Study Of Asymmetric Kinetic Resolution Polymerization Of Rac-LA Catalyzed By Metal Zn And Al Complexes

Chiral polymers,as an emerging special functional material,were used in many special applications,such as chiral separation materials,chiral optoelectronic materials,chiral liquid crystal materials and chiral biofilm materials.Their unique microstructure of polymer chains makes them possess special properties that conventional polymer materials did not possess.The asymmetric kinetic resolution polymerization strategy(AKRP)could be used to synthesize chiral polymers with high stereoregularity in an efficient and selective manner,and to precisely characterize the synthesis of chiral polymers and reveal the control mechanism and chiral effects during the polymerization process.Meanwhile,the PLA materials obtained from ROP of lactide monomer have received a lot of attention in recent years due to their biodegradability.Coupled with the existence of two chiral stereogenic centers within lactide monomer,rac-lactide could be selectively synthesized to chiral PLA materials by AKRP.Therefore,in this paper,we used asymmetric kinetic resolution polymerization strategy and conducted the following studies:1.Chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes were synthesized and utilized for the asymmetric kinetic resolution polymerization of rac-lactide.These polymerizations could proceed rapidly and efficiently under the initiation of these chiral catalysts.The conversion could reach to around 50%in 20 min.And the polymerizations produced highly isotactic PLAs with stereoregular structures and narrow polydispersity[P_m=0.75(CEC)and 0.83(ESC),D=1.09～1.14].The s-factor during the asymmetric kinetic resolution polymerization of rac-LA is up to 4.5.Compared to the polymerization with no initiator,the results of AKRP of rac-LA under initiator adding showed that the molecular mass of resulting PLA keep in line with the theoretical molecular mass but with a lower P_m which equal to 0.65(CEC)and 0.75(ESC).Asymmetric kinetic resolution polymerization and block polymerization experiments showed that the polymerization of rac-LA was controlled by enantiomorphic site control mechanism and chain end control mechanism,the two control mechanisms are both involved during the AKRP of rac-LA.2.A series of chiral cyclohexanediamine-based Salen-aluminum complexes were designed and synthesized and used in the asymmetric kinetic resolution polymerization of rac-LA.It was found that these chiral complexes could initiate the AKRP of rac-LA in controllable manner and showed selectivity reversion for the AKRP of rac-LA with the change of substituents.The complexes preferentially polymerized L-LA when no substituent at the 6 position of phenyl ring or tert-butyl group,while the complexes possess methyl,ethyl,isopropyl at the same position,the complexes will polymerize D-LA.The complex(R,R)-Salen[6-Cl]showed the highest activity in the AKRP of rac-LA,and the conversion reached to 50%in 2.4 h.The complex(R,R)-Salen[4,6-Br,~tBu]showed excellent stereoselectivity for the AKRP of rac-LA(P_m=0.88,CEC;P_m=0.93,ESC),s-factor=4.59.Kinetic studies using the complex(R,R)-Salen[4,6-Br,~tBu]as a catalyst showed that the polymerization process exhibited first-order kinetics with a different rate constant of K_L/K_D～30.Mechanistic investigations revealed an enantiomorphic site control mechanism during the AKRP of rac-LA catalyzed by aluminum catalysts with alkyl substituents while the results of polymerization of rac-LA,D-LA and L-LA by catalysts with electron-withdrawing substituents showed both enantiomorphic site control mechanism and chain end control mechanism involved in the polymerization process.