The copolymerization of ethylene (E) and norbornene (NB) was investigated using the commercially available and inexpensive catalyst system, cyclopentadienylzirconium trichloride (CpZrCl3)/isobutyl-modified methylaluminoxane (MMAO), at a moderate polymerization temperature in toluene. For the CpZrCl3 catalyst system activated by aluminoxane with a 40 mol % methyl group and a 60 mol % isobutyl group (MMAO), the quantities of the charged NB and the polymerization temperature significantly affected the molecular weights, polydispersities, and NB contents of the obtained copolymers and the copolymerization activities in all the experiments. As the charged NB increased and thereby the NB/E molar ratio increased, the NB content in the copolymer increased and reached a maximum value of 71 mol %. The CpZrCl3/MMAO ([A1]/[Zr] = 1000) catalyst system with the [NB] of 2.77 mol L-1 and ethylene of 0.70 MPa at 50 degrees C showed the highest activity of 1690 kg mol(zr)(-1) h(-1) and molecular weight of 21,100 g mol(-1). The C-13 NMR analysis showed that the CpZrCl3/MMAO catalyst system produced the E-NB random copolymer with a number of NB homosequences such as the NN dyad and NNN triad. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7411-7418, 2008