Air-Stable Salen–Iron Complexes: Stereoselective Catalysts for Lactide and ε‑Caprolactone Polymerization through in Situ Initiation

A series of iron­(III) chloride complexes based upon Schiff base framework have been synthesized and characterized by mass spectra, elemental analysis, and X-ray crystallography. These bench-stable complexes were for the first time capable as highly efficient catalysts for lactide and ε-caprolactone polymerization in the presence of propylene oxide (PO), greatly surpassing conventional aluminum analogies. Electron-withdrawing substituents as well as elevated temperature boosted the activity while a bulky group on salicylaldehyde moieties abnormally produces the same effect, whereas rigid backbone retarded the reactivity. Polylactide tactics ranging from isotactic to hererotactic enchainment were obtained by tuning the ligand backbone and substituents. The stereoselectivity was confirmed to proceed via a chain-end control mechanism by kinetic studies using different isomers of lactide, and the overall polymerization process was also investigated in detail by the oligomer mass spectrum as well as end group (−OCHMeCH2Cl) analysis of polymer via 1H, 13C, and two-dimensional (2-D) NMR characterizations.