Comparative Study of Aluminum Complexes Bearing N,O- and N,S-Schiff Base in Ring-Opening Polymerization of ε‑Caprolactone and l‑Lactide

A series of Al complexes bearing
Schiff base and thio-Schiff base ligands were synthesized, and their
application for the ring-opening polymerization of ε-caprolactone
(CL) and l-lactide (LA) was studied. It was found that steric
effects of the ligands caused higher polymerization rate and most
importantly the Al complexes with N,S-Schiff base showed significantly
higher polymerization rate than Al complexes with N,O-Schiff base
(5–12-fold for CL polymerization and 2–7-fold for LA
polymerization). The reaction mechanism of CL polymerization was investigated
by density functional theory (DFT). The calculations predicted a lower
activation energy for a process involved with an Al complex bearing
an N,S-Schiff base ligand (17.6 kcal/mol) than for that of an Al complex
bearing an N,O-Schiff base ligand (19.0 kcal/mol), and this magnitude
of activation energy reduction is comparable to the magnitude of rate
enhancement observed in the experiment. The reduction of activation
energy was attributed to the catalyst–substrate destabilization
effect. Using a sulfur-containing ligand to decrease the activation
energy in the ring-opening polymerization process may be a new strategy
to design a new Al complex with high catalytic activity.