Propylene polymerization with sterically hindered unbridged 2-arylindene metallocenes

Article abstract of DOI:10.1021/ma012241c

Metallocene catalyst systems derived from bis(2-phenylindenyl)zirconium dichloride (1) yield elastomeric polypropylenes of low to intermediate tacticity. The sterically hindered metallocene bis(2-(3,5-di-tert-butylphenyl)indenyl)zirconium dichloride (2) produces polypropylenes of much higher isotacticity than 1 ([mmmm] = 78% vs 44% in liquid propylene). Kinetic profiles of polymerizations conducted with 1/MMAO and 2/MMAO reveal that the initial rates of polymerization increase with increasing temperature; the activity of both catalysts decreases with time, but the decrease is less pronounced for 2/MMAO than for 1/MMAO. The polymer microstructure is highly dependent upon polymerization conditions: isotacticity decreases with increasing temperature and decreasing monomer concentration. Lower tacticity polypropylenes ([mmmm] = 40%) generated with 2/MMAO at low monomer concentrations contain a high percentage of [mmrm] stereoerrors, which is indicative of a stereoblock microstructure comprised of isotactic stereosequences of opposite relative configuration. Activation of bis(2-(3,5-di-tert-butylphenyl)indenyl)zirconium dimethyl (3) with [Ph₃C] [B(C₆F₅)₄] (4) or B(C₆F₅)₃ (5) yielded catalysts with quite different polymerization behavior. A modest drop in productivity and isotacticity in propylene polymerization is observed when 3 is activated with 4, relative to MMAO, but catalysts derived from 3/B(C₆F₅)₃ exhibited low productivity and afforded amorphous polypropylenes of very low tacticity ([mmmm] = 12%).

Full text of DOI:10.1021/ma012241c

5
382
Macromolecules 2002, 35, 5382-5387
Propylene Polymerization with Sterically Hindered Unbridged
2
-Arylindene Metallocenes
Gr egg M. Wilm es, Sh ir ley Lin , a n d Rober t M. Wa ym ou th *
Department of Chemistry, Stanford University, Stanford, California 94305-5080
Received December 26, 2001; Revised Manuscript Received March 26, 2002
ABSTRACT: Metallocene catalyst systems derived from bis(2-phenylindenyl)zirconium dichloride (1) yield
elastomeric polypropylenes of low to intermediate tacticity. The sterically hindered metallocene bis(2-
(3,5-di-tert-butylphenyl)indenyl)zirconium dichloride (2) produces polypropylenes of much higher isotac-
ticity than 1 ([mmmm] ) 78% vs 44% in liquid propylene). Kinetic profiles of polymerizations conducted
with 1/MMAO and 2/MMAO reveal that the initial rates of polymerization increase with increasing
temperature; the activity of both catalysts decreases with time, but the decrease is less pronounced for
2
/MMAO than for 1/MMAO. The polymer microstructure is highly dependent upon polymerization
conditions: isotacticity decreases with increasing temperature and decreasing monomer concentration.
Lower tacticity polypropylenes ([mmmm] ) 40%) generated with 2/MMAO at low monomer concentrations
contain a high percentage of [mmrm] stereoerrors, which is indicative of a stereoblock microstructure
comprised of isotactic stereosequences of opposite relative configuration. Activation of bis(2-(3,5-di-tert-
butylphenyl)indenyl)zirconium dimethyl (3) with [Ph C][B(C F ) ] (4) or B(C F ) (5) yielded catalysts
3 6 5 4 6 5 3
with quite different polymerization behavior. A modest drop in productivity and isotacticity in propylene
polymerization is observed when 3 is activated with 4, relative to MMAO, but catalysts derived from
3
/B(C
6 5 3
F ) exhibited low productivity and afforded amorphous polypropylenes of very low tacticity
([mmmm] ) 12%).
In tr od u ction
a wide variety of polypropylene microstructures. One
5
,6,19
strategy that we have employed
is to introduce
The stereoselective polymerization of propylene with
metallocene catalysts has resulted in the synthesis of
new classes of polypropylenes and provided insights into
ligand substitutions that might bias the conformational
equilibria toward the more stereospecific conformations.
This can be done by introduction of electronically
1
-3
the details of stereoselective polymerization reactions.
5
,6
19,20
biased or sterically biased
metallocenes that favor
Isotactic polypropylene, an important industrial ther-
moplastic, is produced by both traditional Ziegler-Natta
type catalysts and, more recently, by metallocene cata-
lyst systems. An attractive feature of metallocene
catalysts is their ability to generate polypropylenes of
lower tacticity in order to modulate the crystallinity,
density, and physical properties of the resulting poly-
the anti conformations. In this contribution, we detail
our investigations on the polymerization behavior of a
modified 2-arylindenyl metallocene, bis(2-(3,5-di-tert-
butylphenyl)indenyl)zirconium dichloride ((2-(3,5-t-Bu2-
Ph)Ind)2ZrCl2) (2), under a variety of polymerization
conditions. Metallocene 2 produces polypropylenes of
higher isotacticity and molecular weight and exhibits a
higher activity than 1 at 60 °C. Analysis of the depen-
dence of polymer microstructure on monomer concen-
tration at 20 °C for 2/MMAO suggests that the two
enantiomeric anti conformations interconvert in com-
petition with enchainment; the syn rotamer may not be
accessible to these sterically hindered metallocenes.
2
mers. For the past several years, we have been inves-
tigating a family of conformationally dynamic metal-
locene systems in an effort to control the number and
distribution of atactic stereocenters in polypropylene.
Some of these low-tacticity polypropylenes have elas-
tomeric properties14-18 and yet retain the high melting
temperatures of more crystalline isotactic polypropy-
lenes.
We have proposed that the low to intermediate
tacticities of polypropylenes prepared by unbridged
4
-13
Resu lts
Syn th esis. Metallocene 1 was prepared as previously
described. Metallocene 2 was prepared in 64% yield by
metalation of ZrCl4 with the lithium salt of 2-(3,5-di-
2
-arylindenyl metallocene catalysts such as bis(2-
phenylindenyl)zirconium dichloride (1) is a consequence
of the competition between propylene enchainment and
the conformational dynamics of the catalyst. Intercon-
version between a stereoselective anti conformation and
a nonstereoselective syn conformation was proposed as
a means of generating blocks of atactic and isotactic
stereosequences in a single polymer chain as shown in
Scheme 1 (here we have explicitly included both enan-
tiomers of the chiral anti (rac) isomers; vide infra).
The strong sensitivity of these catalyst systems to the
nature of the ligands and the polymerization conditions
offers the possibility of tuning these catalysts to prepare
21,22
tert-butylphenyl)indene in diethyl ether.
The ligand
2
-(3,5-di-tert-butylphenyl)indene was prepared in 70%
yield from 2-indanone and 3,5-di-tert-butylbenzene-1-
magnesium bromide in diethyl ether. 1-Bromo-3,5-di-
tert-butylbenzene was prepared in 29% yield by the
bromination of 1,3,5-tri-tert-butylbenzene in carbon
23
tetrachloride. The dimethyl analogue of 2, (2-((3,5-t-
Bu2Ph)Ind)2ZrMe2 (3), was prepared from 2 by treat-
ment with methyllithium in diethyl ether.
In flu en ce of P olym er iza tion Tem p er a tu r e. The
polymerization of propylene was carried out with
1
/MMAO and 2/MMAO in toluene solution ([C3H6] )
*
Corresponding author. E-mail: waymouth@stanford.edu.
1.2 M) and bulk monomer at 20, 40, and 60 °C (Table
1
0.1021/ma012241c CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/04/2002

Macromolecules, Vol. 35, No. 14, 2002
Propylene Polymerization 5383
Sch em e 1. P r op osed Mech a n ism for P r op ylen e En ch a in m en t by 2-Ar ylin d en e Meta llocen es
Ta ble 1. P r op ylen e P olym er iza tion s a t Va r iou s Tem p er a tu r es Usin g 1/MMAO a n d 2/MMAOa
entry
catalyst
[C3H6] (M)
P (psig)
Tp (°C)
rmax (mmol/min)
[mmmm]b
Mn (g/mol)c
Mw/Mnc
1
2
3
4
5
6
7
1
1
1
1
2
2
2
11d
1.2
1.2
1.2
1.2
1.2
1.2
n/a
10
24
48
10
24
48
20
20
40
60
20
40
60
nde
0.44
0.18
0.09
0.05
0.40
0.29
0.17
100 000
35 700
11 200
3 900
70 700
31 900
13 600
3.0
2.6
2.4
2.2
2.8
4.8
4.1
0.63
0.67
0.78
e
nd
0.66
1.37
a
Polymerization conditions: [Zr] ) 1.0 × 10 M; [Al]/[Zr] ) 3160; [C3H6] ) 1.16 M; trxn ) 60 min. By 13C NMR. c By high-temperature
-5
b
GPC. Synthesized in 90 mL of liquid propylene + 10 mL of toluene. e Not determined.
d
1
). The solubility of propylene in toluene was deter-
mined at 40 and 60 °C in a manner described previ-
ously. At a given monomer concentration, an increase
7
in the polymerization temperature results in an increase
in the maximum rate of polymerization, a decrease in
the molecular weight, and a decrease in isotacticity of
the resulting polypropylenes for metallocenes 1 and 2
(
Table 1). For metallocene 1, low molecular weight
atactic polypropylene is obtained from polymerizations
carried out at 60 °C at 1.2 M propylene.
Comparable rates of polymerization are observed for
metallocenes 1 and 2 at 40 °C, but at 60 °C metallocene
2
is more active than 1 (Table 1 and Figure 1). The rate
7
profiles shown in Figure 1 reveal that the polymeriza-
tion rate at 40 °C for catalysts derived from both 1 and
2
reach a maximum after approximately 10 min and
then decrease over the next 50 min. At 60 °C, the rate
of polymerization for metallocene 2 is higher than that
of 1 but decreases more rapidly.
Both metallocenes yield polypropylenes of intermedi-
ate tacticity, but metallocene 2 yields more highly
isotactic polymers than those of 1 under all conditions
investigated: at 20 °C [mmmm] ) 18% for polyprop-
ylenes synthesized by 1 vs [mmmm] ) 40% for 2 ([C3H6]
)
1.2 M, Table 1, entries 2, 5). The molecular weight
distribution of polymers synthesized by 2/MMAO are
comparable to those of 1/MMAO at 20 °C but are higher
at higher polymerization temperatures: Mw/Mn > 4.0
at 40 and 60 °C.
In flu en ce of Mon om er Con cen tr a tion . Propylene
polymerizations with 2/MMAO were conducted at seven
different monomer concentrations ([C3H6] ) 1.2-11 M)
F igu r e 1. Comparison of catalyst activity between (2-
PhInd) ZrCl (1/MMAO) and (2-(3,5-t-Bu Ph)Ind) ZrCl (2/
MMAO) at (a) 40 °C and (b) 60 °C.
2
2
2
2
2

5
384 Wilmes et al.
Macromolecules, Vol. 35, No. 14, 2002
Ta ble 2. P en ta d Distr ibu tion s of P olyp r op ylen es Syn th esized a t Va r iou s Mon om er Con cen tr a tion s Usin g 2/MMAOa
[
C3H6]
(M)
Mn
Mw/ Tm,max
∆Hf
[mmrm]
(J /g) [mmmm]e [mmmr] [rmmr] [mmrr] +[rmrr] [mrmr] [rrrr] [mrrr] [mrrm]
c
c
d
d
entry
prodb (g/mol) Mn
4000 70 700 2.8 45, 120 27.9
1890 100 000 3.2 45, 130 58.0
2490 90 100 3.2 46, 140 60.5
2020 154 000 3.5 51, 149 75.2
2420 162 000 3.6 145 64.4
4850 119 000 3.3 45, 144 67.6
2140 64 700 3.8 155 93.0
(°C)
8
9
1.2
1.7
2.2
2.7
3.3
3.8
11f
0.40
0.59
0.65
0.70
0.76
0.75
0.78
0.17
0.13
0.11
0.10
0.09
0.09
0.08
0.03
0.02
0.02
0.02
0.01
0.01
0.02
0.10
0.06
0.05
0.04
0.03
0.04
0.03
0.15
0.10
0.08
0.07
0.06
0.06
0.04
0.05
0.03
0.03
0.02
0.02
0.02
0.01
0.02
0.01
0.01
0.01
0.01
0.00
0.01
0.04
0.02
0.02
0.01
0.01
0.01
0.01
0.04
0.03
0.03
0.02
0.02
0.02
0.01
1
1
1
1
1
0
1
2
3
4
a
Polymerization conditions: [Zr] ) 2.0 × 10 M; [Al] ) 3.16 × 10-2; [Al]/[Zr] ) 15 800; Tp ) 20 °C; trxn ) 60 min. b Productivity in kg
-6
PP/(mol Zr h). By high-temperature GPC. By DSC. e By C NMR. f Synthesized in 90 mL of liquid propylene + 10 mL of toluene.
c
d
13
([mmmm] ) 70%, entry 16, Table 3). The productivity
of the catalyst system derived from 5 was also consider-
ably lower than those for other activators. Polypropy-
lenes obtained from cocatalyst 4 were also less isotactic
and less crystalline than those from systems activated
by MMAO ([mmmm] ) 47% vs 70%; ∆Hf ) 54 vs 76 J /g
at [C3H6] ) 2.7 M; entries 18 and 16, Table 3). However,
for the catalyst system 3/[Ph3C][B(C6F5)4], the tacticity
of the polypropylenes increased with increasing mono-
mer concentration in a manner similar to 2/MMAO.
Discu ssion
Conformationally dynamic metallocene catalysts based
on 2-arylindenyl metallocenes are remarkably versatile
catalyst systems that can generate a wide range of
polypropylene microstructures (6% < [mmmm] < 76%).
These catalyst systems are very sensitive to ligand
variations and polymerization conditions. Polypropy-
lenes of low to intermediate tacticity (19% < [mmmm]
< 50%) generated from this family of metallocenes
exhibit elastomeric properties; the properties of these
materials depend quite sensitively on their microstruc-
F igu r e 2. Plot of pentad fraction vs propylene concentration
for (2-(3,5-t-Bu Ph)Ind) ZrCl /MMAO (2/MMAO).
2
2
2
using the procedure previously described7 (Table 2).
Under these conditions, the productivities varied and
did not show a clear trend. For polymerizations in
solution (entries 8-13, Table 2), the molecular weights
were greater than those of the polymer obtained in
liquid monomer (∼11 M). Molecular weight distributions
were broader (Mw/Mn ) 2.8-3.8) than those of polyprop-
1
3,24,25
ylenes synthesized by 1/MMAO where Mw/Mn ) 2.0-
ture.
7
2
.6.
A characteristic feature of this class of metallocenes
is the strong influence of substituents on the 3,5-
positions of the 2-aryl substituent on the stereoselec-
The microstructure of the polypropylenes produced
with metallocene 2 is quite sensitive to monomer
5
,6,19
concentration, a characteristic of other metallocenes of
tivity of the catalysts.
Metallocenes containing a 3,5-
7
this class. As shown in Table 2 and Figure 2, [mmmm]
dimethylphenyl substituent yield polypropylenes with
similar microstructures to 1, whereas those substituted
with either trifluoromethyl (bis((2-(3,5-ditrifluorometh-
ylpheny)indenyl)zirconium dichloride 6) or tert-butyl
groups (bis((2-(3,5-di-tert-butyl-4-methoxypheny)inde-
nyl)zirconium dichloride 7) yield propylenes of much
decreases from 78% to 40% as monomer concentrations
decrease from 11 to 1.2 M. This decrease in [mmmm]
occurs abruptly at propylene concentrations below ap-
proximately 4 M and is accompanied by increases in the
[
mmmr], [xrmx], [mmrr], and [mrrm] pentads (Table 2).
5
,6,19
DSC thermograms of the polymers in Table 2 reveal
very broad melting endotherms for polymers exhibiting
higher tacticity.
The different stereoselectivities
observed for differently substituted metallocenes can be
attributed to many factors, including (1) the accessibility
of the various conformations, (2) the stereospecificity of
any given conformation, (3) the relative rate at which
these conformations interconvert relative to the rate of
polymer chain propagation, and (4) the equilibria among
crystallinity, another feature of polypropylenes synthe-
sized by unbridged 2-arylindene catalysts.2
4,25
The
degree of crystallinity of the polymers increases with
increasing tacticity (and increasing monomer concentra-
tion) as reflected in the heats of fusion, ∆Hf.
Effect of Coca ta lyst on P r op ylen e P olym er iza -
tion s w ith (2-(3,5-t-Bu 2P h )In d )2Zr Me2. Polymeriza-
tions were conducted in toluene with (2-(3,5-t-Bu2Ph)-
Ind)2ZrMe2 (3) activated by MMAO, [Ph3C][B(C6F5)4]
5
the different accessible conformations. Our efforts in
preparing sterically hindered metallocenes (2 and 7) or
those with strong dipoles (6) were guided by the
hypothesis that steric or electronic effects might bias
the steady-state equilibria among various conformations
toward the more stereospecific conformations to gener-
ate more highly tactic polypropylenes. The higher ste-
reospecificities of the 3,5-disubstituted metallocenes 2,
6, and 7 are consistent with the interpretation that
conformations that lead to isotactic stereosequences (the
anti rotamers of Scheme 1) are either more active or
more frequently populated than conformations leading
to atactic stereosequences.
(4), or B(C6F5)3 (5) and in bulk propylene with 4.
Triisobutylaluminum (TIBA) was added to each run to
scavenge impurities. The polymers obtained were ana-
lyzed by 13C NMR, DSC, and high-temperature GPC
(
Tables 3 and 4). As observed for polymerizations using
1
2
(2-PhInd)2ZrMe2, the nature of the cocatalyst has a
dramatic influence on the microstructure of the polypro-
pylenes obtained from 3. Activation of 3 with B(C6F5)3
5
)
yielded a very low-tacticity polypropylene ([mmmm]
12%, entry 15, Table 3) under conditions in which the
The dependence of catalyst stereoselectivity on mono-
mer concentration is a signature of a mechanism involv-
MMAO cocatalyst yielded an isotactic polypropylene

Macromolecules, Vol. 35, No. 14, 2002
Propylene Polymerization 5385
Ta ble 3. P r op ylen e P olym er iza tion s w ith (2-(3,5-t-Bu 2P h )In d )2Zr Me2a (3)
activatorb
[Zr] (µM)
prodc
[mmmm]d
Mn (g/mol)e
Mw/Mne
Tm,max (°C)
f
∆Hf (J /g)
f
entry
[C3H6] (M)
1
1
1
1
1
2
5
6
7
8
9
0
2.7
2.7
1.2
2.7
3.8
5
190
6.0
6.0
6.0
6.0
6.0
290
3180
560
2160
3340
1450
0.12
0.70
0.29
0.47
0.52
0.58
76 900
124 000
h
80 000
115 000
88 400
2.4
3.6
h
2.9
3.2
2.9
ndg
49, 144
h
47, 130
49, 135
54, 150
ndg
76
h
34
54
46
MMAO
4
4
4
4
i
11
a
b
Polymerization conditions: [MMAO]/[Zr] ) 15 800; [B]/[Zr] ) 1; 252 µmol TIBA; Tp ) 20 °C, trxn ) 30 min. 4 ) [Ph3C][B(C6F5)4], 5
c
d
13
h
)
B(C6F5)3. Productivity in kg of PP/(mol Zr h). By C NMR. e By high-temperature GPC. f By DSC. g None detected. Not determined.
i
Polymer synthesized in 90 mL of liquid propylene + 10 mL of toluene.
Ta ble 4. F u ll P en ta d Distr ibu tion a of P olyp r op ylen es Syn th esized by (2-(3,5-t-Bu 2P h )In d )2Zr Me2 (3) w ith Va r iou s
Activa tor s
[
mmrm]+
entryb
[C3H6] (M)
activatorc
[mmmm]c
[mmmr]
[rmmr]
[mmrr]
[rmrr]
[mrmr]
[rrrr]
[mrrr]
[mrrm]
1
1
1
1
1
2
5
6
7
8
9
0
2.7
2.7
1.2
2.7
3.8
11
5
0.12
0.70
0.29
0.47
0.52
0.58
0.16
0.10
0.17
0.15
0.15
0.14
0.07
0.02
0.05
0.03
0.03
0.03
0.13
0.05
0.10
0.07
0.06
0.06
0.23
0.07
0.19
0.14
0.13
0.12
0.11
0.02
0.08
0.05
0.04
0.03
0.04
0.01
0.03
0.02
0.02
0.01
0.10
0.02
0.06
0.03
0.02
0.02
0.04
0.02
0.05
0.03
0.03
0.02
MMAO
4
4
4
4
a
By 13C NMR. b Entry numbers from Tables 3 and 4 correspond to the same polypropylene samples. c 4 ) [Ph3C][B(C6F5)4], 5 ) B(C6F5)3.
ing some process that competes with the stereodiffer-
entiating step. The strong concentration dependence on
the stereoselectivity for metallocene 2 (Table 2) implies
that a similar competition between insertion and isomer-
ization exists in this case. For these sterically hindered
metallocenes, isomerization between the two enantio-
meric anti conformations is still possible. A detailed
examination of the microstructure of polymers synthe-
sized by 2 at a variety of monomer concentrations
supports this hypothesis. This metallocene yields iso-
tactic polypropylenes at high monomer concentrations
and lower tacticity polypropylenes at low monomer
concentrations. At a monomer concentration of 1.2 M,
metallocene 2 yields a polypropylene of intermediate
tacticity ([mmmm] ) 40%, entry 8, Table 2). Investiga-
tion of the pentad distribution reveals stereodefects
derived predominantly from [mmmr], [mmrm], and
the 2-aryl rings in a near-eclipsed conformation, is
destabilized to the point where it is no longer populated
or kinetically competent to enchain the monomer. This
is consistent with recent theoretical studies that the anti
conformations are more stable than the syn for both 1
and 2 but that the difference is much more pronounced
2
0
for 2.
The observation of a significant fraction of [mmrm]
stereoerrors in polymers derived from 2 suggests that
the direct interconversion of the enantiomeric anti forms
is an important component of the conformational dy-
namics of these systems (Scheme 1) and that the
mechanistic picture for these metallocenes should in-
clude at least three states (additional conformations are
of course possible) where the metallocene can intercon-
vert between the two enantiomeric anti conformations
A and B as well as the achiral syn conformation C. A
similar conclusion was recently suggested by Busico
[
mmrr] pentads; the percentage of the [mrmr] pentad,
1
3
characteristic of atactic sequences, is negligible. These
results suggest that the stereodefects are primarily
isolated stereoerrors rather than blocks of stereoerrors.
The origin of the isolated [mrrm] stereoerrors could be
ascribed either to a low stereospecificity for the stereo-
selective conformers or to the epimerization of the last
through analysis of the high-resolution C NMR spectra
of polymers derived from 7 at heptad resolution.3
4,35
The
implication that the achiral syn rotamer does not
contribute significantly to the polymerization behavior
of 2 raises the possibility that a similar situation exists
for catalysts based on (2-PhInd)2ZrCl2, 1. Previous
2
6-30
8
inserted stereocenters of the growing chain.
The
studies with bridged metallocenes have provided evi-
fact that the concentration of [mmrr] and [mrrm]
stereoerrors increases with decreasing monomer con-
centration suggests that epimerization is a likely origin
of these stereodefects.
dence that the syn rotamer of metallocene 1 is compe-
tent to enchain the monomer, and analysis of the
microstructure of polymers derived from 1 is consistent
with the presence of long atactic stereosequences.2
Kinetic modeling implies that for 1 the rates of insertion
relative to isomerization differ only by a factor of 10
(depending on the monomer concentration).7 If there
is a distribution of relative rates for these two processes,
it is possible that these metallocenes also isomerize only
between the enantiomeric forms A and B and that the
atactic stereosequences in the polymers are due to
populations of kinetic states where the rates of olefin
insertion are slower than the rate of isomerization
(Curtin-Hammett conditions). Although we have no
direct evidence at this time to differentiate between the
contribution of a syn rotamer and the performance of
the catalyst under Curtin-Hammett conditions, the
similarity (at pentad resolution) between the 13C NMR
spectra of two polypropylenes of comparable tacticity,
one synthesized with 1 in liquid propylene ([mmmm] )
4,34
The presence of a relatively high concentration of
mmrm] stereoerrors is consistent with a microstructure
[
,36
containing a significant fraction of isotactic sequences
of opposite configuration separated by isolated [r] diads.
The increase in [mmrm] pentads with decreasing mono-
mer concentration suggests that the length of the
isotactic blocks separated by these isolated [r] diads
decreases with decreasing monomer concentration. These
results imply that isomerization takes place primarily
between the two enantiomeric anti (rac) conformations,
A and B, of these sterically hindered metallocenes,
leading to an isotactic stereoblock structure (Scheme 1,
p ) 0), similar to that proposed by Kaminsky for
polypropylenes derived from bis(neomenthylcyclo-
pentadienyl)ZrCl2.3
1-33
For the sterically hindered met-
allocene 2, it is possible that the syn rotamer C, with

5
386 Wilmes et al.
Macromolecules, Vol. 35, No. 14, 2002
4
4%, entry 1, Table 1) and one made with 2 at low
tion, leading to short isotactic block lengths and a more
1
2
monomer concentration ([mmmm] ) 40%, entry 8, Table
2
lenes are accessible from either the unsubstituted 1 or
the tert-butyl-substituted 2 under appropriate condi-
tions. Further studies are warranted to assess the role
of the syn rotamers in generating the atactic stereo-
sequences in these polymers.
stereorandom microstructure. The significantly lower
productivity of the catalyst system derived from 3/
B(C6F5)3 is consistent with this interpretation. The high
sensitivity of these 2-arylindene catalyst systems to both
ligand effects and polymerization conditions allows for
a wide range of polymer structures to be prepared.
), reveals that lower tacticity elastomeric polypropy-
Con clu sion s
In addition to the influence on the stereospecificity,
the nature and position of the substituents on the
2
The sterically hindered metallocene bis(2-(3,5-di-tert-
butylphenyl)indenyl)zirconium dichloride (2), when ac-
tivated with MMAO, produces polypropylenes of much
-phenyl ligand also influence other aspects of the
polymerization behavior. The molecular weights of
polymers produced by 2/MMAO are higher than those
derived from 1/MMAO. Both metallocenes show a
decrease in molecular weight with increasing polymer-
ization temperature (Table 1), but for 1, this effect is
quite dramatic, leading to a low molecular weight,
atactic polypropylene at 60 °C. Introduction of t-Bu
groups on the phenyl rings also appears to influence the
temperature performance of these catalysts. While the
rates of polymerization at 40 °C are comparable for 1
and 2, metallocene 2 shows a higher rate at 60 °C than
higher isotacticity than the metallocene (2-PhInd) ZrCl2
2
(1). Analysis of the pentad content of low-tacticity
polypropylenes generated with 2/MMAO at low mono-
mer concentrations is consistent with a stereoblock
microstructure comprised of isotactic stereosequences
of opposite relative configuration, implicating the in-
terconversion of two enantiomeric anti rotamers in
competition with olefin insertion. The strong depen-
dence of the tacticity on the nature of the cocatalyst
implies that catalyst/cocatalyst interactions provide
another means of modulating the stereospecificity of
these conformationally dynamic systems. By varying the
polymerization conditions (monomer concentration, tem-
perature, and cocatalyst), this class of catalysts can
produce a wide variety of polypropylene microstructures.
1
.
The nature of the cocatalyst system also has a
significant influence on the polymerization behavior of
these metallocene catalysts. Activation of the dimethyl
derivative (2-(3,5-t-Bu2Ph)Ind)2ZrMe2 (3) with B(C6F5)3/
TIBA yields an atactic polypropylene ([mmmm] ) 12%)
under conditions where activation by MMAO yields a
highly isotactic polypropylene ([mmmm] ) 70%). In a
separate contribution, we have shown that the stereo-
selectivity of the dimethyl derivative of metallocene 1
is much lower in the presence of B(C6F5)3/TIBA than
in MMAO, but the magnitude of change for 2 is quite
remarkablesthis highly stereoselective catalyst system
is transformed to a stereorandom one in the presence
of the B(C6F5)3/TIBA cocatalyst system! Activation of 3
with [Ph3C][B(C6F5)4]/TIBA (4) also yields less isotactic
polypropylene than that obtained in the presence of
MMAO (entries 18 and 16, Table 3). The strong depen-
dence of the stereoselectivity on the nature of the
cocatalyst implies that the nature of the ion pairs
formed in the activated complexes has a strong influence
on the polymerization behavior of these catalyst sys-
Exp er im en ta l Section
The metallocene (2-phenylindenyl) ZrCl2 was synthesized
2
according to literature procedure and used in elementally pure
4
form. MMAO was obtained from AKZO (AKZO MMAO Type
1
2
4) and was dried in vacuo prior to use. Procedures for
determination of propylene solubility in toluene at 40 and 60
°
C as well as for conducting polymerizations were identical to
7
13
the experimental procedures previously described. C NMR
of the polymers synthesized were collected using a 75 MHz
Varian Inova spectrometer. Samples were prepared using
2 2 4 2 2 4
C H Cl /C D Cl and data collected at 100 °C. Polymer mo-
lecular weights were measured using a Waters 150-C ALC/
GPC at 139 °C in 1,2,4-trichlorobenzene vs polypropylene
standards. Differential scanning calorimetry was performed
using a Perkin-Elmer DSC 7. Polymer samples were annealed
at 180 °C for 10 min, cooled to 20 °C at a rate of 10 deg/min,
and allowed to rest at room temperature for 36 h. Heating
curves were measured with a heating rate of 20 deg/min from
1
2,35
tems, even for these sterically hindered systems.
2
0 to 180 °C.
Possible origins of these effects include ion-pairing
phenomena that influence (1) the relative population
and/or activity of accessible conformations, (2) the
stereospecificity of any given conformation, or (3) the
relative rate at which these conformations interconvert
relative to the rate of polymer chain propagation. The
increase in tacticity with increasing monomer concen-
tration for the 3/[Ph3C][B(C6F5)4]/TIBA catalyst system
implies that the conformational dynamics competes with
olefin insertion, as observed with MMAO cococatalyst.
The lower stereoselectivity of catalysts derived from
P r ep a r a tion of Br om o-3,5-d i-ter t-bu tylben zen e.23 1,3,5-
Tri-tert-butylbenzene (150 g, 0.6 mol) was dissolved in carbon
tetrachloride (300 mL) in a three-necked flask which had been
painted black to avoid light and equipped with an overhead
stirrer, thermometer, and addition funnel under argon. Iron
pellets (36 g, 0.64 mol) were added, and the slurry was cooled
to 5 °C. tert-Butylcatechol (1.0 g) was added, and a solution of
bromine (201.6 g, 1.26 mol) in carbon tetrachloride (75 mL)
was added over a 1 h period. The slurry was stirred for an
additional 4 h at 5 °C and quenched by pouring into ice water.
The layers were separated, and the organics were washed with
1
0% sodium hydroxide solution. The solution was then washed
3
/[Ph3C][B(C6F5)4]/TIBA (Table 3) relative to MMAO
with salt brine and dried over magnesium sulfate. The solvent
was evaporated, and the product was distilled under vacuum
twice to give 75 g of product, which was then recrystallized
from heptane to give 47 g of pure product (29%).
might be explained by change in the relative rates of
propagation and isomerization. Since the productivities
are comparable (entries 16 and 18), this may imply a
higher rate of conformational isomerism with this
cocatalyst system. Further studies are underway to
address this possibility.
2
1,22
P r ep a r a tion of 2-(3,5-d i-ter t-Bu tylp h en yl)in d en e.
-Bromo-3,5-di-tert-butylbenzene (47.2 g, 0.175 mol) was dis-
1
solved in ether (500 mL) and cooled to -70 °C. tert-Butyl-
lithium (200 mL of 1.7 M solution in pentane, 0.34 mol) was
added at -70 °C over a 2 h period. The solution was allowed
to warm to room temperature slowly. Magnesium bromide
etherate (46.5 g, 0.18 mol) was added, and the slurry was
stirred for 1 h. The mixture was then cooled to 5 °C, and
For the B(C6F5)3 cocatalyst system, we posit that the
-
strongly coordinating CH3B(C6F5)3 anion decreases the
rate of propylene insertion3
7,38
to the point that the rate
of olefin insertion becomes competitive with isomeriza-

Macromolecules, Vol. 35, No. 14, 2002
-bromoindene (34.2 g, 0.18 mol) was added. The mixture was
warmed to room temperature and then refluxed for 3 h. The
solution was cooled to room temperature, and the reaction was
quenched carefully with water. The layers were separated, and
the organics were washed with salt brine and dried over
magnesium sulfate. The solvents were evaporated, and the
product was distilled twice and recrystallized from hexane to
give 37.1 g of product (70%).
Propylene Polymerization 5387
2
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P r ep a r a tion of Bis(2-(3,5-d i-ter t-bu tylp h en yl)in d en yl)-
zir con iu m Dich lor ide. 2-(3,5-Di-tert-butylphenyl)indene (13.8
g, 0.045 mol), and anhydrous diethyl ether (250 mL) were
placed in a 1 L three-necked flask under argon. n-Butyllithium
(9) Kravchenko, R.; Masood, A.; Waymouth, R. M. Organome-
tallics 1997, 16, 3635-3639.
(
10) Petoff, J . L. M.; Bruce, M. D.; Waymouth, R. M.; Masood, A.;
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1
6, 5909-5916.
over a 30 min period at 0 °C. The solution was stirred for an
additional 2 h. Zirconium tetrachloride (5.1 g, 0.022 mol), was
added incrementally over a 1 h period. The mixture was then
stirred overnight. The ethereal solution was chilled to -10 °C,
and the solids were collected. The solids were taken up in 300
mL of dichloromethane, and the residual solids were removed
by filtration through Celite. The Celite was washed with an
(
(
11) Bruce, M. D.; Waymouth, R. M. Macromolecules 1998, 31,
2
707-2715.
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991, 113, 8569-8570.
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1
(15) Dreier, T.; Erker, G.; Froehlich, R.; Wibbeling, B. Organo-
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evaporated to give 11.2 g of product (64%). H NMR (CDCl
3
,
5
7
00 MHz, 293 K): δ 7.61 (s, 4 H, Ph-2,6) 7.56 (s, 2 H, Ph-4)
(
16) Dietrich, U.; Hackmann, M.; Rieger, B.; Klinga, M.; Leskelae,
M. J . Am. Chem. Soc. 1999, 121, 4348-4355.
.00 (dd, 4 H, J ) 6.5 Hz, J ) 2.9 Hz, Ind-4,5) 6.78 (s, 4 H,
Cp) 6.67 (dd, 4 H, J ) 6.5 Hz, J ) 2.9 Hz, Ind-3,7) 1.49 (s 36
(
17) Collette, J . W.; Tullock, C. W.; MacDonald, R. N.; Buck, W.
H.; Su, A. C. L.; Harrel, J . R.; Mulhaupt, R.; Anderson, B. C.
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1
3
H, t-Bu). C NMR (CDCl
30.0, 127.1, 126.2, 124.4, 122.9, 121.4, 105.0, 35.3, 31.6.
Zr analysis found (calculated): C, 71.93 (71.84); H,
3
, 100 MHz, 293 K): δ 151.7, 131.4,
1
C
H
54Cl
2
(18) Bravakis, A. M.; Bailey, L. E.; Pigeon, M.; Collins, S.
46
Macromolecules 1998, 31, 1000-1009.
19) Witte, P.; Lal, T. K.; Waymouth, R. M. Organometallics 1999,
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.87 (6.87).
(
(
(
Syn th esis of Bis(2-(3,5-d i-ter t-bu tylp h en yl)in d en yl)-
1
8, 4147-4155.
zir con iu m Dim eth yl (4). (2-(3,5-t-Bu
.28 mmol) was suspended in 50 mL of diethyl ether and
chilled to -78 °C in a dry ice/acetone bath. Methyllithium (1.4
M in Et O, 0.44 mL, 0.62 mmol, 2.2 equiv) was added dropwise.
2 2 2
Ph)Ind) ZrCl (215 mg,
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J .; Myers, C. L. US Patent 6,160,064, Dec 12, 2000.
0
2
The solution was stirred for 30 min at -78 °C, allowed to warm
to room temperature, and stirred at room temperature for 4
h. The ether was removed in vacuo, and the remaining solid
was extracted with toluene and filtered. The toluene was
layered with pentane and placed in a -40 °C freezer. An off-
white microcrystalline powder was obtained when the mother
(22) Ernst, A. P.; Moore, E. J .; Myers, C. L.; Quan, R. W. PCT
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693-1697.
(
(
(
24) Hu, Y.; Krejchi, M. T.; Shah, C. D.; Myers, C. L.; Waymouth,
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liquor was removed and was dried in vacuo to give 0.101 g of
1
product (50%). H NMR (C
6
D
6
, 500 MHz, 293 K): δ 7.65 (d, 4
26) Leclerc, M. K.; Brintzinger, H. H. J . Am. Chem. Soc. 1996,
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H, J ) 1.7 Hz, Ph-2,6) 7.57 (t, 2 H, J ) 1.7 Hz, Ph-4) 6.93 (dd,
H, J ) 6.5 Hz, J ) 2.9 Hz, Ind-4,5) 6.77 (dd, 4 H, J ) 6.5
Hz, J ) 2.9 Hz, Ind-3,7) 6.45 (s, 4 H, Cp) 1.44 (s, 36 H, t-Bu)
4
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1
3
(28) Busico, V.; Cipullo, R.; Caporaso, L.; Angelini, G.; Segre, A.
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0.76 (s, 6 H, Zr-CH
51.2, 132.6, 129.1, 124.6, 123.9, 122.1, 120.4, 98.1, 35.0, 32.3,
1.6. C48 60Zr analysis found (calculated): C, 79.08 (79.17);
3 3
) C NMR (CDCl , 100 MHz, 293 K): δ
1
3
(
(
29) Resconi, L. J . Mol. Catal. A: Chem. 1999, 146, 167-178.
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H, 8.38 (8.30).
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Ack n ow led gm en t. Funding for this work was pro-
vided by BP Chemical Co. We thank Andy Ernst and
Dave MacFarland of BP for catalyst synthesis and high-
temperature GPC analysis, respectively. We thank
Albemarle for the gift of boron activators. G.M.W.
received a William R. and Sarah Hart Kimball Stanford
Graduate Fellowship and S.L. a Veatch Memorial
Fellowship, for which they are grateful.
5
14.
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(
Refer en ces a n d Notes
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MA012241C