Metal-based polymers have unique properties that make them suited for applications such as gels that change shape on oxidation or “self-healing” materials. Some of these properties arise from the bonding between metal ions and ligands in the metallopolymers. The interactions normally involve one of two extremes: persistent covalent bonding, in which the polymers are stable, or labile coordination bonding, in which the polymer properties can be easily manipulated by solvent, heat, or light.
Rebecca A. Musgrave and Ian Manners of the University of Bristol and coworkers have developed a new feature for metallocene-based polymers, showing that the molecules can switch between the stable and dynamic states. This ability could lead to new applications for stimulus-response materials and ease processing and recyclability of metallopolymers.
Polymers built from metallocene units such as ferrocenylsilane fall into the stable category because of the strong iron-cyclopentadienyl covalent bonds in the signature sandwich structures—the polymers are comparable in durability with polyethylene and polystyrene. On the other hand, polymers containing zinc coordination complexes have labile metal-ligand bonds and fall apart, for example, when irradiated by light.
Manners’s team decided to look at crafting metallopolymers that could reversibly switch between these behaviors. This led them to nickelocene. Unlike ferrocene, which has 18 valence electrons, nickelocene has 20 valence electrons. The two extra electrons are unpaired, and their presence leads to longer, weaker, and more easily cleaved metal-cyclopentadienyl bonds relative to ferrocene.
The researchers found they could use the bonding difference to control the material properties in the nickel-based polymer by choice of solvent, dilution level, temperature, and time. For example, polynickelocene remains stable in a low-polarity, noncoordinating solvent such as toluene but undergoes dynamic depolymerization in a polar, coordinating solvent such as pyridine (Nat. Chem. 2017, DOI: 10.1038/nchem.2743).
These switchable properties should add to the range of applications for metallopolymers, Manners says. In particular, the unpaired electrons in the nickel-based polymer offer the potential to create soluble and easily processed magnetic materials. Polyferrocenes, in contrast, lack interesting magnetic properties.
The new work “is really something special,” says metallopolymer specialist Jens Müller of the University of Saskatchewan. Ring-opening polymerization of strained sandwich compounds is an elegant and well-established method to access metal-containing polymers, Müller notes. But even after years of investigation, it’s fascinating that this chemistry “is still good for a surprise,” he says.