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Polymers are huge molecules that are encountered in nature as well as in our modern technology.

This lesson introduces students to polymers, their properties, and some of the many ways polymers are found in the modern world.

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What Are Polymers?

What do DNA, a plastic bottle, and wood all have in common? Give up? They are all polymers!Polymers are very large molecules that are made up of thousands – even millions – of atoms that are bonded together in a repeating pattern. The structure of a polymer is easily visualized by imagining a chain. The chain has many links that are connected together. In the same way the atoms within the polymer are bonded to each other to form links in the polymer chain.The molecular links in the polymer chain are called repeat units that are formed from one or more molecules called monomers. The structure of the repeat unit can vary widely and depends on the raw materials that make up the polymer.

For example, polyethylene, the polymer used to make a wide variety of plastic bags and containers, has a very simple repeat unit, two carbons that are bonded to one another to form a single link.

Polyethylene repeat unit
Polyethylene repeat unit

Synthesizing Polymers

Polymers are created through chemical reactions known as polymerizations, and the majority are produced through two basic reaction types. The first type of polymerization reaction is known as a condensation polymerization. The second type of reaction is known as chain-growth polymerization.Condensation polymerizations, also called step-growth polymerizations, occur when two monomers react to yield a repeat unit and a smaller molecule such as water.

A great example of this type of reaction is the polymerization of nylon from monomers with carboxylic acids and basic amines. The reaction (shown below) creates a link between each monomer and produces water as a by-product and is used to produce nylon fibers for clothing.

Polymerization of nylon that produces water as a by-product
Jacket made with nylon fibers
Jacket made with nylon fibers

Chain growth polymerizations occur when a monomer forms a highly reactive free radical, or molecule with an unpaired electron. The free radical reacts quickly with another monomer and causes a repeat unit with another free radical.

A rapid chain reaction continues the polymerization, and the polymer chain continues to grow longer. One example of a polymer made through a chain-growth polymerization is polystyrene, a polymer commonly found in disposable drinking cups.

Polymerization of styrene to form polystyrene
Polymerization of styrene to form polystyrene
Polystyrene cups
Polystyrene cups

Polymer Properties

Since many polymers are made of long, flexible chains, they become easily tangled, much like a bowl of cooked spaghetti.

The disordered tangling of the polymer chains create what is known as an amorphous structure. Amorphous polymers are typically transparent and much easier to melt to make materials like kitchen cling film.

Kitchen cling wrap
Kitchen cling wrap

Polymer chains do not always form amorphous arrangements. Under proper conditions, such as stretching, the polymer chains can line up side by side to form orderly, crystalline arrangements.

Crystalline arrangements in polymers can also be achieved through slow cooling, where individual polymer chains fold over on themselves.

Aligned polymer chains that create crystalline polymers
Aligned polymer chains that create crystalline polymers
Folded polymer chains that also create crystals
Folded polymer chains that also create crystals

Polymers can also be used to create huge 3-dimensional networks. These networks are made through the reaction of monomers with more than two possible sites for the polymerization to occur. The multiple reaction sites allow for the different chains to connect with each other to form cross-linked chains. The result of the cross-linked chains is a 3-dimensional solid that is essentially one huge molecule.

Polymers in Industry and Nature

Many of the polymers that we are familiar with from our everyday lives are known as plastics. The plastics, or thermoplastics, are polymers that soften when heated and are molded into different forms. Thermoplastics are used to make everything from soda bottles to picnic cutlery.Another application of polymers is the long strands known as fibers. Fibers include many types of synthetic yarn or rope that are made from amorphous materials such as the polyesters. Crystalline polymers can also be used to make fibers, one of the most famous being the fibers found in bullet resistant clothing.The final industrial applications to be discussed here are rubber and foams.

These materials consist of cross-linked polymer chains. Rubber is found in materials such as latex for paints or in vulcanized rubber found in tires. Foams are produced by either blowing air into the monomers during the reaction or through gases produced as reaction by-products. The soft, springy properties of foams make them ideal for bedding like polyurethane memory foam.

Polyurethane memory foam
Wood contains cellulose
Wood contains cellulose
DNA with double helix structure
DNA with double helix structure

Lesson Summary

Polymers are very large molecules that are made up of thousands to millions of atoms that form a sequence of repeat units. Polymers are made through polymerization reactions where molecules called monomers bond together to form repeat units. The polymer chains can either entangle with each other to form amorphous arrangements or align to form crystalline arrangements. Polymers can also be connected through cross-linking to give 3-dimensional networks.

Many modern objects use polymers in the form of thermoplastics as well as fibers. Biological polymers, such as cellulose and DNA, can be found throughout the natural world.

Polymers: Key Terms

chainsofpolymer
Terms Definitions
Polymers very large molecules made up of thousands to millions of atoms that form a sequence of repeat units
Monomers molecules that bond together to form repeat units in a polymer chain
Repeat units molecular links in the polymer chain
Polymerization the chemical reaction that creates polymers
Condensation polymerizations/step-growth polymerizations two monomers react to create a repeat unit and a smaller molecule
Chain growth polymerizations occur when a monomer forms a highly reactive molecule with an unpaired electron
Free radical a molecule with an unpaired electron
Amorphous structure that forms due to the disordered tangling of polymer chains
Cross-linked chains different chains that connect with each other to create a 3-dimensional solids that are essentially one huge molecule
Thermoplastics polymers that soften when heated and can be molded into different forms
Fibers long strands made from amorphous materials or crystalline polymers

Learning Outcomes

Once they finish the lesson, students should be able to:

  • Define polymer
  • Describe how polymers are synthesized
  • Identify examples of polymers used in our daily lives

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