Decrease in activity caused by hydrogen in Ziegler–Natta ethene polymerisation

In this study we prepared seven different Ziegler–Natta catalysts and polymerised them at different hydrogen concentrations in order to investigate their kinetic behaviour during polymerisation. The objective was to see whether the results corresponded to what could be expected on the basis of Kissinʹs β-agostic deactivation theory. According to this theory, hydrogen causes the formation of dormant sites due to the formation of β-agostic coordination from the ethyl groups formed after hydrogen termination. According to this theory, the more hydrogen that is used, the more β-agostic coupling and the smaller percentage of Ti in a polymerising state. This β-agostic coupling would thus explain the lower activity level seen in polymerisation where more hydrogen has been used. The results of this study showed that none of the catalysts showed the kind of behaviour that would correspond to what could be predicted on the basis of Kissinʹs theory. Deactivation could be detected only when a lower amount of hydrogen was used. When higher amounts of hydrogen were used in polymerisation there was a clear delay in activation time of the catalysts. This particularly seemed to be the case for catalysts where Ti was present as Ti(IV). This delay in the activation of the catalyst caused a decrease in activity in addition to the normal decrease in activity due to hydrogen replacing C2′′ in the polymerisation process. The only catalyst showing no delay in activation was a silica-based PE ZN catalyst where the Ti was already in trivalent form. In this case no decrease in activity was observed in addition to the normal decrease in activity caused by hydrogen replacing C2′′ in the polymerisation process.