Main-chain metallopolymers at the static–dynamic boundary based on nickelocene

Interactions between metal ions and ligands in metal-containing polymers involve two bonding extremes: persistent covalent bonding, where the polymers are essentially static in nature, or labile coordination bonding, which leads to dynamic supramolecular materials. Main chain polymetallocenes based on ferrocene and cobaltocene fall into the former category due to the presence of strong metal–cyclopentadienyl bonds. Herein we describe a main chain polynickelocene formed by ring-opening polymerization of a moderately strained [3]nickelocenophane monomer, that can be switched between static and dynamic states as a result of the relatively weak Ni–cyclopentadienyl ligand interactions. This is illustrated by the observation that, at low concentration or at elevated temperature in a coordinating or polar solvent, depolymerization of the polynickelocene occurs. A study of this dynamic polymer-monomer equilibrium by ¹H NMR spectroscopy allowed for determination of the associated thermodynamic parameters. Microrheology data, however, indicated that under similar conditions the polynickelocene is considered to be static on the shorter, rheological timescale.