Living cationic ring-opening polymerization of 2-oxazolines initiated by rare-earth metal triflates

Abstract

The cationic ring-opening polymerization (CROP) of substituted 2-oxazolines using rare-earth metal triflates (RE(OTf)₃) as initiator was investigated for the first time. In this work, we examined the polymerization characteristics of 2-ethyl-2-oxazoline (EtOx) initiated by Sc(OTf)₃ under conventional thermal heating and microwave irradiation, and compared the respective outcomes with those obtained with the most frequently used initiator methyl tosylate (MeOTs). The results indicated that Sc(OTf)₃ exhibits a higher catalytic efficiency to the EtOx polymerization than MeOTs under identical conditions. The controlled/living nature of the Sc(OTf)₃-catalyzed CROP was confirmed by its linear first-order kinetics and the narrow molecular weight distribution of the resultant polymers as well as the block copolymerization of EtOx and 2-phenyl-2-oxazoline (PhOx). Based on in situ NMR spectroscopic studies and SEC analysis of PEtOx samples obtained from the control termination experiments, a possible initiating/propagating mechanism has been proposed for the living cationic ring-opening polymerization. Morever, this rare-earth catalytic system can also be applied to the ring-opening polymerization of some sterically hindered or aryl-substituted 2-oxazolines.