Methacrylic acid, or 2-methyl-2-propenoic acid, is a low molecular weight carboxylic acid that occurs naturally in small amounts in the oil of Roman chamomile. It is a corrosive liquid with an acrid unpleasant odor. It is soluble in warm water and miscible with most organic solvents.

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Methacrylic acid polymerizes readily upon heating or treatment with a catalytic amount of strong acid, such as HCl. The resulting polymer is a ceramic-looking plastic.

Methacrylic acid is used industrially in the preparation of its esters, known collectively as methacrylates, such as methyl methacrylate. The methacrylates have numerous uses, most notably in the manufacture of polymers with trade names such as Lucite and Plexiglas®.

Methacrylic acid was first obtained in the form of its ethyl ester by Edward Frankland and B. F. Duppa (Annalen, 1865, 136, p. 12) by acting with phosphorus pentachloride on oxyisobutyric ester. It is, however, more readily obtained by boiling citra- or meso-brompyrotartaric acids with alkalis. It crystallizes in prisms, which are soluble in water, melts at 16°C, and boils at 160.5°C. When fused with an alkali, it forms propionic acid. Sodium amalgam reduces it to isobutyric acid. A polymeric form of methacrylic acid was described by F. Engelhorn (Ann., 1880, 200, p. 70).