Preparation And Applications Of Polylactide-based Chiral Porous Materials

Polylactide(PLA)materials,especially microspheres or porous materials,have become increasingly important nowadays.Due to its biocompatibility,biodegradability,nontoxicity and good mechanical properties,PLA has found various practical applications as packaging,medical,and tissue engineering materials,among other fields.The methods for directly preparing PLA materials from monomer are quite limited.Pre-formed PLA is usually used as precursor to prepare PLA materials in most of the as-established strategies,and the preparation process is limited by many factors such as the solubility of the polymer.For PLA porous materials,internal structure and surface morphology are important factors affecting their performance and practical uses.PLLA is a well-known chiral polymer.It can show chirality at different levels,such as the structural units of polylactide(lactic acid),the helical structures of the polymer chains in the solution state,and the helical structure in the crystalline state.The helical structure has profound effects on the properties of the materials.Chirally helical polymers such as polyacetylene,poly(phenyl isocyanide)and poly(amino acid)s show significant potential applications in chiral recognition/adsorption,enantioselective release,asymmetric catalysis,and enantioselective crystallization.Unfortunately,PLLA remains scarcely explored as chiral materials.In this dissertation,we prepared PLA porous materials by precipitation polymerization based on different solution systems and established an unprecedented strategy for regulating the particles size and pore structures.Besides,we explored chiral recognition ability of the PLLA materials.The research contents are shown as follows:1.We successfully prepared porous PLLA microspheres(size,?120?m)by precipitation polymerization using L-lactide as monomer and ethyl acetate as solvent.The structure and morphology of microspheres were characterized by FTIR,SEM and CD techniques.The results show the optimal monomer concentration is 0.4 g/mL and the yields of the polymer microspheres are more than 90%.The formation process of microspheres was investigated by SEM,XRD and DSC analyses.Phase separation and crystallization process occurred in the course of precipitation polymerization and endowed the PLA particles with interesting porous structures.The PLLA microspheres demonstrated chiral recognition ability in both chiral adsorption and enantioselective crystallization experiments.Besides,the size of PLLA microspheres and the porous structure can be tuned through controlling the crystallization of PLLA by adding D-LA in the polymerization system of L-LA.2.Based on precipitation polymerization established above,we prepared PLLA particles(size,?20 ?m)with lamellar structures using butanone/n-heptane as the solvent.By adjusting polymerization conditions(solvent and temperature),a series of PLLA particles with tunable morphology was fabricated.The resulting particles were characterized by SEM,CD,XRD,DSC,and GPC measurements.We proposed a mechanism for regulating the particles morphology.We further explored chiral resolution ability of the PLLA particles used as chiral additive to induce enantioselective crystallization.According to SEM,CD,and XRD characterizations,PLLA particles showed considerable enantioselectivity and preferentially induce L-threonine to crystallize from the racemic solution.Besides,we found that enhancing the crystallinity of PLLA particles is beneficial for improving the particles' enantio-differentiating ability.