The polymerization of norbornene (NBE) was investigated in the presence of two novel pyrazolylimine nickel complexes [2-(C3HN2Me2-3,5){C(Ph)=[4-R2C6H2(R-1)(2)-2,6]}NiBr2 (complex 1, R-1 = Pr-i and R-2 = H; complex 2, R-1 = H and R-2 - NO2)] activated by methylaluminoxane. The two catalytic systems showed high activity [up to 1.27 x 10(6) g of polynorbornene (PNBE)/(mol of Ni h)] for NBE polymerization and provided PNBEs with high molecular weights (weight-average molecular weight <= 17.46 x 10(5) g/mol) and narrow Molecular weight distributions (ca. 2). The electron withdrawing of the nitro group in complex 2 Could not enhance the catalytic activity for NBE polymerization; however, the molecular weights of the polymers were increased. The catalytic activity, molecular weight, and molecular weight distribution were influenced by the polymerization conditions, such as the polymerization temperature and Al/Ni molar ratios. The obtained PNBEs were characterized by means of H-1-NMR and Fourier transform infrared spectroscopy techniques. The analysis results for PNBEs indicated that the NBE polymerization was a vinyl-type polymerization rather than a ring-opening metathesis polymerization. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110:3590-3595, 2008