When I first decided to be an organic chemist (more than fifteen years ago), the very first research project I undertook involved the molecular fragment known as alkynes. That project went on to become my Ph.D. thesis, and since graduating I’ve spent more than ten years investigating alkynes. I guess you could say they are my favorite functional group. Organic chemists study carbon-containing compounds, and an alkyne is one of the simplest representations of a carbon molecule: just two carbon atoms, bound together in a straight line. Appearances can be deceiving, however, as the alkyne group is rich in electron density; this gives it a rich and complex reactivity that makes it one of the more interesting atomic arrangements in organic chemistry. Electrons form the “glue” that bonds atoms together, and so having an excess of electron density allows the alkyne unit to reach out and attack nearby atoms, forming new and interesting structures along the way.
One example of this reactivity was at the heart of a research project I saw underway during graduate school. If an alkyne group is brought into close proximity with another alkyne group (at just the right orientation and distance), the two can react together with the assistance of some light rays to form a more complex structure called a poly(acetylene). If the alkyne starting materials are dissolved in a liquid, there is too much jostling and random molecular movement to allow much of this light-catalyzed crosscoupling to take place. For that reason, a long-running research effort has been made towards aligning alkynes perfectly in a solid crystal. It takes a lot of patience, a heavy amount of luck, and some smart molecular design, but alkynes can be designed that crystallize in the perfect orientation and alignment to undergo a reaction when irradiated. This type of thing has been demonstrated hundreds of times, and I’ve used the technique myself on more than one occasion. Sometimes it’s the only way to access a given molecular architecture, but mostly it’s just a neat alternative way of synthesizing a molecule. I thought I knew the scope and limits of this reaction pretty well.
That’s why I was very interested to read a recent article in Nature: Chemistry that discussed the preparation of a two-dimensional polymer using this technique. Polymers can be produced in an array of shapes, but polymerizing in a two dimensional flat sheet has never really been seen before. It’s different from forming a flat sheet of polymer – a plastic clipboard has that shape. This is actually creating the plastic, from scratch, in a flat two-dimensional molecular orientation. Normally, polymers are linear, although they may coil up into a bundle after they’re made. This new class of materials (prepared by German chemists) expands in two directions at the same time. The key to the synthesis was preparing a multifunctional alkyne that had multiple arms extending in the desired orientation. These molecules were then crystallized so that adjaced alkyne-containing molecules would slide into the correct placement within the crystal, just a fraction of a nanometer away from the first molecule. The chemists then shone a light on the crystal, which zipped all of the alkyne units together into a two-dimensional polymer. This article was really a step forward in polymer chemistry. It stretches the limits that many chemists, including myself, thought were in place, and should lead to new plastics and materials with interesting properties.
The source of this article can be found at: Kissle, P., et al. “A two-dimensional polymer prepared by organic synthesis”. Nature: Chemistry 2012, ASAP on Web. DOI: 10.1038/nchem.1265.
