Synthesis Of Novel Zinc Complexes Containing Salphen Ligands And Their Catalysis In The Ring-opening Polymerization Of ε-caprolactone

Polycaprolactone(PCL)is widely used in packaging,medicine and other fields as a significant bio-based degradable polymer material.The main method for synthesizing this material is the catalytic ring opening polymerization of caprolactone.Due to its low toxicity,environmental friendliness and excellent biocompatibility,zinc complexes has been becoming more attractive as catalysts in the ring-opening polymerization(ROP)of cyclic esters.In this study,we aim to design and synthsize active zinc catalysts for ROP process.Firstly,based on the framework of salphen Schiff base,we have synthesized two types of single-site zinc catalysts:bis-ligated zinc complexes and mono-ligated(salphen)Zn with different functional groups.All zinc compounds were characterized by 1H NMR,13C NMR,FT-IR and elementary analysis.The results of characterization showed that the structures are consistent with our expectations:the zinc center in bis-ligated complexes deduced to be six-coordinated and four-coordinated,respectively.Then,we tested the catalytic activity of all zinc complexes in ROP of ε-CL.(salphen)Zn complexes showed much lower catalytic activity,while all bis-ligated complexes were proven to be active.The further exploration of reaction conditions includes temperature,reaction time,type of initiator,ratio of monomer to catalyst,and solvent.The results show that at 80℃,in toluene solvent,the most active zinc catalyst could convert 1500-fold caprolactone monomer into polymer chain within 45 minutes with 93%conversion in the presence of BnOH.The resulting polymer has a Mn of 11.84 kg/mol and a PDI of 1.18.Increasing reaction temperature will accelerate the reaction but decrease the molecular weight of the PCL product.Coordinative solvent such as THF is not good for this reaction.The 1H NMR of polymer reveals the polymerization catalyzed by zinc complex follows a "coordination-insertion"mechanism.The synthesis route of zinc complexes in this paper are economical,simple and yield efficient,avoiding harsh experimental conditions in other reported processes.The obtained zinc complexes show good catalytic activity towards ROP of ε-CL in a relatively mild condition.