O-Chem in Real Life: Conducting Polymers
We don't spend a lot of time in this class talking about polymers (except in "Real Life" web pages!!), however, their basic structures are easy to understand. Consider the following reaction(s) of the simplest alkene, ethylene...
We are not surprised that this reaction "goes", since it converts many high energy pi-bonds into lower energy (stronger) sigma-bonds. This scheme says nothing about the details of how this works (we may return to this on another occasion), or how the reactions terminate (reactions such as these are chain reactions like the photochemical bromination reaction we met in first semester), or what is on the ends of the polymer chains, but the basic principle is clear. Many ethylene molecules are joined together. The polymer of ethylene is polyethylene, a plastic that finds many uses in packaging, toys, housewares, etc. Typically, polyethylene consists of polymer chains with a range of molecular weights, but each chain contains many thousands of ethylene monomers.
Polymerization of ethylene converts all of the pi-bonds into sigma-bonds. Now consider the corresponding polymer you would get from acetylene, polyacetylene....
We are left with a series of alternating double and single bonds. Now consider the p Atomic Orbitals that are used to make the double bonds...
We recognize the pi-bonds arising from overlap of the p A.O.'s. Why don't the p A.O.'s overlap in the direction of the new sigma bonds, indicated in dark red? They do! This is an example of a "Conjugated System". We will look at conjugated systems in more detail later in the semester. For now, all we need to know is that we have effective pi-bonding throughout the entire polymer molecule! Furthermore, we will see that in conjugated systems, electrons can be delocalized over the entire length of the conjugated system, i.e. over the entire molecule. Scientists wondered if this could mean that electrons could be induced to flow along a molecule, like in a copper wire, and so make a molecular, or at least an organic, wire. The answer is yes (sort of!!).
The Nobel Prize in Chemistry in 2000 was awarded for "The Discovery and Development of Conductive Polymers" to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa. These scientists were the first to obtain polymers that could be made to conduct, and their work has lead to an explosion in the study of organic molecules for electronic applications. It is pretty cool when you think about it. Plastic is usually used to insulate copper wires, yet here is a polymer that conducts electricity. And WE understand why!
Sort of! In fact, the situation is much more complex that described above. First, pure polyacetylene is found to be essentially an insulator, it only conducts when it is appropriately "doped". The usual doping does a one-electron oxidation, leaving single unpaired electrons in the polymer. It is these unpaired electrons that move, the electrons in the pi-bonds are held too "tightly". Second, it is obvious that any charge conduction in doped polyacetylene must involve hopping of electrons between polymer chains, since no single polymer chain can be as long as a useful wire. Third, we now know that the polymer is not straight as shown above, but is twisted and in many places is sufficiently twisted that the conjugation becomes broken. Nevertheless, useful electronic properties are found for suitably modified polyacetylene, and now many other conjugated polymers. Even if Heeger, MacDiarmid and Shirakawa didn't understand their systems entirely, they deserve a lot of credit for initiating a huge contemporary field of research
I met Alan MacDiarmid once, and he was a simply delightful gentleman!