Readily Prepared and Tunable Ionic Organocatalysts for Ring-opening Polymerization of Lactones

Highly potent ionic organocatalyst is developed for room-temperature controlled ring-opening polymerization (ROP) of lactones, including delta-valerolactone, epsilon-caprolactone, and delta-hexalactone. The catalysts are prepared by simply mixing tetra-n-butyl ammonium hydroxide and a (thio)urea at elevated temperature under vacuum, and used in cooperation with an alcoholic initiator. The performance of the catalyst is readily adjusted and optimized through variation of the (thio)urea precursor, catalyst composition, and reaction condition. Urea-derived catalysts are generally superior to thiourea-derived ones. Provided with proper N-substituents, the catalyst affords both high polymerization efficiency and high selectivity for monomer enchainment over macromolecular transesterification, even at high monomer conversion and/or substantially extended reaction time. In addition to acidity, structural symmetry of the urea also proves decisive for the catalytic activity, which enables a catalyst-assisted proton transfer process for the ring-opening of lactone and thus provides a novel mechanistic insight for ROP catalyzed by hydrogen-bonding type bifunctional ionic organocatalysts.