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Tin-bridged ansa-metallocenes of zirconium: Synthesis and catalytic performance in olefin polymerization
Abstract
The first tin-bridged zirconocenes [(CH3)2Sn(C5H4)2]Zr[N(CH3)2]2 (3) and Sn{(C5H4)2Zr[N(CH3)2]2}2 (4) have been obtained from tetrakis(dimethylamido)zirconium (1) and the CH-acidic ligands (CH3)2Sn(C5H5)2 (2a) and Sn(C5H5)4 (2b), respectivily. The ansa-zirconocene 3 is formed in quantitative yield, 4 in 81% yield. The bi- and trimetallic complexes 3 (ZrSn) and 4 (Zr2 Sn) polymerize ethylene more efficiently than the well-known silicon-bridged analogue of 3 (Si instead of Sn).
A new catalyst composed of Tin-bridged yttrocene Ph2Sn(MeCp)2YCI (MeCp=methylcyclopentadienyl) and Al (i-Bu)3 was successfully developed for the polymerization of methyl methacrylate (MMA). Detailed study of factors (such as the molar ratio of Al/Cat., catalyst concentration, various solvents, temperature and time) influencing polymerization reaction indicated that the catalytic active species may be still somewhat stable at high temperature and still have a long catalytic lifetime.1H NMR spectrum showed about 65% syndiotactic content in the polymethyl methacrylate (PMMA) prepared. From kinetic studies, the polymerization rate equation may be expressed as Rp=Kp[Cat.]2.4. The overall activation energy of polymerization is 20.9±3.1 kJ/mol.
Ever since their discovery by Sinn and Kaminsky in 1979, the family of high activity homogeneous olefin polymerization catalysts known as metallocenes has been the subject of intense research and commercial development. While a structurally diverse group, metallocene catalysts are generally considered to have a few common features: a highly electrophilic, early transition metal or rare‐earth atom as the point of polymer chain attachment; a π‐bound cyclopentadienyl ring or isoelectronic analogue attached to this atom; and characteristically narrow distributions of polymer molecular weight and short‐chain branching. While polymerization of ethylene and propylene is reviewed in some detail, the nonpolymerization catalysis and (co)polymerization of functional monomers is also surveyed. The current commercial status of metallocene technology is also briefly discussed.
Once narrowly defined as metal complexes of precise chemical formula (η5-C5H5)2M, the word “metallocene” has evolved to represent a versatile class of polymerization technology that is slowly but significantly reshaping the polyolefin industry. Metallocene catalysts enable production of polyethylenes with narrow molecular weight and composition distributions, efficient incorporation of standard and nonstandard comonomers, an excellent spectrum of tacticity control in poly-α-olefins, and remarkable adaptability enabled by accumulated scientific endeavors relating ligand designs to polymer microstructures. Olefin polymerization is reviewed in some detail with emphasis on commercially relevant technologies, and a brief commercial overview is included. Nonpolymerization catalysis and (co)polymerization of functional monomers are also briefly summarized.
Starting from Cp2(Me)Si-Si(Me)Cp2 1 the complexes X2Ti[(C5H4)(C5H 4)(Me)Si-Si(Me)(C5H4)(C5H 4)]TiX2 (X = Cl (2a); X = Me (3)) were synthesized. The compounds were characterized by means of their 1H- and 13C-n.m.r. and MS-spectra. The crystal structure of 2a·PhMe was determined.
The metallocene compounds had been applied to the polymerizations of olefins and vinyl monomers with methylaluminoxane (MAO) cocatalyst, and they have usually one transition metal atom per molecule, i.e., mononuclear metallocene. Recently it has been found that the dinuclear metallocene compounds containing two transition metal atoms exhibit the peculiar polymerization behaviors for olefins and vinyl monomers. In this article, the dinuclear metallocenes are classified into four groups of dinuclear bent-metallocene, dinuclear ansa-metallocene, dinuclear constrained geometry catalyst and dinuclear half-metallocene, and then the synthesis of dinuclear metallocene of each group as well as the polymerization behaviors for ethylene, propylene, and styrene are described.
This review focuses on Group 4 metallocenes and their use in the polymerisation of α-olefins. A brief overview of their history as well as theories concerning the mechanistic details of the polymerisation reaction precedes a discussion on the design of these polymerisation catalysts. This latter section will cover what effect metal, ligand and bridge choice has on polymerisation activity and the physical properties of the polymer produced. A major drawback to the use of these catalysts in industrial processes is related to the cost of their synthesis, the major problem here being the formation of an unwanted diastereomer during the synthetic process. The techniques employed to overcome these problems are therefore also reviewed. Lastly, the large volume of literature dealing with these catalysts makes it difficult to compare the polymerisation data of different catalysts and laboratories. The review therefore contains tables that attempt to collect the details of the most important polymerisation studies in a comparable and easy to reference manner.
Bis(cyclopentadienyl)aluminum and -gallium species of the type (Pytsi)ECp2 [E = Al (2a), Ga (2b); Pytsi = C(SiMe3)2SiMe2(2-C5H4N)] have been prepared from NaCp and respective dichlorides (Pytsi)ECl2 in isolated yields of 65% (2a) and 83% (2b). In addition to applying standard methods of characterization (NMR spectroscopy, CHN analysis, and MS), the molecular structures of both compounds were solved by single-crystal X-ray crystallography. Reactions between 2a or 2b with Zr(NMe2)4 in benzene, followed by addition of an excess of ClSiMe3, gave the first ansa-zirconocene dichlorides with aluminum (4a) or gallium (4b) in bridging position. The gallium compound 4b was isolated in a yield of 40% and characterized by NMR spectroscopy, CHN analysis, and mass spectrometry. The aluminum species 4a could not be obtained in an analytically pure form; it was characterized by NMR spectroscopy, and its molecular structure was determined by single-crystal X-ray analysis. The angle between the two planes of the Cp ligands in 4a was found to be 57.17(11)°, which is larger than in Cp2ZrCl2 (α = 53.5°) and smaller than in silicon-bridged ansa-zirconocene dichlorides (e.g., Me2Si(C5H4)2ZrCl2: α = 60.1°). Crystallization attempts of 4a from DCCl3 solutions gave crystals of the new species (Pytsi)AlCl[(C5H4)ZrCp] (7), which was characterized by NMR spectroscopy and single-crystal X-ray analysis. In comparison to the Pytsi-containing species 2a and 2b, the less sterically protected compound [2-(Me2NCH2)C6H4]AlCp2 (6), which was prepared from NaCp and [2-(Me2NCH2)C6H4]AlCl2 in yields of 94% and characterized by standard methods (NMR, CHN, and MS), reacted with Zr(NMe2)4 to give the known Cp2Zr(NMe2)2. The targeted ansa-species was not formed, as judged by 1H NMR spectroscopy. The gallium-bridged compound 4b and Cp2ZrCl2, respectively, had been tested for polymerizations of ethylene utilizing MAO as an activator (300 equiv of MAO; 5 or 10 μmol of precatalyst; 15 or 70 psi), resulting in activities of 637 to 1080 for 4b and of 640 to 2601 kg PE (mol Zr)−1 h−1 atm−1 for Cp2ZrCl2.
The reaction of Ti(NMe2)4 with the amine functionalised cyclopentadiene C5H5CH2CH2CH2N(H)CMe 3 gives the titanium cyclopentadienyl-amide complex [Ti{5:1-(C5H4CH2CH2 CH2NCMe3)} (NMe2)2] which reacts with aniline to generate the phenylimido bridged dimer [{Ti(5-C5H4CH2CH2CH2N(H)CMe3)(NHPh)}2 (-NPh)2] whose molecular structure has been determined by X-ray diffraction, revealing a slightly asymmetric Ti(-NPh)2Ti core.
The reaction of PriN(H)SiMe2CH2Cl with two equivalents of fluorenyllithium in THF led to [PriN(H)SiMe2CH2(C13H8)]Li(THF) (1) in high yield. X-ray diffraction studies of 1 revealed a monomeric structure with an η2-coordinated fluorenyl ligand. Lithiation followed by the treatment with zirconium or hafnium tetrachloride afforded the corresponding dichloro complexes [(η1-NPri)SiMe2CH2(η5-C13H8)]ZrCl2 (2) and [(η1-NPri)SiMe2CH2(η5-C13H8)]HfCl2 (3). These metal adducts were characterized by NMR spectroscopy, and in the case of 3 also by X-ray diffraction. Compound 3 shows a weak interaction between the formally 14-electron Hf center and the C-H of the isopropyl group. The dialkyl compound [(η1-NBut)SiMe2CH2(η5-C13H8)]Zr(CH2SiMe3)2 (4) was prepared from Me3SiCH2MgCl and [(η1-NBut)SiMe2CH2(η5-C13H8)]ZrCl2. The NMR spectroscopic and X-ray data point to a fairly congested environment around the zirconium atom. In contrast to the —SiMe2— bridged analogs, the —SiMe2CH2— linked amido-fluorenyl complex 4 features a relatively unstrained ligand backbone with a close to ideal trigonal planar geometry at the amido nitrogen atom.
A newly synthesized catalyst, a pentamethylene bridged dinuclear zirconocene ([(CH2)5(C5H4)2][(C9H7)ZrCl2]2), and its in situ supported catalyst were tested in a ethylene/1-hexene copolymerization and their performance was compared with those of Cp2ZrCl2 and rac-Et(Ind)2ZrCl2. When homogeneous [(CH2)5(C5H4)2][(C9H7)ZrCl2]2 catalyst was used in the copolymerization, its activity decreased with comonomer incorporation (a negative comonomer effect) and it produced copolymer with a lower comonomer content than other catalysts, both of which were ascribed to the steric effect of the [(CH2)5(C5H4)2][(C9H7)ZrCl2]2 catalyst. With in situ supported catalysts, the activity did not change much with the increase of 1-hexene content in contrast to the homogeneous catalysts. In situ supported catalysts produced copolymers with higher content of 1-hexene and lower melting point and crystallinity. The changes in the molecular weight and its distribution of the copolymer with in situ supporting were significant and dependant on the nature of the catalyst.
A series of phosphorus-bridged ansa-metallocene complexes of titanium, zirconium, and hafnium, [PhP(XC{sub 5}Me{sub 4}){sub 2}]MX{sub 2} and [Ph(E)P(C{sub 5}Me{sub 4}){sub 2}]MX{sub 2} (X = Cl, Me, CO, (Se{sub 3}){sub 0.5}; E = O, S, Se), has been synthesized. Structural characterization by X-ray diffraction indicates that, in comparison to their non-ansa counterparts (C{sub 5}Me{sub 5}){sub 2}MX{sub 2}, the cyclopentadienyl groups in phosphorus-bridged complexes are displaced from symmetric {eta}{sup 5}-coordination toward {eta}{sup 3}-coordination. Such {eta}{sup 3},{eta}{sup 3}-coordination creates more electrophilic metal centers than those in their permethylcyclopentadienyl counterparts, as judged by the {nu}(CO) stretching frequencies of the zirconium dicarbonyl complexes Cp{sup *}{sub 2}Zr(CO){sub 2} (1946 and 1853 cm{sup {minus}1}) and [PhP(C{sub 5}Me{sub 4}){sub 2}]Zr(CO){sub 2} (1959 and 1874 cm{sup {minus}1}).
The doubly bridged dinuclear (mu-oxo)titanium complex (Me(2)C)(Me(2)Si)(C(5)H(3))(2)(mu-O)(CpTiCl)(2) (1) was synthesized by the reaction of the lithium compound of the doubly bridged bis(cyclopentadienyl) ligand with (CpTiCl(2))(2)O. Treatment of 1 with concentrated HCl or HBr gave the corresponding Cp-decoordinated products (Me(2)C)(Me(2)Si)(C(5)H(3))(2)[CPTiX(mu-O)TiX(2)] (X = Cl (2), Br (3)). All these titanium complexes were characterized by (1)H NMR, (13)C NMR, MS spectra, and elemental analysis. The crystal structures of (CPTiCl(2))(2)O, 1, and 3 were determined by X-ray diffraction. Their catalytic properties for ethylene polymerization were also studied in the presence of MAO.
The syntheses and X-ray crystal structures of the boron-bridged ansa-metallocene complexes [{Ph(Me2S)B(η5-C5H4)2}ZrCl2]·C6D6 (2) and [{Ph(Me3P)B(η5-C5H4)2}ZrCl2] (3) are reported. Complex 2 is obtained as the product of a double dehalodesilylation reaction between PhB(C5H4SiMe3)2 and ZrCl4(SMe2)2. The dimethyl sulfide adduct of 2 is readily replaced with trimethylphosphine to afford 3. Efforts to alkylate 2 were unsuccessful, presumably due to the lability of the dimethyl sulfide and, hence, its inability to protect the boron bridge from nucleophilic attack by alkyl anions. In contrast, the trimethylphosphine adduct in 3 binds tightly enough to the boron bridge to protect it from nucleophilic attack. Also, unlike 2, complex 3 is activated by methylalumoxane toward the polymerization of ethylene.
Heterogeneous ethylene polymerization catalysts were prepared by supporting tetrakis(dimethylamino)titanium, Ti[N(CH3)2]4, on chemically modified silica surface. The modification of silica was made using a silane coupling agent, Cp(CH2)3Si(OCH2CH3)3 and atomic layer chemical vapor deposition (ALCVD). Titanium amide was also immobilized straight on the silica and on a methyllithium-treated modified silica. The aim of methyllithium treatment was to alkylate the unreacted ethoxy groups of the silane coupling agent on the silica. Methyllithium methylates, besides ethoxy groups, also silica surface and surface cyclopentadienyl groups. A model compound for these heterogeneous catalysts was prepared using a reaction of Ti[N(CH3)2]4 with trimethylsilylcyclopentadienyl yielding a monocyclopentadienyl titanium compound, Me3SiCpTi[N(CH3)2]3. The preliminary results of ethylene polymerization using the prepared heterogeneous and homogeneous titanocene amide complexes and the effect of amount of MAO are presented.
Ethylene polymerization studies have been carried out with novel precatalysts of the type: [(η5-C13H8)-X(t-BuOC6H12)Me-(η5-C5H4)]ZrCl2 [X=C [1a], Si [2a]], [(η5-C13H8)-XMe2-(η5-(t-BuOC6H12C5H3))] ZrCl2 [X=C [3a], Si [4a]] in the presence of excess methylalumoxane (MAO) to compare their catalytic activity and to delineate the effect of the 6-t-butoxyhexyl functionality on ethylene polymerization. The precatalysts [1a] and [2a] with the bridge functionality showed higher activity in ethylene polymerization than the corresponding complexes [3a] and [4a] which have it on the Cp ring moiety. On the other hand the silyl bridged complexes [2a] and [4a] produced a higher molecular weight polyethylene than the carbon-bridged one, regardless of the location of functional group.
Stereospecific polymerization of 1-hexene under high pressures (up to 1,000 MPa = ca. 10,000 atm) using metallocene/methylaluminoxane (MAO) catalysts was investigated. Several C2-symmetric ansa-metallocenes, their meso-isomers, and two Cs-symmetric ansa-metallocenes were employed as catalyst precursors. In the course of this study, novel C2-symmetric germylene-bridged ansa-metallocenes, (rac-[Me2Ge(η5-C5H-2,3,5-Me3)2MCl2] (M = Zr, rac-4a; M = Hf, rac-4b), have been prepared. High pressures induced enhancement of the catalytic activity and the molecular weight of the polymers in most of the catalysts. The maximum of both the catalytic activity and the molecular weight of the polymers was mostly observed at 100–500 MPa in each catalyst, although the enhanced ratio was smaller than that observed for nonbridged metallocenes. Isospecificity of the C2-symmetric ansa-metallocene catalysts was essentially maintained even under high pressure. Highly isotactic polyhexene ([mmmm] = 91.6%) with very high molecular weight (Mw = 2,360,000) was achieved by rac-4b under 250 MPa. High pressures slightly decreased syndiotacticity when the Cs-symmetric ansa-metallocene, isopropylidene(1-η5-cyclopentadienyl)(9-η5-fluorenyl)zirconium dichloride 5, was employed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 283–292, 1999
Heterogeneous ethylene polymerization catalysts were prepared by supporting tetrakis(dimethylamino)zirconium on chemically modified silica surface. The silica was modified with a silane coupling agent, Cp(CH2)3Si(OEt)3, using ALCVD-technique. On the resulting cyclopentadienyl surface zirconium amide, Zr(NMe2)4, was immobilized with and without using methyllithium. Depending on the use of methyllithium the immobilization resulted in either mono- or bis-cyclopentadienyl zirconium complexes, which were characterized using chemical analysis, FT-IR, 13C- and 29Si-NMR results. Moreover, a homogeneous model compound, (Me3SiCp)2Zr(NMe2)2, was synthesized with a reaction between Zr(NMe2)4 and Me3SiCp. The preliminary results of ethylene polymerization with these supported and unsupported organometallic complexes are presented.
New double silylene-bridged binuclear zirconium complexes [(η5-RC5H4)ZrCl2]2[μ,μ-(SiMe2)2(η5-C5H3)2] [R = H (1), Me (2), nPr (3), iPr (4), nBu (5), allyl (6), 3-butenyl (7), benzyl (8), PhCH2CH2 (9), MeOCH2CH2 (10)] were synthesized by the reaction of (η5-RC5H4)ZrCl3·DME with [μ,μ-(SiMe2)2(η5-C5H3)2]2− (L2−) in THF, and they were all well characterized by 1H NMR, MS, IR, and EA. The binuclear structure of Complex 3 was further confirmed by X-ray diffraction, where the two zirconium centers are located trans relative to the bridging [μ,μ-(SiMe2)2(η5-C5H3)2] moiety. When activated with methylaluminoxane (MAO), this series of zirconium complexes are highly active catalysts for the polymerization of ethylene even under very low molar ratio of Al/Zr (Complex 7, 5.41 × 105g-PE/mol-Zr·h, Al/Zr = 50) and linear polyethylenes (PEs) with broad molecular weight distribution (MWD, Mw/Mn = 7.31–27.6) was obtained. The copolymerization experiments indicate that these complexes are also very efficient in the incorporation of 1-hexene into the growing PE chain in the presence of MAO (Complex 6, 3.59 × 106g-PE/mol-Zr·h; 1-hexene content, 3.65%). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4901–4913, 2007
Heterogeneous constrained-geometry hafnium complexes were prepared by anchoring Hf(NMe2)4 on a heterogenized ansa-cyclopentadienylamino ligand of type (RN)R′R′′Si(C5R′′′4), surface 1: R=OMe2Si(CH2)3; R′, R′′, R′′′=Me; surface 2: R=OMe2Si(CH2)3; R′, R′′′=Me, R′′=H. Preparation of the constrained-geometry catalyst ligand on silica surface was carried out in two steps: functionalizing of the silica surface with aminopropyldimethylethoxysilane (APDMES) by using saturated gas–solid reactions and immobilizing of (tetramethylcyclopentadienyl)chlorosilanes, Me2Si(C5Me4H)Cl or MeHSi(C5Me4H)Cl, on the functionalized silica by utilizing n-butyllithium (n-BuLi). Hf(NMe2)4 was immobilized on the heterogeneous surfaces by the amine elimination route. The complexes were characterized by 1H, 13C, and 29Si solid state NMR and FTIR spectroscopy.
Heterogeneous constrained-geometry catalysts were prepared by immobilizing Zr(NMe2)4 on a heterogenized ansa-cyclopentadienylamino ligand of type (RN)R‘R‘ ‘Si(C5R‘ ‘‘4): surface 1, R = −O-Me2Si(CH2)3; R‘, R‘ ‘, R‘ ‘‘ = Me; surface 2, R = −O-Me2Si(CH2)3; R‘, R‘ ‘‘ = Me, R‘ ‘= H. The preparation of the constrained-geometry catalyst ligand on a silica surface has been carried out in two steps: first, functionalizing the silica surface with aminopropyldimethylethoxysilane (APDMES) by using saturated gas−solid reactions, and then immobilizing two different (tetramethylcyclopentadienyl)chlorosilanes, Me2Si(C5Me4H)Cl and MeHSi(C5Me4H)Cl, on the APDMES-functionalized silica by utilizing n-BuLi (n-butyllithium). Zr(NMe2)4 was immobilized on the heterogeneous surfaces using the amine elimination reaction. The two heterogeneous monocyclopentadienylamido zirconium catalysts were characterized with 1H, 13C, and 29Si solid-state NMR and FTIR spectroscopy and tested for ethylene polymerization.
Novel aminoboranediyl-bridged zirconocenes [i-Pr2NB(η5-C5H4)2]ZrCl2 (8b) and [i-Pr2NB(η5-1-C9H8)2]ZrCl2 (4b) have been prepared and structurally characterized. When activated by excess methylaluminoxane, 4b and 8b form highly active catalysts for the polymerization of ethylene. The catalyst derived from 4b converts propylene to isotactic polypropylene.
Polymerization of 1-hexene or 1-octene under high pressures (100−1000 MPa) was investigated using permethylated ansa-metallocenes/methylaluminoxane (MAO) as catalyst systems. Besides the known zirconocene complex (1, Me2Si(η5-C5Me4)2ZrCl2), dialkylsilylene- and dimethylgermylene-bridged hafnocene dichlorides, R2E(η5-C5Me4)2HfCl2 (2, R = Me, E = Si; 3, R = Et, E = Si; 4, R = vinyl, E = Si; 5, R = Me, E = Ge), were synthesized and structurally characterized. The catalytic activity of the ansa-metallocenes was remarkably enhanced under high pressures despite their very congested structures. Poly(1-hexene) with unprecedented high molecular weight such as Mw = 1.02 × 107 (Mw/Mn = 3.79, by GPC) was obtained under 750 MPa with 2. Studies on termination reactions revealed that the bimolecular chain-transfer process (β-hydrogen transfer to olefin) is much accelerated under 500 MPa in 1, while β-hydrogen elimination to metal is still major termination process in 2. This indicated that two complexes that have same structure but central metals showed quite different high-pressure effects.
The first examples of tin-bridged [1]ferrocenophanes, Fe(η- C5H4)2SntBu2 (7a) and Fe(η-C5H4)2SnMes2 (7b) have been synthesized by the low-temperature reaction of Fe(η-C5H4Li)2 · n TMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) with tBu2SnCl2 and Mes2SnCl2 (Mes=2,4,6-trimethylphenyl), respectively. They were isolated in 65% (7a) and 85% (7b) yield as orange crystalline solids, which were characterized by multinuclear NMR and UV/Vis spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction. The tilt angles between the planes of the cyclopentadienyl rings are 14.1(2)°for 7a and 15.2(2)°(average) for the three independent molecules of 7b in the unit cell. Although they have significantly smaller tilt angles than analogous [1]ferrocenophanes with the lighter Group 14 elements silicon or germanium in the bridge, 7a and 7b still readily undergo ring-opening polymerization (ROP) by thermal reaction in the solid state (7a at 150°C; 7b at 180°C), to give high-molecular-weight poly(ferrocenylstannane)s [Fe(η-C5H4)2SntBu2](n) (8a) and [Fe(η-C5H4)2SnMes2](n) (8b). Remarkably, 7a and 7b were also found to polymerize in solution at room temperature in the absence of externally added initiators. ROP is much more rapid for 7a than for 7b in solution. The cyclic dimers [Fe(η-C5H4)2SnR2]2 (3; R=tBu, Mes) were formed as by-products in amounts which depended on the solvent. Electrochemical studies of the cyclic dimers and polymers indicated the presence of significant Fe···Fe interactions that are mediated by the tin- atom spacer. When benzene solutions of 7a and 7b were treated with small amounts of Karstedt's catalyst, slower polymerization was observed. Stoichiometric reaction of Pt(1,5-cod)2 (cod=cyclooctadiene) with 7a yielded: the novel trimetallic 1-stanna-2-platina[2] ferrocenophane Fe (η- C5H4)2Pt(1,5-cod)SntBu2 (9), which functioned as a sluggish catalyst for the ROP of 7a and 7b.
Novel double silylene-bridged binuclear Group 4 metallocene complexes [(η5-C5H5)MCl2]2[μ, μ-(SiMe2)2 (η5-RC5H2)2] [R = Me, M = Zr (1); R = allyl, M = Ti (2); R = allyl, M = Zr (3); R = nBu, M = Ti (5); R = Me3Si, M = Zr (6)] and [(η5-CH3C5H4)ZrCl2]2[μ, μ-(SiMe2)2 (η5-allyl-C5H2)2, (4)] were synthesized by the reaction of CpMCl3 (M = Ti, Zr) with [μ, μ-(SiMe2)2 (η5-RC5H2)2]2- (L2-) in THF, and they were all well characterized by 1H-NMR, MS, and EA. When activated with methylaluminoxane (MAO), this series complexes are active catalysts for Ethylene polymerization (Complex 3, 3.86 × 106 g-PE/mol-M h, Al/Zr = 500) and copolymerization of ethylene with 1-hexene (Complex 3,30.87 × 105 g-polymer/mol-M h, 1-hexene content in polymer 2.34 mol %, Al/Zr = 1000). The result of polymerization implies that there is a cooperative effect between the two metals in the double-bridged binuclear molecules. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
Ever since their discovery by Sinn and Kaminsky in 1979, the family of high activity homogeneous olefin polymerization catalysts known as “metallocenes” has been the subject of intense research and commercial development. While a structurally diverse group, metallocene catalysts are generally considered to have a few common features: a highly electrophilic, early transition metal or rare-earth atom as the point of polymer chain attachment; a π-bound cyclopentadienyl ring or isoelectronic analogue attached to this atom; and characteristically narrow distributions of polymer molecular weight and short-chain branching. While polymerization of ethylene and propylene is reviewed in some detail, the nonpolymerization catalysis and (co)polymerization of functional monomers is also surveyed. The current commercial status of metallocene technology is also briefly discussed.
Metallocene catalysts refer to the combination of bis(cyclopentadienyl)metal complexes of Group 4 (IVB) (especially zirconium), or cyclopentadienyl-substituted derivatives thereof, and a cocatalyst, typically methylalumoxane (MAO) or a perfluorated arylborane. Metallocene catalysts are employed in olefin polymerization as a new generation of Ziegler–Natta catalysts. The single-site character of the active species enables the syntheses of polymers with narrow molar mass, comonomer distribution and tacticity distribution as industrially important features. Through a rational substitution pattern at the cyclopentadienyl ligands, metallocene catalysts allow for a unprecedented control and variation of the tacticity in α-olefin polymerization. Furthermore, metallocene catalysts made the polymerization of cyclic olefins without ring opening possible.
IntroductionHistorical DevelopmentMethylalumoxane - Characteristics and FunctionThe Borane Activators or Cationic Catalysts and other Cocatalyst-free SystemsGeneral Mechanism of Chain Growth. Chain Transfer and DeactivationEthene HomopolymerizationPropene HomopolymerizationHomopolymerization of Other α-OlefinsOlefin OligomerizationCopolymerizationPolymerization and Copolymerization of Cyclic OlefinsDiene- and CyclopolymerizationPolymerization of Polar MonomersRecent Trends in Ligand and Metallocene Catalyst Design
[CpR(RPNEt2)]M (CpR=t-BuC5H3, C5(CH3)4, indenyl, fluorenyl; M=Li, K) smoothly react with VCl3(Me3P)2 and CrCl3(THF)3 systems giving paramagnetic complexes [CpR(R1PNEt2)]MCl2 (M=V(Me3P)2, Cr). After reaction with MAO these complexes are active in the polymerisation of ethylene yielding highly crystalline, high-density products of high molecular weight (Mw ranging from 100 000 to 4.5×106 g mol−1, 20≤Tp≤100 °C). Polymerisation with chromium complexes leads to the formation of polyethylenes with broad molecular weight distribution.
The organo-tin compounds, Me2Sn(C5H4R-1)2 (R=Me (1), Pri (2), But (3), SiMe3 (4)) and Me2Sn(C5Me4R-1)2 (R=H (5), SiMe3 (6)), were prepared by the reaction of Me2SnCl2 with the lithium or sodium derivative of the corresponding cyclopentadiene. Compounds 1–6 have been characterized by multinuclear NMR spectroscopy (1H, 13C, 119Sn). In addition the molecular structures of 5 and 6 were determined by single crystal X-ray diffraction studies. The transmetalation reaction of 1–6 with ZrCl4 or [NbCl4(THF)2] gave the corresponding metallocene complexes in high yields.
The modification of a mesoporous silica surface with Si(Ind)(CH3)2Cl and the immobilization of CpZr(NMe2)3 on this surface was studied via IR-spectroscopy. To reduce side reactions, the indenyl-modified silica was reacted with hexamethyldisilazane (HMDS) under IR-control before the CpZr(NMe2)3-immobilization. The role of the hydroxyl group protection with HMDS is discussed. The surface modifications have been repeated via Schlenk technique at the same conditions and the surface modifications were studied with CP MAS–NMR, MAS–NMR, elemental-, SEM- and BET-analysis. The surface species of the resulting catalysts are discussed. The precatalysts have been treated with methylaluminoxane (MAO) (Al:Zr (mol:mol)=500:1) and the resulting Zr contents (leaching-effect) are discussed. All catalysts have been tested in ethylene and propylene polymerization.
A new catalyst composed of Tin-bridged yttrocene Ph2Sn(MeCp)2YCI (MeCp=methylcyclopentadienyl) and Al (i-Bu)3 was successfully developed for the polymerization of methyl methacrylate (MMA). Detailed study of factors (such as the molar
ratio of Al/Cat., catalyst concentration, various solvents, temperature and time) influencing polymerization reaction indicated
that the catalytic active species may be still somewhat stable at high temperature and still have a long catalytic lifetime.1H NMR spectrum showed about 65% syndiotactic content in the polymethyl methacrylate (PMMA) prepared. From kinetic studies,
the polymerization rate equation may be expressed as Rp=Kp[Cat.]2.4. The overall activation energy of polymerization is 20.9±3.1 kJ/mol.
Five new SiNSi bridged metallocene dichloride complexes of the type [CpSiMe2N(R)SiMe2Cp]MCl2 (Cp=C5H4, C9H6; R=alkyl, M=Ti, Zr, Hf) were synthesized using a new type of amine elimination reaction. All complexes were characterized by 1H-, 13C-, 15N- and 29Si-NMR spectroscopy. An X-ray structure analysis was performed for complex [C5H4SiMe2N(nBu)SiMe2C5H4]HfCl2 (4)
This review describes the different synthetic strategies employed in the preparation of group 4 and 5 ansa-bis(cyclopentadienyl) complexes containing a group 14 single-atom bridge. The general preparative routes can be summarized in: (i) metathesis reactions using alkali-metal ansa-ligand precursors; (ii) transmetalation with tin substituted cyclopentadiene compounds; (iii) reaction of the metal amide derivatives with ansa-bis(cyclopentadiene) compounds. Excluding fused ring systems, such as indenyl or fluorenyl, from this study, it is still nevertheless evident the wide variety of complexes that have been reported. Substituents can be varied in nature, position and/or number at the C5 ring and/or at the single-atom ansa-bridge. Functional groups have also been introduced into the ansa-ligand framework and their reactivity in, for example, hydroboration or hydrosilylation is described in this review.
Ansa-metallocene complexes have been extensively studied as the precatalysts for olefin polymerization. The bridges have important effects on the activities of the catalysts and polymer properties such as molecular weight and tacticity, mainly by altering the frame work of the catalysts. Here, we review the effects of the structures of ansa-metallocene complexes on the catalytic activities, especially the effects of the bridges on the catalytic activities. (c) 2005 Elsevier B.V. All rights reserved.
This review covers research in olefin polymerization by metallocene catalysts published in 1996. While metallocenes are generally defined as metal complexes with two cyclopentadienyl rings, this review will additionally cover catalysts containing only one cyclopentadienyl ring, as well as catalysts with ligands isolobal to cyclopentadienide. Included will be certain disclosures concerning cocatalyst preparation and use. Polymer properties and uses will not be covered.
The gas-phase reaction between MoO3-x and H(2)S in a reducing atmosphere at elevated temperatures (800 degrees to 950 degrees C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS(2). A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions.
Since the metal alkoxidesM(OR), were first comprehensively reviewed in 1960, considerable progress has been maintained and the next major review in 1967 quoted over three hundred references. The most significant advances have involved the chemistry of the transition metal alkoxides with the emphasis on the ligand field aspects of the alkoxo group (e.g., electronic spectra, magnetism, etc.) and on X-ray crystallographic and NMR structural determinations. Industrial applications have been concerned with metal alkoxides as components of soluble Ziegler-Natta catalysts for olefin polymerization and also as sources for the production of pure metal oxides. The metal dialkylamides M(NR,) are of special interest, in that they contain covalent metal-nitrogen bonds, and occupy a position between the metal alkoxides and the metal alkyls. The field has not been fully reviewed, hitherto. However, excellent reviews have recently appeared on titanium dialkylamides and the reactivity of metal-dialkylamido bonds.
Tin (Sn) compounds are well-recognized both in basic research and in industrial applications. Tin NMR spectroscopy includes three magnetically active tin isotopes namely, 115Sn, 117Sn , and l19Sn. However, most tin NMR parameters refer to the 119Sn nucleus owing to its properties such as abundance, magnetic moment, and NMR frequency. This chapter describes: (1) the compilation and updation of 119Sn chemical shifts (δ 119Sn) from studies on organotin compound and inorganic tin compounds, (2) indirect nuclear spin-spin couplings, (3) relaxation mechanisms concerning the 119Sn nucleus, and (4) experimental details for 1I9Sn-NMR measurements. It is observed that with modem equipment there are, in principle, no serious experimental difficulties in observing 119Sn resonances either directly or by heteronuclear double resonance. δ 119Sn data, available for all kinds of tin compounds, significantly allows using this parameter in an empirical way in order to find models for the qualitative interpretation of δ 119Sn. This also applies to couplings involving the l19Sn nucleus, which serve as a sensitive tool in the discussion of structure and bonding. All 119Sn-NMR parameters of tin compounds largely concern measurements in the liquid state.
The preparation of Cp substituted dialkylamido titanium cyclopentadienyls (R2N)3TiC5H3′, R″ = H, alkyl, SiMe3, GeMe3) by the reaction of C5H4R′R″ with (R2N)4Ti is described. The physical properties of the compounds obtained are listed and their IR and Raman spectra discussed on the basis of a h5-CpTi structure. The analysis of the NMR spectra is the subject of the subsequent communication.ZusammenfassungDie Darstellung Cp-substituierter Dialkylamido-titan-cyclopentadienyle (R2N)3TiC5H3H′R″ (R2N = Me2N, Piperidido, R′, R″ = H, Alkyl, SiMe3, GeMe3) aus C5H4R′R″ und (R2N)4Ti wird beschrieben. Die physikalischen Eigenschaften der erhaltenen Verbindungen werden angegeben und ihre IR- sowie Raman-Spektren unter Zugrundelegung einer h5-CpTi-Struktur diskutiert. Die Analyse der Kernresonanzspektren ist Gegenstand der folgenden Arbeit.
The 1H-NMR spectra of stannanes of the type (C5H5)4-nSnRn, with RCH3, C2H5, n-C4H9, and C6H5 and n=0, 1, 2, and 3, are reported and discussed. The five cyclopentadienyl protons show only one sharp signal at approx. τ=4, which has satellites at both sides originating from H117Sn- and H119Sn-coupling. These coupling constants 117SnC5H5 and 119SnC5H5 show a dependence on the type and number of additional substituents of the stannanes.
Enantioselective cyclopolymerization represents a novel strategy for the synthesis of optically active main-chain chiral polymers. Cyclopolymerization of 1,5-hexadiene using the optically active catalyst precursor, (R,R)-(EBTHI)ZrBINOL ((R,R)-1) [EBTHI = ethylene-1,2-bis([eta][sup 5]-4,5,6,7-tetrahydro-1-indenyl); BINOL = 1,1[prime]-bi-2-naphtholate], yields optically active poly(methylene-1,3-cyclopentane) (PMCP) with a molar optical rotation of [[Phi]][sup 28][sub 405] +51.0[degrees] (c = 0.80, CHCl[sub 3]). The molar optical rotation for the polymer derived from (R,R)-1 is higher than that of the model compound trans-(1R,3R)-1,3-dimethylcyclopentane and is also temperature dependent, suggesting that the polymer adopts chiral conformations which contribute to the observed optical rotation. The microstructure of the polymer was interpreted by [sup 13]C NMR at tetrad resolution. A statistical model for the microstructure based on an enantiomorphic site control mechanism provided good agreement with the experimental data. On the basis of this model, the enantioface selectivity for the cyclopolymerization of 1,5-hexadiene in the presence of catalysts derived from (R,R)-1 is 91% at 20[degrees]C, indicative of a highly isotactic microstructure. The molar optical rotation of poly(methylene-1,3-cyclopentane) is independent of molecular weight, which provides experimental support for an enantiomorphic site control mechanism. The absolute configuration of PMCP was tentatively assigned on the basis of the sign of the optical rotation of the model compound trans-(1R,3R)-1,3-dimethylcyclopentane and the known enantioface selectivity of 1 with similar [alpha]-olefins. 46 refs., 12 figs., 5 tabs.
Ziegler-Natta catalysts are remarkable in their ability to polymerize {alpha}-olefins to high molecular weight, stereoregular polyolefins. One of the major limitations of conventional Ziegler-Natta catalysts is their intolerance to Lewis bases; catalysts based on titanium halides and alkylaluminum cocatalysts are poisoned by most types of monomers containing ethers, esters, amines, and carboxylic acids. The absence of functionality in hydrocarbon polymers seriously affects their adhesive properties, affinity for dyes, permeability, and compatibility with more polar polymers. Previous attempts to polymerize sterically hindered amines, esters and amides, alkyl halides, and carboxylic acids using catalysts derived from TiCl{sub 3} and AlR{sub 3-n}Cl{sub n} have achieved limited success due to the severe loss of catalytic activity in the presence of these monomers. This work reports that cationic, group four metallocenes are active catalysts for the homo-polymerization of {alpha}-olefins containing silyl-protected alcohols and tertiary amines. Employing different monomers and conditions, a table shows the starting monomer, reaction time and temperature, and spectroscopic analysis of the end products. A major advanatage of these metallocene-based catalysts is that the ligand system can be modified to proved the optimal combination of catalystic activity, stereospecificity, and tolerance to functionality. 32 refs., 1 tab.
Bis 94% Anteil an isotaktischen Pentaden wird bei der Polymerisation von Propen mit den ansa ‐Zirconocen‐Katalysatoren 1 erreicht. Es zeigt sich, daß sperrige β‐Substituenten für die Stereo‐ und α‐Substituenten für die Regioselektivität entscheidend sind. α‐Substituenten begünstigen darüber hinaus den Aufbau langer Ketten ( M W bis 9190). R ¹ = t Bu, i Pr; R ² = H, Me; μ‐X = Me 2 C‐CMe 2 , Me 2 Si. magnified image
In the presence of excess butylmagnesium chloride and in diethyl ether at 25-degrees-C, zirconocene dichloride is an effective catalyst precursor for the stereoselective cyclization of nonconjugated dienes containing a terminal alkoxide substituent. The products are carbocyclic rings containing adjacent vinyl and methyl substituents with trans stereochemistry.
ansa-Metallocene Derivatives, Part 19. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. We wish to thank Frau E. Barsties (Universität Konstanz), Dr. W. Ball, Dr. Merthes and Dr. P. Simak (BASF AG) for the polymer analyses, BASF AG for gifts of chemicals, and Prof. J. E. Bereaw (California Institute of Technology) and one of the referees for valuable comments. Part 18: P. Burger, H. U. Hund, K. Evertz, H. H. Brintzinger, J. Organomet. Chem. 378 (1989) 153.
This paper is concerned with the existence of the travelling wave solutions for some cross-diffusion systems with small parameters. By using singular perturbation method, we prove the existence of the travelling wave solutions with a speed c(ϵ) ϵ, which extend the results of [22] and [23].
Quantitativ und stereoselektiv konnten rac ‐Metallocene nun erstmals synthetisiert werden. Die neuartigen rac ‐Metallocene 1 wurden aus einfach zugänglichen homoleptischen Metallamiden und nicht‐deprotonierten Cyclopentadienen erhalten. Komplexverbindungen dieses Types kommen als Katalysatoren fü die iso‐taktische Olefin‐Polymerisation in Betracht. M = Zr, Hf; Y = NPh, C 5 H 4 . magnified image
The 1H and 13C NMR spectra of various cyclopentadienyl titanium dialkylamides (R2N3TiC5H3R′R″ (R2N = Me2N, piperidido; R′, R″ = H, alkyl, SiMe3, GeMe3) and some related compounds have been recorded and analyzed concerning the structure of these compounds. Increasing the size of R, R′ and R″ causes a bending of the ring from a h5 type structure as in (R2N)3TiC5H5 in the direction of a h2 type in (R2N)3TiC5H3R′R″. The indenyl compound (Me2N)3TiInd probably has a h3 structure.ZusammenfassungDie 1H- und 13C-KMR-Spektren einiger Cyclopentadienyl-titan-dialkylamide (R2N)3TiC5H3R′R″ (R2N = Me2N, Piperidido; R′, R″ = H, Alkyl, SiMe, GeMe3) und einiger verwandter Verbindungen wurden aufgenommen und im Hinblick auf Strukturinformationen ausgewertet. Wachsende Grösse von R, R′ und R″ verursacht eine Abwinkelung des Ringes von einer h5-Struktur im (R2N)3TiC5H5 in Richtung auf einen h2-Bindungstyp im (R2N)3TiC5H3R′R″. Die Indenyl-verbindung (Me2N)3 TiInd besitzt wahrscheinlich eine h3-Struktur.
Treatment of dimethyl(tetramethylcyclopentadieny)chloro silane with S-(-) pyrolidine methanol provided the O-silylated ligand(C5Me4H)SiMe2OCH2(C4H7NH), 1, (Cp∗SiProH2) as a 90% pure, thermally unstable oil in 65% yield. Reaction of 1 with Zr(NMe2)4 resulted in attachment of 1 to the zirconium center with elimination of HNMe2, yielding (Cp∗SiPro)Zr(NMe2)2, 2, as a viscous oil in high yield (95%). Compound 2 was converted to the trichloride derivative (Cp∗SiProH)ZrCl3, 3, in 75% yield by treatment with three equivalents of HCl·HNMe2: compound is a mixture of diastereomers; the major species was characterized by X-ray crystallography, revealing a Cp∗ON coordination mode for the Cp∗SiProH ligand. (3: orthorhombic; space group P212121, a = 10.0009(13), b = 12.7597(12), c = 16.2749(15) Å, V=2076.8(4) Å3, Z = 4, R = 0.043, rw = 0.041.) Deprotonation of 3 (diastereomeric mixture) with LiN(SiMe3)2 produced the dichloride (Cp∗SiPro)ZrCl2, 4, in 71% yield. Alkylation of either 3 or 4 resulted in SiO bond cleavage in the Cp∗SiPro ligand and gave a dimeric complex 5 which was characterized by X-ray crystallograhy. (5: monoclinic, space group P21, a = 9.1285(10), b = 20.2197(22), c = 11.0214(14) Å, β = 90.38(7)°, V = 2034.2(4) Å3, Z = 2, R = 0.040, Rw = 0.042. Limited ethylene polymerization activity was observed for 3 and 4 in the presence of MAO co-catalyst.
The homoleptic metal amides Ti(NEt2)4, Ti(NMe2)4, and Zr(NEt2)4 react with silyl-substituted cyclopentadienes (1–3) and the indene 4 to yield the new half-sandwich complexes 8–16 of type Me2 Si[CpR][NR′]M[NR′'2)2 (M = Ti, Zr). The new ligands have been characterized by 1H, 13C, and 29 Si NMR spectroscopy, IR, and GC-MS with regard to the sigmatropic rearrangements caused by hydrogen and silicon migration. The titanium and zirconium complexes 8–16 were characterized by their 1H, 13C, and 29Si NMR spectra, IR, and mass spectrometry. The capabilities and limitations of the “salt-free” procedure employed here are described.
The synthesis of the amine-substituted cyclopentadiene C5H5CH2CH2CH2N(H)Me (2, HCpNH) is described. The reaction of 2 with M(NMe2)4 (M = Zr, Hf) gives the ring-closed species [M(eta5:sigma-C5H4CH2CH2CH2NMe)(NMe2)2] (M(CpN)(NMe2)2; M = Zr (3), Hf (4)) in high yields (60-95 %). Aminolysis of 3 and 4 with 2 equiv of Me2NH.HX (X = Cl, I) provides a route to amine adducts of the dihalide complexes [M(CpN)X2(NHMe2)] (M = Zr, X = Cl (5), I (6); M = Hf, X = I (7)). Aminolysis of 3 with 1 equiv of Me2NH.HI gives the mixed amido-iodide [Zr(CpN)I(NMe2)] (8). The dichloride 5 is an excellent precursor to alkyl complexes; thus, reaction of 5 with C6H5CH2MgCl (2 equiv) gives [Zr(CpN)(CH2Ph)2] (9), that of 5 with Me3-SiCH2Li gives [Zr(CpN)(CH2SiMe3)2] (10), and that of 5 with C6H5CH2MgCl (1 equiv) gives the mixed complex [Zr(CpN)(CH2Ph)Cl] (11). The H-1 and C-13 NMR data for 9 and 10 are discussed in terms of eta2-benzyl (9) and agostic (10) interactions in these formally 14-valence-electron compounds. The reaction of 5 with LiBH4 gives [Zr(CpN)(eta3-BH4)2] (13). Aminolysis of 3 with cyclopentadiene gives [Zr(CpN)(eta-C5H5)(NMe2)] (14), while 3 also reacts with phenylacetylene to give the poorly stable bis(acetylide) [Zr(CpN)(CCPh)2] (15). Compound 11 has been characterized as a benzene-d6 solvate by X-ray crystallography: (C32H40Cl2N2Zr2)0.5-(C6D6), M(r) = 437.17, monoclinic, P2(1)/c, a = 11.649(1) angstrom, b = 10.360(1) angstrom, c = 17.802(1) angstrom, 106.750(4)-degrees, V = 2057.3(3) angstrom3, Z = 4, D(x) = 1.411 g cm-3, lambda(Mo KalphaBAR) = 0.710 73 angstrom, mu = 6.6 cm-1, F(000) = 888, T = 295 K, and R(F) = 0.035 for 3691 unique observed reflections with I greater-than-or-equal-to 2.5sigma(I) and 260 parameters. The crystal structure consists of dimer molecules of 11, obeying symmetry 1BAR and coupled by two bridging Cl atoms, with two different Zr-Cl distances of 2.6288(8) and 2.6797(9) angstrom, respectively.
In this study valence force field parametrizations for zirconocene dichloride complexes are reported. The three models developed and investigated provide complete sets of parameters that can be used either in standard force fields without software extensions for the treatment of pi-ligands or in force fields that can handle the multiple-reference problem encountered in highly symmetric coordination compounds as well as in force fields where a distributed-forces model for the treatment of centrosymmetric ligands is available. The new parameters have been tested with a series of 21 nonbridged and 12 ansa-metallocene complexes. The agreement between X-ray structural data and the results of the three model calculations is excellent. On the average, bond lengths are reproduced to better than 2 pm, bond angles to 1-degrees, and the torsion angles defining the relative orientations of the ligands in the bent-metallocene conformers to about 60 for complexes exhibiting no significant intermolecular interaction in the solid state. Additionally, the new series of parameter sets presented allows for a good reproduction of the experimental vibrational data.
Nonlocal density functional (DF) calculations have been carried out on the insertion of ethylene into the metal-CH3 bond of the Kaminsky type metallocenes: CP2ZrCH3+ (2a), (SiH2CP2)ZrCH3+ (3a), CP2ScCH3 (4a), and the constrained geometry catalyst (CGC) (SiH2(Cp)NH)ZrCH3+ (5a). The objective has been to study the energy profile for the insertion involving 2a and how it is modified by introducing the silane bridge in 3a, replacing one Cp with NH in 5 or switching to the neutral system 4a. The DF calculations reveal that the insertion into the bis-Cp systems proceeds with a modest barrier of 3, 4, and 14 kJ/mol for 2-4, respectively. This barrier is marginally influenced by going from the charged species CP2ZrCH3+ (2a) to the neutral system CP2ScCH3 (4a) and is unchanged in going from the unbridged system CP2ZrCH3+ (2a) to the silane bridged system (3a). The systems 2a, 3a, and 4a all form a pi-complex with a very shallow minimum at the beginning of the insertion process. The insertion process for the zirconium CGC (5a) exhibits a clear activation barrier and a pronounced minimum for the pi-complex. The deeper potential well of the pi-complex in 5a compared to 2a, 3a, and 4a can be related to a reduction in the steric interaction between ethylene and the ligands on the metal center as one Cp ring is replaced by a NH ligand. The higher barrier in the case of 5a is a consequence of the stable pi-complex which has to be abandoned in order to proceed to the insertion product. Calculations have also been carried out on two chain terminating steps. The first step involved beta-hydride elimination. It is concluded that this process is unfavorable for early transition metal centers with an endothermicity of 176 kJ/mol. The second step is concerned with the activation of an ethylene C-H bond by the metallocene to form an alkane and a vinylzirconocene. It is concluded that this process is viable as a chain terminating step.
Synthesis of rac-[((1,2,3,4-tetraphenyl-1,3-butadiene-1,4-diyl)germylene)bis(1-η 5-indenyl)]dichlorozirconium (1) was achieved with substantial suppression of the undesired meso diastereomer as predicted by MM2 force field calculations. The corresponding zirconocenium ion of 1 polymerizes propylene with high catalytic activity and stereospecificity to very high molecular weight. The characteristics of this catalysis are essentially independent of polymerization temperature over a broad temperature range.
Nonlocal density functional (DF) calculations have been carried out on the insertion of ethylene into the metal-CH3 bond of the Kaminsky type metallocenes (KTM) Cp2ZrCH3+ (2), (SiH2CP2)ZrCH3+ (3), Cp2ScCH3+ (4), and (SiH2(Cp)NH)ZrCH3+ (5).
The amine elimination reaction of 1,2-bis(3-indenyl)ethane (3) and Zr(NMe(2))(4) (2) affords pure rac-(EBI)Zr(NMe(2))(2) (4; EBI = 1,2-ethylenebis(1-indenyl)) in. 68% isolated yield. Treatment of 4 with 2 equiv of Me(2)-NH.HCl affords rac-(EBI)ZrCl2 (1) in 92% isolated yield. Compound 1 can, also be prepared directly from 2 and 3 in a ''one-pot'' synthesis in 69% isolated yield.
Ab-initio Molecular Dynamics simulations on ethylene insertion in a bridged di(cyclopentadienyl) methylzirconocene have revealed that the entire reaction path starting from the pi-coordinated ethylene-zirconocene complex up to and including propyl formation takes place in about 150 femto-seconds, which is unexpectedly fast, and suggesting the absence of any significant barrier of activation. Starting from a reactant structure without alpha-H agostic interaction, at T=400 K such an interaction evolves during the course of the insertion reaction and before the propyl is formed. The product state exhibits gamma-H agostic interaction.
Models for the likely active catalysts in homogeneous Ziegler-Natta systems have been studied using ab initio quantum chemical methods. We investigated the geometries of the isoelectronic model complexes, X[sub 2]M-R where X = Cl or Cp = ([eta][sup 5]-C[sub 5]H[sub 5]); where M = Sc and Ti[sup +] (and also Ti); and where R = H, CH[sub 3], or SiH[sub 3]. The general trend is that the M = Sc compounds strongly prefer a planar configuration, whereas the M = Ti[sup +] cases generally prefer pyramidal geometries. This difference in geometry can be related to the differing ground-state electronic configurations for the metals: Sc is (4s)[sup 2](3d)[sup 1], whereas Ti[sup +] is (4s)[sup 1](3d)[sup 2]. The nonplanar geometry for [Cp[sub 2]Ti-R][sup +] suggests an explanation for the origin of stereospecificity in the syndiotactic polymerization by unsymmetric metallocene catalysts. These results suggest that ([eta][sup 5]-C[sub 5]H[sub 4])CMe[sub 2]([eta][sup 5]-fluorenyl)Sc-R would not catalyze syndiotactic polymerization under these conditions. 31 refs., 5 figs., 7 tabs.
Investigation of the polymerization process and property modification of metallocene-based cycloolefin copolymers (COC) shows that this monomer-catalyst combination enables synthesis of a new class of transparent thermoplastic polymers. Metallocene catalysis offers much opportunity to vary the composition and microstructure of the copolymers. The breadth of the polymer spectrum which can be further widened by polymer modification enables many applications for this new class of thermoplastics. The key parameters that control structure and properties in the COC family are discussed in this paper.Untersuchungen wesentlicher Aspekte der Polymerisation und der Eigenschaftsmodifikation auf Metallocen-Katalyse basierender Cycloolefin-Copolymere (COC) zeigen, daß mit dieser Monomer-Katalysator-Kombination eine neue Klasse transparenter Thermoplasten zugänglich gemacht wird. COC können mit Metallocenen in beliebiger zusammensetzung und mit kontrollierter Mikrostruktur hergestellt werden. Die somit erreichte und durch zusätzliche Modifikationen noch erweiterbare Breite des Eigenschaftsspektrums ermöglicht vielfältige Anwendungen dieser neuen Polymerklasse. Die wichtigsten Schlüsselfaktoren im rohstofflich-technologischen Bereich, die diese Struktur- und Eigenschaftsvielfalt ergeben, werden beispielhaft beschrieben.
Stereocontrol energy (ΔE0) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η5-indenyl) - (1) and rac-ethylenebis (1-η5-4,5,7,8-tetrahydroindenyl) (2) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is -agostic interaction; it is lowered in the cases of β and agostic interactions. The 13C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE0. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc.