Ethylenetetracarboxylic dianhydride
Ethylenetetracarboxylic dianhydride is a chemical compound with formula C6O6, that can be seen as the twofold anhydride of ethylenetetracarboxylic acid C6H4O8. It has a bicyclic molecular structure consisting of two maleic anhydride rings fused by their respective alkene units.
Source: Wikipedia — Ethylenetetracarboxylic dianhydride (CC BY-SA 4.0)