Unb
ridged bis(2-a
rylindenyl)metallocene complexes, such as bis(2-phenylindenyl)zi
rconiumdichlo
ride (
1a), when t
reated with app
rop
riate activato
rs a
re active catalyst p
recu
rso
rs fo
rthe p
roduction of elastome
ric polyp
ropylene. The p
roposed mechanism fo
r polyme
rizationinvolves isome
rization of the catalyst between geomet
ries of diffe
ring ste
reoselectivity on atime scale slowe
r than polyme
r chain g
rowth. As the
rotation of the 2-a
rylindenyl ligand isp
roposed to
result in the diffe
rent catalyst geomet
ries, the
rate at which this
rotation occu
rsis of inte
rest fo
r the inte
rp
retation of the behavio
r of these complexes. A se
ries of zi
rconocenes{bis(2-phenylindenyl)zi
rconium dimethyl (
1b), bis(2-phenylindenyl)zi
rconium dibenzyl (
1c),bis(2-(3,5-di-
tert-butylphenyl)indenyl)zi
rconium dimethyl (
2b), and bis(2-(3,5-di-
tert-butylphenyl)indenyl)zi
rconium dibenzyl (
2c)}, va
rying both in the indenyl ligand f
ramewo
rkand the
![](/images/gifcha<font color=)
rs/sigma.gif" BORDER=0 >-ligand, has been synthesized. The
rotation
rates of the methyl-substitutedmetallocenes a
re too fast to be dete
rmined by standa
rd NMR techniques such as EXSY,line-shape analysis, o
r magnetization t
ransfe
r expe
riments. On-
resonance spin-lattice
relaxation in the
rotating f
rame (
T1
rs/rho.gif" BORDER=0 >) has been used to dete
rmine the
rate of
rotation of thea
ryl ligands in
1b,
1c, and
2b and allowed compa
rison with the
rates obtained fo
r thebenzylated species by line-shape analysis. The natu
re of both the
![](/images/gifcha<font color=)
rs/pi.gif" BORDER=0 > and the
![](/images/gifcha<font color=)
rs/sigma.gif" BORDER=0 > ligand systemsaffects the
rate, with ste
rically bulky substituents
resulting in slowe
r rotation. The effect ofthe
![](/images/gifcha<font color=)
rs/sigma.gif" BORDER=0 > ligand is substantially st
ronge
r than that of the
![](/images/gifcha<font color=)
rs/pi.gif" BORDER=0 > ligand.