Self-healing materials have been an objective of extensive research for obvious reasons. Most polymers, regardless of their application, are exposed to external stress that may cause them to undergo substantial wear-and-tear over their lifetimes. The development of polymers that fix themselves would help to create materials that are more durable, improving their overall safety and reliability. To date, most intrinsically self-healing polymers are limited to systems that possess relatively weak mechanical properties. The development of a cross-linked polymer system containing sacrificial hydrogen bonds, featured in a recent publication by the Guan group, is an important advancement towards the creation of self-healing materials with more robust mechanical properties. Olefin metathesis is essential both to the synthesis of this polymer and to its ability to repair itself after damage.
The preparation of this polymer involves the co-polymerization of cyclooctene and a substituted cyclooctene derivative containing a secondary amide capable of hydrogen bonding. A second-generation Grubbs catalyst was employed in this ring-opening metathesis polymerization (ROMP) to produce linear polymers with high molecular weight. The functional group tolerance of the Grubbs catalyst, e.g. its stability in the presence of secondary amide groups, is important to the success of this reaction. After ROMP, a radical crosslinking procedure was performed, and the polymer network was impregnated with more second-generation Grubbs catalyst.
A series of tests to determine the mechanical and self-healing properties of this polymer shows that its toughness and self-repairing capability are substantial. For example, tensile tests show that the tensile elongation at break for this polymer is almost 950% and that the enhanced toughness provided by the sacrificial hydrogen bonds is maintained over time. Furthermore, the presence of the second-generation Grubbs catalyst within the polymer matrix enables rapid self-healing under relatively mild conditions (50°C in air); for example, samples that were damaged by cutting through ca. 75% of their width were repaired via olefin cross-metathesis at the cut interface, and the original toughness of the polymer samples after self-reparation was largely retained. These results demonstrate the significance of olefin metathesis in creating self-healing polymers that are mechanically robust and resilient.