Preparation Of High Molecular Weight Polylactide By Organocatalytic Ring-opening Polymerization Of Propylene Glycol Ester And Its Toughening Modification

As the most widely studied degradable polymer,polylactide（PLA）is regarded as the most promising alternative to petroleum-based polymers because of its biocompatibility,excellent mechanical properties and ease of processing.Currently,traditional PLA synthesis methods usually use metal catalysts to prepare polymers by ring-opening polymerization under high temperature conditions,but this method suffers from metal residue,which is unfavorable for applications in biomedical materials and electronic devices.To solve this problem,organocatalysts have been widely studied and applied in recent years,which have the advantages of simple synthesis,easy operation and high stability,and the prepared PLA is white and can be used directly without further purification.Although the mechanical properties of PLA are comparable to those of commercially available polystyrene,PLA is a brittle material by nature,limiting its range of applications,making toughening modifications particularly important.This paper focuses on the preparation and modification of PLA as follows:（1）Polylactide with high molecular weight was prepared with excellent mechanical properties by ring-opening polymerization of L-lactide（L-LA）catalyzed by organic base/urea.It was found that the MTBD/diurea catalytic system exhibited high catalytic activity and controllability,and it was possible to obtain polylactide with controllable molecular weight and narrow molecular weight distribution;the structures were characterized by ¹H NMR,MALDI-TOF MS;polylactide with molecular weight up to 76.8 k Da can be prepared at 0.05%catalyst content,which exhibits good tensile strength comparable to commercial polylactide;the kinetics of polymerization was studied and the apparent rate constant was calculated to be 0.0054 s^-1;different molecular weights of polylactide were synthesized by changing the ratio of monomer to initiator;the interaction between MTBD/diurea was studied and the catalytic mechanism of MTBD/diurea catalytic system was elucidated;the thermal properties of polylactide were characterized by TGA and DSC tests.（2）Toughening modification of high molecular weight polylactide using degradable triblock copolymers.The triblock copolymers PLLA-PδCL-PLLA with different molecular weights were prepared using lactide andδ-caprolactone as monomers,the structures were characterized by ¹H NMR.The prepared triblock copolymer PLLA-PδCL-PLLA was used for the toughening modification of polylactide materials,and it was found that the mechanical properties of the composites showed a transition from brittleness to toughness as the molecular weight of PLLA-PδCL-PLLA increased;the elongation at break of the composite increased with the increase of PLLA-PδCL-PLLA content;the impact strength of the composites further increased with the increase of PLLA-PδCL-PLLA content;compared with PLLA-PδCL-PLLA,the PDLA-PδCL-PDLA composite prepared by using D-LA as monomer has better mechanical properties under the same conditions.