Fluorimetric and Potentiometric Study of the Conformational Transition of Isotactic and Atactic Poly(methacrylic Acid) in Mixed Solvents

Boštjan Jerman and Ksenija Kogej*

Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, P. O. Box 537, SI-1000 Ljubljana, Slovenia E-mail: ksenia.kogej@fkkt.uni-lj.si

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Fluorimetric and potentiometric titrations of isotactic (i-PMA) and atactic poly(methacrylic acid) (a-PMA) were performed in methanol-water and in dioxane-water mixtures with varying contents of organic solvent. Fluorimetric investigation of a-PMA confirmed previous reports that the conformational transition of the atactic chain disappears when organic solvent content exceeds 40 and 20% of methanol and dioxane, respectively. For the i-PMA chain, potentiometric data revealed that the conformational transition occurs in methanol-water mixtures with up to 60% of methanol in approximately the same region of degree of neutralization (α_N) as in water. Besides, the irreversibility of this transition does not disappear. On the other hand, the discontinuity in the potentiometric titration curve in dioxane-water mixtures is no longer detectable in 50% dioxane. Concurrently, i-PMA dissolves in 50% dioxane at α_N = 0, whereas it is not soluble in unneutralized state in any of the pure solvents. The observed behavior is attributed to high tendency of dioxane for hydrogen bond-formation both with water and with PMA. The nature of the change in chain dimensions upon neutralization for i-PMA in 50% dioxane is discussed. Results are discussed also in view of a high intermolecular association tendency of PMA chains.

Keywords: isotactic poly(methacrylic acid), irreversible conformational transition, mixed solvents,
methanol-water, dioxane-water