Comparison of Catalytic Activities of Aluminum Complexes with Sulfur and Oxygen Containing Ligands for the Ring-Opening Polymerization of ε‑Caprolactone

A series of aluminum complexes bearing various ligands, including tert-butanolate, tert-butyl thiolate, 2,6-dimethylphenolate, 2,6-dimethylbenzenethiolate, 2,6-di-tert-butyl-4-methylphenolate, 2,6-di-trimethylsilylbenzenethiolate, furan-2-ylmethanolate, thiophen-2-ylmethanolate, pyridin-2-olate, pyridine-2-thiolate, quinolin-8-olate, and quinoline-8-thiolate were synthesized and assessed for their catalytic activity in the ring-opening polymerization of ε-caprolactone (CL). Polymerization results revealed that SSi-Al exhibited a significantly higher catalytic activity ([CL] : [SSi-Al] : [BnOH] = 100 : 1 : 2; [CL] = 2 M; conv. = 92% at 25 °C after 35 min with kobs = 0.0733 min-1) compared to OBHT-Al (kobs = 0.0196 min-1), with a 3.7-fold increase in kobs values. Other sulfur-containing Al complexes also demonstrated superior catalytic performance compared to their oxygen-containing counterparts: 7.1-fold (Sdm-Alvs.Odm-Al), 2.9-fold (StBu-Alvs.OtBu-Al), 10.0-fold (SPy-Alvs.OPy-Al), 2.2-fold (Sthio-Alvs.Ofu-Al), and 16.4-fold (SQu2-Alvs.OQu2-Al). Although StBu-Al demonstrated the highest catalytic activity, its capacity to control CL polymerization was found to be limited. In contrast, the Al complexes bearing quinoline type ligands revealed the lowest catalytic activity. Sdm-Al revealed the second highest catalytic activity and provided good controllable CL polymerization. These findings demonstrate that incorporating sulfur into ligand structures may enhance the catalytic performance of Al complexes in CL ring-opening polymerization.