Aspects of hydrogen activation in propene polymerization using MgCl2/TiCl4/diether catalysts

13C NMR analysis of the chain‐end distribution of poly(propylenes) prepared using the highly active catalyst system MgCl2/TiCl4/diether—AlEt3 has revealed particularly high proportions of butyl chain‐ends in polymers prepared at relatively low hydrogen pressures. This indicates that the high sensitivity of this type of catalyst to hydrogen, both 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 increasing hydrogen pressure, indicating that a stereoirregular insertion may also slow down the chain propagation, again leading to chain transfer and resulting in the conversion of a potential chain defect into an isobutyl chain‐end. Analysis of highly isotactic polymer fractions isolated via temperature rising elution fractionation revealed the presence of both butyl and isobutyl chain‐ends, indicating that even the most highly stereo‐specific sites in MgCl2‐supported catalysts are not totally regiospecific.