Sandwich SiMe2-bridged ansa-complexes of zirconium, containing heterocycle-fused η5-cyclopentadienyls, derivatives of 2,5-dimethyl-7H-cyclopenta1,2-b:4,3-b'dithiophene (2), 5,10-dihydroindeno1,2-bindole (3) and 5,6-dihydroindeno2,1-bindole (4) – 'heterocenes' – catalyzed ethylene homopolymerization and ethylene/oct-1-ene (E/O) copolymerization in the presence of triisobutylaluminum (TIBA) without MAO and perfluoroaryl borate activators, the benchmark bis(η5-fluorenyl) complex 1 was virtually inactive under these conditions. In the absence of MAO, E/O copolymerization resulted in a formation of copolymers not containing EOOE fragments even at E/O ratio of 2.4. Addition of 10 eq. of MMAO-12 resulted in increase of both catalytic activities of 2–4 and oct-1-ene incorporation. E/O copolymers POE-05 (obtained on TIBA activated precatalyst 2) and POE-17 (obtained on TIBA/MMAO-12 activated precatalyst 4) were not inferior to the best commercial E/O copolymers in physico-mechanical characteristics at −45 °C. Extrusion blends of POE-05 and POE-17 with high density polyethylene (HDPE) demonstrated high impact strength and frost resistance. Display omitted •TIBA-activated heterocenes catalyze ethylene/oct-1-ene (E/O) copolymerization.•Copolymers without EOOE fragments are formed up to E/O ratio of 2.4•Random E/O copolymers have promising low-temperature mechanical characteristics.•Blends of E/O copolymers with HDPE have improved frost resistance.