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International audience; A SiO 2 /MgCl 2-bisupported Ziegler−Natta-type catalyst is studied in gas-phase ethylene/1-butene copolymerization. With a minimal amount of alkylaluminum required to activate the catalyst, polymerization kinetics is profoundly affected by the dosage of it. High dosage of alkylaluminum retards the polymerization but does not kill the catalyst. Through a comprehensive characterization on the samples by differential scanning calorimetry, crystallization elution fractionation, thermal gradient interaction chromatography, cross-fractiona-tion chromatography, and nuclear magnetic resonance, it has been shown that alkylaluminum also alters the short-chain branching distribution (SCBD) of the copolymer but barely affects the total comonomer incorporation. It is proposed that alkylaluminum alters the active species from producing statistical copolymer chains to producing block polymer chains with C4-enriched and C4-scattered segments. It provides an example of a bimetallic structure of active species as well as an approach to tailor SCBD between a statistical copolymer and block copolymer bearing C4-enriched and C4-scattered segments.