ASPECTS OF HYDROGEN ACTIVATION IN PROPENE POLYMERIZATION USING MGCL2 TICL4/DIETHER CATALYSTS/

C-13 NMR analysis of the chain-end distribution of poly(propylenes) pr epared using the highly active catalyst system MgCl2/TiCl4/diether - A lEt(3) has revealed particularly high proportions of butyl chain-ends in polymers prepared at relatively low hydrogen pressures. This indica tes that the high sensitivity of this type of catalyst to hydrogen, bo th with respect to catalyst activity and polymer molecular weight, can largely be ascribed to chain transfer following regioirregular (2,1-) insertion, such an insertion leading to a species having low activity in chain propagation. Isotactic stereoregularity increases with incre asing hydrogen pressure, indicating that a stereoirregular insertion m ay also slow down the chain propagation, again leading to chain transf er and resulting in the conversion of a potential chain defect into an isobutyl chain-end. Analysis of highly isotactic polymer fractions is olated via temperature rising elution fractionation revealed the prese nce of both butyl and isobutyl chain-ends, indicating that even the mo st highly stereo-specific sites in MgCl2-supported catalysts are not t otally regiospecific.