Structural aspects and ring opening polymerization of ε-caprolactone using mono- and di-aluminum compounds incorporating bidentate pyrrole-morpholine ligands

A series of Al-compounds incorporating the bidentate pyrrole-morpholine precursor C₄H₃NH [2-CH₂N(CH₂CH₂)₂O] (1) and aluminum-alkyl have been conveniently synthesized and characterized by ¹H and ¹³C NMR spectroscopy. Reacting one or two equivalents of 1 with AlMe₃ in toluene afforded AlMe₂{C₄H₃N [2-CH₂N(CH₂CH₂)₂O]} (2) and AlMe{C₄H₃N [2-CH₂N(CH₂CH₂)₂O]}₂ (3), respectively, in moderate yield. Furthermore, compound (4), AlMe₂{C₄H₃N [2-CH₂N(CH₂CH₂)₂O]}·AlMe₃, was acquired either by the combination of 2 and one equivalent of AlMe₃ or directly from the reaction between 1 and two equivalents of AlMe₃. The molecular structures of 1, 2, and 4, were determined by single crystal X-ray diffractometry in solid state, showing a distorted tetrahedral arrangement around the Al atom in 2 and 4. All the aluminum compounds could effectively initiate the ring-opening polymerization (ROP) of ε-caprolactone in a well-controlled manner with molecular weight distributions (PDI = 1.03-1.49).