Uranium-mediated ring-opening polymerization of ε-caprolactone: a comparative study

Abstract

The ring-opening polymerization (ROP) of the cyclic ester ε-caprolactone was studied using the uranium(IV) complexes [(Im^DippN)₂U(NMeEt)₂] (3), [(C₅Me₅)₂U(NMe₂)₂] (4) and [(C₅Me₅)₂U(NCMePh)₂] (5) as initiators. While the bis(imidazolin-2-iminato) complex 3 displayed a surprisingly high catalytic activity of 1.2 × 10⁷ g (PCL) mol⁻¹ h⁻¹ at room temperature, compounds 4 and 5 exhibited lower catalytic activities even at 90 °C. The activity of the uranium complex 3 was further compared to the imidazolin-2-iminato uranium(IV) complexes [(Im^tBuN)₄U] (1) and [(Im^MesN)₃U(NMeEt)] (2), which display catalytic activities of 7.9 × 10³ g (PCL) mol⁻¹ h⁻¹, and 5.3 × 10³ g (PCL) mol⁻¹ h⁻¹, respectively at an elevated temperature of 90 °C. In order to shed light on the operative mechanisms, kinetic studies were carried out with complexes 3–5.