Water-Soluble Polymers. 73. Electrolyte- and pH-Responsive Zwitterionic Copolymers of 4-[(2-Acrylamido-2-methylpropyl)- dimethylammonio]butanoate with 3-[(2-Acrylamido-2-methyl- propyl)dimethylammonio]propanesulfonate

Amphoteric copolymers of the carboxybetaine monomer 4-[(2-acrylamido-2-methylpropyl)dimethylammonio]butanoate (AMPDAB), 1, with the sulfobetaine monomer 3-[(2-acrylamido-2-methylpropyl)dimethylammonio]propanesulfonate (AMPDAPS), 2, have been synthesized by free radical polymerization in 0.5 M NaBr. The feed ratios of 1:2 were varied from 100:0 to 0:100 mol %. Molecular weights were determined by low angle laser light scattering and range from 1.65 × 106 to 3.95 × 106. The homopolymer of 2 was insoluble in deionized water but soluble in aqueous solutions containing a critical concentration of added electrolytes. In contrast, the homopolymer of 1 was soluble in deionized water. The solubility behavior of copolymers of 1 and 2 is complex and varies with copolymer composition, pH, as well as the concentration and nature of the added electrolytes. Coulombic interactions and hydration effects are suggested to be the main factors affecting the solubility behavior of these amphiphilic copolymers. Low shear viscosity studies were conducted at varying electrolyte concentrations and pH values. The copolymers with a higher incorporation of AMPDAPS, 2, mer units showed greater enhancement in viscosity with increased electrolyte concentration. Copolymers containing AMPDAB, 1, mer units exhibit polyelectrolyte behavior at low pH in deionized water as the carboxylate groups are protonated, resulting in an overall cationic charge along the polymer chain. The solution behavior of these responsive systems is rationalized on the basis of competitive hydration of the zwitterionic moieties under testing conditions.