Estudos praticos e teoricos da influencia das bases de Lewis em polimerização e co-polimerização de olefinas por TiCL4/TiO2(reduzido)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Heterogeneization of titanium tetrachloride on chemically reduced titanium dioxide, developed in our group, has been used as a very active catalyst for ethylene polymerization. This catalytic system although polymerizing propylene with lesser activity , yields a polymer with low polydispersity , high molar mass and isotacticity of 78%. The isotacticity of this polymer can be increased by addition of a Lewis base to the catalytic system, TiCl4 on TiO2 chemically reduced with Al(C2H5)3 as co-catalyst. In this work, we studied the behavior of the active site, in presence of Lewis bases such as 2,2,6,6-tetrametilpiperidine (TMP), feniltriethoxisiloxane (PTS), ethyl benzoate (BE) and pyridine (PY). With the use of Lewis s base BE the isotacticity of polypropylene increased to 95% without loss of the catalytic activity of the catalyst. Copolymers of ethylene-propylene were also obtained in gaseous phase using the same catalytic system shown above. It was possible to get copolymers with crystallinity of to 60 % determined a infrared spectroscopy. DSC analyses showed that we had the formation of polymer blocks. Chromatography permeation gel showed that the copolymer was obtained with high average molarweight and low polydispersity. The behavior of the active site, in the presence of this base, was also studied by computational ab initio calculations using Hartree-Fock (HF) and Theory Functional Density (DFT) levels. Pseudo-potentials were used to represent the electrons of the internal layers of the Ti, O, Cl, Al and C atoms. The values for chemical potential (m), Activation Energy (AE), electronegativity (c) and ionization potential (IP) were obtained by calculations of the enegy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital LUMO. The analysis of the atomic charges showed that the active site of the catalytic system TiCl4/TiO2 (reduced)/Al(CH3)3/BE becomes more positive and in this form more susceptive to the coordination with an a-olefin. The energy of the iso-butyl dissociation of active site was calculated for two deferent systems, (TiCl4-Al(CH3)3 and TiCl4/TiO2 (reduced)/Al(CH3)3/BE. These in this calculations show that the dissociation, energy the last system is higher than for the homogeneous system.

ASSUNTO(S)

funcionais de densidade copolimeros grafitizados

ACESSO AO ARTIGO

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