Synthesis Of Unsymmetrical Multidentate Phenolate Zinc, Magnesium Complexes And Their Catalytic Application In The Ring-Opening Polymerization Of Rac-Lactide

Five new unsymmetrical multidentate phenols have been synthesized. Based upon these ligands framework, totally five zinc (II) and magnesium (II) complexes have been synthesized and characterized, which were further used to catalyze the ring-opening polymerization (ROP) of rac-lactide.Firstly, five new unsymmetrical multidentate phenols L1-L5 were obtained and fully characterized with 1H NMR,13C NMR spectroscopy and elementary analysis (EA), L1-L4 possess craw-type framework, L5 is linear. The reaction of these ligands with Zn[N(SiMe3)2]2 or Mg[N(SiMe3)2]2 afforded the corresponding zinc and magnesium complexes bearing multidentate phenolate ligand. Zinc complexes Z2, Z3 and magnesium complex M3 were characterized with 1H NMR,13C NMR spectroscopy and elementary analysis (EA). Due to the poor solubility of complexes Z4, M4 in common deuterated solvents, they were characterized with EA and MS. Complexes Z2, Z3, M3 and M4 are silylamido complexes with one multidentate phenolate ligand, while complex Z4 possesses two phenolate ligands.These complexes were used to catalyze the ring-opening polymerization (ROP) of rac-lactide, and it was found that all of these complexes except for Z4 are good initiators for the ROP of rac-latide, which show enhanced activity upon addition of isopropanol. In THF at 25℃, without addition of isopropanol, complexes Z2 and Z3 show similar catalytic activities; with the addition of isopropanol, the catalytic reactivity of Z2 is enhanced significantly in comparison with Z3. The catalytic reactivity of magnesium complexes was always M3>M4. This might be due to the increased steric effect of M4, leading to the decrease of catalytic activity. For a given ligand, magnesium complex was more active but less isotactic selective for the ROP of rac-lactide when compared to the corresponding zinc complex.