Mikhail A. Matsko

New experimental data were obtained on the effect of internal donors composition (di‐n‐butyl phthalate, diethyl 2,3‐diisobutyl succinate and 2‐isopropyl‐2‐isopentyl‐1,3‐dimethoxypropane) in the titanium–magnesium catalysts (TMCs) on the molecular weight distribution of polyethylene, polypropylene, and polyhexene produced over these catalysts during polymerization in the absence and in the presence of hydrogen. These data show the great effect of monomer structure and the presence of hydrogen at the polymerization of α‐olefins on the molecular weight distribution (polydispersity) of polymers. We propose these results are determined by the heterogeneity of active sites with different regiospecificity in the chain transfer reaction with hydrogen at the polymerization of α‐olefins.

In this review, we summarize and discuss our experimental data published in a number of papers on the transfer reactions of polymer chains in the polymerization of ethylene, propylene, and hexene-1, and the copolymerization of ethylene with α-olefins over multisite supported titanium–magnesium catalysts (TMC). Three groups of transfer reactions are discussed in the review: (1) transfer reactions with AlEt₃ cocatalyst, (2) transfer reactions with hydrogen, and (3) transfer reactions with participation of α-olefins in the case of ethylene copolymerization with α-olefins. We have found polymerization conditions where it is possible to observe heterogeneity of active sites of TMC for all three groups of the indicated reactions. It is shown that (1) the transfer reaction with AlEt₃ proceeds with higher reactivity on the active sites that produce polymers with low molecular weight; (2) the transfer reaction with hydrogen, in the case of α-olefin polymerization and copolymerization of ethylene with α-olefins, proceeds with higher reactivity on the active sites which produce polymers with high molecular weight; (3) the transfer reaction with α-olefins proceed with higher reactivity on the active sites that produce high molecular weight polymers.

Polymeric floating photocatalysts based on polyethylene (PE)/TiO₂ compositions were synthesized by in situ ethylene polymerization in the presence of a titanium–magnesium catalyst synthesized by the sequential deposition of a magnesium–aluminum complex and TiCl₄ on commercial TiO₂ P25. The optical band gap of the synthesized PE/TiO₂ composites was shown to be 3.1–3.3 eV, which allowed for their use as photocatalysts for the utilization of solar light. The photocatalytic activity of the PE/TiO₂ composites was studied for the degradation of methyl orange (MO) under irradiation with UV light (λ_max = 384 nm). The composites containing 20–50 wt.% of PE were found to have an optimum combination between floatability and photocatalytic activity. The maximum photodegradation rate was observed at an MO concentration below 5 ppm. The polymeric PE/TiO₂ floating photocatalysts could be used repeatedly, but the long-term exposure of the composites to UV radiation was accompanied by oxidation of the polymer.

Data were obtained on the copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts (TMC) prepared on the same magnesium dichloride support but differing in the composition and oxidation state of titanium. The copolymerization kinetics of ethylene with 1-hexene over TMC of different compositions were studied. Data on the composition of the produced ethylene–1-hexene copolymers, their molecular weight distribution, thermophysical characteristics, and branching distribution were presented. The constants of ethylene–1-hexene copolymerization over catalysts with different compositions were calculated. The TMC containing only Ti(II) compounds as the active component exhibited increased copolymerizing ability compared to the conventional TiCl₄/MgCl₂ catalyst containing Ti(III) compounds as the active component. In addition, TMC with Ti(II) as an active component produces copolymers with a more uniform branching distribution. It was shown that the TMC containing isolated Ti(II) ions could be used to produce X-ray amorphous ethylene-propylene elastomers with a high yield.

Data are presented on the great differences of the kinetics of hexene-1 and propylene polymerization over the same supported titanium–magnesium catalyst, as well as molecular weight and molecular weight distribution of the polymers produced. It is found that the composition of cocatalysts (AlEt₃ or Al(i-Bu)₃ greatly affects the kinetics of hexene-1 polymerization and molecular weight distribution of polyhexene, contrary to data obtained at propylene polymerization. The presence of hydrogen at hexene-1 polymerization leads to a much higher increase of activity in comparison with propylene polymerization. Possible reasons for these differences are discussed on the basis of experimental results.

Ethylene polymerization with bis(imino)pyridlyiron precatalysts generally produces linear polyethylene (PE) even with the presence of α-olefins because α-olefins are not incorporated into polymeric products. Interestingly, α-olefins, such as hexene-1 or butene-1, have been found to act as effective chain transfer agents in the ethylene polymerization promoted by nonsymmetrical bis(imino)pyridyliron complexes with modified methylalumoxane (MMAO), resulting in higher catalytic activities with higher amounts of polymers with lower molecular weights, and, more importantly, narrower molecular weight distributions of the resultant polyethylenes (PE). This phenomenon confirms the assistance of α-olefins in the chain-termination reaction of iron-initiated polymerization and regeneration of the active species for further polymerization. Besides higher activities of the catalytic system, the formation of linear PE with trans-vinylene terminal groups and lower molecular weights are explained. The observation will provide a new pathway for enhancing catalytic activity and improving the quality of polyethylenes obtained by regulation of molecular weights and molecular weight distribution.

Представлены данные о возможностях регулирования молекулярной структуры полиэтилена (ПЭ), получаемого на нанесенных катализаторах, которые содержат бис(имино)пиридильные комплексы Fe(II) (LFeCl2) и бис(иминные) комплексы Ni(II) (*LNiBr2), закрепленные на силикагеле, модифицированном введением оксида алюминия (SiO2(Al)). При варьировании условий полимеризации на катализаторах LFeCl2 /SiO2(Al) получен линейный ПЭ с различной молекулярной массой и регулируемым молекулярно-массовым распределением (ММР). При гомополимеризации этилена на катализаторах *LNiBr2 /SiO2(Al) синтезирован разветвленный ПЭ с молекулярно-массовыми и термофизическими характеристиками, близкими полиэтилену низкой плотности, получаемому при сополимеризации этилена с α-олефинами на нанесенных металлоценовых катализаторах и нанесенных катализаторах циглеровского типа. Предложен метод конструирования нанесенных бикомпонентных катализаторов, содержащих комплексы LFeCl2 и *LNiBr2 закрепленные на носителе SiO2(Al), для целенаправленного получения полиэтилена с требуемой молекулярной структурой. Представлены примеры синтеза линейного ПЭ с бимодальным ММР на бикомпонентном нанесенном катализаторе, содержащем два различных комплекса LFeCl2, ПЭ с контролируемым распределением разветвлений на бикомпонентном катализаторе, приготовленном путем закрепления комплексов LFeCl2 и *LNiBr2 на носителе SiO2(Al), а также пример модификации промышленного оксидно-хромового катализатора путем введения в его состав комплекса LFeCl2 для регулирования ММР и распределения разветвлений в получаемом полиэтилене.

Ethylene polymerization and ethylene/1-hexene copolymerization over the titanium–magnesium catalytic system in the presence of chlorocyclohexane (CHC) and hexachloro-p-xylene (HCPX) has been studied. Modification of TMC with chlorocyclohexane and hexachloro-p-xylene increased catalyst activity severalfold for both ethylene polymerization and ethylene/1-hexene copolymerization. The key kinetic regularities of ethylene homopolymerization and ethylene/1-hexene copolymerization in the presence of CHC and HCPX were determined, and the copolymerization constants were measured. Molecular characteristics and the copolymer composition were determined for the synthesized samples of ethylene homopolymers and ethylene/hexene copolymers. Modification of the titanium–magnesium catalyst with chlorinated organic compounds reduced 1-hexene content in the copolymer; polymerization was sensitive to 1-hexene as a regulator of polymer molecular weight. The potential mode of action of chlorinated organic modifiers on catalytic properties of titanium–magnesium catalyst is discussed.

Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (M_w ≥ 10⁶ g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (K_tr^H) and polymer chain propagation (K_p) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.