Phenyl ether polymers are a class of polymers that contain a phenoxy or a thiophenoxy group as the repeating group in ether linkages. 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. The structure of an oxygen-containing PPE is provided in Figure 1 and that of a 2, 6-xylenol derived PPO is shown in Figure 2. Either class can have the oxygen atoms attached at various positions around the rings. Structure and synthesis The proper name for a phenyl ether polymer is poly(phenyl ether) or polyphenyl polyether, but the name polyphenyl ether is widely accepted. Polyphenyl ethers (PPEs) are obtained by repeated application of the Ullmann Ether Synthesis: reaction of an alkali-metal phenate with a halogenated benzene catalyzed by copper. PPEs of up to 6 phenyl rings, both oxy and thio ethers, are commercially available. See Table 1. They are characterized by indicating the substitution pattern of each ring, followed by the number of phenyl rings and the number of ether linkages. Thus, the structure in Figure 1 with n equal to 1 is identified as pmp5P4E, indicating para, meta, para substitution of the three middle rings, a total of 5 rings, and 4 ether linkages. Meta substitution of the aryl rings in these materials is most common and often desired. Longer chain analogues with up to 10 benzene rings are also known. The simplest member of the phenyl ether family is diphenyl ether (DPE), also called diphenyl oxide, the structure of which is provided in Figure 4. Low molecular weight polyphenyl ethers and thioethers are used in a variety of applications, and include high-vacuum devices, optics, electronics, and in high-temperature and radiation-resistant fluids and greases. Figure 5 shows the structure of the sulfur analogue of 3-R polyphenyl ether shown in Figure 3. Physical properties Typical physical properties of polyphenyl ethers are provided in Table 2. Physical properties of a particular PPE depend upon the number of aromatic rings, their substitution pattern, and whether it is an ether or a thioether. In the case of products of mixed structures, properties are hard to predict from only the structural features; hence, they must be determined via measurement. The important attributes of PPEs include their