Synthesis of Aluminum Complexes Based on 2,6-Bis(2-hydroxyphenyl)Pyridines: Efficient Initiators for Ring-Opening Polymerization of Cyclic Esters

The search for new initiators based on nontoxic metal complexes, allowing for the controlled production of polycaprolactone (PCL) and polylactide (PLA) is an actual task. In the present work, we report the synthesis of aluminum complexes 6a and 6b based on substituted 2,6-bis(2-hydroxyphenyl)pyridine pro-ligands. The compositions and structures of the novel compounds were established by elemental analysis and 1H, 13C, NMR spectroscopy in the solid state by X-ray diffraction analysis (6a and 6b). All the synthesized aluminum complexes are monomeric. Complexes 6a and 6b (in presence of benzyl alcohol (BnOH)) turned out to be active in the ring opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL), L-, rac-lactide (L-, rac-LA) gave PCL and PLA with high molecular weights. The mechanism of the polymerization reaction for compounds & Scy;, 6a, and 6b was also investigated using the density functional theory (DFT) method. The substituent effect analyzed in terms of percent buried volume and non-covalent interactions. The values of calculated Gibbs free activation energies correspond to the trend found experimentally.