Anionic Iron(II) Alkoxides as Initiators for the Controlled Ring-Opening Polymerization of Lactide

Hetero-bimetallic Fe(II) alkoxide/aryloxides were evaluated as initiators for the ring-opening polymerization of rac-lactide. [(THF)NaFe(OtBu)3]2 (1) and [(THF)4Na2Fe(2,6-diisopropylphenolate)4] (2) (THF = tetrahydrofuran) both polymerized lactide efficiently at room temperature, with complex 1 affording better control over the molecular weight parameters of the resultant polymer. At conversions below 70%, a linear increase in molecular weight with conversion was observed, indicative of a well-controlled polymerization process. Complex 2 is the first example of a dianionic Fe(II) alkoxide and has been structurally characterized to reveal a distorted square planar FeO4 array in which both Na counterions bridge two aryloxide ligands and are further complexed by two THF ligands.