Synthesis Of Group-4-metal And Indium Complexes Containing Chiral N Atoms, And Their Catalytic Properties For Polymerization Of Lactide

Development of Lewis acid catalysts for polymerization of lactide (LA) is a lasting interest over past two decades as polylactide (PLA) is a biodegradable and biocompatible polyester derived from biorenewable feedstocks. Stereochemistry of polylactide is one of the main factors that determine the physical and mechanical properties of a polymeric material, as well as its rate of chemical and biological degradation. A promising and practical strategy for the convenient production of the stereoregular polymers is stereoselective ring-opening polymerization (ROP) of D,L-LA. Most known catalysts do not bias towards different enantiomers in the ROP of D,L-LA, therefore furnishing atactic amorphous PLA. Many efforts have been made to develop catalysts for stereoselective ROP of D,L-LA, while the reports on efficient catalysts are limited to Salen and Salan aluminium complexes.The investigation of group-4-metal complexes remains an interesting topic for decades, due to their remarkable catalytic properties in polymerization and in some special reactions Although group-4-metal complexes have been used as initiators for ROP of D,L-LA, most reported complexes displayed good catalytic activity with no stereoselectivity.In this dissertation, we report the preparation and structures of group-4-metal complexes (1-6) containing the unsymmetric tertiary amine ligands (H2L1-E2L3). The hydroxy- and phenoxy-bridged dinuclear Zr and Hf complexes 2 and 3 were prepared from the reaction of Na2L2 and MC14 (M= Zr or Hf) in the presence of 0.5 equiv of water in THF, while the reaction of H2L2 with NaH and then with ZrCl4 in the freshly distilled THF gave an eight-coordinate bis(homoleptic) mononuclear complex 1. When TiCl4 was adopted as reactant with the identical procedure, an oxo-bridged symmetric dinuclear Ti complex 4 was obtained instead of the hydroxy- and phenox-bridged dinuclear complex like 2 and 3.Two homochiral N atoms were created in each of the complexes 2 and 3 due to the restriction on the conformation inversion of the unsymmetric tertiary amine upon coordination. Complexes 2 and 3 both undergo spontaneous resolution (conglomeration) during crystallization. To further confirm the chirality of the dinuclear complexes, CD spectra of N(R)N(R)-2 and N(S)N(S)-2 were recorded, respectively. The mirror-image CD spectra, reflecting opposite Cotton effects, were observed for the individual grains of single crystals in the solid state. The CD spectra further support that spontaneous resolution occurs in crystallization and there is only one type of enantiomer in each single crystal. The complexes 2,3,4 and 5 are active in polymerization of D,L-LA. The racemates of 2 and 3 catalyzes controlled polymerization of D,L-LA, forming isotactic-rich polylactide in high yields with a narrow polydispersity. Conversions of the monomer to PLA reach 91-94% in 10 h with 2 or 3 as initiator at a D,L-LA/initiator molar ratio of 200. The narrow polydispersities (Mw/Mn= 1.01-1.13) and the linear correlation between the observed Mn and the conversion of monomer imply a living polymerization system. More evidence for the stereoselective polymerization of D,L-LA is given by the homonuclear decoupled]H NMR spectra of the methine region of the PLA samples derived from 2 and 3 (Pm= 0.65-0.73). Polymerization performed with enantiopure L-LA results in the entirely isotactic PLA and the epimerization in this catalytic system can be ruled out.To extend the study on the complexes of unsymmetric tertiary amine ligands and their catalytic properties, a dinuclear indium complex (7) was prepared. The X-ray analysis shows that complex 7 is an unusual C2-symmetric dinuclear indium complex with a sole water bridge. Complex 7 was used as initiator for ring-opening polymerization of D,L-LA, giving heterotactic-rich polylactide (Pr= 0.63-0.71). The ring-opening polymerization of D,L-LA was explored in the presence of air, water, and alcohol. When the polymerization was carried out in air, only a slight decrease in conversion of D,L-LA and in the Mn value of PLA was observed. The catalytic activity of 7 was not affected by an equiv of PhCH2OH or EtOH. In addition, improved polydispersities (Mw/Mn= 1.13-1.17) were obtained as compared to the PLA formed in the absence of alcohol (Mw/Mn= 1.23-1.31). Complex 7 keeps its catalytic activity even in the presence of an equiv of water in the reaction solution, affording heterotactic-rich PLA (Pr= 0.64).