Synthesis and characterization of polymer-supported calix[4]arenes and bifunctional ion-exchange resins for selective metal ion complexation

Bifunctional ion-exchange resins and polymer-supported calix[4]arenes have been synthesized and characterized. The ion-selectivity of these polymers was evaluated.

Phosphonic acid polymers were synthesized from polystyrene beads at varying levels of crosslinking and studied for their ability to complex Eu(III) and Fe(III) from varying acid concentrations. The percent complexed decreased with increasing crosslinking and acid concentrations regardless of the type of support used. However, by introducing a hydrophilic sulfonic acid ligand into the polymer matrix, rapid kinetics was obtained even with a highly crosslinked gel from high ionic strength solutions due to increased accessibility. The principle of bifunctionality is thus proposed as an alternative to macroporosity for enhanced complexation kinetics.

Complexation of metal ions through a mechanism of inter- and intra-ligand cooperation for enhanced ionic selectivities was evaluated. The diphosphonic acid resin, due to the possibility of intra-ligand cooperation of the phosphoryl groups, had high affinities for all metal ions studied especially in less acidic solutions. The dicarboxylic acid resin was ineffective at the pH studied due to protonation of the carbonyl oxygen. The complexation behavior of the phosphonoacetic acid resin was comparable to the monophosphoric acid resin, leading to the belief initially that cooperation of the phosphoryl-carbonyl groups was not possible by intra-ligand cooperation. However, the ability of a β-ketophosphonate resin to complex high amounts of Cd(II) from a 0.10 N HNO₃ solution showed that they can in fact cooperate by an intra-ligand mechanism for enhanced affinities. Hydrogen bonding of the carboxylic acid proton to the phosphoryl oxygens was proposed as the reason for poor affinities displayed by the phosphonoacetic acid resin.

Calix[4]arene and phosphorylated calbc[4]arene were successfully immobilized onto crosslinked polystyrene supports. The former was able to complex Cs(I) from a 1 N NaOH solution in amounts greater than 96% while the latter was effective in complexing transition metal ions fi"om acidic solutions. The effective cooperation of the ligating groups arranged around the cavity was responsible for the high selectivities displayed by these resins.