Comparison of catalytic activities of aluminum complexes with sulfur and oxygen containing ligands for the ring-opening polymerization of ε-caprolactone

Abstract

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 S^Si–Al exhibited a significantly higher catalytic activity ([CL] : [S^Si–Al] : [BnOH] = 100 : 1 : 2; [CL] = 2 M; conv. = 92% at 25 °C after 35 min with k_obs = 0.0733 min⁻¹) compared to O^BHT–Al (k_obs = 0.0196 min⁻¹), with a 3.7-fold increase in k_obs values. Other sulfur-containing Al complexes also demonstrated superior catalytic performance compared to their oxygen-containing counterparts: 7.1-fold (S^dm–Alvs.O^dm–Al), 2.9-fold (S^tBu–Alvs.O^tBu–Al), 10.0-fold (S^Py–Alvs.O^Py–Al), 2.2-fold (S^thio–Alvs.O^fu–Al), and 16.4-fold (S^Qu₂–Alvs.O^Qu₂–Al). Although S^tBu–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. S^dm–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.