Chain Growth Process Research Articles

Synthesis and Characterization of Fully Conjugated Donor–Acceptor–Donor Triblock Copolymers

A synthetic approach is established to provide a monofunctional telechelic poly(3-octylthiophene) (P3OT) bearing a single bromine-substituted end group that is of potential use in the preparation of well-defined block copolymers. Telechelic P3OT was prepared via a chain growth process by a catalyst-transfer condensation polymerization (CTCP) of 5-bromo-4-octyl-2-thienylmagnesium iodide initiated by a phenylnickel(II) initiator. Optimization of the conditions for quenching the reaction allowed for the installation an α-bromo functionality at the terminus of the polymer. We demonstrate the utility of this well-defined monofunctional polymer, Ph–P3OT–Br, by coupling it to a poly(quinoxaline) (PQ) bearing boronate ester end groups to provided a new class of donor–acceptor–donor (D–A–D) triblock copolymers. The formation of the triblock copolymers was confirmed by gel-permeation chromatography (GPC) and 1H NMR spectroscopy. The optical properties of the polymers were investigated using UV–visible absorption an...

ChemInform Abstract: Mechanistic Kinetic Treatment of the Chain Growth Process in Higher Alcohol Synthesis Over a Cs-Promoted Zn-Cr-O Catalyst.

Abstract A kinetic analysis has been performed of higher alcohol synthesis over ZnCrO + 15% Cs2O (w/w) using a reaction network strictly based on mechanistic evidence. This includes crossed aldol condensations of aldehydes and ketones and stepwise C, linear additions, both occurring in normal and “oxygen retention reversal” modes, as well as reversible ketonization reactions, and is consistent with chemical equilibrium constraints affecting the following reactions: methanol synthesis, water-gas shift, hydrogenation of carbonyl compounds, ester formation, and ketonization. Kinetic assumptions include reversible equilibrium adsorption of the reacting species on the catalyst sites, equilibrium of the C1 intermediates with CO/H2, and competition of water in adsorbing on the active sites. In line with available chemical data, different reactivities are attributed to the species participating in the chain growth process depending on their molecular structure (aldehydes versus ketones, linear versus branched). Using nine rate parameters, the model successfully describes the concentrations of 20 primary alcohols and ketones (and of the species related by chemical equilibria) as functions of contact time, temperature, pressure, and feed composition. The formation of about 50 additional compounds in trace amounts is also predicted, which are indeed not detected in the synthesis products. The parameter estimates show that aldol condensations are one order of magnitude faster than C1 additions, indicate a greater reactivity of aldehydes than ketones, attribute a significant role to reverse ketonizations in the chain growth process in parallel to aldol condensations, and demonstrate the specific inhibiting action of water on condensations with oxygen retention reversal and ketonization. The apparent activation energies are very small for aldol condensations, ∼63 kJ/mol for C2 linear additions, and ∼155 kJ/mol for ketonizations.

Theoretical Studies of the Formation and Reactivity of C2 Hydrocarbon Species on the Fe(100) Surface

The adsorption and reactions of organic fragments containing C2 unit on the iron surface have been investigated employing periodic density functional theory with a plane-wave basis set and pseudopotentials, as well as a slab model representing the p(2 × 2) Fe(100) surface topology. The calculations demonstrate that the most favorable C2 species on the Fe(100) surface are those containing the acetylenic carbon at the α position (i.e., C−CH, C−CH2, and C−CH3). Both the hydrogenation reactions and C−C bond coupling that are responsible for the chain growth process have been explored; it is observed that the most likely mechanism of C−C bond propagation involves the recombination of adsorbed C and CH2/CH3 followed by the migratory insertion of hydrogen at the α C atom. The subsequent β-hydride or reductive elimination results respectively in the evolution of ethylene or ethane. The present computational study indicates that the production of ethane is preferred over ethylene, which agrees with the product sel...

Fischer–Tropsch synthesis on sulphur poisoned Co/Al2O3 catalyst

Abstract The effect of sulphur poisoning (in the range 0–2000 ppm) on the characteristics and catalytic performances in the Fischer–Tropsch synthesis (FTS) of a bench-scale alumina supported cobalt catalyst is investigated in this study. It is found that sulphur did not lead to appreciable variations in the catalyst morphological characteristics; however, the catalyst reducibility/hydrogenating capability is significantly modified upon increasing the sulphur loading. The comparison between the catalytic performances of the sulphured samples pointed out that the presence of sulphur remarkably affected the productivity and the selectivity of the reaction. In particular for low S amounts ( These effects have been tentatively associated with different effects of sulphur on the catalyst active sites, i.e. on the sites (or site ensembles) responsible for CO hydrogenation and for the chain growth processes. Finally, the experimental data of CO conversion decay with S-loading were nicely described according to a simple deactivation model which implies a strong effect of sulphur on the catalyst active sites.

Palladium(0)-Catalyzed Trimerization of Arylisocyanates into 1,3,5-Triarylisocyanurates in the Presence of Diimines: A Nonintuitive Mechanism

We show here that palladium(0) (dibenzylideneacetone) complexes bearing 1,10-phenanthroline constitute efficient catalysts for the cyclotrimerization of aromatic isocyanates. For the first time, the mechanism of this reaction has been investigated experimentally and theoretically with group 10 catalysts. This investigation provides a very consistent picture of the catalytic cycle. Notably, we establish that the reaction does not proceed by stepwise cycloadditions or ring insertions involving metallacyclic intermediates, as might have been anticipated. Rather, in our proposal, the initial steps of the mechanism resemble the chain-growth process operative during the anionic polymerization of isocyanates and feature charge-separated intermediates. These steps are then followed by ring closure on the metal center of the last intermediate formed to yield a seven-membered metallacycle that reductively eliminates the cyclotrimer and re-forms the active species. In addition, we conclusively show that the (known) palladacycles that could be isolated during the experimental investigations are not catalytic intermediates but result from catalyst deactivation. Thus, with Pd(0) diimine catalysts, the actual trimerization mechanism appears to be a blend between the two types of mechanisms proposed thus far for the oligomerization of heterocumulenes with very different catalysts. In conclusion, this work contributes to a better understanding of the reactivity of arylisocyanates in the vicinity of late group 10 metal centers in low oxidation state and sheds some light on the detrimental self-poisoning processes observed during the reductive carbonylation of nitroaromatic substrates catalyzed by related catalysts in non-nucleophilic media.

Toward Controlled Gilch Synthesis of Poly(p-phenylenevinylenes): Synthesis and Thermally Induced Polymerization of α-Bromo-p-quinodimethanes

It is general consensus that in Gilch polymerizations the 1,4-bis(halomethylene)benzene starting material first changes into an α-halo-p-quinodimethane intermediate which then acts as the real active monomer in the subsequent chain growth process. Recently, we could verify the formation of α-chloro-p-quinodimethane directly via in-situ NMR spectroscopy at low temperatures. However, quantitative formation of this p-quinodimethane was not possible there. Now, we show that even such quantitative conversion into the active monomer is possible if bromomethylene-functionalized starting materials are used instead of their chloromethylene counterparts. Moreover, it is even possible to induce chain growth leading to PPV in a very controlled way by carefully warming the obtained solution of p-quinodimethane. In this manner, the temperature can be determined where chain growth startsand hence thermal energy is sufficient for the initiating process. Finally, we could reconfirm that the chain growth is a radical polym...

Lessons from simulation regarding the control of synthetic self-assembly

We investigated the role of particle potential symmetry on self-assembly by Monte Carlo simulation with a particular view toward synthetically creating structures of prescribed form and function. First, we established a general tendency for the rotational potential symmetries of the particles to be locally preserved upon self-assembly. Specifically, we found that a dipolar particle potential, having a continuous rotational symmetry about the dipolar axis, gives rise to chain formation, while particles with multipolar potentials (e.g., square quadrupole) having discrete rotational symmetries lead to the self-assembly of “random surface” polymers preserving the rotational symmetries of the particles within these sheet structures. Surprisingly, these changes in self-assembly geometry with the particle potential symmetry are also accompanied by significant changes in the thermodynamic character and in the kinetics of the self-assembly process. Linear chain growth involves a continuous chain growth process in which the chains break and reform readily, while the growth of the two-dimensional polymers only occurs after an “initiation” or “nucleation” time that fluctuates from run to run. We show that the introduction of artificial seeds provides an effective method for controlling the structure and growth kinetics of sheet-like polymers. The significance of these distinct modes of polymerization on the functional character of self-assembly growth is illustrated by constructing an artificial centrosome structure derived from particles having continuous and discrete rotational potential symmetries.

Model Studies Towards Alternative Cross-Linking of Unsaturated Polyesters

Abstract: Fumaric and maleic acid esters are functionalised with propargylamine. A subsequent Pd/Cu-catalysed oxidative homo-coupling of two alkyne moieties yields the corresponding dimers. Hence, a model ofan unsaturated polyester chain is cross-linked in a well-defined, metal-mediated reaction in a very high yield. Keywords : Unsaturated polyester, cross-linking, catalytic, alkyne coupling. INTRODUCTION Unsaturated polyester resins are heterochain polymers,containing repeating ester groups and aliphatic double bondsin the backbone [1]. These chains are generally co-polymerised in a free-radical chain-growth process, with avinyl monomer, usually styrene, to yield the final product, athree-dimensional cross-linked network [1, 2]. Sinceenvironmental legislation becomes more and more stringentconstantly reducing the amount of monomeric components,which can be used in unsaturated polyester products, thedevelopment of alternative cross-linking methods, reducingor completely avoiding possible emissions of harmful,volatile compounds, e.g. of styrene, is of particular interest.Radical cross-linking processes do not yield very welldefined products, instead they give rise to different chainlengths of the linkages, irregular, incomplete consumptionof the double bonds of the polyester heterochain and directlinking of the latter. In addition, side-reactions such ashomopolymerisation of styrene occur [3]. So, we opted for awell-defined metal-mediated reaction that could circumventthese problems. Further considering an unsaturation in thelinkage a desirable feature, since it provides the possibilityfor additional cross-linking if demanded to alter theproperties of the final product, the metal-catalysed couplingof alkynes is highly attractive for its wide scope [4]. Here wereport a model reaction for an alternative cross-linking ofunsaturated polyesters

Enhanced Incorporation of α-Olefins in the Fischer−Tropsch Synthesis Chain-Growth Process over an Alumina-Supported Cobalt Catalyst in Near-Critical and Supercritical Hexane Media

Operating Fischer−Tropsch synthesis (FTS) in supercritical fluid (SCF) media offers advantages over conventional gas-phase FTS operation including in situ extraction of heavy hydrocarbons from the catalyst pores coupled with enhanced incorporation of α-olefins in the chain growth process. In this study, hydrocarbon product distributions in near-critical and supercritical hexane phase FTS (SCH-FTS) was studied over a 15% Co/Al2O3 in a high-pressure fixed-bed reactor system. The critical point of the hexane−syngas−products reaction mixture as collected from the reactor outlet was measured using a variable-volume view cell apparatus. All reactions were carried out in near-critical and supercritical regimes by tuning either the reaction temperature (230−260 °C) or the reaction pressure (30−80 bar). Deviations from the standard Anderson−Schultz−Flory (ASF) chain growth model were observed in most cases; however, the degree of deviation depends on the reaction conditions within the near-critical and supercritic...

Lessons from Simulation Regarding the Control of Synthetic Self-Assembly

AbstractWe investigate the role of particle potential symmetry on self-assembly by Monte Carlo simulation with the particular view towards synthetically creating structures of prescribed form and function. First, we establish a general tendency for the rotational potential symmetries of the particles to be locally preserved upon self-assembly. Specifically, we find that a dipolar particle potential, having a continuous rotational symmetry about the dipolar axis, gives rise to chain formation, while particles with multipolar potentials (e.g., square quadrupole) having discrete rotational symmetries led to the self-assembly of random surface polymers preserving the rotational symmetries of the particles within these sheet structures. Surprisingly, these changes in self-assembly geometry with the particle potential symmetry are also accompanied by significant changes in the thermodynamic character and in the kinetics of the self-assembly process. Linear chain growth involves a continuous chain growth process in which the chains break and reform readily, while the growth of the two-dimensional polymers only occurs after an ‘initiation’ or ‘nucleation’ time that fluctuates from run to run. We show that the introduction of artificial seeds provides an effective method for controlling the structure and growth kinetics of sheet-like polymers. The significance of these distinct modes of polymerization on the functional character of self-assembly growth is illustrated by constructing an artificial centrosome structure derived from particles having continuous and discrete rotational potential symmetries.

Characterization and catalytic properties of cobalt supported on delaminated ITQ-6 and ITQ-2 zeolites for the Fischer–Tropsch synthesis reaction

Cobalt catalysts (ca. 20 wt% Co) supported on all-silica delaminated ITQ-2 and ITQ-6 zeolites have been prepared by impregnation with aqueous Co(NO 3) 3 solutions. The catalysts have been characterized by X-ray diffraction (XRD), nitrogen adsorption, transmission electron microscopy (TEM), temperature-programmed reduction (TPR), and infrared spectroscopy of adsorbed CO. The catalytic properties for the Fischer–Tropsch synthesis (FTS) reaction under typical FTS conditions (493 K, 20 bar, H 2/CO = 2) have been evaluated, and the results compared with those obtained over a conventional Co/SiO 2 and a mesoporous Co/MCM-41 catalyst with comparable cobalt loading. Co/ITQ-6 was the most active catalyst, with a FTS reaction rate which was about 1.5 and 1.8 times higher than that of Co/MCM-41 and Co/SiO 2, respectively. The high activity of Co/ITQ-6 is ascribed both to a relatively good dispersion (as observed by TEM) and to a high reducibility (determined by TPR) of the supported Co 3O 4 particles. The dispersion and reducibility of cobalt particles in Co/ITQ-2 were very close to those in Co/SiO 2. Consequently, both catalysts presented similar FTS reaction rates. In addition, Co/ITQ-6 and Co/ITQ-2 presented a higher selectivity toward the formation of C 5+ hydrocarbons than Co/SiO 2 and Co/MCM-41. The higher C 5+ selectivity of the catalysts based on the delaminated zeolites was ascribed to a higher concentration of coordinatively unsaturated Co 0 sites which are characterized by a band at 1897 cm −1 in the infrared spectrum of adsorbed CO. This type of site having an enhanced electron density might stabilize surface hydrocarbon intermediates favoring chain growth processes.

Iron catalyzed polyethylene chain growth on zinc: a study of the factors delineating chain transfer versus catalyzed chain growth in zinc and related metal alkyl systems.

The bis(imino)pyridine iron complex, [[2,6-(MeC=N-2,6-iPr2C6H3)2C5H)N]FeCl2] (1), in combination with MAO and ZnEt2 (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR2 product, a Poisson distribution of linear alkanes is obtained; linear alpha-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe2 and ZniPr2 also show catalyzed chain growth; in the case of ZnMe2 a slight broadening of the product distribution is observed. The products obtained from Zn(CH2Ph)2 show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh2 shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR3 (R = Me, Et, octyl, IBu) and GaR3 (R = Et, nBu) act as highly efficient chain transfer agents, whereas GaMe3 exhibits behavior close to catalyzed chain growth. LinBu, MgnBu2 and BEt3 result in very low activity catalyst systems. SnMe4 and PbEt4 give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt2 compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn-C bond, and (4) a reasonably close match in Zn-C and Fe-C bond strengths.

Characterization of poly(trimethylene/butylene terephthalate) copolymer prepared by the copolycondensation of bis(3-hydroxypropyl terephthalate) and bis(4-hydroxybutyl terephthalate)

Homopolymers and copolymers were synthesized by polycondensation and copolycondensation, with varying feed ratios of bis(3-hydroxypropyl terephthalate) (BHPT) and bis(4-hydroxybutyl terephthalate) (BHBT) at 270°C. In addition, in the mol ratio of 1:1, copoly(trimethylene terephthalate/butylene terephthalate) [P(TT/BT)], with reaction times of 5, 10, 20, 30, and 60 min, was synthesized to identify the chain-growth process of the copolymers. From differential scanning calorimetry (DSC) data, it was found that a random copolymer might be formed during copolycondensation. The molecular structure of copolymers, formed through the interchange reaction of BHPT and BHBT, was investigated using carbon nuclear magnetic resonance spectroscopy (13C-NMR). We calculated the sequence-length distributions of trimethylene and butylene sequences and randomness in the copolymers using 13C-NMR data. From the values of the number-average sequence length calculated, it was determined that a random copolymer was produced: This result coincides with previous DSC data. The lateral spacing of the unit cell of the copolymer increased slowly when the mol percent of one monomer was increased to that of the other monomer, indicating broadening of the unit cell by lateral distortion. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2200–2205, 2003

Anti-ASF Distribution of Fischer–Tropsch Hydrocarbons in Supercritical-Phase Reactions

Direct wax synthesis from syngas was implemented by supercritical-phase Fischer–Tropsch synthesis (FTS) reaction. By introducing supercritical-phase n-pentane into the reaction, waxy hydrocarbon formation rate was increased significantly while light hydrocarbon formation rate was suppressed effectively. Anti-Anderson–Schultz–Flory distribution of FTS hydrocarbons was realized at lower temperatures, such as 473 K. The intermediate, olefins, readily readsorbed onto the catalyst surface and significantly stimulated the carbon chain-growth process.

First Principles Study of Propene Polymerization in Ziegler−Natta Heterogeneous Catalysis

In this work we address the problem of isotacticity in a realistic heterogeneous Ziegler−Natta system by means of ab initio molecular dynamics. We focus on a previously identified 5-fold catalytic center, and we inspect its ability to select the appropriate olefin enantioface in the chain growth process. We study the first steps in the propene polymerization process and determine the energetics of the initial complexation phase for the different stereochemical orientations of the incoming propene. Then we analyze the subsequent insertions, which represent the crucial issue for the formation of a stereospecific polymer chain, and we find that the 5-fold catalytic center possesses a high degree of stereoselectivity. We examine the role of the agostic interaction, which can switch from α to β and allow, even in the presence of a substrate, processes that can lead to chain termination.

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H2O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differences in physisorption, solubility, or diffusivity. Preferential physisorption of longer hydrocarbons and increase of the solubility with chain length influences the product distribution and results in a decreasing olefin-to-paraffin ratio with increasing chain length. Process development and reactor design should be based on reliable kinetic expressions and detailed selectivity models.

Higher Alcohol Synthesis over Double Bed Cs−Cu/ZnO/Cr2O3 Catalysts: Optimizing the Yields of 2-Methyl-1-propanol (Isobutanol)

Higher alcohol synthesis from H2/CO has been carried out over two tandem beds of a cesium-promoted Cu/ZnO/Cr2O3 catalyst with a temperature difference between the beds. Lower alcohols produced in the low temperature top bed are supplied as oxygenated precursors to the higher temperature bottom bed to react further in the chain growth process leading to the formation of higher alcohols. In this configuration, a high space time yield of 202 g/kg of catalyst/h of 2-methyl-1-propanol (isobutanol) was obtained. This presently observed yield of isobutanol in a double bed configuration with the copper-based catalyst in both beds represents a very significant enhancement over the earlier reported isobutanol yields in a double bed confiuration with a copper-free catalyst in the second bed (Beretta et al., 1996). A kinetic reaction network (Smith et al., 1991) was modified to model this double bed catalyst configuration. This model was then used to obtain kinetic parameters and predict product yields for the reaction system in the current study. It was also demonstrated that the kinetic model could be used as a predictive tool to optimize experimental parameters. In particular, the model was used to predict the optimum mass ratio of the two catalyst beds to produce the maximum yield of isobutanol.

Supercritical-phase process for selective synthesis of heavy hydrocarbons from syngas on cobalt catalysts

Supercritical-phase Fischer–Tropsch synthesis co-fed with 1-tetradecene was carried out over Co/SiO 2 catalysts. It was found that a supercritical fluid could extract the product from the catalyst bed efficiently, and as a result, the mass transfers for both reactants and products were enhanced. 1-Tetradecene added as a chain initiator could participate in the chain growth process. This consequently increased the rate of formation of hydrocarbons larger than C 14 significantly, and decreased the yield of C 1–C 13 hydrocarbons, leading to a flatter carbon number distribution of product than that obtained without olefin addition. In addition, wax composition analysis also showed that the average molecular weight and degree of saturation of wax increased due to addition of 1-tetradecene. The effects of catalyst pore size, concentration of co-fed 1-olefin, and CO/H 2 ratio of syngas were investigated.

Quantitative formation of the intermediate of alkene insertion in the copolymerization of p-methylstyrene and carbon monoxide catalyzed by [(PriDAB)Pd(Me)(NCMe)]+BAr4−

The intermediate after the first CO and p-methylstyrene insertion in the chain growth process of the copolymerization of these monomers has been quantitatively obtained from [(PriDAB)Pd(Me)(NCMe)]+[{3,5-(CF3)2C6H3}4B]– in CHCl3 at 20 °C.

Non-ASF Product Distributions Due to Secondary Reactions during Fischer–Tropsch Synthesis

Since Fischer–Tropsch (FT) synthesis is a chain growth reaction, its total product yield decreases exponentially with chain length forming a so-called Anderson–Schulz–Flory (ASF) distribution. Such a distribution is unselective toward middle distillates for all possible chain growth probabilities. Chain-length-dependent secondary reactions, however, cause deviations from the ASF-type distribution and can thus be used to improve the selectivity to the desired product range. To investigate secondary reactions we set out to study FT synthesis on flat model catalysts, a cobalt foil and cobalt particles on a SiO2wafer, allowing a much better definition of the experiments than porous ones. On a Co foil olefin hydrogenation is the main chain-length-dependent secondary reaction, causing an exponential increase in the paraffin-to-olefin ratio with carbon number, but not resulting in a deviation from the ASF distribution. On Co/SiO2model catalysts chain-length-dependent reinsertion of α-olefins into the chain growth process is the main secondary reaction, causing an increase of the growth probability with chain length. To a lesser extent, hydrogenolysis also plays a role, shortening long hydrocarbons by successive demethylation. On Co/SiO2the interplay of chain-length-dependent reinsertion and hydrogenolysis results in sigmoid product distributions with a high selectivity to middle distillates. These product distributions can be fitted with a simple model in which a chain growth reaction is combined with chain-length-dependent secondary hydrogenation, reinsertion, and hydrogenolysis.