Cononsolvency of poly(carboxybetaine methacrylate) in water–ethanol mixed solvents

Cononsolvency refers to the phenomenon in which a polymer collapses in mixtures of good solvents with specific compositions. The coil-to-globule-to-coil transitions of a zwitterionic polymer, poly(2-[( N -2-methacryloyloxyethyl- N,N -dimethyl)ammonio]acetate) (PCB2), in water, ethanol, and water–ethanol mixed solvents were investigated and compared with those of poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC). PCB2 showed cononsolvency in water–ethanol mixed solvents with specific ethanol volume fractions. The reentrant square well-type sharp coil–globule transition was reproduced by a statistical mechanical model involving competitive hydrogen bonding and cooperative hydration. PCB2 showed a broader cononsolvency range than PMPC because of its lower association constants of water and ethanol and the marked competition for hydrogen bonding. The zwitterion-specific cononsolvency characteristics were rationalized with the electrostatic potentials and van der Waals energies of the zwitterions.