RESUMO
The blend of butadiene and acrylonitrile copolymer (NBR) with natural poly-cis-isoprene (NR) shows increased resistance to swelling in solvents in comparison to the individual components. In aerospace, NBR rubber is used as thermal protection for rockets and shall not contain other polymers, even in low contents, otherwise, it can affect the protection performance and rocket safety by causing detachment of the elastomer/propellant interface; therefore, this investigation presents methodologies to determine the NR/NBR contents. This study explores different analytical techniques, such as Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy, in the mid-infrared (MIR) by reflection and in the near-infrared by reflectance (NIRA) modes, Furthermore, quantification strategies by univariate, bivariate and multivariate (chemometric) models are evaluated and compared. A proposed methodology, based on multivariate Raman microscopy with partial least squares regression (PLS), showed high linearity (R2 > 0.99) and low error (< 0.82 %). The validation of FT-MIR data for the CH3, which presented lower error (1.3%) than vinylidene band (6%), showed that both methodologies (reflection and NIRA reflectance) can be used for the quantification of NR in NR/NBR. These results constitute a contribution to the state of the art in researching industrial and aerospace elastomeric applications.
Assuntos
Borracha , Análise Espectral Raman , Análise Espectral Raman/métodos , Borracha/química , Espectroscopia de Infravermelho com Transformada de Fourier/métodos , Butadienos/química , Butadienos/análiseRESUMO
Many heterocyclic compounds can be synthetized using diaza-1,3-butadienes (DADs) as key structural precursors. Isolated and in situ diaza-1,3-butadienes, produced from their respective precursors (typically imines and hydrazones) under a variety of conditions, can both react with a wide range of substrates in many kinds of reactions. Most of these reactions discussed here include nucleophilic additions, Michael-type reactions, cycloadditions, Diels-Alder, inverse electron demand Diels-Alder, and aza-Diels-Alder reactions. This review focuses on the reports during the last 10 years employing 1,2-diaza-, 1,3-diaza-, 2,3-diaza-, and 1,4-diaza-1,3-butadienes as intermediates to synthesize heterocycles such as indole, pyrazole, 1,2,3-triazole, imidazoline, pyrimidinone, pyrazoline, -lactam, and imidazolidine, among others. Fused heterocycles, such as quinazoline, isoquinoline, and dihydroquinoxaline derivatives, are also included in the review.
Assuntos
Imidazolidinas , Imidazolinas , Butadienos/química , Hidrazonas , Iminas/química , Indóis , Isoquinolinas , Lactamas , Pirazóis , Pirimidinonas , Quinazolinas , TriazóisRESUMO
Isoprene is the dominant non-methane organic compound emitted to the atmosphere1-3. It drives ozone and aerosol production, modulates atmospheric oxidation and interacts with the global nitrogen cycle4-8. Isoprene emissions are highly uncertain1,9, as is the nonlinear chemistry coupling isoprene and the hydroxyl radical, OH-its primary sink10-13. Here we present global isoprene measurements taken from space using the Cross-track Infrared Sounder. Together with observations of formaldehyde, an isoprene oxidation product, these measurements provide constraints on isoprene emissions and atmospheric oxidation. We find that the isoprene-formaldehyde relationships measured from space are broadly consistent with the current understanding of isoprene-OH chemistry, with no indication of missing OH recycling at low nitrogen oxide concentrations. We analyse these datasets over four global isoprene hotspots in relation to model predictions, and present a quantification of isoprene emissions based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural nitrogen oxide emissions bias modelled OH and hence isoprene. Over southern Africa, we find that a prominent isoprene hotspot is missing from bottom-up predictions. A multi-year analysis sheds light on interannual isoprene variability, and suggests the influence of the El Niño/Southern Oscillation.
Assuntos
Atmosfera/química , Butadienos/análise , Butadienos/química , Mapeamento Geográfico , Hemiterpenos/análise , Hemiterpenos/química , Imagens de Satélites , África , Austrália , Brasil , Conjuntos de Dados como Assunto , El Niño Oscilação Sul , Formaldeído/química , Radical Hidroxila/análise , Radical Hidroxila/química , Ciclo do Nitrogênio , Óxidos de Nitrogênio/análise , Óxidos de Nitrogênio/química , Oxirredução , Estações do Ano , Sudeste dos Estados UnidosRESUMO
Copolymerization of isoprene (IP) with glycidyl methacrylate (GMA) was performed under RAFT (reversible addition-fragmentation chain-transfer) polymerization conditions in a platform for high-output experimentation. Covering the range between 1 and 0.2 molar fraction of IP in the feed, four sets of reactions were carried out at 10, 15, 20, and 30 h at 115 °C. The kinetic data obtained were used to estimate the reactivity ratios using a nonlinear least-squares approach (NLLS). Reactivity ratios rGMA = 0.61 and rIP = 0.74 indicate that both monomers tend to crosspropagate in agreement with known literature values. Concerning the RAFT study, relatively good control and livingness of the copolymerization was observed except for the experiment in which IP represents 20 mol % in the feed. 1H NMR characterization confirmed the presence of both monomers in the final copolymer, particularly the presence of the epoxy ring of GMA which is susceptible to post polymerization reactions. Finally, preliminary results on the hydrogenation of various polymers are discussed.
Assuntos
Butadienos/química , Compostos de Epóxi/química , Hemiterpenos/química , Metacrilatos/química , Polímeros/síntese química , Técnicas de Química Combinatória , Estrutura Molecular , Polimerização , Polímeros/químicaRESUMO
The polymeric blend of poly (lactic-co-glycolic acid) (PLGA) and polyisoprene (PI) has recently been explored for application as stents for tracheal stenosis and spring for the treatment of craniosynostosis. From the positive results presented in other biomedical applications comes the possibility of investigating the application of this material as scaffold for tissue engineering (TE), acquiring a deeper knowledge about the polymeric blend by exploring a new processing technique while attending to the most fundamental demands of TE scaffolds. PLGA/PI was processed into randomly oriented microfibers through the dripping technique and submitted to physical-chemical and in vitro characterization. The production process of fibers did not show an effect over the polymer's chemical composition, despite the fact that PLGA and PI were observed to be immiscible. Mechanical assays reinforce the suitability of these scaffolds for soft tissue applications. Skeletal muscle cells demonstrated increases in metabolic activity and proliferation to the same levels of the control group. Human dermal fibroblasts didn't show the same behaviour, but presented cell growth with the same development profile as presented in the control group. It is plausible to believe that PLGA/PI fibrous three-dimensional scaffolds are suitable for applications in soft tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2581-2591, 2017.
Assuntos
Butadienos/química , Derme/metabolismo , Fibroblastos/metabolismo , Hemiterpenos/química , Ácido Láctico/química , Teste de Materiais , Mioblastos Esqueléticos/metabolismo , Pentanos/química , Ácido Poliglicólico/química , Engenharia Tecidual , Alicerces Teciduais/química , Animais , Linhagem Celular , Derme/citologia , Fibroblastos/citologia , Humanos , Camundongos , Mioblastos Esqueléticos/citologia , Copolímero de Ácido Poliláctico e Ácido PoliglicólicoRESUMO
Volatile organic compounds (VOCs) play a central role in atmospheric chemistry. In this work, the kinetic and mechanistic reactivities of VOCs are analyzed, and the contribution of the organic compounds emitted by anthropogenic and natural sources is estimated. VOCs react with hydroxyl radicals and other photochemical oxidants, such as ozone and nitrate radicals, which cause the conversion of NO to NO2 in various potential reaction paths, including photolysis, to form oxygen atoms, which generate ozone. The kinetic reactivity was evaluated based on the reaction coefficients for hydroxyl radicals with VOCs. The mechanistic reactivity was estimated using a detailed mechanism and the incremental reactivity scale that Carter proposed. Different scenarios were proposed and discussed, and a minimum set of compounds, which may describe the tropospheric reactivity in the studied area, was determined. The role of isoprene was analyzed in terms of its contribution to ozone formation.
Assuntos
Poluentes Atmosféricos/química , Butadienos/química , Hemiterpenos/química , Ozônio/análise , Pentanos/química , Compostos Orgânicos Voláteis/química , Poluentes Atmosféricos/análise , Florestas , Cinética , Nitratos/química , Compostos Orgânicos Voláteis/análiseRESUMO
The Diels-Alder (DA) reaction provides an attractive route to increase the number of six member rings in substituted Polycyclic Aromatic Hydrocarbons (PAHs). The density functional theory (DFT) B3LYP method has been used in this work to inquire if the substitution of H over the edge of triindenetriphenylene (pristine hemifullerene 1) and pentacyclopentacorannulene (pristine hemifullerene 2), could improve the DA cycloaddition reaction with 1,3-butadiene. The substituents tested include electron-donating (NH2, OMe, OH, Me, i-Pr) and electron-withdrawing groups (F, COOH, CF3, CHO, CN, NO2). The electronic, kinetic and thermodynamic parameters of the DA reactions of the substituted hemifullerenes with 1,3-butadiene have been analyzed. The most promising results were obtained for the NO2 substituent; the activation energy barriers for reactions using this substituent were lower than the barriers for the pristine hemifullerenes. This leads us to expect that the cycloadditions to a starting fullerene fragment will be possible.
Assuntos
Butadienos/química , Fulerenos/química , Modelos Moleculares , Alcenos/química , Reação de Cicloadição , Elétrons , Cinética , TermodinâmicaRESUMO
A number of model Diels-Alder (D-A) cycloaddition reactions (H2C=CH2 + cyclopentadiene and H2C=CHX + 1,3-butadiene, with X = H, F, CH3, OH, CN, NH2, and NO) were studied by static (transition state - TS and IRC) and dynamics (quasiclassical trajectories) approaches to establish the (a)synchronous character of the concerted mechanism. The use of static criteria, such as the asymmetry of the TS geometry, for classifying and quantifying the (a)synchronicity of the concerted D-A reaction mechanism is shown to be severely limited and to provide contradictory results and conclusions when compared to the dynamics approach. The time elapsed between the events is shown to be a more reliable and unbiased criterion and all the studied D-A reactions, except for the case of H2C=CHNO, are classified as synchronous, despite the gradual and quite distinct degrees of (a)symmetry of the TS structures.
Assuntos
Butadienos/química , Reação de Cicloadição , Ciclopentanos/química , Butadienos/síntese química , Simulação por Computador , Ciclopentanos/síntese química , Modelos Químicos , Modelos Moleculares , TermodinâmicaRESUMO
The effect of an extra EWG in the reactivity and regioselectivity in Diels-Alder reactions of ß-cyanolevoglucosenone and 4 different dienes was studied by a joint computational and experimental study. Conceptual DFT analysis successfully predicted an important enhancement in the reactivity, and correctly anticipated the regioselectivity in the reactions with isoprene. However, this static treatment failed when dealing the regiochemical preference of the reactions involving a substituted anthracene as diene. MPW1K/6-31G* calculations correctly reproduced the experimental observations. Based on the collected data, we found that when dealing with dienes and dienophiles with no clear electronically activated position, the ease of pyramidalization of the interacting atoms dictates the regioselectivity of the DA reaction.
Assuntos
Alcenos/química , Butanos/química , Carboidratos/química , Biologia Computacional , Reação de Cicloadição , Cicloexenos/química , Antracenos/química , Butadienos/química , Ciclopentanos/química , Hemiterpenos/química , Cetonas/química , Pentanos/química , EstereoisomerismoRESUMO
Bioactive food components (BFACs) represent promising candidates for liver cancer chemoprevention. Among them, isoprenic derivatives (carotenoids, retinoids, perillyl alcohol, limonene, geraniol, farnesol, geranylgeraniol and ß- ionone) can be highlighted. The relevance of animal models for the investigation of chemopreventive agents is supported by comparative functional genomic studies that reinforce the similarities between rodent and human hepatocarcinogenesis. Thus, characterization of BFACs in animal models as blocking and/or suppressing agents allows the establishment of the theoretical basis for the development of chemoprevention strategies. Dietary isoprenic derivatives actions on hepatocarcinogenesis may involve a block in carcinogen activation, induction of phase 2 enzymes and an antioxidant activity, as well as interference with cellular processes including cell communication, proliferation, apoptosis, differentiation and remodeling of preneoplastic lesions. Dietary isoprenic derivatives modulate molecular targets including HMG-CoA-reductase, Rho, nuclear receptors, c-myc, connexin 43, NF-κB and Nrf2. Several networks related to these targets are altered in early phases of hepatocarcinogenesis. This emphasizes the importance of such agents for the chemoprevention of hepatocellular carcinoma. Combinations of isoprenic derivatives or of these substances with other BFACs classes should be further investigated. Also, toxicity and bioavailability and pharmacokinetic aspects of these derivatives represent relevant issues in their development as chemopreventive agents. One major current limitation of the adoption of dietary isoprenic derivatives for liver cancer chemoprevention is the challenge in overcoming the initial preclinical phase in agent development. Dietary isoprenic derivatives that present liver cancer chemopreventive properties should be further explored in clinical trials, begining with the phase 0 approach.