Norbornene polymerization using the commercially available and inexpensive catalyst system, cyclopentadienylzirconium trichloride (CpZrCl3) and isobutyl-modified methylaluminoxane (MMAO), were carried out over a wide range of polymerization temperatures and monomer concentrations. For the CpZrCl3 catalyst system activated by aluminoxane with a 40 mol % methyl group and a 60 mol % isobutyl group (MMAO(40/60)), the polymerization temperature and monomer concentration significantly affected the molecular weight (M-n) of the obtained polymer and the catalytic activity. With an increase in the polymerization temperature from 0 to 27 degrees C, the catalytic activity and M. increased, but these values dramatically decreased with the increasing polymerization temperature from 27 to 70 degrees C, meaning that the most suitable temperature was 27 degrees C. The CpZrCl3/MMAO(40/60) ([Al]/[Zr] = 1000) catalyst system with the [NB] of 2.76 mol L-1 at 27 degrees C showed the highest activity of 145 kg mol(Zr)(-1) h(-1) and molecular weight of 211,000 g mol(-1). The polymerization using the CpZrCl3/MMAO(40/60) catalyst system proceeds through the vinyl addition mechanism to produce atactic polynorbornene, which was soluble in chloroform, toluene, and 1,2-dichlorobenzene, but insoluble in methanol. (C) 2008 Wiley Periodicals, Inc.