Polyampholyte terpolymers of amphoteric, amino acid-based monomers with acrylamide and (3-acrylamidopropyl)trimethyl ammonium chloride

Low-charge-density amphoteric copolymers and terpolymers composed of acrylamide, (3-acrylamidopropyl)trimethyl ammonium chloride, and the amino acid derived monomers (e.g., N-acryloyl valine, N-acryloyl alanine, and N-acryloyl aspartate) were prepared via free-radical polymerization in aqueous media to yield terpolymers with random charge distributions and homogeneous compositions. Sodium formate (NaOOCH) was employed as a chain transfer agent during the polymerization to suppress gel effects and broadening of the molecular weight distribution. Terpolymer compositions were determined by ¹³C and ¹H. NMR spectroscopy. Terpolymer molecular weights and polydispersity indices were obtained via size exclusion chromatography/multi-angle laser light scattering, and hydrodynamic diameter values were obtained via dynamic light scattering. The solution properties of low-charge-density amphoteric copolymers and terpolymers have been studied as a function of solution pH, ionic strength, and polymer concentration. The low-charge-density terpolymers display excellent solubility in deionized (DI) water with no phase separation. The charge-balanced terpolymers exhibit antipolyelectrolyte behavior at pH values ≥(6.5 ± 0.2). As solution pH is decreased, these charge-balanced terpolymers become increasingly cationic because of the protonation of the anionic repeat units. Charge-imbalanced terpolymers generally demonstrate polyelectrolyte behavior, although the effects of intramolecular electrostatic interactions (e.g., polyampholyte effects) on the hydrodynamic volume are evident at certain values of solution pH and salt concentration. The aqueous solution behavior (i.e., globule-to-coil transition at the isoelectric point in the presence of salt and globule elongation with increasing charge asymmetry) of the terpolymers in the dilute regime correlates well with that predicted by the polyampholyte solution theories of Dobrynin and Rubinstein as well as Kantor and Kardar. Examination of comonomer charge density, hydrogen-bonding ability, and spacer group (e.g., the moiety separating the ionic group from the polymer chain) indicates that conformational restrictions of the amino acid comonomers result in increased chain stiffness and higher solution viscosities in DI water and brine solutions.