Impact of the method of Mg 2+ cation introducing into the titanium post-metallocene catalytic system on their activity

doi:10.21203/rs.3.rs-3982011/v1

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Impact of the method of Mg 2+ cation introducing into the titanium post-metallocene catalytic system on their activity

https://doi.org/10.21203/rs.3.rs-3982011/v1

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This work shows for the first time the influence of the method of introducing Mg compounds into a post-metallocene-type catalytic system on its catalytical performance and the properties of the resulting polymer, and it is concluded that the real catalytic center is heterometallic Ti/Mg complexes. Compares three different approaches to introducing Mg compounds into catalytic systems containing a titanium(IV) dichloride complex with an (O^O)2- type ligand and an organoaluminium compound - the use of Al/Mg co-catalysts, the immobilization of a Ti complex on the surface of anhydrous MgCl2 and in situ preparation of heterometallic Ti/Mg complexes. All three approaches lead to the production of ultra-high molecular weight polyethylene, but systems with externally introduced magnesium chloride showed the lowest productivity. UHMWPE samples synthesized on Ti/Mg precatalysts and conventional OAC are suitable for solvent-free solid-phase processing into high-strength, high-modulus film threads.

Among the vast majority of various post-metallocene polymerization catalysts developed over the past decades, [1–6] perhaps the least studied are complexes of group 4 metals with (O^O) type ligands, in which one or both hydroxyl groups are not phenolic, but alcoholic. These pre-catalysts have shown fairly high efficiency in the synthesis of ultra-high molecular weight polyethylene (UHMWPE), [7–13] a promising structural material of industrial interest. [14–16] However, the large molecular weight of the polymer and the presence of a three-dimensional network of macromolecule entanglements determines the very high viscosity of its melt, which greatly complicates its processing. Carrying out the polymerization of ethylene on single-site catalysts under fairly mild conditions (low temperature, low concentrations of catalyst and monomer) makes it possible to obtain UHMWPE with a low degree of entanglements (so called disentangled UHMWPE). [17] Such a polymer can be processed by a solvent-free solid-phase method into high-strength, high-modulus oriented tapes. [17–20] Using this method instead of gel spinning technology has significant advantages, since it does not require the use of large volumes of high-boiling solvents. The above-mentioned Ti(IV) complexes with O^O-type ligands are capable of producing dis-UHMWPE. [7–13] However, these pre-catalysts have their own specifics, which have not yet received a reasonable explanation, and perhaps to some extent complicate their industrial use: in the presence of traditional aluminum-containing activators used in classical Ziegler catalysis (trialkylaluminum, alkylaluminum chlorides, or alkylaluminoxanes), these complexes demonstrate extremely low activity. But the same precatalysts, when activated by Al/Mg cocatalysts {Alk_nAlCl_3-n+Bu₂Mg}, dramatically change their properties and exhibit very high activity, reaching 1–4 tons of polymer / mol Ti hour atm.

According to [21, 22], upon interaction of the components of Al/Mg co-catalysts, anhydrous highly dispersed magnesium chloride is formed as one of the reaction products (reaction 1):

Bu₂Mg + 3Et₂AlCl = MgCl₂ + 2Et₂BuAl + Et₂AlCl (1)

This, in principle, brings these post-metallocene catalyst systems closer to classical Ziegler-Natta catalysts. However, in the case of post-metallocene catalytic system, the Ti/Mg molar ratio is orders of magnitude lower than in classical heterogeneous titanium-magnesium catalysts.

One of the possible reasons for the increase the catalytic activity of the pre-catalysts being discussed is the formation of heterometallic Ti/Mg complexes as precursors of “true catalyst”. This version is supported by DFT calculations of interaction products in the LTiCl₂-MgCl₂-AlR₃ system, [23–27] (Fig. 1A) but no experimental evidence has been obtained to date. Thus, in this work, an attempt was made to more unambiguously clarify the effect of magnesium chloride on the productivity of the studied catalytic systems and on the properties of the resulting polymers, depending on the method of its introduction into these systems.

2.1. Materials and Methods

All manipulations with air-sensitive materials were performed using standard Schlenk techniques. Argon and ethylene of special-purity grade (Linde gas) were dried by purging through a Super Clean™ Gas Filters. Toluene and nefras (a hydrocarbon, predominantly aliphatic solvent, one of the types of straight-run gasoline) were distilled over Na/benzophenone ketyl and the water content was periodically controlled by Karl-Fischer coulometry by using a Methrom 756 KF apparatus. Diethylaluminum chloride and di-n-butylmagnesium (Aldrich) were used as 1.0 M solution in heptane without further purification. The ligand L – 2,4-di-tert-butyl-6-(1,1,1,3,3,3-hexafluoro-2-hydroxypropane-2-yl)-phenol and corresponding complex 1 – LTiCl₂·2iPrOH were prepared following literature procedure; [28] their properties corresponded to literature data.

Complex 2 (L₂Mg·2THF). In a 50 mL flame-dried Schlenk flask, ligand L1 (0.37 g, 1.00 mmol) was dissolved in anhydrous THF (5 mL). A solution of (n-Bu)₂Mg (0.50 ml, 0.50 mmol) in heptane (10 mL) was added to the resulting solution under stirring in an argon atmosphere. The solution was stirred for 14 h at room temperature; the resulting white precipitate was filtered off, washed with hexane (5 mL) and dried in vacuum. The yield of the complex was 0.67 g (74%). Calculated (%) for C₄₂H₅₈F₁₂MgO₆ (911.19): C, 55.36; H, 6.42; F, 25.02; Mg, 2.67. Found (%): C, 55.32; H, 6.37; F, 24.98; Mg, 2.63. ¹H NMR (CDCl₃), (ppm): 1.31 (s, 18H, t-Bu); 1.34 (s, 18H, t-Bu); 2.28 (m, 8H, CH₂); 2.45 (m, 8H, CH₂); 5.72 (2H, OH); 7.36 (s, 2H, Ar); 7.49 (s, 2H, Ar). ¹³C NMR (CDCl₃), (ppm): 20.41; 30.21; 31.54; 35.43; 37.89; 87.43; 120.11; 121.07; 122.43; 137.57; 142.28; 151.52; 152.65. ¹⁹F NMR (CDCl₃), (ppm): 83.20.

Complex 3 (L₂Mg₂·3THF). The yield of the complex was 0.86 g (86%). Calculated (%) for C₄₆H₆₄F₁₂Mg₂O₇ (1005.59): C, 54.89; H, 6.38; F, 22.71; Mg, 4.83. Found (%):C, 54.85; H, 6.32; F, 22.67; Mg, 4.80.

Complex 4 (L₄Mg₅(n-BuO)₂·4THF). The yield of the complex was 0.67 g (74%). Calculated (%) for C₉₂H₁₃₀F₂₄Mg₅O₁₄ (2034.83): C, 54.16; H, 6.42; F, 22.37; Mg, 5.92. Found (%): C, 54.40; H, 6.42; F, 22.18; Mg, 5.87. ¹H NMR (CDCl₃), (ppm): 1.05 (s, 6H, CH₃); 1.25 (s, 18H, t-Bu); 1.29 (s, 18H, t-Bu); 1.34 (s, 18H, t-Bu); 1.42 (s, 18H, t-Bu); 2.72 (m, 24H, CH₂); 2.45 (m, 12H, CH₂); 3.57 (m, 8H, CH₂); 7.36 (m, 8H, Ar). ¹³C NMR (CDCl₃), (ppm): 12.93; 20.41; 21.88; 29.53; 30.88; 31.12; 35.90; 37.04; 45.10; 47.26; 48.35; 50.36; 107.69; 121.09; 122.97; 124.41; 126.95; 132.09; 134.22; 140.04; 140.89; 141.15; 143.03. ¹⁹F NMR (CDCl₃), (ppm): 86.88.

Complex 5 (LTiCl₂·MgCl₂). A 10-ml toluene solution of the complex 3 (L₂Mg₂·3THF, 1.01 g, 1.00 mmol) was placed under argon in the flask equipped with a magnetic stirrer. Then we added TiCl₄ (0.38 g, 2.00 mmol). The solution was stirred for 5 hours at room temperature. The reaction mixture was evaporated in vacuum. The yield of the complex was 1.04 g (89%). Calculated (%) for C₁₇H₂₀F₆MgO₂Ti (584.31): C, 34.87; H, 3.44; F, 19.37; Mg, 4.18; Ti, 8.16. Found (%): C, 34.72; H, 3.31; F, 19.30; Mg, 4.11; Ti, 8.12.

Complex 6 (LTiCl₂·2MgCl₂). A 10-ml toluene solution of the 2,4-di-tert-butyl-6-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenol, (0.186, 0.50 mmol) was placed under argon in the flask equipped with a magnetic stirrer. n-Heptane solution of dibutylmagnesium (1.05 ml, 1.05 mmol) was added dropwise while stirring at 78 ᵒC. Reaction mixture was slowly brought to room temperature and stirred for 4 h. It was cooled again to -78 ᵒC and TiCl₄ (0.095g, 0.50 mmol) was added. The solution was stirred for 5 hours at room temperature. The reaction mixture was evaporated in vacuum. The yield of the complex was 0.50 g (88%). Resulting product was used for ethylene polymerization without further purification. Calculated (%) for C₁₇H₂₀Cl₂F₆O₂Ti·2Cl₂Mg (679.52): C, 30.05; H, 2.97; Cl, 31.30, F, 16.78; Mg, 7.15; Ti, 7.04. Found (%): C, 30.01; H, 2.95; Cl, 31.28; Mg, 7.11; Ti, 7.00.

NMR spectra were recorded on Bruker AMX-400 instrument. THF-d₈ was stored over Na/K alloy and was degassed by freeze–pump-thaw cycles. Chemical shifts are reported in ppm vs. SiMe₄ and were determined by reference to the residual solvent peaks. All coupling constants are given in Hertz. IR spectra were recorded on a Magna-IR 750 spectrophotometer.

Elemental analysis (C, H, Cl) was performed by the microanalytical laboratory at A. N. Nesmeyanov Institute of Organoelement Compounds on Carlo Erba-1106 and Carlo Erba-1108 instruments. The content of Ti was performed by X-ray fluorescence analysis on the VRA-30 device (Karl Zeiss, Germany).

X-ray crystal structure determination

X-ray diffraction data for 2 were collected at 120 K with a Bruker APEX DUO diffractometer, using graphite monochromated Cu-Kα radiation (l = 1.54178 Å). Using Olex2, [29] the structure was solved with the ShelXT [30] structure solution program using Intrinsic Phasing and refined with the XL [31] refinement package using Least Squares minimization against F²_hkl in anisotropic approximation for non-hydrogen atoms. The hydrogen atoms of the OH groups were located from difference Fourier synthesis while the positions of other hydrogen atoms were calculated, and they all were refined in the isotropic approximation within the riding model. Crystal data and structure refinement parameters are given in Table S1.

The single-crystal X-ray diffraction data for compounds 3 and 4 were collected on a four-circle XtaLAB Rigaku Synergy-S diffractometer equipped with a HyPix-6000HE area-detector (T = 100 K, λ(CuKα)-radiation, graphite monochromator, shutterless ω-scan mode). The data were integrated and corrected for absorption by the CrysAlisPro program. [32] For details, see Table S1. The structures were solved by intrinsic phasing modification of direct methods [30] and refined by a full-matrix least-squares technique on F² with anisotropic displacement parameters for all non-hydrogen atoms. In the crystal of 3, one of the four tert-butyl groups is disordered over two sites with the occupancies of 0.7:0.3. In the crystal of 4, two of the four tetrahydrofuran ligands are disordered over two sites each with the equal occupancies. The crystal of 4 contained two solvate heptane molecules in the asymmetric unit, and the crystal of 3 contained a solvate pentane molecule in the asymmetric unit, which is disordered over two sites with the occupancies of 0.8:0.2. The hydrogen atoms in both compounds were placed in calculated positions and refined within the riding model with fixed isotropic displacement parameters [U_iso(H) = 1.5U_eq(C) for the methyl groups and 1.2U_eq(C) for the other groups]. All calculations were carried out using the SHELXTL program suite. [33] Crystallographic data for 2, 3·C₅H₁₂ and 4·2C₇H₁₆ have been deposited with the Cambridge Crystallographic Data Center, CCDC 2308808, CCDC 2307415 and CCDC 2307414, respectively. The supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Polymer synthesis and Polymer evaluation methods are described in detail [11]. The technique for manufacturing oriented films from UHMWPE nascent reactor powder and determining their mechanical characteristics is described in. [20]

Synthesis and Characterization.

A titanium dichloride complex stabilized by substituted saligenin − 2,4-di-tert-butyl-6-(1,1,1,3,3,3-hexafluoro-2-hydroxypropane-2-yl)-phenol (Complex 1), the synthesis, structure and catalytic properties of which are described in [28], was used in this work. Its activation with the “standard” co-catalyst for such complexes - {3Et₂AlCl + Bu₂Mg} leads to the production of UHMWPE with a productivity of 1830 kg of PE/mol Ti h atm with a viscosity-average molecular weight of 2.35 million Da (Table 1, entry 1). We used these results as references when discussing experimental data obtained on new catalytic systems of this type. In the presence of conventional organoaluminum activators - Me₃Al, Et₃Al, i-Bu₃Al, Et₂AlCl and MAO, this complex did not show catalytic activity in the polymerization of ethylene, as was already noted. [7, 28] Moreover, adding magnesium chloride to these catalytic systems also does not lead to its activation. However, mechanochemical activation of anhydrous magnesium chloride, obtained by dehydration of its crystal hydrate in a mixture with ammonium chloride, in a ball mill for 5 hours in absolute toluene medium and in the presence of Et₃Al somewhat changes this previously obtained result. Complex 1, immobilized on this mechanically activated MgCl₂ showed activity in ethylene polymerization in the presence of Et₃Al, Et₂AlCl or their mixture (Ti/Al ratio ~ 1:300), which, however, was much inferior to the reference system, but still reached 309 kg of PE /mol Ti h atm for Et₃Al, (entries 2–4). Interestingly, the molecular weight of polyethylene obtained using this heterogenized system is noticeably higher than that obtained using an Al/Mg cocatalyst (entry 1 vs entries 2–3, Table 1). The identified significant differences in the productivity of Mg-containing catalytic systems indicate that the method of their formation can affect the actual content of heterometallic complexes. Quite possibly that MgCl₂ formed during the interaction of the Al/Mg activator components in statu nascendi, reacts with the titanium precatalyst and quite easily forms a Ti/Mg heterometallic complexes. This possibility is confirmed by DFT calculation showing a high probability of the formation of neutral complexes A and B as a result of activation of complex 1 by Al/Mg activator (Fig. 1A). [23–24, 26] In our opinion, structure B seems more stable, since the fluorine atoms of the ligand are also capable of coordination with the metal. The possibility of stabilizing heterometallic complexes with the same ligand L through coordination of a non-transition metal with the fluorine atoms of the ligand (Fig. 1B) has been experimentally proven; ^7,10 complexes C-E showed activity in the polymerization of ethylene. We conducted a series of experiments aimed at facilitating the formation of such heterometallic complexes. At the first stage, magnesium complexes L₂Mg·2THF, L₄Mg₅(n-BuO)₂·4THF and L₂Mg₂·3THF were obtained by interaction of ligand L and dibutylmagnesium solution in ratios 2:1, 1:1 and 1:2 (Scheme 1).

The composition and structure of the complexes were confirmed by elemental analysis, NMR spectroscopy and X-ray diffraction (Figs. 2–3).

According to X-ray diffraction data, the complex 2 (Fig. 2) crystallizes in the monoclinic space group P21/c. The magnesium(II) ion coordinates two oxygen atoms (Mg-O 2.0214(13) and 2.0328(14) Å) from two symmetry-independent ligands and two oxygen atoms from two THF molecules (Mg-O 1.9145(11) and 1.9225(11) Å). Two fluorine atoms (Mg-F 2.5090(13) and 2.6716(12) Å) from the same two ligands complete the coordination environment of the magnesium(II) ion, which is close to an octahedron, as gauged by the ‘continuous symmetry measures’. [34] They latter measure how far the coordination polyhedron is from a reference shape, such as an ideal octahedron (OC-6). For the complex 2, the corresponding symmetry measure S(OC-6) estimated with the Shape 2.1 program [34] is 3.776. For comparison, the ‘symmetry measure’ quantifying the deviation of the polyhedron of the magnesium(II) ion from another ideal polyhedron with six vertices, a trigonal prism (TPR-6), is 12.067.

The compound 3 ([L₂Mg₂(THF)₃] is a binuclear magnesium(II) complex including the central Mg₂O₂ ring (Fig. 3A). The Mg₂O₂ ring adopts a butterfly conformation, with the fold angle of 12.65(9)°. One of the two L-ligands is tridentate, with one µ²-bridged oxygen atom from the ─C(CF₃)₂O─ substituent as well as the phenoxy oxygen atom and one fluorine atom from one of the two CF₃-groups coordinated to the two different Mg atoms. The other L-ligand is bidentate by the both oxygen atoms, with the µ²-bridged oxygen atom from the ─C(CF₃)₂O─ substituent. Thus, the Mg atoms adopt the distorted MgO₄F- and MgO₅-tetragonal-pyramidal coordination, respectively (Table S1). The two six-membered chelate rings have a boat conformation, and the five-membered chelate ring has an envelope conformation.

The compound 4 ([L₄Mg₅(n-BuO)₂(THF)₄] is a pentanuclear magnesium(II) complex containing the four spiro-bonded Mg₂O₂ rings (Fig. 3B). The Mg₂O₂ rings are almost planar and arranged at the angles of 61.78(5), 83.02(5) and 63.13(3)° relative to each other, respectively. The internal L-ligands are tetradentate, with the both µ²-bridged oxygen atoms binding three Mg atoms and two fluorine atoms from the different CF₃-groups coordinated to the central and terminal Mg atoms. The terminal L-ligands are tridentate, with one µ²-bridged oxygen atom from the ─C(CF₃)₂O─ substituent binding two ending Mg atoms as well as the phenoxy oxygen atom and one fluorine atom from one of the two CF₃-groups coordinated to the terminal Mg atom. The both butoxy ligands are µ²-bridged between the middle Mg atoms, and all four THF ligands are terminal. Thus, the central and two terminal Mg atoms adopt the significantly distorted MgO₄F₂-octahedral geometry with the two cis-disposed fluorine atoms, whereas the two other Mg atoms have the distorted MgO₅-tetragonal-pyramidal coordination (Table S2). Moreover, the L-ligands form the four six-membered and four five-membered chelate rings. The two internal six-membered chelate rings have a boat conformation, while the two terminal L-ligands have a sofa conformation. All four five-membered chelate rings adopt an envelope conformation.

The interaction of complex 3 - L₂Mg₂·2THF with titanium tetrachloride resulted in the synthesis of a heterometallic Ti/Mg complex, which, according to elemental analysis, corresponds to the composition LTiCl₂·MgCl₂. To obtain a Ti/2Mg precatalyst, ligand L was deprotonated with 2 equivalents of Bu₂Mg, after which TiCl₄ was added to the reaction mixture. Attempts to grow single crystals of heterometallic complexes were unsuccessful, so they were tested in situ in ethylene polymerization. The results are presented in the Table 1 and Fig. 4.

Table 1

Catalytic activity of the compared precatalysts
Entry	Activator, [Ti]/[Al]	m _polym., g	A^c	Bulk density, g/cm³	T_m ^d, °С	Deg. of сrystal^e, %	М_v, 10⁶ Da
Reference catalytic system: Complex 1 (LTiCl₂·2iPrOH) + Al/Mg activator
1	Et₂AlCl + Bu₂Mg 1:300/100	1.6	1830	0.072	139.3/135.6	70.1/52.3	2.35
Complex 1 + MgCl₂ (mechanically activated)
2	Et₂AlCl, 1:300	0.07	80	0.069	140.2/136.9	69.9/49.8	3.04
3	Et₃Al, 1:300	0.27	309	0.059	139.3/134.3	79.3/60.7	3.54
4	Et₂AlCl/Et₃Al 1:100/200	0.10	114	0.068	142.4/138.7	79.0/60.9	1.53
Precatalyst 5 – {LTiCl₂·MgCl₂}
5	Me₃Al, 1:500	3.0	1714	0.056	140.7/132.6	72.1/46.2	2.94
6 ^b	Me₃Al, 1:500	2.6	1273	0.050	140.7/131.1	71.5/49.8	3.69
7	Et₃Al, 1:500	4.1	2343	0.056	140.5/133.2	68.1/46.9	2.80
8 ^b	Et₃Al, 1:500	2.6	1486	0.059	140.8/133.3	69.6/49.2	3.76
9	iBu₃Al, 1:500	1.2	686	0.061	142.4/133.4	67.9/41.1	2.73
10	MAO, 1:500	2.6	1486	0.047	140.8/132.6	68.6/46.5	4.55
11 ^b	MAO, 1:500	2.2	1257	0.044	140.2/130.9	71.7/46.9	3.98
Precatalyst 6 – {LTiCl₂·2MgCl₂}
12	Me₃Al, 1:500	4.5	2571	0.070	140.8/132.9	75.4/54.4	4.00
13 ^b	Me₃Al, 1:500	2.0	1143	0.077	141.8/134.0	71.1/50.7	2.76
14	Et₃Al, 1:500	4.0	2286	0.064	143.9/139.2	58.6/45.9	2.30
15 ^b	Et₃Al, 1:500	3.3	1886	0.072	141.3/134.0	72.9/56.4	3.21
16	iBu₃Al, 1:500	3.5	2000	0.078	141.7/134.7	74.9/54.9	2.72
17 ^b	iBu₃Al, 1:500	0.6	343	0.077	142.7/135.6	76.6/54.3	3.61
18	МАО, 1:500	4.6	2629	0.051	140.4/131.3	70.2/53.3	3.53
19 ^b	МАО, 1:500	2.5	1429	0.053	141.4/133.8	67.2/47.7	4.00

^a Polymerizations were carried out in 100 ml of toluene with 5⸱10^− 6 mol of precatalyst at a constant 1.7-atm excessive ethylene pressure for 30 min, temperature 30 °C.

^b Polymerizations were carried out in 100 ml of nefras.

^c Activity, kg of PE/mol_Ti h atm.

^d Melting points were determined by DSC; the values for the first and second heating runs are given.

^e Degree of crystallinity was calculated by use of value ΔHm^100% = 288 J g^− 1; the values for the first and second heating runs are given.

The most important result obtained when testing heterometallic precatalysts 5–6 is their ability to be effectively activated by conventional trialkylaluminum derivatives and methylaluminoxane (MMAO-12). A precatalyst with a Ti/Mg ratio = 1:2 is the most effective and, in most cases, exceeds the activity of the reference system – 1+{3Et₂AlCl + Bu₂Mg}. The molecular weights of the resulting polymers in almost all cases exceed the value for the reference system. In this case, the highest molecular weight polymers were obtained using precatalyst 5 activated by MMAO-12 (4.55·10⁶ Da) and precatalyst 6 in the presence of Me₃Al (4.0·10⁶ Da). For both precatalysts, the use of sterically loaded i-Bu₃Al leads to a decrease in activity.

The possibility of replacing the expensive solvent - toluene with technologically more acceptable aliphatic solvents largely determines the prospects for the industrial use of new catalysts. For this reason, the productivity of catalytic systems and the properties of the polymer during polymerization in nefras (one of the types of straight-run gasoline) were investigated. As can be seen from the data presented in Table 1 (entries 6, 8, 11, 13, 15, 17, 19) and in Fig. 4B, the activity of both precatalysts (5–6) is significantly reduced. Its most acceptable values are maintained when using Et₃Al as an activator (entry 8–1486 kg of PE/mol Ti h atm. and entry 15–1886 kg of PE/mol Ti h atm.), that is, at the level of activity of the reference system. The molecular weights of UHMWPE samples obtained in nefras, in some cases, even exceeded the values for samples synthesized in toluene (Fig. 4B). The maximum molecular weight values in this series of experiments were obtained with precatalyst 5 activated by MMAO-12 (4.55·10⁶ Da, entry 10) and precatalyst 6 in the presence of Me₃Al (4.0·10⁶ Da, entry 12).

The polymerization conditions used in this work (low temperature, low precatalyst and monomer concentrations) facilitate the production of a polymer with a low density of the macromolecule’s entanglement network (Disentangled UHMWPE). [17] Such a polymer can be processed by a solid-phase solventless method into high-strength, high-modulus films and threads. Processing of UHMWPE reactor powders into high-modulus oriented films was carried out by producing monolithic samples under pressure and shear deformation at a temperature below the melting point of the polymer, followed by uniaxial stretching. [20] The quality criteria for the resulting oriented film tapes were: uniformity in width and homogeneity of sample masses within the same multiplicity at the same length. Unfortunately, not all compacted samples turned out to be suitable for orientation drawing. Table 2 and Figure S17 shows the mechanical characteristics of film tapes prepared from reactor powders. The tensile curves of oriented UHMWPE film tapes are shown in Fig. S6. In terms of tensile strength values, they were slightly inferior to materials obtained using a catalytic system (1/ Et₂AlCl + Bu₂Mg, entry 1), but in terms of average tensile modulus values they were superior to the reference system.

Table 2

Mechanical properties of UHMWPE oriented film tapes ^a
Entry	Catalytic system	σ,^b GPa	E,^c GPa
1	1/ Et₂AlCl + Bu₂Mg	1.75	81.2
7	5 /Et₃Al	1.42	92.47
8	5 /Et₃Al (nefras)	1.27	99.65
12	6 /Me₃Al	1.56	116.55
13	6 /Me₃Al (nefras)	1.20	98.73
14	6 /Et₃Al	1.66	114.1
15	6 /Et₃Al(nefras)	1.61	125.57
16	6 /iBu₃Al	1.51	107.30

^a Numbering corresponds to Table 1.

^b σ – tensile strength, GPa;

^c E – average tensile modulus, GPa.

According to literature data, the highest values of mechanical characteristics were recorded for oriented UHMWPE films obtained on titanium complexes with polyfluorinated phenoxyimine ligands in the presence of MAO. [17–20] It is interesting that in our case, precatalysts 5–6, activated by MMAO-12, catalyze the synthesis of UHMWPE with the highest molecular weight values in this series (up to 4.5·10⁶ Da), but the resulting reactor powders could not be processed into oriented film threads.

We studied the surface morphology of particles of nascent reactor UHMWPE powders successfully processed into oriented film filaments in comparison with unsuitable for solid-phase processing polymers obtained in the presence of MAO (Figs. 5, S18-S36). At low magnification, a highly porous surface of the particles is visible, which explains the anomalously low values of bulk density, and at high magnification, a highly porous surface of the particles is visible, which explains the anomalously low values of bulk density, and at high magnification, conglomerates of spherical particles with a small number of strands can be seen. Thus, the nature of the activator - Me₃Al or MAO - does not significantly affect either the type of particle morphology - broccoli-shaped - or the size of the spherical elements.

The positive effect of non-transition metal additives on olefin polymerization processes has been well studied using the example of late transition metal complexes - nickel and palladium, [35–39] but for early transition metals it has been studied to a much lesser extent. The results of present and our earlier works [10, 40–42] indicate that the introduction of a non-transition metal compound - magnesium chloride into the composition of post-metallocene catalytic systems, in particular into a system based on a titanium complex with an O^O^2- type ligand, also has a significant positive effect on their activity in olefin polymerization reactions.

Based on the working hypothesis about the heterometallic nature of catalytically active sites of the considered type, it is possible to explain the observed influence of the method of introducing magnesium compounds into the catalytic system on their productivity by the real content of Ti/Mg complexes. Immobilization of titanium complex 1 on the surface of crystalline MgCl₂ leads to the least productive catalytic systems, since MgCl₂ is insoluble in toluene, and the overwhelming number of Mg atoms in crystallites are coordination-saturated. The probability of the formation of Ti/Mg complexes and, accordingly, an increase in system productivity increases when complex 1 interacts with MgCl₂ at the moment of its formation from the components of the Al/Mg activator (in statu nascendi).

However, with this approach, a significant excess of Mg compounds is present in the system (the usual Ti/Al/Mg ratio is 1:300:100), which leads to the formation of a set of heterogeneous catalytic centers and broadening of the polymer MWD. In our opinion, the most preferable is the third approach, which consists in the interaction of ligands deprotonated by magnesium alkyls with titanium tetrachloride. Pre-catalysts obtained in this way are successfully activated by traditional activators (trialkylaluminum, alkylaluminum chlorides and alkylalumoxanes), which opens up another possibility for regulating the properties of the polymer product. The Ti/Mg molar ratio in such systems can be reduced from 1:100 to 1:1–2 without reducing the productivity of the catalytic systems. In addition, this approach makes it possible to use more accessible solutions of organomagnesium compounds in solvating solvents. The above clearly demonstrates the genetic continuity of post-metallocene catalysis with classical Ziegler catalysis in its so-called titanium-magnesium version using internal donors as modifiers. It can be assumed that the main function of these donors is to activate the surface of magnesium chloride by forming complexes of type 2–4, the treatment of which with titanium chloride leads to the formation of an effective catalytic system, as in our case.

An obvious direction for further development of this line of research is the modification of O^O-type ligands aimed at stabilizing heterometallic complexes and studying their catalytic properties, which will be the subject of our next report.

Author Contributions

Vladislav A. Tuskaev - Conceptualization, Writing – original draft; Dmitry A. Kurmaev - Conceptualization, Methodology, Investigation; Svetlana Ch. Gagieva – Investigation; Evgenii K. Golubev – Investigation; Mikhail I. Buzin – Investigation; Viktor N. Khrustalev - Investigation, Svetlana A. Aksenova – Investigation; Alexandra V. Gracheva – Investigation, Boris M. Bulychev – Conceptualization, Project administration, Writing – review & editing

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Russian Science Foundation (Project № 23-13-00089). X-ray diffraction data for complex 2 were collected using the equipment of Center for molecular composition studies of INEOS RAS with the financial support from Ministry of Science and Higher Education of the Russian Federation (Contract/agreement No. 075-03-2023-642). X-ray diffraction data for complexes 3, 4 were collected using the equipment of the RUDN University Strategic Academic Leadership Program. This publication has been prepared with support of the RUDN University Strategic Academic Leadership Program.

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Scheme 1 is available in Supplementary Files section.

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Impact of the method of Mg 2+ cation introducing into the titanium post-metallocene catalytic system on their activity

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