Polymerization of ethylene: Some aspects of metallocene catalyst stabilization under homogeneous and heterogeneous reaction conditions

The development of metallocene-based catalysts is an important advance on the study of polyolefinic materials. However, due to the rather different conditions that are established in actual applications, only around 3% of these polymers are obtained from metallocene technology. In view of this, novel strategies must be developed to produce metallocene-based catalysts that are more thermally stable, which is a fundamental requirement to establish metallocene technologies. Homogeneous and heterogeneous polymerizations of ethylene were compared, using the Ph₂C(Cp)(Flu)ZrCl₂/MAO system. Homogeneous polymerizations were more active than the corresponding supported reactions. At low ethylene pressure, the addition of 1-hexene increases the activity under homogeneous conditions. Nevertheless, this is not observed on the respective supported systems. At higher pressure conditions, all polymerizations attained higher yields. However, when the reaction temperature increases the activity significantly decreases under homogeneous conditions. Furthermore, when the polymerization was performed under heterogeneous conditions the deactivation was lower. The homogeneous and supported catalytic systems show different characteristics and, in all attempted reactions, immobilization of the molecular catalyst reduces the activity. However, the deactivation ratio was lower when the polymerization was performed under heterogeneous conditions. This means that immobilization of Ph₂C(Cp)(Flu)ZrCl₂ on silica can improve the thermal stability of the catalytic species.