Explainer: What are polymers? | Science News for Students

Explainer: What are polymers?

These materials, many of them artificial, are built from repeating chains of smaller chemical units
Oct 13, 2017 — 6:50 am EST
polymer model

Polymers, whether artificial (such as the plastic shown) or natural, are made of repeating chains of smaller chemical units. Here, carbon atoms are shown as black, oxygen as red and hydrogen as white.

Molekuul/iStockphoto

Polymers are everywhere. Just look around. Your plastic water bottle. The silicone rubber tips on your phone’s earbuds. The nylon and polyester in your jacket or sneakers. The rubber in the tires on the family car. Now take a look in the mirror. Many proteins in your body are polymers, too. Consider keratin (KAIR-uh-tin), the stuff your hair and nails are made from. Even the DNA in your cells is a polymer.

By definition, polymers are large molecules made by bonding (chemically linking) a series of building blocks. The word polymer comes from the Greek words for “many parts.” Each of those parts is scientists call a monomer  (which in Greek means “one part”). Think of a polymer as a chain, with each of its links a monomer. Those monomers can be simple — just an atom or two or three — or they might be complicated ring-shaped structures containing a dozen or more atoms.

In an artificial polymer, each of the chain’s links will often be identical to its neighbors. But in proteins, DNA and other natural polymers, links in the chain often differ from their neighbors.

DNA molecule
DNA, life’s storehouse of genetic information, is a lengthy molecule made from a series of smaller, repeating chemical units. As such, it’s a natural polymer.
Ralwel/iStockphoto

In some cases, polymers form branching networks rather than single chains. Regardless of their shape, the molecules are very big. They are so big, in fact, that scientists classify them as macromolecules. Polymer chains can include hundreds of thousands of atoms — even millions. The longer a polymer chain, the heavier it will be higher. And, in general, longer polymers will give te materials made from them a higher melting and boiling temperature. Also, the longer a polymer chain, the higher its viscosity (or resistance to flow as a liquid). The reason: They have a greater surface area, which makes them want to stick to neighboring molecules.

Wool, cotton and silk are natural polymer-based materials that have been used since ancient times. Cellulose, the main component of wood and paper, also is a natural polymer. Others include the starch molecules made by plants. [Here’s an interesting fact: Both cellulose and starch are made from the same monomer, the sugar glucose. Yet they have very different properties. Starch will dissolve in water and can be digested. But cellulose doesn’t dissolve and can’t be digested by humans. The only difference between these two polymers is how the glucose monomers have been linked together.]

Living things build proteins — a particular type of polymer — from monomers called amino acids. Although scientists have discovered some 500 different amino acids, animals and plants use only 20 of them to construct their proteins. In the lab, chemists have many options as they design and construct polymers. Chemists may build artificial polymers from natural ingredients. Or they can use amino acids to build artificial proteins unlike any made by Mother Nature. More often, chemists create polymers from compounds made in the lab.

The anatomy of a polymer

Polymer structures can have two different components. All start with a basic chain of chemically bonded links. This is sometimes called its backbone. Some may also have secondary parts that dangle from some (or all) of the chain’s links. One of these attachments may be as simple as a single atom. Others may be more complex and referred to as pendant groups. That’s because these groups hang off the main chain of the polymer just as individual charms hang off the chain of a charm bracelet. Because they are exposed to the surroundings more than are the atoms that make up the chain itself, these “charms” often determine how a polymer interacts with itself and other things in the environment.

Sometimes pendant groups, instead of hanging loose from one polymer chain, actually connect two chains together. (Think of this as looking like a rung that stretches between the legs of a ladder.) Chemists refer to these ties as crosslinks. They tend to strengthen a material (such as a plastic) made from this polymer. They also make the polymer harder and more difficult to melt. The longer the crosslinks, however, the more flexible a material becomes.

A chemical bond is what holds atoms together in a molecule and some crystals. In theory, any atom that can form two chemical bonds can make a chain; it’s like needing two hands to link with other people to make a circle. (Hydrogen wouldn’t work because it can form only one bond.)

But atoms that typically form only two chemical bonds, such as oxygen, don’t often make long, polymer-like chains. Why? Once  oxygen forms two bonds, it ibecomes stable. That means its two "outstretched hands" are already taken. None are left to hold a pendant group. Since many atoms that are part of a polymer’s backbone generally have at least one pendent group, the elements that typically appear in the polymer chain are ones that become stable with four bonds, such as carbon and silicon.

pvc structure
Polymers are made by chemically linking up many copies of simpler groups called monomers. For example, polyvinyl chloride (PVC) is made by linking long chains of monomers (shown in the bracket). It’s made of two carbon atoms, three hydrogens and one chlorine atom.  
Zerbor/iStockphoto

Some polymers are flexible. Others are very stiff. Just think of the many types of plastics: The material in a flexible soda bottle is very different from that in a rigid pipe made from polyvinyl chloride (PVC). Sometimes materials scientists add other things to their polymers to make them flexible. Known as plasticizers (PLAA-stih-sy-zurs), these take up space between individual polymer chains. Think of them as acting like a molecular-scale lubricant. They let the individual chains slide across each other more easily.

As many polymers age, they may lose plasticizers to the environment. Or, aging polymers may react with other chemicals in the environment. Such changes help explain why some plastics start out flexible but later become stiff or brittle.

Polymers don’t have a definite length. They usually don’t form crystals, either. Finally, they usually don’t have a definite melting point, at which they immediately switch from a solid into a pool of liquid. Instead, plastics and other materials made from polymers tend to soften gradually as they heat up.

Power Words

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amino acids     Simple molecules that occur naturally in plant and animal tissues and that are the basic building blocks of proteins.

anatomy     (adj. anatomical) The study of the organs and tissues of animals. Or the characterization of the body or parts of the body on the basis of its structure and tissues. Scientists who work in this field are known as anatomists.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cellulose     A type of fiber found in plant cell walls. It is formed by chains of glucose molecules.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical bonds     Attractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

component     Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).

compound     (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

dissolve     To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

glucose     A simple sugar that is an important energy source in living organisms. As an energy source moving through the bloodstream, it is known as “blood sugar.” It is half of the molecule that makes up table sugar (also known as sucrose).

keratin     A protein that makes up your hair, nails and skin.

lubricant     A substance used to cut friction between surfaces that are in contact with one another.

macromolecule     A molecule that contains a large number of atoms. (The prefix macro comes from the Greek and means “large” or “long.”) Polymers, including natural proteins (such as DNA) and artificial materials (such as nylon and polyester), are examples of macromolecules.

materials scientist     Someone who studies the ways in which the atomic and molecular structure of a material relates to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

monomer    A molecule that is used as the basic building block of some larger molecule, known as polymers. From the Greek language, monomer means “one part.” (Polymer, also from Greek, means “many parts.”)

network     A group of interconnected people or things.

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

nylon     A silky material that is made from long, manufactured molecules called polymers. These are long chains of atoms linked together.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

pendant group      (in chemistry) A group of atoms that dangle from the main links of a polymer (a type of chain-like molecule). Such pendant groups of atoms (as well as any single atoms that might be attached to a polymer) often determine how the polymer interacts with other substances in its environment.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

plasticizer     Any of several chemicals added to certain synthetic materials to make them soft and/or pliable.

polyester     A synthetic material used chiefly to make fabrics. The actual chemical name for the material used is polyethylene terephthalate.

polymer     A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

polyvinyl chloride (PVC)     A plastic formed by using heat to turn a liquid resin into a solid. The plastic can be soft and flexible or rigid and hard. The raw ingredients consist primarily of chlorine and carbon.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

resistance      (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.

silicon     A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.

silicone     Heat-resistant substances that can be used in many different ways, including the rubber-like materials that provide a waterproof seal around windows and in aquariums. Some silicones serve as grease-like lubricants in cars and trucks. Most silicones, a type of molecule known as a polymer, are built around long chains of silicon and oxygen atoms.

starch     A soft white chemical made by all green plants. It’s a relatively long molecule made from linking together a lot of smaller, identical building blocks — all of them glucose, a simple sugar. Plants and animals use glucose as an energy source. Plants store that glucose, in the form of starch, as a reserve supply of energy. Animals that consume starch can break down the starch into glucose molecules to extract the useful energy.

viscosity     The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.