The high resolution C-13 NMR microstructure of polypropene fractions containing m-rich and r-rich sequences, obtained by solvent extraction from polymer samples prepared in the presence of typical MgCl2-supported Ziegler-Natta catalysts, was analyzed in terms of a Coleman-Fox type ’two-site"statistical model taking into account the possibility (only postulated in the literature) of chemical junctions between the two different types of stereosequences. The results confirmed the copresence of m-rich and r-rich sequences (stereoblocks) in single polypropene molecules. This indicates the ability of isospecific and syndiospecific active centers to interconvert reversibly in times which are shorter than the average growth times of the polymer chains.