Ionic thermo‐responsive copolymer with multi LCST values: easy and fast LCST‐change through anion exchange

An ionic thermo‐responsive copolymer with multiple lower critical solution temperatures (multi‐LCSTs) has been developed, and the multi‐LCSTs were easily changeable according to the various counter anion types. The multi‐LCST values were achieved by introducing an ionic segment with an imidazolium moiety within the p‐NIPAAm polymer chain to produce poly(NIPAAm‐co‐BVIm) copolymers, p‐NIBIm]⁺Br]^?, and changing the counter anion type to produce p‐NIBIm]⁺X]^? (X = Cl, AcO, HCO₃, BF₄, CF₃SO₃, PF₆, SbF₆). The as‐prepared temperature‐responsive copolymers were physicochemically characterized via proton nuclear magnetic resonance spectroscopy (¹H‐NMR), Fourier‐transform infrared, X‐ray photoelectron spectroscopy, and thermogravimetric analysis. Their various LCST values, micelle sizes, and surface charges were determined using an Ultraviolet‐visible spectrophotometer and a Zeta (ξ) sizer, which were fitted with temperature and stirring control. The copolymers showed a broad LCST spectrum between 39°C and 52°C. The Zeta (ξ) potential values at a pH = 7 decreased from about +9.7 for p‐NIBIm]⁺X]^? (X = Cl ≈ Br) to about +2.0 mV for p‐NIBIm]⁺X]^? (X = PF₆ ≈ SbF₆). The micelle size (or volume) of the copolymers with different anionic species gradually increased from 181.2 nm (or 2.49 × 10^?17 cm^?3) for p‐NIBIm]⁺Br]^? to 229.2 nm (or 5.04 × 10^?17 cm^?3) for p‐NIBIm]⁺CF₃SO₃]^?, showing a clear effect of the anion on the micelle size (or volume) at a constant temperature, such as body temperature. The fact that the most important physicochemical properties for the thermo‐responsive copolymers, such as the LCST value, micelle size (or volume), and surface charge, could be easily controlled only through the anion exchange suggests these are highly applicable as ionic thermo‐responsive copolymers in a drug (or gene, protein) delivery system. Copyright © 2015 John Wiley & Sons, Ltd.