Polymer is a macromolecule formed by linking small molecules together. They can be natural, like cellulose, or synthetic, produced using chemicals. Polymerization reactions include addition polymerization via free radical or ionic mechanisms. During free radical polymerization, radicals are generated through initiation and then propagate by adding monomers in a chain reaction that terminates when radicals combine. Ionic polymerization similarly initiates with ions that stabilize and propagate a growing chain until termination. Common polymers include polyethylene, nylon, epoxy resin, and synthetic fibers that find uses in various applications.