Enhanced Catalytic Activity of Aluminum Complexes for the Ring-Opening Polymerization of ε-Caprolactone

A series of dinuclear aluminum (Al₂Pyr₂) complexes bridged by two pyrazole ligands were synthesized, and their catalytic activity toward ring-opening polymerization of ε-caprolactone (CL) was investigated. Different types of the Al-N-N-Al-N-N skeletal ring were found among these Al₂Pyr₂ complexes. The butterfly form, L^Thio₂Al₂Me₄, exerted the highest catalytic activity for CL polymerization. κ²-CL coordination with both Al centers within the butterfly form L^Thio₂Al₂Me₄ facilitates the initiation process. Generally speaking, the Al₂Pyr₂ complexes exhibited substantially higher catalytic activity for CL polymerization than literature examples such as β-diketiminate- or traiaza-bearing aluminum complexes. In fact, the Al₂Pyr₂ complexes can even carry out CL polymerization at room temperature.