Mechanism Of Chain Growth Research Articles

A study of the mechanism of curing of the dicyandiamide-epoxydiane oligomer system in the presence of asymmetrical urea

Catalyzed epoxy systems have been studied consisting of an ED-20 epoxy diane oligomer ED-20, a curing catalyst (dicyandiamide (DCDA)), and an accelerator (an asymmetric urea based on toluene diisocyanate). The chemical composition and structure of the catalyzed epoxy systems have been established by IR and NMR spectroscopy in the initial and cured states. The influence of the composition of the complex catalyst system of curing on the mechanisms of chemical transformation of epoxy system has been considered. It is shown that the products of interaction of epoxydiane oligomers with DCDA in the presence of the accelerator-asymmetric urea of Dyhard UR500 brand-is a complex system consisting of chains of polyesters produced by a chain growth mechanism and amine sites, as well as interaction products of epoxy groups with imines, isocyanates, and hydroxyl groups with nitriles.

Iron-catalyzed dehydropolymerization: a convenient route to poly(phosphinoboranes) with molecular-weight control.

The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.

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

Controlled Pd(0)/t-Bu3P-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers via the chain-growth mechanism with a series of ArPd(t-Bu3P)X (X = I, Br, Cl) complexes as initiators were described. Both isolated PhPd(t-Bu3P)X (X = I, Br) complexes and ArPd(t-Bu3P)X (X = I, Br, Cl) complexes in situ generated from Pd2(dba)3/t-Bu3P/ArX (X = I, Br, Cl) were employed as initiators for the controlled Pd(0)/t-Bu3P-catalyzed Suzuki cross-coupling polymerization. The in situ generated ArPd(t-Bu3P)X complexes were found to be better initiators in general. Among them, the combinations of p-BrC6H4I, p-HOCH2C6H4Br and p-PhCOC6H4Br with Pd2(dba)3/t-Bu3P were identified as highly robust initiator systems, resulted in polymers with narrow PDIs (1.13–1.20). In addition, the Pd2(dba)3/t-Bu3P/p-HOCH2C6H4Br and Pd2(dba)3/t-Bu3P/p-PhCOC6H4Br initiator systems also offered heterobifunctional chain ends with high fidelity. Our study showed that an additional amount of t-Bu3P in the initiator system helped to achieve the narrow PDIs, likely by stabilizing Pd(0) species in the initiator system via coordination to form more stable Pd(t-Bu3P)n (n ≥ 2) complexes. Our study opens a new avenue toward well-defined conjugated polymers.

Popping open one by one into polymers

Noncovalent Assembly We rarely board airplanes by joining the back of a single well-ordered line. More often, we jostle around in one of several bulging crowds that merge haphazardly near the gate. Roughly speaking, these processes are analogous to the chain growth and step growth mechanisms of

Chain growth polymerization mechanism in polyurethane-forming reactions

A new approach simulates alcohol–isocyanate reactions in polyurethane systems: catalytic mechanisms dominate the kinetics and chain growth mechanisms overwhelm step growth.

Chain growth mechanism on bimetallic surfaces for higher alcohol synthesis from syngas

Density function theory calculations are performed to investigate the chain growth mechanism on bimetallic surfaces during the syngas conversion. The weighted d-band center correlates well with the adsorption energy of two reactants on bimetallic surface. The boundary between Cu and Co domains facilitates the association reaction of chain growth. Particularly, the reduction of barrier for CO insertion step accelerates the formation of acyl intermediate and thus provides paths to higher alcohol synthesis. The present work demonstrates the synergistic effect in the bimetallic surface from the microscopic view.

One-Pot Synthesis and Characterization of All-Conjugated Poly(3-alkylthiophene)-block-poly(dialkylthieno[3,4-b]pyrazine)

The Kumada catalyst transfer polymerization (KCTP) was applied on dialkyl-substituted thieno[3,4-b]pyrazine monomer systems—a low-bandgap system consisting of a thiophene ring with a fused pyrazine ring which displays promising properties for use in electronic applications. The poly(thieno[3,4-b]pyrazine)s were synthesized through a chain-growth mechanism with best results using an external o-tolyl initiator. A block copolymer with poly(3-hexylthiophene) was successfully formed without impurities. It is shown by UV–vis, fluorescence, DSC, and AFM measurements that the electronically fundamentally different blocks influence each other substantially in terms of the self-assembly and the optical properties, generating materials with significantly different properties than the constituting homopolymers.

Theoretical and Experimental Approach to Accurately Predict the Complex Molecular Weight Distribution in the Polymerization of Strainless Cyclic Esters

Polyesters obtained by the catalytic ring-opening polymerization of macrolactones in many aspects resemble the properties of polyethylene. However, the molecular weight distribution is intrinsically different and equals the molecular weight distribution observed for a step-growth process even though macrolactone ring-opening polymerization follows a chain-growth mechanism. The concurrent occurrence of transesterification reactions leading to the formation of cyclic polymers is responsible for the deviation from the molecular weight distribution characteristic for chain-growth polymerization. To explain and extent on the theoretical principles forming the basis of this peculiar molecular weight distribution, in this work the cyclization process during the polymerization of the 17-membered macrolactone ambrettolide has been analyzed. Liquid chromatography under critical conditions has been applied to semiquantitatively analyze the fractions of cyclic and linear products. In addition, low molecular weight si...

Polymerization of Hydroxy-Containing Diazoacetates: Synthesis of Hydroxy-Containing “Poly(substituted methylene)s” by Palladium-Mediated Polymerization and Poly(ester–ether)s by Polycondensation through O–H Insertion Reaction

Two types of polymerization of hydroxy-containing diazoacetates are described. The polymerization of hydroxy-containing diazoacetates using palladium complexes proceeded successfully under chain-growth mechanism even without a protecting group to give C–C main chain polymers bearing a hydroxy-containing ester substituent on each carbon of the backbone. The resulting polymers had a slightly branched structure due to chain transfer reaction with the hydroxy groups, while the polymers obtained by polymerization of silyl-protected diazoacetates and subsequent deprotection had a completely linear structure. The hydroxy-containing polymers with an appropriate hydrophilic/hydrophobic balance showed a lower critical solution temperature-type phase separation in an aqueous medium. On the other hand, the polymerization of hydroxy-containing diazoacetates using InCl3 as a catalyst proceeded under step-growth mechanism to give oligomers having a distinct repeating unit (ester–ether), where a new ether bond was generated through O–H insertion reaction of diazocarbonyl groups into hydroxy groups.

Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine-boranes using a [Rh(Xantphos)]⁺ catalyst.

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2B═NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.

Study on the formation of a supramolecular conjugated graft copolymer in solution

The formation in solution of a supramolecular graft copolymer bearing conjugated blocks is demonstrated using diffusion ordered NMR spectroscopy (DOSY). A tailor-made poly(3-(2-ethylhexyl)thiophene) (P3EHT) with a phenol end group is synthesized. For this purpose, the chain-growth mechanism of the polymerization of 2-bromo-5-chloromagnesio-3-alkylthiophenes in the presence of a Ni(dppp) catalyst (dppp = 1,3-bis(triphenylphosphino)propane) is exploited, as it enables the use of functionalized initiators to introduce specific end groups. The so-obtained polythiophene was subsequently mixed in solution with poly(4-vinylpyridine) (P4VP) to enable phenol-pyridine hydrogen bonding. The formation of the supramolecular graft copolymer is studied using DOSY-measurements. Based on the results thereof, the amount of P3EHT attached to the P4VP is calculated and the association constant of the hydrogen bond is estimated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 804–809

Fundamental issues on practical Fischer–Tropsch catalysts: How surface science can help

The present article highlights the contribution of surface science and molecular modeling to the understanding of Fischer–Tropsch catalysis, in particular related to carbon-induced Co Fischer–Tropsch catalyst deactivation. The role of atomic and graphitic carbon in surface restructuring is discussed. Both forms of surface carbon stabilize surface roughness, while molecular CO promotes mobility of Co surface atoms. In a proposed chain growth mechanism on Co(0001) chain elongation proceeds via alkylidyne+CH. The resulting acetylenic species is hydrogenated to alkylidyne, the route to further growth. (Cyclo-)polymerization of acetylenic species produces (aromatic) forms of polymeric surface carbon, a slow side reaction.

The Controlled Polymerization of Poly(cyclopentadithiophene)s and Their All-Conjugated Block Copolymers

In this manuscript, the Kumada catalyst transfer polymerization (KCTP) of cyclopenta[2,1-b;3,4-b′]dithiophene (CPDT), a monomer consisting of two fused thiophene entities, is investigated. It is shown that this polymerization follows a controlled chain-growth mechanism. Furthermore, the formation of block copolymers with poly(3-alkylthiophene)s is investigated, and it is shown that these block copolymers can be formed if 3-alkylthiophene is used as the first block and CPDT as the second. The resulting all-conjugated block copolymers consist of two blocks with substantially different electronic and physical properties and it is shown that the blocks influence each other, resulting in a unique material with different properties compared to a blend.

Polymers of propylene and higher 1-alkenes produced with post-metallocene complexes containing a saligenin-type ligand

Polymerization reactions of propylene and three higher linear 1-alkenes, 1-hexene, 1-octene and 1-decene, were carried out with post-metallocene catalysts derived from Ti complexes I and II with a bidentate phenol-alcohol (saligenin-type) ligand derived from 2,4-di-tert-butyl-6-(1,1,1,3,3,3-hexafluoro-2-hydroxy-propan-2-yl)phenol, in the presence of two cocatalysts, MAO and a combination of AlEt2Cl and MgBu2. All catalyst systems contain a large variety of active centers and produce both amorphous atactic polymers and partially crystalline isotactic material. The AlEt2Cl-MgBu2 cocatalyst produces significantly more active catalyst systems. Analysis of 13C NMR data for the propylene polymers shows that the mechanism of isotactic chain growth is principally different for the active centers formed in the presence of the two cocatalysts.

Writing on Polymer Chains

Synthetic polymer materials are currently limited by their inability to store information in their chains, unlike some well-characterized biopolymers. Nucleic acids store and transmit genetic information, and amino acids encode the complex tridimensional structures and functions within proteins. To confer similar properties on synthetic materials, researchers must develop"writing" mechanisms, facile chemical pathways that allow control over the primary structure of synthetic polymer chains. The most obvious way to control the primary structure is to connect monomer units one-by-one in a given order using iterative chemistry. Although such synthesis strategies are commonly used to produce peptides and nucleic acids, they produce limited yields and are much slower than natural polymerization mechanisms. An alternative strategy would be to use multiblock copolymers with blocks that have specified sequences. In this case, however, the basic storage element is not a single molecular unit, but a longer block composed of several repeating units. However, the synthesis of multiblock copolymers is long and tedious. Therefore, researchers will need to develop other strategies for writing information onto polymer chains. In this Account, I describe our recent progress in the development of sequence controlled polymerization methods. Although our research focuses on different strategies, we have emphasized sequence-regulation in chain-growth polymerization processes. Chain-growth polymerizations, particularly radical polymerization, are very convenient methods for synthesizing polymers. However, in most cases, such approaches do not lead to controlled monomer sequences. During the last five years, we have shown that controlled/living chain-growth polymerization mechanisms offer interesting advantages for sequence regulation. In such mechanisms, the chains form gradually over time, and therefore the primary structure can be tuned by using time-controlled monomer additions. For example, the addition of small amounts of acceptor comonomers, such as N-substituted maleimides, during the controlled radical polymerization of a large excess of donor monomer, such as styrene, allows the writing of information onto polymer chains in a robust manner. Even with these advances, this strategy is not perfect and presents some of the drawbacks of chain-growth polymerizations, such as the formation of chain-to-chain sequence defects. On the other hand, this approach is experimentally easy, rapid, scalable, and very versatile.

Chain Growth by CO Insertion in the Fischer–Tropsch Reaction

AbstractThe molecular kinetics of the mechanism of chain growth through CO insertion of the Fischer–Tropsch reaction is analyzed. The maximum chain growth within the CO insertion chain growth model is predicted if the rate of CO activation to give the C1 species that initiates chain growth balances the rate of chain growth termination. The overall rate of chain growth, determined by the elementary rates of CO insertion, hydrogen‐transfer reaction steps, and CO bond cleavage, has to be fast compared to the rate of methanation and the rate of chain growth termination, which gives an oxygenate or hydrocarbon product. However, estimates of rate constants based on quantum‐chemical data predict low chain growth within this CO insertion mechanism, which is mainly caused by the relatively slow rate of CO insertion into the growing chain compared to the rate of product desorption. Such a high barrier for CO insertion is consistent with oxygenate formation through the carbide mechanism pathway. A comparison of the derived expressions for CO consumption shows that the rate of chain growth is limiting within the mechanism of chain growth through CO insertion, whereas within the carbide mechanism it is rate controlled by the rate of CO to CHx monomer formation.

Environmental Uncertainty on Supply Chain Growth Mechanism of Action Research-based on the Supply Chain Information System Alignment and Supply Chain Integration

Environmental Uncertainty on Supply Chain Growth Mechanism of Action Research-based on the Supply Chain Information System Alignment and Supply Chain Integration

Role of the transition metal in Grignard metathesis polymerization (GRIM) of 3-hexylthiophene

Regioregular poly(3-alkylthiophene)s are widely used in organic electronics applications such as solar cells and field effect transistors. Nickel, palladium, and platinum diphenylphosphinoethane complexes were tested as catalysts for the Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-hexylthiophene and 2-bromo-5-iodo-3-hexylthiophene. Nickel-mediated polymerization generated regioregular, low-polydispersity poly(3-hexylthiophene) with well-defined molecular weight consistent with a “living” chain-growth mechanism. By contrast, palladium-mediated polymerization proceeded by a step-growth mechanism and generated polymers with less than 80% head-to-tail couplings. Platinum-mediated polymerization gave very low molecular weight products. Kinetic and computational results suggested that the nickel catalyst acts as an initiator and remains associated with the growing polymer chain, while palladium dissociates from the growing chain. Computational and experimental evidence was provided for various side reactions of dissociated Pd(0) catalyst, which could yield a step growth mechanism and lower regioirregularity.

Synthesis and Characterization of Novel Soybean-Oil-Based Elastomers with Favorable Processability and Tunable Properties

A new series of soybean-oil-based elastomers poly(epoxidized soybean oil-co-decamethylene diamine) (PESD) was synthesized by ring-opening polymerization from epoxidized soybean oil (ESO) and decamethylene diamine (DDA) in different molar ratios. The effect of the molar ratio on the structure and properties of PESD was identified by various methods. According to the results of Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (1H NMR) and thermogravimetry (TGA), the glycerol center of ESO was broken by ammonolysis as expected in the process of polymerization, which resulted in un-cross-linked elastomers with low glass transition temperatures (Tg) ranging from −30 to −17 °C. PESD-3 (molar ratio of DDA to ESO is 2:1) was found to have the highest molecular weight and was most suitable for further processing. Then, PESD-3 was successfully cross-linked through succinic anhydride by a general rubber processing method to obtain a cross-linked bioelastomer. The mechanism of chain growth, ...

Surfactant-Controlled Polymerization of Semiconductor Clusters to Quantum Dots through Competing Step-Growth and Living Chain-Growth Mechanisms

This article reports control of the competition between step-growth and living chain-growth polymerization mechanisms in the formation of cadmium chalcogenide colloidal quantum dots (QDs) from CdSe(S) clusters by varying the concentration of anionic surfactant in the synthetic reaction mixture. The growth of the particles proceeds by step-addition from initially nucleated clusters in the absence of excess phosphinic or carboxylic acids, which adsorb as their anionic conjugate bases, and proceeds indirectly by dissolution of clusters, and subsequent chain-addition of monomers to stable clusters (Ostwald ripening) in the presence of excess phosphinic or carboxylic acid. Fusion of clusters by step-growth polymerization is an explanation for the consistent observation of so-called "magic-sized" clusters in QD growth reactions. Living chain-addition (chain addition with no explicit termination step) produces QDs over a larger range of sizes with better size dispersity than step-addition. Tuning the molar ratio of surfactant to Se(2-)(S(2-)), the limiting ionic reagent, within the living chain-addition polymerization allows for stoichiometric control of QD radius without relying on reaction time.