Journal of Molecular Catalysis A: Chemical 396 (2015) 328–334 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical jou rnal h om epa ge: www.elsevier.com/locate/molcata Kinetic features of ethylene polymerization over titanium-magnesium Ziegler-Natta catalysts: Effect of monomer concentration on the number of active centers and propagation rate constant Artem A. Barabanov a,∗ , Viktoria V. Sukulova a , Mikhail A. Matsko a , Vladimir A. Zakharov a,b a Boreskov Institute of Catalysis SB RAS, pr. academika Lavrentieva, 5, 630090 Novosibirsk, Russia b Novosibirsk State Univercity, Pirogova., 2, 630090 Novosibirsk, Russia a r t i c l e i n f o Article history: Received 20 August 2014 Received in revised form 6 October 2014 Accepted 8 October 2014 Available online 18 October 2014 Keywords: Ethylene polymerization Polymerization kinetics Propagation rate constant Number of active centers Ziegler-Natta catalysts a b s t r a c t 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 signifi- cantly 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 14 CO. An increase in ethylene concentration was found to increase the number of active centers. In some cases the increase of ethylene concentration pro- ceeds 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 (AlEt 3 ). Based on the experimen- tal data, we proposed a scheme of reactions to explain the effects of ethylene and AlR 3 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Studying the kinetics of olefin polymerization over modern highly active supported titanium-magnesium catalysts (TMC) is an important step towards understanding the mechanism of action of these systems. A number of reviews have been devoted to the analysis of the kinetic features of olefin polymerization over Ziegler-Natta catalysts [1–6]. However, some kinetic findings remain uninterpreted because of the versatility of the catalysts of this type, complex composition of the active component, several types of active centers, and changes they undergo during polymer- ization. In particular, dependence of the polymerization rate on monomer concentration is a key feature of polymerization. Accord- ing to the two-stage mechanism of the propagation reaction at catalytic polymerization proposed by Cossee [7], the propagation rate can be linearly dependent on monomer concentration and described by the first-order equation. Indeed, these dependences were observed in a large number of studies devoted to olefin ∗ Corresponding author. Tel.: +7 3833269770; fax: +7 3833308056. E-mail address: barabanov@catalysis.ru (A.A. Barabanov). polymerization over different catalysts [1–6]. However, some studies showed that a mixed (between the first and second) order of the polymerization rate with respect to ethylene is observed at low ethylene pressure [8–12]. Several hypotheses have been proposed to interpret this effect. Thus, Ystenes proposed the so-called “trigger” mechanism of the propagation reaction suggesting that an ethylene molecule coor- dinated on titanium ion at the titanium–polymer bond is inserted when the active center interacts with the second ethylene molecule [13,14]. The features of the “trigger” mechanism were also dis- cussed by other authors [9,15–17]. A number of studies suggested that the observed second order of the polymerization rate with respect to monomer is due to the presence of a slow initiation stage, which occurs when a monomer interacts with the precursor of active centers contain- ing the titanium–alkyl bond [18–20]. Resconi et al. suggested that the centers with different reactivities with respect to monomer may differ in conformation of the growing polymer chain (agostic interactions of - and -hydrogen and titanium atom) [21,22]. The assumptions in the existing literature regarding the reasons for the observed shift in the order of the polymerization rate with respect to monomer (from the first towards the second order) are http://dx.doi.org/10.1016/j.molcata.2014.10.011 1381-1169/© 2014 Elsevier B.V. All rights reserved.