Synthesis And Catalytic Properties Research Of Heterocyclic Schiff-base Rare Earth Complexes

There is a growing interest in polycaprolactone and polylactone as their biodegradability, biocompatibility and low toxicity. They have been used in biomedicine as surgical suture, nerve catheter, skeletal support and fixtures, and as a delivery medium for the controlled release of drugs. An efficient way to synthesize those polyesters is the metal complexes catalyze ring-opening polymerization of lactones and carbonates; therefore, the development of simple, convenient and efficient metal complexes initiators for the ring-opening polymerization of cyclic esters is important from both practical and fundamental viewpoints. In this dissertation, several Schiff-base lanthanum complexes were synthesized. And their catalytic activity for the ring-opening polymerization of ε-caprolactone (CL) and 2, 2-dimethyltrimethylene carbonate (DTC) was investigated. It was found that lanthanum complexes supported by the Schiff base ligand (N-(2-pyridyl)-3, 5-di-tert-butyl-Salicylaldimine) has high catalytic activity for ring-opening polymerization of CL and DTC.The complex shows high catalytic activity for the ring-opening polymerization of CL: for example, when CL polymerization was carried out in toluene at 60°C with [CL]/[La] = 2000 and solvent volume/monomer volume = 2:1 for 2h, PCL with 96% conversion and Mv = 4.57× 10⁴ was obtained. Further study found that molecular weight increased linearly with the monomer conversion as polymerization time prolonged, it showed some characteristic of living polymerization. Mechanism study revealed that the polymerization proceeds via acyl-oxygen bond cleavage.Ring-opening polymerization of DTC has been carried out by using this initiator, the optimum reaction conditions were as follows: [DTC] = 2 mol/L, [DTC] / [La] = 500, T = 40℃, t = 40min, in toluene. But the molecular weight of PDTC obtained by this initiator was somewhat low, it was only 1～2 × 10⁴, possibly due to the incomplete ring-opening polymerization and transesterification. ¹H-NMR study revealed that PDTC obtained by this initiator didn't contain ether unit which resulted from CO₂ elimination.