Ethylene uptake

Hartley and Wagner found no evidence for PdCl2(olefin)2 and suggested that Cruikshank and Davies needed to postulate this complex because they did not take into account the formation of Na[Pd2Cl5-(CHgCOOH)]. The results of a study of ethylene uptake by Na2PdCl4 were most consistent with the following equilibrium 106) ... 

When the chloride ion concentration was varied, not only did the rate of the oxidation change but also the amount of ethylene initially taken up in excess of that required to saturate the solution. By assuming that the net initial ethylene uptake resulted from the following equilibrium,... 

Theoretical calculations at DFT level for ethylene insertion into Ti-Me bonds of cationic alkylamidinato complexes [TiMe(R1NGRNR1)2]+ (R = H, Ph R1 = H, SiMe3) have been performed,320 as have calculations for a bis(/5-diketonato)titanium model system (Scheme 137) in the presence of ethylene. Special attention is paid to the possible occurrence of agostic alkyl complexes and to the mechanism of ethylene uptake, chain propagation, and termination.321... 

Problem 9.11 Metal-oxide catalyzed polymerization of ethylene was carried out in benzene solution in a stirred autoclave with a suspension of hydrogen-reduced molybdena-alumina catalyst (Friedlander and Oita, 1957). The pressure was maintained nearly constant by repressuring the autoclave with ethylene as it was consumed in the polymerization process. Temperatures of 200-275°C were studied. The ethylene concentration in solution was controlled by adjusting the pressure (in the range 625 to 1000 psi) at any particular temperature. The ethylene uptake rate (rate of pressure drop, dPIdt) was mea-... 

It is assumed that the monomers are present as aluminates. Although separately pretreating the alcoholic monomers with MAO prior to polymerization did not improve comonomer conversion of incorporation (ethylene uptake was improved), the polymerization procedure involves mixing of the polar comonomer with MAO prior to zirconocene addition, and higher total levels of MAO (ca. 10 000 equiv per Zr) were found to have a more favorable effect on conversion. 

The second insertion in the productive cycle of Fig. 14 would involve the chelated acyl complex (5). Again it might have been difficult to convert this to the olefin complex (6) in the nickel case as the strong chelate Ni-oxygen bond has to be weakened. However, for nickel it seems that (5) is replaced with the five-coordinated acyl complex 5a by uptake of one additional CO. However, 5a is not amenable for ethylene uptake as a first step in the insertion of ethylene into the metal-acyl bond since ethylene will have to replace the more strongly bound CO (>10 kcal mol ). It is thus Ukely that the CO/ethylene polymerization cycle is blocked by a species such as 5a or the four-coordinated chelate (5) of Fig. 14. 

Fig. 16 Ethylene uptake profiles symbols) in a batch reactor at 23°C, over (a) Cr(II) grafted on S948-500 (102.7 mg, 1.71 wt% Cr, 34.5 pmol Cr) and (b) Cr(II) grafted on S948-800 (314.3 mg, 0.62 wt% Cr, 38.1 pmol Cr). The lines are three-parameter fits to the first-order integrated kinetic rate equation...

This method was first reported by Heinemann et al. [1] for the polymerization of ethylene with the catalysts depicted in Figure 3.15 in the presence of clays with different types of organic modifications. Figure 3.16 compares the ethylene uptake curves with homogeneous and clay-supported MBI catalyst. Lower activity and more stable polymerization rate were observed for the clay-supported system. As water present on the clay surface acts as a catalyst poison [71], the low polymerization activity observed for the clay-supported system was attributed to water traces remaining on the organoclay surface. 

Figure 3.16 Comparison of ethylene uptake for homogeneous polymerization (a) and polymerization in presence of clay (modified with dimethyl stearyl benzyl ammonium chloride-DMDS) and (b) with MBI (reproduced from Ref. [1]).

In 1957, D. B. Ludlum and coworkers at the E. I. DuPont Company published an early paper on the characteristics of the Mulheim catalyst [11], Ludlum investigated the polymerization of ethylene in a high-boiling inert solvent (decalin) using TiCl and tri-isobutylaluminum (TIBA), Tri-n-hexyl aluminum (THA) or triethylaluminum (TEAL) at 100 C and 1-1.5 atm of ethylene pressure. The rate of ethylene uptake was found to decrease shortly after polymerization was initiated and Lunlum postulated that this was due to the encapsulation of polymerization sites by the polyethylene. The polymerization was also found to be first-order with regard to the ethylene pressure and the concentration of TiCl. The effect of the Al/Ti ratio on catalyst activity and the type of aliuninum alkyl used (TIBA or TEAL) in the polymerization are shown in Figure 2.1. 

The ethylene uptake of semicrystalline SPS films presenting its nanoporous host 5 phase is much higher than for other polymeric materials, since it is related to the formation of a co-crystalline phase, where ethylene molecules are oriented nearly perpendicular to the crystalline polymer helices [98]. This ethylene uptake is much higher than for industrial polypropylene samples also containing large amounts (up to 25wt%) of silica. 

However, this behavior cannot be depended on. It is safer to monitor hydrogen uptake. Reduction of 4-oxo-slowed down at the ethylene stage although in other compounds, the same catalyst did give spontaneously selective reductions. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Higher phytoextraction coefficients indicate higher metal uptake. The effectiveness of phytoextraction can be limited by the sorption of metals to soil particles and the low solubility of the metals however, metals can be solubilized through the addition of acids or chelating agents and so allow uptake of the contaminant by the plant. Ethylene diamine tetra-acetic acid (EDTA), citric acid, and ammonium nitrate have been reported to help in the solubilization of lead, uranium, and cesium... 

Yamaoka T, Tabata Y, Ikada Y (1994) Distribution and tissue uptake of poly(ethylene glycol) with different molecular weights after intravenous administration to mice. J Pharm Sci 83 601-606... 

Rate measured as gas uptake in ml min-1. 1 1 1 Mixtures of alkene, hydrogen, and carbon monoxide at 600 mm total pressure gave uptakes for ethylene and propylene of 4.55 and 1.60 ml min-1, respectively. 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

The permeability of human skin to n-hexane has been determined in vitro in flow-through diffusion cells (Loden 1986). Pieces of full-thickness human skin were exposed to 3H -hcxane in human serum, and the appearance of label in the trans compartment measured for 0.5 or 12 hours. The skin was then sectioned with a microtome into 0.25 mm slices and the quantity of label in the skin measured. The rate of resorption (uptake of substance by the receptor fluid beneath the skin [i.e., the amount that passes through the skin]) was calculated. The rate of resorption for n-hexane through human skin was calculated to be 0.83 ( g cm2/hr). The permeability of n-hexane through human skin was much lower (approximately 100-fold) than for other chemicals tested in this study. For example, rates of resorption (in g cm2/hr) were 99 for benzene and 118 for ethylene glycol. 

We have discussed the structure and synthesis of the library of molecular catalysts for polymerization in Section 11.5.1. In the present section we want to take a closer look at the performance of the catalyst library and discuss the results obtained [87], The entire catalyst library was screened in a parallel autoclave bench with exchangeable autoclave cups and stirrers so as to remove the bottleneck of the entire workflow. Ethylene was the polymerizable monomer that was introduced as a gas, the molecular catalyst was dissolved in toluene and activated by methylalumoxane (MAO), the metal to MAO ratio was 5000. All reactions were carried out at 50°C at a total pressure of 10 bar. The activity of the catalysts was determined by measuring the gas uptake during the reaction and the weight of the obtained polymer. Figure 11.40 gives an overview of the catalytic performance of the entire library of catalysts prepared. It can clearly be seen that different metals display different activities. The following order can be observed for the activity of the different metals Fe(III) > Fe(II) > Cr(II) > Co(II) > Ni(II) > Cr(III). Apparently iron catalysts are far more active than any of the other central metal... 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

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