Abstract: Novel liquid curable gelling material are disclosed and based on the reaction between a polyol-capped isocyanate and an adduct of an anhydride and a polyol. The liquid gelling material forms a gel characterized by both urethane and ester linkages. In a specific embodiment, the two part curable liquid potting composition comprises in part A, a hydroxyl capped polyisocyanate, and less than 1000 ppm of free isocyanate, and in part B, an anhydride adduct of polybutadiene comprising a polybutadiene segment. The preferred polybutadiene segment has a molecular weight of from 500 and 20,000. In a second embodiment, the 2-part curable liquid potting composition comprises in part A: a polybutaidene polyol capped polyisocyanate containing less than 1000 ppm of free isocyanate, and in part B an anhydride adduct of a polyol having a molecular weight of from 500 and 20,000.