Regio‐ and stereospecificity in propene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: effects of hydrogen and the external donor

Investigation of the effects of hydrogen and different external donors on the stereoregularity and chain‐end distribution of polypropylene prepared using MgCl2/TiCl4/phthalate ester‐AlEt3‐alkoxysilane catalyst systems has not only confirmed the importance of regiospecificity in relation to hydrogen activation but has also indicated a significant effect of stereospecificity. Alkoxysilanes giving high isospecificity also give high molecular weight polypropylene. Polymer stereoregularity also increases with increasing hydrogen concentration in polymerization. These effects indicate that not only regioirregular but also stereoiregular insertion slows down chain propagation. In each case the probability of chain transfer with hydrogen is higher than that following a stereoregular insertion, and in each case chain transfer with hydrogen leads to the regeneration of isospecific propagation.