A composition is described which includes A) a chain-extended prepolymer, which can be obtained from the reaction of a) a polymer containing at least one amino, thiol or hydroxyl group, b) at least one polyisocyanate and c) at least one alkoxylated bisphenol as chain extender, and optionally d) an epoxide compound containing a primary or secondary hydroxy group, said compound containing a primary or secondary hydroxyl group, or at least one epoxy resin A which contains this epoxide compound, and optionally B) an epoxy resin B. The composition is suitable as a toughener or as an A component of a 2k epoxy resin adhesive which contains this toughener. The adhesive is suitable particularly for bonding in windmills.