A composition, comprising(a) a strained cycloolefin,(b) a filler, and(c) 0.01-10.0% by weight, based on the sum of components (a)+(b), of a copolymer selected from the group consisting of core/shell polymers and microgels, is distinguished by having high storage stability and gives polymers having high heat stability, toughness and mechanical strength, which polymers are suitable as encapsulating material for electrical and electronic components.