A new class of isospecific and highly regiospecific C-2-symmetric ansa-zirconocenes, characterized by a bisindenyl ansa ligand with bulky substituents in the 3 position of indene and a single carbon bridge is disclosed : variation of the size of the substituent in C(3) has a strong effect on the extent of chain transfer and isospecificity in propene polymerization. In fact, while rac-[Me2C(1-indenyl)(2)]ZrCl2 produces low molecular weight and moderately isotactic polypropene (iPP) also containing some regioirregularities ((M) over bar(n) = 6500, mmmm ca. 81% and 2,1(tot) = 0.4% at 50 degrees C in liquid monomer), rac-[Me2C(3-tert-butyl-1-indenyl)(2)]ZrCl2 produces iPP with molecular weights between 25000 (T-p = 70 degrees C) and 410000 (T-p = 20 degrees C) and a fairly high isotacticity (mmmm ca. 95% at 50 degrees C), with no detectable 2,1 units. The influence of polymerization temperature on the catalyst performance has been investigated by polymerizing liquid propene in the temperature range of 20-70 degrees C : the experimental Delta Delta E(not subset of)values for enantioface selectivity have been estimated for two members of the new class (rac-[Me2C(3-tert-butyl-1-indenyl)(2)]ZrCl2 Delta Delta E-enant(not subset of) = 4.6 kcal/mol; rac-[Me2C(3-(trimethylsilyl)1-indenyl)(2)]ZrCl2 Delta Delta E-enant(not subset of) = 2.6 kcal/mol). For comparison, Brintzinger＇s moderately isospecific, benchmark catalyst rac-[ethylene(1-indenyl)(2)]ZrCl2 (Delta Delta E-enant(not subset of) = 3.3 kcal/mol), the single carbon bridged, unsubstituted rac-[Me2C(1-indenyl)(2)]ZrCl2 (Delta Delta E-enant(not subset of) = 2.8 kcal/mol), and the C-2-symmetric, practically aspecific, rac-[ethylene(3-methyl-1-indenyl)(2)]ZrCl2 (Delta Delta E-enant(not subset of) = 1.9 kcal/mol) are also reported. The molecular structures of rac-[Me2C(3-tert-butyl-1-indenyl)(2)]ZrCl2 and rac-[Me2C(3-(trimethylsilyl)-1-indenyl)(2)]ZrCl2 have been determined.