CROSS-LINKABLE ETHYLENICALLY UNSATURATED POLYMER COMPOSITIONS ARE DESCRIBED, WHICH COMPOSITIONS COMPRISE AN ETHYLENICALLY UNSATURATED POLYMER, A PRECURSOR OF A POLYFUNCTIONAL NITRILE N-OXIDE OR NITRILE IMINE AND A LACTONE OR SULTONE. THESE COMPOSITIONS CROSS-LINK WHEN HEATED. SUITABLE LACTONES ARE THOSE CONTAINING 3 OR 4 CARBON ATOMS IN THE LACTONE RING SUCH AS PROPIOLACTONES AND BUTYROLACTONES AND SUITABLE SULTONES ARE THOSE CONTAINING 3 CARBON ATOMS IN THE SULTONE RING SUCH AS 1,3-PROPANE SULTONE. THESE COMPOSITIONS ARE USEFUL AS SEALANTS AND ADHESIVES.