Sunlight Controls Polymer Growth

A novel living polymerization process that is governed by light could lead to the generation of functional and complex metal-rich polymer architectures.

The growth of polymer chains by the repeated addition of monomers normally involves three steps: initiation of the process, propagation of the polymer chain, and chain termination. If there is no termination step, the process is known as a living polymerization, because the chains continue to grow as long as the chains are fed with monomers. The process can be employed to generate macromolecular materials with predetermined molecular weights and controlled architectures.

Chemists in England and Canada now show that, in the presence of bright sunlight or light from a mercury lamp, ferrocene derivatives with a silicon atom bridging the two cyclopentadienyl ligands can be polymerized by using a cyclopentadienyl anion as a nucleophilic initiator (Nature Mat., published online May 14, dx.doi.org/10.1038/nmat1649).

“To the best of our knowledge, this is the first example of a living polymerization of any monomer—organic or metal-containing—where the monomer needs to be photoexcited for polymerization to take place,” says Ian Manners, chemistry professor at the University of Bristol, England. He carried out the work with postdoc Makoto Tanabe and coworkers at the University of Toronto.

Photoexcitation selectively weakens the monomer’s iron-cyclopentadienyl bond, allowing attack of the cyclopentadienyl anion in the initiation and chain propagation steps, Manners explains. As a result, the polymerization can be halted and then restarted by simply switching the light source off and on.

“The work is a beautiful example of photocontrolled living anionic polymerization,” comments living polymerization expert Krzysztof Matyjaszewski of Carnegie Mellon University, Pittsburgh. “The photoexcitation of ferrocenophane monomers changes the polymerization mechanism. This technique can be used for patterning and is especially interesting for the preparation of iron-containing materials with precisely controlled nanostructured morphologies.”

The mild photopolymerization route, Manners suggests, offers unprecedented opportunities to access functional and complex polymer architectures containing metal atoms. “For example, block copolymers can be prepared when the light source is alternately switched on and off in between the sequential addition of different monomers,” he says. “Metal-containing block copolymers are attracting extensive current interest as a result of their self-assembly to give metal-rich nanodomains in thin films and micelles in solution.”

His group is now attempting to elucidate the nature of the photoexcited monomer and is exploring the properties and applications of new metal-rich block copolymers. “We are also attempting to extend the new photocontrolled ring-opening polymerization method to other cyclic monomers that contain a range of metals and also other ligands,” Manners says.