Highly efficient magnesium initiators for lactide polymerization

Two monomeric, six-coordinated magnesium complexes with bulky aminophenolate ligands [(Htbpoa)(2)Mg] (1) and [(Htbpca)(2)Mg(THF)(2)] (2), where Htbpoa = N-[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-methyl-1,3-dioxolaneamine and Htbpca = N-[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-cyclohexylamine have been prepared, characterized and employed as initiators for lactide polymerization. The crystal structure of the homoleptic compound I has been determined and shows that the six-coordinate magnesium atom in 1 is surrounded by two tridendate tbpoa ligands. In the solution, however, complex 1 exists in equilibrium with a five-coordinate species la having one oxolane fragment dangling. The tbpoa and tbpca ligands in I and 2 play a dual role, as the ancillary ligand stabilizing the monomeric magnesium species and as the initiating polymerization group.