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 ?-caprolactone (CL). The ionic liquid containing Ce(OTf)₄ was used, at least three times, in the ROP of CL without losing catalytic activity.