pH-Responsive Gelation in Metallo-Supramolecular Polymers Based on the Protic Pyridinedicarboxamide Ligand

Pyridinedicarboxamide (PDCA) ligands have been widely integrated into different polymer chemistries and provided promising self-healing properties in bulk upon adding specific metal ions. However, their application in the development of synthetic hydrogels is limited due to the lack of clear kinetics and thermodynamic data. To fill this gap, we study the dynamics of polymer hydrogels formed by the condensation of linear poly(ethylene glycol) (PEG) precursors and the PDCA ligand, followed by the complexation with different metal ions at various pH values. Rheological studies reveal an unprecedented ability of PDCA in complexation with Co2+/3+ and Ni2+ at elevated pH values, but in contrast with former reports, not with Fe3+ and Zn2+. Moreover, Co2+/3+ gels demonstrate lower equilibrium constants resulting in faster and more efficient self-healing compared to those made by Ni2+. Spectroscopic UV and Fourier transform infrared (FTIR) studies also suggest structural modifications in the presence of the gel-forming metal ions but not the others. To explain these results, we employ static and periodic density functional theory (DFT) simulations. Static DFT indeed reveals that the binding interaction is clearly stronger for Co and Ni, and Fe shows the poorest values by far. The most stable complex geometry is octahedral, in which the central pyridyl, one lateral deprotonated amide, and one carbonyl participate in the metal coordination. Moreover, the periodic calculations unveil an overstabilization effect due to the noncovalent interactions between the PEG segments, which results in their torsion and formation of a helicoidal-like structure.