## 1 Introduction  
At present, an essential amount of polyethylene (PE) in the world is produced with the use of highly active Ziegler-Natta catalysts, which contain titanium chlorides deposited on the MgCl\({}_{2}\)-containing support (supported titanium-magnesium catalysts, TMC), in combination with organoaluminum co-catalysts (OAC) [1; 2; 3]. The majority of produced PE grades, for example, pipe and film grades of high-density polyethylene (HDPE) and also linear low-density polyethylene (LLDPE), are copolymers of ethylene with \(\alpha\)-olefins. In particular, LLDPE contains up to 5 mol.% of \(\alpha\)-olefin. The physico-mechanical and rheological properties of the produced copolymers depend on the type and content of the comonomer and its distribution in the obtained polymer [4; 5; 6; 7]. Naturally, the ability of the comonomer to efficiently and uniformly insert itself into the polymer chains is determined primarily by the characteristics of the catalyst active sites and by the polymerization conditions [8; 9; 10; 11; 12].  
It is known that conventional multisite Ziegler-Natta catalysts, particularly the TMC, produce copolymers with broad molecular weight distribution (MWD) and non-uniform molecular weight distribution of the comonomer [8; 9; 10; 11; 12; 13]. Supposedly [14; 15], the formation of copolymers with the indicated properties may be caused by the non-uniformity of the TMC active sites with respect to the oxidation state of titanium.  
According to our data [16; 17; 18], the oxidation state of titanium in a titanium-magnesium catalyst exerts virtually no effect on the molecular weight characteristics of the produced homopolyethylene. However, the literature provides virtually no data concerning the effect of the titanium oxidation state in TMC on the copolymerization ability of such catalysts or the compositional heterogeneity of the obtained ethylene-\(\alpha\)-olefin copolymers. Refs. [16; 19] demonstrate that the composition of TMC, in particular the titanium oxidation state in the active component, affects the efficiency of comonomer insertion into the polymer chain during polymerization of ethylene with 1-hexene. Note that the authors of refs. [16] and [19] reached opposite conclusions on the copolymerization ability of the Ti(II)-based catalyst. In ref. [16], where the TMC active component was represented by the divalent titanium complex n\({}^{6}\)-BenzeneTiAl\({}_{2}\)Cl\({}_{8}\) (the TiCl\({}_{2}\) compound bound with two AlCl\({}_{3}\) molecules), an increased copolymerization ability was observed in the polymerization of ethylene with 1-hexene as compared to the standard TMC (TiCl\({}_{4}\)/MgCl\({}_{2}\)). In ref. [19], the TMC containing only titanium(II) chloride (TiCl\({}_{2}\)) showed a decreased copolymerization ability in comparison with the TiCl\({}_{4}\)-based TMC. Most likely, the reactivity of the catalyst active site is affected not only by the oxidation state of Ti\({}^{n+}\) (the electronic state of titanium) but also by its coordination environment.  
In this study, we investigated the copolymerization kinetics of ethylene with 1-hexene and propylene over four modifications of TMC, in which the active component was represented by titanium compounds in different oxidation states (Ti(II), Ti(III), or Ti(IV)) and in different coordination environments. Data were obtained on the copolymerization ability of these catalysts, their molecular weight characteristics, and the compositional heterogeneity of the produced copolymers.