N-heterocyclic carbene-induced zwitterionic ring-opening polymerization of ethylene oxide and direct synthesis of α,ω-difunctionalized poly(ethylene oxide)s and poly(ethylene oxide)-b-poly(ε-caprolactone) block copolymers

An N-heterocyclic carbene (NHC), namely, 1,3-bis-(diisopropyl)imidazol-2- ylidene (1), was demonstrated to bring about the metal-free ring-opening polymerization of ethylene oxide at 50 °C in dimethyl sulfoxide, in absence of any other reagents. Poly(ethylene oxide) (PEO) of polydispersities < 1.2 and molar masses perfectly matching the [monomer]/[(1)] ratio could thus be obtained in quantitative yields, attesting to the controlled/living character of such carbene-initiated polymerizations. It is argued that (1) adds to ethylene oxide to form a zwitterionic species, namely 1,3-bis-(diisopropyl)imidazol-2- ylidinium alkoxide, that further propagates by a zwitterionic ring-opening polymerization (ZROP) mechanism. Through an appropriate choice of terminating agent NuH or NuSiMe₃ at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, α,ω-bis(hydroxy)-telechelic PEO, α-benzyl,ω-hydroxy, and α-azido,ω-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H₂O, PhCH₂OH, and N ₃SiMe₃ as terminating agent, respectively. Characterization of these α,ω-difunctionalized PEOs by techniques such as ¹H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both α and ω positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(ε-caprolactone) diblock copolymer by sequential ZROP of the corresponding monomers was successfully achieved using (1) as organic initiator without isolation of the PEO block intermediate.