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

Abstract

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 degrees 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 NuSiMe(3) at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, alpha,omega-bis(hydroxy)-telechelic PEO, alpha-benzyl,omega-hydroxy, and alpha-azido,omega-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H(2)O, PhCH(2)OH, and N(3)SiMe(3) as terminating agent, respectively. Characterization of these alpha,omega-difunctionalized PEOs by techniques such as (1)H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both alpha and omega positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(epsilon-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.

Versatile Route to Synthesize Heterobifunctional Poly(ethylene glycol) of Variable Functionality for Subsequent Pegylation

“…In order to avoid this dimer formation and to synthesize PEG with protected thiol end groups, a two-step synthesis was adopted. The tosylate group was first displaced by reaction with the in situ formed potassium thioacetate in DMF to yield α-thioacetate-ω-hydroxyl PEG (6). The introduction of the thioester end group was confirmed by 1 H and 13 C-NMR (see Appendix, Figure S12 and S13).…”

“…Synthesis of α-thioacetate-ω-hydroxyl PEG (6). α-Tosyl-ω-hydroxyl PEG (1, 10 g, 6.23 mmol) was dissolved in 150 mL of dry DMF, evacuated and purged with argon.…”

“…PEG (6) and PEG (7). In order to avoid this dimer formation and to synthesize PEG with protected thiol end groups, a two-step synthesis was adopted.…”

Multifunktionelle Poly(ethylenglycole)

“…[96,97] Eine interessante Erweiterung, insbesondere im Hinblick auf Biokonjugation, ist die Verwendung eines Initiators mit einer orthogonalen Funktionalität, die selektiv adressiert werden kann. [97][98][99] Neben Hydroxygruppen wurde bislang nur die Einführung von Aminogruppen durch Einsatz von geschützten Monomeren beschrieben (über N,N-Dibenzylaminoglycidol (DBAG)).…”

N-heterocyclic carbenes as organocatalysts in controlled/living ring-opening polymerization ofO-carboxyanhydrides derived froml-lactic acid andl-mandelic acid

“…[19][20][21] NHCs have been applied in a wide range of monomers such as lactones, 20,22 cyclic carbonates, 23 epoxides, 24,25 and cyclic siloxanes. 26,27 The diverse structures and tunable catalytic performance of NHCs provide a far-ranging choice of catalyst in ROP, and these merits inspired us to investigate their application in OCAs.…”