Controlled Ring-Opening Polymerization of e-Caprolactone Catalyzed by a Rare Earth Schiff-Base Complex

Lanthanide Schiff-base complexes with the formula [3,5-tBu2-2-(O)C6H2CH=NC6H5]3Ln(THF) (Ln = Sc, Y, La, Nd, and Gd) were synthesized by the metathesis reaction of anhydrous LnCl3 with a Schiff-base sodium salt in good yields (> 80%). The complex containing a neodymium center was characterized by X-ray diffraction and the structure around the neodymium atom could be described as a pentagonal bipyramid. The ring-opening polymerization (ROP) of ɛ-caprolactone (CL) was successfully carried out using the lanthanide Schiff-base complexes as catalysts, and the neodymium complex leads to a controlled ROP of CL. The influence of the reaction conditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. The polymerization rate was first-order with respect to the monomer concentration. End-group analyses of the oligomer of CL showed that the polymerization underwent a coordination-insertion mechanism.