Applications Of Schiff-base Aluminum Catalysts In The Ring Opening Polymerization Of Rac-Lactide

In a variety of biodegradable materials, biodegradable polyesters refer particularly to one type of aliphatic polyesters which can be obtained by the ring opening polymerization of lactide, glycolide,?-caprolactone as monomer under given conditions. Biodegradable polyesters have a wide range of applications which mainly consist of environmental protection and biomedical application. Since polyesters can be degraded under natural environment conditions such as the existence of microorganism and enzyme, it's been considered as an eco-friendly material. Furthermore, the monomers of polyesters are non-toxic, and they have excellent biocompatibility, and being commonly used in the biomedical field, such as the carrier of the controlled anti-cancer drug release, bone nails, bone plates, sutures. There're two types of polymerization applied in polymerization of esters. The first method is the polycondensation of hydroxyl acid. The second method is the ring opening polymerization of lactones or cyclic diesters. The by-pruduct of the polycondensation of hydroxyl acid is water, and since this approach is a reversible reaction, and the water removal is difficult, therefore it's almost impossible to obtain polymers with high molecular weight. However, under certain given conditions, such as water-free, oxygen-free schlenk technique, and appropriate catalysts, the ring opening polymerization of the lactide and ?-caprolactone can be a well-controlled polymerization. In recent years, catalysts utilized in the polymerization of lactide and ?-caprolactone have received the most research attention in this area.The catalysts being investigated in this thesis are utilized in the ring opening polymerization of lactide monomer. Among many catalyst systems, shiff base-metal coordination compound catalyst system outshines others due to its outstanding catalytic properties, and have become a relatively hotter research area. With Schiff base being the ligand moiety of the coordination compound, different substituents on the Schiff base have different impact on the ring opening polymerization of the lactide, as different substituent has different electro-negativity and steric hindrance. The changing of coordination metals also plays huge impact on the polymerization, such as initiation period, and the rate of polymerization. The lactide monomer which were applied in the polymerization of polylactide (PLA) has two stereogenic centers, and hence lactide has 3 different optical isomers:L-LA, D-LA, meso-LA. Therefor, PLA have several different sequence configurations. There are isotactic sequence configuration, syndiotactic sequence configuration, atatic sequence configuration, heterotactic sequence configuration, block sequence configuration. The mechanical properties and physical properties of poly lactide (PLA) are deeply influenced by the PLA chain configuration. Since different poly lactide (PLA) chain configurations can be obtained by using appropriate catalysts, therefore the stereoselectivity control of the ring opening polymerization process of lactide monomer has become a top priority.We have designed and synthesized a series of Schiff base-aluminium coordination compound catalysts. Through several rigorous characterizations, we have confirmed the structure of the catalysts. A comprehensive investigation on their stereoselective and kinetic features in the ring opening polymerization of lactide was carried out afterwards. Linear correlation between the lactide monomer conversion and molecular weight indicated a well controlled polymerization. The steric hindrance of the substituents adjacent to hydroxyl on the Schiff base has a great influence on the stereoselectivity control of the polymerization. The polymerization proved to be first-order in both catalysts and lactide monomer, furthermore we studied the effect of morpholinomethyl groups in the polymerization of Rac-lactide.