Precatalysts For Polymerization Research Articles

Quantum-chemical calculations of the effect of cycloaliphatic groups in α-diimine and bis(imino)pyridine ethylene polymerization precatalysts on their stabilities with respect to deactivation reactions

For the first time, it is attempted to interpret an experimentally found enhancing effect of cycloaliphatic substituents in aromatic rings of Ni II- and Pd II-α-diimine and Fe II-bis(imino)pyridine ethylene polymerization precatalysts on their catalytic activities at elevated temperatures (60–80 °C), using quantum chemical density functional theory calculations of relative stabilities of the complexes with respect to different deactivation processes, including thermal decomposition and one-electron reduction. It was shown that the effect correlates with the calculated higher thermal stabilities of cycloalkyl-substituted Fe II-, Ni II- and Pd II-complexes as compared to the corresponding alkyl-substituted ones. Ni II- and Pd II-α-diimine complexes with cycloalkyl substituents are shown to be more stable than their alkyl-substituted analogues with respect to both thermal decomposition and one-electron reduction. The averaged difference of the thermal decomposition energies between the complexes with cycloaliphatic substituents on one side and aliphatic ones on the other side is ∼2.3 kcal/mol, corresponding to ∼30 times lower equilibrium constant of the thermal decomposition reaction for the cycloalkyl-containing complexes. For the Fe II- and Pd II-complexes, the thermal stability correlates with the calculated overlap population of the metal–nitrogen bonds. It was shown that the structure of o-substituents (cycloalkyls vs. alkyls) in the phenyl rings of the ligands does not affect the reaction energies for the transformation reactions of the precatalysts into their corresponding active catalytic cationic forms.

Iminohydroxamato Early and Late Transition Metal Halide Complexes − New Precatalysts for Aluminoxane‐Cocatalyzed Olefin Insertion Polymerization

AbstractWe report on new families of non‐metallocene metal precatalysts for olefin polymerization with titanium, zirconium, vanadium and nickel as the active metal sites. The novel ligand design concept is based on iminohydroxamic acids and their derivatives as the principal chelating units. Various anionic and neutral [N,O] and [N,N] ligand systems are easily accessible by a modular synthetic sequence of imidoyl chlorides with substituted hydroxylamines or hydrazines, respectively. Steric protection of the metal coordination site, a necessary requirement for suppression of chain termination pathways of non‐metallocene catalysts, is brought about by bulky aryl substituents on the imino nitrogen atoms. Crystal structures of some of the hydroxamato ligands reveal interesting intermolecular hydrogen‐bridged structures, whereas in the solid‐state structure of one titanium precatalyst a five‐membered chelate was observed, in line with the design principle of these systems. Preliminary ethylene polymerization studies with methylaluminoxane‐activated metal complexes (M = Ti, Zr, V, Ni) show that the most active systems are [N,O]NiBr2 catalysts containing neutral O‐alkyl iminohydroxamate ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Homo- and heterometallic carboxylate cage complexes as precatalysts for olefin polymerization—Activity enhancement through “inert metals”

The polymerization behavior of novel polynuclear cage complexes as precatalysts in the vinyl or addition polymerization of norbornene has been investigated and correlated with the results of the known mononuclear precatalysts M(acac) x (M=Ni II, Co III, Cr III, and Fe III, acac=acetylacetonate, x=2 or 3) and mixtures thereof. The cage complexes can be activated with the Lewis acids methylalumoxane (MAO) as well as with tris(pentafluorophenyl)borane, B(C 6F 5) 3, in combination with or without triethylaluminum (AlEt 3). It is shown that nickel is the most active metal in the polymerization of norbornene with heterometallic precatalysts. The homo- and heterometallic nickel cage compounds reveal a maximum activity per nickel in the presence of “inert metal” atoms—an effect which is not seen when Ni(acac) 2 is only physically mixed with other metal-acac salts. The inert metal effect may be advantageously applicable to other valuable catalysts when used as heterometallic polynuclear compounds.

Ferrous rather than ferric species are the active sites in bis(imino)pyridine iron ethylene polymerization catalysts

1H NMR and EPR spectroscopic studies show that the iron centers in bis(imino)pyridine iron(II) and bis(imino)pyridine iron(III) olefin polymerization pre-catalysts LFeCl 2 and LFeCl 3 (L=bis(imino)pyridine) are converted upon treatment with methylaluminoxane (MAO) into ferrous complexes of the type [L(Me)Fe(II)(μ-Me) 2AlMe 2] or [LFe(II)(μ-Me) 2AlMe 2] +[Me-MAO] −.

New unsymmetrical thioether- and thiolate-substituted ferrocenediyl ligands and their metal complexes

A series of new unsymmetrical 1,1′-disubstituted ferrocenediyl ligands featuring thioether or thiolate substituents have been conveniently synthesised. The chelating nature of the ligands is shown by coordination with a range of transition metal centres and bridged metal dimeric species are observed with platinum and palladium. Electrochemical studies show that the diferrocene species is a partially delocalised Robin–Day Class II system, whilst on coordination of metal centres to the monoferrocene ligands, the oxidation potential of the ferrocene subunit is greatly increased ranging from 0.25 to 0.60 V upon passing from platinum(II) to palladium(II) ions. Interestingly, the outer ferrocenediyl subunits of the dimeric complexes indicate electronic interaction through the diplatinum or dipalladium backbones. The chromium- and vanadium-containing species act as low activity, pre-catalysts for ethylene polymerisation.

Olefin Polymerization with [{bis(imino)pyridyl}Co(II)Cl(2)]: Generation of the Active Species Involves Co(I) This research was sponsored by CW/STW and Basell Technology Company.

Activated by reduction: On treatment with methylaluminoxane (MAO), the Brookhart/Gibson cobalt-based polymerization precatalyst [LCoIICl2] is first reduced to [LCoICl], subsequently alkylated to [LCoIMe], and finally converted into the (as yet unknown) active species (see scheme, PE=polyethene).

A Density Functional Study of Ion-Pair Formation and Dissociation in the Reaction between Boron- and Aluminum-Based Lewis Acids with (1,2-Me2Cp)2ZrMe2

Calculations have been carried out on the reaction between the olefin polymerization precatalyst (1,2-Me2Cp)2ZrMe2 (P) and a number of Lewis acids (A) to form the methide-bridged (contact) ion pair (1,2-Me2Cp)2ZrMe(μ-Me)A (I; P + A → I + ΔHipf). This is the first step in the activation of P to becoming an olefin polymerization catalyst. The calculated enthalpies of formation (ΔHipf) for I were as follows (A, ΔHipf in kcal/mol): B(C6F5)3 (1a), −23.8; B(C6F5)2(C6H3F2) (1b), −21.5; B(C6F5)2(C6H5) (1c), −18.3; B(C6F5)2(C6H3(CH3)2 (1d), −18.0; B(C6H5)3 (1e), −6.7; B(C10F7)3 (1f), −25.8; (MeAlO)6 (2a), −15.9; (MeBO)6 (2b), −22.3; AlMe3 (2c), −8.1; Al(C6F5)3 (2d), −30.8. The charge separation between the (1,2-Me2Cp)2ZrMe+ and AMe- fragments in I was calculated for all A, and it was found that the charge separation as well as −ΔHipf increases through the series 1e, 1c, 1b, and 1a with the number of fluorine atoms. A good activating Lewis acid (A) has the equilibrium shifted strongly from P and A toward I, and this is the case for all A except 1e and 2c. Also considered was the complete dissociation in solution (toluene) of I into the counterions [(1,2-Me2Cp)2ZrMe]+ and AMe- with the dissociating enthalpy ΔHips as well as the formation from I of the solvent-separated ion pair (S = toluene) [(1,2-Me2Cp)2ZrMe]+−S−[AMe]- with the reaction enthalpy ΔHss. The two types of separation processes have both been postulated as the second and final steps in the activation of P. The calculated values are as follows (A, ΔHips and ΔHss in kcal/mol): 1a, 38.0, 18.7; 1f, 43.6, 18.9; 2a, 57.0, 32.4; 2b, 46.9, 35.3; 2c, 69.2, 35.3; 2d, 48.3, 20.6. It is concluded that the formation of [(1,2-Me2Cp)2ZrMe]+−S−[AMe]- is the more likely separation process. Consideration has also been given to the influence of solvent polarity on the separation process, with S = toluene, chlorobenzene, and 1,2-dichlorobenzene. Finally discussed are ΔHips and ΔHss for the ion pair [(1,2-Me2Cp)2ZrMe]+[A]- (III‘); A- = B(C6F5)4- (3a), Al(C6F5)4- (3b), [(C2B9H11)2Co]- (3c), and {tBuCH2CH[B(C6F5)2]H}- (3d)), where III‘ is formed from the reaction of P with the activator [CPh3+][A-]. Here the calculated values are as follows (A-, ΔHips and ΔHss in kcal/mol): 3a, 22.1, −4.2; 3b, 26.2, 0.7; 3c, 34.9, 7.5; 3d, 26.7, −0.5. It is found that III type ion pairs are easier to dissociate than I held together by a methide bridge.

Polymerization of olefins with titanium and zirconium complexes containing hydrotris(pyrazolyl)borate or hydrotris(3,5-dimethylpyrazolyl)borate

Zirconium and titanium complexes containing hydrotris(pyrazolyl)borate (Tp) or hydrotris(3,5-dimethylpyrazolyl)borate (Tp *) as a ligand have been examined as polymerization precatalysts for ethylene, propylene and styrene. Activation of the group 4 transition-metal complexes by methylaluminoxane exhibits a greater catalytic activity in ethylene polymerization for TpMCl 3 (M=Zr, Ti) than for the corresponding mono-cyclopentadienyl analogue, CpMCl 3. Dimethyl substitution on the pyrazolyl groups enhances the activity for the Zr complex (Tp *ZrCl 3), but not for the Ti complex (Tp *TiCl 3). Complete substitution of O tBu or OPh groups for Cl ligands in Tp *ZrCl 3 greatly decreases the activity. Mono substitution of an OPh group for one Cl ligand in Tp *TiCl 3 does not affect the activity. The addition of propylene comonomer reduces the catalytic activity of Zr complexes containing Tp or Tp * to a considerable degree. The Tp and Tp * complexes of Zr and Ti produce atactic polymers from styrene.

The synthesis of olefin polymerization precatalysts: [TiCl 4(C 4H 7OCH 2CO 2CH 3)] and [(C 4H 7OCH 2CO 2Et) 2 Mg(μ-Cl) 2TiCl 4] · 2CH 2Cl 2. Crystal structures and properties

Direct reactions of TiCl 4 or of a 1:1 mixture of TiCl 4 and MgCl 2 with chiral tetrahydrofurfuryl C 4H 7OCH 2CO 2CH 3 and C 4H 7OCH 2CO 2Et esters, respectively, gave [TiCl 4(C 4H 7OCH 2CO 2CH 3)] ( 1) and [(C 4H 7OCH 2CO 2Et) 2Mg(μ-Cl) 2TiCl 4] · 2CH 2Cl 2 ( 2) which are good precatalysts for ethylene polymerization. The crystal structures of 1 and 2 have been solved by X-ray methods. In 1 the titanium atom is octahedrally coordinated by four chlorine atoms and two oxygen atoms of the chelate tetrahydrofurfuryl acetate ligand. In the heterobimetallic complex 2, formed by slightly distorted edge-sharing octahedra, the titanium and magnesium atoms are separated by Ti ⋯ Mg of 3.725(16) Å and linked through two bridging Cl atoms.