Ethylene polymerization with catalyst systems based on supported metallocenes with varying steric hindrance

• Published in: Journal of molecular catalysis. A, Chemical Chemical, 2003-10, Vol.206 (1), p.353-362
• Main Authors: Guimarães, Rafael, Stedile, Fernanda C, dos Santos, João H.Z
• Format: Article
• Language: English
• Subjects: Applied sciences; Ethylene polymerization; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts; Supported metallocenes; XPS
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/S1381-1169(03)00411-4

A series of metallocenes differing in the metal center (M=Zr, Ti, Hf), in the coordination sphere (RCp, with R=H, Me, iBu, nBu; Indenyl) and the bridge (Et, Me 2Si) were studied as homogeneous systems and supported on silica and MAO-mediated (2 wt.% Al/SiO 2) silica. The effect of these parameters were evaluated in the grafted metal content, in catalyst activity and in polymer properties. A series of metallocenes differing in the metal center (M=Zr, Ti, Hf), in the coordination sphere (RCp, with R=H, Me, iBu, nBu; Indenyl) and the bridge (Et, Me 2Si) were studied as homogeneous systems and supported on silica and MAO-mediated (2.0 wt.% Al/SiO 2) silica. The effect of these parameters were evaluated in the grafted metal content, in catalyst activity and in polymer properties. Metal content was determined by Rutherford backscattering spectrometry (RBS) and X-ray photoelectronic spectroscopy (XPS). Alkyl ligands seemed not to affect sensitively the final metal content, but indenyl complexes led to higher grafted metal content. Immobilization on MAO-mediated systems afforded higher metal contents. XPS measurements evidenced a reduction in Zr, Ti or Hf binding energy (BE) after immobilization on silica or on MAO-modified silica, in the latter two different species being observed after signal deconvolution. Monitoring of the surface reaction by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that at this aluminum content residual silanol groups still remains, bearing probably two kind of species: one supported on silanol groups and another on MAO counterpart. Inductive effects from substituent on ligands influence in the catalyst activity, but this electronic effect is reduced in comparison to the steric effect played by the silica surface. Resulting polymers showed higher molecular weight than those produced by the soluble systems.