Comparative Study of Lactide Polymerization with Lithium, Sodium, Potassium, Magnesium, Calcium, and Zinc Azo­naphthoxide Complexes

Graphical Abstract

Lithium, sodium, potassium, magnesium, calcium and zinc complexes stabilized by monoanionic bidentate and tridentate azonaphthoxide ligands are synthetized. Although the extremely active alkali-metal complexes follow an activated-monomer mechanism, the alkaline-earth and zinc complexes follow a coordination–insertion mechanism assisted by the ligand.

Abstract

A series of lithium, sodium, potassium, magnesium, calcium and zinc azonaphthoxide complexes [M(κ^m-{2-O(C₁₀H₆)N=N(C₆H₄R)})]_n (R = H, m = 2; M = Li 1, Na 2, K 3; R = OMe, m = 3; M = Li 4, Na 5, K 6) and [M(κ^m-{2-O(C₁₀H₆)N=N(C₆H₄R)})₂]_n (R = H, m = 2; M = Mg 7, Zn 8, Ca 9; R = OMe, m = 3; M = Mg 10, Zn 11, Ca 12) have been synthesized and fully characterized. The solid-state molecular structures of the zinc complexes 8 and 11 and the magnesium derivative 10 have been determined by X-ray diffraction studies. The alkali-metal compounds 1–6 are extremely active catalysts for the ring-opening polymerization (ROP) of L-lactide. A catalytic-activity study has been performed by varying the metal complex and the coordination sphere of the metal in the presence and absence of a coinitiator. Stoichiometric reactions involving the catalysts, BnOH and lactide have been performed to elucidate mechanistic aspects related to the polymerization of L-lactide.