Abstract
Ring-opening polymerization (ROP) of lactones and cyclic carbonates catalyzed by (super) strong Bronsted 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 Bronsted 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 Bronsted 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 delta-valerolactone (VL), epsilon-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 (D 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. H-1 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.