Meg has taught college-level science. She holds a Ph.D. in biochemistry.
Monomers and Polymers
In this lesson, we're going to be talking about monomers and polymers. You may not know it, but polymers are something that you interact with every single day. You can find them in the plastic milk jug that you drunk out of this morning or in the fleece jacket that you're wearing. You can find them in the trees outside your window. You can also find them in your own body as proteins.
What I mean when I say polymer is a long chain molecule that is made up of many smaller units or that is made from many smaller units. Each unit is called a monomer or single, while polymers are many. Each unit can be combined to make a many unit molecule, or polymer.
In the case of the plastic milk jug that we talked about, it's made up of polyethylene. Polyethylene is made up of a smaller molecule called ethylene, which is four hydrogen atoms with two carbon atoms connected with a double bond. Now, this is pretty amazing to think about, because ethylene is a gas at room temperature. Instead of just relying on these ethylene molecules to get together and make something interesting for us, we take these molecules and cram them into a small container to pressurize them, and then we add a catalyst to start the polymerization process.
The catalyst will cause one of the hydrogen atoms to break loose, and this hydrogen atom will go form a bond with one of the carbons. Carbon can only have four bonds, so the double carbon-carbon bond breaks. We go find another ethylene molecule to form a bond with and so on. So, first it forms a dimer with another ethylene molecule, then the double carbon bonds in that ethylene molecule break and go find another ethylene molecule to interact with and form a trimer, or three unit molecule, and so on until you end up with a very long polymer.
The reason these milk jugs stay together so well is that their polymers are so long, which means that they can have really strong intermolecular forces occurring between them. If you consider something like Christmas lights, you can put them together, and they come out as a big jumble, intertwined around one another so that you can't pull them apart. That's what happens with these polymers; they get wrapped around each other and form really strong bonds between molecules so you can't pull them apart.
In this case, we showed a polymer that is made up of one monomer, but it's also possible to have polymers that are made up of different monomers. One polymer that is made of different monomer units is nylon, which is a combination of hexamethylenediamine and adipic acid. The nitrogen in the hexamethylenediamine reacts with the carbon in the carbonyl group of the adipic acid, creating water as a byproduct and forming very long chains with alternating units of hexamethlyenediamine and adipic acid.
The same is true of kevlar, which is a copolymer of 1,4-phenylene-diamine and terephthaloyl chloride. Those names aren't important, but what's important is to realize that polymers can be made up of different monomers. The nitrogen in the 1,4-phenylene-diamine bonds to the carbonyl carbon in terephthaloyl chloride, forming hydrochloric acid as a byproduct. The monomers come together to form a very strong polymer that is used in bulletproof vests.
In addition to all of these man-made polymers, they can also occur naturally as well. One example of natural polymer is cellulose. Cellulose is a natural polymer that's made up of repeating glucose monomers. And of all these glucose molecules, cellulose is one molecule that is responsible for helping to hold plants upright.
Finally, we have proteins. Proteins are another natural polymer, and they have many possible monomer units because there are 20 different amino acids, which are the building blocks of proteins. They're formed when the nitrogen of one amino acid bonds to the carbonyl carbon of another amino acid, similar to the formation of nylon, and creates peptide bonds. In this case, we can form a small polymer. The nitrogen on alanine can bond to the carbonyl carbon on glycine, with water as a byproduct. Similarly, serine can come along, and its nitrogen can bond to the carbonyl carbon on alanine with water as a byproduct, forming peptide bonds. Similar bonds can occur between all the amino acids, forming very long molecules known as proteins.
We've learned that polymers are long chain molecules that are made from small, single units, known as monomers. We know that plastics are a good example of polymers that can be formed from the same monomers or from different monomers forming copolymers. Additionally, we know that there are natural polymers such as cellulose, which is a long glucose molecule found in plants, and proteins, which are made from the 20 different amino acids.
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