The block copolymerization polypropylene under highly isotactic stereocontrol, using a new catalyst obtained by means of rational ligand design, was discussed. Moderately isotactic polypropylenes of very low average molecular mass in reasonable yields was obtained. NMR analysis of samples prepared at different propene concentrations revealed a 1,2 insertion regiochemistry and pointed to trans-alkylation by the Al-alkyl and β-H transfer to the monomer as the dominating chain transfer pathways. The results are in agreement with quantum mechanics/molecular mechanics (QM/MM) calculations.