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A polymer is a chain of small molecules called monomers held together by covalent bonds. Monomers are the individual units that link to form larger polymer molecules. Organisms synthesize biological polymers like DNA and proteins, while scientists create synthetic polymers such as plastics, nylon, polyethylene, and teflon in laboratories.

Condensation polymerization occurs when monomers with reactive hydrogen and hydroxyl groups link together, releasing water as a byproduct. Most natural polymers form this way. Addition polymerization involves monomers with carbon-carbon double bonds joining without releasing byproducts. The overview notes that addition polymerization requires an initiator molecule and is faster than condensation polymerization, which typically requires heat and a catalyst.

Homopolymers consist of the same type of monomer repeated throughout the chain, such as cellulose, which is composed entirely of glucose units. Heteropolymers contain two or more different types of monomers, like agarose, a marine polymer made of galactose and 3,6-anhydro-L-galactopyranose units linked together.

Linear polymers form long, unbranched chains of monomers, such as cellulose held by beta 1,4-glycosidic bonds. Branched polymers have chains splitting from the main backbone, like amylopectin in starch. Crosslinked polymers have two or more chains connected at multiple points, forming ladder-like or network structures similar to pectins in jellies and jams.

Natural polymers include DNA, the genetic material in all living organisms, and proteins, polysaccharides, and lipids, which serve as building blocks of life. Cellulose is the most abundant natural polymer on Earth. These biological polymers are typically formed through condensation polymerization reactions within cells.

Addition polymerization requires an external initiator molecule with an unpaired valence electron that reacts with a monomer's double bond, creating a reactive chain. Each newly added monomer retains an unpaired electron, allowing the chain to grow. The reaction terminates when two chains with unpaired electrons bond together, eliminating all unpaired electrons and stopping polymer growth.

Synthetic polymers such as nylon, polyethylene, and teflon are among the most common polymers because they can be engineered in laboratories with specific properties suited for various applications. Unlike natural polymers, synthetic polymers can be designed through controlled addition or condensation reactions to achieve desired characteristics for industrial, commercial, and consumer products.