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Introduction to Organic Molecules II: Monomers and Polymers

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  • 0:06 Monomers and Polymers
  • 0:50 Plastics
  • 3:40 Natural Polymers
  • 4:45 Summary
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Lesson Transcript
Instructor: Meg Desko

Meg has taught college-level science. She holds a Ph.D. in biochemistry.

From everyday man-made items like milk jugs and styrofoam to natural proteins and plant materials, the world is full of polymers! Check out this lesson to learn how polymers are constructed on a molecular level.

Monomers and Polymers

Ethylene is a gas that is polymerized to make milk jugs
Milk Jugs Made of Polyethylene

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.

Plastics

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.

Ethylene reacts to form the polymer known as polyethylene
Ethylene Forms Polymer

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.

Kevlar is a copolymer that is made of different types of monomers
Kevlar Made of Different Types of Monomers

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.

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