Stereoselective zwitterionic ring-opening polymerization of rac-lactide

Stereoselective zwitterionic ring-opening polymerization (ZROP) poses a potentially great challenge in polymer chemistry because of its unique ion-pairing catalysis mechanism. Herein, a stereoselective ZROP of rac-lactide (rac-LA) was explored for synthesis of isotactic cyclic polylactides by using commercially available N-heterocyclic carbene (NHC) catalysts at low temperature in tetrahydrofuran. The epimerization of lactide during ZROP can be inhibited at low temperature. The best isoselectivity can reach to a high value of P-m = 0.91 associated with a high melting point (T-m) of 193 degrees C. The resulting polylactides exhibit narrow molecular weight distributions (M-w/M-n < 1.2) and inhibited transesterification as evidenced by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The resulting polylactide is predominantly cyclic as evidenced by H-1 NMR and MALDI-TOF MS analyses.