Study On The Polymerization Of Rac-Lactide And The Catalysts

Biodegradable and biocompatible polymers formed from inexpensive renewable resources are attracting considerable attention in both academic and industrial circles. Polyesters derived from the ring-opening polymerization (ROP) of lactides and lactones constitute one important class of such polymers. In this paper, Schiff base ligands L1-L5were synthesized from the reaction of salicylaldehyde with alcohol amine. The aluminium complexes were obtained from the reaction of the ligands with aluminium isopropoxide. The ligands and the complexes obtained were characterized with the infrared spectroscopy (IR) and the nuclear magnetic resonance (NMR).The paper studies the polymerization of lactide with Schiff base aluminium compleses cat.1-4. The effects of the feed ratio of the monomer and the catalyst, polymerization temperature and the reaction time were investigated. Catalysts cat.1and cat.3show low catalytic activity in the polymerization of lactide at130℃. High polymer yield above80%were obtained from long time reaction, which indicate that few active species were produced at the low temperature. The molecular weight distributions of the polylactide are in the range of1.2to1.3. Higher polymer yield above85%were obtained at140℃. However, the raise of the temperature result in broaden of the molecular weight distributions from1.5to1.7. The ideal feed ratio of the monomer and the catalyst is200(mol/mol). The presences of the melt endothermic peak in the DSC curves indicate the crystalline structure of polylactide obtained with cat.1and cat.3.Ring opening polymerizations of lactide were also studied with cat.2and act.4. Polymerization, results indicate that these two catalysts showed higher activity than cat.1and cat.3under same polymerization condition, this can explained by the structure of the catalyst. For cat.2and cat.4, bulky tertiary butyl constituents in the ortho and para of phenolic hydroxyl provide protection of the metal center and inhibit the side reaction such as the exchange of ester, which increases the catalyst activity. Narrow molecular weight distributions of polylactide were obtained with cat.2and cat.4.Catalysts cat.5showed the highest catalytic activity in lactide polymerization, high polymer yield above80%were obtained within2hours at130℃or140℃, suggesting that the structure of the ligand of the complexes affect the catalytic activity extensively. The absence of phenyl or benzenyl substituents in methene and the presence of tertiary butyl constituents in the ortho and para of phenolic hydroxyl of cat.5increase the electronic effect of the catalyst and increase the catalyst activity. Narrow molecular weight distributions of polylactide were obtained with cat.5. The presence of the melt endothermic peak in the DSC curves indicates the crystalline structure of polylactide obtained with cat.5.In short, Schiff base aluminium compleses cat.1-5studied in this work show catalytic activity in lactide polymerization, and the activity decreases in the follow of cat.5>cat.2≈cat.4>cat.1≈cat.3. This result indicates that the activity of the Schiff base aluminium compleses and the microstrucre of polylactide could be controlled by the design of the ligand structure and the control of the reaction condition.