Comparison of Ti, Zr, and Hf as Cations for Metallocene-Catalyzed Olefin Polymerization (original) (raw)
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Inorganic Chemistry, 2007
This article describes the syntheses of two covalently linked oxygen-bridged hybrid metallocene−nonmetallocene polymetallic catalysts bearing two different catalytically active group 4 metals. The reactions of Cp* 2 (Me)Zr(OH) (2) with Ti(NMe 2 ) 4 and Hf(NMe 2 ) 4 led to the formation of a heterobimetallic compound Cp* 2 (Me)Zr(µ-O)Ti(NMe 2 ) 3 (7) and a trimetallic derivative Cp* 2 (Me)Zr(µ-O)Hf(NMe 2 ) 2 (µ-O)Zr(Me)Cp* 2 (9), respectively, under the elimination of Me 2 NH. The crystal data confirm the molecular structures of 7 and 9, which crystallize in the space groups P1 h and P2 1 /n, respectively. 9 is the first example of a crystallographically characterized heterotrimetallic complex having a Zr−O−Hf−O−Zr core. 7 bearing two active catalytic centers, namely, zirconium and titanium, exhibits bimodal activity in olefin polymerization when activated with methylalumoxane (MAO). It produces polyethylene largely controlled by the zirconium center, and polystyrene seems to be formed predominantly by the titanium center. DFT calculations were performed on the supposed cationic intermediates, revealing that a cation generated on the titanium center is sterically more accessible for monomer binding, though it is energetically less-favorable than that generated on the zirconium center.
Comptes Rendus Chimie, 2005
Cp 2 Zr(CH 3 )R and its derivatives can serve as powerful olefin polymerization catalysts after activation by a Lewis acid A to form the ion-pair [Cp 2 ZrR] + [CH 3 A] -(I) which is held together by a Zr-µ-CH 3 Abridge. It is generally assumed that the cation Cp 2 ZrR + (II) of I is the active species whereas the influence of the anion CH 3 A -(III) is less well understood. We have conducted an extensive study based on density functional theory (DFT) of ethylene polymerization catalyzed by both I and II in order to probe the influence of the anion CH 3 Afor the case where A = B(C 6 F 5 ) 3 . The reaction between ethylene and the cation II leads in the first place to a p−complex in a highly exothermic and exogonic reaction without any (uptake) barrier. Even the subsequent insertion process has a transition state that is lower in energy than II and C 2 H 4 at full separation. The only (internal) barrier is the modest energy required to proceed from the p-complex to the insertion transition state. For the reaction between the ion-pair and ethylene the monomer can approach cis or trans to the Zr-µ-CH 3 Abridge. In addition with R = Pr, cis and trans approaches are possible for 4 different orientations of the propyl chain. We find for all of these approaches that the rate determining step is the (partial) displacement of the anion CH 3 A -. For the first insertion (R = Me) the total insertion barrier is 13.0 kcal/mol for the most favorable (trans) approach. The second (R = Pr) insertion (which likely also is a good model for subsequent propagation steps) prefer a cis-approach in which the ethylene uptake barrier of 9.5 kcal/mol is rate determining whereas the barrier for the subsequent insertion process only is 6.8 kcal/mol. Displacement of the anion was found to be more pronounced for R = Pr than R = Me and larger for the insertion transition state than the uptake transition state. Solvation effects were seen to stabilize anion displacement and thus reduce especially insertion barriers. Thus for the favored cis-path of the second propagation, the insertion was the rate determining step in the gas-phase with a barrier of 10.0 kcal/mol whereas the corresponding uptake process became the rate determining step in solution with a barrier of 8.6 kcal/mol. After the insertion, the ion-pair was found to recombine completely so that ethylene will have to displace the anion in the next propagation step. Considerations were also given to the chain termination step by transfer of hydrogen to the monomer. Here the rate determining step is again cis-uptake off ethylene with a barrier of 12.5 kcal/mol whereas the subsequent hydrogen transfer barrier only is 10.6 kcal/mol.
Polymer Science Series B, 2011
The polymerizations of propylene and ethylene with two postmetallocene catalysts [(4R,5R)-2,2dimethyl-α,α,α',α'-tetra(perfluorophenyl)-1,3-dioxolane-4,5-dimethanol]TiCl 2 · MgCl 2 and [(4R,5R)-2,2-dimethyl-α,α,α',α'-tetra-(perfluorophenyl-1,3-dioxolane-4,5-dimethanol]TiCl 2 ⋅ (LiCl) 2 are studied. The first catalyst shows higher activity in both reactions and forms a lower molecular mass PP. This PP is characterized by a wide molecular-mass distribution that can be described by a set of five or six Flory components with different average molecular masses. Along with heterogeneity with respect to kinetic parameters, there is heterogeneity with respect to stereospecificity. Some of the sites form a high-molecular-mass highly isotactic polymer whose melting point is ≥150°C, whereas other sites produce syndiotactic and atactic PPs. For the most isospecific sites, a stereocontrol mechanism similar to the mechanism typical for metallocene catalysts with C 1 -symmetry is advanced. The catalysts under study are composed of the [(4R,5R)-2,2dimethyl-α,α,α',α'-tetra(perfluorophenyl)-1,3-dioxolane-4,5-dimethanol]TiCl 2 complex supported on LiCl and MgCl 2 , respectively.
2004
New group IV compounds containing a 9-dialkylaminofluorenide ligand have been prepared. Their properties and reactivity have been investigated. For example, single crystal X-ray analysis of the metallocene (9-(N,N-dimethylamino)fluorenyl)2ZrCl2 (4) reveals a bonding mode that includes a zirconium-nitrogen bond. In combination with methylaluminoxane (MAO), several aminofluorenide complexes afford atactic polypropylene. A statistical model has been developed that quantifies the unidirectional site epimerization probability, e, for singly- and doubly-bridged C1- symmetric metallocene polymerization catalysts. The unidirectional site epimerization model allows deconvolution of the site sequence probability and the stereochemical probability for producing a given pentad. Highly stereoregular syndiotactic polypropylene is obtained with the catalyst systems Ph2C(Oct)(C5H4)ZrCl2/MAO (8/MAO) (Oct = octamethyloctahydrodibenzofluorenyl, C-29H36) and Me2C(Oct)(C5H4)ZrCl2/MAO (12/MAO). Melting t...
From Unstable to Stable: Half-Metallocene Catalysts for Olefin Polymerization †
Inorganic Chemistry, 2008
The reaction of LAlMeOH [L) CH(N(Ar)(CMe)) 2 , Ar) 2,6-i-Pr 2 C 6 H 3 ] with CpTiMe 3 , Cp*TiMe 3 , and Cp*ZrMe 3 was investigated to yield LAlMe(µ-O)TiMe 2 Cp (2), LAlMe(µ-O)TiMe 2 Cp* (3), and LAlMe(µ-O)ZrMe 2 Cp* (4), respectively. The resulting compounds 2-4 are stable at elevated temperatures, in contrast to their precursors such as CpTiMe 3 and Cp*ZrMe 3 , which already decompose below room temperature. Compounds 2-4 were characterized by singlecrystal X-ray structural analysis. Compounds 2 and 3 were tested for ethylene polymerization in the presence of methylaluminoxane. The half-metallocene complex 3 has higher activity compared to 2. The polydispersities are in the range from 2.8 to 4.2. A copolymerization with styrene was not observed.
Macromolecules, 1993
In order to study the formation and the nature of WAl adducts that play a role in the olefin polymerization activity of the group 4 rnetdlocene-methylaluminoxam catalytic systems, the reectiom of AMand/or methylaluminoxam with CplTiMeCl and CplTiMe have been monitored by ' H and '42 NMR spectroecopy. Temperatures between 203 and 283 K and Al/Ti ratios from 1 to 40 have been used. The change in 142 NMR signals has been found the most diagnostic. Several adducts have been obeerved and identified. Thekindofadductachangeebothwiththe reactiontemperatureandthe WTiratio. By comparing the MA0 and AlM-reactions with CplTiMeCl it is poeeible to deduce that, with respect to AM-, MA0 is a better alkylating agent and it has a greater capacity for producing and stabilizing cation-like complexes. At the Al/Ti ratio of 40 a complex that we tentatively assign as a solvent separated ion pair prevails. The fact that, after the addition of CH&l2 to the toluene solution, this complex is present in greater amounta is consistent with this assignment.
Macromolecular Rapid Communications, 1997
The thermodynamics of the reaction of an ethylene molecule with the Cp2TiCH3Cl/ A1(CH3),C1 system (Cp = q5-C5H5), as a model for olefin polymerization with homogeneous Ziegler-Natta catalysts, was investigated via quantum mechanical DFF calculations. The comparison of the calculated energies for three possible titanium-olefin coordinated intermediates, the ionic complex Cp,TiCH3(C,&)'//A1(CH3)zCl;, the bimetallic complex Cp,TiCH,(C,&)" Al(CH3)2CI~-and the olefin-separated ion pair Cp,TiCH:/ C2H4/Al(CH3)2Cl;, shows that the most feasible polymerization mechanism occurs via olefin-separated ion pair.
Macromolecules, 2000
A kinetic model was developed to describe the propylene polymerization behavior of fluxional, two-state metallocene catalysts. In particular, the pentad and molecular weight distributions can be described as well as other parameters of interest, such as the weight fraction of crystallizable sequences and the isotactic sequence length distribution, in terms of fundamental kinetic constants and polymerization conditions that pertain to these two-state catalyst systems. The model was used in an attempt to describe the polymerization behavior of two, prototypical, fluxional catalyst systems, (2-PhInd) 2ZrCl2/ MAO (1) and (2-p-CF3PhInd)2ZrCl2/MAO (2). The model can accurately reproduce the pentad distributions observed in PP prepared using these catalysts and the response of the distribution to changes in polymerization conditions, specifically changes in [C3H6] at constant T. These studies illustrate that the rate of state-to-state interconversion is slower but of comparable magnitude to the rate of monomer insertion and that the states have similar stability and reactivity. The broad molecular weight distributions previously observed with this family of catalysts can be described by the model. However, the model predicts that the state-to-state interconversion rate has to be significantly slower than the rate of formation of dead polymer chains, and this is inconsistent with the rate estimated from the response of the pentad distribution to changes in the rate of propagation (i.e., [C 3H6]). Recent work where propylene polymerizations using 1 were carried out to low conversion indicate that the broad MWD seen in earlier studies is partly related to variations in [C3H6] during polymerization.