Active Olefin Polymerization Research Articles

Titanium Complexes Bearing NNO‐Tridentate Ligands: Highly Active Olefin Polymerization Catalysts with Great Control on Molecular Weight and Distribution†

Comprehensive SummaryA series of titanium amido complexes (1‐Ti(NMe2)2 to 4‐Ti(NMe2)2) bearing NNO‐tridentate ligands was synthesized from the reactions of phenoxy‐imine‐amine ligands with one equivalent of Ti(NMe2)4. Subsequently, titanium dichloride complexes (1‐TiCl2 to 4‐TiCl2) were prepared by treatment of titanium amido complexes with excess Me3SiCl in toluene. All metal complexes were characterized by 1H and 13C NMR spectroscopy, and the molecular structures of complexes of 2‐Ti(NMe2)2, 2‐TiCl2, and 4‐TiCl2 in the solid state were determined by single‐crystal X‐ray diffraction. Upon activation with MAO, titanium dichloride complexes as single‐site catalysts were extremely active toward ethylene homopolymerization with great control on molecular weight and distribution. In particular, 3‐TiCl2/MAO exhibited an activity as high as 1.35 × 108 g(PE)·mol−1(Ti)·h−1 at 70°C and produced polyethylene with high molecular weight (41.7 × 104 g∙mol−1) and very narrow distribution (Đ = 1.23). In the presence of 1‐octene as comonomer, linear low‐density polyethylenes (LLDPEs) with ultrahigh molecular weights (Mw = 136—326 × 104 g∙mol−1) and narrow distributions (Đ = 1.20—1.50) were successfully synthesized.

Methylaluminoxane’s Molecular Cousin: A Well-defined and “Complete” Al-Activator for Molecular Olefin Polymerization Catalysts

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al+[B(C6F5)4]−, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al+ = [iBu2(DMA)Al]2(μ-H)+). Unlike the simpler [AliBu2]+[B(C6F5)4]−, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al+[B(C6F5)4]− can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al+ is the true activating species formed in situ in the binary cocatalyst [PhMe2NH]+[B(C6F5)4]−/AliBu3. As little as 50 equiv of Al-H-Al+[B(C6F5)4]− are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu3. The high, yet “tamed,” Lewis acidity of cationic Al-H-Al+ is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al+[B(C6F5)4]− offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.

Techniques in the synthesis of organometallic compounds of Hafnium

Abstract Hafnium is a transition metal and it is the 45th most abundant transition element present on the earth. Hafnium has been successfully alloyed with several metals including titanium, iron, and niobium. Hafnium complexes are less active olefin polymerization catalysts. In the current review synthesis of hafnium complexes involving bonding through different linkages like “carbon, nitrogen, oxygen, carbon and oxygen, nitrogen and cobalt nitrogen and oxygen, nitrogen and phosphorus, nitrogen and sulfur, phosphorus and carbon, phosphorus and oxygen, sulfur carbon and oxygen, carbon–nitrogen and oxygen, carbon–nitrogen and phosphorus, carbon–nitrogen oxygen, sulfur and phosphorus, carbon–oxygen phosphorus and nitrogen”. The commonly used solvents for the synthesis of Hafnium complexes are tetrahydrofuran, n-hexane, and toluene, etc. These complexes were mostly reported at different temperatures ranges from −35 to 110 °C with continuous stirring, according to the nature of ligands. An overview of techniques in the synthesis of Hafnium complexes through various routes has been compiled.

Calcium Amido Complexes Coordinated by Tridentate Amidinate Ligands: Synthesis, Structures and Catalytic Activity in Olefin Hydrophosphination and Polymerization of Cyclic Esters

The reactions of [(Me3Si)2N]2Ca(thf)2 with amidines 2‐(Ph2P=NPh)C6H4NHC(tBu)=N(2,6‐R2C6H3) {R = iPr (L1H); R = Me (L2H)} afford heteroleptic calcium amido complexes [2‐(Ph2P=NPh)C6H4NC(tBu)=N(2,6‐R2C6H3)]Ca[N(SiMe3)2](thf) {R = iPr (1); R = Me (2)} featuring tridentate coordination of the amidinate ligands. Complexes 1 and 2 proved to be efficient catalysts for intermolecular hydrophosphination of styrene, α‐methylstyrene, divinylbenzene and phenylacetylene with Ph2PH and PhPH2. Compounds 1 and 2 exhibit high catalytic activity in the ring‐opening polymerization of ε‐caprolactone and enable quantitative conversion of 500 equiv. of monomer in 5 seconds (22 °C). Polymerization of rac‐lactide initiated by 1 and 2 proceeds much slower: 52–72 % conversion ([M]0/[Ca]0 = 100) was reached in 12 h (20 °C). The obtained polymers are characterized by narrow polydispersities (Mw/Mn = 1.22–1.28). Catalytic activity of complexes 1 and 2 in rac‐lactide polymerization increases significantly in the presence of 1–3 equiv. of iPrOH. Nearly quantitative (86–98 %) conversion of 100 equiv. of monomer was reached in 1 h and the obtained polymer samples were characterized by relatively narrow polydispersity (Mw/Mn = 1.51–1.68).

Synthesis of post-metallocene catalyst and study of its olefin polymerization activity at room temperature in aqueous solution followed by prediction of yield

In the research work presented here, the following work has been carried out: (1) Synthesis of the post-metallocene complex (2) Investigation of its olefin polymerization activity at room temperature in aqueous solution (3) Characterizations to verify the synthesized catalyst (4) Calculation of the yield of the synthesized polymer by varying the moles/amount of catalyst, co-catalyst, and monomer (5) Prediction of an appropriate proportion of catalyst, co-catalyst, and monomer, which can result in maximum yield. The polymer sample has also been characterized by different instrumental techniques viz. 1H NMR spectroscopy and dynamic light scattering (DLS) to investigate the properties of the polymer. Moreover, for identifying the best combination of complexes, response surface methodology has been adopted. From the analyses, a safe zone has been predicted, which can result in optimum yield. The obtained zone has been validated by performing more experiments. From the results, it has been perceived that the developed methodology has the capability to predict the suitable amount of complexes to be taken so that the yield is maximum.

Preparation of Pincer Hafnium Complexes for Olefin Polymerization.

Pincer-type [Cnaphthyl, Npyridine, Namido]HfMe2 complex is a flagship among the post-metallocene catalysts. In this work, various pincer-type Hf-complexes were prepared for olefin polymerization. Pincer-type [Namido, Npyridine, Namido]HfMe2 complexes were prepared by reacting in situ generated HfMe4 with the corresponding ligand precursors, and the structure of a complex bearing 2,6-Et2C6H3Namido moieties was confirmed by X-ray crystallography. When the ligand precursors of [(CH3)R2Si-C5H3N-C(H)PhN(H)Ar (R = Me or Ph, Ar = 2,6-diisopropylphenyl) were treated with in situ generated HfMe4, pincer-type [Csilylmethyl, Npyridine, Namido]HfMe2 complexes were afforded by formation of Hf-CH2Si bond. Pincer-type [Cnaphthyl, Sthiophene, Namido]HfMe2 complex, where the pyridine moiety in the flagship catalyst was replaced with a thiophene unit, was not generated when the corresponding ligand precursor was treated with HfMe4. Instead, the [Sthiophene, Namido]HfMe3-type complex was obtained with no formation of the Hf-Cnaphthyl bond. A series of pincer-type [Cnaphthyl, Npyridine, Nalkylamido]HfMe2 complexes was prepared where the arylamido moiety in the flagship catalyst was replaced with alkylamido moieties (alkyl = iPr, cyclohexyl, tBu, adamantyl). Structures of the complexes bearing isopropylamido and adamantylamido moieties were confirmed by X-ray crystallography. Most of the complexes cleanly generated the desired ion-pair complexes when treated with an equivalent amount of [(C18H37)2N(H)Me]+[B(C6F5)4]−, which showed negligible activity in olefin polymerization. Some complexes bearing bulky substituents showed moderate activities, even though the desired ion-pair complexes were not cleanly afforded.

NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d0 group 4 metal complexes

d0 metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the 13C NMR chemical shift tensors of the α-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the π-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. The 13C NMR chemical shifts, the α-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

The Multifaceted Role of Methylaluminoxane in Metallocene-Based Olefin Polymerization Catalysis

In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe2+ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe2+, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.

Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced 89Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Environmentally benign stereoselective polymerizations of polar as well as nonpolar olefins by a new postmetallocene Ti(IV) salicylate complex at ambient temperature in aqueous emulsion

A new Titanium (IV) complex, dichlorobis(salicylato)titanium(IV) [TiL2Cl2] (1) {LH = Salicylic acid} has been synthesized and characterized by elemental analysis, 1H NMR, 13C NMR, IR and UV-visible spectroscopy. As a post-metallocene complex it’s olefin polymerization activity has been investigated at room temperature in aqueous emulsion. In presence of tetraphenylborate as cocatalyst, it has been found to exhibit moderate to high activity in the range of 103 in homopolymerization and copolymerization of styrene and methyl methacrylate to produce syndiotactic rich, ultrahigh molecular weight polymers with low molecular weight distribution. The polymer latex produced has been analyzed by DLS, HRTEM and FESEM which revealed that the latex is highly stable and contains polymer particles having size around 50 nm. The microstructures, molecular weights and thermal properties of the polymers have also been determined by 1H-NMR, 13CNMR, GPC and DSC analysis. The polymers have also been characterized by 1H-NMR, 13C-NMR, GPC and DSC analysis.

A New Post-Metallocene-Ti Catalyst with Maltolate Bidentade Ligand: an Investigation in Heterogeneous Polymerization Reactions in Different Mesoporous Supports

A new titanium catalyst easily synthesized from ethylmaltol bidentate chelator ligand was studied in homogeneous and heterogeneous ethylene polymerization. The dichlorobis(3-hydroxy-2-ethyl-4-pyrone)titanium(IV) complex was characterized by 1H and 13C NMR (nuclear magnetic resonance), UV-Vis and elemental analysis. Theoretical study by density functional theory (DFT) showed that the complex chlorines exhibit cis configuration, which is important for the activity in olefin polymerization. The complex was supported by two methods, direct impregnation or methylaluminoxane (MAO) pre-treatment, in five mesoporous supports: MCM-41 (micro and nano), SBA-15 and also the corresponding modified Al species. All the catalytic systems were active in ethylene polymerization and the catalytic activity was strongly influenced by the method of immobilization of the catalyst and the type of support.

Titanium (IV) complexes stabilized by 2,6-bis-(α,α′-diphenyl(hydroxy)methyl)-pyridine: Catalytic activity in olefin polymerization and impact of lithium and magnesium chlorides

A series of mono- and bi-nuclear complexes were synthesized by combining 2,6-bis-(α,α′-diphenyl(hydroxy)methyl)-pyridine and halide or alkoxide Ti(IV) derivatives. Crystal structures of synthesized products were determined. It was demonstrated that individual complexes 2–5 are active in ethylene and 1-hexene polymerization only in the presence of Et2AlCl/Bu2Mg binary co-catalyst. The systems formed from the lithium or magnesium salts of 2,6-bis(α,α′-diphenyl(hydroxy)methyl)-pyridine and TiCl4, containing respective Ti complexes and reaction byproducts (LiCl or MgCl2), are able to catalyze ethylene polymerization when activated by methylalumoxane or trimethylaluminum (TMA). The highest efficiency of catalysis was observed in the presence of binary co-catalysts Et2AlCl/Bu2Mg. The results of this study show the unique ability of Li and Mg chlorides to improve the catalytic ability of post-metallocene pre-catalysts.

ChemInform Abstract: Carborane‐Based Transition Metal Complexes and Their Catalytic Applications for Olefin Polymerization: Current and Future Perspectives

AbstractReview: 34 refs.

On the Origin of Living Polymerization over o-Fluorinated Post-titanocene Catalysts

Group 4 bis(phenoxyimine) and related complexes are highly active olefin polymerization catalysts of the post-metallocene family. When activated with methylalumoxane, titanium complexes bearing o-fluoroaryl substituents can conduct living polymerization of ethylene and propylene. In spite of more than 10-year history of such catalysts, the origin of living behavior of such catalysts remains open to questions. This work presents the state of the art of the discussion on the living nature of single-site post-metallocene titanium based catalysts bearing o-fluoroaryl substituents. Recent experimental and theoretical results are discussed.

Zirconium and hafnium complexes based on 2-aryl-8-arylaminoquinoline ligands: synthesis, molecular structure, and catalytic performance in ethylene copolymerization

A general and efficient approach toward new zirconium and hafnium complexes based on 2-aryl-8-arylaminoquinoline ligands was developed. These precursors, when activated with MAO/borate cocatalyst and supported on silica, result in active olefin polymerization catalysts. The ethylene copolymers produced under industrially relevant conditions show very high molecular weights and unique microstructures defined by the multisite nature of the catalyst. A site-diversification mechanism is proposed to explain the presence of at least five individual sites, as deduced from 3D-TREF analysis of ethylene-butene copolymers.

Hafnium Amidoquinoline Complexes: Highly Active Olefin Polymerization Catalysts with Ultrahigh Molecular Weight Capacity

The preparation and characterization of a new class of polyolefin procatalysts is described. Hafnium tribenzyl procatalysts, supported by amidoquinoline ligands, were prepared in two steps from commercially available materials. Solid-state structures, determined by single-crystal X-ray analyses, revealed that all the hafnium complexes display approximate trigonal-bipyramidal geometry around the metal center. The complexes were evaluated in an ethylene/1-octene copolymerization study and were found to be highly active at elevated temperatures (120 °C). The best catalyst, derived from ((2,6-dimethylphenyl)(2,4-dimethylquinolin-8-yl)amino)tribenzylhafnium (6d), compares favorably to previously reported systems supported by bidentate nitrogen-based ligands. In particular, this catalyst exhibits very high molecular weight capacity and high catalytic activity, with a moderate 1-octene response. Alkyl substitution at the carbon ortho to the quinolino nitrogen was found to be an important factor for improving pol...

Validation of BET specific surface area for heterogeneous Ziegler-Natta catalysts based on αS-plot

Specific surface area based on the BET method in N2 adsorption has historically posed poor correlation with olefin polymerization activity of heterogeneous Ziegler-Natta (ZN) catalysts. We have investigated the validity of the BET surface area for ZN catalysts based on the αS-plot method, where non-porous core–shell MgO/MgCl2/TiCl4 catalysts were used as a reference material. The αS-plots for typical industrial ZN catalysts clarified the presence of two classes of micropores and resultant difficulty to isolate a multilayer adsorption region for the BET method. The results cast doubt on the application of the BET method to evaluate the surface area of heterogeneous ZN catalysts.

Discovery of Methylaluminoxane as Cocatalyst for Olefin Polymerization

The discovery of methylaluminoxane (MAO) was the start for investigations and innovations of new classes of highly active olefin polymerization catalysts. Different transition metal complexes together with MAO as cocatalyst allow the synthesis of polymers with a highly defined microstructure, tacticity, and stereoregularity as well as new cycloolefin, long chain branched, or blocky copolymers with excellent properties. These new polyolefins could not be obtaind with such a purity before by Ziegler–Natta catalysts. The single site catalyst character of metallocene and other transition metal complexes activated by MAO leads to a better understanding of the mechanism of the olefin polymerization.

Synthesis and olefin polymerization activity of (quinolin-8-ylamino)phenolate and (quinolin-8-ylamido)phenolate Group 4 metal complexes

New Zr and Hf complexes bearing either a tridentate monoanionic 2- tert-butyl-6-[(quinolin-8-ylamino)methyl]phenolate [N,N,O −] or a dianionic 2- tert-butyl-6-[(quinolin-8 ylamido)methyl]phenolate [N,N −,O −] ligand have been synthesized and characterized. The geometries of the zirconium complexes have been studied using DFT methods. The complexes were tested as catalysts in the polymerization of ethylene and α-olefins after activation with methylalumoxane, resulting in the production, with moderate activities, of high molecular weight linear polyethylene and prevailingly isotactic polypropylene, poly(1-butene), poly(1-pentene) and poly(1-hexene). The content of isotactic pentads of these poly(α-olefins) increases while the size of the monomer increases, ranging between 50% for polypropylene and 99% for poly(1-hexene).

Group 4 metal bis(chelate) complexes of 2-anilidomethylpyridine ligands: Synthesis and catalytic activity for olefin polymerization

A series of monoanionic, bidentate 2-anilidomethylpyridine ligands were synthesized and used to prepare bis(chelate) Zr(IV) and Hf(IV) bis(dimethylamido) complexes . All complexes were characterized by NMR spectroscopy and elemental analysis. The solution structures of the complexes were more or less fluxional, depending on the substituent on the ligands. The complexes were tested as catalysts for the polymerization of ethylene and propene, in combination with different activators. Use of Al iBu 2H – methylaluminoxane as the co-catalyst resulted in the generation of catalysts producing high molecular weight polyethylenes with good activities, but yielding only stereoirregular and poorly regioregular polypropylenes.