Zwitterionic organocatalysis for ring-opening polymerization of cyclic esters

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, delta-valerolactone and epsilon-caprolactone were successful. PLLA was prepared with controlled molecular weight (1.4 to 21.2 kg mol-1) and narrow dispersion (& Dstrok; 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. Zwitterionic catalytic model for ring-opening polymerization (ROP) was unveiled. Among the designed quaternary ammonium carboxylate zwitterionic catalysts, carnitine, a natural product zwitterion, showed optimal performance in ROPs of cyclic esters.