The Polymerization of norbornene (NB) with catalysts obtained by combining cobalt-(II)-phosphine complexes (i.e., CoCl2(PR3)(2), COCl2(PRPh2)(2), R = alkyl or cycloalkyl group, Ph = phenyl) with methylaluminoxane (MAO) is reported. These systems exhibited excellent activity when used in an halohydrocarbon solvent, yielding amorphous, Soluble vinyl-type poly(norbornene)s (PNB)s. The NB copolymerization with ethylene (E) was also examined. Norbornene macromonomers (M-n in the range 740-8740 g mol(-1)) with a terminal vinyl group, likely originating from insertion of one ethylene molecule followed by a beta-hydrogen elimination reaction, were obtained in almost 100% yield. By varying the NB starting concentration as well as the time and the polymerization temperature, it was possible to tune the Molecular weight of the macromonomers which exhibited different T-g values Upon the NB polymerization degree. The PNBs and the macromonomers were fully characterized by NMR analysis, and a reaction mechanism for the formation of the vinyl-end NB macromonomers in the copolymerization of NB with E is proposed.