The propylene-ethylene copolymers with a very small amount of ethylene (from 0.01 to 1.8 mol%) obtained with different catalyst systems at 100 degrees C or 120 degrees C were characterized by C-13 NMR. The chain end group n-propyl formed through the chain transfer reaction of species to alkylaluminum following "one" ethylene molecule insertion was detected in these copolymers of low molecular weight. C-13 NMR chain end group analysis of these copolymers implied that most of syndiotactic species in the MgCl2-supported Ziegler-Natta catalysts would be "dormant" for propylene insertion while highly isotactic species would remain highly active for propylene insertion when these two species were inserted by "one" ethylene molecule. For isotactoid species occasional "one" ethylene molecule insertions remarkably slowed down propagation of propylene. The active species could be divided into three classes according to their behavior following "one" ethylene molecule insertion, which fit Busico [1] three-state model of active species to a great degree. The chain end group ethyl (Et') formed by chain transfer reactions by alkylaluminum at "one" ethylene molecule insertion followed by a 2-1-last-insertion was also detected by C-13 NMR. Thus, the fraction of "dormant" polypropylene chains with a 2,1-last inserted unit was estimated to be in the 3-6 mol% range for a TiCl4/MgCl2/diester-silane catalyst while for a TiCl4/MgCl2/diether catalyst the content of such the fraction was ca. 7-9 mol%. C-13 NMR analysis indicated that a large part of the "dormant" 2-1-inserted sites revived by "one" ethylene insertion were still "dormant" for propylene insertion at high polymerization temperature. (C) 2014 Elsevier B.V. All rights reserved.