A series of ethylene, propylene homopolymerizations, and ethylene/propylene copolymerization catalyzed with rac-Et(Ind)(2)ZrCl2/modified methylaluminoxane (MMAO) were conducted under the same conditions for different duration ranging from 2.5 to 30 min, and quenched with 2-thiophenecarbonyl chloride to label a 2-thiophenecarbonyl on each propagation chain end. The change of active center ratio ([C-*]/[Zr]) with polymerization time in each polymerization system was determined. Changes of polymerization rate, molecular weight, isotacticity (for propylene homopolymerization) and copolymer composition with time were also studied. [C-*]/[Zr] strongly depended on type of monomer, with the propylene homopolymerization system presented much lower [C-*]/[Zr] (ca. 25%) than the ethylene homopolymerization and ethylene-propylene copolymerization systems. In the copolymerization system, [C-*]/[Zr] increased continuously in the reaction process until a maximum value of 98.7% was reached, which was much higher than the maximum [C-*]/[Zr] of ethylene homopolymerization (ca. 70%). The chain propagation rate constant (k(p)) of propylene polymerization is very close to that of ethylene polymerization, but the propylene insertion rate constant is much smaller than the ethylene insertion rate constant in the copolymerization system, meaning that the active centers in the homopolymerization system are different from those in the copolymerization system. Ethylene insertion rate constant in the copolymerization system was much higher than that in the ethylene homopolymerization in the first 10 min of reaction. A mechanistic model was proposed to explain the observed activation of ethylene polymerization by propylene addition. (C) 2016 Wiley Periodicals, Inc.