A polymer is a general term for any molecule that is a long string of smaller repeating parts. The difference between linear and branched polymers is based on their structure.

Poly is a prefix that means “many.” A mer is a suffix meaning “part” or “unit.”

In manufacturing, polymers are often thought of as plastics because many artificial substances like plastics are polymers derived from petroleum. However, there are many different polymers (both naturally occurring and artificial) that are made of different parts. The way the units join together to form the polymer chain determines the polymer's properties along with its name. Polymers with different structures are named as linear polymers, branched polymers or cross-linked polymers.

Polymers are made from long, repeating chains of carbon-carbon bonds joining monomers, which are the smallest unique part of the chain. Many common polymers are made from petroleum and other hydrocarbons but others occur naturally. For example, artificial polyethylene is formed from a chain of ethylene molecules. Naturally occurring starch is made from long chains of glucose molecules. Some polymer chains are only a few hundred units long, while others have the potential of being infinitely long. For example, the molecules in natural rubber are so entwined that a whole rubber band can be considered one large polymer molecule.

The simplest polymer is a linear polymer. A linear polymer is simply a chain in which all of the carbon-carbon bonds exist in a single straight line. An example of a linear polymer is Teflon, which is made from tetrafluoroethylene. It is a single strand of units made from two carbon atoms and four fluorine atoms. When formed, these linear polymers can create strands of fibers or form a mesh that can be very strong and hard to break through.

Branched polymers occur when groups of units branch off from the long polymer chain. These branches are known as side chains and can also be very long groups of repeating structures. Branching polymers can be further categorized by how they branch off from the main chain. Polymers with many branches are known as dendrimers, and these molecules can form a webbing when cooled. This can make the polymer strong in the ideal temperature range. However, when heated, both linear and branched polymers soften as the temperature vibration overcomes the attractive forces between the molecules.

The cross-linked polymer forms long chains, either branched or linear, that can form covalent bonds between the polymer molecules. Because cross-linked polymers form covalent bonds that are much stronger than the intermolecular forces that attract other polymer chains, the result is a stronger and more stable material. An example of this is when natural rubber is vulcanized, which means it is heated so the sulfur molecules in the rubber polymer chains form covalent bonds with each other. This difference in strength is noticeable when you compare the stiffness, rigidity and durability of a car tire with that of a rubber band.