Maleic acid

Maleic acid is the cis-isomer of butenedioic acid, an unsaturated organic dibasic acid. Maleic acid shows reactions typical of both olefins and carboxylic acids. Commercially important reactions of the acid groups include esterification with glycols to polyesters and dehydration to the anhydride. The double bond is involved in conversions to fumaric acid, to sulfosuccinic acid, and to Malathion.

Maleic acid has been adopted as a high performance organic anode material for lithium ion batteries. In has been reported to have an ultrahigh reversible capacity of ca. 1500 mAhg^–1 at 46.2 mAg^–1 current density, and a relatively high capacity of 570.8 mAhg^–1 at current density of 46.2 Ag^–1. When cycled at high current density of 2.31 Ag^–1, a capacity retention of 98.1% was obtained after 500 cycles.
Maleic acid has been used in constructing polymeric solid electrolytes for battery and fuel cell research. Maleic acid based polymer blend were reported to increase the ionic conductivity of PEO basd polymer solid electrolytes by suppressing crystallization.
Maleic acid has also been used to make superswelling acrylamide (AAm)/maleic acid (MA) hydrogels by free radical polymerization in aqueous solution of AAm with MA as comonomer with some multifunctional crosslinkers such as trimethylolpropane triacrylate and 1,4-butanediol dimethacrylate. In this hydrogel system, the uptake of cationic dye increased with an increase of MA content in the AAm/MA hydrogels.