Stereoselective ring-opening polymerization of rac-lactide catalyzed by the proton sponge bisphosphazene 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene

Abstract

Synthesis of highly isotactic polylactide (PLA) via organocatalyzed stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA) is a key strategy to improve the physico-chemical properties of PLA. In this context, the proton sponge bisphosphazene 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene (HMPN) was synthesized and applied to catalyze the stereoselective ROP of rac-LA. Semicrystalline PLA with a P_m of 0.75 was conveniently prepared at room temperature. PLAs with higher tacticity (P_m up to 0.93) and higher melting temperature (T_m up to 189 °C) were obtained at lowered temperature. Binary catalysts composed of HMPN and ureas exhibited both high activity (99% monomer conversion within 5 minutes) and high stereoselectivity (P_m up to 0.83) at room temperature. In situ NMR study of HMPN, urea, and BnOH demonstrated the weak interactions between HMPN and BnOH, verifying the kinetic inactivity to deprotonation of HMPN. Kinetic studies verified the controlled manner and chain end control mechanism for the HMPN/U-2 catalyzed ROP of rac-LA.