The chain-end groups of the polypropylene (PP) polymerized without addition of molecular hydrogen over a MgCl2. TiCl4. dioctylphthalate/Et3Al catalyst system consisted of n-butyl, n-propyl, vinylidene and vinyl groups in addition to ethyl and i-butyl groups which were detected in the PP prepared under the same polymerization conditions except for the addition of diphenyldimethoxysilane (DPDMS) as an external donor. The newly detected chain-end groups indicate that the additional chain-transfer reactions were brought about by Et3Al at 2,1-inserted sites and by beta-hydrogen elimination at both 1,2- and 2,1-inserted sites. These chain-transfer reactions could account for the observed drops of the activity and the molecular weight in the absence of DPDMS. In addition, the detected chain ends in the PP polymerized with addition of molecular hydrogen over this catalyst system having no DPDMS suggest that the molecular hydrogen addition leads not only to the conversion of the dormant 2,1-inserted sites into the active sites, but also to a decrease in the frequency of 2,1-insertion.