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A polymer is a molecule made up of many repeating units. The reaction that creates the polymer for the many smaller subunits (called monomers) is called a polymerization reaction. Polymers are widely used both by nature and man due to the amazing variety of properties they can exhibit. One of the simplest polymers is polyethylene, which is used for a variety of things such as trash bags and sandwich wrap. The polyethylene is made up of many ethylene molecules, CH2=CH2. Two ethylene molecules can form a bond to make
The molecule above is an example of a linear polymer- there's only one long chain. The properties of bulk polyethylene are due to the long string-like nature of the molecules: it acts much like a bowl of spahgetti, stretching and bending. With the proper choice of monomers, you can also have branched polymers, where the molecule looks more like a giant spider, or even cross-linked polymers where all the molecules are bonded to each other. The latter type of polymers tend to be hard and non-ductile. By varying the amount of branching and cross-linking, polymer chemists can develop materials which are very closely suited to the needs at hand. There are numerous ways of making polymers. Two of the most common are addition and condensation. Polyethylene is an addition polymer: each unit adds onto the chain without leaving any residue behind. A condensation polymer is one where the reaction leaves behind a small molecule, often water. The formation of peptide bonds in proteins is an example of a condensation polymerization. Glycine is the simplest amino acid: the reaction below shows the reaction between two glycine molecules to form a glycine dimer. |
| the atoms in the red box drop away to form the water molecule on the right. This is the reaction that is used for all amino acids in the formation of proteins and is fundamental to life as we know it. |
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