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

A series of dinuclear aluminum (Al(2)Pyr(2)) complexes bridged by two pyrazole ligands were synthesized, and their catalytic activity toward ring-opening polymerization of e-caprolactone (CL) was investigated. Different types of the Al-N-N-Al-N-N skeletal ring were found among these Al(2)Pyr(2) complexes. The butterfly form, (L2Al2Me4)-Al-Thio, exerted the highest catalytic activity for CL polymerization. K-2-CL coordination with both Al centers within the butterfly form (L2Al2Me4)-Al-Thio facilitates the initiation process. Generally speaking, the Al(2)Pyr(2) complexes exhibited substantially higher catalytic activity for CL polymerization than literature examples such as beta-diketiminate- or traiaza-bearing aluminum complexes. In fact, the Al(2)Pyr(2) complexes can even carry out CL polymerization at room temperature.