Polimerização 1,4-cis de butadieno com sistemas catalíticos Ziegler-Natta à base de neodímio

Nowadays the catalyst systems based on neodymium are the ones most used in the high cis polybutadiene production. These systems contain a neodymium compound (catalyst), an alkylaluminium compound (cocatalyst) and a halogen compound (halogenating agent). The microstructure, molecular weight characteristics and the reaction activity are influenced by the nature and concentration of catalyst system components. Those characteristics are also affected by the polymerization conditions. This paper presents a brief review on 1,4-cis-butadiene polymerization on neodymium catalysts.

Violencia en los estadios de fútbol de Primera División en España

Se enmarca dentro del ámbito de la violencia en el deporte, y tiene como finalidad el estudio de ésta, llevada a cabo por los aficionados radicales en los estadios de fútbol de Primera División (1ª Div. de aquí en adelante) en España. Con la intención de abordar el tema enunciado, partimos de las siguientes hipótesis: ¿Es posible la ausencia total de violencia por parte de los seguidores en los partidos de fútbol profesional de 1ª Div.?, ¿Son pertinentes y suficientes, las medidas actuales de seguridad para prevenir la violencia en las gradas y en los aledaños de los estadios?, ¿Es posible reconvertir a los grupos de seguidores violentos en grupos de animación?, y ¿Son reconvertibles todos los miembros que componen estos grupos?. A través de la elaboración y posterior análisis de los epígrafes que componen el presente escrito, la intención es dar respuesta a estas hipótesis iniciales y extraer conclusiones que puedan ser relevantes para el estudio y gestión de la seguridad, en concreto de la violencia en el fútbol profesional. El interés por la temática presentada emana de un vínculo profesional, desarrollado en los estadios de fútbol. Y la seguridad, abordada desde la vertiente laboral y académica.

Emprego de catalisadores à base de níquel para homo- e copolimerização de estireno

This review deals with the homo- and copolymerization of styrene with nickel catalysts. The catalytic activity, polymer stereoregularity, polymer molecular weight and polydispersity are dependent upon nickel ligands and reaction parameters. Catalysts supported on silica, treated with methylaluminoxane (MAO), have shown higher stereospecificity and activity compared to homogeneous ones. The influence of these parameters is discussed focusing on the elucidation of some aspects of the polymerization mechanism.

Decomposição catalítica da hidrazina sobre irídio suportado em compósitos à base de nanofibras de carbono para propulsão espacial: testes em condições reais

The aim of this work is to present the catalytic performance of iridium supported on carbon nanofibers with macroscopic shaping in a 2 N hydrazine microthruster placed inside a vacuum chamber in order to reproduce real-life conditions. The performances obtained are compared to those of the commercial catalyst Shell 405. The carbon-nanofiber based catalyst showed better performance than the commercial catalyst from the standpoint of activity due to its texture and its thermal conductivity.

Lipossomas: propriedades físico-químicas e farmacológicas, aplicações na quimioterapia à base de antimônio

The use of organoantimonial complexes in the therapeutic of leishmaniasis and schistosomiasis has been limited mainly by the need for daily parenteral administration, their adverse side-effects and the appearance of drug resistance. Liposome encapsulation has been so far the most effective means to improve the efficacy of pentavalent antimonials against visceral leishmaniasis. Pharmacologically- and pharmaceutically-acceptable liposomal compositions are still being investigated through manipulation of preparation method, lipid composition and vesicle size. Recently, the encapsulation of a trivalent antimonial within "stealth" liposomes was found to reduce its acute toxicity and effectively deliver this compound to the parasite in experimental schistosomiasis.

Metodologias analíticas para determinação das fumonisinas em milho e alimentos à base de milho

Fumonisins are mycotoxins occurring worldwide, mainly in maize and maize-based food products, which could affect animal and human health. This paper reviews analytical methodologies for the determination of these fungal toxins in foods. It includes extraction, cleanup, derivatization procedures, detection, quantification, and confirmation procedures. Initial attempts at gas chromatographic methods and thin layer chromatography were supplanted by liquid chromatographic methods, mainly performed with fluorometric detection, or mass spectrometry detection, enabling the analysis of polar and thermolabile chemicals without chemical derivatization, which results in lower limits of detection. Alternative methods, such as enzyme linked immunosorbent assay or zone capillary zone electrophoresis, are also described.

Elaboració d'una base de dades per a l'autoformació i autoavaluació de coneixements i competències en estudis científics superiors

L'objectiu principal és oferir uns recursos educatius multimèdia (en format HTML i JAVA) utilitzables en línea (Internet) o en suport autònom (CD-ROM) per a l'autoformació i l'autoevaluació ...

O pKa de indicadores ácido-base e os efeitos coloidais

An experiment is proposed that can be included in undergraduate courses of chemistry. The subject is the acidity of organic compounds, which are employed as pH indicators, particularly in acid-base titrations. The indicators used are methyl orange, bromophenol blue and bromocresol green in aqueous medium. The influence of colloidal systems on the equilibrium is evaluated by the pKa. The colloids employed are surfactants like sodium dodecyl sulfate, cetyl-trimethylammonium bromide and a polymeric non-ionic F127 (pluronics). The effect of stabilization promoted by the system on the acidic or basic structureof the indicator establishes the action mechanism of the colloid on the pKa values.

Degradación de T-patterns en estudios observacionales: Un estudio sobre la eficacia en el ataque de fútbol sala

Una de las técnicas analíticas más novedosas en el ámbito de la metodología observacional es la que permite la detección de T-patterns (estructuras regulares de conducta) mediante el software THEME, que analiza datos observacionales en búsqueda de relaciones de intervalo crítico en sentido ascendente (desde T-pattern simples, dos eventos que se relacionan en un primer nivel, a T-patterns complejos constituidos por diferentes eventos relacionados en distintos niveles). En el presente artículo se analiza esta cuestión, la degradación de los T-patterns, que hasta el momento ha sido puntualmente estudiada. Para ello, se utiliza el muestreo observacional constituido por el conjunto de las secuencias que acaban en gol (n= 23) protagonizadas por el F.C. Barcelona durante el Play Off Final de la liga española 2012-13 de fútbol sala. De esta forma este trabajo satisface un segundo objetivo, en este caso disciplinar, de analizar la e'cacia en la construcción de la fase ofensiva en el fútbol sala; habiéndose tipi'cado que las secuencias que 'nalizan en gol se caracterizan por conseguir, mediante juego combinativo, un lanzamiento desde sector de'nición-corredor central.

Titulações espectrofotométricas de sistemas ácido-base utilizando extrato de flores contendo antocianinas

Considering the attraction of the students' attention by the changes in the colors of vegetable crude extracts caused by the variation of the pH of the medium, the use of these different colors in order to demonstrate principles of spectrophotometric acid-base titrations using the crude extracts as indicators is proposed. The experimental setup consisted of a simple spectrophotometer, a homemade flow cell and a pump to propel the fluids along the system. Students should be stimulated to choose the best wavelength to monitor the changes in color during the titration. Since the pH of the equivalence point depends on the system titrated, the wavelength must be properly chosen to follow these changes, demonstrating the importance of the correct choice of the indicator. When compared with the potentiometric results, errors as low as 2% could be found using Rhododendron simsii (azalea) or Tibouchina granulosa (Glory tree, quaresmeira) as sources of the crude extracts.

Propriedades ácido-base e de complexação de ácidos húmico e fúlvico isolados de vermicomposto

Proton binding properties of humic and fulvic acids were studied by potentiometric titration. Carboxylic groups were the predominant ionizable sites in comparison to phenolic and amine groups. Total acidity of fulvic acid was 12 x 10-3 mol g-1, a number significantly higher than that obtained for humic acid (5.2 x 10-3 mol g-1). Copper ion binding was evaluated at pH 4, 5 and 6 by potentiometric titration with an ion selective electrode for Cu(II). Differential stability constants and complexation capacities were systematically higher for humic acid, despite its lower number of ionizable sites in comparison with fulvic acid.

Diseño e implementación de la base de datos para la gestión de regalos grupales de una red social

El presente trabajo consiste en la creación de una base de datos para generar una aplicación que permita la gestión de un regalo grupal.

Tempo de vida útil de propelentes base-simples

The purpose of this work was to determine the safe shelf life of single-base propellants. The kinetic parameters relative to the consumption of the stabilizer diphenylamine (DPA) added to the propellant were determined as a function of the storage and ageing time. High Performance Liquid Chromatography (HPLC) with spectrophotometric detection was used to determine the DPA percentage before and after the artificial ageing at 60, 70 and 80 ºC. The experimental data were very well adjusted to a pseudo-first order kinetic model and the respective kinetic constants are 8.0-10-3 day-1 (60 ºC); 1.9-10-2 day-1 (70 ºC); 1.2-10-1 day-1 (80 ºC). The activation energy was calculated as 130 kJ mol-1 and the half-time for depletion of the DPA at the hypothetical temperature of 40 ºC of storage was estimated as being 6 years.

Operador dipolo-dipolo na base de momento angular: um complemento ao estudo de ressonância magnética nuclear

The relationship between the magnetic dipole-dipole potential energy function and its quantum analogue is presented in this work. It is assumed the reader is familiar with the classical expression of the dipolar interaction and has basic knowledge of the quantum mechanics of angular momentum. Except for these two points only elementary steps are involved.

Aprendizaje dialógico : base teórica de las comunidades de aprendizaje

El aprendizaje dialógico y los siete principios que lo componen sientan las bases de las comunidades de aprendizaje como proyecto de transformación global y también las prácticas educativas concretas que se realizan en este marco