The data on the effect of polymerization duration on molecular weight (MW), molecular weight distribution (MWD), and content of a-olefin were obtained for ethylene/1-hexene copolymers produced on a supported titaniummagnesium catalyst (TMC) upon polymerization in the absence and presence of hydrogen. An increase in MW, narrowing of the MWD, and a decrease in 1-hexene content in the copolymer takes place with increasing polymerization duration. It was shown by molecular weight fractionation of copolymers obtained at different polymerization duration followed by analysis of narrow fractions to determine the butyl branching content that the decrease comonomer content in copolymers with increasing polymerization duration is accounted for by the decrease in the share of low molecular weight fractions with an increased butyl branch content. These data, in combination with the data concerning the narrowing of the MWD of copolymers and the decrease in activity with polymerization duration allow arriving at conclusion that active sites producing low molecular weight polymer with an increased butyl branching content is predominantly deactivated with polymerization. Active sites producing high molecular weight polymer has a low ability to copolymerize ethylene with a-olefins; however, these sites show higher stability during copolymerization. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012