A novel approach to RE-OR bond from in situ reaction of rare earth triflates and sodium alkoxides: A versatile catalyst for living ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.polymer.2014.03.032

A series of rare earth triflates (RE(OTf)₃, RE = Sc, Y and Lu) were used for the first time as moisture-stable precursors to generate rare earth alkoxide complexes through an in situ reaction with sodium alkoxides (NaOR) in tetrahydrofuran. ¹H NMR and ¹³C NMR results confirmed the fast ligands exchange process and the formation of rare earth-oxygen (RE-OR) bond. The in situ formed catalysts displayed high reactivity toward living ring-opening polymerization (ROP) of ε-caprolactone (CL). For instance, Lu(OTf)₃/sodium isopropoxide (NaOⁱPr)-catalyzed ROP of CL with the [CL]₀/[NaO ⁱPr]₀/[Lu(OTf)₃]₀ feeding ratio of 300/3/1 produced poly(ε-caprolactone) (PCL) with controlled molecular weight (M_n,exp = 11.9 kDa vs M_n,theo = 11.8 kDa) and narrow polydispersity (PDI) of 1.08 within 3 min at 25°C. The kinetic studies and chain extension confirmed the controlled/living nature for the Lu(OTf)₃/NaOⁱPr-catalyzed ROP of CL. In addition, end-functionalized PCLs bearing vinyl or alkynyl group with narrow PDIs were obtained by using functional sodium alkoxides in the presence of Lu(OTf) ₃. ¹H NMR and MALDI-ToF MS analyses of the obtained PCLs clearly indicated the presence of the residue of OR groups at the chain ends. A coordination-insertion polymerization mechanism was proposed including a fast ligand exchange between Lu(OTf)₃ and NaOR giving the respective lutetium alkoxide complexes, and a CL insertion into RE-OR bond via acyl-oxygen cleavage.