Water-soluble reversible coordination polymers: Chains and rings

The formation of soluble reversible coordination polymers with Zn2+ ions in aqueous solution was studied for two bifunctional ligands, differing in spacer length. Viscosity measurements were used to follow the formation of polymers as a function of the ratio between metal ions and ligands, the total ligand concentration, and the temperature. All the experimental findings could be reproduced and interpreted with a theoretical model that accounts for the formation of chains and rings. At low concentrations and at a 1: 1 metal-to-ligand ratio, a large fraction of the ligand monomers are incorporated in small rings, with a small contribution to the viscosity. Rings are less important at higher concentrations or if one of the two components is in excess. The fraction of monomers in chains and rings could be estimated from H-1 NMR measurements, which were in good agreement with the model predictions. With increasing temperature, the fraction of monomers in rings decreases. As a result, the reduced viscosity increases with increasing temperature.