8. Active Center Selection and Propene Polymerization Control with the New Supported Ziegler-Natta Catalysts

Internal Lewis bases (ILB) and external Lewis bases (ELB) are both necessary with the supported Ziegler-Natta catalysts to achieve the requirements of high activity and stereospecificity. The ILB is not directly involved in the active center. During the catalyst preparation the ILB masks the magnesium chloride surface, blocking the acidic defects. The catalyst is then formed by a selective replacement of the weakest bonded ILB by the titanium tetrachloride, giving thus access to low acidic active centers. The scheme can be considered as an active center selection. High activity is observed when a dynamic selection process results in an accumulation of trapped defects.