Abstract:
Heterogeneous "aluminoxane-free" catalysts for ethylene polymerization based on in-situ generation of weakly coordinating metallocenium salts were developed through functionalizing silica support with tris (pentafluorophenyl) borane (B(C6F5)3) stabilized with anilinium ([HNMe2Ph]+[BC6F5)3-SiO2]-) or carbenium salt ([CPh3]+[B(C6F5)3-SiO2]-). The presence of these salts prevented the extraction of borane from silica surface during polymerization catalyzed by zirconocene dichloride (Cp2?rCl2)/ tri-isobutylaluminum (TIBA). In addition, TIBA was used as the alkylating agent, scavenger and activator for the zirconocene dichloride. When used with the carbenium salt, the catalyst produced a higher molecular weight polymer than the anilinium salt and homogeneous zirconocene systems. The produced polymer possessed good morphology, high bulk density and free-flowing granules of polyethylene. The polydispersity index did not change with the type of the salt but increased with increasing loading of zirconocene. It was found experimentally that supported and in-situ activated zirconocene catalyst systems worked very well for ethylene polymerization. The polymerization profiles of both catalysts showed constant catalytic activity after the initial period up to 30 min. Moreover, the pretreatment of silica with alkylaluminum compounds, such as tri-isobutylaluminum (TIBA) and tri-ethylaluminum (TEA), dramatically enhanced the co-catalytic performances of the [CPh3]+[B(C6F5)3-SiO2]. The productivity and ethylene consumption profiles for [CPh3]+[B(C6F5)3-SiO2]- co-catalysts and Cp2ZrCl2/TIBA were measured after pre-treating the support with TEA and TIBA. TEA treatment gave higher productivity by at least a factor of two. Both of the treated co-catalyst systems improved the average molecular weight of the product over the untreated co-catalyst system. TIBA-treated co-catalyst provided a narrow moleculare weight distribution while TEA-treated co-catalyst system gave the broad distribution.
