AbstractThe stereochemistry of polypropenes obtained in the presence of several MgCl2‐supported Ziegler‐Natta catalyst systems comprising different Lewis bases has been investigated by13C NMR spectroscopy, with special attention paid to the “less tactic” (heptane‐soluble) fractions. These have been found to consist of “tendentially isotactic” and “tendentially syndiotactic” stereosequences, in largely variable amounts (depending on the catalyst system). The microstructure of the former is in accordance with the enantiomorphic site model, whereas that of the latter suggests a stereocontrol arising from the asymmetry of the growing chain end. In both cases, propene insertion is primary (1–2). Polymerization conditions leading to the formation of “syndiotacticrich” polymers showing syndiotactic crystallini