Polyethers are polymers consisting of monomers joined together by ether linkages (two carbon atoms bonded to an oxygen atom). A variety of polyethers are produced, ranging from elastomers to engineering plastics. The structures of the compounds vary significantly, yet they all preserve the C—O—C connection. They can be either aliphatic or aromatic polyethers. Polyethylene oxide (PEO), polypropylene oxide (PPO), and polytetrahydrofuran are aliphatic polyethers and are flexible and relatively noncrystalline. Because they have hydroxyl groups at the chain ends, they are sometimes called polyether glycols. Polyethylene glycol (PEG) and polypropylene glycol are alternative names for the first two compounds. The production of polyurethanes is the greatest market for all three. Lubricants, hydraulic fluids, and surfactants are some of the other applications.