Zr And Ti Complexes Containing Multidentate Aminophenolate Ligands: Synthesis, And Catalytic Activities In Ring-Openging Polymerization Of Lactide

The development of homogeneous catalysts/initiators for synthesizing ecofriendly biodegradable polymers,such as poly?lactic acid??PLA?,poly?caprolactone??PCL?and so on,to tackle the environmental pollution caused by abandoned waste composed of petrochemical materials,from renewable resources is an attractive area of research.In recent years,such polymers have been extensively utilized as drug delivery excipients,adsorbable sutures,medical implants and scaffolds for tissue engineering.Ring-opening polymerization?ROP?of lactides is considered as the most suitable route to produce PLA.Titanium complexes?LM?OⁱPr?₂,L =-OC₆H₂-4-R¹-6-R₂-2-CH₂-N[?CH₂?₃-N?CH₂CH₂?₂O]-CH₂-4-R¹-6-R²-C₆H₂O-,R¹ = Me,R² = ^<sup>tBu,M = Ti,complex 1;R¹ = R² = ^tBu,M = Ti,complex 2?based on symmetrical amino-bis?phenolate?s ligands were synthesized.Moreover,zirconium complexes?L₂M,L =-OC₆H₂-4-R¹-6-R²-2-CH₂-N[?CH₂?₃-N?CH₂CH₂?₂O]-CH₂-4-R¹-6-R²-C₆H₂O-,R¹ = Me,R² = ^tBu,M = Zr,complex 3;R¹ =R² = ^tBu,M = Zr,complex 4?containing two symmetrical amino-bis?phenolate?s ligands were prepared.These complexes were characterized by ¹H,¹³ C NMR spectroscopy and elemental analyses.The solid state structure of complex 1 and complex 4 was determined by X-ray diffraction.The variable temperature ¹H NMR of complexes 2 were examined,which indicated rotation of pendant arm probably caused the change of peaks.Complexes 1,2,3,4 were highly active initiators for the ring-opening polymerization of L-lactide?L-LA?and rac-lactide?rac-LA?.Experimental results show that complexes 1 and 2 are highly active and excellent catalysts for immortal ring-opening polymerization?i ROP?of L-lactide and rac-lactide in the melt.The monomers to complex ratio reached to 200 000/1,which is the highest feeding molar ratio of lactide,to the best of our knowledge so far.Meanwhile,narrow polydispersity index?1.11–1.58?of polymers were obtained,the numbe-average molecular weight approached 8.64×10⁴ g/mol.The turnover frequency?TOF?reached to 6593 h^-1,and the polymerization proceeded smoothly,even in an air environment.Moreover,we verified that the mechanism of the polymerization initiated by complexes 1 & 2 was coordination-insertion mechanism by OⁱPr via ¹H NMR,MALDI-TOF MS of polymers.We inferred that the mechanism of the polymerization catalyzed by complexes 3 & 4 was coordination-insertion mechanism initiated by ligand in the absence of Bn OH and by Bn O-group in the presence of Bn OH.