Ethylene/propylene (E/P) and ethylene/propylene/5-ethylidene-2-norbornene (E/P/ENB) copolymers were obtained on -Et(2-MeInd)ZrMe activated by a number of isobutylaluminium aryloxides: (2,6-BuPhO-)AlBu (1-DTBP) (2,6-Bu,4-Me-PhO-)AlBu (1-BHT), (2,4,6-BuPhO-)AlBu (1-TTBP), (2,6-Bu,4-Me-PhO-)AlBu (2-BHT), (2,6-BuPhO-)2AlBu (2-DTBP), [(2-Me,6-Bu-CHO)AlBu] (1-MTBP), [(2,6-Ph-PhO)AlBu] (1-DPP). This study shows how the structure of an activator influences catalytic activity and polymer properties, such as the copolymer composition, molecular weight characteristics, and thermophysical and mechanical properties. It has been shown that both the introduction of a bulky substituent in the para-position of the aryloxy group and the additional aryloxy group in the structure of an activator lead to a significant decrease in activity of the catalytic system in all studied copolymerization processes. Moreover, activation by bulkier aryloxides leads to lower levels of comonomer insertion and gives rise to higher molecular weight polymers. Broad or multiple endothermic peaks with different values of melting points are observed on the DSC curves of the copolymers obtained with different catalytic systems. The DSC of the thermally fractionated samples makes it possible to reveal the heterogeneity of the copolymer microstructure, which manifests itself in the presence of a set of lamellar crystallites of different thickness. The results also present the mechanical properties of the copolymers, such as the tensile strength (σ), elongation at break (ε), and engineering strain (EL). The synthesized E/P and E/P/ENB copolymers contain about 1-4 wt.% of the sterically hindered phenols obtained in situ as a residue of the hydrolyzed activators in the course of reaction quenching. This determines the increased thermooxidative stability of the copolymers.