In copolymerization of ethylene (E) with tri-alkenyl-silsesquioxanes (POSS-6-3 and POSS-10-3) were used bis (phenoxy-imine) titanium, zirconium and vanadium catalysts, as well as salen-type vanadium catalysts. There were obtained copolymers with POSS-R-3 units as side branches of polymer chains. The type of employed bis (phenoxy-imine) titanium, zirconium and vanadium catalysts and salen-type catalyst of vanadium determined the copolymer chain termination reactions, as well as the mechanism of modification of the active species by the POSS-R-3 comonomer. POSS-R-3 comonomers caused selective poisoning of bis(phenoxy-imine) zirconium catalyst and they provided protection against tight contact with [Me-MMAO](-) counter anion. Concentration of the precursor of the active site and Ti(III) complex was found decreased in the system with bis(phenoxy-imine) titanium catalyst and POSS-R-3. There were formed different catalytic species characterized by diamagnetic properties for bis(phenoxy-imine) vanadium due to the presence of POSS-R-3. Our findings were based on results from the following methods: FT-IR, H-1 NMR, H-1-H-1 NOESY, EPR and GPC.