Synthesis and structures of aluminum ion-pair complexes that act as L- and racemic-lactide ring opening polymerization initiators

Abstract

Seven aluminum ion-pair complexes (1–7) were synthesized by reaction of NNO Schiff base ketoimines with differing substituents with tris(2,6-dimethylphenoxy) aluminum dimer in an inert atmosphere glovebox at room temperature. The complexes were characterized with 1H, 13C, and 19F nuclear magnetic resonance spectroscopy, absorbance spectroscopy, and elemental analysis. Furthermore, the dinuclear nature of the complexes was elucidated with single crystal X-ray crystallography and 27Al NMR. The ion-pair complexes consisted of a bis-ligating homoleptic ketoiminate with octahedral coordination as an aluminum cation complex and a tetrahedral aluminum anion complex of 2,6-dimethylphenoxide. Each ion-pair was used to initiate the ring opening polymerization (ROP) of L- and rac-lactide to poly-lactic acid (PLA) in toluene at 100 °C at a 100:1 [LA]:[ion-pair] ratio. Proton NMR spectra showed percentage conversions of monomer to polymer of 96–99% in 18 h. The isolated polymeric materials were determined to have moderate molecular weights with low to moderate polydispersity index values using gel permeation chromatography. The bis-ligated aluminum cation and tetraphenoxide aluminate anions of 1 were isolated into separate compounds (8 and 9) with unreactive counter ions, and each yielded low molecular weight PLA under the polymerization conditions. Thus, lactide polymerization appeared to require both ion-pair components to yield the moderately high molecular weight PLA, and this was investigated spectroscopically.