an unsaturated oligomer (oligoesters)— for example, polymaleinates and oligoester acrylates. Mixtures of these oligoesters and their solutions in monomers that are capable of copolymerization (styrene, methyl methacrylate, dial-lylphthalate, and so on) are also called polyester resins.

Oligoesters are produced by polycondensation in a melt or inert solvent. Polymaleinates,

are produced from maleic acid, HOOCCH =CHCOOH, or its anhydride (sometimes mixed with another dicarboxylic acid or anhydride) and glycol. Oligoester acrylates,

are produced from an unsaturated monocarboxylic acid, usually acrylic acid, CH₂=CHCOOH, or methacrylic acid, CH₂ = C(CH₃)COOH; glycol; and a dicarboxylic acid. In the above formulas A and A′ are divalent radicals forming parts of the glycol and dicarboxylic acid molecules, respectively; X = — H, — CH₃, or — Cl; x = 1–5, y = 0–5, and n = 1–20. The most commonly used glycols are ethylene, diethylene, triethy-lene, and 1,2—propylene; sometimes, mainly during the preparation of oligoester acrylates, the glycols are partially or completely replaced by glycerol, pentaerythritol, or xylitol. The dicarboxylic acids used in this process include adipic, sebacic, phthalic, isophthalic, terephthalic, and tetrachlorophthalic acids.

Unsaturated oligoesters are viscous liquids or solid substances with softening point 30°–150°C, molecular weight 300–3,000, and density 1.1–1.5 g/cm³(20°C).

Oligoester solutions are usually liquids with viscosity 75–7,000 millinewton - sec per sq m, or centipoise, which can convert to cross-linked solid polymers upon curing. Cured polyester resins have high strength; good adhesion to glass fiber, paper, and metals; resistance to water, mineral and organic acids, gasoline, oils, and oxidizing agents; and good electrical insulating properties.

Most polyester resins are used as binders for fiber glass reinforced plastics. They are also widely used in the preparation of paints and varnishes, as poured insulation for sealing components in radio engineering and electrotechnical equipment, and in the impregnation of porous metal castings for hermetic sealing, as well as in the manufacture of haberdashery. Polyester resins also serve as the basis of compounds for poured flooring and of cements and adhesives for bonding fiber glass reinforced plastics to one another and to asbestos cement, fiberboard, and cellular plastics. Polyester resins are manufactured in the USSR (PN-1, PN-3, PN-8), the USA (Atlac, Selectron, Hetron), Great Britain (Crystic, Cellobond), France (Rhodester, Sterpon), and the Federal Republic of Germany (Leguval, Palatal). World production of polyester resins in 1973 was about 1.3 million tons.