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methacrylic acid[¦meth·ə¦kril·ik ′as·əd]
α-methyl-acrylic acid, CH2=C(CH3)—COOH; a colorless liquid with a sharp odor. Melting point, 16°C; boiling point, 160.5°C; density, 1.0153 g/cm3 (at 20 °C). Soluble in water and organic solvents.
Methacrylic acid is reduced to isobutyric acid by sodium amalgam; it reacts with bases and alcohols to form methacrylates, CH2=C(CH3)COOR, which may be salts (if R is a metal) or esters (if R is an organic radical). It polymerizes readily, yielding polymethacrylic acid, which is colorless, brittle, infusible, and highly hygroscopic and is a typical weak polyelectrolyte.
Methacrylic acid is prepared commercially by the addition of hydrocyanic acid, HCN, to acetone, with subsequent dehydration to methacrylonitrile, CH2=C(CH3)—CN, which then undergoes saponification. Methacrylic acid and its derivatives are used in the manufacture of important industrial polymeric products. Methyl methacrylate, which is used in the preparation of Plexiglas, is the most valuable methacrylic acid derivative. Methacrylic acid is also used in the manufacture of carboxylated rubber, safety glass, adhesives, and ion-exchange resins; polymethacrylic acid salts serve as emulsifiers.