Propylene has been polymerized with mono(eta(5)-cyclopentadienyl)titanium tribenzyloxide activated with methylaluminoxane (MAO). It was found that the content of residual trimethylaluminium (TMA) in MAO has a determinative effect on the polymerization. An excess of TMA in the catalyst system reduces the Ti species to inactive lower valent states. The catalyst system gives medium molecular-weight atactic polypropylene (M-v = 2-7 x 10(4)) with narrow molecular weight distribution (M-w/M-n = 1.4-1.8). The polymer has a stereoirregular structure described by Bernoullian statistics. Statistical analysis of the regiotriad distribution of the polypropylene chains indicates a regioblock microstructure.