Polymer Chemistry
What is a Polymer?
The word "polymer" is a combination of "poly-," which means "many," and "-mer" which literally means "part". In this context we'll translate as "repeat unit." So, a polymer is a molecule made of many repeating units. You'll notice that this idea of repeating units is analogous to the the crystal structures we introducted in previous chapters - although here, it simply tells us about the chemical repeating unit along a polymer chain, not how a polymer chain packs to fill all space (because, indeed, polymer chains often do not do this).
Most polymers are hydrocarbons, made up primarily of hydrogen and carbon. Some examples of hydrocarbon polymers with their repeat units highlighted are shown below in Figure 9.3.1. These polymers are ones that you may encounter on a daily basis: polyethylene, poly(vinyl chloride), and polypropylene. Note that while these polymers are made up primarily hydrogen and carbon, but the presence of other elements in the repeat units (i.e., chlorine) can dramatically alter properties. Poly(vinyl chloride), for example, has improved resistance to chemical degradation and fire.
There are very important non-hydrocarbon polymers as well such as polysiloxanes (silicones), and polyphosphates (common on biological processes), although we'll focus on hydrocarbon polymers, here.
Figure 9.3.1 Some common polymers. Left: Polyethylene is one of the simplest polymers consisting of just carbon and hydrogen. Most plastic bags are made from it. Middle: Poly(vinyl chloride) or PVC has one chlorine atom in the repeat unit. Many plumbing pipes are made from it. Right: Polypropylene has another carbon group in the repeat unit. It is commonly used to make rope.
Why do Polymers Form?
So, polymers are long chain molecules made of repeating units. Why do these long chains form? The repeat units are typically their own separate molecules before they get combined into a polymer. For example, polyethylene with a repeat unit of $\ce{C2H4}$ is synthesized from ethylene molecules which have the same chemical formula but in the structure $\ce{H2C=CH2}$. (Note - one might think that the repeat unit for polyethylene is $\ce{H2C}$. This is sometimes used, but because polyethylene is typically made from ethylene, chemists prefer the repeat unit $\ce{H2C=CH2}$. This is similar to a crystallographer selecting a larger unit cell to better see symmetry.)
We will address some of the processes through which polymers form later (Section 9.7), but here we want to address why they form. Phrased differently, why do the mers combine into a polymer rather staying remaining in their small molecule form? You should be able to generate some idea for this based on your understanding of the electrostatic interpretation of atomic bonding we covered in Section 3.5.