Chelating diamide complexes of titanium: new catalyst precursors for the highly active and living polymerization of α-olefins

The reaction of RHN(CH₂)₃NHR (1a,b) (a, R=2,6-ⁱPr₂C₆H₃; b, R=2,6-Me₂C₆H₃) with 2 equiv of BuLi followed by 2 equiv of ClSiMe₃ yields the silylated diamines R(Me₃Si)N(CH₂)₃N(SiMe₃)R (3a,b). The reaction of 3a,b with TiCl₄ yields the dichloride complexes [RN(CH₂)₃NR]TiCl₂ (4a,b) and two equiv of ClSiMe₃. An X-ray study of 4a (P2₁/n, a=9.771(1) Å, b=14.189(1) Å, c=21.081(2) Å, β=96.27(1)°, V=2905.2(5) ų, Z=4, T=25°C, R=0.0701, R_w=0.1495) revealed a distorted tetrahedral geometry about titanium with the aryl groups lying perpendicular to the TiN₂-plane. Compounds 4a,b react with 2 equiv of MeMgBr to give the dimethyl derivatives [RN(CH₂)₃NR]TiMe₂ (5a,b). An X-ray study of 5b (P2₁2₁2₁, a=8.0955(10) Å, b=15.288(4) Å, c=16.909(3) Å, V=2092.8(7) ų, Z=4, T=23°C, R=0.0759, R_w=0.1458) again revealed a distorted tetrahedral geometry about titanium with titanium–methyl bond lengths of 2.100(9) Å and 2.077(9) Å. These titanium dimethyl complexes are active catalysts for the polymerization of 1-hexene, when activated with methylaluminoxane (MAO). Activities up to 350,000 g of poly(1-hexene)/mmol catalyst·h were obtained in neat 1-hexene. These systems actively engage in chain transfer to aluminum. Equimolar amounts of 5a or 5b and B(C₆F₅)₃ catalyze the living aspecific polymerization 1-hexene. Polydispersities (M_w/M_n) as low as 1.05 were measured. Highly active living systems are obtained when 5a is activated with {Ph₃C}⁺[B(C₆F₅)₄]⁻. A primary insertion mode (1,2 insertion) has been assigned based on both the initiation of the polymer chain and its purposeful termination with iodine.     

β-Diketiminate complexes of Group 4: active complexes for the isomerization of α-olefins and the polymerization of propylene towards elastomeric polypropylene

The β-diketiminate lithium ligand [{N(SiMe₃)C(Ph)}₂CH][Li] reacted with Group 4 metal salts (Ti and Zr) to yield the complexes [{N(SiMe₃)C(Ph)}₂CH]₂TiCl₂ (5) and [{N(SiMe₃)C(Ph)}₂CH][N(SiMe₃)C(Ph)NC(Ph)CH(SiMe₃)]ZrCl₂ (6). The crystal structure of 6 shows that one of the two ketamidinate ligands undergoes an isomerization to the corresponding substituted benzamidinate. A mechanism for the catalyzed isomerization of the β-diketiminate ligand is presented. Complex 5 was found to be active in the polymerization of propylene, producing remarkably high-molecular weight (>100,000 g mol⁻¹) elastomeric polymer, whereas the zirconium complex was found inactive. Complex 5, and surprisingly complex 6, were found to be active catalysts, in the presence of MAO (methylalumoxane), for the isomerization of aliphatic olefins (1-octene, allylbenzene). Each complex catalyzed the olefins by different mechanisms. Kinetic studies for the isomerization of allylbenzene by complex 5 show that the reaction rate follows a first order in both catalyst and olefin concentrations with ΔH^‡=14.7(4) kcal mol⁻¹ and ΔS^‡=−33(1) e.u. These findings support the epimerization mechanism of the last inserted monomer that is proposed for as the primary cause for the elastomeric properties of polypropylene produced by this complex. In addition, we show how complementary isomerization studies of α-olefins shed light on the polymerization mechanism.     

A low-symmetry nickel(II) α-diimine complex for homopolymerization of ethylene: study of interactive effects of polymerization parameters

A low-symmetry nickel(II) α-diimine complex [(2,4,6-trimethylphenyl)imino)-((2,6-diisopropylphenyl)imino)-acenaphthene nickel(II) dibromide] (C1) was synthesized and characterized crystallographically. Response surface method (RSM) was employed to study the interactive effects of critical factors on ethylene polymerization. The maximum activity of C1, when activated by methylaluminoxane (MAO) cocatalyst {4960 kgPE [(mol Ni)^?1 h^?1]} was achieved at 10 °C, 7 bar and CC (cocatalyst-to-catalyst ratio) equal to 1550, which is higher than most similar counterparts. Considering surface plots of MW, it is concluded that at high levels (>1000) of CC, there is competition between chain transfer to aluminum and reinsertion of macromonomer to newly formed metal-methyl bonds.     

Highly active zinc alkyl cations for the controlled and immortal ring-opening polymerization of ε-caprolactone

Zinc alkyl cations supported by N,N-BIAN-type bidentate ligands were found to be highly active in the immortal ROP of ε-caprolactone to yield narrowly disperse and chain length-controlled poly(ε-caprolactone), whether in solution or bulk polymerization conditions.     

Multi-walled carbon nanotubes as a ligand in nickel α-diimine based ethylene polymerization

Two novel heterogeneous nickel ?-diimine based polymerization catalysts, containing MWCNT as the main ligand, were synthesized by novel in situ catalyst preparation technique. The in situ synthesis was performed by covalent attachment of the acenaphthenic ligand core to amine functionalized MWCNT ligand arms through diimine bonding and further nickel dibromide chelation. The prepared catalysts were fully characterized and their structures and supporting efficiencies were determined. Single or double introduction of the MWCNTs through their ends or sidewall(s) in the catalytic system, as a ligand, influenced the catalytic performance, microstructure and morphology of obtained polyethylenes. MWCNT sidewall bonding to para-aryl position of the tetramethylphenyl moiety performed as more electron-donating ligand than MWCNT ends linked to the imine bond and protected the catalytic system to retain its activity. This character resulted in the maintenance of the resulting polymer topology at elevated temperatures so that the catalytic activity and the obtained polymer melting points remained around 110 g PE?mmol?1 Ni?h?1 and 123 ℃ in all polymerization temperatures respectively. In polymerization trials, molecular weight fall against temperature was not as sharp as what had been observed in sequentially prepared catalysts insofar as the molecular weight of resultant polymer at 60 ℃ reached to 310000 g?mol?1 which was close to the highest value had been reported at 30 ℃ for sequentially prepared catalysts. TEM observations showed the presence of the stopped-growth polymer chains due to geometrical constrains or ligand debonding for both catalytic systems.     

Highly active rhodium/phosphorus catalytic system for the hydroformylation of α-methylstyrene

Rhodium catalyzed hydroformylation of a-methylstyrene was investigated in the presence of monodentate phosphine ligands L1–L6. We found that the phosphine with good p-acceptability could efficiently improve the activity of the a-methylstyrene hydroformylation. The big steric hindrance of a-C in a-methylstyrene enhanced the regioselectivity towards the linear aldehyde, which resulted in3-phenylbutanal as the predominant product(99.0%). When tris(N-pyrrolyl)phosphine(L1) modified Rh(acac)(CO)_2was employed as the catalyst, the TOF could reach up to 5786 h~(-1)in the a-methylstyrene hydroformylation at relatively mild conditions(110 8C, 6 MPa).     

Polymerization of linear higher α-olefins with a modified Ziegler catalyst

The polymerization of hexene-1, octene-1 and decene-1 with a modified Ziegler catalyst based on the product of interaction of TiCl₄, Et₂AlCl, and n-Bu₂O in toluene has been studied. Et₂AlCl, i-Bu₂AlCl, and a combination of Et₂AlCl with MgBu₂ were used as cocatalysts. The addition of a small amount of MgBu₂ to Et₂AlCl resulted in a sharp increase in the catalytic system activity along with decreases in the molecular masses of the formed polymers. It has been shown that a change of [Mg]/[Al] ratio makes it possible to produce polyolefins in a wide range of molecular masses with high effectiveness. The above mentioned polymers are amorphous ultrahigh molecular materials with predominantly isotactic structure.     

Highly controlled immortal polymerization of β-butyrolactone by a dinuclear indium catalyst

An ethoxy-bridged dinuclear indium catalyst was used for the ring opening polymerization of the cyclic ester β-butyrolactone to form the biodegradable polyester poly(hydroxybutyrate) (PHB). The catalyst shows remarkable activity and control during polymerization, allowing for formation of diblock polymers. Addition of high ratios of alcohols to the catalyst leads to fast chain transfer and immortal polymerization.     

Environmentally friendly and efficient method for the synthesis of the new α,α′-diimine ligands with benzimidazole moiety

The new α,α′-diimine ligands with benzimidazole moiety were synthesized based on the rearrangement of 3-aroylquinoxalin-2(1H)-ones when exposed to 4,5-diamino-2,1,3-benzoxadiazole, 4,5-diamino-2,1,3-benzothiadiazole and 5,6-diaminoquinoxaline. Among them, we report the first examples of the new heterocyclic system namely benzo[4′,5′]imidazo[1′,2′:1,2]quinolino[3,4-b and 4,3-b][1,2,5]oxadiazolo[3,4-f]quinoxalines, which exhibits an interesting electrochemical behavior. All compounds were fully characterized by IR, ¹H and ¹³C NMR spectroscopies, and mass spectrometry.     

A highly active and selective palladium pincer catalyst for the formation of α-aryl ketones via cross-coupling

Several air and moisture stable Pd(II) pincer complexes were synthesized via oxidative addition of Pd(0) to novel PheBox pincer ligand precursors. Low loadings (1 mol%) of the Pd complex [t-BuPhebox-Me(2)]PdBr are capable of efficiently promoting the selective α-monoarylation of a variety of ketones with numerous aryl bromides in only 1 h at 70 °C with 82-99% yields.     

Ethylene polymerization on a SiH4-modified Phillips catalyst: detection of in situ produced α-olefins by operando FT-IR spectroscopy

Ethylene polymerization on a model Cr(II)/SiO(2) Phillips catalyst modified with gas phase SiH(4) leads to a waxy product containing a bimodal MW distribution of α-olefins (M(w) < 3000 g mol(-1)) and a highly branched polyethylene, LLDPE (M(w) ≈ 10(5) g mol(-1), T(m) = 123 °C), contrary to the unmodified catalyst which gives a linear and more dense PE, HDPE (M(w) = 86,000 g mol(-1) (PDI = 7), T(m) = 134 °C). Pressure and temperature resolved FT-IR spectroscopy under operando conditions (T = 130-230 K) allows us to detect α-olefins, and in particular 1-hexene and 1-butene (characteristic IR absorption bands at 3581-3574, 1638 and 1598 cm(-1)) as intermediate species before their incorporation in the polymer chains. The polymerization rate is estimated, using time resolved FT-IR spectroscopy, to be 7 times higher on the SiH(4)-modified Phillips catalyst with respect to the unmodified one.     

A new route of the reaction of EtAlCl2 with α-olefins catalyzed by Ti complexes

A new method for the synthesis of dialkyl(ethyl)alanes by the reaction of EtAlCl₂ with -olefins in the presence of Mg and a catalytic amount of Cp₂TiCl₂ (Ti(OPrⁱ)₄, Ti(OBuⁿ)₄) in THF was developed.     

Highly active and stereoselective olefin polymerization catalysts generated by the transfer-epimetallation of olefins or acetylenes with dialkyltitanium(IV) complexes: three-membered metallocycles as active catalyst sites‡

Efficient transfer-epimetallations of simple olefins and acetylenes by R₂TiL₂ reagents (R = Buⁿ, Bu^t; L = X) are readily achieved in THF at −78°C to generate titanacyclopropa(e)ne intermediates, readily capable of inserting various unsaturated addends (olefin, acetylene, nitrile). Analogous epimetallations conducted in hydrocarbons lead to the isotactic stereoselective polymerization of 1-alkenes and the cyclotrimerization of acetylenes. In place of the widely accepted Arlman-Cossee model for the active catalytic site, namely a Ti-C bond on the TiCl₃ crystal lattice, the 2-substituted-1-halotitanacyclopropyl cation formed in hydrocarbon media is proposed as the active site for stereoselective olefin polymerization.     

Polar comonomer incorporation using cationic Ni α-diimine olefin polymerization catalysts

<正>Polyolefins are an indispensable class of materials that have become the most widely produced and utilized polymers today. They are readily synthesized from cheap and abundant monomer feedstocks, such as ethylene and propylene, and are capable of achieving a vast array of thermal and mechanical properties based upon their composition and to-     

Polymerization of styrene with tetradentate chelated α-diimine nickel(II) complexes/MAO catalyst systems: Catalytic behavior and microstructure of polystyrene

The recently reported N,N,N,N-tetradentate chelated α-diimine Ni(II) complexes 1–3 were demonstrated to be efficient precatalysts for the polymerization of styrene in the presence of MAO, producing isotactic-enriched heterotactic polystyrenes (isotactic contents in the range 73–75.5%). The corresponding sequence distributions were determined by NMR spectroscopy, and fitted to six probabilistic models of chain propagation. A better conformity to a Bovey–first order Markov chain propagation model was obtained, indicating a chain-end control mechanism. A comparison of the microstructural features with other polystyrenes obtained by several different types of initiation indicates that the polystyrenes are formed by a coordination insertion polymerization mechanism at the nickel centers.     

Bimetallic aluminum alkyl complexes as highly active initiators for the polymerization of ε-caprolactone

Two dinuclear aluminum alkyl complexes supported by a piperazidine-bridged bis(phenolato) group were prepared, and both complexes exhibited extremely high activity for the ring-opening polymerization of ε-caprolactone. In the presence of benzyl alcohol (BnOH), the polymerization accelerated dramatically.     

Practical and theoretical study on the α-substituent effect on α-diimine Nickel(II) and Cobalt(II)-based catalysts for polymerization of ethylene

The Late transition metal catalysts based on Ni(II) and Co(II) were synthesized and their structure and activity in polymerization of ethylene were compared. Methylaluminoxane (MAO) was used as a co-catalyst. To discover the optimum polymerization conditions, the effect of polymerization temperature, monomer pressure, [Al]: [Ni] molar ratio and time of polymerization were studied. Activity of the catalysts was promoted by increasing of the monomer pressure. The viscosity average molecular weights M_v of the synthesized polymers using 1,2-bis(2,4,6-trimethyl phenyl imino) acenaphthene Nickel(II) dibromide were increased with increasing of the monomer pressure from 1 up to 6 bar which studied. Explicitly, the ortho-substituent has a significant effect on the catalyst behavior. Melting point and crystallinity of the obtained polyethylene using 1,2-bis(2,4,6-trimethyl phenyl imino) acenaphthene Nickel(II) dibromide catalyst were increased with enhancing monomer pressure. The optimum and stable structures were computed and some factors related to the activity were studied. Catalyst 1,2-bis(2,4,6-trimethyl phenyl imino) acenaphthene Nickel(II) dibromide had the highest activity with the highest quantities of dipole moment (18.29 Debye), charge of Mullikan on metal atom (1.48) and Sum of electronic and thermal Energies (–7906.52 e.u.).