Indium alkoxide complexes supported by constrained Schiff-base ligands for the ring-opening (co)polymerization of cyclic esters

Abstract

Indium tert-butoxide complexes supported by constrained Schiff-base ligands with different diamine backbones were successfully synthesized (2a–c) and developed for the ring-opening polymerization (ROP) of lactide (LA), glycolide (GA) and ε-caprolactone (CL). Single-crystal X-ray studies reveal that all complexes are monomeric containing a five-coordinate indium metal center. All complexes were highly active for ROP of cyclic esters. The indium tert-butoxide complex having a trans-cyclohexyl backbone (2c) showed very high activities finishing 98% conversion of 200 equiv. of L-lactide (L-LA) in 3 min at room temperature (TOF = 3920 h⁻¹). Polymerization of rac-LA gave stereoblock isotactic-enriched PLA with P_m values of up to 0.85. The kinetic studies of the ROP of cyclic esters revealed a pseudo first-order dependence with respect to monomer concentrations and the rate order of rac-LA ≈ D-LA > L-LA ≫ CL. From the studies of monomer reactivity ratios using complex 2c, the copolymerization of LA with GA gave block copolymers while the copolymerization of LA with CL gave gradient copolymers.