Solution behaviour of poly(N-acryloyl-L-leucine) and its copolymers with N-isopropylacrylamide

A vinyl polymer carrying COOH groups and the groups (amido and isopropyl) present in the temperature-sensitive poly(N-isopropylacrylamide) (pNIPAAm) was synthesized along with two of its copolymers incorporating NIPAAm units. The protonation of COO- groups, studied in aqueous media over a range of temperatures (25-35°C) that included the lower critical solution temperature of pNIPAAm, showed polyelectrolyte behaviour with a decreasing pattern of the basicity constant (log K) and enthalpy (-ΔHo) changes in relation to the degree of protonation (α). Beyond a critical α value (α = 0.4), the log K/α slope drastically changed and the reduced viscosity (η/C) sharply decreased. Under these conditions the Fourier transform infra-red spectra revealed a strong band at 1625 cm-1 presumably reflecting the interaction of the amido groups of several monomer units. For the copolymers the decreasing pattern of log K values decreased with increasing NIPAAm content, and beyond the critical α, the log K/α slope increased again, showing a greater endothermic effect superimposed on the protonation of the COO- group. The phenomenon was attributed to hydrophobic forces between isopropyl groups outweighing the repulsive electrostatic interactions of the ionized polymer. At higher temperatures, the -ΔHo values decreased in a sensitive manner and the critical α shifted to a lower value because of enhanced hydrophobic interactions. A reduced amount of structured water molecules on the polymer was revealed by the lower entropy changes (ΔSo). A hydroxo-complex species of Cu(OH)2L2 2- (L- is the repeating unit of the polymer) stoichiometry was revealed at low pH with a linear decrease in stability constants (log β) in relation to pH. The pH range for a single complex species was consistent with the electronic spectra. © 1997 Elsevier Science Ltd.