Dialkylaluminium 2-imidazolylphenolates: Synthesis, characterization and ring-opening polymerization behavior towards lactides

The stoichiometric reaction of the 2-imidazolylphenols (L1–L9) with the trialkylaluminium reagents AlR3 (R = Me, Et and iBu), afforded the corresponding dialkylaluminium 2-imidazolylphenolate complexes [R2Al(L1–L9)] (C1–C11), which were characterized by 1H/13C NMR spectroscopy and by elemental analysis. The molecular structures of the representative complexes C1, C2, C4, C6 and C11 were determined by single-crystal X-Ray diffraction, and revealed a distorted tetrahedral geometry at aluminum. These dialkylaluminium 2-imidazolylphenolates (C1–C11) could efficiently catalyze the ring-opening polymerization of lactides to afford high molecular weight polylactide, both in the presence and absence of BnOH, and as such represent rare examples of the use of bi-dentate ligation at aluminum in such lactide polymerization systems. On the basis of the polymerization results for l-lactide, d-lactide and rac-lactide, the nature of the ligands and the aluminum bound alkyls were found to significantly affect the catalytic activity as well as the properties of the resultant polylactides.