Thermoresponsive polycations

Abstract

Aqueous solutions of polyelectrolytes are known to be complex; however, when the charged polymer carries a simple counterion such as sodium or chloride it is usually soluble in water. By modifying the hydrophobic/hydrophilic balance of the polymer structure with substituents in the charged repeating unit or by changing the counterion, the solubility and thermal behavior of the polymer can be varied. Recent interest in polymeric ionic liquids has moved much of the focus towards asymmetric systems where the polymer may bind a bulky, often hydrophobic, counterion. Polyelectrolytes with hydrophobic substituents and/or counterions are often thermoresponsive and, interestingly, by refining the structure, it is possible to change the LCST behavior into UCST, and even the coexistence of both LCST and UCST is possible. In this review, we summarize recent progress made on investigating the thermoresponsive behavior of polycations and cationic copolymers in aqueous and non-aqueous solvents. The main emphases are on tuning the solution properties of polycations with alkyl substituents, counterions, and copolymer structures.