Ring-opening polymerization of lactide initiated by magnesium and zinc alkoxides

A mono methylether Salen-type ligand, SalenMe-H (1) is prepared in a one flask reaction by condensation of trans-1,2-diaminocyclohexane with 2-methoxybenzenaldehyde and followed by the addition of 2,4-di-tert-butylsalicylaldehyde. Further reaction of 1 with Mg(OBn)2 in THF produces a magnesium alkoxide, [(SalenMe)Mg(OBn)]2 (2). Compound 1 reacts with ZnEt2 yields monomeric complex (SalenMe)ZnEt (3), which further reacts with 1 molar equiv of benzyl alcohol giving [(SalenMe)Zn(OBn)]2 (4). Experimental results show that complexes 2 and 4 efficiently initiate the ring-opening polymerization of l-lactide and rac-lactide in a controlled fashion, yielding polymers with very low polydispersity indexes. Kinetic studies show a second-order dependency on [LA] and a first-order on [2] with magnesium complex 2 as an initiator. While zinc complex 4 is used as an initiator, the polymerization rate has a first order dependency on both [LA] and [4].