In polymer chemistry, a chain extender is a low molecular weight (MW) reagent that converts polymeric precursors to higher molecular weight derivatives. Often, it is convenient to prepare a polymer at an intermediate MW, which are suitable for solution- or melt-processing. At or near the final stages of production, the material is treated with a chain extender. Typically, chain extenders are bifunctional, i.e., they have two functional groups, which can link together two polymers. Representative classes of chain extenders are diglycidyl ethers, diols, diamines, or dianhydrides. Chain extenders are often applied to polyurethanes, where they usually string together diisocyanates within the polyurethane's "hard segment".
Chain extenders are often added to polymers improve their physical properties, as well as adding to their molecular weight. In particular, the addition of a chain extender will improve the thermal stability and viscosity of a polymer.