Three is company: dual intramolecular hydrogen-bond enabled carboxylic acid active in ring-opening polymerization
Hydrogen-bond organocatalysis using (thio)urea and base has achieved massive success. Intramolecular H-bond (IMHB) assisted Brønsted acid (BA) catalysis, especially in polymerizations, was not explored. Here we suggested an IMHB–BA model in ring-opening polymerization (ROP) with γ-resorcylic acid (RA) as a representative catalyst, promoting ROPs of δ-valerolactone (VL) and ε-caprolactone (CL). The exceptional carboxylic acid RA showed efficient activation and precise control with high conversions (93–98%), predicted molecular weights from 3090 to 13 000 g mol−1, narrow dispersities (Đ 1.02–1.08), and nearly Poisson distributions (Đ ≤ 1.03) at higher molecular weights. A dual IMHB in RA was estimated using computational calculations, which predicted short H-bond lengths and near 180° bond angles, denoting strong H-bonding. A cationic monomer activation mechanism was proposed and supported by NMR titrations. The controlled/living nature of the ROPs was confirmed by kinetics and chain extension experiments. 1H NMR, SEC, and MALDI-ToF MS analyses strongly indicated that the obtained PVL and PCL were exactly the designated ones. Synthesis of well-defined PVL-b-PCL and clickable end-functionalized PVLs again verified that the catalytic ROPs proceeded in a controlled/living manner, and suggested that IMHB–BA catalysis was a generally applicable method.