Living Cationic Polymerization of ε‑Caprolactone Catalyzed by a Metal-free Lewis Acid of Trityl Tetrafluoroborate

Research on metal-free catalytic synthesis of polycaprolactone (PCL) is fueled by the many practical needs for biodegradable and biocompatible materials. In this regard, the pursuit of effective and economical catalysts as well as precise control over the polymerization has become the main challenge. Herein, we report a living cationic ring-opening polymerization of ε-caprolactone catalyzed by a safe, inexpensive, and metal-free Lewis acid of trityl tetrafluoroborate. The polymerization is initiated by alcohol or water, which is corroborated by the presence of initiator segments at the chain ends of PCL polymers via 1H NMR and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Kinetic experiments show that polymerization is a first-order reaction to monomers. This result, together with the predictable molar mass, narrow molar-mass dispersity, and controllable chain growth and extension, confirms the synthesis to be a living polymerization. This work expands the catalytic toolbox of PCL synthesis and represents a rare example of cationic living polymerization of ε-caprolactone catalyzed by Lewis acids.