Studies On Direct Synthesis Of Poly (Lactic Acid) And Its Co-polymer

Poly(lactic acid) ( PLA ) polymers are one of the most important aliphatic polyesters because of their biodegradability, renewable resources, and excellent properties. PLA polymers can be employed as a promising cost-effective alternative to commodity petrochemical-based plastics so as to lessen the dependence of plastics on oil and diminish the pollution caused by waste plastic products. Thus the interest in direct polycondensation of LA and/or other hydroxyl-acids is increasing.Both direct synthesis and ring-opening polymerization (ROP) can be utilized to prepare homo- and copolymers of lactic acid (LA). ROP relates to the polyaddition of L-lactide, D-lactide, D, L-lactide, and glycolide. However the synthesis and isolation of these lactones lead to high-priced polymers of LA and environmental pollution. In the present work, direct synthesis with melt polymerization in the absence of organic solvent is utilized to prepare poly(L-lactic acid) (PLLA), poly(D,L-lactic acid) (PDLLA) and poly(L-lactic acid -co- glycolic acid) (PLGA) from respective hydroxy-acids. The yields, molecular weight (Mw), solubility in chloroform, and optical purity of polymers obtained were determined. FT-IR, DSC, and X-ray diffraction (XRD) were employed for the characterization of polymers. The 1H NMR and 13 NMR were performed for microstructure analysis.The PLLA with Mw more than 35000 Da and PLGA (90/10) with Mw more than 50000 Da can be prepare through melt polymerization catalyzed by SnCl2 and p-tolulenesulfonic acid system (Sn(II)/TSA) at 180 for 15 hours under 100Pa. Moreover, a molecular weight of PLGA(90/10) higher than 73000 Da can be got through solid-state polycondensation of such melt polymerized PLGA. The microstructure analysis from 1H and 13C NMR spectra shows the racemization of L-lactic acid (LLA) blocks both in the homopolymerization of LLA and in the copolymerization of LLA with other hydroxy-acids such as glycolic acid (GA) and D-lactic acid (DLA). Moreover, the racemization of LLA blocks can be enhanced with long reaction time, high temperature, and the addition of comonomers. In the copolymerization of LLA with GA5 GA has higher reactivity than LLA, which leads to sequential block copolymers instead of random copolymers of LLA with GA under present reaction conditions. Transesterification in the copolymerization of LLA and GA can decrease sequence lengths of GA and LA.PLGA samples with GA fractions lower than 30% possess crystallinity, and show crystallization of relatively long LLA sequences and no crystallization of GA sequence in copolymer chains. The crystallinity can be adjusted by the addition of GA with various contents. The transesterification and the racemization can also result in lower crystallinity, which are further verified by DSC and WAXD measurements. Thus the transesterification and the racemization must be paid special attention, because they are enhanced in the present reaction conditions.Both PLLA and PLGA melt were paeudoplasic fluids and had higher values of the viscous flow activation energy. At the same time, the flow activation energy of PLGA was higher than that of PLLA, which indicated that the viscosity of PLGA was more sensitive to temperature.Thus PLGA samples with various GA fractions show different properties, and will be useful as drug delivery systems, films, fibre and other application.