A polymer is a chain of small molecules, called monomers, that are held together by covalent bonds. Organisms can synthesize biological polymers, or scientists can synthesize polymers in the laboratory.

Natural polymers include DNA, the genetic material found in all living organisms, and proteins, polysaccharides, and lipids, the building blocks of life.

Plastics are some of the most common synthetic polymers and include nylon, polyethylene, and teflon.

The process of monomers linking to each other to form a larger molecule is known as polymerization. Monomers polymerize by two distinct reaction mechanisms, condensation and addition.

Condensation polymerization is when two monomers with a reactive hydrogen atom and hydroxyl group, respectively, link to each other, with the release of water as a by-product. Most natural polymers are formed through condensation reactions.

In addition polymerization, monomers with carbon-carbon double bonds are added to each other to form a polymer without the generation of any by-product through the release of atoms.       

Polymers that are made up of the same type of monomers are known as homopolymers, and those made up of different types of monomers are known as heteropolymers. Cellulose, the most abundant natural polymer, is a homopolymer of glucose, whereas agarose, a polymer of marine origin, is a heteropolymer of galactose and 3,6-anhydro-L-galactopyranose.

Polymers have various structures depending on their type of monomers. Linear polymers are long chains of monomers. For example, cellulose is a linear polymer held together by beta 1,4-glycosidic bonds. 

Branched polymers have chains splitting off of the main chain, such as amylopectin, a component of starch that branches out after 24 to 30 monomers through alpha 1,6-glycosidic bonds.

Crosslinked polymers have two or more chains held together at several points like a ladder, such as a synthetically constructed silicon ladder polymer, or form a complex network of interlinked chains—like the gel formed by pectins in jellies and jams.