Mechanical Force Triggers Catalyst

Unusual approach promises polymers that repair themselves.

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"Let the force be with you" may take on a meaning beyond "Star Wars," due to work at the Department of Chemical Engineering and Chemistry at the Eindhoven University of Technology, Eindhoven, the Netherlands. A research team there has discovered a way to activate dormant homogeneous catalysts by exerting force.

"Latent catalysts that are activated by an external agent (e.g., acid) or by raising the temperature are well known, and they are useful because they provide a method to have the catalyst present in a stable form that can be activated when necessary. The present mode of activation is novel and without precedent, and opens entirely new possibilities in the range of applications, the most important of which I consider its use in self-healing polymeric materials," says Rint Sijbesma, a professor in the department, who worked on the development with Alessio Piermattei and Karthik Sivasubramanian.

By incorporating a latent polymerization catalyst in a material, it might be possible to use the mechanical stresses that cause cracks to repair them — by initiating polymerization to reinforce the material at the exact time and place needed, explains Sijbesma.
Besides their use for self-healing polymers, reactions that can be switched on by strong shear forces can be extremely useful in reaction injection molding of polymers and in reaction control in lab-on-a-chip applications, he notes.
Figure 1 -- Ripcord effect
Liquid flow pulling on polymer tail attached to metal ion releases the ion,
which then serves as a catalyst for polymerization.
Source: Eindhoven University of Technology.

The researchers enveloped a catalyst (a metal ion) using two molecular caps to which they attached a long polymer tail and then dissolved the material in a liquid. Applying ultrasound, they created a strong flow that pulled hard enough on the polymer chains to separate the molecular caps from the metal ion (Figure 1). The ion, once released, functions as a catalyst in building a polymer chain. Similar constructions could be built into all kinds of materials, believes Sijbesma. "The principle is very general and can be applied to any latent homogeneous catalyst," he says.

"We have shown in two distinct systems that a catalyst could be 'switched on' when one of the ligands was pulled off from an inactive precursor metal complex through polymer chains attached to the ligands. This resulted in catalysts with high activity for transesterification, ring-closing metathesis, and ring-opening metathesis polymerization, all of these being useful chemical transformations," notes Sijbesma.

"The most exciting aspect of the work is that it shows how catalytic activity — one of the most fundamental concepts in chemistry — can be controlled by mechanical force in a productive way," he adds.

Because the researchers consider self-healing polymers the most important potential application, they now are focusing on using deformation instead of ultrasound as the source of mechanical force. Deformation of a solid promises better control than ultrasonication of a liquid, they note. Experiments with solids already have started. "They are very promising, and we expect to have established feasibility within a year," says Sijbesma. If all goes very well, the technique could be commercialized in three years, he adds.
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