Living Polymerization of Chiral <i>O</i>-Carboxyanhydride of Mandelic Acid and Precise Stereoblock Copolymer Syntheses Using Highly Active OOO-Tridentate Bis(phenolate) Zinc Complexes

Abstract

The synthesis of highly stereoregular poly(mandelic acid) (PMA) via controllable polymerization is a considerable challenge because of the easy racemization of monomers during polymerization; consequently, precise stereoblock copolymers of PMA have not been reported so far. In this work, highly stereoregular PMA was synthesized through a living ring-opening polymerization of O-carboxyanhydrides (OCAs) of mandelic acid using highly active OOO-tridentate bis(phenolate)/zinc catalysts. In this polymerization system, the side reaction of racemization was suppressed very well via decreasing the basicity of ligands in zinc complexes. The molecular weight increases linearly as the ratio of monomer to initiator increases and can be high up to 32.5 kg/mol with narrow molecular weight distributions. Meanwhile, for the living polymerization feature, stereodiblock and stereotriblock copolymers with controllable molecular weights were obtained upon sequential one-pot monomer addition within 45 min. Thermal and X-ray diffraction characterizations indicated that these stereoblock copolymers crystallized in a stereocomplex phase with a distinct melting temperature (Tm) up to 178 °C, and the solubility behaviors of stereoblock PMAs and enantiomeric homopolymers of PMAs are remarkably different.