A series of ethylene-norbomene copolymers were synthesized using Me2Si(Me4Cp)(NtBu)TiCl2 as the metallocene catalyst and methylaluminoxane (MAO) as the cocatalyst, with the same molecular characteristics except the molecular weight, to evaluate its influence on the determination of the glass transition temperature (T-g). The polymers were characterized using wide-angle X-ray scattering, differential scanning calorimetry, microhardness measurements, and dynamic mechanical thermal analysis. The value of the T-g, for the same norbornene content and determined from the last three mentioned methods, increases significantly up to a limit of M-n about 6-10 x 10(4) (g/mol). Above this value, T-g remains practically constant. (C) 2003 Wiley Periodicals, Inc.