Macromolecules, Vol.41, No.17, 6299-6308, 2008
Synthesis by RAFT and ionic responsiveness of double hydrophilic block copolymers based on ionic liquid monomer units
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 degrees 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 H-1 NMR spectroscopy from chain ends analysis. These hydrophific 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 H-1 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.