New Mechanism for the Ring-Opening Polymerization of Lactones? Uranyl Aryloxide-Induced Intermolecular Catalysis

The uranyl aryloxide [UO₂(OAr)₂(THF)₂] (Ar = 2,6-^<i>t</i>Bu₂-C₆H₂) is an active catalyst for the ring-opening <i>cyclo</i>-oligomerization of ε-caprolactone and δ-valerolactone but not for β-butyrolactone, γ-butyrolactone, and <i>rac</i>-lactide. ¹H EXSY measurements give the thermodynamic parameters for exchange of monomer and coordinated THF, and rates of polymerization have been determined. A comprehensive theoretical examination of the mechanism is discussed. From both experiment and theory, the initiation step is intramolecular and in keeping with the accepted mechanism, while computational studies indicate that propagation can go via an intermolecular pathway, which is the first time this has been observed. The lack of polymerization for the inactive monomers has been investigated theoretically and C–H···π interactions stabilize the coordination of the less rigid monomers