Efficient initiators for the ring-opening polymerization of L-lactide: Synthesis and characterization of NNO-tridentate Schiff-base zinc complexes

A series of efficient catalysts, based on zinc alkoxides coordinated with NNO-tridentate Schiff-base ligands ((LH)-H-1-(LH)-H-6), for ring opening polymerization Of L-lactide have been prepared. The reactions of diethyl zinc (ZnEt2) with (LH)-H-1-(LH)-H-6 yielded [(mu-L)ZnEt](2) (1a-6a), respectively. Further reaction of compounds 1a-6a with benzyl alcohol (BnOH) produced the corresponding compounds of [LZn(mu-OBn)](2) (1b-6b), respectively. X-ray crystal structural studies reveal that all of these compounds 1a-6a are dimeric bridging through the phenolato oxygen atoms of the Schiff-base ligand. However,, the molecular structures of 1b-6b show a dimeric character bridging through the benzylalkoxy oxygen atoms. Ring-opening polymerization of L-lactide, initiated by 1b-6b, proceeds rapidly with good molecular weight control and yields polymer with a very narrow molecular weight distribution. Experimental results show that the substituents on the imine carbon of the NNO-ligand affect the reactivity of zinc complexes dramatically. (C) 2008 Wiley Periodicals, Inc.