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
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
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 creates 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
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 amorphousstructure. Amorphous polymers are typically transparent and much easier to melt to make materials like kitchen cling film.
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
Folded polymer chains that also create crystals
Polymers can also be used to create huge three-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 three-dimensional solid that is essentially one huge molecule!
Properties of 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.
