Ethylene/propylene copolymerization was catalyzed under the same conditions with unbridged metallocene bis(2,4,7-trimethylindenyl)zirconium dichloride (Cat.1) activated by aluminoxane type cocatalysts having different content of trimethylaluminum (TMA). Chain structure of the copolymers was characterized by C-13 NMR and thermal analysis on copolymer fractions separated based on crystallinity. Changing the cocatalyst from commercial methylaluminoxane (MAO, containing both free TMA and bond TMA) to dried MAO (dMAO, containing only bond TMA) and BHT-treated MAO (BMAO, containing almost no TMA) caused only slight differences in polymerization activity and moderate increase in copolymer molecular weight, but marked differences in copolymer chain structure. The copolymers produced using MAO (sample 1) and dMAO (sample 2) as cocatalysts contain high amount of fractions with very low propylene content and almost isolated propylene units. The copolymer produced using BMAO as cocatalyst (sample 3) has very broad composition distribution, but its fraction with the highest crystallinity has much higher content of PP dyads than the crystalline fractions of the other two samples. This sample has the lowest crystallinity among the three samples. Phase morphology and tensile properties of the copolymer changed markedly when different cocatalyst was used. Tensile properties of sample 1 and 2 were similar to mixtures of elastomer and crystalline plastic materials, but tensile properties of sample 3 were more or less like a thermoplastic elastomer. Influences of TMA on the micro scale environment of active centers were considered the main reason for the effects of cocatalyst on copolymer structure and properties. (C) 2017 Elsevier Ltd. All rights reserved.