Phenoxy–Thioether Aluminum Complexes as -Caprolactone and Lactide Polymerization Catalysts

A series of new phenoxy-thioether (OS) proligands havebeen synthesized. They were found to readily react with 1 equiv of AlMe3to 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)) inquantitative yields. All the aluminum methyl complexes are stablemonomeric species. In the solid state, as determined from X-raycrystallographic studies, complex 2 consists of a four-coordinatealuminum species in which the metal center is chelated by the sulfurand oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and L- andrac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of thecyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of rac-LA wereconverted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by 1−4 are first order with respect tomonomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on thepolymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the orthoposition 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.