Two MgCl2/THF/TiCl4 (THF = tetrahydrofuran) catalysts were prepared with MgCl2/THF supports having an amorphous (Sup-A) or a molecular compound structure, [Mg3Cl5(THF)(4)Bu](2) (Bu = n-butyl) (Sup-B). Amorphous catalyst A had a normal supported Ziegler-Natta-type structure with low donor and titanium contents (the molar ratios, THF/Mg = 0.14 and Ti/Mg = 0.17), while catalyst B had a molecular type of composition with high donor and titanium contents (the molar ratios, THF/Mg = 0.84 and Ti/Mg = 0.80). Despite the different composition, Cat-A and Cat-B had some structural similarities as evidenced from the powder X-ray diffraction data and nuclear magnetic resonance and infrared spectroscopy results. Polymerization studies showed that these catalysts are highly active in copolymerization of ethylene and 1-butene. Although there were differences in the composition and the structure between the catalysts, they produced almost similar copolymers as indicated by the molecular weight, molecular weight distribution, comonomer content, melting point, and crystallinity of the copolymers. (C) 2014 Elsevier B.V. All rights reserved.