Ethylene Polymerization Catalysts Research Articles

Synthesis of α-Diimine Complex Enabling Rapidly Covalent Attachment to Silica Supports and Application of Homo-/Heterogeneous Catalysts in Ethylene Polymerization.

For covalent attachment-supported α-diimine catalysts, on the basis of ensuring the thermal stability and activity of the catalysts, the important problem is that the active group on the catalyst can quickly react with the support, anchoring it firmly on the support, shortening the loading time, reducing the negative impact of the support on the active centers, and further improving the polymer morphology, which makes them suitable for use in industrial polymerization temperatures. Herein, we synthesized a α-diimine nickel(II) catalyst bearing four hydroxyl substituents. The hydroxyl substituents enable the catalyst to be immobilized firmly on silica support by covalent linkage in 5-10 min. Compared with the toluene solvent system, the homogeneous catalysts show high activity and thermal stability in hexane solvent at the same conditions. Compared with homogeneous catalysts, heterogeneous catalysis leads to improvements in catalyst lifetime, polymer morphology control, catalytic activity, and the molecular weight of polyethylene (up to 679 kg/mol). The silica-supported catalysts resulted in higher melting temperatures as well as lower branching densities in polyethylenes. Even at 70 °C, the polyethylene prepared by S-CatA-2 still exhibits dispersed particle morphology, and there is no phenomenon of reactor fouling, which is suitable for industrial polymerization processes.

DFT insight into metals and ligands substitution effects on reactivity of phenoxy-imine catalysts for ethylene polymerization

The reaction mechanism of ethylene (ET) polymerization catalyzed by the phenoxy-imine (FI) ligands using DFT calculations was studied. Among five possible isomers, isomer A which has an octahedral geometry and a (cis-N/trans-O/cis-Cl) arrangement is the most stable pre-reaction Ti-FI dichloride complex. The isomer A can be activated by MAO to form the active catalyst and the active form was used for the study of the mechanism for Ti-FI. The second ethylene insertion was found to be the rate-determining step of the catalyzed ethylene polymerization. To examine the effect of group IVB transition metals (M = Ti, Zr, Hf) substitutions, calculated activation energies at the rate-determining step (EaRDS) were compared, where values of EaRDS of Zr < Hf < Ti agree with experiments. Moreover, we examined the effect of substitution on (O, X) ligands of the Ti-phenoxy-imine (Ti-1) based catalyst. The results revealed that EaRDS of (O, N) > (O, O) > (O, P) > O, S). Hence, the (O, S) ligand has the highest potential to improve the catalytic activity of the Ti-FI catalyst. We also found the activation energy to be related to the Ti-X distance. In addition, a novel Ni-based FI catalyst was investigated. The results indicated that the nickel (II) complex based on the phenoxy-imine (O, N) ligand in the square-planar geometry is more active than in the octahedral geometry. This work provides fundamental insights into the reaction mechanism of M − FI catalysts which can be used for the design and development of M − FI catalysts for ET polymerization.

N,N-Bis(2,4-Dibenzhydryl-6-cycloalkylphenyl)butane-2,3-diimine-Nickel Complexes as Tunable and Effective Catalysts for High-Molecular-Weight PE Elastomers.

Four examples of N,N-bis(aryl)butane-2,3-diimine-nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr2 (where Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2 (Ni1), Ar = 2-(C6H11)-4,6-(CHPh2)2C6H2 (Ni2), 2-(C8H15)-4,6-(CHPh2)2C6H2 (Ni3) and 2-(C12H23)-4,6-(CHPh2)2C6H2 (Ni4)), disparate in the ring size of the ortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of Ni2 and Ni4 highlight the variation in the steric hindrance of the ortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl2, Et2AlCl or MAO as activators, Ni1-Ni4 displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order Ni2 (cyclohexyl) > Ni1 (cyclopentyl) > Ni4 (cyclododecyl) > Ni3 (cyclooctyl). Notably, cyclohexyl-containing Ni2/MAO reached a peak level of 13.2 × 106 g(PE) of (mol of Ni)-1 h-1 at 40 °C, yielding high-molecular-weight (ca. 1 million g mol-1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with 13C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl2); 81.1% (Et2AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (Xc) and molecular weight (Mw) were the main factors affecting tensile strength and strain at break (εb = 353-861%). In addition, the stress-strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4-71.2%), properties that align with thermoplastic elastomers (TPEs).

Enhancing the thermal stability of α-Diimine Ni(II) catalysts for ethylene polymerization via symmetrical axial shielding strategy

Targeting practical complex catalyst for ethylene polymerization, the catalytic system requires a good thermo-stability along with high activity as well as reasonable molecular weights of resultant polyethylene. Herein, a series of symmetrical α-diimine-nickel complexes bearing functionalized 2,6-dibenzhydryl-4-t-butylphenyl N-aryl were prepared and examined in detail for ethylene polymerization, and compared the catalytic performance with their unsymmetrical analogues. The symmetrical catalysts displayed high activities in the level of 106 g mol-1 h−1, producing high molecular weight branched polyethylene (Mw: > 1.79 × 105 g mol−1) at all the reaction temperatures ranging from 30 °C to 100 °C, in comparison, both classical and unsymmetrical α-diimine-nickel analogues gave low activities and became completely inactive at 100 °C under similar conditions. It was proposed that π-π interactions between the acenaphthylene and phenyl of benzahydral group suppress the N-aryl rotation and thereby generate a blockage around the metal center by interlocking the axial positions, which in result significantly improve the thermal stability of the nickel complex catalysts.

Role of Titanium in Ti/SiO2‐Supported Metallocene‐based Olefin Polymerization Catalysts. Part 2: Particle Fragmentation

AbstractA commercial SiO2 modified by the addition of a TiO2 layer on its surface and pore walls offers a support for metallocene‐based ethylene polymerization catalysts with a 35 % improved catalytic activity. The effect of such support modification on the catalyst particle fragmentation during polymerization reactions were assessed. This was performed by exposing the internal cross‐section of several SiO2 and Ti/SiO2‐based metallocene catalyst particles, polymerized at a wide range of reaction conditions, with a Focused‐Ion Beam Scanning Electron Microscope (FIB‐SEM), followed by a semi‐quantitative analysis of polymer, catalyst and macropores of each 2D image obtained. The titanation of the SiO2 lead to a support framework that fractures earlier and more efficiently. This was evidenced by the presence of several empty (polymer‐free) cracks along the support dense‐shell, found more extensively in the Ti‐modified polymerized catalyst particle, which also resulted in a more sustained polymerized particles macroporosity during the course of the reaction.

Iron ethylene polymerization catalysts incorporating trifluoromethoxy functionality: Effects on PE molecular weight and productivity

The capacity to broaden the range of molecular weights displayed by polyolefinic materials is an important factor to be considered in the design of polymerization catalysts. Herein, the 2,6-dibenzhydryl-4-trifluoromethoxy modified bis(imino)pyridyl-ferrous chlorides, [2-[CMeN{2,6-{(C6H5)2CH}2-4-(F3CO)C6H2}]-6-(CMeNAr)C5H3N]FeCl2 [Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-i-Pr2C6H3Fe3, 2,4,6-Me3C6H2Fe4, 2,6-Et2-4-MeC6H2Fe5], are used as precatalysts in the solution polymerization of ethylene. On the activation with either MAO or MMAO, all complexes displayed high productivity [up to 18.4 × 106 g (PE) mol-1 (Fe) h-1 for Fe5/MAO], generating highly linear polyethylenes with a wide range of molecular weights (Mw range: 0.85 × 103 to 8.80 × 105 g mol-1). Notably, higher activity was achieved in hexane than in toluene under MAO activation, while the opposite trend was seen with MMAO, highlighting the key role played by solvent in the polymerization process. By comparison with structurally related iron catalysts, the presence of the electron withdrawing para-trifluoromethoxy group has the effect of increasing the molecular weight of the polyethylene. In addition to the polymerization studies, full synthetic and characterization details are presented for Fe1-Fe5 including the X-ray structures of Fe1 and Fe2.

Synthesis, characterization and ethylene oligomerization of MIL-125(Ti)-NH2 grafted salicylaldehyde nickel complexes

Metal-organic framework MIL-125(Ti)-NH2 was prepared and coordinated with salicylaldehyde and NiCl2·6H2O to create a heterogeneous catalyst for ethylene polymerization. The structure of the products was characterized by a variety of physical methods, such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma mass spectrometry (ICP-MS), and nitrogen adsorption-desorption. The catalytic performance of MIL-125(Ti)-NH2-Sal-Ni was also investigated. The results showed that the reaction temperature, Al/Ni molar ratio, and ethylene pressure influenced on the catalytic activity and product selectivity. Under optimal reaction conditions, the catalytic activity of MIL-125(Ti)-NH2-Sal-Ni for ethylene oligomerization could reach 5.90 × 104 g·(mol Ni·h)−1 using cyclohexane as the solvent and MAO as the co-catalyst; the main products were C4 and C6. In addition, MIL-125(Ti)-NH2-Sal-Ni had good catalytic stability and recycling, which could be reused at least twice without significant loss of activity. Moreover, based on the evaluation of catalytic performance, the catalytic ethylene oligomerization mechanism of MIL-125(Ti)-NH2-Sal-Ni was also deduced.

Developments in Binuclear Late Transition Metal Catalysts for Ethylene Homo- and Copolymerization

Recent advances in the research of late transition metal (Ni) salicylaldimine bimetallic catalysts in ethylene polymerization and copolymerization of ethylene with comonomers have been introduced. The effect of positioning two polymerization-active metal centers in close proximity on catalytic activity, molecular weight, molecular weight distribution, and levels of branching are thoroughly documented. The synergistic effect of these binuclear catalysts compared to their mononuclear analogs is the subject of this review.

Stability and reactivity of dinuclear iron(III)-salen complexes as catalysts in ethylene polymerization: A computational approach

The increasing demand for olefin polymers has prompted extensive research on catalysis, processes, and structural characterization to explore their economic potential. In this study, N,N'-bis(salicylidene)-1,2-ethylenediamine (salen) was synthesized, complexed with Fe(III) ion, and the complex was bridged by simple condensation reaction with five different dicarboxylic acids to produce dinuclear complexes. Different methods, such as 1H NMR, FT-IR, UV–Vis, and EDX elemental analysis, were used to analyze and identify the ligand and complexes. Spectroscopic results supported by molecular modeling studies, revealed that the complexes had an octahedral geometry. Structural optimization of the ligand and metal complexes was performed using Avogadro (version 1.2.0) and the Universal Force Field molecular mechanics level. Density functional theory was used to conduct computational calculations on the stability and reactivity of the zeolite-supported and unsupported complexes as catalysts for the polymerization of ethylene. Interestingly, the supported complexes were found to exhibit lower stability and higher reactivity than the neat metal complexes. As such, the synthesized complexes are promising catalysts for the production of polyethylene, and their use could significantly reduce the cost of industrial polyethylene production.

A strategy for high ethylene polymerization performance using titanium single-site catalysts.

The synthesis of heterogeneous Ti(IV)-based catalysts for ethylene polymerization following surface organometallic chemistry concepts is described. The unique feature of this catalyst arises from the silica support, KCC-1700. It has (i) a 3D fibrous morphology that is essential to improve the diffusion of the reactants, and (ii) an aluminum-bound hydroxyl group, [(Si-O-Si)(Si-O-)2Al-OH] 2, used as an anchoring site. The [(Si-O-Si)(Si-O-)(Al-O-)TiNp3] 3 catalyst was obtained by reacting 2 with a tetrakis-(neopentyl) titanium TiNp4. The structure of 3 was fully characterized by FT-IR, advanced solid-state NMR spectroscopy [1H, 13C], elemental and gas-phase analysis (ICP-OES and CHNS analysis), and XPS. The benefits of combining these morphological (3D structure) and electronic properties of the support (aluminum plus titanium) were evidenced in ethylene polymerization. The results show a remarkable enhancement in the catalytic performance with the formation of HDPE. Notably, the resulting HDPE displays a molecular weight of 3 200 000 g mol-1 associated with a polydispersity index (PD) of 2.3. Moreover, the effect of the mesostructure (2D vs. 3D) was demonstrated in the catalytic activity for ethylene polymerization.

Exploring fluoride effects in sterically enhanced cobalt ethylene polymerisation catalysts; a combined experimental and DFT study.

The fluoro-substituted 2,6-bis(arylimino)pyridine dichlorocobalt complexes, [2-{CMeN(2,6-(Ph2CH)2-3,4-F2C6H)}-6-(CMeNAr)C5H3N]CoCl2 (Ar = 2,6-Me2C6H3 Co1, 2,6-Et2C6H3Co2, 2,6-iPr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et-4-MeC6H2Co5), were synthesized in good yield from the corresponding unsymmetrical N,N,N'-ligands, L1-L5. Besides characterization of Co1-Co5 by FT-IR spectroscopy, 19F NMR spectroscopy and elemental analysis, the molecular structures of Co2 and Co5 were also determined highlighting the unsymmetrical nature of the terdentate ligand and the pseudo-square pyramidal geometry about the metal center. When either MAO or MMAO were employed as activators, Co1-Co5 were able to achieve a wide range of catalytic activities for ethylene polymerisation. Co5/MAO exhibited the highest activity of the study at 60 °C (7.6 × 106 g PE mol-1 (Co) h-1) which decreased to 3.3 × 106 g PE mol-1 (Co) h-1 at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the ortho groups was enhanced (for MMAO: Co3 > Co5 > Co2 > Co1 > Co4), a finding that was supported by DFT calculations. Furthermore, it was shown that particularly high molecular weight polyethylene could be generated (up to 483.8 kg mol-1) when using Co5/MMAO at 30 °C, while narrow dispersities (M w/M n range: 1.8-4.7) and high linearity (T m > 131.4 °C) were a feature of all polymers produced. By comparison of Co3 with its non-fluorinated analogue using experimental data and DFT calculations, the substitution of fluorides at the meta- and para-positions was demonstrated to boost catalytic activity and improve thermal stability.

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 &gt; Ni4 &gt; Ni3 &gt; Ni1 &gt; Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.

Ti-iminocarboxylate catalyzed polymerization of ethylene to highly crystalline, disentangled, ultrahigh molecular weight polyethylene

Disentangled ultrahigh molecular weight polyethylene (d-UHMWPE) is a niche, highly sought-after, and an emerging class of polyethylene. However, the synthesis of d-UHMWPE is uncommon and is equally challenging. Here, we report a Ti-iminocarboxylate complex that produces d-UHMWPE under ambient polymerization conditions. Treatment of tetrakis(dimethylamido-titanium) [Ti(NMe2)4] with 2-((4-methoxybenylidene)amino)benzoic acid produced a novel titanium complex Cat.1 [bis-dimethylamido-Ti-(bis-iminocarboxylate)]. The identity of Cat.1 was unambiguously ascertained using a combination of 1-2D NMR spectroscopy and mass analysis. Cat.1 was activated using various cocatalysts, and MMAO (modified methylaluminoxane) outperformed the others. Screening of polymerization parameters suggested an Al/Ti ratio of 1000, 4 bar ethylene pressure, 35 °C temperature, and a polymerization time of 30 min as the optimal condition. Under the optimized polymerization conditions, Cat.1 polymerizes ethylene to ultrahigh molecular weight polyethylene with a weight average molecular weight (Mw) of 2.5 × 106 g/mol. The thermal behavior of the thus prepared material disclosed a highly crystalline [crystallinity χ (DSC) 96 %], strictly linear polyethylene. Detailed thermal analysis of the nascent polyethylene using a specifically designed DSC method revealed the existence of a high melting peak at 139–140 °C, which is a hallmark of disentangled UHMWPE. Thus, a homogeneous, single-site ethylene polymerization catalyst has been developed, which upon activation with MMAO produced highly crystalline d-UHMWPE.

New Titanium(IV)-Alkoxide Complexes Bearing Bidentate OO Ligand with the Camphyl Linker as Catalysts for High-Temperature Ethylene Polymerization and Ethylene/1-Octene Copolymerization.

In order to increase the thermal stability of olefin polymerization precatalysts, new titanium(IV) complexes with diolate ligands differing in the degree of steric hindrances were synthesized from readily available precursor (±)camphor. The structures of the complexes 1-2 were established by X-ray diffraction. Complexes 1-4 in the presence of an activator {EtnAlCl3-n + Bu2Mg} catalyzed the synthesis of UHMWPE with an Mv up to 10 million and a productivity of up to 3300 kg/molTi·atm·h. The obtained polymers are obviously characterized by a low density of macromolecular entanglement, which makes it possible to use the solid-phase method for their processing. The mechanical characteristics of the oriented UHMWPE films had a breaking strength up to 2.7 GPa and an elastic modulus of up to 151 GPa. The precatalysts 1-4 were also active in ethylene/1-octene copolymerization. The comonomer content was in the range of 1.4-4.6 mol%. The use of a rigid linker and an increase in the steric load of the diolate complexes ensured the thermal stability of the catalytic system in the range of 50-70 °C.

Ethylene Polymerization through Neutral Nickel Complexes Bearing Cyclic Imides

The catalyst synthesis of salicylaldimine Ni(II) complexes with bulky imide moieties, ethylene polymerization, and characterization of synthesized polyethylenes are described in this paper. These Ni(II) complexes are designed to bear 2-aminobiphenyl and 4-tritylaniline. Results confirmed relatively high activity (up to 4 × 10 4 g PE mol Ni-1 h-1) of these catalysts in ethylene polymerization. Moreover, Ni(II) complexes demonstrated enhanced thermal stability, maintaining activity level up to 80°C. The generated polyethylenes possess moderate branching density and high melting temperatures. Less bulky 2-aminobiphenyl group resulted in higher branch content, while in Ni(II) complexes bearing 4-tritylaniline, more linear structure was observed. These semicrystalline polyethylenes showed mechanical properties similar to thermoplastics.

Novel Imido‐Cr/Silica Ethylene Polymerization Catalysts Modified from the Phillips Catalyst

AbstractIn this work, novel imido‐Cr/silica catalysts are synthesized by modifying the Phillips catalyst CrOx/silica with isocyanates. The imido‐Cr/silica, which has moderate activities of 200–300 kgPE molCr−1 bar−1, shows lower efficiency than CrOx/silica for C2H4 polymerization. On the other hand, regarding to the modified catalysts, the productivity can be improved almost linearly with increasing C2H4 pressure, and very stable activities are observed during the 1 h run of C2H4 polymerization. When compared with CrOx/silica, the modified catalysts can produce polyethylene (PE) with much higher molecular weight (MW) and broader molecular weight distribution (MWD), which is featured by a distinct ultrahigh MW shoulder. Meanwhile, the MW and MWD of the PE products can be reduced obviously by adding a small amount of H2 before C2H4 polymerization. In addition, it is found that more 1‐hexene can be copolymerized by CrOx/silica, and it is preferentially incorporated into a portion of the PE chains. While such a preference is much less significant for the modified catalysts.

A Ziegler-type spherical cap model reveals early stage ethylene polymerization growth versus catalyst fragmentation relationships

Polyolefin catalysts are characterized by their hierarchically complex nature, which complicates studies on the interplay between the catalyst and formed polymer phases. Here, the missing link in the morphology gap between planar model systems and industrially relevant spherical catalyst particles is introduced through the use of a spherical cap Ziegler-type catalyst model system for the polymerization of ethylene. More specifically, a moisture-stable LaOCl framework with enhanced imaging contrast has been designed to support the TiCl4 pre-active site, which could mimic the behaviour of the highly hygroscopic and industrially used MgCl2 framework. As a function of polymerization time, the fragmentation behaviour of the LaOCl framework changed from a mixture of the shrinking core (i.e., peeling off small polyethylene fragments at the surface) and continuous bisection (i.e., internal cleavage of the framework) into dominantly a continuous bisection model, which is linked to the evolution of the estimated polyethylene volume and the fraction of crystalline polyethylene formed. The combination of the spherical cap model system and the used advanced micro-spectroscopy toolbox, opens the route for high-throughput screening of catalyst functions with industrially relevant morphologies on the nano-scale.

Lewis acids modulation in phosphine‐sulfonate palladium catalyzed ethylene polymerization

Lewis acid modulation in metal‐catalyzed olefin polymerization have received increasing attention. In this work, a series of N‐containing phosphine‐sulfonate palladium complexes were synthesized, characterized and used as stimuli‐responsive type catalysts. The introduction of metal Lewis acids results in significantly improved catalytic performances of N‐containing phosphine‐sulfonate Pd catalysts in ethylene polymerization, resulting in improved catalytic activities and high‐molecular‐weight polyethylenes (Mn up to 369.2 kg mol−1). It was indicated that the metal Lewis acids can form interactions with the N‐containing moieties on the phosphine‐sulfonate Pd catalysts. This provides a simple and versatile strategy to tune the properties of olefin polymerization catalysts.

Effects of Silica Support Properties on the Performance of Immobilized Metallocene Catalysts for Ethylene Polymerization

AbstractThe choice of the silica support for α‐olefin polymerization catalysts has overwhelming ramifications on the overall metal loading, catalyst performance, and properties of the resulting polymer. There are several physical properties of a silica support to consider but the pore volume and pore diameter are the two of the most important support properties in such considerations. During the catalyst immobilization process and the subsequent polymerization reaction, the change in the pore volume can either facilitate or inhibit the mass transfer of catalytic compounds and monomers within and throughout the catalyst particle. This work presents the experimental study on the effects of pore diameter and pore volume in porous silica particles used to support rac‐Et(ind)2ZrCl2 catalyst on ethylene polymerization. The amount of immobilized methylaluminoxane (MAO) is varied for three commercially available silica micro‐particles with similar surface areas and particle sizes. It has been observed that each type of silica exhibits different effects of the immobilized MAO on their pore characteristics, although a general trend is observed. Ethylene polymerization in slurry phase with the prepared supported catalysts also shows that the polymerization activities can be correlated with pore diameter and surface area for the three silica supports.

Highly branched polyethylene used as sorbents for oil‐spill cleanup and separation

AbstractHighly branched polyethylene (HBPE) was developed into practical application as highly efficient sorbent material for oil‐spill cleanup and oil/water separation. To obtain large‐scale production of HBPE, a thermal stable Ni(II)‐α‐diimine catalyst for ethylene polymerization in our previous work has been employed to prepare the polyolefin material in a 10‐liter polymerization reactor with a high activity (&gt;106 gPE/mol[Ni] h). The structure, molecular weight and distribution, thermal and mechanical properties were systematically characterized by NMR, ATR‐FTIR, GPC, DSC, DMA, and TGA, respectively. Through simple but feasible cross‐linking process, the HBPE‐based oil‐absorption materials with contact angles up to 111.5° were directly applied into absorption test using various oil and pure hydrocarbons. Reusability and recovery of the absorption materials and oil or solvents were probed by drying or using the distillation method. Oil/water separation was made to determine the hydrophobic and oleophilic nature of this material. Flory‐Rehner polymer swelling theory is employed to study the structure–property relationship via determining the network structure including cross‐linking density ρc and average molecular weight Mc between two cross‐links. The mixture of sorbent material after absorbing oil was regarded as crude oil component to simulate oil refinery process by thermogravimetric analysis, providing an alternative approach for oil‐collection.