Thermoresponsiveness of Carboxylated Polyallylamines Induced by Divalent Counterions as Ionic Effectors

We describe novel ionic effector-induced lower critical solution temperature (LCST)-type thermoresponsive polymers in an aqueous system. Carboxylated polyallylamines (carboxylated PAAs) are conjugates between polyallylamine and various carboxylic anhydrides possessing six-membered rings such as phthalic anhydride and 1-cyclohexene-1,2-dicarboxylic anhydride. These polymers showed LCST-type thermoresponsiveness in the presence of divalent metal ions (Ca2+, Mg2+, or Sr2+) in aqueous media, where a reversible phase transition between soluble and insoluble states was observed with a change in the temperature, although these polymers did not show thermoresponsiveness without divalent ions or with monovalent ions (Li+, Na+, or K+). The cloud points of the carboxylated PAAs could be precisely controlled by not only salt concentration, pH, salt species, and side-chain structures but also the mixing ratio of different salt species.