same time, such a polylactic acid copolymer can facilitate production and processing. It is necessary to research and develope simple process, good solubility, low-cost and more practical value of the star-shaped Polylactic acid.First, linear poly D,L-lactic acid (LPDLLA) was synthesized by directly melt polycondensation from D,L-lactic acid. It was found that the appropriate reaction conditions are as follows: 0.5wt% of SnCl2.2H2O/TSA catalyst, polymerization temperature of 165℃, 3kPa at 18h.Star-shaped polyD,L-lactic acid (SPDLLA) was synthesized by directly melt polycondensation from D,L-lactic acid using pentaerythritol as a core. When the molar ratio of D,L-lactic acid / pentaerythritol is 1000, star-shaped SPDLLA with the higher specific viscosity about 0.556 dL.g-1 was successfully synthesized. But the polymerization time is shortened to 12 hours.In the same molecular weight, as the dynamic volume of the star-shaped polymer is lower than the linear polymer, the molecular weight of star-shaped polymer is significantly greater than the linear polymer in the same specific viscosity. In the same reaction conditions, the specific viscosity of linear polylactic acid was lower than that of star-shaped polylactide, indicating that adding a small amount of pentaerythritol to polycondensation is an effective way to synthesize higher molecular weight of polylactic acid.Through NMR analysis, indicating that the hydroxyl pentaerythritol have been involved in the condensation reaction with the lactic acid to form a four-arm SPDLLA. Using gel permeation chromatography (GPC) determine molecular weight of SPDLLA, maximum weight average molecular weight (Mw) = 51 408, the number average molecular weight (Mn)=37250, polydispersity index (Mw / Mn) = 1.37. Mw / Mn is less than1.5~2 is one of the important symbols for obtained polymer is the copolymer, so obtained polymer is indeed star-shaped copolymers.The differential scanning calorimetry (DSC) of SPDLA and LPDLLA have shown that the glass transition temperature (Tg) of SPDLLA is 45.65℃, is significantly lower than that of LPDLLA (which Tg is 50~60℃). The introduction of pentaerythritol in the polymer change the regularity of molecular chain, so that in PLA copolymer movement is more easy.Significant melt absorption peak of the X-ray diffraction (XRD) and DSC analysis of SPDLLA do not appear, that indicate, the synthesized star-shaped polymer is amorphous, SPDLLA still keep low crystalline of LPDLLA material.