Valentina N. Panchenko

Polymerization of ethylene catalyzed by iron complex bearing 2,6-bis(imine)pyridyl ligand : 1H and 2H NMR monitoring of ferrous species formed via catalyst activation with AlMe3, MAO, AlMe3/B(C6F5)3 and AlMe3/CPh3(C6F5)4

1 H and 2 H NMR spectroscopic monitoring of ferrous species formed via interaction of 2,6-bis[1-2,6-dimethylphenylimino)ethyl]pyridineiron (II) chloride (1) with AlMe 3 , MAO, AlMe 3 /B(C 6 F 5 ) 3 and AlMe 3 /CPh 3 (C 6 F 5 ) 4 is reported. At interaction of 1 with MAO in toluene solution, the new stable heterodinuclear neutral complexes with proposed structures LFe(II)(Cl)(u-Me) 2 AlMe 2 and LFe(II)(Me)(u-Me) 2 AlMe 2 are formed (L is initial tridentate ligand). Complex LFe(II)(Cl)(u-Me) 2 AlMe 2 predominates at low Al/Fe ratios (less than 50), while LFe(II)(Me)(U-Me) 2 AlMe 2 at high Al/Fe ratios (more than 500). Complex assigned to LFe(II)(Me)(u-Me) 2 AlMe 2 can be prepared via interaction of 1 with AlMe 3 . Activation of LFe(II)(Me)(u-Me) 2 AlMe 2 by B(C 6 F 5 ) 3 and CPh 3 B(C 6 F 5 ) 4 gives rise to formation of new complexes with proposed structures [LFe(u-Me) 2 AlMe 2 ] + [MeB(C 6 F 5 ) 3 ] - and [LFe(u-Me) 2 AlMe 2 ] + [B(C 6 F 5 ) 4 ] - . Unexpectedly, the activity at ethylene polymerization was even higher for 1/AlMe 3 than for 1/MAO catalytic system. The co-catalytic activity of MAO towards 1 dramatically decreased with the diminishing of AlMe 3 content in the composition of MAO. Activity of the catalyst 1/AlMe 3 and the molecular structure of polyethylene produced do not change noticeably at the addition of B(C 6 F 5 ) 3 to 1/AlMe 3 . These data allow to suggest, that active species of 1/AlMe 3 and 1/MAO systems are neutral methylated ferrous complexes but not cationic intermediates. Probably, complex LFe(II)(Me) 2 AlMe 2 is the closest precursor of these active species.

The metallocene/methylaluminoxane catalysts formation : EPR spin probe study of Lewis acidic sites of methylaluminoxane

Abstract Using stable nitroxyl radical 2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPO) as a spin probe, Lewis acidic sites of methylaluminoxane (MAO) were identified. It was found that MAO contains two types of acidic sites. TEMPO, coordinated to the sites I and II, exhibits in the EPR spectra triplet ( g o =2.0047, a N =18.6 G) and triplet of sextets ( g o =2.0045, a N =19.6 G and a Al =1.7 G), respectively ( a N and a Al are constants of hyperfine structure from the corresponding nucleus). According to EPR measurements, concentration of sites I is close to that of sites II, and MAO contains one site of each type per 100±30 aluminium atoms. The adducts of TEMPO with sites I are less stable than those with sites II. Based on the values of a Al and relative stabilities of the adducts with TEMPO, the acidic sites I and II were attributed to coordinatively unsaturated aluminium atoms in AlOMe 2 and AlO 2 Me environment, respectively. From the EPR spectra of coordinated TEMPO, the average radius of MAO oligomers (AlOMe) n was evaluated to be 5.8 A at 20°C, which corresponds to the value of n =15–20. Thus, the major part of MAO contains not more than one Lewis acidic site per one oligomeric (AlOMe) n molecule.

IRS study of ethylene polymerization catalyst SiO2/methylaluminoxane /zirconocene

Abstract IR spectroscopy has been used to study the interaction of silica with two methylaluminoxane (MAO) samples differed by trimethylaluminium (TMA) content and with TMA. MAO and TMA have been shown to react with silica in a different way. Whereas TMA interacts with terminal hydroxyl groups of silica via the protolysis reaction, MAO mainly adsorbs on the surface hydroxyl groups of silica without noticeable protolytical reaction with them. When silica is treated with commercial grade MAO with significant TMA content, the silica surface hydroxyl groups mainly interacts with TMA and MAO adsorbs on the surface of SiO 2 /TMA sample. Lewis acidic sites (LAS) of silica, modified with TMA and MAO samples differed by TMA content, have been investigated by IR spectroscopy (CO adsorption as probe molecule at 77 K). Two types of LAS were found on the surface of silica modified with MAO and TMA: LAS of moderate strength ( ν CO =2204–2212 cm −1 ) and weak LAS ( ν CO =2194 cm −1 ). The concentration of these acidic sites was estimated. By anchoring of Cp 2 ZrCl 2 on silica, modified with TMA and MAO, the corresponding catalysts SiO 2 /TMA/Cp 2 ZrCl 2 and SiO 2 /MAO/Cp 2 ZrCl 2 were prepared and tested at ethylene polymerization. Some correlations between the amount and strength of surface LAS of supports, catalysts composition and their activity are discussed. It is proposed that the surface active species are formed at zirconocene interaction with the most strong LAS.

Formation of Trivalent Zirconocene Complexes from ansa-Zirconocene-Based Olefin-Polymerization Precatalysts: An EPR- and NMR-Spectroscopic Study

Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. The chloro-bridged heterodinuclear ansa-zirconocenium cation [(SBI)Zr(μ-Cl)2AlMe2](+) (SBI = rac-dimethylsilylbis(1-indenyl)), present in toluene solution as its B(C6F5)4(-) salt, thus gives rise to an EPR signal assignable to the complex (SBI)Zr(III)(μ-Cl)2AlMe2, while (SBI)Zr(III)-Me and (SBI)Zr(III)(μ-H)2Al(i)Bu2 are formed by reduction of [(SBI)Zr(μ-Me)2AlMe2](+) B(C6F5)4(-) and [(SBI)Zr(μ-H)3(Al(i)Bu2)2](+) B(C6F5)4(-), respectively. These products can also be accessed, along with (SBI)Zr(III)-(i)Bu and [(SBI)Zr(III)](+) AlR4(-), when (SBI)ZrMe2 is allowed to react with HAl(i)Bu2, eliminating isobutane en route to the Zr(III) complex. Further studies concern interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation.

Novel Zirconocene Hydride Complexes in Homogeneous and in SiO2‐Supported Olefin‐Polymerization Catalysts Modified with Diisobutylaluminum Hydride or Triisobutylaluminum

Reactive species in SiO 2 -supported, zirconocene-based olefin-polymerization catalysts have been characterized by comparison of their UV-vis spectra with those of related, NMR-spectroscopically identified catalyst species in homogeneous solution. Neutral zirconocene dihydride complexes are found to arise in hydrocarbon solutions as well as on SiO 2 supports when catalyst systems that contain rac-Me 2 si(ind) 2 ZrCl 2 and methylaluminoxane (MAO) are modified by addition of diisobutylaluminum hydride or triisobutylaluminum. These complexes, tentatively formulated as adducts with Lewis-acidic alkylaluminum species AlR 2 X, rac-Me 2 Si(ind) 2 ZrH 2 ·{nAlR 2 X}, are reconverted into the initial reactive zirconocene cations upon addition of isobutene to these reaction systems.

Zirconium Allyl Complexes as Participants in Zirconocene‐Catalyzed α‐Olefin Polymerizations

In a search for the hitherto elusive catalyst resting state(s) of zirconocene-based olefin polymerization catalysts, a combination of UV/Vis and NMR spectrometric methods reveals that polymer-carrying cationic Zr allyl complexes make up about 90 % of the total catalyst concentration. Other catalyst species that take part in the polymerization process have to be generated from this allyl pool into which they appear to relapse rather frequently.

Determination of the number of active centers and of the propagation rate constant for ethylene polymerization on supported Ni-containing catalysts by using 14CO

Ethylene polymerization on supported Ni-containing catalysts was studied using 14 CO as the inhibitor and as a label for determining the number of active centers (AC). The polymerization rate decreases sharply after introducing a mole ratio 14 CO/Ni = 0.5/1 and recovers after the removal of 14 CO. The number of labels in the polymer and in the liquid phase (hexane) increases with the amount of 14 CO introduced and with 14 CO contact time (τ co ). In the absence of ethylene the number of labels in the polymer does not increase with τ co and the number of labels in the liquid phase is essentially less than in the presence of ethylene. The rate of the label accumulation in the polymer during short contact times (τ co ≤ 5 min) is essentially higher than that during the following periods of time (τ co = 5-135 min). Fast label accumulation occurs through first 14 CO molecule insertion into the Ni-polymer bond, and the slower one through copolymerization of 14 CO with ethylene, i.e. when the second and more 14 CO molecules insert into the polymer chain. Catalyst activity and the number of AC (in the range of 1.5 to 6% of Ni content) decrease with polymerization time. For both catalysts the values of the propagation rate constant are maximum at short polymerization time (5 to 15 min) and equal to (3 ± 0.8). 10 3 L/(mol. s) at 65°C.

IR spectroscopic study of the basicity of aminated silica gels

The aminated silica gels SiO2/SOCl2/NH3 (I), SiO2/SiCl4/NH3 (II), SiO2/BCl3/NH3 (III), and SiO2/γ-aminopropyltriethoxysilane (SiO2/APTES, IV) have been synthesized. According to DRIFT spectroscopy and chemical analysis data, the surface amino groups of I–III are “free,” while those of IV interact with the surface OH groups of the silica gel and with one another. The strength of basic sites has been measured on the proton affinity (PA) scale as the shift of the ν(CD) band of adsorbed deuterochloroform. The basicity of an aminated silica gel depends on its chemical composition. Silica gel IV (PA = 938 kJ/mol) is a stronger base than I–III (PA = 829 kJ/mol). As the basicity of the NH2 group decreases, the N-H stretching band shifts to higher frequencies.

Zirconium-allyl complexes as resting states in zirconocene-catalyzed α-olefin polymerization.

UV-vis spectroscopic data indicate that zirconocene cations with Zr-bound allylic chain ends are generally formed during olefin polymerization with zirconocene catalysts. The rates and extent of their formation and of their re-conversion to the initial pre-catalyst cations depend on the types of zirconocene complexes and activators used.

Nickel phosphate molecular sieves VSB-5 as heterogeneous catalysts for synthesis of monosaccharides from formaldehyde and dihydroxyacetone

Condensation of dihydroxyacetone with formaldehyde was found to be effectively catalyzed by nickel phosphate molecular sieves (VSB-5 and Fe–VSB-5) in neutral aqueous medium. In the presence of VSB-5 the main products are erythrulose and 3-pentulose, while Fe–VSB-5 favours the formation of only erythrulose.