United We Stand: The Tale of a Polymer
What is a polymer?
Polymers are all around us. They are in our cars, they are in our adhesives, they are in our food, and they are in our bodies. Plastics, rubbers, glues, starches, and even DNA are all polymers. What could all of these things possibly have in common? The answer lies in the name. Polymer is a word stemming from the Greek words for many, poly, and parts, meros. A polymer is simply a molecule which is made up of many parts. These parts, called monomers, are often many repetitions of only one or two molecules, though it is conceivable that a polymer in which every monomer differs from every other can be produced.
The process of making a polymer, called polymerization, is similar to playing the game Barrel of Monkeys. Think of each monkey as a monomer. You take the first monkey out of the barrel. Then, you link the arm of that monkey around the arm of another. This is like the chemical reaction that takes place between two monomers linking them together. You keep doing this with the monkeys until you have a long chain, a polymer. In a Barrel of Monkeys there may be only 30 or so monkeys in the final chain, but in a polymer there may be hundreds or even thousands of monomers linked together to form the final chain!
An example of a polymer which we depend on for our very being is DNA. DNA is made up of little molecules called nucleotides, of which there are four being Adenine, Thymine, Cytosine, and Guanine (A, T, C, and G for short). These are the four different monomers which made up DNA. Like the monkeys and any other monomer, these nucleotides bind together one after another in a long chain. These monomers can be put in almost any sequence, AAAGCT, ATTCGTAA, GAGACTCT, etc., and the end result is always a polymer.
Two other examples of polymers you might know well are polystyrene, commonly known as styrofoam, and dimethyl siloxane, better known as Silly Putty. In Figure 1 you can see the monomer and resulting polymer of styrofoam. As you can see, polymers can have many different properties, from bouncy Silly Putty to brittle styrofoam to powdery DNA (it does become a stringy white solid when dry!). Why do so many different properties exist? One reason, out of the many, many possibilities, is cross linking.
Cross linking refers to the linking of multiple polymer chains. Perhaps the best analogy for this is to think of a suit of chain mail armor. Chain mail is made by taking several metal chains and linking them together side by side. See Figure 2 for an example. While each individual chain may be flimsy, and a suit of long chains would be almost useless in battle, by linking the chains together you can achieve a stiffer, more protective “fabric.” Cross linking of polymers does the same thing. By taking many individual polymer chains and linking them together with chemical bonds between the two chains, a stiffer polymer can be formed.
Synthetic polymers have been around since the mid 1800’s with the invention of polystyrene. Since then we have invented a huge numbers of man-made polymers with all kind of properties, from shatter-proof safety glass to epoxy resins to Silly Putty. Look around your house, what polymers do you see? What is your favorite polymer?