Zwitterionic organocatalysis for ring-opening polymerization of cyclic esters

Abstract

Zwitterionic catalysis is powerful in simultaneously activating electrophiles and nucleophiles in organic transformations; however, its potential in polymerizations remains untapped. Herein, a zwitterionic catalytic model for ring-opening polymerization (ROP) is introduced and a series quaternary ammonium carboxylate zwitterionic catalysts have been designed and evaluated in the ROPs of cyclic esters. Among the zwitterionic catalysts, L-carnitine, a commercially available and inexpensive natural product, showed the optimal catalytic performance. ROPs of L-lactide (L-LA), trimethylene carbonate, δ-valerolactone and ε-caprolactone were successful. PLLA was prepared with controlled molecular weight (1.4 to 21.2 kg mol⁻¹) and narrow dispersion (Đ 1.02 to 1.21). The zwitterionic catalytic mechanism is proposed where quaternary ammonium and hydroxyl cooperatively activated the carbonyl of monomers, while the carboxylic anion activated the chain-end/initiator. NMR titrations validated the bifunctional activation mechanism. Organocatalysts of zwitterionic nature provide a new design tool for wider scope investigation of catalytic polymerizations.