A Conformational Change of Poly(2-vinylpyridine) in Solution as Revealed by Potentiometric Titration

Abstract

The ionization behavior of isotactic and atactic poly(2-vinylpyridine) (P2VP) in water–ethanol mixtures containing dilute hydrochloric acid is investigated by means of potentiometric titration. The degree of ionization of P2VP, β, is directly determined by measuring a UV absorbance at 2625 A. Stoichiometric proton concentrations (pHc) are determined by applying Gran’s procedure proposed for nonaqueous systems. Potentiometric titration curves established for the polymer species with different steric isomerism indicate that the atactic polymer exists in solution as a stronger base than the isotactic polymer. The electrostatic free energies required to ionize the polymers are calculated to be 2000 and 1800 cal/equiv mol for isotactic and atactic polymer, respectively. A plot of pHc vs. β constructed with data for the atactic polymer shows a plateau region in a range of β between 0.2 and 0.4, while such a region is not observed for the isotactic polymer. This feature suggests that a conformational transition from one assumed at a low degree of ionization to another assumed at a high degree of ionization occurs in the atactic polymer chain. The free energy change for the conformational transition at the discharged state is estimated to be ca. 300 cal/equiv mol according to the approach proposed by Nagasawa–Holtzer. In connection with the above findings, the viscometric behavior of the polymer species with different steric isomerism at β=0.3 is also investigated. No significant difference in the hydrodynamic behavior between the isotactic and atactic species is detected.