| Currently, environment pollution caused by the hardness and long period of degradation of the polymers increasingly attracts people’s attention. While at the same time, the preparation of polymers made by petroleum is facing great challenges because the fossil flues based on the petroleum is running out. Therefore, the development of the biodegradable, environment-friendly, low cost polymer materials will become the trend in the future.Polylactide (PLA) is considered as the new "ecological material" because of its wide source, low cost, biodegradation, excellent biocompatibility, non-toxicity and harmlessness. As we all known, Lactide has three different stereoisomers:D-lactide, L-lactide and meso-lactide. At the same time, rac-lactide is composed of equivalent D-lactide and L-lactide, while PLA has many kinds of stereoisomers which have a great influence on its physical properties. For instance, stereoblock-PLA has a regular arrangement back bone so it has a high degree of crystallinity and good mechanical performance, and naturally it has a wider application. Presently, literatures about the efficient catalysts for the ROP of rac-lactide to stereoblock-PLA are mainly limited to aluminum and indium complexes, while they often have low catalytic activity.As is mentioned above, we designed tetradentate [ONNO] ligand in this paper. In order to achieve higher catalytic activity of the polymerization, we introduced hydrogen, tert-butyl and phenyl as substituents to ortho-position of phenolic hydroxyl group. Meanwhile, the length of carbochain between two nitrogen atoms was designed as three-CH2-. Then, three kinds of different substituent tetradentate [ONNO] ligands were prepared by some conventional organic synthesis methods, Suziki coupling, group protection and removal, high pressure reduction, Schlenk line, etc. At last, we successfully got target ligand with purity over 99%, and 1H NMR,13C NMR and MS data were obtained to characterize all the synthesized compounds.After synthesizing and characterizing tetradentate [ONNO] ligand, we further obtained four unreported yttrium complexes by reacting these ligands with alkyl yttrium and amino yttrium, which were measured by 1H NMR. Meanwhile, coordination polymerization of D-lactide and rac-lactied were carried out by using these yttrium complexes in certain reaction conditions. Experimental results indicated that these yttrium complexes are highly active for the ring-opening polymerization of rac-lactide. |
