Browsing by Issue Date, starting with "1997"
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- Homogeneous Ziegler-Natta polymerisation: A kinetic approach .2. Transient-state kineticsPublication . Marques, M.M.; Dias, A.R.; Justino, Jorge; Costa, C.; Lemos, F.; Ribeiro, F.R.An integrated view is presented of several possible mechanisms applicable to the interpretation of homogeneous Ziegler-Natta polymerisation. In this paper, the transient aspects related to the kinetics of Ziegler-Natta polymerisation are investigated. Extensive data are used to construct kinetic profiles (rM vs. t) from a theoretical approach. Special attention is given to the duration of the transient period as a function of the different kinetic parameters. The kinetic models developed are fitted to experimental data, either directly obtained by the authors or published in the literature. These general models have a broad range of application.
- Study of low catalytic activity systems (Biscyclopentadienyl complexes-aluminoxane) on olefin polymerizationPublication . Justino, Jorge; Marques, M.M.; Correia, S.; Dias, A.R.; Lemos, F.; Ribeiro, F.R.Olefin polymerization using low activity catalyst systems: Cp2WCl2-methylaluminoxane, [Cp2Mo(CH3)2][PF6]-methylaluminoxane and Cp2Mo(CH3)2-methylaluminoxane, is described. All these catalyst systems were studied in ethylene and propylene polymerization; however, they only showed catalytic activity for ethylene. Only one type of methylaluminoxane (MAO) was used, prepared in an Al/H2O ratio of 2. The kinetics of polymerization were examined and the experimental data (either presented here or in previous papers) were fitted to the kinetic models developed previously and now extended to account for the methylaluminoxane effect.
- A kinetic approach to homogeneous Ziegler type polymerization. Transient statePublication . Marques, M.M.; Costa, C.; Justino, Jorge; Lemos, F.; Ribeiro, F.R.; Dias, A.R.In this paper we present a kinetic approach to the analysis of transient-state homogeneous Ziegler-Natta polymerization activity data. The main features of the experimental data are discussed and fitted to transient kinetic models.
- Diversidade metabólica dos microrganismosPublication . Pinto, AnaSabemos hoje, com base em estudos científicos feitos desde meados do século passado, que os microrganismos estão presentes e todos os ambientes que nos rodeiam e em quase todos os lugares concebíveis do nosso planeta. A esta grande variedade de ambientes corresponde um grupo muito heterogéneo de seres que apresenta uma enorme diversidade metabólica. Os vários sistemas de classificação dos organismos celulares, ate a década de 90, apresentam pouca clareza e correcção na ordenação dos microrganismos. A árvore filogenética proposta por Woese et al. (1990), que divide os seres celulares em três domínios Bacteria, Archaea, Eucarya, veio fornecer um modo mais completo e natural de classificação dos organismos celulares e, em particular, dos microrganismos. Atendendo as diferentes fontes de energia e de carbono e à natureza do dador de electrões ou de átomos de hidrogénio, podem classificar-se os organismos celulares em vários tipos nutricionais. Uma certa monotonia nos seres superiores que pertencem basicamente a dois tipos nutricionais, as plantas são foto-autotróficas, os animais são ornoquímio-heterotróficos, contrasta com uma enorme variedade nos microrganismos. Estes, para além de poderem pertencer a dois tipos já referidos, podem ainda ser foto-hetetotróficos, litoquímio-autotróficos, litoquímio-heterotróficos, orgnoquímio-autotróficos. Esta enorme diversidade e viabilizada por alguns processos e vias metabólicas que só existem nos microrganismos. Estes seres conseguem desenvolver processos bioquímicos e viabilizar a vida em ambientes outrora considerados com incapazes de a manter. Todas estas actividades dos microrganismos são fundamentais para o funcionamento dos ciclos biogeoquímicos e sem eles a vida no nosso planeta não seria possível.
- Polymerization of ethylene using metallocene and aluminoxane systemsPublication . Justino, Jorge; Dias, A.R.; Ascenso, J.; Marques, M.M.; Tait, P.J.T.This paper describes ethylene polymerization using a number of metal-locene and aluminoxane catalyst systems, Cp2MR2 and methylaluminoxane [M = Zr, W, Nb; R = Cl, CH3]. Two types of methylaluminoxane, MAO (1) and MAO (2), were used as cocatalysts. The polymerization activities of the complexes Cp2WCl2 and Cp2NbCl2 were compared with that of Cp2ZrCl2. The Nb and W complexes were found to be less active than the Zr complex. Polyethylene characterization was also carried out by the following methods: gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR).