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 (rac-LA) for the synthesis of stereoregular polylactide (PLA). In this context, a series of binary organocatalysts consisting of different phosphazenes (CTPB, C₃N₃-Me-P₃, C₃N₃-Py-P₃, t-BuP₂, and t-BuP₄) and achiral ureas (U1-U6) were applied for the stereocontrolled ROP of rac-LA under mild conditions. It is remarkable that C₃N₃-Py-P₃/U4 with the compatible basicity/acidity showed both high activity (92% conversion within 10 min) and great stereoselectivity (P_m = 0.92) at room temperature (20 °C), and the resultant stereoblock PLA had predictable molar mass, narrow distribution (Đ = 1.07), and high melting temperature (T_m = 190 °C). Interactions involved among phosphazene, urea, and initiator were investigated by an in situ NMR technique. It was found that C₃N₃-Py-P₃ 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 k_obs^-1 (rac-LA) = 0.212 min^-1, k_obs^-1 (D-LA) = 0.311 min^-1, and k_obs^-1 (L-LA) = 0.317 min^-1, indicative of the chain end control mechanism for stereocontrolled ROP.