Phenyl ether polymers is a high-strength engineering plastic developed in the 1960s. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers (PPEs) and polyphenylene oxides (PPOs). The phenoxy groups in the former class of polymers do not contain any substituents whereas those in the latter class contain 2 to 4 alkyl groups on the phenyl ring. PPEs have excellent high temperature properties and good oxidation stability. PPEs are used in applications requiring low volatility and excellent thermo-oxidative and ionizing radiation stability. Such applications include use as diffusion pump fluids; high vacuum fluids; and in formulating jet engine/turbine lubricants, high-temperature hydraulic lubricants and greases, and heat transfer fluids. In addition, because of excellent optical properties these fluids have found use in optical devices.