Ring-opening polymerization of lactones by rare-earth metal triflates and by their reusable system in ionic liquids

The ring-opening polymerizations (ROPs) of lactones catalyzed by rare-earth metal trifluoromethanesulfonates (triflates) (RE(OTf)3) were examined. Among various complexes, scandium triflate (Sc(OTf)3) emerged as an effective catalyst in toluene. The ROP of lactones by Sc(OTf)3 proceeded in a living fashion, and the number of polymer molecules was controlled by the amount of protic additives such as benzyl alcohol and H2O. In other words, one molecule of Sc(OTf)3 catalytically produced a number of polymer molecules (up to 40 molecules) depending on the amount of protic additives. The plausible mechanism was depicted as an activated monomer mechanism. The polylactones with a number-average molecular weight over 25,000 were successfully synthesized. Immobilization of RE(OTf)3 was investigated in three ionic liquids, and cerium triflate (Ce(OTf)4) showed relatively high catalytic activity in a biphasic system of 1-butyl-3-methylimidazolium hexafluoroantimonate and toluene in the ROP of ɛ-caprolactone (CL). The ionic liquid containing Ce(OTf)4 was used, at least three times, in the ROP of CL without losing catalytic activity.