Synthesis by RAFT and Ionic Responsiveness of Double Hydrophilic Block Copolymers Based on Ionic Liquid Monomer Units

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Abstract

Three imidazolium-based ionic liquid (IL) monomers, namely, 3-(1-ethyl imidazolium-3-yl)propylmethacrylamido bromide (IL-1), 2-(1-methylimidazolium-3-yl)ethyl methacrylate bromide (IL-2), and 2-(1-ethylimidazolium-3-yl)ethyl methacrylate bromide (IL-3), and methacrylic acid (MAA) were polymerized by the reversible addition fragmentation chain transfer (RAFT) process in methanolic solutions at 70 °C, using either 2-cyanopropyl dithiobenzoate (CTA-1) or (4-cyanopentanoic acid)-4-dithiobenzoate (CTA-2) as chain transfer agents (CTAs). Under these conditions, polymers exhibited molar masses predetermined by the initial molar ratio of the monomers to the dithioester precursor, as evidenced by 1H NMR spectroscopy from chain ends analysis. These hydrophilic polymers were subsequently used as macro-CTAs in chain extension experiments in aqueous or in alcoholic solutions, affording IL-based double hydrophilic block copolymers (DHBCs) of the type PIL-1-b-PAm, PMAA-b-PIL-2 and PMAA-b-PIL-3, where PAm and PIL stand for polyacrylamide and polymeric ionic liquid. These DHBCs could be further manipulated and made to self-assemble in micelle-like structures in water by exchanging the bromide (Br−) counteranion of IL blocks for −N(SO2CF3)2. This anion exchange indeed turned the solution properties of the PIL blocks from hydrophilic to hydrophobic, as verified on the corresponding IL-based homopolymers which were immiscible with water after the anion switch. Investigations by 1H NMR evidenced that the diblock copolymers exhibited salt-responsive behavior in aqueous solutions: anion exchange induced the formation of water-soluble micellar aggregates consisting of hydrophobic − N(SO2CF3)2-based IL blocks at the core stabilized by water-soluble PAm or PMAA at the shell.

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