Bifunctional Coordinating Polymers: Auxiliary Groups as a Means of Tuning the Ionic Affinity of Immobilized Phosphate Ligands

A series of coordinating polymers are synthesized by immobilizing polyols (ethylene glycol, glycerol, 1,1,1-tris(hydroxymethyl)ethane, pentaerythritol, and pentaerythritol triethoxylate) onto cross-linked poly(vinylbenzyl chloride) and then monosubstituting with diethyl phosphate ligands. Ionic affinities are determined with a series of divalent transition metal ions: Pb 2+, Cd 2+, Cu 2+, Ni 2+, and Zn 2+ . For each polymer, the variable controlling the affinity, as measured by the distribution coefficient, D, is the polarizability of the transition metal ion, as measured by the Misono softness parameter, σ. For D ≥ 0, the correlation is D = S σ + σ min, where S is the selectivity and σ min is the minimum softness of a divalent transition metal ion which must be exceeded for coordination to occur. The values of S are 3810 (pentaerythritol), 1480 (pentaerythritol triethoxylate), 1340 (glycerol), 474 (tris(hydroxymethyl)ethane), and 21 (glycol); increasing S indicates greater selectivity. It is proposed that, though coordination occurs through the phosphate ligand, selectivity varies as a function of the polyol: the −OH groups act as auxiliary groups in modifying the polarizability of the phosphate ligands (the primary ligand) through hydrogen bonding and, as a result, affecting the selectivity of the primary ligand. Interaction between the −OH and phosphate moieties is evident in FTIR spectra of the polymers with the absorbance of a band within the range 874−895 cm -1 .