Copolymerization of ethylene and 1-butene were performed using the catalyst system TiCl3.1/3 AlCl3/MgCl2/THF with or without partial activation by AlEt(2)Cl. The catalyst and the copolymers were characterized by IR spectroscopy. The experimental results reveal that partial activation strongly affects the catalytic sensitivity toward 1-butene and obviously leads to a two-step polymerization rate profile.