Low Catalyst Concentration Research Articles

Diphosphines with expandable bite angles: highly active ethylene dimerisation catalysts based on upper rim, distally diphosphinated calix[4]arenes.

The binding properties of two large diphosphines, cone-5,17-dibromo-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (1) and cone-5,17-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (2) toward Ni(II) centres have been investigated. Whatever the starting complex, NiBr2 or [NiCp]BF4, quantitative formation of a chelate complex was observed, illustrating the preorganisation of the ligands. An X-ray structure determination was carried out for [NiCp1]BF4 which revealed that the nickel atom is positioned to one side of the calixarene axis, the PNiP plane being roughly parallel to the calixarene reference plane. The molecule has C(1) symmetry in the solid state, a feature which is also observed in solution at low temperature. As shown by variable-temperature 1H and 31P NMR studies, the complex undergoes two distinct motions: 1) a fan-like swinging of the coordination plane which displaces the metal from one side of the calixarene axis to the other, a motion during which the PNiP angle is likely to undergo a significant enlargement; 2) a rapid oscillation of each PPh2 unit about the corresponding Ni--P bond. In the latter dynamics the two endo-oriented PPh rings alternately occupy the calixarene entry. The two flexible ligands were assessed in ethylene oligomerisation. Activation with methylaluminoxane of the paramagnetic complexes [NiBr2.(1 or 2)] afforded highly active ethylene dimerisation catalysts, with turnover frequencies up to 10(6) (mol C2H4) (mol Ni)(-1) h(-1). The selective formation of 1-butene can be rationally controlled by using low catalyst concentrations.

Mechanism of mass transfer from bubbles in dispersions: Part I. Danckwerts’ plot method with sulphite solutions in the presence of viscosity and surface tension changing agents

The paper presents physical properties and kinetics data of sodium sulphite solution, pure or with additives that change surface tension and viscosity. Sokrat44 (copolymer of acrylonitrile and acrylic acid) and short-fiber carboxymethylcellulose (CMC) for the Newtonian and long-fiber CMC for the non-Newtonian viscosity enhancement and Ocenol ( cis-9-octadecen-1-ol) or polyethylenglycol (PEG) 1000 for surface tension change were used. The kinetics data are presented under the form of dependencies of oxygen absorption rate (measured in stirred cell) on catalyst (Co 2+) concentration, oxygen concentration in gas, pH and temperature of the solution. The method based on Danckwerts’ plot for the separate determination of mass transfer coefficient k L and gas–liquid interfacial area a in gas−liquid dispersions is described using the kinetics data. Problematic features of the method both at low cobalt catalyst concentrations (deceleration of the chemical oxidation of sulphite in presence of viscosity enhancing additives and non-zero concentration of oxygen in the liquid) and at high catalyst concentrations (shrinking of bubbles in dispersion due to high absorption rates) are taken into account. The method is used in Part II for the determination of k L in gas–liquid dispersions produced in mechanically agitated vessel and bubble column.

Synthesis, Structures, and Olefin Polymerization Behavior of Sterically Crowded Tris(pyrazolyl)borate Zirconium and Hafnium Complexes

The synthesis, molecular structures, and olefin polymerization behavior of the sterically crowded tris(pyrazolyl)borate complexes TpMsZrCl3 (2a; TpMs = HB(3-mesitylpyrazolyl)3-), TpMs*ZrCl3 (2b; TpMs* = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-), (TpMs**ZrCl2)2(μ-O) (2c; TpMs** = HB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)2-), and TpMs*HfCl3 (3b) are described. The molecular structures of TlTpMs (1a) and TlTpMs* (1b), the reagents used to prepare 2a−c and 3b, are also described. The core structures of 1a,b, and of 2a−c and 3b, are very similar, but the steric properties of these compounds differ markedly due to the different placements of the mesityl substituents on the tris(pyrazolyl)borate ligands. Under MAO activation conditions at low catalyst concentrations, 2a exhibits extremely high activity for ethylene polymerization and ethylene/hexene copolymerization. 2a/MAO produces ultrahigh-molecular-weight polyethylene and ethylene/hexene copolymers (up to 27% hexene) with narrow molecular weight distributions (Mw/Mn = 1.8−2.3), characteristic of single-site catalysis. 2a/MAO is more active for ethylene polymerization than the less crowded analogues 2b/MAO and 2c/MAO.

Pronounced Enhancement Brought in by Substituents on the Cyclopentadienyl Ligand: Catalyst System (C5Me4R)2Sm(THF)x/MMAO (R = Et, iPr, nBu, TMS; MMAO = Modified Methylaluminoxane) for 1,4-Cis Stereospecific Polymerization of 1,3-Butadiene in Cyclohexane Solvent

The C5Me4R-ligated samarocene(II) complexes, (C5Me4R)2Sm(THF)x (1, R = Me; 2, R = Et; 3, R = iPr; 4, R = nBu, 5; R = TMS; x = 1 or 2), in combination with modified methylaluminoxane (MMAO) were examined in cyclohexane with respect to their catalytic performances for the polymerization of butadiene. Complexes 2−5 showed considerable promotion in both catalytic activity and regularity of the resulting polymer as compared to parent 1: activity, 3 ≫ 5 ∼ 4 > 2 > 1; 1,4-cis selectivity at 50 °C, 3 ∼ 5 (98.6%) > 4 (97.5%) > 2 (97.1%) > 1 (96.2%). The system 3/MMAO showed an excellent catalytic activity even at very low catalyst concentration ([butadiene]0/[Sm]0 = 150 000), where the turnover number (TON) reached about 115 600 butadiene-mol/(Sm-mol h) at 50 °C (78% polymer yield in 1 h). At room temperature, the polymerization reaction in cyclohexane using 3/MMAO catalyst gave polybutadiene with extremely high 1,4-cis selectivity of 99.1% in reasonable rate (polymer yield = 87%, in 10 min) while the performance ...

Dramatic increase in the rate of the Mukaiyama aldol reaction by ‘fluorous nano flow’ system in the lowest concentration of a fluorous catalyst

The reactivity of the Mukaiyama aldol reaction is significantly increased by ‘fluorous nano flow’ system even in the low concentration (<0.0001 M) of a lanthanide fluorous catalyst and the reaction completes within seconds as a contact time in the micro cell.

Macrocycles, 22

AbstractThe phosgenation of bisphenol A with diphosgene in the CH2Cl2/NaOH system was studied using the pseudo‐high‐dilution method either with triethylamine, with triethylbenzyl ammonium chloride (TEBA‐Cl), or with tetraphenylphosphonium chloride as catalyst. For Ph4PCl, only low‐molecular‐weight polycarbonates containing various by‐products were obtained. With the TEBA catalyst, the fraction of cycles and the average molecular weights increased with higher feed ratios of the catalyst. Both the fraction of cycles and the molecular weights steeply decreased at higher temperatures. With triethylamine as catalyst both the fraction of cycles and molecular weight passed through a maximum at low catalyst concentration, when the feed ratio of triethylamine was varied. Higher temperatures favored slightly lower fractions of cycles and lower molecular weights. Samples containing ≥ 95 mol‐% of cycles were obtained. The cycles were identified by matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry up to 22 000 Da in the case of virgin samples, and up to 55 000 Da after fractionation by size exclusion chromatography (SEC). Selected samples were characterized by SEC measurements and evaluated by the triple‐detection method. High polydispersities and a tendency towards a bimodal mass distribution were found, when triethylamine was used as catalyst.MALDI‐TOF mass spectrum of a polycarbonate prepared by triethylamine‐catalyzed phosgenation at 20 °C by the pseudo‐high‐dilution method.magnified imageMALDI‐TOF mass spectrum of a polycarbonate prepared by triethylamine‐catalyzed phosgenation at 20 °C by the pseudo‐high‐dilution method.

Kinetics of surface‐initiated atom transfer radical polymerization

AbstractAlthough atom transfer radical polymerization (ATRP) is often a controlled/living process, the growth rate of polymer films during surface‐initiated ATRP frequently decreases with time. This article investigates the mechanism behind the termination of film growth. Studies of methyl methacrylate and methyl acrylate polymerization with a Cu/tris[2‐(dimethylamino)ethyl]amine catalyst system show a constant but slow growth rate at low catalyst concentrations and rapid growth followed by early termination at higher catalyst concentrations. For a given polymerization time, there is, therefore, an optimum intermediate catalyst concentration for achieving maximum film thickness. Simulations of polymerization that consider activation, deactivation, and termination show trends similar to those of the experimental data, and the addition of Cu(II) to polymerization solutions results in a more constant rate of film growth by decreasing the concentration of radicals on the surface. Taken together, these studies suggest that at high concentrations of radicals, termination of polymerization by radical recombination limits film growth. Interestingly, stirring of polymerization solutions decreases film thickness in some cases, presumably because chain motion facilitates radical recombination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 386–394, 2003

Negatively curved spongy carbon

We describe the production and characterization of a form of nanostructured carbon consisting of fully connected, three-dimensional (3D) sp2 networks. This form of carbon is characterized by interconnected thin layers forming a spongy structure with meso- and macroporosity. It is produced by a pulsed microplasma cluster source in the presence of a very low concentration of metallorganic catalyst. The unique 3D structure and topology of the spongy carbon are very interesting for catalysis and electrochemical applications and for the investigation of negatively curved forms of carbon such as schwarzites.

Kinetic Determination of Trace Amounts of Cu(II) in Water Based on its Catalytic Effect on the Reaction of Mercaptosuccinic Acid and Cr(VI)

A kinetic method is presented for the determination of Cu(II) in water. It is based on the catalytic effect of Cu(II) on the oxidation of mercaptosuccinic acid by chromate in acidic media. The extent of the reaction is followed spectrophotometrically at 345 nm and pseudo-first order rate coefficients are determined as a function of catalyst concentration. The optimum operating conditions regarding ionic strength, temperature and concentration of reagents were established. Interference by several ionic species was studied and the effects of Fe(III) and Pb(II) were suppressed by complexation with pyrophosphate. Calibration lines were obtained for both low (30–640 µg · l−1) and high (640–1500 µg · l−1) catalyst concentrations. The relative standard deviation for 625 µg · l−1 Cu(II) is 6.1% (n = 5). The detection limit is 22 µg · l−1. The method was applied to real samples of river water of the mining region of Baia-Mare, Northern Romania. The results were compared to those obtained by an officially standardized AAS method. Good agreement was attained. The method is inexpensive, fairly rapid and sensitive. Moreover, its working range covers the exact range of concentrations usually encountered in the mentioned geographic area.

Synthesis of novel oxo complexes of tungsten and molybdenum with various chalcogen-bridged chelating bis(aryloxo) ligands and their catalytic behavior for ring-opening metathesis polymerization

Oxo tungsten complexes coordinated by OEO (E=S, Te, CH 2) chelating bis(aryloxo) ligands, WO[E(4-Me-6- t BuC 6H 2O) 2]Cl 2, ( 1: E=S, 2: E=Te, 3: E=CH 2), were synthesized by the reaction of WOCl 4 with 2,2′-E(4-Me-6- t BuC 6H 2OH) 2 [E=S ( t Bu 2tbpH 2), E=Te (TebpH 2), E=CH 2 (mbpH 2)]. Similarly a molybdenum complex, MoO[S(4-Me-6- t BuC 6H 2O) 2]Cl 2 ( 4), was also prepared. The structures of the oxodichloro tungsten complexes 1 and 3 were determined by single-crystal X-ray analysis to have pseudo C s symmetry. A μ-oxo binuclear tungsten complex with two t Bu 2tbp ligands, {WO[S(4-Me-6- t BuC 6H 2O) 2]Cl} 2(μ-O) ( 5), was synthesized by refluxing a mixture of W 2O 3Cl 6 and two equivalents of t Bu 2tbpH 2 in THF, whose structure was determined by single-crystal X-ray analysis. We study here the catalytic behavior of these complexes for the ring-opening metathesis polymerization (ROMP) of norbornene. In the catalytic systems, 1–AlEt 3 and 2–AlEt 3, cis– trans selectivity varies depending on the catalyst concentration and on the polymerization temperature. The resulting poly(norbornene) tend to have higher cis-content with increasing catalyst concentration and with lowering polymerization temperature. Oppositely, the polymerization under low catalyst concentration and at high polymerization temperature gave the trans-rich poly(norbornene). On the other hand, the complex 3 with the methylene-bridged bis(aryloxo) ligand did not show such a dependence of cis– trans selectivity. We proposed that these observations might come from oxotungsten–aluminum interaction.

Perhalogenated 2-pyridylporphyrin complexes: synthesis, self-coordinating aggregation properties, and catalytic studies

The synthesis of 2, 3, 7, 8, 12, 13, 17, 18-octabromo-meso-tetrakis(2-pyridyl)porphyrin, H 2 Br 8 T 2 PyP , is described, including the comparison of four attempted methods for the demetallation of ZnBr 8 T 2 PyP . One of the methods represents a strategy of demetallation based on the acid-base properties of the macrocycle, the solvent-dependent kinetics of metal insertion into porphyrins and the pH -dependent solubility of the 2-pyridylporphyrin derivatives in water. Self-coordinating aggregation of ZnBr 8 T 2 PyP in non-coordinating solvents was verified by 1 H NMR spectroscopy. The Mn(III)/Mn(II) redox potential for MnBr 8 T 2 PyPCl is 0.38 V higher than the reduction potential measured for its first-generation analogue, MnT 2 PyPCl . Cyclohexane hydroxylation by iodosylbenzene was performed in CH3 CN catalyzed by MnBr 8 T 2 PyPCl and MnT 2 PyPCl . MnBr 8 T 2 PyPCl was highly active, even at low concentration (5 × 10−5 M ), but perhalogenation did not account for oxidative robustness. At such a low catalyst concentration, MnT 2 PyPCl exhibited no activity as inferred by comparison to blank experiments.

Ethylene polymerization with novel silicon bridged homo-dinuclear cyanoethyl cyclopentadienyl complexes of titanium and zirconium

Novel homo dinuclear silicon bridged cyanoethyl cyclopentadienyl complexes of titanium and zirconium (CH 3) 2Si((η 5-C 5H 3CH 2CH 2CN)(C 5H 5)MCl 2) 2 (M=Ti(1), Zr(2)) were synthesized and developed for the polymerization of ethylene. Compared with their corresponding mononuclear complexes (η 5-C 5H 4CH 2CH 2CN)(C 5H 5)MCl 2 (M=Ti(3), Zr(4)), the dinuclear complexes had higher catalytic activity. And the polyethylene produced had a higher molecular weight than that obtained with mononuclear catalysts. Effects of conditions on the ethylene polymerization catalyzed by (2)/MAO (methylaluminoxane) were studied in detail. The catalyst showed a very high activity ( 10 6g PE/mol Zr·h) under low catalyst concentration and high molar ratio of Al/Zr.

An Improved Procedure for the Mild Racemization of N-Acyl α-Amino Acids

An improved procedure for the mild racemization of N-acyl α-amino acids has been developed, using Pd(PPh3)4 either as the pre-formed complex or by its in situ formation from Pd(OAc)2 or Pd2(dba)3 and triphenylphosphine. The concentration of the palladium complex catalyst required for effective racemization is much lower than for the previously reported rhodium based catalytic system. In addition, the racemization also proceeds more rapidly at this lower catalyst concentration.

Acylation of aromatic compounds using moisture insensitive InCl 3 impregnated mesoporous Si-MCM-41 catalyst

Acylation of aromatic compounds (benzene, toluene, p-xylene, mesitylene, anisole, naphthalene, methylnaphthalene and methoxynaphthalene) by an acyl chloride (benzoyl chloride, phenylacetyl chloride, propionyl chloride or butyryl chloride) in high yield, in a short reaction period (3 h), even in the presence of moisture in the aromatic substrate or solvent (dichloroethane), can be accomplished at low temperature (80±1°C) using an InCl 3 impregnated Si-MCM-41 catalyst in low catalyst concentration.

Synthesis of a Novel Oxotungsten(VI) Complex Having a Chelating Bis(aryloxo) Ligand and Its Catalytic Behavior for Ring-Opening Metathesis Polymerization

Abstract An oxotungsten complex coordinated with O–S–O chelating bis(aryloxo) ligand, WO{S(4-Me-6-tBuC6H2O)2}Cl2 (1), was synthesized by the reaction of WOCl4 with 2, 2′-S(4-Me-6-tBuC6H2OH)2. The structure of the complex 1 was determined by single-crystal X-ray analysis to have pseudo Cs symmetry. The catalytic behavior of 1/AlEt3 system for the ring-opening metathesis polymerization of norbornene was studied. In this catalytic system, strong dependencies of cis/trans selectivity on the catalyst concentration and on the polymerization temperature were observed. With increasing catalyst concentration and with lowering polymerization temperature, the resulting poly(norbornene)s tend to have higher cis-regularity. Oppositely, trans-rich poly(norbornene)s were produced by the polymerization under low catalyst concentration and at high polymerization temperature. We propose that these observations might come from oxotungsten–aluminum interaction.

The influence of catalyst remnants on thermal degradation during melt processing of high melting ethylene–carbon monoxide copolymers

The thermal stability of alternating ethylene–carbon monoxide copolymers (POK-C2), synthesised using a palladium-bidentate catalyst, is predominantly determined by the amount of ‘active’ catalyst species present in as-synthesised polymer for a palladium content >6ppm. For a lower active palladium content the intrinsic limitations to the thermal stability of the chemical structure become noticeable. It is demonstrated that the thermal stability of polymers prepared under optimised conditions, (e.g. low catalyst content) is sufficient to allow melt processing for at least a few minutes of residence time above the melting temperature. Homogeneous transparent melt crystallised films are obtained and small strands were drawn at 225°C to a ratio of 8. The mechanical properties of these oriented samples are comparable with commercially available PET and PA-6,6 melt spun fibres. The crystalline fraction in these samples is mainly present in the very dense POK-α form, indicating that the formation of a significant amount of bulky chain defects during thermal exposure, like Paal–Knorr and aldol type condensation products, is prevented.

Polyesters ofω-Unsaturated Fatty Acid Derivatives

Unsaturated polyesters, based on ω-unsaturated fatty acid esters from oleochemical resources and petrochemical diols, were synthesized via two different synthetic pathways. 9-decenoic and 10-undecenoic acid methyl ester I a, b were transesterified with petrochemical diols to produce the α,ω-alkylene di(un)decenoates II a–d, which were subsequently converted into unsaturated polyesters by acyclic diene metathesis polymerization. A screening of ten homogeneous and heterogeneous metathesis catalysts was performed, implying optimum results for B2O3—Re2O7/Al2O3—SiO2 + SnBu4. Molecular weights (M̄w) up to 100 000 g/mol were determined by MALLS-GPC. On the other hand, C10 , C11 and C14-ω-unsaturated fatty acid methyl esters Ia–c were metathetically dimerized using homogeneous ruthenium and tungsten carbene complexes at an extremely low catalyst concentration. The resulting symmetrical, internally unsaturated, dicarboxylic acid dimethyl esters IIIa–c were polycondensated in bulk with conventional transesterification catalysts achieving molecular weights (M̄w) up to 110 000 g/mol. Melting temperatures of 6 to 92°C were determined by DSC.

Effect of catalysts on thin-film polymerization of thermotropic liquid crystalline copolyester

Using a novel thin-film polymerization technique, we have investigated in situ uncatalyzed and catalyzed polycondensation reaction systems for 73/27 (mol ratio) poly(p-oxybenzoate/2,6-oxynaphthoate) [P(OBA/ONA)] thermotropic liquid crystalline copolyester. We have also determined the effect of catalysts on the kinetics and morphological changes of the reactions. Because the thin-film polymerization is conducted on the heating stage and the morphology is observed in situ by a polarizing microscope, we can directly observe and determine the accurate onset time for LC phase generation. The number-average degree of polymerization (DP) at the onset of this morphological change decreases with decreasing reaction temperature in the range of 230-290 °C. The LC phase may form at a DP as low as 2 at 230 °C. Most importantly and surprisingly, the reaction rate constant obtained from the thin-film polymerization is much greater (20-30 times) than the previous reported value obtained from the bulk polymerization reaction because the release of acetic acid in the former is much easier and quicker than in the latter. Clearly, the thin-film polymerization may be a better and accurate technique to observe the approximately inherent properties of polymerization kinetics than the traditional bulk polymerization reaction. Three kinds of catalysts, namely, sodium acetate, calcium acetate, and antimony oxide, have been studied. Sodium acetate has obvious acceleration effect on the reaction. Reaction rate constant increases almost proportionally to the catalyst content in the low catalyst content range, and activation energy slightly decreases with an increase in sodium acetate percentage. Calcium acetate has a higher catalytic effect than sodium acetate when the catalyst content is high, but the trend reverses when the catalyst content is low. Polymerization with high content of calcium acetate produces LCP with undesirable morphology because it suppresses the coalescence process among LC domains. Antimony oxide is a polymerization inhibitor for this reaction. It slows down the reaction, but does not alter the sequence of the morphological changes during the polymerization.

Selectivity of the OsHCl(CO)(O2)(PCy3)2 catalyzed hydrogenation of nitrile-butadiene rubber

A new homogeneous catalyst precursor has been discovered for the hydrogenation of carbon-carbon unsaturation resident within acrylonitrile-butadiene copolymers. The hydrido-phosphine complex OsHCl(CO)(O 2 )(PCy 3 ) 2 (1) selectively and quantitatively saturates olefin, leaving the copolymer's nitrile functionality intact. However, the process suffers from an undesirable crosslinking reaction that is not demonstrated by the established rhodium technology. The extent of this crosslinking is dependent on the process conditions and can be minimized by operating with a low catalyst concentration and high H 2 pressure. Kinetic studies have identified a previously unknown unexpected influence of olefin on the polymer crosslinking process. In light of this new information, the prevailing mechanism for this class of reactions has been reconsidered.

Effect of Oxygen on the Belousov−Zhabotinsky Reaction at Low Cerium Concentrations

An effect of oxygen on the cerium-catalyzed Belousov−Zhabotinsky reaction, which occurs only at low catalyst concentrations and consists of an abrupt drop in the average Ce4+ concentration, was investigated. Perturbations of this effect by addition of acrylonitrile or silver nitrate are also described. An attempt was made to model the observations by modifying the Gao−Försterling mechanism (Gao; Försterling. J. Phys. Chem. 1995, 99, 8638−8644) for the [Ru(bipy)3]2+/bromomalonic acid/bromate system by replacing the ruthenium reactions by those of cerium and adding the reactions of malonic acid in the presence of oxygen. The modified mechanism yields a qualitative interpretation of several important features of the experiments.