Temperature-regulated H-bonding organocatalysis enables highly controllable ring-opening polymerization of lactide at elevated temperatures

Organocatalytic ring-opening polymerization (ROP) is a more sustainable approach for synthesizing renewable and biodegradable polymers compared to conventional metal-based catalysis. A key challenge in the field has been applying organocatalysis under the harsh conditions of industrial ROP while maintaining high activity and selectivity. Herein, we present an efficient N-heterocyclic carbene/thiourea binary organocatalyst system (NHC/TU-2) for the ROP of lactide (LA) at elevated temperatures, offering excellent control by almost completely suppressing transesterification and epimerization. It was found that NHC/TU-2 operates according to the imidate H-bonding catalysis model; however, its notable feature is that the strength of the H-bonding is regulated by temperature, enabling the activation of the monomers with high selectivity. Additionally, the large size of the NHC/TU-2 complex introduces steric hindrance at the living chain ends, further preventing attacks on the polymer chain and thereby eliminating transesterification. The mechanism of temperature-regulated H-bonding catalysis was validated through nuclear magnetic resonance (NMR) titration experiments, H-bonding constant studies, and density functional theory (DFT) calculations. The NHC/TU-2 (0.5:1) achieved stereoretention during the ROPs of L-LA and D-LA, affording stereoregular poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) with predicted molecular weights, narrow dispersity (Ð Pm > 0.99), and no coloration. The thermal properties of the synthesized PLLA, PDLA, and their stereocomplexes were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Furthermore, cytotoxicity tests demonstrated that the PLLA synthesized with NHC/TU-2 was nontoxic.