Comparative study of lactide polymerization by zinc alkoxide complexes with a β-diketiminato ligand bearing different substituents

A series of β-diketiminate zinc complexes has been synthesized and their reactivity for the ring-opening polymerization (ROP) of lactide has been studied. The reaction of β-diketimines (LH) with diethyl zinc forms the monomeric [LZnEt] complexes which further react with benzyl alcohol (BnOH) in toluene/hexane yielding dinuclear or trinuclear zinc complexes. These complexes have been characterized by single crystal X-ray diffraction, which showed the tri- and tetra-coordinated zinc complexes in which zinc atoms exhibit trigonal and tetrahedral geometry, respectively. All complexes have been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is heavily dependent on the N-aryl substituents with the order: alkyl group ∼ alkoxy > halide group > nitro group. The β-diketiminate zinc complexes allow controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight. On the basis of literature reports, a mechanism for ROP of lactide has been proposed.

Generalized mechanism for ROP of lactide (EDG = electron donating group and EWG = electron withdrawing group).Figure optionsDownload high-quality image (145 K)Download as PowerPoint slideHighlights
► The β-diketiminate zinc alkoxide complexes have been synthesized.
► Their reactivity for the ring-opening polymerization (ROP) of lactide has been studied.
► The electron withdrawing substituents have an inclination to become dinuclear forms.
► The rate of polymerization order: alkyl group ∼ alkoxy > halide group > nitro group.