Polymers: What They Are and How They Form

So what are polymers, exactly? A polymer is a very large molecule made of many small, repeating units joined together in a long chain. Each of these small repeating units is called a monomer. Think of a polymer like a paper chain made from many identical loops: each loop is a monomer, and the whole strip of connected loops is the polymer. The word itself comes from the Greek "poly" (many) and "meros" (parts), which is a good way to remember what does polymer mean at its core: many parts linked into one big structure.

Polymers can contain anywhere from a few hundred to many thousands of monomer units, and the monomers are held together by strong covalent bonds along the backbone of the chain. To understand why those bonds form the way they do, it helps to be comfortable with introduction to bonding before working through polymer chemistry in detail.

Polymerization is the chemical process that links individual monomers into a polymer chain. Understanding what are monomers and polymers, and how one becomes the other, is the key idea in this topic: monomers are the small building-block molecules, and polymers are the long chains those building blocks form once they react together.

There are two main types of polymerization reactions:

• Addition polymerization: monomers containing a carbon-to-carbon double bond, such as \(CH_2=CH_2\) (ethylene), open up that double bond and join directly to one another. No atoms are lost in the process. This is exactly what happens when producing what is a polyethylene, one of the most common plastics: thousands of ethylene monomers link into a long \(-[CH_2-CH_2]_n-\) chain.
• Condensation polymerization: monomers join together while releasing a small molecule, usually water, as a byproduct. This method is used to make polymers such as nylon and polyester, where two different types of monomers alternate along the chain.

The figure below shows an example of polymerization: an ethylene monomer opening its double bond to become one repeating unit inside a polyethylene chain.

Polymers occur naturally as well as being manufactured in factories. Common natural polymers include starch, cellulose, proteins, DNA, and natural rubber. If you have ever wondered what is natural rubber made of, the answer is a natural polymer called polyisoprene, made of repeating isoprene monomer units that come from the sap (latex) of rubber trees.

Synthetic polymers are made by chemists in the lab or in industry, usually starting from small molecules derived from petroleum. Polymer examples in this category include polyethylene (used in plastic bags and bottles), polystyrene (used in foam packaging), PVC (used in pipes), and nylon (used in fabrics and rope).

Because polymer chains are so long, the way neighboring chains attract one another has a big effect on the material's properties, such as melting point and flexibility. Reviewing intermolecular forces will help explain why some polymers are soft and stretchy while others are rigid and brittle.

Polymers are often classified by how they behave when heated. This distinction matters a lot in everyday materials, and it is exactly what the terms thermosetting polymer and thermosetting plastic refer to.

In a thermoplastic, the individual chains are held together only by intermolecular forces, so heating can loosen those attractions enough for the chains to slide past one another and flow. In a thermosetting polymer, permanent covalent cross-links form between chains during manufacturing, locking the structure in place so it cannot be melted and reshaped again.

Most polymer backbones are built from carbon atoms bonded to one another, because carbon can form four stable covalent bonds and link into very long, stable chains. Looking at the structures of carbon shows why this element is uniquely suited to forming the long, repeating backbones found in almost every synthetic and natural polymer.

A polymer is a large molecule built from many repeating monomer units joined through polymerization, either by addition (as in polyethylene) or condensation (as in nylon). Polymers can be natural, like the polyisoprene in natural rubber, or synthetic, like polyethylene and PVC. Thermoplastic polymers soften and reshape when heated, while thermosetting polymers form permanent cross-links and cannot be re-melted once cured.

Related lessons

• Chemistry Help