Synthesis and Characterization of Magnesium Alkoxides Incorporated into Bulky Aluminium Tetraphenolate Helices and Application in the Ring-Opening Polymerization of Lactides

Two aluminium and magnesium heterobimetallic complexes of [(LAl)MgOBn]2 (2) and [(LAl)Mg(OC2H4OCH3)]2 (3), in which L = N1,N1,N2,N2-tetrakis(2-hydroxy-3,5-dimethylbenzyl)-1,2-ethanediamine, have been synthesized and characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. The magnesium alkoxide of complex 2 supported by the bulky aluminium tetraphenolates, as a rare helical heterobimetallic initiator, can effectively initiate the ring-opening polymerization (ROP) of L-lactide (L-LA) leading to polymers with good molecular weight control and narrow molecular weight distributions. Kinetic studies have shown the overall rate expression is -d[lactide]/dt = k[lactide][complex] for the ROP of L-LA. Furthermore, complex 2 shows modest isotactic selectivity in the ROP of rac-lactide.