Aluminum Complexes Bearing Bidentate Amido–Phosphine Ligands for Ring-Opening Polymerization of ε-Caprolactone: Steric Effect on Coordination Chemistry and Reactivity

The aluminum complexes LAlMe2 (Al1–Al5: LH = 2,6-(R1)2-4-R2-C6H2NHCH2C6H4-2-PPh2; Al1, R1 = H, R2 = H; Al2, R1 = Me, R2 = H; Al3, R1 = iPr, R2 = H; Al4, R1 = Ph2CH, R2 = iPr; Al5, R1 = Cl, R2 = H) have been synthesized and characterized by elemental analysis and 1H, 13C, and 31P NMR. NMR analysis in solution reveals an interesting hemilabile coordination of the soft P donor. The molecular structures of Al2–Al4 were defined by X-ray diffraction studies, showing a distorted-tetrahedral geometry around the aluminum center in all structures. Careful comparison of these crystal structures suggested that different substituents on the ligands could lead to unignorable changed coordination environments around the Al center, thus affecting their catalytic properties. In the presence of BnOH, complexes Al1–Al5 efficiently catalyzed the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with high conversions in a controlled manner, and high molecular weights (Mn up to 118.6 kg mol–1) of polycaprolactones (PCLs) were readily prepared. Immortal polymerizations by Al4 having bulky Ph2CH groups were also studied with up to 20 equiv of alcohols and 2000 equiv of monomers, without sacrificing polymerization control.