Polypropenes were prepared, both in and without the presence of molecular hydrogen, using the six ansa-zirconocene catalysts rac-Et(Ind)(2)ZrCl2 (I), rac-Et(IndH(4))(2)ZrCl2 (II), rac-Me2Si(Ind)(2)ZrCl2(III), rac-Me2Si(2-Me-Ind)(2)ZrCl2 (IV), rac-Me2Si(Benz[e]Ind)(2)ZrCl2(V), and rac-Me2Si(2-Me-Benz[e]Ind)(2)ZrCl2 (VI) and also Cp2ZrCl2 (VII) [Ind = indenyl], under standardized conditions (0.40 bar partial pressure of propene in toluene solution, at 30 degrees C, zirconocenes activated by methylaluminoxane). The microstructures of the polymers were investigated in detail by using H-1 and C-13 NMR and GPC, and relationships between catalyst structures and polymer microstructural features are discussed. Hydrogen was found to activate all of the catalysts to differing degrees. Trends in hydrogen activation were compared with trends in (a) the overall numbers of n-butyl terminals (resulting from chain transfer to hydrogen following secondary insertion), (b) the ratios, in the presence of hydrogen, of chain transfer to hydrogen to the other mechanisms which lead to renewed chain growth following secondary insertion, and (ci the ratios, in the presence of hydrogen, af sites following primary insertion to sites following secondary insertion. The absence of any clear correlations under these three headings leads to the conclusion that the release by chain transfer to hydrogen of resting states following secondary monomer insertion cannot be the only mechanism giving rise to hydrogen activation with these catalysts.