Ethylene/1-hexene copolymerizations with disiloxane-bridged metallocenes, rac- and meso-1,1,3,3-tetramethyldisiloxanediyl-bis(l-indenyl)zirconium dichloride (rac-1, meso-1) activated by modified methylaluminoxane were performed to investigate the influence of conformational dynamics on comonomer selectivity. Although H-1 NOESY (nuclear Overhauser and exchange spectroscopy) analysis indicated that the most stable conformation for the meso isomer in solution was that in which both indenes project over the metal coordination site, this isomer showed higher 1-hexene selectivity in copolymerization (r(e) = 140 +/- 30, r(h) = 0.024 +/- 0.004) than the rac isomer with only one indene over the coordination site (r(e) = 240 +/- 20, r(h) = 0.005 +/- 0.001). The meso isomer showed high 1-hexene selectivity, a high product of reactivity ratios (r(e)r(h) = 3.3 +/- 0.5) and produced copolymers that could be separated into fractions with different ethylene content suggesting that the active species exhibited multisite behavior and populated conformations with different comonomer selectivities during the copolymerization. (C) 2004 Wiley Periodicals, Inc.