1-Pentene and 4-methyl-1-pentene (4M1P) have been polymerised using several C2-symmetric ansa-zirconocene catalysts rac-X(2-R 1,4-R2-Ind)2ZrCl2 [X = C 2H4, R1 = R2 = H (1); X = SiMe 2, R1 = R2 = H (2); X = SiMe2, R1 = Me, R2 = H (3); X = SiMe2, R1 = H, R2 = Ph (4); X = SiMe2, R1 = Me, R 2 = Ph (5)] with MAO as cocatalyst. The effects of polymerisation conditions as well as substituents on the indenyl ligand were studied. Except for the poly-1-pentenes synthesized with 3 and 5 at low temperatures, low molecular weight isotactic polymers were generally obtained. Compared to their behaviour in propylene polymerisation, the relative activity and selectivity of catalysts 1-5 are considerably different for 1-pentene and 4M1P polymerisation. Of the five catalysts, 1 and 4 showed the highest activities for both 1-pentene and 4M1P polymerisation, while 5 resulted in the lowest activities, especially for 4M1P polymerisation. Subsequently, a Cs- and several C 1-symmetric zirconocenes, (R1)2C(3-R 2-Cp)(2,7,-R3-Flu)ZrCl2 (R1 = Me, R2 = R3 = H (6); R1 = R2 = Me, R3 = H (7); R1 = Me, R2 = t-Bu, R3 = H (8); R1 = Me, R2 = R3 = t-Bu (9); R 1 = Ph, R2 = t-Bu, R3 = H (10); R1 = Ph, R2 = R3 = t-Bu (11), were tested in 1-pentene and 4M1P polymerisation with MAO as cocatalyst. The effect of substituents on the bridge, the cyclopentadienyl (Cp) and fluorenyl (Flu) ligand, was studied relative to the polymerisation temperature and type of monomer. The molecular weights of the polymers were considerably higher than those of the poly-1-pentenes and P4M1Ps obtained with C2-symmetric zirconocenes (1-5). The catalytic activities and polymer molecular weights strongly depend on the fluorenyl substituent and the bridge, while the type of substituent on the Cp ligand has a strong influence on the tacticity of the polymers. © 2011 The Royal Society of Chemistry.