Bis(imino)phenoxide complexes of zirconium possess spectacular reactivity towards the solvent-free polymerization of cyclic ester monomers and lactides. Polymerization of lactide is living and the resulting product has exceptionally high number average molecular weights (Mn). Polymerization of β-butyrolactone gives syndiotactic poly(3-hydroxybutyrate) with good Mn and molecular weight distribution (MWD).
The synthesis of titanium(IV), zirconium(IV) and hafnium(IV) complexes of Trost's semi-crown ligand is described. All complexes were fully characterized by 1H and 13C NMR and mass spectroscopy. The molecular structures of the representative complexes 2, 3 and 4 were determined by single-crystal X-ray diffraction studies. The X-ray diffraction studies reveal that complexes 2, 3 and 4 crystallized in an orthorhombic crystal system. Complexes 2 and 3 have a monomeric structure in the solid state with distorted octahedral geometry around the metal center, whereas complex 4 was found to crystallize in a trimeric structure bridging with an oxygen atom, where the geometry around the titanium is distorted trigonal bipyramidal. The activities and stereoselectivities of these complexes toward the ring-opening polymerization (ROP) of lactides (L-LA and rac-LA) and ε-caprolactone (CL) have been investigated. Complexes 1–4 were found to be efficient single-component initiators for the ring-opening polymerization of cyclic esters and yielded high molecular weight polymers (Mn) with narrow molecular weight distributions (MWD).The microstructure of the resultant polylactides (PLAs) from rac-LA was determined. Complexes 2 and 3 afforded isotactic-enriched PLA (Pm = 0.78–0.71) with narrow MWD (1.07–1.04), on the other hand complexes 1 and 4 produced atactic PLA. Kinetic and post polymerization studies confirm that the polymerization proceeds through the coordination–insertion mechanism.
A series of homoleptic iminophenoxide and bis(imino) phenoxide supported Zr and Hf complexes 1-10 has been synthesized and characterized by NMR, mass spectrometry and elemental analysis. Structural analysis of the selected complexes by single crystal X-ray diffraction method revealed that the complexes are monomeric in their solid state with distorted dodecahedral geometries. These complexes are found to be potential single-site catalysts for ring-opening polymerization (ROP) of lactide (LA) and e-caprolactone (e-CL) and afforded controlled molecular weight (M n ) polyesters with narrow molecular weight distributions (MWDs) of 1.12-1.28. Kinetic studies revealed that the polymerization rate have shown to be first order dependency on lactide. The low molecular weight oligomer analysis imparted that the polymerization proceeds through the coordination-insertion mechanism.
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