It was found that the observed order of the polymerization rate with respect to ethylene concentration at ethylene polymerization over two titanium-magnesium catalysts of different compositions is significantly higher than 1(1.6-2.1). The data on the effect of ethylene concentration on the number of active centers (C-p) and the propagation rate constant (k(p)) at ethylene polymerization over these catalysts were obtained by method of polymerization quenching with (CO)-C-14. An increase in ethylene concentration was found to increase the number of active centers. In some cases the increase of ethylene concentration proceeds to the narrowing of the molecular weight distribution of the resulting polyethylene and an increase in the calculated value of propagation rate constant. These effects were shown to be most pronounced at low ethylene pressure and increased concentration of an activator (AlEt3). Based on the experimental data, we proposed a scheme of reactions to explain the effects of ethylene and AlR3 concentrations on the number of active centers, the average values of propagation rate constant and molecular weight distribution of polymers produced over these multi-site catalysts. (C) 2014 Elsevier B.V. All rights reserved.