Ring-Opening Polymerization of Cyclic Esters and Trimethylene Carbonate Catalyzed by Aluminum Half-Salen Complexes

A series of ONO-tridentate Schiff base ligands derived from chiral and achiral amino alcohols and amino acids were synthesized and reacted with AlEt3 to provide dimeric aluminum complexes. These complexes were tested for the ring-opening polymerization (ROP) of rac-lactide at 70 °C in toluene, producing poly(lactide) with up to 82% isotacticity. The most active of these aluminum complexes was chosen to perform ring-opening homopolymerizations of rac-lactide, trimethylene carbonate (TMC), rac-β-butyrolactone (rac-β-BL), δ-valerolactone (δ-VL), and ε-caprolactone (ε-CL). Kinetic parameters were investigated, and each polymerization was found to be first order with respect to monomer concentration. Fractional orders were observed with respect to catalyst concentration, indicating catalyst aggregation during the polymerization processes. Activation parameters were determined for all monomers, with their ΔG⧧ values at 90 °C being in the order rac-lactide ≈ rac-β-BL > δ-VL > TMC ≈ ε-CL. Fineman–Ross and kinetic studies of the copolymerization of rac-lactide and δ-VL both indicate that the rate of rac-lactide enchainment is higher than that of δ-VL, resulting in a tapered copolymer. In addition, single crystals of one of these aluminum complexes were grown in the presence of rac-lactide and characterized using X-ray crystallography. The unit cell contains two lactide monomers, one d- and one l-lactide, adding further proof that polymerization takes place via an enantiomorphic site control mechanism.