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.