Synthesis And Properties Of Poly(L-Lactic Acid) And Its Amphiphilic Block Copolymers

Poly(lactic acid) (PLA) is important aliphatic polyester in the biomedical polymer materials field, it has good biocompatibility and biodegradability and a wide range of applications such as drug controlled release systems, tissue engineering, cell culture and medical surgical suture lines. Poly(ethylene glycol) (PEG) is an excellent biocompatible, hydrophilic, non-toxic and non-immunogenicity biomedical materials, it is often introduced to poly(lactic acid) molecules as a hydrophilic component to form amphiphilic block copolymer material. The block copolymer composition can be controlled by copolymerizable methods to regulate copolymers hydrophilicity and degradation rate, it could be prepared with core-shell structure polymer microspheres in selective solvents system.In this paper, lactide was prepared from lactic acid with stannous octoate as catalyst by means of condensation polymerization and depolymerization by using the improved synthetical technology, the crude yield can reach 81.2%, purification of lactide by recrystallization three times with the combined methods of alternating solvents and concentration gradient, Both total yield and the purity of the L-lactide were raised. Ploy(L-lactic acid) microspheres were prepared by using emulsion-solvent evaporation method. The ring-opening polymerization of L-lactide with different molar ratios of monomer/initiator ([M]0/[I]0) was initiated successfully by potassium/naphthalene and potassium ethoxide respectively, The number average molecular weight (Mn) of poly(L-lactic acid) can be up to 27.1×103 and 36.5×103 respectively with narrow molecular weight distribution, the Mn of PLLA can be controlled by changing the [M]0/[I]0 in this range.Poly(L-lactic acid)-poly(ethylene glycol)-poly(L-lactic acid) triblock copolymers (PLLA-PEG-PLLA) with different block content were obtained by L-lactide ring-opening copolymerization, using macroinitiator which was made from potassium/naphthalene and poly(ethylene glycol). The structures of these copolymers were characterized by FTIR and 1H-NMR. Their Contact angle and water absorption were tested, It was found that the hydrophilic enhanced with EO/LA increasing. Thermal properties and crystalline morphology of triblock copolymers were studied by means of differential scanning calorimetry(DSC) and polarized optiacal microscope(POM), the results showed that melting peak of each block were lower than their homopolymers and only one Tg was observed in each copolymer. Depending on composition of the copolymers, only one block crystallized or two blocks didn’t crystallize. The isothermal crystallize morphology of copolymer changed as the blocks’content altering. The crystallize morphology changed with the isothermal crystallize temperature, copolymers nanoparticles for drug-delivery system can be formed by self-assembled in selective solvents.Therefore, the hydrophilicity, thermal properties and crystalline morphology of these triblock copolymer could be availably improved by adjusting the composition of triblock copolymers. Available pharmaceutical and biomedical materials could be gained according the practical requirement which enlarged the applications of the PLA material.