Mechanism Of Chain Growth Research Articles

Nickel-Catalyzed Copolymerization of Ethylene and Vinyltrialkoxysilanes: Catalytic Production of Cross-Linkable Polyethylene and Elucidation of the Chain-Growth Mechanism.

Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CH3CN)+ complexes 4a,b/B(C6F5)3 yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of -Si(OR)3 groups whose ratios depend on temperature and ethylene pressure. Detailed low-temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt2)+ reacts rapidly at -60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(C2H4)+ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for formation of chain-end -Si(OR)3 groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the Ni center to the ether moiety, R-O-Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene homopolymerization, but still remarkably high TOFs of up to 4.5 × 105 h-1 and overall productivities can be achieved. Activation of readily available (α-diimine)NiBr2 complexes 2 with a combination of AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4] yields a highly active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example, copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are precursors to the commercial cross-linked polyethylene, PEX-b.

Mechanism of Cobalt-Catalyzed CO Hydrogenation: 2. Fischer-Tropsch Synthesis.

Fischer–Tropsch (FT) synthesis is one of the most complex catalyzed chemical reactions in which the chain-growth mechanism that leads to formation of long-chain hydrocarbons is not well understood yet. The present work provides deeper insight into the relation between the kinetics of the FT reaction on a silica-supported cobalt catalyst and the composition of the surface adsorbed layer. Cofeeding experiments of 12C3H6 with 13CO/H2 evidence that CHx surface intermediates are involved in chain growth and that chain growth is highly reversible. We present a model-based approach of steady-state isotopic transient kinetic analysis measurements at FT conditions involving hydrocarbon products containing up to five carbon atoms. Our data show that the rates of chain growth and chain decoupling are much higher than the rates of monomer formation and chain termination. An important corollary of the microkinetic model is that the fraction of free sites, which is mainly determined by CO pressure, has opposing effects on CO consumption rate and chain-growth probability. Lower CO pressure and more free sites leads to increased CO consumption rate but decreased chain-growth probability because of an increasing ratio of chain decoupling over chain growth. The preferred FT condition involves high CO pressure in which chain-growth probability is increased at the expense of the CO consumption rate.

Simultaneous step-growth and chain-growth cationic polymerization of styrenic monomers bearing carbazolyl groups

Cationic polymerization of two styrene derivatives containing carbazole moiety with different linking topology, 9-(4-vinylphenyl)carbazole (M1) and 3-(4-vinylphenyl)-9-ethylcarbazole (M2), using 1-chloro-1-phenylethane (PhEtCl)/SnCl4 initiating system has been investigated. It was shown that polymerization of M1, which is characterized by higher electron density on the carbon at the 3 position of carbazolyl group than on vinyl group, proceeds predominantly via step-growth (SG) pathway through addition of protonated monomer/oligomers to 3-position of carbazolyl group (electrophilic aromatic substitution). On the contrary, M2, in which 3-position of carbazolyl group is protected via introduction of 4-vinylphenyl group, polymerized predominantly via chain-growth (CG) pathway through addition of carbocation to double bond of monomer. The polymers synthesized via SG mechanism showed different photophysical characteristics in comparison with corresponding monomer, while in case of polymers prepared via CG mechanism the polymer and monomer have the same absorption and fluorescence profiles.

Controlled ring-opening polymerization of α-amino acid N-carboxyanhydrides in the presence of tertiary amines

The mechanism of the primary ammonium/tertiary amine-mediated ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride (BlG-NCA) was investigated. Kinetic analyses revealed that the normal amine mechanism (NAM) together with a dormant-active chain end equilibrium were responsible for the controlled nature of this polymerization pathway, but that the polymerization also proceeded via the activated monomer mechanism (AMM). Mixtures of primary amines (1 equiv) and tertiary amines (0–1.5 equiv) were therefore tested to confirm the co-existence of the NAM and AMM and determine the limits for a controlled polymerization. For tertiary amine molar fractions smaller than 0.8 equiv, the reaction times were greatly reduced (compared to primary amine-initiated polymerization) without compromising the control of the reaction. Hence, the polymerization of NCA can proceed in a controlled manner even when the AMM contributes to the overall chain growth mechanism.

Insight into CH4 formation and chain growth mechanism of Fischer−Tropsch synthesis on θ-Fe3C(0 3 1)

θ-Fe3C has been reported to play a key role in C2−C4 olefins production for Fischer−Tropsch synthesis. In this work, (031) is confirmed as the most exposed facet of θ-Fe3C due to its thermodynamic stability. Investigation on CH4 formation and C1−C1 coupling reveals that CH4 formation exhibits a high effective barrier and CH+CH and CH2+CH2 are the dominant chain growth pathways. ΔEeff was employed to quantify the selectivity of CH4 and C2+. The high value of ΔEeff indicates the preference of C2+ formation to CH4. These results will help for the design of selective Fe-based FT catalysts.

High-Molecular-Weight Polynucleotides by Transferase-Catalyzed Living Chain-Growth Polycondensation.

We present terminal deoxynucleotidyl transferase-catalyzed enzymatic polymerization (TcEP) for the template-free synthesis of high-molecular-weight, single-stranded DNA (ssDNA) and demonstrate that it proceeds by a living chain-growth polycondensation mechanism. We show that the molecular weight of the reaction products is nearly monodisperse, and can be manipulated by the feed ratio of nucleotide (monomer) to oligonucleotide (initiator), as typically observed for living polymerization reactions. Understanding the synthesis mechanism and the reaction kinetics enables the rational, template-free synthesis of ssDNA that can be used for a range of biomedical and nanotechnology applications.

A density functional theory based elementary reaction mechanism for early steps of Fischer-Tropsch synthesis over cobalt catalyst. 2. Microkinetic modeling of liquid-phase vs. gaseous-phase process

The process of Fischer-Tropsch synthesis (FTS) is conducted commercially under both gaseous- and liquid-phase conditions. For decades, numerous studies have been performed to better understand this heterogeneous catalytic process, where some of them have tried to compare the reaction rates and product distributions of FTS under different phase conditions. In spite of several provided models, the actual reaction mechanisms and kinetics of important elementary steps at each phase condition are still under investigation, especially in the liquid-phase. In this work, we have utilized the generalized-gradient approximation method of density functional theory (DFT-GGA) to estimate the rate parameters of elementary reactions in liquid-phase FTS on a flat Co(0001) catalyst surface. In this regard, using the continuum-based conductor-like screening model (COSMO) for the liquid solvent environment, the activation energies and rate constants of individual reactions, suggested in an existing microkinetic model, have been calculated in the presence of n-hexane solvent at T=493K and P=25bar. The preferred reaction pathway has been discussed and the quantitative results have been compared to that of the gaseous-phase in order to find the impact of liquid solvent, as well as differences in the probable surface reactions under each phase condition. It has been found that while the carbide mechanism is the dominant mechanism of chain growth under both phase conditions, the presence of the liquid solvent enhanced the CO insertion sub-mechanism of chain growth, which can describe some of the experimental observations in FTS studies.

CAL-B catalyzed regioselective bulk polymerization of l-aspartic acid diethyl ester to α-linked polypeptides.

This paper reports that the bulk polymerization of l-aspartic acid diethyl ester catalyzed by immobilized CAL-B at 80 °C for 24 h gives primarily (∼95%) α-linked poly(l-aspartate) in 70% yield with DPavg = 50 and regioselectivity (α/β) = 94 : 6. Plots of log{[M]0/[M]t} vs. time and DPavgvs. conversion indicate that this polymerization proceeds in a controlled manner by a chain-growth mechanism up to 90% conversion. Thereafter, competition occurs between chain growth and step mechanisms.

Enzymatic synthesis of poly(ε-caprolactone-co-ε-thiocaprolactone)

This study presents the successful enzymatic ring opening copolymerization of ε-caprolactone (ε-CL) and ε-thiocaprolactone (ε-TCL) catalyzed by Novozyme 435, immobilized lipase B from Candida antarctica. Various monomers feed ratio (from 0 to 100% in TCL content) were tested using two polymerization strategies. The first one is a direct one-step strategy, starting with both monomers mixed in the feed, while the two-steps strategy consists in a first step of ε-CL homopolymerization followed by a second step with ε-TCL addition in the medium and its polymerization from the previously formed PCL chains. The obtained copolymers have been fully characterized by NMR, DSC, SEC and MALDI-ToF MS spectroscopy. Main results of the one-step strategy showed a decrease in the final copolymer molar mass with the increase in ε-TCL content as well as minimum values of crystallinity and melting temperature for the 50:50 feed composition. An in-depth analysis of the MALDI-TOF mass spectra of the copolymers showed interesting patterns that suggest a micro-structuration of polymers from the one step strategy resulting from an uncommon multistep chain growth mechanism of trimers addition. The two-steps strategy produced copolymers showing a more blocky structure, as confirmed by their two distinct melting temperatures.

A Novel Chain-Growth CuAAC Polymerization: One-pot Synthesis of Dendritic Hyperbranched Polymers with Well-Defined Structures

This highlight presents an overview of our recent achievements on developing a chain-growth copper-catalyzed azide–alkyne cycloaddition (CuAAC) polymerization of multifunctional ABm (m ≥ 2) monomers to produce structurally defined hyperbranched polymers in one pot. The chain-growth mechanism is attributed to the dedicate complexation between copper(I) catalyst and triazole groups that confine the copper catalyst in the polymers and selectively favor the polymer–monomer reaction rather than the monomer–monomer reactions. The living nature of this CuAAC polymerization was extensively explored to demonstrate the intriguing features of multibatch addition of various AB2 monomers to produce hyperbranched polymers with high molar mass, low dispersity, core-shell segmented structures, and tunable solubility. 1 Background 2 Development of Living Chain-Growth CuAAC Polymerization 3 Functionalization and Segmented Structure in Hyperbranched Polymers 4 Conclusion

Impact of Preferential π-Binding in Catalyst-Transfer Polycondensation of Thiazole Derivatives.

Polymerizing electron-deficient arenes in a controlled, chain-growth fashion remains a significant challenge despite a decade of research on catalyst-transfer polycondensation. The prevailing hypothesis is that the chain-growth mechanism stalls at a strongly associated metal-polymer π-complex, preventing catalyst turnover. To evaluate this hypothesis, we performed mechanistic studies using thiazole derivatives and identified approaches to improve their chain-growth polymerization. These studies revealed a surprisingly high barrier for chain-walking toward the reactive C-X bond. In addition, a competitive pathway involving chain-transfer to monomer was identified. This pathway is facilitated by ancillary ligand dissociation and N-coordination to the incoming monomer. We found that this chain-transfer pathway can be attenuated by using a rigid ancillary ligand, leading to an improved polymerization. Combined, these studies provide mechanistic insight into the challenges associated with electron-deficient monomers as well as ways to improve their living, chain-growth polymerization. Our mechanistic studies also revealed an unexpected radical anion-mediated oligomerization in the absence of catalyst, as well as a surprising oxidative addition into the thiazole C-S bond in a model system.

Thiol‐Ene versus Binary Thiol–Acrylate Chemistry: Material Properties and Network Characteristics of Photopolymers

The versatile nature of photo‐induced thiol‐ene chemistry has made it one of the most popular reactions in the design of polymer networks. In the present work, the structure of uniform thiol‐ene and complex binary thiol‐acrylate networks are studied in detail by photopolymerizing cycloaliphatic and linear alkyl thiols with an allyl monomer and its acrylic counterpart. Along with the crosslink kinetics, the balance between step growth and chain growth mechanism of the two different systems is investigated by FT‐IR spectroscopy. Low field NMR experiments are carried out to obtain an insight into the chain dynamics and structure of the photopolymer networks. In addition, tensile properties and glass transition temperature are determined to correlate network properties with mechanical performance.

Synthesis of Highly Fluorescent All-Conjugated Alternating Donor–Acceptor (Block) Copolymers via GRIM Polymerization

Although controlled polymerization procedures for conjugated polymers have considerable advantages with respect to molar mass and end group control, the material scope has been very limited, in particular considering block copolymers and donor–acceptor type all-conjugated polymers, imposing considerable challenges upon the synthetic polymer community. In this work, a push–pull monomer consisting of a thiophene (donor) and a pyridine (acceptor) unit is synthesized and subsequently polymerized via Kumada catalyst-transfer polymerization using a nickel catalyst (GRIM polymerization). In this way, an alternating donor–acceptor copolymer is obtained via a chain-growth mechanism. Furthermore, an all-conjugated block copolymer containing a poly(3-hexylthiophene) block and the alternating copolymer is successfully prepared in a one-pot procedure as well. The diblock structure is confirmed by comparison of the thermal, electrochemical, and spectroscopic properties of the block copolymer and its constituting polyme...

CO Hydrogenation to Higher Alcohols over Cu–Co-Based Catalysts Derived from Hydrotalcite-Type Precursors

Cu–Co-based catalysts derived from hydrotalcite (HT)-type precursors were applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a H2/CO ratio of 1/1. Catalysts with higher Cu/Co ratios were found to provide the best trade-off between selective alcohol formation and moderate Fischer–Tropsch synthesis (FTS) activity. Within the alcohols and hydrocarbons formed the productivities decreased exponentially with increasing chain length according to the ASF distribution indicating a chain growth mechanism. Thermal analysis revealed the presence of different bivalent cations in one single HT-type precursor phase. After calcination at lower temperatures (Tcalc < 600 °C) a carbonate-modified ZnAl2O4 matrix was obtained. Within this amorphous matrix Cu2+ and Co2+ were found to be partially embedded resulting in an impeded ion reduction. After HAS the presence of bulk Co2C was detected by XRD. Both close contact of Cu0 and Co0 as well as Co2C–Co0 interfaces are known to provide the mechanistic requirements for higher alcohol formation. For comparison HAS was performed over a physical mixture consisting of the Al-containing HTs of Cu, Co or Zn. For the simultaneously co-precipitated samples the major roles of Cu are to decrease the FTS activity of metallic Co and to lower the alcohol chain growth probability by intimate Cu0–Co0 interactions. With increasing Cu content the alcohol selectivities were found to increase at the expense of high conversion, with ethanol being the major oxygenate product for all HT-based catalysts.

Ring-Opening Alternating Copolymerization of Epoxides and Dihydrocoumarin Catalyzed by a Phosphazene Superbase

3,4-Dihydrocoumarin (DHC), a six-membered phenolic lactone derived from natural resources, which does not undergo homopolymerization, was subject to ring-opening anionic copolymerization with several epoxides (ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide, and 2-ethylhexyl glycidyl ether) from a mono- or dialcohol catalyzed by a phosphazene superbase (t-BuP4). Spectroscopic analysis revealed that the products were aromatic poly(ether–ester)s with perfect alternating monomeric sequences, consisting of both linear alternating copolymers and cyclic ones with relatively lower molar masses caused by intramolecular transesterification reactions. The alternating copolymers showed good thermal stability and higher glass transition temperatures as compared with the homopolymers of the corresponding epoxides. The in situ initiator-activation and chain-growth mechanism of the t-BuP4-catalyzed polymerization also allowed for copolymerization of DHC and epoxides from multifunctional initiators, su...

Mesoporous Hybrid Polypyrrole-Silica Nanocomposite Films with a Strata-Like Structure

Using a single-potential-step coelectrodeposition route, Ppy-SiO2 nanocomposite films characterized by a multimodal porous structure were cathodically deposited from ethanolic solutions on oxidizable and nonoxidizable substrates for the first time. The materials produced have an interesting and unique strata-like pore structure along their depth. With the exception of a silica-rich inner region, the nanocomposite films are homogeneous in composition. Because the region closest to the electrode surface is silica-rich, the fabrication of Ppy-SiO2 and Ppy free-standing films become possible using a multistep etching strategy. Such films can be captured on a variety of different supports depending on the application, and they maintain their conductivity when interfaced with an electrode surface. These mesoporous composite films form through a unique mechanism that involves the production of two catalysts, OH(-) and NO(+). Through the process of understanding the reaction mechanism, we highlighted the effect of two simultaneous competing redox reactions occurring at the electrode interface on the morphology of the electrodeposited Ppy nanocomposite films and how solvent can influence the Ppy electropolymerization reaction mechanism and hence control the morphology of the final material. In an ethanolic solvent system, the pyrrole monomers undergo a step-growth polymerization, and particulate-like nanostructured films were obtained even upon changing the monomer or acid concentration. In an aqueous-based system, nanowire-like structures were produced, which is consistent with a chain-growth mechanism. Such materials are promising candidates for a wide range of applications including electrochemical sensing, energy storage, and catalysis.

Comparison between polymerized ionic liquids synthesized using chain-growth and step-growth mechanisms used as stationary phase in gas chromatography

In this study the merits of polymerized imidazolium based ionic liquid (PIL) stationary phases obtained via condensation and free radical polymerizations are compared as stationary phases in gas chromatography (GC). Poly(1-vinyl-3-butyl-imidazolium – bis(trifluoromethane)sulfonamide) (poly(ViC4Im+ NTf2−)) was obtained via a chain-growth mechanism while poly(propylimidazolium-NTf2) (poly(C3Im+ NTf2−)) was synthesized via a step-growth polymerization. The thermal stability of both polymers was assessed using thermal gravimetric analysis and compared with bleeding profiles obtained from the statically coated GC columns (30m×0.25mm×0.25μm). The performance was compared to what could be obtained on commercially available 1,5-di(2,3-dimethylimidazolium)pentane2+ 2NTf2− (SLB-IL111) ionic liquid based columns. It was observed that the step-growth polymer was more thermally stable, up to 325°C, while the chain-growth polymer showed initial degradation at 250°C. Both polymers allowed reaching minimal plate heights of 0.400–0.500mm for retained solutes such as benzaldehyde, acetophenone, 1-methylnaphthalene and aniline. Assessment of the McReynolds constants illustrated that the polarity of the step-growth polymer was similar to the SLB-IL111 column, while displaying improved column stability. The PIL phases and particularly the so far little studied condensation based polymer shows particular retention and satisfactory column performance for polar moieties such as esters, amine and carbonyl functionalities.

Preparation of an Aurylated Alkylthiophene Monomer via C-H Activation for Use in Pd-PEPPSI-iPr Catalyzed-Controlled Chain Growth Polymerization.

In the search for new synthetic routes toward greener and more facile syntheses of conjugated polymers, C-H functionalization provides a promising solution by minimizing the production and processing of aryl halide monomer precursors used in traditional organometallic coupling reactions. In this paper, we investigate the use of Au(I) and its ability to directly C-H activate 2-bromo-3-hexylthiophene to form a reactive monomer species, bypassing the typical Grignard monomer formation from a dihalogenated thiophene. Addition of Pd-PEPPSI-iPr as a palladium catalyst source in the presence of the resultant aurylated thiophene monomer yielded poly(3-hexylthiophene) as observed by both NMR and GPC. Studies on the growth of these polymers show linear dependence between Mn and monomer conversion, low dispersities, as well as Mn predicted by catalyst loading, which is supportive of a living-type chain growth mechanism. This Au-Pd system represents a novel methodology for incorporating C-H activation into the synthesis of P3HT with control over Mn.

Semi-renewable latex via step-photopolymerization of dithiol and dienic terpenes

Abundance, low-cost and ene functionality are the key assets of terpenes in view of a large-scale use as renewable monomers in radical polymerization. However, their sterically hindered ene structure has been mostly ineffective in a chain-growth mechanism, leading to sluggish activity. To address this challenge, the alternative of a step-growth diene–dithiol linear photopolymerization is investigated here. Four non-modified dienic terpenes – geranyl acetate, linalool, limonene and geraniol – are reacted with ethylene glycol dithiol in various conditions. In bulk and solution, limited conversions prevent the formation of high-molecular-weight polymers. By contrast, a mini-emulsion containing the two difunctional comonomer yields higher conversions (&gt;98%) in less than 10 min of ultraviolet irradiation, making the formation of polythioether chains possible. The authors assume that a thiol–ene polymerization confined in nanodroplets may drive high conversion thanks to a radical compartmentalization effect. The resultant aqueous polymer dispersions show an average diameter ranging from 100 to 140 nm, low glass transition temperature (less than −30°C), but only moderate molecular weights between 4 and 5 kDa for the fraction soluble in tetrahydrofuran. These promising results pave the way for a novel semi-synthetic and eco-efficient approach to polymer latexes based on natural terpene monomers.

Chain-growth click polymerization of AB2 monomers for the formation of hyperbranched polymers with low polydispersities in a one-pot process.

Hyperbranched polymers are important soft nanomaterials but robust synthetic methods with which the polymer structures can be easily controlled have rarely been reported. For the first time, we present a one-pot one-batch synthesis of polytriazole-based hyperbranched polymers with both low polydispersity and a high degree of branching (DB) using a copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization. The use of a trifunctional AB2 monomer that contains one alkyne and two azide groups ensures that all Cu catalysts are bound to polytriazole polymers at low monomer conversion. Subsequent CuAAC polymerization displayed the features of a "living" chain-growth mechanism with a linear increase in molecular weight with conversion and clean chain extension for repeated monomer additions. Furthermore, the triazole group in a linear (L) monomer unit complexed Cu(I) , which catalyzed a faster reaction of the second azide group to quickly convert the L unit into a dendritic unit, producing hyperbranched polymers with DB=0.83.