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 [(ImDippN)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 [(ImtBuN)4U] (1) and [(ImMesN)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.