Fast and living ring-opening polymerization of ε‑caprolactone by aluminum complexes bearing amino-quinoline ligands

Catalysts drive innovation in sustainable polymers because of their reactivity and selectivity. In this context, the synthesis and characterization of a series of Al complexes (Al1 − Al5) bearing N,N-bidentate amino-quinoline ligands are described, as well as their applications in ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The amido-coordination Al complexes were directly synthesized by reactions of imino-quinoline compounds with AlMe3 via methyl migration from Al to the carbon atom of imine, as indicated by both NMR spectroscopy and X-ray diffraction. In the presence of BnOH, complexes Al1 − Al5 showed significant reactivity for ROP of ε-CL with good control over molar mass (Mn) and dispersity (Đ < 1.2), and polycaprolactones (PCLs) with high molecular weights (Mn up to 129 kDa) were readily synthesized. Remarkably, immortal catalysis with excess of BnOH (up to 20 equiv. to Al) was achieved to prepare PCLs with predictable Mns, low dispersities, and end groups using an extremely low loading of Al complex (0.05 mol% to ε-CL monomer).