pH-induced solubility transition of sulfonamide-based polymers.

As an approach to designing new pH-sensitive polymers for bio-related application, we have modified selected sulfonamides, with various pK(a), to polymerizable monomers. The pK(a) of the monomers, homopolymers, and copolymers with N,N-dimethylacrylamide were examined, and pH-induced phase transition behavior, particularly in solubility, was investigated. The pK(a) of sulfonamide monomers and polymers at 25 degrees C was slightly higher than those of corresponding sulfonamides but the enthalpy of the ionization was influenced due to interfering resonance structures that are present in the mother compounds. The solubility transition of each homopolymer in aqueous solutions occurred at a degree of ionization of 85-90%. For the copolymers, the solubility transition observed by light transmittance completed in a narrow pH range (0.2-0.3 pH units) and this transition pH shifted to a higher pH region with the increasing sulfonamide unit in the copolymer, though the pK(a) was not considerably changed. This polymer precipitation occurs because of the relative balance of overall hydrophilicity/hydrophobicity along the polymer chain. Because the unionized form of sulfonamide units is considered to be hydrophobic, the copolymer with higher content of sulfonamide unit requires a higher degree of ionization for solubilization and thus solubility transition occurred at higher pH.