SynopsisKinetic studies of the pH-induced conformational transition of the hydrophobic copolyelectrolyte of maleic acid and styrene in aqueous NaCl at ionic strength 0.09 were performed by an optical method a t 25.OoC with a rapid reaction analyzer of high resolving power. A pH jump applied to a solution, in which the molecule is in the compact form, gives a kinetic trace from which, by appling the general kinetic theory of cooperative transitions, two mean relaxation times are determined. Plots of these relaxation times against the degree of ionization of the primary carboxyl groups of the maleic acid units agree well with theoretical predictions from the linear Ising model, where the molecule in the compact form is described as a random coil with hydrophobic "pockets." The initiation parameter for a pocket obtained from the kinetic data is 0.036. On the other hand, the parameter calculated from the previous pH-titration results by applying Zimm-Rice theory is 0.015 a t the ionic strength 0.09 and 25.0°C. Such values are compared with those determined from pH-titration data on other hydrophobic polyelectrolytes.transition. The standard enthalpy change, AH:, estimated by calorimetry at 25.0"C was reported to be similar to the heat of transfer of benzene from the hydrophobic to the aqueous medium.13 The difference spectrum between the

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Abstract:The conformation of poly(ethacrylic acid) was studied in aqueous salts by means of potentiometric, viscometric, optical and iH-NMR measurements. The modified potentiometric titration curves in aqueous NaC1 at ionic strengths 0.01 -0.3 and 5-35 ~ indicated the pH-induced conformational transition from the compact to extended coil. The standard free energy changes and the difference in molar heat capacity between the two conformers were estimated from the potentiometric data. The viscometric and optical behaviours did also show the existence of the compact form at acid pH's and the pHinduced conformational transition to the extended coil in aqueous NaC1 and NaC10 4. From comparison with the data for poly(methacrylic acid), the results were concluded to be consistent with the existence of the compact form stabilized by hydrophobic interaction between ethyl groups. The methyl widths in 400 MHz 1H-NMR spectra indicated that the side chains in the polymer in the compact form are in a more restricted motional state than in the coil form. Intermolecular aggregations were found at very low degrees of ionization of carboxyl groups.

Kinetic studies of the pH‐induced conformational transition of the hydrophobic copolyelectrolyte of maleic acid and styrene in aqueous NaCl at ionic strength 0.09 were performed by an optical method at 25.0°C with a rapid reaction analyzer of high resolving power. A pH jump applied to a solution, in which the molecule is in the compact form, gives a kinetic trace from which, by appling the general kinetic theory of cooperative transitions, two mean relaxation times are determined. Plots of these relaxation times against the degree of ionization of the primary carboxyl groups of the maleic acid units agree well with theoretical predictions from the linear Ising model, where the molecule in the compact form is described as a random coil with hydrophobic “pockets.” The initiation parameter for a pocket obtained from the kinetic data is 0.036. On the other hand, the parameter calculated from the previous pH‐titration results by applying Zimm–Rice theory is 0.015 at the ionic strength 0.09 and 25.0°C. Such values are compared with those determined from pH‐titration data on other hydrophobic polyelectrolytes.

An alternating copolymer of maleic acid and perdeuteriostyrene was prepared, and the pH-induced conformational transition from the compact to extended coil form was observed by pH, optical, viscometric, and NMR titrations in 0.01-0.27 mol/L aqueous NaCl at 5-45 °C. The transition curves were calculated from the pH and optical data in terms of the coil fraction in the molecule vs. degree of ionization of the carboxyl groups. Also, thermodynamic parameters of the conformational transition were determined from the pH-titration curves and their temperature dependence, taking into consideration dissociation of the secondary carboxyl groups. The transition curve, transition free energy, and difference of specific heats between the compact and coil forms at 0.03 M NaCl were compared with those for the maleic acid-nondeuterated styrene copolymer. All results are consistent with a more enhanced hydrophobic stabilization of the compact form in the deuterated copolymer than in the nondeuterated copolymer. Possible reasons for the enhancement of the hydrophobic interaction between the aromatic substances by deuteration are discussed.