Brّnsted acid-free controlled polymerization of tetrahydrofuran catalyzed by recyclable rare earth triflates in the presence of epoxides

We report a series of novel, highly efficient, hydrolytically stable and recyclable Lewis acid rare earth triflate catalysts for the living/controlled bulk ring-opening polymerization (ROP) of tetrahydrofuran (THF) in the presence of epoxides. The rare earth triflates [RE(OTf)3] include that of Sc, Y, La, Nd, Dy and Lu. Epoxides include propylene oxide (PO), cyclohexene oxide (CHO) or styrene oxide (SO). Especially RE(OTf)3/PO shows high activities, producing PTHF with up to 62 percent conversion and relatively narrow molecular weight distributions (MWDs) (as low as 1.14). Molecular weights (MWs) can be readily controlled from 1.7 to 56.1 kDa by changing monomer to catalyst molar ratios. The Sc(OTf)3/PO catalyst gives an α,ω-dihydroxyl telechelic PTHF with MWs close to calculated values and MWDs below 1.2. The rare earth catalysts are easily recovered by simple water extractions. For instance, Sc(OTf)3 can be used at least six times without an obvious decrease of catalytic activity. A polymerization mechanism involving the first two propagation steps of the alkyltetrahydrofuranium cation is proposed to account for the induction periods observed.