## 3 Results and Discussion
#### 3.2.1 Copolymerization of Ethylene with 1-Hexene

Table 2: Data on the copolymerization of ethylene with 1-hexene over Ti-Mg catalysts containing titanium compounds in different oxidation states as the active component. In the case of ethylene-1-hexene copolymerization over TMC-1, TMC-1-2, and TMC-3 catalysts (Figure 1A,B), a high initial activity followed by a considerable decrease in polymerization rate was observed. In the case of copolymerization over TMC-2, the activity sharply increases in the initial period (10 min) until a stable level is achieved (Figure 1C). It was supposed [29; 30; 31] that the increase in activity of different catalysts in the copolymerization of ethylene with \(\alpha\)-olefin relative to the homopolymerization of ethylene may be associated with the higher concentration of monomers on the surface of catalysts. This effect was explained by an increase in the portion of amorphous material in the produced copolymers in comparison with homopolyethylene, which has a higher crystallinity. As seen in Table 2, indeed, all the copolymers of ethylene with 1-hexene have a lower crystallinity (26-47%) with respect to PE (60-66%) and a lower melting point of the crystalline part. The gel permeation chromatographic analysis of the C\({}_{2}\)/C\({}_{6}\) copolymer samples produced on different TMC under similar polymerization conditions revealed that the polymers have close molecular weights (M\({}_{\rm w}\) = 170-230 \(\times\) 10\({}^{3}\)) and MWD (M\({}_{\rm w}\)/M\({}_{\rm n}\) = 4.1-4.4) (Table 2, exps. 5, 9, and 12). An increase in the concentration of 1-hexene in the reaction medium leads to a decrease in the molecular weight (M\({}_{\rm w}\) drops from 170-230 to 100-140 kg/mol) of copolymers obtained over all the TMC. As was shown in refs. [32; 33], \(\alpha\)-olefin is an efficient polymer chain transfer agent during the copolymerization of ethylene with \(\alpha\)-olefins (propylene, 1-hexene) over TMC. Figure 2 displays the dependences of 1-hexene content in copolymers obtained over TMC of different compositions on the [C\({}_{6}\)H\({}_{12}\)]/[C\({}_{2}\)H\({}_{4}\)] ratio in the reaction medium (heptane). Figure 1: Kinetic curves of ethylene homopolymerization and ethylene–1-hexene copolymerization over TMC with different compositions of the active component (**(A)**—TMC-1 and TMC-1-2; **(B)**—TMC-3; **(C)**—TMC-2) (the numbering of kinetic curves corresponds to the numbering of experiments in Table 2). According to the data presented in Figure 2 and Table 2, which corresponds to the inverse \(r_{1}\) value, the copolymerization ability changes in the following series of catalysts: TMC-1 > TMC-1-2 > TMC-2 > TMC-3. Thus, the highest copolymerization ability (\(r_{1}\) = 49) was observed for the TMC-1 catalyst, which contains isolated monomeric compounds of Ti(II) as the active component. The copolymerization ability of this catalyst exceeds that of TMC-2 and TMC-3 by a factor of 2-3 (\(r_{1}\) = 99 and 130, respectively). The intermediate copolymerization ability (\(r_{1}\) = 68) was exhibited by the TMC-1-2 catalyst, which contains isolated monomeric Ti(III) compounds obtained by the oxidation of Ti(II) compounds in the TMC-1 catalyst. It is known that one of the main factors affecting the properties of copolymers is the distribution of comonomer units in the polymer chain (in the fractions of copolymers with different molecular weights). To analyze the compositional heterogeneity of copolymers obtained on different catalysts and, respectively, the heterogeneity of active sites of these catalysts in the copolymerization of ethylene with 1-hexene, copolymers with a close content of 1-hexene (17-21 Bu/1000C) were separated into fractions with a narrow molecular weight distribution.