The Synthesis, Characterization And Application Of A Series Of New Schiff-base Aluminum Catalysts

Polylactic acid(PLA) is a biodegradable material which is the most extensivelystudied by far, and PLA has been proven to be the most attractive and usefulbiodegradable polyester. Due to the presence of two sterogenic centers in a lactidemonomer, there are three different lactide stereoisomers exist. Due to the lactic acidhas two kinds of optical isomers, the corresponding lactide has three stereoisomers, sopoly (lactic acid)(PLA) can have a variety of sequence structure, including isotacticstructure, syndiotaxy structure, atactic structure, heterotactic structure and blockstructure. Since the study showed that the sequence structure of poly (lactic acid)(PLA), which can be controlled through stereoselective polymerization of lactide, hasa great influence on its performance, the development of the catalysts ofstereoselective polymerization of lactide has become one of the hot research topic inrecent years.Based on the above ideas and previous research foundation, we designed,synthesized and charactered a series of single-center asymmetric schiff-basealuminum catalysts. And we successfully applied them in polymerization ofrac-lactide, L-lactide. Kinetic studies have shown that all the monomers and catalystsare the first order. When the substituents on the benzene ring of catalysts iselectrophilic, the activity of catalyst can be significantly improved; and when thesubstituents is big steric hindrance, the stereoselective of catalyst can be significantlyimproved. Stereoselectivity study for polymerization of rac-lactide indicated that wegot block PLA, and we infered that the true initiator should be schiff-baseisopropoxide aluminum compound according to end-group analysis.We also designed, synthesized and charactered a series of bimetallic schiff-basealuminum complexes. And we successfully applied them in the polymerization ofrac-lactide, which obtained highly isotactic PLA(Pm up to0.97). Kinetic studies haveshown that all the monomers and catalysts are the first order and the electronic effectsand the steric effects of substituents on the benzene rings have a significant impact on the catalytic effect. In a certain temperature range, the higher polymerizationtempera-ture, the faster the polymerization rate, which can be proved in the study ofpolymerization temperature. Comparison of the catalytic effect of the ethyl aluminumcomplexes and isopropoxide alumium complexes indicated that the real initiator arethe latter one in this series polymerization.