A thickened composition which is cured by free-radical initiated polymerization is composed of a compound containing at least two ethylenically unsaturated double bonds per molecule; an active hydrogen-containing homopolymer or copolymer of a diene having a number average molecular weight of from 500 to 15,000; the active hydrogens being derived from hydroxyl, amino, or mercapto groups; and an organic polyisocyanate present in an amount sufficient to thicken said composition without gelation.