Stereo-selective Ring-opening Polymerization Of Rac-lactide Catalyzed By S-block Metal Complexes

Degradable polyesters play an important role in our daily life. For example, Polylactide(PLA), polycaprolactone(PCL), and polycarbonate(PC) have been widely used in the fields of film, clothing, fiber and so on. Because of its biocompatibility, polylactide(PLA) has also been used as biomedical polymer material, like surgical sutures, bone nail, drug carriers and tissue scaffold. Considering the prospects in the application of PLA, scientists have being focusing on improving its performance. From the point of molecular, the property of the polymer chain is closely related to its microstructure. Lactide possesses two chiral centres in its monomer, and polylactides can exhibit different property depending on its microstructure. For example, when poly-rac-lactide chain without high tacticity normally exhibited a lower Tm than that of the enantiopure isotactic PLLA or PDLA, but a 1 : 1 mixture of PLLA and PDLA displays a comparable Tg to PLLA and a significantly high Tm of 230 °C. This increased melting point is attributed to the formation of a stereocomplex. If a stereoselective catalyst was used, we can get a polymer with high melting point from rac-lactide. So it is worth to search catalysts with high stereoselectivities.This thesis focues on the synthesis of non-toxic metal complexes, and its applications in ring-opening polymerization(ROP) of rac-lactide. The following is the four chapters of this thesis.The first chapter gives a brief review about the stereoselective ring-opening polymerization of rac-lactide and meso-lactide.The second chapter reports two crown ether complexes of sodium and potassium naphthalenolates, and their application in ROP of rac-lactide. The attractions in this chapter were that the two complexes can iso-selectively catalyze the ROP of rac-lactide at room temperature, giving a Pm value of 0.73. This is the highest Pm value at room temperature obtained by alkali metal complexes at that time.The third chapter systematically studied the metal-size influence of lithium, sodium, potassium, and rubidium complexes for the ROP activity and selectivity of rac-lactide. Notably, this was the first time for rubidium complex used in ROP of lactide, offering more possibilities for the ROP of cyclic esters.The chapter 4 differs from chapter 2 and 3, using non-toxic zinc and magnesium complexes as catalysts to the ROP of rac-lactide. Inspired by the high selectivity of aluminium schiff base complexes, we designed a similar Schiff base Zinc and magnesium complexes. The selectivities of these two catalysts were not good as we expected, probably because of the bending structures caused by unexpected reduction of C=N bond.In short, the main point of this paper was the study of the stereoselective ROP of rac-lactide with non-toxix metal complexes, and a good improvement has been achived. This would be a great referencial value in the future research.