Fundamental implications of occasional secondary (2,1-) insertions in propene polymerization with different MgCl2-supported Ziegler-Natta catalysts are discussed, with reference to the properties of the polymers obtained. The relatively narrow molecular weight distribution of polypropylene prepared using catalysts of type MgCl2/TiCl4/diether-AlR3 is ascribed to the fact that the active species in this system are relatively uniform, in the sense that significant 2,1-insertion takes place at both highly isospecific and weakly isospecific active species. In contrast, the isospecific species in the catalyst system MgCl2/TiCl4/diisobutyl phthalate-AlEt3-alkoxysilane undergo less 2,1-insertion and are therefore less responsive to chain transfer with hydrogen. The presence of (some) highly regiospecific active sites in such catalysts will therefore lead to the formation of a high molecular weight polymer fraction and, overall, a broad molecular weight distribution. The presence of high molecular weight chains leads to relatively slow molecular relaxation and a more rapid onset of crystallization of the polymer from the melt, as evidenced by rheological studies of polypropylenes prepared using different catalysts and having different molecular weight distributions.