Controlled Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Rare-Earth Perfluoroalkanesulfonates and Perfluoroalkanesulfonimides

Abstract

We examined ring-opening polymerizations (ROPs) of ε-caprolactone in toluene at 25−60 °C catalyzed by scandium perfluoroalkanesulfonates and perfluoroalkanesulfonimides. Using these scandium catalysts, which have strong electron-withdrawing ligands, the ROPs proceeded quickly. Under identical conditions, polymerizations were completed more rapidly for reactions catalyzed by scandium trifluoromethanesulfonimide [Sc(NTf2)3] and scandium nonafluorobutanesulfonimide [Sc(NNf2)3] than for those catalyzed by scandium trifluoromethanesulfonate [Sc(OTf)3]. It was possible to synthesize poly(ε-caprolactone)s (M n = 2.6 × 103−9.8 × 103) with low polydispersities (M w/M n = 1.12−1.40). After polymerization, the catalysts were easily recovered by simple filtration or by extraction with H2O and could be reused. Other rare-earth nonafluorobutanesulfonimides [M(NNf2)3; M = Tm, Sm, and Nd] were also tested. Samarium nonafluorobutanesulfonimide [Sm(NNf2)3], as well as Sc(NNf2)3, catalyzed living polymerizations of ε-caprolactone. The reaction kinetics and activation thermodynamics of certain scandium catalysts and Sm(NNf2)3 were investigated. Increasing the perfluoroalkane chain length decreased the values of both the activation energy and the activation enthalpy. The ranked values of the activation free energies parallel those of the times required for completion of the polymerizations. 13C NMR spectroscopy was used to characterize the relative Lewis acidities of certain catalysts. Our findings indicate that Sc(NNf2)3, which accelerated the ROP than Sc(OTf)3 or Sc(NTf2)3, is an excellent catalyst for the ROP of ε-caprolactone.

The effect of microwave irradiation on the kinetics and activation thermodynamics of ring‐opening polymerization of ε‐caprolactone

“…Because no other mass peaks were observed, cyclic structures were not formed as by‐products. The spectral patterns are similar to those found when the polymer was synthesized using conventional heating …”

“…The values of the activation parameters are summarized in Table . The tendency of the relationship of catalysts and E a s for ROPs using microwave heating coincided with that using conventional heating, in which the faster polymerization system showed not lower but higher E a values. We expected that microwave heating would cause the value of E a to be smaller than that for conventional heating, if the microwaves enhanced the catalytic activities.…”

“…We performed ROPs of ε‐caprolactone under microwave‐ and conventional‐heating conditions that were catalyzed by the highly active Lewis acids Sc(NTf 2 ) 3 and Sc(NNf 2 ) 3 at 50–70 °C (Table , Runs 1–13). This temperature range was set to eliminate the possibility of thermal degradation and transesterification.…”

“…(d), Nf 2 NH]. Using the values of the slopes and eq , in which [M], [I], [C] are concentrations of monomer, initiator, and catalyst, respectively, we calculated the k p , according to our reported procedure . Du et al calculated the k p using eq for ROP of lactide using Sn(Oct) 2 initiate by poly(ethylene glycol) .…”

“…We have reported on the ROP of ε‐caprolactone catalyzed by Lewis and super Brønsted acids with the temperature of the polymerization controlled by conventional (oil bath) heating. We also determined the Arrhenius‐ and Eyring‐derived thermodynamic activation parameters by assessing the reaction kinetics at various temperatures . To the best of our knowledge, no study has dealt with the activation thermodynamics of microwave‐assisted polymerizations, which forms the basis of the study reported herein, in which we characterize the ROP of ε‐caprolactone at relatively low temperatures (50–70 °C) controlled by microwave heating and catalyzed by the super Brønsted acids, trifluoromethanesulfonimide (Tf 2 NH), and nonafluorobutanesulfonimide (Nf 2 NH), and by the Lewis acids, scandium trifluorobutanesulfonimide (Sc(NTf 2 ) 3 ) and scandium nonafluorobutanesulfonimide (Sc(NNf 2 ) 3 ) .…”