Weak Lewis Pairs as Catalysts for Highly Isoselective Ring-Opening Polymerization of Epimerically Labile rac-O‑Carboxyanhydride of Mandelic Acid

Ring-opening polymerization (ROP) of rac-O-carboxyanhydride of mandelic acid (rac-manOCA) is a promising method to synthesize isotactic biodegradable polyester of poly­(mandelic acid), which has a high glass-transition temperature (Tg) and a high melting temperature (Tm) due to the formation of a stereocomplex. However, it is still a big challenge to synthesize isotactic poly­(rac-manOCA) so far because of the easy epimerization of manOCA in the polymerization progress. In this work, a highly isoselective ROP of the rac-manOCA system as the first stereoselective example is reported using weak Lewis pairs of zinc complexes and 3-fluoropyridine (3-F-Py) as catalysts with the highest Pm value of 0.92, and no obvious epimerization happens in the polymerization progress. Tg of poly­(rac-manOCA) (∼5.3 kg/mol) increases from 84 to 92 °C when the isotacticity increases from Pm = 0.60 to 0.92. Isothermally crystallized isotactic poly­(rac-manOCA) (5.3 kg/mol, Pm = 0.92) demonstrates a melting point at 116 °C, and no obvious melting point can be found for the almost atactic poly­(rac-manOCA) (5.4 kg/mol, Pm = 0.60). The Tm value of a 1:1 mixture of enantiopure poly­(l-manOCA) and poly­(d-manOCA) can reach 173 °C after isothermal crystallization.