Copolymerization of lactide, epoxides and carbon dioxide: a highly efficient heterogeneous ternary catalyst system

Abstract

The controlled copolymerization of mixed monomers that usually imparts distinct advantages into single polymer chains and requires a single versatile catalyst remains a key challenge in polymer chemistry. Herein, a novel heterogeneous ternary catalyst system (TCS), which consists of SalenCo^III, zinc glutarate (ZnGA) and PPNCl, is proposed. The intermolecular cooperation between cobalt and zinc realizes the combination of the ring-opening polymerization (ROP) of lactide, and the ring-opening copolymerization (ROCOP) of epoxides and CO₂, forming multiblock copolymers with polylactide (PLA) and poly(propylene carbonate) (PPC) blocks. The unique synergistic effect between cobalt and zinc can further enhance the activity and selectivity of the TCS. The high conversion of monomers (over 99% PO and LLA) and a high molecular weight copolymer (698.0 kg mol⁻¹) were achieved. The high activity of heterogeneous TCS results in the reduction of the cobalt catalyst, and this potentially opens up exciting new possibilities for industrial application. Homogeneous TCSs were also explored, and the comparison of the homo- and heterogeneous systems contributes to a reliable understanding of the mechanisms of intermolecular cooperation and the synergistic effect of TCSs.