Hafnium Catalysts Research Articles

Mechanism of Activation of a Hafnium Pyridyl−Amide Olefin Polymerization Catalyst: Ligand Modification by Monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.

FINDING CATALYSTS FASTER

A FOUR-YEAR COLLABORATION between Symyx Technologies and Dow Chemical has borne fruit in the discovery of a new class of single-site catalysts for olefin polymerization. new amide-ether-based hafnium catalysts were found using a fully integrated high-throughput screening methodology { J. Am. Chem. Soc. , 125, 4306(2003)}. The results are stunning, says one of the coauthors, Dow chemist James C. Stevens, because the new screening methodology enabled them to do in days or weeks what used to take many months or years. Dow-Symyx researchers set out to discover new catalysts with potential for the production of linear low-density polyethylene, which is a copolymer of ethylene and an α-olefin, such as 1-octene. They first synthesized a library of hafnium and zirconium complexes containing 23 different bidentate and tridentate ligands. To screen these complexes under different activation conditions, they carried out 384 polymerization experiments in just a few hours. work uncovered many catalytical...

A fully integrated high-throughput screening methodology for the discovery of new polyolefin catalysts: discovery of a new class of high temperature single-site group (IV) copolymerization catalysts.

For the first time, new catalysts for olefin polymerization have been discovered through the application of fully integrated high-throughput primary and secondary screening techniques supported by rapid polymer characterization methods. Microscale 1-octene primary screening polymerization experiments combining arrays of ligands with reactive metal complexes M(CH(2)Ph)(4) (M = Zr, Hf) and multiple activation conditions represent a new high-throughput technique for discovering novel group (IV) polymerization catalysts. The primary screening methods described here have been validated using a commercially relevant polyolefin catalyst, and implemented rapidly to discover the new amide-ether based hafnium catalyst [eta(2)-(N,O)[bond](2-MeO[bond]C(6)H(4))(2,4,6-Me(3)C(6)H(2))N]Hf(CH(2)Ph)(3) (1), which is capable of polymerizing 1-octene to high conversion. The molecular structure of 1 has been determined by X-ray diffraction. Larger scale secondary screening experiments performed on a focused 96-member amine-ether library demonstrated the versatile high temperature ethylene-1-octene copolymerization capabilities of this catalyst class, and led to significant performance improvements over the initial primary screening discovery. Conventional one gallon batch reactor copolymerizations performed using selected amide-ether hafnium compounds confirmed the performance features of this new catalyst class, serving to fully validate the experimental results from the high-throughput approaches described herein.

Dynamic Response of Stereoblock Elastomeric Polypropylene Studied by Rheooptics and X-ray Scattering. 2. Orthogonally Oriented Crystalline Chains

A combination of tensile stress, rheooptical birefringence, and wide-angle X-ray scattering (WAXS) was used to probe the dynamic response of the low-tacticity ether-soluble (ES) fraction of elastomeric polypropylene (ePP) derived from metallocene 2-arylindene hafnium catalyst. The ES fraction has isotactic pentad distribution [mmmm] = 21% and a very low amount of crystallinity (≤2% by differential scanning calorimetry and WAXS). In tensile stretching and step-strain shearing, ES exhibits unusual deformation behavior of crystalline chains preferentially oriented orthogonal relative to the deformation axis. Under deformation, WAXS shows arcing along the meridian axis at a scattering angle 2θ = 16.0° (d = 0.551 ± 0.002 nm) which coincides with one of the characteristic reflections of the β-form, but the higher order reflection for the β-form at 2θ = 21.3° is not observed. The meridional arcing, which signifies crystallization of the low-tacticity fraction of ePP, is also observed when ES is blended with high...

Dynamic Response of Stereoblock Elastomeric Polypropylene Studied by Rheooptics and X-ray Scattering. 1. Influence of Isotacticity

The dynamic response of elastomeric polypropylene (ePP) prepared by metallocene 2-arylindene hafnium catalyst was investigated by rheooptical birefringence as well as wide- and small-angle X-ray scattering (WAXS and SAXS). Solvent extraction of ePP ([mmmm] = 34%) results in three fractions with increasing tacticity, crystallinity, and molecular weight in the following order: ether soluble ([mmmm] = 21%), heptane soluble (44%), and heptane insoluble (76%). Unstretched ePP reveals a crystalline phase of the α-form isotactic polypropylene (i-PP). Tensile stretching of ePP yields three sets of scatterings (equatorial, off-axis diagonal, and meridional arc). High-tacticity fractions contribute to the equatorial and off-axis diagonal scatterings, revealing molecular-scale orientation parallel to the strain axis and crystalline phase transformation from the α-form to the mesomorphic form. The meridional arc is contributed by the low-tacticity ether-soluble fraction with crystalline chains oriented with a preferred direction orthogonal relative to the strain direction. SAXS, which probes long-range ordering, exhibits a broad and diffuse meridional peak for the intermediate-tacticity fraction. This implies that lamellae are oriented orthogonal to the strain direction upon stretching. Permanent deformation of ePP after stretching, as measured by the residual strain (tensile set), originates from permanently oriented crystallites and chains pinned within crystalline networks.

Morphology of Thermoplastic Elastomers: Elastomeric Polypropylene

The morphology of low-density (0.86 g/cm3), low-crystallinity (10%) elastomeric polypropylene (ePP) derived from a bis(2-arylindenyl)hafnium catalyst was investigated using a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), Fourier transform infrared (FT-IR) spectroscopy, and tapping mode atomic force microscopy (TM-AFM). This thermoplastic elastomer, when crystallized isothermally from the melt, exhibits morphologies reminiscent of classical semicrystalline polymers. The presence of lamellae, crosshatching, hedrites, and spherulites was revealed by high-resolution TM-AFM. POM confirmed the presence of hedrites and spherulites. The parent ePP can be fractionated into components of different average tacticities ([mmmm]%: 21%−76%) and crystallinities (1%−40% as determined by DSC and WAXS) but more similar molecular mass (Mw: 147−432 kg/mol) and polydispersity (Mw/Mn: 2.1−2.5). The analysis of the morphologies of these fraction...

A Ligand-Accelerated Chiral Lewis Acid Catalyst in Asymmetric Michael Addition of Thiols to α,β-Unsaturated Carbonyls

A novel chiral hafnium catalyst, which was readily prepared from Hf(OTf)4 and chiral ligand 1, has been developed in asymmetric Michael reactions of thiols with 3-(2-alkenoyl)-2-oxazolidinones, affording the corresponding adducts in high yields and enantiomeric excesses. Although chiral Lewis acids are less reactive than their original Lewis acids in many cases, ligand-acceleration has been demonstrated in this asymmetric Michael addition reaction.

Group 4 alkyl complexes as precursors of silica anchored molecular catalysts for the reduction of ketones by hydrogen transfer

The silica anchored mononuclear isopropoxides of the elements of group 4, SiOM(O iPr) 3, M=Ti, Zr, Hf, were synthesized via a two-step procedure, comprising: (i) the reaction of the tetraneopentyl complexes MNp 4 with a silica partially dehydroxylated at 500°C; (ii) the reaction of the surface complexes thus obtained with isopropanol. The supported Ti complex is totally inactive for the reduction of ketones by isopropanol; furthermore a significant fraction of titanium leaches into the solution. On the contrary, the zirconium and hafnium complexes are efficient catalysts for the same reaction. Their properties are dependent upon the nature of the ketone, the nature of the solvent and are sensitive to the presence of water. Under all conditions so far tested, the supported hafnium catalyst exhibits a higher activity than the zirconium one. The very similar catalytic behaviors of SiOHf(O iPr) 3 and SiOHf(OH) 3, a complex obtained by mild hydrolysis of SiOHfNp 3, are interpreted by the easy substitution of the hydroxy ligands by isopropanol, evidenced by in situ IR spectroscopy; unexpectedly, the reverse reaction is much more difficult. Finally, the better performances of SiOZr(O iPr) 3 when compared to those of a formally similar solid synthesized from Zr(O iPr) 4 and silica highlights the importance of the choice of the precursor and of the surface state of silica to obtain a stable, mononuclear active species.

Effect of operating conditions on the molecular weight distribution of polyethylene synthesized by soluble metallocene/methylaluminoxane catalysts

Investigations of the effects of polymerization conditions on the molecular weight distribution (MWD) of polyethylene synthesized with soluble metallocene/methylaluminoxane (MAO) catalysts have been performed. The following variables were investigated in this study: catalyst type, polymerization temperature, catalyst concentration, MAO concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MAO with trimethylaluminium (TMA), and of different catalyst activities of polyethylene. Similarities and differences with other published results are highlighted. In all cases, an effort was made to illustrate the significance of the effects by presenting replicate measurements. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2 (1)), its titanium and hafnium analogues (Cp2TiCl2 (2) and Cp2HfCl2 (3)), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2 (4) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(H4Ind)2ZrCl2 (5)). According to a 22 factorial experiment, independent increases in the concentrations of catalyst or MAO causes a decrease in average molecular weight, with no interaction between these two factors. Replacing MAO with TMA at constant overall aluminium concentration causes a drastic decrease in average molecular weight. Extremely high polymerization rates were observed to impart only a slight increase in the breadth of the MWD. The effects of ethylene partial pressure suggest that for the zirconium catalysts, transfer to monomer is the main chain transfer mechanism, while for hafnium catalysts, this is not the case.

Comparison of zirconocene and hafnocene catalysts for the polymerization of ethylene and 1‐butene

AbstractIn polymerizations with ethylene and copolymerizations with ethylene and 1‐butene the catalyst system rac‐ethylene(bisindenyl)zirconium dichloride (rac‐Et(Ind)2ZrCl2)/methylaluminoxane (MAO) shows lower activities than the analogous Hafnium catalyst. The polymers obtained with the hafnium‐catalyst have average molar masses up to ten times higher than those synthesized with the zirconium compound. It is remarkable that rac‐Et(Ind)2HfCl2 is much more able to incorporate the comonomer. This is expressed by the copolymerization parameters. At a polymerization temperature of +30°C, e.g., the r1‐parameter of the hafnium‐catalyst is 5,4, the corresponding parameter of the zirconium catalyst is 19,4. Polymerizations with a catalyst mixture rac‐Et(Ind)2(Hf, Zr)Cl2 (containing 95% rac‐Et(Ind)2HfCl2 and 5% rac‐Et(Ind)2ZrCl2) show that each catalyst compound produces its own polymer independently.

Comparative behaviour of hafnium and titanium catalysts in copolymerization of α-olefins

Supported hafnium and titanium catalysts were prepared under the same conditions starting from the corresponding tetrakisbutanolates, which were supported on MgCl 2 by reaction of a suspension of Hf/Mg catalyst with aluminum alkyl chlorides. The systems were employed in the copolymerization of 1-hexene with 4-methyl-1-pentene. The extent of polymerization, as well as the copolymer composition, sequence distribution, stereoregularity, and molecular weight were evaluated. The hafnium catalysts gave higher molecular weights and better isotactic stereoregularity but displayed lower activity than the titanium catalysts, but the co-monomer distributions were similar for the two systems. These results are discussed in terms of the number and type of active sites.

Supported hafnium catalysts for ethylene polymerization

Hafnium polymerization catalysts supported on spray-dried magnesium chloride have been prepared by precipitation with alkylaluminium chlorides from paraffin solutions of hafnium(tetra-n-butanolate). These catalysts have been used for ethylene polymerization after activation with different aluminium alkyl derivatives such as tri(isobutyl) aluminium, triethylaluminium and diethylaluminium chloride. The influence of the polymerization conditions, i.e. hydrogen pressure, 1-butene content, temperature, cocatalyst and ethyl benzoate, on the specific activity, polymerization rate and molecular weight has been investigated. The behaviour of the Hf catalyst has been compared to that of similar systems based on titanium or zirconium. The results obtained suggest that the selection of appropriate transition metal derivatives provides the further possibility of tailoring the characteristics of polyethylene.

Oligomerization of Isoprene by Hafnium Catalysts

Abstract Oligomerization of isoprene by the catalyst system consisting of hafnium(IV) n-butoxide and diethylaluminum chloride was studied. Isoprene was oligomerized to a mixture of dimers, trimers and higher oligomers. The compound, 2,6-dimethyl-1-trans-3,6-octatriene, was the main dimer of isoprene in this reaction. The maximum yield of this dimer in the reaction at 130°C for 2 hr was accomplished when the molar ratio of diethylaluminum chloride to hafnium(IV) n-butoxide was 4.7. The addition of tetrahydrofuran or triphenylphosphine kept the selectivity for the dimer nearly constant. These results were compared with those obtained by the catalyst system consisting of titanium (IV) n-butoxide or zirconium(IV) n-butoxide and diethylaluminum chloride.