This thesis aims at the investigation of the cross over step on ROMP polymers in block copolymerization reactions as well as termination reactions. For the investigation of the cross over step in block copolymerization reactions, living polymer chains were reacted with three structurally different cycloolefins. After quenching, the resulting block copolymers were studied by GPC and MALDI-TOF MS. To obtain information about the catalyst species in this process, co-oligomerization reactions were conducted and investigated via ESI-TOF MS. The results show that the efficiency of the process depends on the monomer/catalyst couple. For the investigation of the cross over step in termination reactions, the living polymer chains were reacted with symmetric olefins. The generated polymers were investigated via GPC, NMR and MALDI-TOF MS. The efficiency of the process depends on the terminating agent, the catalyst as well as the reaction time and the ratio of terminating agent/living chain.