Alkaline-Earth Metal Complexes:Synthesis,Characterization And Catalytic Properties Towards Polymerization Of Lactide

Two kinds of novel and efficient alkaline-earth metal amido complexes were synthesized via amine elimination reaction.The catalytic behaviors of these Complexes for the ring-opening of rac-Lactide(rac-LA)were investigated.These complexes were characterized by NMR spectroscopy and single crystal X-ray diffraction analysis.All these complexes showed extremely high activity as initiators for the ROP of rac-LA in a constant feeding ratio of[rac-LA]:[Catalyst]=200:1,For example,employing complex 1 as the initiator,the polymerization proceeded to a 89%yield in 2 min,even at low temperature,79%PLA was obtained with complex 2 as the initiator in 9h.Those complexes both show no stereoselectivity toward the ROP of rac-Lactide at room temperature.But at low temperature(-75?),For complex 1,the polymerization afforded PLA with P_r=0.84,whereas for complex 2,the isotacticity of the resultant PLA improved up to P_m=0.78.Through the observation of Temperature-varying 1H NMR spectrum,we concluded that OMe and the central metal have a rapid coordination dissociation process at room temperature,and this process is suppressed at low temperatures.