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, comprising breaking crosslinks formed as a result of the hydrogenation by adding after the hydrogenation a compound that satisfies formula I or by adding before, during or after the hydrogenation a compound that satisfies formula II

• 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 selected from the group consisting of —R3, —OR4, —SR4, and —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 selected from the group consisting of —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 wherein R3-R12 optionally contain one or more heteroatoms from the groups 13, 14, 15, 16, or 17 from the Periodic System of the Elements, and
• whereby the process is absent carrying out ozonolysis to break said crosslinks.