The flow of electrons is a central topic in organic chemistry. One of the most important “bookkeeping” activities for chemists such as myself is keeping track of all the molecules that are gaining and losing electrons. If a compound gains electrons, it’s overall positive charge is reduced, because electrons are negatively charged. This is called a reduction. If a molecule loses electrons, it’s overall positive charge is increased; the loss of a negative is a positive. This is called oxidation. The two processes together is called redox, and normally, one cannot take place without it’s partner. As one compound is reduced, another becomes oxidized. The molecule performing the reduction is the reducing agent, and it becomes oxidized even as it drives the reduction. The reasoning is that it’s actually the transfer of electrons causing the redox and since those electrons have to come from somewhere, there must be another chemical changing it’s oxidation state.
One example of a reaction that uses this principle is called atom-transfer radical polymerization, or ATRP. It was developed by a scientist at Carnegie Mellon and it allows chemists to stitch a few molecules at a time onto the end of a rapidly growing polymer chain. I’ve studied polymers for fifteen years, and I’ve used ATRP dozens of times. It’s a powerful technique. The active catalyst / driving force for the formation of the plastic is an atom of copper with a “+1″ positive charge. Copper can exist in a number of different oxidation states. Copper(I) is the species required for ATRP, and as the reaction proceeds, it is gradually consumed; the metal gets oxidized into copper(II). This species has two positive charges on the atom and it doesn’t participate in the polymerization reaction. Up until now, the only solution to this problem was to simply add more copper(I) salt over the course of the reaction. This does drive the process to completion but it results in a buildup of copper ions in the reaction mixture, which can get trapped in the plastic as it forms. While we normally don’t think about copper as being a toxic metal, in amounts much above 5000 parts per million it can be extremely toxic to living cells.
Trying to reduce the amount of copper required for the ATRP reaction has been an important goal for organic chemists. I’ve followed some of the developments closely, and I was especially interested to read a recent article published in Science. Science is one of the top chemistry journals in the world, and they had news of an ATRP modification devised by the inventor of the reaction. Called “electrochemically mediated” ATRP, the technique applies pulses from a battery to a reaction vessel that contains the ingredients for a polymerization. Electrochemistry is a tool I’ve used throughout my career and it allows chemists to obtain extremely detailed information on a real time basis regarding the flow of electrons inside the reaction cell. When the researchers saw that the concentration of copper(I) was dropping (corresponding to a production of doubly charged copper, which was inactive for the polymerization), they were able to apply electrons in the form of direct current. This way, the electrons didn’t have to come from an external reducing agent and there was no need to continually add fresh copper salt to the reaction. Just a tiny amount of copper could allow the reaction to proceed, and this copper was then regenerated with each pulse of electricity. The resulting electrochemical ATRP proceeded to completion with only a few percent of the normally required amount of copper salt.
This type of approach gives scientists a fine degree of control over the polymerization reaction. Normally, synthetic chemists like myself have to rely on temperature or additional chemical reagents in order to halt a reaction or force its progression. With this new technique, the reaction can be halted or pushed forward with a simple computer command to the electrochemical analyzer. It’s a nice step forward, and although it takes some of the “art” out of a chemical reaction, I have to admit that it gives chemists a much needed additional degree of control over ATRP.
The source of this article can be found at:
Magenau, A., et al. “Electrochemically mediated atom transfer radical polymerization”. Science 2011, 332, 81-84.