Process for the hydrogenation of a polymer composed of diene monomer units and nitrile group containing monomer units and hydrogenated polymer

Process for the hydrogenation of a polymer composed of diene monomer units and nitrile group containing monomer units, in which the hydrogenation in the form of an aqueous dispersion is carried out in the presence of hydrazine and an oxidizing compound characterized in that crosslinks formed as a result of the hydrogenation are broken by adding after the hydrogenation, a compound that satisfies formula I or by adding before, during or after the hydrogenation a compound that statisfies formula II

• 2
• where
• R1 is a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms and
• R2 is an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms,
• X is chosen from the group comprising -R3, —OR4, —SR4, —NR5R6, where R3, R4 and R5 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms and R6 is an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms
• Y is chosen from the group comprising -R7, —OR8, —SR8, —NR9R10 and —N═CR11R12, where R7, R8, R9, R10, R11 and R12 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms, and it being possible for R3-R12 to also contain one or more heteroatoms from the groups 13, 14, 15, 16 or 17 of the Periodic System of the Elements.