The structural details of isotactic polypropene (iPP) produced with the moderately isospecific racemic-ethylenebis(l-indenyl)zirconium dichloride/methylaluminoxane (rac-(EBI)ZrCl2/MAO) catalyst are strongly dependent on monomer concentration. Polymerizing propene at 50 degrees C in toluene, at propene concentrations in the experimentally measurable range 0.4-11 mol/L, rac-(EBI)ZrCl2/MAO shows activities in the range 2-350 kg(PP)/(mmol(Zr) . h) and yields polypropenes with M(v)’s from 8800 to 36 600, percent mmmm pentads ranging from 54 to 86%, and corresponding melting temperatures from 86 to 136 degrees C. Two types of regioirregular (secondary) placements in an isotactic sequence of primary propene insertions are observed in iPP synthesized in liquid monomer, erythro (E, ca. 0.4%) and three (T, ca. 0.2%). These secondary units are gradually converted into 1,3 propene units as the monomer concentration is lowered. In "starved catalyst" conditions, that is, as [M] --> 0, rac-(EBI)ZrCl2/MAO produces atactic propene oligomers ((M) over bar(n) = 1080, mmmm = 9.1%). These effects are the most probable cause for the discordance of literature data on metallocene-catalyzed propene polymerization. Three chain transfer mechanisms have been detected : beta-hydrogen transfer to the metal and beta-hydrogen transfer to the monomer, both occurring after a primary insertion, and beta-hydrogen transfer to the monomer after a secondary insertion with the exclusive formation of a cis-2-butenyl end group.