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.

A new programed upper critical solution temperature-type thermoresponsive polymer was developed using water-soluble anionic polymer conjugates derived from polyallylamine and phthalic acid with cleavage-induced phase transition property. Intrinsic charge inversion from anion to cation of the polymer side chain is induced through a side chain cleavage reaction in acidic aqueous media. With the progress of side chain cleavage under fixed external conditions, the polymer conjugates express a thermoresponsive property, followed by shifting a phase boundary due to the change in polymer composition. When the phase transition boundary eventually reached the examined temperature, phase transition occurs under fixed external conditions. Such new insight obtained in this study opens up the new concept of time-programed stimuli-responsive polymer possessing a cleavage-induced phase transition.