(also polysulfide elastomer; Thi-okol), a synthetic rubber that is a product of the polycondensation of dihalides of aliphatic compounds (for example, ethylene dichloride or propyl dichloride) with polysulfides of alkali metals (for example, Na₂S_x, where x = 2–4). Polysulfide rubbers are divided into polytetrasulfides and polydisulfides, with the general formulas [—R—S—S—S—S—]_n and [—R—S—S—]_n where R is an organic radical. Polysulfide rubbers are produced in solid and liquid form (high-molecular-weight and low-molecular-weight rubbers, respectively), and also as aqueous dispersions, or latices.

Polysulfide rubbers are special-purpose polymers characterized by high resistance to swelling in solvents, fuels, and oils; resistance to sunlight; moisture and gas impermeability; and stability during storage. These properties result from the high sulfur content and the absence of unsaturated bonds in the macromolecules. The molecular weight of solid rubbers is (200–500) × 10³; density, 1.27–1.60 g/cm³; glass transition point, –23° to – 57°C. Zinc oxide, p-quinone dioxime, and a mixture of Altax and diphenylguanidine are used to vulcanize polysulfide rubbers. Vulcanized polysulfide rubbers are inferior in mechanical properties to those made from other synthetic rubbers—for example, their tensile strength is 6–10 meganewton per sq m, or 60–100 kilograms-force per sq cm; relative elongation, 200–400 percent.

Polysulfide rubbers are used in rubberizing fuel storage tanks and in the manufacture of oil- and gasoline-resistant tubing and of gastight diaphragms for gas meters. Sealing compounds are prepared from liquid Thiokols. The most important commercial polysulfide rubbers are Tiokol DA (USSR) and Thiokol A, FA, and ST (USA).