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Modelagem matematica da polimerização via radical livre controlada usando mecanismo RAFT (transferencia de cadeia reversivel por adição-fragmentação); Mathematical modeling of living free radical polymerization using mechanism (reversible addition-fragmentation chain transfer)

Ivan Carlos Franco
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 15/02/2007 PT
Relevância na Pesquisa
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Polimerização via radical livre controlada (CRP) tem recebido cada vez mais atenção como uma técnica para produção de polímeros com micro estrutura altamente controlada. Em particular, distribuições de pesos moleculares estreitas são obtidas, com polidispersidade muito próxima de um, sendo um campo promissor na ciência de polimerização com estruturas controladas. Um importante requisito para engenharia de polimerização e desenvolvimento de polímeros é a construção de modelos matemáticos úteis, especialmente aqueles de natureza mecanística, com validação experimental. O objetivo deste trabalho é o desenvolvimento de modelos matemáticos abrangentes para simulação de polimerização para mecanismo RAFT. Metacrilato de metila em benzeno a 60°C na presença de 2,2-cianopropil 1- pirrolecarboditioato (agente RAFT) e com AIBN como iniciador em um reator batelada será considerado como estudo. Os pesos moleculares foram calculados utilizando o método dos momentos. Uma análise paramétrica considerando o efeito da concentração de iniciador e da concentração de agente RAFT foi realizada. O modelo predito está de acordo com os dados experimentais da literatura, o que atesta sua validade para ser utilizado no controle de estruturas poliméricas obtidas no processo RAFT; Lately...

Desenvolvimento de um modelo matematico mais rigoroso para polimerização radicalar controlada mediada por radicais nitroxido de estireno; Development of more detailed mathematics model for nitroxide mediated living free radical polymerization (NMRP)

Andre Rodriguez Murari
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 18/12/2007 PT
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A polimerização radicalar livre controlada - LFRP ("Living Free-Radical Polimerization") tem sido alvo de muita atenção nos últimos anos, pois permite obter polímeros com uma microestrutura altamente controlada, ou seja, com uma distribuição de pesos moleculares médios estreita e polidispersidade (PDI) perto de uma unidade, e com estruturas moleculares complexas (por exemplo: copolímeros bloco puros), com diversas arquiteturas complexas e com funcionalidades variadas. A polimerização iônica também gera microestruturas controladas, contudo as condições de reação para a polimerização iônica requerem elevadas purezas, o que torna inviável economicamente suas aplicações em alguns processos industriais. A polimerização radicalar livre controlada mediada por radicais nitróxido - NMRP ("Nitroxide-Mediated Living Free-Radical Polymerization") é uma das técnicas da LFRP, sendo que é mais robusta às impurezas comparada com a polimerização iônica. Muitos pesquisadores têm desenvolvido modelos cinéticos para predizer o comportamento da NMRP, sendo que muitos deles não consideram em seus modelos os efeitos difusionais. Entretanto, seu mecanismo ainda não é totalmente conhecido. Acredita-se que existam outras reações paralelas não previstas nos mecanismos apresentados em literatura. Isso explicaria o fato de o modelo predizer conversões máximas de 100 %...

Investigação da polimerização via radical livre usando iniciadores trifuncionais; Investigation of free radical polymerization using trifunctional initiators

Paula Forte de Magalhaes Pinheiro Bonassi Machado
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 10/03/2009 PT
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A utilização de iniciadores químicos com funcionalidade superior a um, na polimerização via radical livre, tem crescido muito no meio acadêmico, científico e industrial. Estes tipos de iniciadores têm a capacidade de aumentar a velocidade da reação sem causar a redução do peso molecular do polímero formado. Além disso, iniciadores com funcionalidade superior a dois (multifuncionais), que não sejam cíclicos, podem ter a capacidade de gerar ramificações nas cadeias, alterando a microestrutura do produto. Existem alguns poucos trabalhos em literatura que abordam, em nível experimental e/ou de simulação, a polimerização via radical livre utilizando-se iniciadores bifuncionais (com dois grupos funcionais). Em Machado (2004), encontram-se modelos matemáticos utilizando iniciadores bifuncionais para a produção de polímeros lineares (poliestireno) e ramificados (poliacetato de vinila), este último, inédito em literatura aberta. O grau de complexidade envolvido no mecanismo cinético aumenta de forma significativa à medida que se aumenta a funcionalidade do iniciador. O objetivo deste trabalho foi estudar a polimerização via radical livre utilizando um iniciador cíclico trifuncional (TRIGONOX 301, T301). Este trabalho foi desenvolvido em nível experimental (polimerização em ampolas) e computacional (desenvolvimento de modelo matemático...

Polymerization of ɛ-caprolactone using ruthenium(II) mixed metallocene catalysts and isopropyl alcohol: Living character and mechanistic study

Mendes, Paulo J.G.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
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A series of ruthenium(II) complexes with the general formula [Ru(η5-C5H5) (η6-substituted arene)]+[PF6]− (substituted arene = 2-phenylpyridine (1), dibenzosuberone (2) and toluene (3)), in combination with isopropyl alcohol were used for the polymerization of ɛ-caprolactone. The polymerization was found to be quantitative and controlled, with PDI in the range 1.1–1.3. By means of MALDI-ToF analyses, functionalization studies with d,l-lactide and NMR monitoring techniques, it has been found that the polymerization proceeds via a living Activated Monomer mechanism (AM) involving an η6–η4 change of the coordination mode of the arene. These experimental results were corroborated by DFT studies. The growth of several polymer chains per ruthenium atom highlights interesting potentialities for molecular weight control and catalyst economy. The stability of the ruthenium complexes allows their recovery at the end of the polymerization, which can be viewed as a further advance in a green chemistry frame.

Polymerization of ɛ-caprolactone using Ru(II) Catalysts: mechanistic insights

Mendes, Paulo J.
Fonte: 3rd EUCHEMS, Nuremberg, Germany Publicador: 3rd EUCHEMS, Nuremberg, Germany
Tipo: Aula
ENG
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Ring-opening polymerization of cyclic esters is an important field in polymerization chemistry due to the biocompatibility and biodegradability of the obtained materials. Even if the literature concerning the ROP of lactones is vast, there is not detailed research for Group 8 transition metal based catalysts or initiators. It was only in 2006 that was reported the first example regarding the use of ruthenium derivatives in ROP of lactones. In this contribution we present the results of the polymerization of ε-caprolactone by [Ru(η5-C5H5)(η6-substituted arene)][PF6] complexes in the presence of isopropyl alcohol. By in situ NMR techniques it was found that this polymerization proceeds via an activated monomer mechanism. A DFT study of the polymerization initiation step will also be presented in order to corroborate this mechanism.

Quantum mechanics/molecular mechanics investigation of the ethene polymerization mechanism catalyzed by a bulky diimine-Ni(II) complex

Ferreira,Daví A. C; Meneghetti,Simoni M. P; Oliveira Neto,Marçal de; Rocha,Willian R; Meneghetti,Mario R
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2011 EN
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In this work we describe a theoretical investigation of the ethene polymerization reaction catalyzed by a bulky cationic α-diimine Ni(II) complex. We employed the combination of density functional theory (DFT) and molecular mechanics (MM), within the ONIOM approach, to evaluate the structures and energies involved on the most representative reactions observed on ethene polymerization, using the Brookhart catalytic system. All intermediates and transition state structures along these elementary steps were treated as representatives of the polymerization process. We discuss the conformation of the ligands around the active site along the polymerization reaction steps, the coordination angle of the coordinated olefin, p-complexes, and branch formation of the growing chain during the polymerization process.

Ethylene Polymerization by 2-Methyl-8-(benzimidazol)quinolyliron(II) Pre‑Catalyst: a DFT Understanding its Chain Propagation and Transfer

Yang,Wenhong; Taniike,Toshiaki; Terano,Minoru; Chena,Yan; Sun,Wen-Hua
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2014 EN
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The ethylene polymerization mechanism of the 2-methyl-8-(benzimidazol) quinolyliron(II) pre-catalyst is investigated by the DFT calculations, illustrating the possible intermediates with their geometrical and spin configurations. Regarding either methyl or ethyl group bonding on iron cores, the energy barriers for ethylene insertion have been extensively calculated. Within the iron‑methyl species, both resting state and transition state favor the configurations at high-spin state (quintet); whilst the iron-ethyl species prefer the low-spin state. According to the energy barriers, the chain propagation is more favorable than chain transfer for the bidentate iron pre-catalyst, which is well consistent with the experimental observation.

Metallocene-catalyzed alkene polymerization and the observation of Zr-allyls

Landis, Clark R.; Christianson, Matthew D.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
EN
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Single-site polymerization catalysts enable exquisite control over alkene polymerization reactions to produce new materials with unique properties. Knowledge of catalyst speciation and fundamental kinetics are essential for full mechanistic understanding of zirconocene-catalyzed alkene polymerization. Currently the effect of activators on fundamental polymerization steps is not understood. Progress in understanding activator effects requires determination of fundamental kinetics for zirconocene catalysts with noncoordinating anions such as [B(C6F5)4]−. Kinetic NMR studies at low temperature demonstrate a very fast propagation rate for 1-hexene polymerization catalyzed by [(SBI)Zr(CH2SiMe3)][B(C6F5)4] [where SBI is rac-Me2Si(indenyl)2] with complete consumption of 1-hexene before the first NMR spectrum. Surprisingly, the first NMR spectrum reveals, aside from uninitiated catalyst, Zr-allyls as the sole catalyst-containing species. These Zr-allyls, which exist in two diastereomeric forms, have been characterized by physical and chemical methods. The mechanism of Zr-allyl formation was probed with a trapping experiment, leading us to favor a mechanism in which Zr-polymeryl undergoes β-H transfer to metal without dissociation of coordinated alkene followed by σ-bond metathesis to form H2 and Zr-allyl. Zr-allyl species undergo slow reactions with alkene but react rapidly with H2 to form hydrogenation products.

Importance of the release of strand 1C to the polymerization mechanism of inhibitory serpins.

Chang, W. S.; Whisstock, J.; Hopkins, P. C.; Lesk, A. M.; Carrell, R. W.; Wardell, M. R.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /01/1997 EN
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Serpin polymerization is the underlying cause of several diseases, including thromboembolism, emphysema, liver cirrhosis, and angioedema. Understanding the structure of the polymers and the mechanism of polymerization is necessary to support rational design of therapeutic agents. Here we show that polymerization of antithrombin is sensitive to the addition of synthetic peptides that interact with the structure. A 12-m34 peptide (homologous to P14-P3 of antithrombin reactive loop), representing the entire length of s4A, prevented polymerization totally. A 6-mer peptide (homologous to P14-P9 of antithrombin) not only allowed polymerization to occur, but induced it. This effect could be blocked by the addition of a 5-mer peptide with s1C sequence of antithrombin or by an unrelated peptide representing residues 26-31 of cholecystokinin. The s1C or cholecystokinin peptide alone was unable to form a complex with native antithrombin. Moreover, an active antitrypsin double mutant, Pro 361-->Cys, Ser 283-->Cys, was engineered for the purpose of forming a disulfide bond between s1C and s2C to prevent movement of s1C. This mutant was resistant to polymerization if the disulfide bridge was intact, but, under reducing conditions, it regained the potential to polymerize. We have also modeled long-chain serpin polymers with acceptable stereochemistry using two previously proposed loop-A-sheet and loop-C-sheet polymerization mechanisms and have shown both to be sterically feasible...

Rad51 Polymerization Reveals a New Chromatin Remodeling Mechanism

Dupaigne, Pauline; Lavelle, Christophe; Justome, Anthony; Lafosse, Sophie; Mirambeau, Gilles; Lipinski, Marc; Piétrement, Olivier; Le Cam, Eric
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 04/11/2008 EN
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Rad51 protein is a well known protagonist of homologous recombination in eukaryotic cells. Rad51 polymerization on single-stranded DNA and its role in presynaptic filament formation have been extensively documented. Rad51 polymerizes also on double-stranded DNA but the significance of this filament formation remains unclear. We explored the behavior of Saccharomyces cerevisiae Rad51 on dsDNA and the influence of nucleosomes on Rad51 polymerization mechanism to investigate its putative role in chromatin accessibility to recombination machinery. We combined biochemical approaches, transmission electron microscopy (TEM) and atomic force microscopy (AFM) for analysis of the effects of the Rad51 filament on chromatinized templates. Quantitative analyses clearly demonstrated the occurrence of chromatin remodeling during nucleoprotein filament formation. During Rad51 polymerization, recombinase proteins moved all the nucleosomal arrays in front of the progressing filament. This polymerization process had a powerful remodeling effect, as Rad51 destabilized the nucleosomes along considerable stretches of DNA. Similar behavior was observed with RecA. Thus, recombinase polymerization is a powerful mechanism of chromatin remodeling. These remarkable features open up new possibilities for understanding DNA recombination and reveal new types of ATP-dependent chromatin dynamics.

Barnacle cement: a polymerization model based on evolutionary concepts

Dickinson, Gary H.; Vega, Irving E.; Wahl, Kathryn J.; Orihuela, Beatriz; Beyley, Veronica; Rodriguez, Eva N.; Everett, Richard K.; Bonaventura, Joseph; Rittschof, Daniel
Fonte: Company of Biologists Publicador: Company of Biologists
Tipo: Artigo de Revista Científica
EN
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Enzymes and biochemical mechanisms essential to survival are under extreme selective pressure and are highly conserved through evolutionary time. We applied this evolutionary concept to barnacle cement polymerization, a process critical to barnacle fitness that involves aggregation and cross-linking of proteins. The biochemical mechanisms of cement polymerization remain largely unknown. We hypothesized that this process is biochemically similar to blood clotting, a critical physiological response that is also based on aggregation and cross-linking of proteins. Like key elements of vertebrate and invertebrate blood clotting, barnacle cement polymerization was shown to involve proteolytic activation of enzymes and structural precursors, transglutaminase cross-linking and assembly of fibrous proteins. Proteolytic activation of structural proteins maximizes the potential for bonding interactions with other proteins and with the surface. Transglutaminase cross-linking reinforces cement integrity. Remarkably, epitopes and sequences homologous to bovine trypsin and human transglutaminase were identified in barnacle cement with tandem mass spectrometry and/or western blotting. Akin to blood clotting, the peptides generated during proteolytic activation functioned as signal molecules...

The 8 and 5 kDa Fragments of Plasma Gelsolin Form Amyloid Fibrils by a Nucleated Polymerization Mechanism, while the 68 kDa Fragment is Not Amyloidogenic†

Solomon, James P.; Yonemoto, Isaac T.; Murray, Amber N.; Price, Joshua L.; Powers, Evan T.; Balch, William E.; Kelly, Jeffery W.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 08/12/2009 EN
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Familial amyloidosis of Finnish type (FAF), or gelsolin amyloidosis, is a systemic amyloid disease caused by a mutation (D187N/Y) in domain 2 of human plasma gelsolin, resulting in domain 2 misfolding within the secretory pathway. Upon passage through the Golgi, furin endoproteolysis within domain 2 occurs as a consequence of the abnormal conformations that enable furin to bind and cleave, resulting in the secretion of a 68-kDa C-terminal plasma gelsolin fragment (amino acids173–755, C68). The C68 fragment is cleaved upon secretion from the cell by membrane type 1 matrix metalloprotease (MT1-MMP), affording the 8 and 5 kDa fragments (amino acids 173–242 and 173–225, respectively) comprising the amyloid fibrils in FAF patients. Herein, we show that the 8 and 5 kDa gelsolin fragments form amyloid fibrils by a nucleated polymerization mechanism. In addition to demonstrating the expected concentration dependence of a nucleated polymerization reaction, the addition of preformed amyloid fibrils, or “seeds”, was shown to bypass the requirement for the formation of a high energy nucleus, accelerating 8 and 5 kDa D187N gelsolin amyloidogenesis. The C68 fragment can form small oligomers, but not amyloid fibrils, even when seeded with preformed 8 kDa fragment plasma gelsolin fibrils. Because the 68 kDa fragment of gelsolin does not form amyloid fibrils in vitro or in a recently published transgenic mouse model of FAF...

Metal-catalyzed reversible deactivation radical polymerization:mechanistic studies and application on the design of polymer drugs

Mendonça, Patrícia
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Tese de Doutorado
ENG
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O principal objectivo deste trabalho consistiu no desenvolvimento de novas técnicas de polimerização radicalar por desactivação reversivel (RDRP) catalisada por metais, mais ecológicos do que os existentes, para a polimerização de monómeros com interesse para aplicações biomédicas. Como prova de conceito, este trabalho teve também como objectivo a preparação de novas estruturas poliméricas, para utilização como sequestradores de ácidos biliares (BAS), recorrendo aos métodos RDRP desenvolvidos. O mecnaismo que rege a RDRP catalisada por metais tem sido alvo de um intenso debate na literatura, existindo a proposta de dois mecanismos distintos: polimerização radicalar viva por transferência de electrão (SET-LRP) e polimerização radicalar por transferência de átomo com activador suplementar e agente redutor (SARA ATRP). Na fase inicial do trabalho pretendeu-se contribuir para a compreensão do mecanismo da polimerização referida, por ser um factor crucial para o desenvolvimento de novos sistemas catalíticos. O rácio ligante/cobre solúvel foi avaliado durante a polimerização do acrilato de metilo (MA) por RDRP catalisada por Cu(0) e os resultados sugeriram que a polimerização é governada pelo mecanismo de SARA ATRP proposto. Uma das maiores limitações da técnica de ATRP prende-se com o uso de catalisadores metálicos...

Polymerization of methyl acrylate and as comonomer with ethylene using single-site catalysts

Vadhavkar, Sameer S.
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
EN_US
Relevância na Pesquisa
47.21198%
Because of the excellent control over polymerizations provided by single-site catalysts (SSCs), numerous research groups are trying to find such catalysts, which would also be efficient for the polymerization of polar monomers. However, many SSCs are deactivated by undergoing reactions with the electronegative atoms in the polar groups. In the present work, we attempted to copolymerize ethylene with methyl acrylate using three SSCs: (1) a bis(imino)pyridyl iron(II) chloride / methylaluminoxane catalyst, (2) a chiral metallocene / tris(pentafluorophenyl) borane catalyst, and (3) an in situ trifluromethyl substituted nickel (II) enolate catalyst. Only the first two catalysts led to polymer formation. The metallocene had relatively low activities and formed polymethyl acrylate. The most efficient catalyst for the synthesis of the copolymers was the ironbased catalyst 1. The ethylene consumption indicates that its activity reaches a maximum after which it steadily declines. By kinetic measurements, 1H and 13C NMR spectroscopy and the measurement of glass and melting temperatures, we could show that copolymers of ethylene and methyl acrylate were not formed. The products formed were blends, of high density polyethylene and methyl acrylate. Homopolymerizations of methyl acrylate using the three catalyst systems were also studied. Again it was found that the catalyst 1 had the highest activities. Atactic polymers were formed. There was a clear dependence of the activity and molecular weight on the Al/Fe-ratio...

Propagation mechanisms in ring-opening polymerization of small phosphorus heterocycles: toward free-radical polymerization of phosphines?

Hodgson, Jennifer; Coote, Michelle
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
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The mechanism and kinetics of the propagation step in the radical ring-opening polymerization of methylphosphirane, -phosphetane, and -phospholane, and also phenylphosphetane, were studied via high-level ab initio calculations. It was found that radical ring-opening polymerization should occur via attack of the carbon-centered propagating radical at the phosphorus center of the ring. This is a facile process (with reaction rates of the order of 104-106 L mol-1 s-1 at 298.15 K), driven by the creation of a transition structure that resembles a (relatively stable) stretched phosphoranyl radical. The reaction is fastest for the four-membered phosphetanes, reflecting the best compromise between the strain in the transition structure and the strain released by the partially broken ring bond. Though slightly slower, radical ring opening of the three-membered phosphirane should also occur, but the reaction of the five-membered phospholane is not thermodynamically favorable. On the basis of the calculations for the methyland phenyl-substituted heterocycles, we predict that radical ring-opening polymerization should be possible for phosphetane monomers and may also be feasible for phosphiranes, though in some situations it may be hindered by a β-scission reaction. Since the preferred propagation mechanism involves attack by a carbon-centered propagating species...

Mechanism and Kinetics of Dithiobenzoate-Mediated RAFT Polymerization. 1. The Current Situation

Barner-Kowollik, Christopher; Buback, Michael; Charleux, Bernadette; Coote, Michelle; Drache, Marco; Fukuda, Takeshi; Goto, Atsushi; Klumperman, Bert; Lowe, Andrew B; Mcleary, James B; Moad, Graeme; Monteiro, Michael J; Sanderson, Ronald D; Tonge, Matthew
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
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Investigations into the kinetics and mechanism of dithiobenzoate-mediated Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerizations, which exhibit nonideal kinetic behavior, such as induction periods and rate retardation, are comprehensively reviewed. The appreciable uncertainty in the rate coefficients associated with the RAFT equilibrium is discussed and methods for obtaining RAFT-specific rate coefficients are detailed. In addition, mechanistic studies are presented, which target the elucidation of the fundamental cause of rate retarding effects. The experimental and theoretical data existing in the literature are critically evaluated and apparent discrepancies between the results of different studies into the kinetics of RAFT polymerizations are discussed. Finally, recommendations for further work are given.

Quantum Chemical Mapping of Initialization Processes in RAFT Polymerization

Coote, Michelle; Izgorodina, Ekaterina; Krenske, Elizabeth; Busch, Markus; Barner-Kowollik, Christopher
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
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We present the first ab initio simulation of a reversible addition fragmentation chain transfer (RAFT) polymerization. Using ab initio molecular orbital theory, we calculate the equilibrium constants for the first eight addition-fragmentation steps in the cyanoisopropyl dithiobenzoate-mediated polymerization of styrene. We then simulate the concentration profiles for the RAFT agent, and its unimeric and dimeric adducts, assuming standard experimental parameters for styrene homopolymerization and the addition of the styryl radical to the RAFT agent. The simulated data show excellent agreement with published experimental data, high-lighting the accuracy of quantum chemistry. In contrast, the currently used chain-length independent models fail to describe even the qualitative trends in the data, regardless of whether the fragmentation reaction is assumed to be fast or slow. The calculated chain-length dependent equilibrium constants are large, in agreement with the earlier proposed slow fragmentation model.

Molecular weight distributions in ideal polymerization reactors: An introductory review

Meira,G.R.; Oliva,H.M.
Fonte: Latin American applied research Publicador: Latin American applied research
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2011 ES
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The ultimate aim of polymerization reaction engineering is the production of polymers with tailor-made properties. An introductory review into this field is presented, with emphasis on the effects on the molar mass distribution (MMD), of the sought combination of polymerization mechanism, reactor type, and reactor control. Three ideal polymerization mechanisms are analyzed: free-radical, "living" anionic, and step-growth. "Living" anionic and step-growth polymerizations are similar in that their growing chains remain reactive while inside the reactor; and for these systems the narrowest MMDs are produced in reactors with narrow residence time distributions (RDT); i.e.: batch or continuous tubular reactors. In contrast, in conventional free-radical polymerizations, the polymer molecules grow in a fraction of a second and thereafter remain inactive while inside the reactor. In this case, the RTD does not affect the MMD, and the homogeneous continuous stirred-tank reactors provide the narrowest MMDs. Representative mathematical models of polymerization reactors are useful for: a) quantifying the interrelationships between their numerous inputs and outputs; and b) developing open- and closed-loop strategies for increasing reactor productivity and product quality.

Limit of applicability of the monomer-enhanced mechanism for radical generation in persulfate initiated mechanism for radical generationin persulfate initiated polymerization of acrylamide

Rintoul,I.; Wandrey,C.
Fonte: Latin American applied research Publicador: Latin American applied research
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2010 EN
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The limit of applicability of the monomer-enhanced mechanism as dominant mechanism ruling the initiation process of the polymerization of acrylamide (AM) initiated with potassium persulfate (K2S2O8) has been determined. The kinetics and mechanisms for radical generation were studied in solution at very low monomer and initiator concentrations. The transition of the initiation mechanism from dominant monomer-enhanced dissociation to dominant thermal decomposition of persulfate was observed. Both mechanisms contribute equally to radical generation at [AM] / [K2S2O8] ≈ 4.5. Thermal decomposition dominates at lower [AM] or higher [K2S2O8] and opposite variations promote the monomer-enhanced dissociation. This work also includes quantification of the rate coefficients, efficiencies and activation energies of the mentioned mechanisms, and the kinetics of AM polymerization when photochemically initiated with phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide.

Ring-opening polymerization of lactides by (pyrazol-1-ylmethyl)pyridine Zn(II) and Cu(II) complexes: Kinetics, mechanism and tacticity studies

Ojwach,Stephen O.; Zaca,Thembisile P.
Fonte: South African Journal of Chemistry Publicador: South African Journal of Chemistry
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2015 EN
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The kinetics, mechanism and polymer microstructure studies of ring-opening polymerization (ROP) of lactides (LA) by Zn(II) and Cu(II) complexes of (pyrazolylmethyl)pyridine ligands, 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) is described. The complexes [Zn(Ac)2(L1)] (1), [Cu(Ac)2(L1)] (2), [Zn(Ac)2(L2)] (3) and [Cu2(Ac)4(L2)2](4) form active initiators in the ROP of D,L-LA and L-LA monomers. Generally Zn(II) complexes 1 and 3 exhibit higher activities compared to the corresponding Cu(II) complexes 2 and 4. Polymerization kinetics of D,L-LA show higher rates compared to the L-LA reactions. All the polymerization reactions follow pseudo first-order kinetics with respect to monomer while 1 shows second-order dependency on the polymerization reactions. Molecular weights of the polymers range from 813 g mol-1 to 9207 g mol-1 and exhibit relatively narrow molecular weight distributions between 1.2 to 1.6. While poly(D,L-LA) are predominantly atactic, poly(L-LA) are largely isotactic. All polymerization reactions proceed through coordination insertion mechanism followed by hydrolysis of the end groups.