Stereoselective ring-opening polymerization of rac-lactide catalyzed by phenyl diphosphazene/urea binary catalytic system

Polylactide (PLA) is a fully bio-derived polyester with great biodegradability, biocompatibility, and mechanical properties. Synthesis of stereoregular PLA by highly stereoselective organocatalyzed ring opening polymerization (ROP) of racemic lactide (rac-LA) at room temperature is challenging despite some important developments in the past few years. In this contribution, two bulky phenyl diphosphazene bases, p-PDPB and m-PDPB, were conveniently synthesized by the Staudinger reaction. In combination with different hydrogen-bond donors such as ureas and squaramides, they could mediate the stereocontrolled ROP of rac-LA in THF at room temperature. Semicrystalline PLAs with narrow dispersity and high tacticity (P_m up to 0.84) were successfully synthesized in a well-controlled manner using phenyl diphosphazene based/urea binary catalytic system. Structural analysis verified the linear structure and high end-group fidelity of the resulting polymers. Moreover, the minimal transesterification of the polymer backbone proved by the MALDI-TOF MS analysis indicated the good controllability of the phenyl diphosphazene based binary catalytic system.