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

A series of lithium, sodium, potassium, magnesium, calcium and zinc azonaphthoxide complexes [M((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((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.