From the nature of zirconocene/activator catalysts to polymerization performance and polyethylene properties

The influence of the activation procedure applied to a metallocene on its catalytic behavior in ethylene polymerization is discussed and the effect of the zirconocene/activator nature on the properties of PE is demonstrated. MAO and TIBA/CPh3B(C6F5)(4) are intercompared as cocatalysts. The catalytic behavior and the kinetics of ethylene polymerization catalyzed by TIBA/CPh3B(C6F5)(4)-activated metallocene complexes allowed to infer that, in ternary catalytic systems with dichloride precatalysts, TIBA acts not only as an impurity scavenger or simply an alkylating agent, but also as a cocatalyst giving rise to heteronuclear cationic species providing lower chain propagation and chain transfer rates than those generated upon MAO action. When activated with TIBA/CPh3B(C6F5)(4) dichloride zirconocenes were moderately active as compared with MAO and yielded PEs of considerably higher M. A halide or alkyl ligand present in the outer coordination sphere of active sites affects considerably both the catalytic performance of active species and the properties of the resulting PE. The C1-keeping Al-Zr heteronuclear active sites give rise to high-M linear PE; alkyl holders are highly capable of chain propagation and chain transfer to monomer. The high reactivity of the Zr-Cl bond in the latter active sites manifests itself also in the high ability to incorporate alfa-olefins to yield PE chains containing branches with 4-10 incorporated ethylene units.