In the last twenty five years the role of the metallocenes in Ziegler-Natta homogeneous catalysis has become more and more relevant. Their single-centre nature and the availability of a large variety of ?-ligands structures has allowed the preparation of polymers with a wide range of microstructures and properties. Despite the interesting results achieved, they are still seldom applied in industrial processes since their homogeneous nature prevents their use in conventional gas phase or slurry reactors. Therefore, they must be heterogenized trying to preserve their activity and/or stereoselectivity as much as possible. The most widely used supports are either organic (mainly polystyrene) or inorganic (silica, alumina, and MgCl2) materials. The main drawback of the approaches attempted thus far is the loss of activity and stereocontrol of the catalysts due to the reduced accessibility of the metal centers and to the interference of the solid matrix surfaces. One of the most elegant and recent solutions to this problem is the development of self-immobilizing catalysts,1 i.e. catalysts with an unsaturated function that can be used as a comonomer in a prepolymerization step. In practice, the catalyst is "copolymerized" in presence of MAO with low pressure ethylene, isolated by precipitation and reused after filtration and washing with the appropriate solvent. In the framework of this kind of research, we report here the synthesis, characterization, immobilization and ethylene polymerization experiments for several new norbornene-bridged zirconium catalysts.
Self-immobilizing precatalysts to polymerize a-olefins: norbornene-bridged zirconium ansa-metallocenes
Forlini F;Sacchi MC;Polo E
2007
Abstract
In the last twenty five years the role of the metallocenes in Ziegler-Natta homogeneous catalysis has become more and more relevant. Their single-centre nature and the availability of a large variety of ?-ligands structures has allowed the preparation of polymers with a wide range of microstructures and properties. Despite the interesting results achieved, they are still seldom applied in industrial processes since their homogeneous nature prevents their use in conventional gas phase or slurry reactors. Therefore, they must be heterogenized trying to preserve their activity and/or stereoselectivity as much as possible. The most widely used supports are either organic (mainly polystyrene) or inorganic (silica, alumina, and MgCl2) materials. The main drawback of the approaches attempted thus far is the loss of activity and stereocontrol of the catalysts due to the reduced accessibility of the metal centers and to the interference of the solid matrix surfaces. One of the most elegant and recent solutions to this problem is the development of self-immobilizing catalysts,1 i.e. catalysts with an unsaturated function that can be used as a comonomer in a prepolymerization step. In practice, the catalyst is "copolymerized" in presence of MAO with low pressure ethylene, isolated by precipitation and reused after filtration and washing with the appropriate solvent. In the framework of this kind of research, we report here the synthesis, characterization, immobilization and ethylene polymerization experiments for several new norbornene-bridged zirconium catalysts.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
