Ring opening polymerization of ε caprolactone catalyzed by (ArO) 2Sm(THF) 4 (ArO=2

6 ditertbutyl 4 methyl phenoxo ) was systematically studied.The polymerization system has “living ” characteristic and shows high activity

the conversion is as high as 98% in toluene at 60℃ for 1h under /=2000.The central ions and ligands have effect on the activity of catalysts.The initiation mechanism by divalent lanthanide complexes has been studied through oligomer end group analysis by NMR.The results demonstrate that catalyst reacts first with ε-caprolactone to give a trivalent lanthanide enolate which then catalyzes caprolactone polymerization.