Polymerization of both (S)-3-methyl-1-pentene and (R,S)-3-methyl-1-pentene in the presence of two metallocene catalysts prepared from metallocene precursors of different symmetry (C-2 and C-s) and C-13-enriched MAO shows that the insertion of the monomer in the initiation step is diastereoselective to an extent similar to that previously observed in the presence of the heterogeneous isotactic specific catalyst delta-TiCl3-Al((CH3)-C-13)(3). Surprisingly, the catalyst prepared with the metallocene precursor of C-s symmetry promotes coisotactic polymerization of the two enantiomers of 3-methyl-1-pentene but only dimerization of (S)-3-methyl-1-pentene.