Chain Shuttle between Dual Sites of a Monomolecular Organocatalyst Enables Controlled Ring-Opening Copolymerization

Abstract

The development of a metal-free sequence-controlled polymerization method to yield sustainable polymers is at the forefront of polymer science. Here, we demonstrate the organocatalytic sequence-controlled ring-opening copolymerization (ROCOP) of lactide (LA) and ethylene oxide (EO), which is challenging due to the large difference in polymerization reactivity between LA (ΔH ≈ −25 kJ mol^–1) and EO (ΔH ≈ −95 kJ mol^–1). In the ROCOP process, the multifunctional organocatalyst Cat. 2 generates dual active sites of amine-thiourea and boron, which can selectively enable LA and EO polymerization, respectively. The growing-chain shuttle between the dual sites affords poly(LA-co-EO). The polymer sequence is largely adjusted covering statistics, gradients, and blocks, clarified by reactivity ratios (by three models) and visualizations of copolymer microstructure (by Monte Carlo simulation). We also conduct an exploration of how the monomer sequence affects thermal and mechanical properties of resultant oxygen-rich copolymers. The results provide insights into organocatalytic and sequence-controlled polymerization methods.