Synthesis Of Unsymmetrical Salphen-Al And Urea/Alkoxides Complexes And Stereoselective Ring-Opening Polymerization Of Rac-Lactide

Polylactide?PLA?,as a biodegradable polyester material,has broad application prospects in the filed of biomedicine,food packaging and agricultural production.At present,polylactide with high molecular weight and narrow molecular weight distribution is mainly obtained by ring-opening polymerization of metal-organic catalyst,but problems such as metal residues and slow polymerization rates do exist with this approach.Meanwhile there are few catalyst that can achieve stereoselective ring-opening polymerization,so the development of both high efficiency,controllability,low toxicity,high stereoselectivity of the catalyst is of great significance.In this paper,unsymmetric salphen aluminum complexes and urea-alkoxide complexes were designed and synthesized and the stereoselective ring-opening polymerization of racemic lactide was systematically studied.The main contents are as follows:In the first part of our work,we use o-phenylenediamine and salicylaldehyde with different substituents as raw material to synthesize unsymmetric salphen aluminum complexes:^tBu-PhLAlMe.L=?3,5-^tBu-1-OH-C₆H₂?CH=N-C₆H₄-N=CH?3-Ph-1-OH-C₆H₃?(^tBu-PhLH₂)and the comparative symmetric Salphen aluminum complexes:^tBuLAlMe.L=?3,5-^tBu-1-OH-C₆H₂?CH=N-C₆H₄-N=CH?3,5-^tBu-1-OH-C₆H₂?(^tBuLH₂)and ^PhLAlMe.L=?3-Ph-1-OH-C₆H₃?CH=N-C₆H₄-N=CH?3-Ph-1-OH-C₆H₃?(^PhLH₂).Through the methane elimination reaction,the above three methylaluminum complexes were reacted with benzyl alcohol to synthesize the corresponding benzyloxy aluminum complexes:^tBu-PhLAlOBn,^tBu-LAlOBn,^PhLAlOBn.When the ligand(^tBu-PhLH₂)was reacted with aluminum isopropoxide to obtain the monometallic aluminum complex:^tBu-PhLAlOⁱPr,it was found that the complexes of two bridged dimer structures were produced:?-O₂-(cis-^tBu-PhLAlOⁱPr)₂ and?-O₂-(trans-^{t Bu-Ph}LAlOⁱPr)₂.The above complexes were characterized by ¹H NMR,¹³C NMR,elemental analysis and X-ray diffractions and the preliminary investigation of the ring-opening polymerization of racemic lactide showed that the greater steric hindrance of benzene ring,the higher stereoselectivity of racemic lactide,but the polymerization rate will slow down.In the second part of our work,we use urea-alkoxide as organic catalyst system to investigate stereoselective ring-opening polymerization of racemic lactide.We synthesized a series of ureas successfully,which can reacted with different alkoxides to form a binary catalytic system based on hydrogen bond.Through the study of stereoselective ring-opening polymerization of racemic lactide,we founded that the catalytic system can achieve high efficiency?over 90%conversion in 1-2 mints?,controllability?narrow molecular weight distribution,end group fedeity,chain extension experization?,high stereoselectivity?P_i up to 0.93?simultaneously.