Mechanism Of Chain Growth Research Articles

Mechanistic Insight into Catalyst-Transfer Polymerization of Unusual Anion-Radical Naphthalene Diimide Monomers: An Observation of Ni(0) Intermediates

Ni-catalyzed polymerization of anion-radical complexes formed upon mixing of 2,6-bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-octyldodecyl) diimide and activated Zn powder was investigated. We provide experimental evidence that the polymerization involves the chain-growth mechanism and proceeds via Ni(0) complexes which were previously proposed to be key intermediates in other chain-growth catalyst-transfer polycondensations (CTPs), such as Kumada CTP of thiophenic monomers. DFT calculations predict a remarkable stability of the naphthalene diimide-based Ni(0) complexes. A plausible polymerization mechanism is proposed.

Controlled Pd(0)/t-Bu3P-Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with PhPd(t-Bu3P)I or Pd2(dba)3/t-Bu3P/ArI as the Initiator

Controlled Pd(0)/t-Bu(3)P-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers via the chain-growth mechanism with an ArPd(t-Bu(3)P)I complex as the initiator are described. ArPd(t-Bu(3)P)I complexes, either prepurified or generated in situ from Pd(2)(dba)(3)/t-Bu(3)P/ArI (dba = dibenzylideneacetone) without separation/purification, were found to be efficient initiators in general for the controlled Suzuki cross-coupling polymerization, with narrow polydispersity indexes (PDIs) of 1.13-1.35 being observed. The Pd(2)(dba)(3)/t-Bu(3)P/p-BrC(6)H(4)I combination was identified as a highly robust initiator system, with PDIs of ≤1.20 in general and as low as 1.13 being obtained. Higher number-average molecular weights (M(n)) were achieved without a significant increase in the PDI (from 1.14 for a polymer with a M(n) = 9500 to 1.20 for a polymer with M(n) = 31,400) by using a smaller amount of the Pd(2)(dba)(3)/t-Bu(3)P/p-BrC(6)H(4)I initiator in the polymerization.

Molecular Dynamics Simulations of Methanol to Olefin Reactions in HZSM-5 Zeolite Using a ReaxFF Force Field

A ReaxFF force field has been extended and used in the present work to simulate methanol to olefin (MTO) reactions in H-ZSM-5 zeolite. By explicitly considering multibody interactions and thermodynamic conditions, the initial reaction network of MTO in acidic zeolite has been obtained. New reaction mechanisms are proposed based on the simulations. For the activation of methanol, a less possible but very important CH3 radical mechanism is identified in addition to the commonly accepted methoxyl mechanism. The commonly accepted chain-growth mechanism, in which ethene interacts with methyloxide, has been observed. However, it is a small contribution to the total production. The more popular route for the chain growth is attributed to the presence of deprotonated Bronsted sites, which are produced via the activation of methanol molecules. Therefore, the hydrocarbon pool is working with the methanol molecules involved. With the hydrocarbon pool, the chain growth is significantly accelerated. Considering the co...

Ultra-precise insertion of functional monomers in chain-growth polymerizations

Chain-growth polymerizations are popular methods because they allow synthesis of high-molecular weight polymers in high yields and in short times. However, copolymers prepared by such processes generally exhibit uncontrolled monomer sequences. The controlled radical copolymerization of styrene with N-substituted maleimides is an interesting exception allowing preparation of controlled primary structures. However, because of the statistical nature of chain-growth mechanisms, sequence deviations are still present in these copolymers. Here we describe a specific range of experimental conditions that allows ultra-precise incorporation of a single N-substituted maleimide unit in a polystyrene chain. This occurs in a given kinetic regime where the styrene/N-substituted maleimide comonomer ratio is very low. This situation usually only arises in the later stages of a chain-growth polymerization. Nevertheless, we show that it is possible to restore these particular kinetic conditions multiple times during a single polymerization by using successive feeds of donor and acceptor comonomers.

Reaction pathways of halocarbon catalytic oligomerization

The hydrodechlorination of CH2Cl2, CHCl3, and their mixtures catalyzed by a Pt-Co/C catalyst has been investigated in an effort to elucidate the chemistry associated with the generation of hydrocarbon oligomerization products. Both saturated and unsaturated C2 + hydrocarbons were observed with all three reaction mixtures, with the selectivity following the order CH2Cl2 + H2 < CHCl3 + H2 < CH2Cl2 + CHCl3 + H2. The results suggest that C2 + paraffins are formed by means of the alkyl mechanism of hydrocarbon chain growth; whereas, the alkenyl mechanism is responsible for olefin formation.

Detailed Kinetics of the Fischer–Tropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

A complete mechanistic kinetic model of the Fischer–Tropsch synthesis (FTS) over a Co/Al2O3 state-of-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H2 consumption and n-paraffins, α-olefins and H2O formation are derived. Such expressions are functions of the molar fraction of CO, H2 and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by non-linear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8–25 bar, T = 210–235 °C, H2/CO feed ratio = 1.8–2.7 mol mol−1, GHSV = 2,000–7,000 cm3(STP) h−1 gcat−1). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer–Tropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.

Kumada Catalyst‐Transfer Polycondensation: Mechanism, Opportunities, and Challenges

Kumada catalyst-transfer polycondensation (KCTP) is a new but rapidly developing method with great potential for the preparation of well-defined conjugated polymers (CPs). The recently discovered chain-growth mechanism is unique among the various transition metal-catalyzed polycondensations, and has thus attracted much attention among researchers. Most progress is found in the areas of mechanism and external initiation via new initiators, but also the number of monomers other than thiophene that can be polymerized is steadily increasing. Accordingly, the variety of CP chain architectures is increasing as well, and a considerable contribution of KCTP toward more efficient materials can be expected in the future. This review critically focuses on very recent progress in the synthesis of CPs and the mechanism of KCTP, and is finally aimed at providing a comprehensive picture of this exciting polymerization method.

COPOLYMERIZATION OF CYCLOHEXENE OXIDE AND CARBON DIOXIDE CATALYZED BY ALUMINUM PORPHYRIN

Chloro (5, 10, 15, 20-tetraphenyl-porphyrinato) -aluminum (TPPA1C1) in combination with bis(triphenylphosphine) iminium chloride (PPNCl) was used as binary catalyst for the copolymerization of cyclohexene oxide and CO,. After 9 h reaction at 80 C, poly (1, 2-cyclohexylene carbonate) (PCHC) with narrow molecular weight distribution (M(w)/M(n) = 1. 12) was obtained and the yield was as high as 97. 2%. However, the number average molecular weight was only 6.8 x 10(3). It' s found that the concentration and polarity of solvents have little effect on the chain transfer reaction, whereas a decrease in monomer concentration leads to a remarkable reduction in the chain transfer reactions, indicating that there exists obvious chain transfer to monomers. Furthermore, the structure of PCHC was analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS) technique, upon which the mechanism of chain growth and chain transfer reaction in the polymerization process was proposed: during the copolymerization of cyclohexene oxide and CO, catalyzed by TPPAlCl/PPNCl system, besides the added initiator, newly active centers were generated by chain transfer, thereby the number of the polymer molecules exceeded the number of the added aluminium porphyrin molecules. Furthermore, the narrow molecular weight distribution of the obtained polymer indicates that the rate of chain exchange is much faster than that of chain propagation. The reversibility of the rapid exchange reaction is the key of alternation of chain termination and chain propagation.

One-Pot Multimolecular Macrocyclization for the Expedient Synthesis of Macrocyclic Aromatic Pentamers by a Chain Growth Mechanism

POCl(3)-mediated one-pot macrocyclization allows the highly selective formation of five-residue macrocycles that are rigidified by internally placed intramolecular hydrogen bonds. Mechanistic investigation by using tailored competition experiments and kinetic simulation provides a comprehensive model, supporting a chain-growth mechanism underlying the one-pot formation of aromatic pentamers, whereby the successive addition of a bifunctional monomer unit onto either another monomer or the growing oligomeric backbone is faster than other types of bimolecular condensations involving oligomers longer than monomers. DFT calculations at the B3LYP/6-31G* level reveal the five-residue pentamer to be the most stable with respect to alternative four-, six-, and seven-residue macrocycles. These novel mechanistic insights may become useful in analyzing other macrocyclization, oligomerization, and ploymerization reactions.

Chain growth mechanism of Fischer–Tropsch synthesis on Fe 5C 2(0 0 1)

The chain growth mechanisms of Fischer–Tropsch synthesis on Fe 5C 2(0 0 1) were investigated at the levels of density functional theory. On the H 2 and CO co-adsorbed surface, the formation of CH and CCO is the most favored initial steps. The subsequent steps of CCO coupling with C into CCCO and CCO hydrogenation into CCH 2 and CHCH are competitive. Furthermore, CCH from CCH 2 and CHCH dehydrogenation can couple with C to form CCCH. Since chain initiation from CO insertion obeys insertion mechanism, and chain propagation from CCH coupling obeys carbide mechanism, both mechanisms are operative and co-operative in Fischer–Tropsch synthesis. The carburized active surfaces can be regenerated and maintained by CO adsorption on the vacancy site, followed by hydrogenation into surface formyl (CHO) and successive dissociation into surface CH and O. In addition surface O can be hydrogenated into surface OH, and H 2O formation from surface OH disproponation is energetically more favored than surface OH hydrogenation.

Synthesis, Electronic Structure, and Ethylene Polymerization Activity of Bis(imino)pyridine Cobalt Alkyl Cations

A new spin on polymers: the title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.

Ligand-Based Steric Effects in Ni-Catalyzed Chain-Growth Polymerizations Using Bis(dialkylphosphino)ethanes

The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. Three different Ni(L-L)Cl2 catalysts were synthesized using commercially available bis(dialkylphosphino)ethane ligands with varying steric properties. One of these catalysts (Ni(depe)Cl2) outperformed the others for this polymerization. The polymer characterization data were consistent with a chain-growth mechanism. Rate and spectroscopic studies revealed a rate-limiting reductive elimination for both initiation and propagation with Ni(depe)Cl2. In contrast, less hindered Ni(dmpe)Cl2 and more hindered Ni(dcpe)Cl2 were ineffective polymerization catalysts; NMR spectroscopic studies indicated that competing decomposition and uncontrolled pathways intervene. For other monomers, Ni(depe)Cl2 performed similar to the conventional catalysts. Copolymerization studies revealed that block copolymers could be effectively prepared. Overall, these studies indicate that altering the ligand-based steric properties can have a significant impact on the chain-growth polymerization.

One-Pot Synthesis of Hybrid Macrocyclic Pentamers with Variable Functionalizations around the Periphery

Rather than four- or six-residue macrocylces, one-pot macrocyclization allows for the highly selective formation of five-residue macrocycles rigidified by intramolecular hydrogen bonds. Variable functionalizations around the pentameric periphery were achieved by reacting monomers with higher oligomers bearing different exterior side chains. The formation of these hybrid pentamers suggests a chain-growth mechanism for the one-pot macrocyclization where the successive addition of monomers onto higher oligomers is faster than those between two monomers or two higher oligomers.

Surface-mediated chain reaction through dissociative attachment

Chain reactions on a surface offer an important route to linear nanopatterning. We recently reported cooperative reactions on a silicon surface in which the reaction of one halogen atom with a silicon atom of a silicon dimer induced the halogenation of its neighbouring silicon atom through surface-mediated charge transfer. The reaction was unable to propagate further but here we describe how, by chemically bridging the gaps between the rows of these silicon dimers, this mechanism is able to form extended chains. The agents for chain growth are CH(3)Cl molecules that dissociatively attach CH(3) groups and chlorine atoms to silicon atoms from different dimers. By means of charge transfer through the surface, this gives rise to dangling bonds adjacent to the CH(3) groups and chlorine atoms (in effect, 'free radicals') that dissociate further incoming CH(3)Cl molecules, thereby providing the growing points for chains of indefinite length. This versatile mechanism of chain growth is examined in experiments and using ab initio theory.

Unraveling the Mechanism of Polymerization with the Phillips Catalyst

The mechanism of polymer chain growth at the Phillips catalyst has been studied, with the three prevailing mechanistic proposals, the Cosee−Arlman, Green−Rooney, and metallacycle mechanisms, considered. Through analysis of low molecular weight oligomers/polymers formed during ethylene/a-olefin copolymerization with labeled monomers, it is shown that the isotopomer distribution is inconsistent with a metallacycle mechanism. Further analysis of polymer formed by copolymerization of labeled ethylene was used to rule out a Green−Rooney mechanism. The results support the notion of chain growth via a Cossee−Arlman process.

Aspekte und Probleme der Synthesegaschemie

Synthesis gas or methanol (or other C1 components) are sources for the production of all important basic or intermediate products of the organic chemical industry. CO hydrogenation leads to a broad product distribution given by an unselective chain growth mechanism. Therefore, an important target of catalyst research in the field of CO hydrogenation is the limitation of the selectivity problem. Following this essential task chain growth is to be limited, the range of the C number of products is to be narrowed, hydrogenation activity is to be varied, and oxygenated products are to be formed selectively. Approaches to circumvent some selectivity problems may be possible by the application of shape selective catalysts, by the modification of some classical CO hydrogenation catalyst systems or by use of unconventional catalysts. At present selective formation is possible by the separation of the hydrogenation step to the C1 component methanol and of the step of chain growth. In connection with carbonylation and hydrocarbonylation reactions the heterogenisation of complex catalysts may be of future importance.

Kumada chain-growth polycondensation as a universal method for synthesis of well-defined conjugated polymers

Kumada chain-growth polycondensation (KCGP) is a novel method for the synthesis of well-defined conjugated polymers. Because the Ni-catalyst can transfer in an intramolecular process to the propagating chain end, the polymerization follows chain-growth mechanism. With this newly developed method, various conjugated polymers, such as polythiophenes, poly(p-phenylene) (PPP), polypyrrole (PPy), and polyfluorene with controlled molecular weights and relatively narrow polydispersities (PDIs), have been prepared. Especially, the polymerizations for poly(3-alkylthiophene)s (P3ATs), PPP, and PPy exhibited quasi-living characteristics, which allows preparing polymer brushes, fully-conjugated block copolymers, and macroinitiators and macro-reactants for the synthesis of rod-coil block copolymers. In the current review, the progress in this new area is summarized.

The effect of electrostatic and electrohydrodynamic forces on the chaining of carbon nanofibres in liquid epoxy

The formation of chains of aligned carbon nanofibres (CNFs) in polymers is a subject of great interest in the field of multifunctional nanocomposites. The mechanism of CNF chain assembly and growth in a low viscosity epoxy is investigated by developing a finite element model of a chain attached to an electrode. The model examines the combined effects of electrostatic and electrohydrodynamic forces on chain morphology. The electrohydrodynamic forces are modelled using the theory of ac electro-osmosis. The predictions of the model are supported by experimental results. The experiments were conducted on a CNF/epoxy/amine mixture by applying an ac field at frequencies ranging from 100 to 100 000 Hz. The predictions of the model qualitatively capture the variations of chain morphology and growth rate as functions of ac frequency. Higher frequencies promote a more uniform and denser network of chains. The rate of growth of chains is highest at an intermediate frequency. A uniform network of chains was observed at frequencies of 1 kHz and greater in the experiments. The rate of growth of chains was maximized at a frequency of 1 kHz for a liquid viscosity of 0.03 Pa s.

Chain-Growth Synthesis of Polyfluorenes with Low Polydispersities, Block Copolymers of Fluorene, and End-Capped Polyfluorenes: Toward New Optoelectronic Materials

Here we present the kinetics on the polymerization of poly(9,9-dioctylfluorene) using the Grignard metathesis method (GRIM). By adjusting the reaction conditions, we observed a chain-growth mechanism evidenced by the relatively high molecular weight polymer produced early in the reaction. Well-defined fluorene polymers with average molecular weights of about 10 kDa and with PDI values <1.2 have been produced. We have also found that under certain conditions end-capping of the polymer chain can be obtained. Using these conditions, we have also been able to synthesize two new block copolymers, poly(9,9-dioctylfluorene)-b-poly(3-hexylthiophene) (PF8-b-P3HT) and poly(9,9-dioctylfluorene)-b-poly(1,4-dioctyloxyphenylene) (PF8-b-PPP), that have been produced through chain extension by monomer addition.

Polymerization Mechanism of α-Linear Olefin

The density functional theory on the level of B3LYP/6–31G was empolyed to study the chain growth mechanism in polymerization process of α-linear olefin in TiCl3/AlEt2Cl catalytic system to synthesize drag reduction agent. Full parameter optimization without symmetry restrictions for reactants, products, the possible transition states, and intermediates was calculated. Vibration frequency was analyzed for all of stagnation points on the potential energy surface at the same theoretical level. The internal reaction coordinate was calculated from the transition states to reactants and products respectively. The results showed as flloes: (i) Coordination compounds were formed on the optimum configuration of TiCl3/AlEt2Cl. (ii) The transition states were formed. The energy difference between transition states and the coordination compounds was 40.687 kJ/mol. (iii) Double bond opened and Ti–C(4) bond fractured, and the polymerization was completed. The calculation results also showed that the chain growth mechanism did not essentially change with the increase of carbon atom number of α-linear olefin. From the relationship between polymerization activation energy and carbon atom number of the α-linear olefin, it can be seen that the α-linear olefin monomers with 6–10 carbon atoms had low activation energy and wide range. It was optimum to synthesize drag reduction agent by polymerization.