N-Heterocyclic Carbene-Organocatalyzed Ring-Opening Polymerization of Ethylene Oxide in the Presence of Alcohols or Trimethylsilyl Nucleophiles as Chain Moderators for the Synthesis of alpha,omega-Heterodifunctionalized Poly(ethylene oxide)s
Résumé
The present study describes innovations in the ring-opening polymerization (ROP) of ethylene oxide (EO) using N-heterocyclic carbenes (NHCs) as organocatalysts, which enables the synthesis of alpha,omega-heterodifunctionalized poly(ethylene oxide)s (PEOs). Two representative NHC catalysts, namely, 1,3-bis(diisopropyl)imidazol-2-ylidene (1) and 1,3-bis(di-tert-butyl)imidazol-2-ylidene (2), were efficiently employed in conjunction with a variety of chain regulators of general structure NuE, where Nu and E are the nucleophilic and the electrophilic part, respectively, with E = H or SiMe3 (e.g., PhCH2OH, HC CCH2OH, N3SiMe3, and PhCH2OSiMe3). Catalytic amounts of the NHC (typically [NHC]/[NuE]/[EO] = 0.1/1/100 in moles) were indeed utilized to trigger the metal-free ROP of EO at 50 C in dimethyl sulfoxide, allowing the polymerization to proceed to completion. In this way, PEOs of dispersities lower than 1.2 and molar masses perfectly matching the [EO]/[NuE] ratio were obtained, attesting to the controlled/living character of these NHC-catalyzed polymerizations. Characterization of alpha,omega-difunetionalized PEOs by combined techniques such as H-1 NMR spectroscopy, MALDI-TOF mass spectrometry, and size exclusion chromatography confirmed the quantitative introduction of the nucleophilic moiety (Nu) and its electrophilic component (E = H Or SiMe3) in the alpha- and omega-position of the PEO chains, respectively, and the formation of polymers with narrowly distributed molar masses. These results are discussed in the light of the existence of two possible mechanisms. The first one involves a direct attack of the NHC catalyst onto EO and the formation of a zwitterionic intermediate (activated monomer mechanism). The second possibility is the activation by the NHC of the E moiety of the NuE chain regulator first and then of the alpha-Nu,omega-OE PEO chain (activated chain end mechanism).