Tunable intramolecular H-bonding promotes benzoic acid activity in polymerization: inspiration from nature
Ring-opening polymerization (ROP) of lactones and cyclic carbonates catalyzed by (super)strong Brønsted acids offers a valuable approach to generate biodegradable aliphatic polyesters. However, these strong acids usually lead to backbiting and decarboxylation; thus a mild and effective acidic catalysis for these ROPs becomes necessary. Inspired by weak Brønsted acidic catalysis in squalene–hopene cyclases, we propose that ortho-amido group(s) on benzoic acids would increase the acidity of the carboxylic moiety by intramolecular H-bonding, and make carboxylic acid active in promoting the ROPs. A series of o-amido- and o,o′-bis(amido)-benzoic acids are evaluated as typical intramolecular H-bonding enhanced Brønsted acidic catalysts in the ROPs. Both o-amido- and o,o′-bis(amido)-benzoic acids exhibited good to excellent performances in the rate and control of ROPs of δ-valerolactone (VL), ε-caprolactone (CL), and trimethylene carbonate (TMC) at room temperature in solutions. An exceptional carboxylic acid, o,o′-bis(pivalamido)benzoic acid, showed efficient activation in solution and precise control with high conversions (91–96%), predicted molecular weights from 3.09 to 10.31 kg mol−1, and narrow dispersities (Đ 1.03–1.12) in ROPs of CL and TMC. Well-defined diblock copolymers consisting of PTMC, PVL and PCL segments were synthesized. The controlled/living characteristics of the ROPs were verified by chain extension experiments. 1H NMR, SEC, and MALDI-TOF MS analyses strongly indicated that the obtained polymers were exactly the designated ones. A cationic monomer activation mechanism was proposed and was supported by NMR titrations. The experimental results indicated that mild and tunable ortho-amido benzoic acid with intramolecular H-bonding is a competent organocatalyst in living polymerization.