Complexation of bisphenol A with calix[6]arene-polymer conjugates

Sodium calix[6]arenehexasulfonic acid (SCX₆) was introduced into polymer molecules by the cross-linking of SCX₆ with epichlorohydrin (PolySCX₆) or by the covalent binding of carboxymethylated SCX₆ to an amino group-carrying vinyl polymer (Poly(SCX₆HMA-co-AAm)). The complexation of bisphenol A (BPA) with the SCX₆ group in the polymers was examined fluorimetrically using acridine red (AR) as a probe. From the inhibitory effect of BPA on the complexation of AR with the SCX₆ group in the polymers, the association constant (K_assoc) of BPA with the SCX₆ group was evaluated and the value for BPA with the SCX₆ group in Poly(SCX₆HMA-co-AAm) was much larger than that with the non-conjugated SCX₆, probably because of the support of hydrophobic interactions between the guest and the hydrocarbon moieties in the polymer (polymer main chain and hexamethylene groups introduced for the conjugation of SCX₆). The K_assoc value for bisphenol B (BPB) with a nonconjugated SCX₆ was larger than those for bisphenol A and bisphenol F, because of the higher hydrophobicity of BPB among the bisphenols examined. Thermodynamic parameters indicated that the inclusion of BPA into the cavity of sodium propyloxycalix[6]arenehexasulfonic acid (SPCX₆) was enthalpy driven, and the entropy change was largely negative (-92 J · mol^-1 · K^-1). This is probably because of a tight fit of guest molecules in the SPCX₆ cavity, resulting in the loss of freedom of both the SPCX₆ and the guest molecules. The entropy change for the complexation of BPA with the SCX₆ group in the polymer was less negative, probably because of a partial release of water molecules around the SCX₆ group upon the complexation. The effect of the structure of guest and host molecules on the complexation was also examined.