Abstract: A method for converting a polymer or oligomer derived from an ethylenically unsaturated monomer into alkanes or into a hydrocarbon fraction or a lower oligomer fraction by controlled hydrocracking, wherein the polymer or oligomer is exposed to a catalyst based on a metal hydride or an organometallic complex supported on a mineral carrier, the complex having at least one hydrocarbon ligand and optionally at least one hydride ligand, and the resulting mixture is reacted with hydrogen to cause catalytic hydrocracking of the polymer or oligomer. The polymer or oligomer is broken down into reclaimable products with a lower molecular weight for use, e.g., in the field of polymers, particularly controlled molecular weight polymers, fuels or lubricants.