Cationic alkyl-rare-earth-metal complexes (i.e., Group 3 and the lanthanides) have recently attracted much interest as homogeneous polymerization catalysts. [1][2][3][4][5][6][7][8][9][10] A number of cationic alkyl-rare-earth-metal complexes supported by various ancillary ligands such as deprotonated aza crowns, [2] benzamidinates, [3] b-diketiminates, [4] anilido imines, [5] amidefunctionalized triazacyclononanes, [6] phosphides, [7] cyclopentadienyls, [8] and crown ethers [9] have been synthesized and their reactivity has been studied. Despite these extensive efforts, however, the olefin-polymerization chemistry of the cationic alkyl-rare-earth-metal complexes is still limited solely to that of ethylene, [2][3][4][5][6][7][8][9][10] whereas the development of active rare-earth-metal catalysts for the efficient polymerization/copolymerization of higher olefins has remained a challenge.The copolymer of ethylene with a cyclic olefin such as norbornene (COC) is one of the most important highperformance polymer materials with many desirable properties. Since Kaminsky et al. first described the copolymerization of ethylene (E) with norbornene (NB) by using zirconocene-based catalysts in 1991, [11] extensive studies have been carried out in this area. Most of the catalysts reported so far are complexes based on transition metals such as those of Group 4 and Group 10, [12] whereas no rare-earthmetal complex has been previously used for the copolymerization of ethylene with norbornene. [13] Herein we report an excellent cationic half-sandwich scandium catalyst for the copolymerization of ethylene with norbornene. This catalyst not only represents the first example of a rare-earth-metal catalyst for ethylene-norbornene copolymerization, but it also shows several unique characteristics, such as extremely high activity for the alternating ethylene-norbornene copolymerization and unprecedented formation of novel poly(ethylene-alt-norbornene)-bpolyethylene block copolymers.