A polymer is a substance made up of many repeating units (called monomer units). Polymers are usually distinguished by a high molar mass (formula weight), often ranging into thousands or millions of grams per formula unit. Polymerization is the process by which monomers (smaller chemical units) are combined to form a polymer. Additional Info: Examples of everyday monomers and their polymers include: Ethylene (C2H4) is a highly flammable gaseous molecule with a formula weight (molar mass) of 32 grams per mol. When polymerized using a catalyst, it forms an insoluble solid comprised of straight chains of CH2 units called polyethylene. Polyethylene is a widely used commodity plastic. Nylon(tm) is a formed from the condensation of hexamethylenediamine and adipic acid (and loss of water). MSDS Relevance: Polymerization of monomers is often an exothermic process (one that evolves heat). If polymerization begins when it is not desired, the result could be a fire or explosion. Materials that have this kind of behavior are often described as Hazardous Polymerization on their MSDS. Undesired polymerization of a monomer alters the physical properties of the material and could result in undesired changes. One fortunate example of this was the discovery of Teflon (DuPont trademark for poly(tetrafluoroethylene) as a white solid in what was supposed to be a cylinder of gaseous tetrafluoroethylene. See also: Exothermic