Phenoxy-Thioether Aluminum Complexes as ε‑Caprolactone and Lactide Polymerization Catalysts

A series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe3 to afford the corresponding Al chelate complexes {4,6-tBu2-OC6H2-2-CH2S(2-R-C6H4)}AlMe2 (R = H (1), Br (2), CH3 (3), CF3 (4)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex 2 consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and L- and rac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of rac-LA were converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by 1−4 are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide.