Two MgCl2-supported Ziegler-Natta model catalysts were prepared by contacting activated MgCl2 with deficient or excess amount of TiCl4. Ethylene/1-hexene copolymerization with the catalysts was conducted under different 1-hexene feed, and active center concentration of the reaction system was determined by quench-labeling method using thiophene-2-carbonyl chloride as the quencher. The catalytic activity was only moderately enhanced (increment <50%) by adding 1-hexene in ethylene polymerization with the model catalyst (Cat-1) of 0.1 wt% Ti content, and the active center ratio ([C*]/[Ti]) was only slightly increased (from 64 to 72 mol%). The comonomer activation effect in Cat-1 catalyzed copolymerization was mainly attributed to increase of apparent propagation rate constant by the comonomer, which can be explained by reduction of mass transfer barrier in the polymer/catalyst particles because of larger monomer diffusion coefficients in the copolymer phase than in the more crystalline polyethylene phase. In contrast, the activity was nearly tripled by adding 1-hexene when the catalyst of 1.47% Ti content (Cat-2) was used, meanwhile its [C*]/[Ti] was also tripled (from 15 to 49 mol%). The strong comonomer activation effect in Cat-2 catalyzed copolymerization was caused by marked increase of active center concentration. In this case, fragmentation of the polymer/catalyst particles was intensified by adding the comonomer, and more active centers that were originally inaccessible to the cocatalyst and monomer were exposed through the particle fragmentation. According to the results of this work, the comonomer effect in conventional supported Ziegler-Natta catalysts can be explained by the combination of physical factor (increase in diffusion coefficient) and chemical factor (exposure of inaccessible active center precursors through particle fragmentation). (C) 2018 Elsevier Inc. All rights reserved.