Highly Active Organocatalysts for Stereoselective Ring-Opening Polymerization of Racemic Lactide at Room Temperature.

Although significant advances have been achieved, highly stereocontrolled polymerization using organocatalysts is still a great challenge, such as ring-opening polymerization of racemic lactide (-LA) for the synthesis of stereoregular polylactide (PLA). In this context, a series of binary organocatalysts consisting of different phosphazenes (CTPB, CN-Me-P, CN-Py-P, -BuP, and -BuP) and achiral ureas (U1-U6) were applied for the stereocontrolled ROP of -LA under mild conditions. It is remarkable that CN-Py-P/U4 with the compatible basicity/acidity showed both high activity (92% conversion within 10 min) and great stereoselectivity ( = 0.92) at room temperature (20 °C), and the resultant stereoblock PLA had predictable molar mass, narrow distribution ( = 1.07), and high melting temperature ( = 190 °C). Interactions involved among phosphazene, urea, and initiator were investigated by an in situ NMR technique. It was found that CN-Py-P reacted with benzyl alcohol (BnOH) to form a relatively loose ion pair in the presence of U4, which accounted for both high activity and stereoselectivity. Kinetics studies for different LA monomers at 20 °C showed (-LA) = 0.212 min, (-LA) = 0.311 min, and (-LA) = 0.317 min, indicative of the chain end control mechanism for stereocontrolled ROP.