Potentiometric Titrations of Maleic Acid Copolymers in Dilute Aqueous Solution: Experimental Results and Theoretical Interpretation

Abstract

Summary: The paper reports a study on the flexibility of a family of 1:1 hydrolysed maleic anhydride (maleic acid, MA)–olefin copolymers in dilute aqueous solution. The copolymers were MA–ethene, MA–propene and MA–isobutene. The study was carried out in the absence and in the presence of monomonovalent salts and at different polymer concentrations. Experimental data showing the negative logarithm of the ‘apparent’ dissociation constant, pKa versus the degree of dissociation, α, experimental data were obtained from potentiometric titrations using NaOH, KOH or tetramethylammonium hydroxide as the base. The pKa data were fully described with calculated curves obtained using an extension of the counterion condensation theory of linear polyelectrolytes, in which a semiflexible model for the polymers was introduced. Under the present experimental condition, no relevant specificity of the monovalent counterions was apparently observed for the different copolymers. The calculated pKa versus α curves allowed the derivation of both the intrinsic dissociation constants of the first and the second dissociation steps for the different copolymers and the corresponding stiffness parameters built into the model. The agreement between the experimental and calculated data shows an appreciable success of the model. The results pointed to an increase of stiffness parallel to the increase of size of the olefin comonomer, in qualitative agreement with already published findings. Furthermore, for all copolymers the chain rigidity was larger in the α range of the first dissociation than in that of the second one. The former rigidity was attributed to the formation of intramolecular hydrogen bonds upon the first ionisation of the MA repeating units, followed by an increase of rotational freedom upon breaking of the H‐bond in the second dissociation step. Comparison of the rigidity parameters of the MA copolymers with the data obtained for other polyelectrolytes, both natural—poly(L‐glutamic acid) and pectic acid—and synthetic—poly(acrylic acid) and poly(methacrylic acid)—was also performed. magnified image

Synthesis, characterisation and properties of α,β-poly(N-2-hydroxyethyl)-dl-aspartamide-graft-maleic anhydride precursors and their stimuli-responsive hydrogels

“…In the 0 < c < 1 range, PHEA-MA-a (DD = 25%) shows a gradual monotonic increase of pH with the degree of neutralisation, whereas PHEA-MA-b (DD = 50%) presents a distinct two-step behaviour with two intermediate plateaus. Analogous trend is reported in the literature for other polyvalent electrolytes, described using site binding (SB) models [38,44] or extensions of the Counterion Condensation (CC) theory of linear polyelectrolytes [43]. A two-step protonation has been generally explained as follows: in the first deprotonation step, at lower pH, all the neighbour pair interactions are avoided and deprotonation, with Na + binding, is likely to involve every second site.…”

“…The initial decrease of pK a up to approximately the condition of half neutralisation can be interpreted postulating an extra-interaction due to the presence of intra-repetitive units H-bonding, that takes into account a stiffer polymer conformation at low a values, eventually destroyed by the increase of protonation degree, as already observed for other similar carboxylated polymers [43,45,46]. The comparison between the FTIR spectra of the two precursors in the m(C@O) spectral region, reported in Fig.…”

“…Often the corresponding ''apparent" equilibrium constants present unusual features that have been related either to abrupt changes in the molecular conformations or to aggregation-disaggregation phenomena occurring varying the pH. Several fascinating theoretical approaches have been developed to interpret the experimental evidence of potentiometric titrations, starting with the pioneristic work of Katchalsky [39] further developed by different authors [37,[40][41][42][43]. In this case, the specific polyaminoacid structure of PHEA backbone, as well as the ''random" graft-copolymerisation with MA, complicate the description of the proton-binding reactions with respect to other homopolymers or copolymers.…”

The Electrostatic Origin of Low‐Hydration Polymorphism in DNA

“…If no transition is existent a monotonic increasing function can be observed (i.e., poly(acrylic acid)) 43. It is already well established that the shapes of the p K app versus α for polymers such as poly(methacrylic acid),44–46 xanthan,47 poly( L ‐aspartic acid),48 poly(maleic acid)49–53 and poly(fumaric acid)52, 53 show an unusual dissociation behavior, where the curve shape itself directly reflects the additional work required for a conformational change due to ionization.…”

Potentiometric Behavior of Polyampholytes Based on N,N′‐diallyl‐N,N′‐dimethylammonium Chloride and Maleamic Acid Derivatives

“…If no transition is existent a monotonic increasing function can be observed (i.e., poly(acrylic acid)). [ 43 ] It is already well established that the shapes of the p K app versus α for polymers such as poly(methacrylic acid), [44][45][46] xanthan, [ 47 ] poly( L -aspartic acid), [ 48 ] poly(maleic acid) [49][50][51][52][53] and poly(fumaric acid) [ 52 , 53 ] show an unusual dissociation behavior, where the curve shape itself directly refl ects the additional work required for a conformational change due to ionization. Figure 2 -4 illustrate the dependence of p K app for PalH, PalPh and PalPhBisCarb as a function of α at different ionic strengths (an example titration curve obtained for PalH in 1 M NaCl can be found in the Supporting Information).…”