Effects of TMA and MAO on Ethylene–Propylene Copolymer Using Supported Zirconocene Catalysts

Abstract

Copolymerization of ethylene and propylene was carried out in the presence of rac-ethylenebis(indenyl)zirconium dichloride (Et[Ind]₂ZrCl₂) with SiO₂ as the support for the catalyst. The effect of trimethylaluminum (TMA) and methylaluminoxane (MAO) was investigated using two methods of providing a supported system i.e., in situ supported system and a preformed supported system. Catalyst activity increased with increasing the [Al]_TMA/[Zr] ratio to a maxima whereafter an adverse effect was observed in both the in situ and preformed supported system, while activity continuously increased with the increase of the [Al]_MAO/[Zr] ratio. With respect to the molecular weight of the copolymers, an increase of TMA in the system reduced Mw only slightly, while an increase in MAO appeared to have no effect. This suggested that chain transfer to alkylaluminum is not the major chain transfer reaction for this copolymerization system. The molecular weight distribution of copolymer obtained for these supported systems is higher than for the copolymer normally produced from the corresponding homogeneous system. These supported systems might yield plural active species. Copolymers produced have ethylene incorporation close to the ethylene molar feed ratio with random distribution of comonomer insertion. A very low level of the PPP triad was observed. The amount of Al loaded on silica depended linearly with [Al]_MAO added, whereas almost all the loaded Zr were immobilized on the support. From SEM-EDX analysis, Al was well dispersed on the silica support, no effect of zirconocene catalyst loaded on the Al distribution was observed at the reaction condition used. In ethylene/propylene copolymer produced, Al was detected throughout the particles supporting the possibility of catalyst fragmentation mechanism.

Keywords:

• Supported metallocene
• Metallocene catalyst
• Ethylene–propylene copolymerization
• Trimethylaluminum

Acknowledgments

Thanks are due to Ms. Wanna Sririnnuth for NMR analysis. We are grateful to the Thailand Research Fund (TRF) for financial support of this work.