Preliminary investigations on a simple polyelectrolyte derived from (CH2CH2C(COOH)2)n: Unexpected solubility-insolubility pattern controlled selectively by the nature of the alkali counterion

- A polyelectrolyte is described that exhibits an uncommon water solubility pattern when its counterions are alkali ions.
- The solubility pattern depends highly on the nature of the alkali counterion, in the order: Li > K > Na.
- Practical use can be expected in processes where selective recognition of alkali ions is requested.

sMonodisperse samples of alkali poly(trimethylene-1,1-dicarboxylate) polyelectrolytes of (CH2CH2C(COO−)2)n structure and of low to moderate molecular weights have been synthesized and characterized. These polymers are densely and evenly decorated by carboxylate groups, with the anionic moieties extending symmetrically in transverse directions on every third carbon alongside the backbone. In sharp contrast to other polyelectrolytes, they easily precipitate with sodium and potassium ions in water. The precipitation is selective, the solubility varying with the nature of the alkali cation in the order: Li+ > K+ > Na+. Precipitation is fast and reversible, the solubility-insolubility transition being rapidly crossed in both directions when the proper alkali ion is added in excess to the solution/suspension. Experimental investigations (WAXS, FTIR) on the precipitated polymer and DFT theoretical calculations on model 1:1 complexes of malonate subunits with alkali cations have been carried out in order to cast some light on the above phenomena. In addition, a model molecule (CH3C(COOH)2CH2CH2C(COOH)2CH3) has been prepared, supporting the structure of the obtained polyelectrolyte as well as the formation of specific structures at half-deprotonation of the poly(carboxylic acid).

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