We have kinetically elucidated the origins of activity enhancement because of the addition of comonomer in Ziegler-Natta propylene polymerization, using stopped-flow and continuously purged polymerization. Stopped-flow polymerization (with the polymerization time of 0.1-0.2 s) enabled us to neglect contributions of physical phenomena to the activity, such as catalyst fragmentation and reagent diffusion through produced polymer. The propagation rate constant k(p) and active-site concentration [C*] were compared between homopolymerization and copolymerization in the absence of physical effects. k(p) for propylene was increased by 30% because of the addition of a small amount of ethylene, whereas [C*] was constant. On the contrary, both k(p) (for propylene) and [C*] remained unchanged by the addition of 1-hexene. Thus, only ethylene could chemically activate propylene polymerization. However, continuously purged polymerization for 30 s resulted in much more significant activation by the addition of comonomer, clearly indicating that the activation phenomenon mainly arises from the physical effects. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4005-4012, 2011