Counteranion effects on the rate
and stereochemistry of syndiotactic propylene enchainment bythe archetypal
Cs-symmetric precatalyst [Me
2C(Cp)(Flu)]
ZrMe
2 (
1; Cp = C
5H
4; Flu = C
13H
8, fluorenyl) areprobed using the cocatalysts MAO (
2), B(C
6F
5)
3 (
3)
, B(2-C
6F
5C
6F
4)
3 (
4)
, Ph
3C
+B(C
6F
5)
4- (
5),
and Ph
3C
+FAl(2-C
6F
5C
6F
4)
3- (
6), offering greatly different structural
and ion
pairing characteristics. Reaction of
1 with
3 affords [Me
2C(Cp)(Flu)]ZrMe
+ MeB(C
6F
5)
3- (
7). In the case of
4, this reaction leads to formation the
-methyl dinuclear diastereomers {([Me
2C(Cp)(Flu)]ZrMe)
2(
-Me)}
+ MeB(2-C
6F
5C
6F
4)
3- (
8). A similarreaction
with
6 results in diastereomeric [Me
2C(Cp)(Flu)]ZrMe
+ FAl(2-C
6F
5C
6F
4)
3- (
10) ion
pairs. Themolecular structures of
7 and 10 have been determined by single-crystal X-ray diffraction. Reorgani
zationpath
ways available to these species have been examined using EXSY
and dynamic NMR, revealing thatthe cation-MeB(C
6F
5)
3- interaction is considerably
weaker/more mobile than in the FAl(2-C
6F
5C
6F
4)
3--derived analogue. Polymeri
zations mediated by
1 in toluene over the temperature range of -10
to+60
C
and at 1.0-5.0 atm propylene pressure (at 60
C) reveal that activity, product syndiotacticity,
mand mm stereodefect generation,
and chain transfer processes are highly sensitive to the nature of the ion
pairing. Thus, the complexes activated
with
4 and 5, having the
weakest ion
pairing, yield the highestestimated propagation rates,
while
with
6, having the strongest
pairing, yields the lo
west. The stronglycoordinating, immobile FAl(2-C
6F
5C
6F
4)
3- anion produces the highest/least temperature-dependent productsyndiotacticity, lo
west/least temperature-dependent
m stereodefect abundance,
and highest productmolecular
weight. These polypropylene microstructural parameters,
and also
Mw, are least sensitive toincreased propylene pressure for FAl(2-C
6F
5C
6F
4)
3-, but highest
with MeB(C
6F
5)
3-. In general,
mmstereodefect
production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacialpropylene misinsertion to be the prevailing
mm-generating process in all systems at 60
C, being mostdominant
with
6,
where
mm stereodefect abundance is lo
west. For 1,3-dichloroben
zene as the polymeri
zationsolvent, product syndiotacticity, as
well as
m and mm stereodefects, become indistinguishable for allcocatalysts. These observations are consistent
with a scenario in
which ion
pairing modulates the rates ofstereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity,
and also dictates the rate of termination relative to propagation
and the preferred terminationpath
way. In comparison to
3-
6, propylene polymeri
zation mediated by MAO (
2) +
1 in toluene reveals anestimated ordering in site epimeri
zation rates as
5 >
4 >
2 >
3 >
6,
while product syndiotacticities rankas
6 >
2 >
5 ~ 4 >
3.