Ring-opening polymerization of rac-lactide catalyzed by Al(III) and Zn(II) complexes incorporating Schiff base ligands derived from pyrrole-2-carboxaldehyde

A series of Al(III) chloride [LAl-Cl]; Al(III) alkoxide [LAl-OR](2); and Zn(II) [LZn](2) complexes with Schiff base ligands were obtained. H-1 NMR and X-ray diffraction studies indicate that [LAl-Cl] complexes have C-s symmetry and the Al center is penta-coordinated. The Al(III) alkoxide complex [(LAl)-Al-5-(OPr)-Pr-i](2) is a dimer bridged by (OPr-)-Pr-i with the Al center in a distorted octahedral environment. Zn complexes [LZn](2) are double helix dimers with tetra-coordinated Zn centers. The catalytic activity for the ring-opening polymerization of rac-lactide was evaluated. The best activity in this series is shown by the aluminium chloride complex with a flexible three-carbon bridge; more flexible four-carbon bridges lower the activity.