Endowing aspecific, unbridged-metallocene propylene-polymerization catalysts with isospecificity: The unprecedented role of MAO

Article abstract of DOI:10.1002/anie.200600666

(Figure Presented) Unbridged yet isospecific metallocene polypropylene catalytic systems can be generated in situ during the MAO-assisted activation step (see picture; MAO = methyl aluminum oxane), as demonstrated with a dimethylaminophenyl-substituted zi

Full text of DOI:10.1002/anie.200600666

Angewandte
Chemie
Polymerization Catalysts
cationic active species, thereby endowing aspecific,
unbridged-metallocene precatalysts with isospecificity.
Herein, we report a novel example of a sterically unhindered,
unbridged zirconocene system that is able to produce highly
isotactic polypropylene through the unprecedented role of
methyl aluminum oxane (MAO).
The amine-functionalized, unbridged zirconocenes [{1-(p-
Me2NC6H4)-3,4-Me2C5H2}2ZrX2] [(AP)2ZrX2; X = Cl (2), X =
Me (3)] were obtained from newly synthesized ligand 1 as
outlined in Scheme 1. The molecular structure of 2 has
C2 symmetry in the solid state (Figure 1). The polymerization
DOI: 10.1002/anie.200600666
Endowing Aspecific, Unbridged-Metallocene
Propylene-Polymerization Catalysts with
Isospecificity: The Unprecedented Role of
MAO**
Seong Kyun Kim, Hwa Kyu Kim, Min Hyung Lee,
Seung Woong Yoon, and Youngkyu Do*
The relationship between the nature of catalyst systems and
the resulting polymer has been well established in single-site
olefin-polymerization systems and now provides the oppor-
tunity to tailor the polymerꢀs properties.[1–4] In particular,
chiral ansa-metallocene catalysts, which follow an enantio-
morphic site-control mechanism, have been intensively inves-
tigated to obtain stereochemical control of propylene poly-
merization by varying the ligand structure.[5–7] Unbridged-
metallocene-based systems, on the other hand, have received
little attention owing to their aspecific nature[8,9] in spite of
their ability to produce iso-rich[10,11] or isotactic–atactic block
polypropylene[12,13] when containing rotationally hindered
ligands. As unbridged metallocenes are far easier to synthe-
size than ansa-metallocenes, we have been investigating
isospecific, unbridged-metallocene catalytic systems that can
be generated in situ during the activation step. To this end, we
have designed “class I” unbridged metallocenes, a new class
analogous to the known aspecific, unbridged metallocenes of
“class II”.[8,9] The Lewis basic sites E in class Icomplexes are
found to interact with [Me-MAO]À to generate rigid, rac-like
Scheme 1. Synthetic routes to zirconocenes 2 and 3.
Figure 1. Molecular structure of 2. Top: top view; bottom: side view.
of propylene with 2/MAO ([Al]/[Zr] = 1000) was performed
at various temperatures (Tp = 0, 25, 50, and 708C; Table 1,
entries 1–4, respectively). The 2/MAO system shows lower
catalytic activity but produces higher molecular weight
polypropylenes than the well-known isospecific catalyst rac-
[Et(Ind)2ZrCl2] (EBIZr)/MAO (entry 9) under identical
reaction conditions. The differential scanning calorimetry
(DSC) and gel permeation chromatography (GPC) traces
indicate that all the crude polypropylenes from the 2/MAO
system show multiple melting transitions (Tm) and broad
molecular-weight distributions (Mw/Mn) of between 4.5 and 11
(Figure 2).
[*] Dr. S. K. Kim, H. K. Kim, Dr. M. H. Lee, S. W. Yoon, Prof. Dr. Y. Do
Department of Chemistry
School of Molecular Science BK-21
Center for Molecular Design and Synthesis
and
Polyolefin Materials Research Center
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (Republic of Korea)
Fax: (+82)42-869-2810
E-mail: ykdo@kaist.ac.kr
Homepage: http://xray.kaist.ac.kr/
The crude polypropylenes were fractionated by stepwise
solvent extraction[14] into three portions for further analysis,
1) diethyl ether soluble, 2) diethyl ether insoluble and n-
heptane soluble, and 3) diethyl ether insoluble and n-heptane
insoluble. The mmmm methyl pentad values in Table 2
suggest that these portions correspond to atactic-like, mod-
[**] The authors are grateful for financial support from CMDS, POMRC,
the BK-21 project, and the Honam Petrochemical Co. S.K.K. thanks
Mr. H. S. Shin for performing the NMR spectroscopy experiments.
MAO=methyl aluminum oxane.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Propylene polymerization data.[a]
erately isotactic, and highly isotactic poly-
propylenes, respectively. The amount of n-
heptane insoluble polypropylene decreases
Entry
Cat.
Cocat.
Tp [8C]
Yield [g]
Act.[b]
Mw [”10À3
]
Mw/Mn
1
2
2
2
MAO
MAO
0
25
50
70
25
25
25
0
25
25
25
25
1.02
6.24
7.25
3.41
6.38
6.95
2.37
2.83
12.4
1.02
2.18
1.22
102
624
725
341
638
695
237
566
5930
204
435
244
227
166
4.56
10.7
9.50
5.27
11.4
2.10
2.10
1.87
2.13
12.4
2.66
2.00
as
the
polymerization
temperature
increases, while that of the diethyl ether
soluble portion increases. The n-heptane
insoluble portions have remarkably high
mmmm values of around 86% (Figure 3A)
and a Tm of 1518C (Table 2, entry 2). The
class II complex [{1-(p-C6H5C6H4)-3,4-
Me2C5H2}2ZrCl2] (BPZr)[9] gives atactic
polypropylene even at 08C (entry 8). These
results clearly demonstrate that the
unbridged zirconocene 2/MAO system is
capable of producing highly isotactic poly-
propylene and also that the simple function-
3
2
MAO
32.0
10.1
168
15.2
16.6
58.9
19.1
126
4
2
MAO
5
3
MAO
6[c]
7[c]
8[d]
9[e]
10[f]
11[f]
12[f]
3
3
borate/TIBA
borate/TOA
MAO
MAO
MAO
BPZr
EBIZr
3
3
3
MAO/borate
borate/MAO
13.3
22.4
[a] Polymerization conditions: P(propylene)=1 bar, [Zr]=5.0 mmol, [Al]/[Zr]=1000, solvent=50 mL of
toluene, tp =120 min; Entry numbers 1–9 correspond to those in Table 2. [b] Activity, in units of
(kg of PP)/[(mol of Zr)hbar]. [c] borate=[Ph3C][B(C6F5)4]; TIBA=triisobutylaluminum; TOA=trioctyl-
aluminum; [B]/[Zr]=1, [Al]/[Zr]=200. [d] tp =60 min; BPZr=[{1-(p-C6H5C6H4)-3,4-Me2C5H2}2ZrCl2]. alization of class II to class I metallocenes
[e] tp =25 min; EBIZr=rac-[Et(Ind)2ZrCl2]. [f] tp =60 min; [Al]/[Zr]=200, [B]/[Zr]=1.
should be an effective route for endowing
Figure 3. Methyl region of the 13C NMR spectra of polypropylenes
obtained from the n-heptane insoluble portion from: A) the reaction
with 2/MAO given as entry 2 of Table 1 and Table 2 and B) for the
crude polymer obtained from the reaction with 3/borate/TIBA given as
entry 6 of Table 1 and Table 2.
Figure 2. Molecular-weight distribution of the polymers obtained with
2/MAO.
aspecific, unbridged-metallocene precata-
lysts with isospecificity.
Table 2: Detailed analysis of each extracted portion of the polymer samples.[a]
Entry[b]
Extraction portion
Wt%
Tm [8C]
Mw [”10À3
]
Mw/Mn
mmmm [%]
The use of 3, the dimethyl analogue of 2,
1
EE-sol
EE-insol/C7-sol
C7-insol
EE-sol
EE-insol/C7-sol
C7-insol
EE-sol
EE-insol/C7-sol
C7-insol
EE-sol
EE-insol/C7-sol
C7-insol
EE-sol
EE-insol/C7-sol
C7-insol
EE-sol
EE-sol
13
21
66
20
21
59
50
32
18
64
22
14
21
44
35
100
100
100
97
–
151.1
155.7
–
131.8
151.3
–
146.4
148.3
–
127.3
140.6
n/d[c]
n/d[c]
n/d[c]
–
n/d[c]
85.8
337
27.2
68.4
273
10.7
39.4
113
n/d[c]
2.45
3.44
5.23
4.20
4.26
4.66
3.80
2.59
3.35
2.59
2.18
n/d[c]
n/d[c]
n/d[c]
2.10
2.10
1.87
2.13
14
55
85
19
53
86
17
67
86
provides clues to the nature of the isospe-
cific catalytic species in the 2/MAO system.
When 3 is activated with [Ph3C][B(C6F5)4] in
the presence of TIBA or TOA ([Al]/[Zr] =
200),[15] for example, the resulting catalytic
systems give completely diethyl ether solu-
ble, atactic polypropylenes (Figure 3B; see
also Tables 1 and 2 entries 6 and 7), while 3/
MAO affords polypropylene whose tacticity
distribution is similar to that obtained with
2/MAO (see Tables 1 and 2, entry 5). There-
fore, the involvement of MAO appears to be
essential in achieving isospecificity in pro-
pylene polymerization. Furthermore, when
the activation of 3 is effected by initial
treatment with MAO and subsequent treat-
ment with [Ph3C][B(C6F5)4] (see Table 1,
entry 11), the resulting catalytic species
gives completely atactic, diethyl ether solu-
2
3
4
5
5.3
19
53
86
17.8
48.1
n/d[c]
n/d[c]
n/d[c]
15.2
16.6
58.9
19.1
n/d[c]
n/d[c]
n/d[c]
5.4
3.2
n/d[c]
76
6
7
8
9
–
–
129
EE-sol
EE-insol/C7-sol
[a] EE=diethyl ether, C7=n-heptane; See Supporting Information for detailed analysis. [b] Entry
numbers correspond to those in Table 1. [c] Not determined.
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Chemie
ble polypropylene whose Mw/Mn value of 2.66 reflects its
single-site catalytic nature. 1H NMR spectroscopy experi-
ments revealed that the addition of [Ph3C][B(C6F5)4] to 3/
MAO causes the formation of Ph3CMe, as judged by the
appearance of a signal at d = 1.96 ppm,[16] while the treatment
of neutral MAO with [Ph3C][B(C6F5)4] at room temperature
does not produce Ph3CMe. Since [Me-MAO]À is the generally
assumed counteranion formed in metallocene/MAO sys-
tems,[17] the formation of Ph3CMe can be understood in
terms of MeÀ abstraction by [Ph3C]+ from [Me-MAO]À.[18]
Therefore, the treatment of 3/MAO with [Ph3C][B(C6F5)4]
causes the transformation of the isospecific [(AP)2ZrMe]+-
[Me-MAO]À in 3/MAO to aspecific [(AP)2ZrMe]+[B(C6F5)4]À
and neutral MAO. This observation, and the additional
finding that the catalytic system generated by treating 3
initially with [Ph3C][B(C6F5)4] ([B]/[Zr] = 1) and then with
MAO (Al/Zr= 200) (see Table , entry 12) also gives com-
pletely atactic polypropylene, implies that the influence of
MAO on the isospecificity occurs due to [Me-MAO]À rather
than neutral MAO.
ligand rotation is restricted on the NMR timescale, up to
343 K, and the formation of Al···N interactions, respectively.
Although coalescence was not observed up to 343 K, the
ligand rotation rate of the active species in 3/MAO can be
calculated as 122 sÀ1 at the coalescence temperature by taking
273 K as the no-exchange limit;[22] it is therefore slower than
122 sÀ1 at the polymerization temperatures.
The position of the upfield-shifted Me2N signal shows a
large temperature dependency such that at the higher
temperature limit the chemical-shift value approaches that
of neutral 3. Therefore, the Al···N Lewis acid–base interaction
is stable at the low-temperature limit but is likely to be
cleaved at the higher temperature limit, thereby leading to a
thermal equilibrium between the species with and without
this interaction. 3/TMA and 3/TOA (Al/Zr= 3) systems also
show an upfield shift of the Me2N signal but only one Cp
proton signal. Thus, trialkylaluminum is capable of forming an
Al···N interaction but is not able to prevent the ligand
rotation.
Overall, the above observations support the formation of
a rac-like ion-pair of the type [(AP)2ZrP]+[Me-MAO]À (IS) as
the isospecific catalytic species (Scheme 2). The simultaneous
presence of the cation–anion pairing and interactions
between the Lewis acid sites in [Me-MAO]À and the nitrogen
atoms of the amine-functionalized, unbridged-zirconocene
cation in IS is effective in preventing the ligands from rotating
rapidly, which leads to the prevalence of a racemic, C2-
symmetric-like active species in solution. The fact that the
hydrodynamic diameter (25 Š)[23] of [Me-MAO]À is larger
than the separation (ca. 15 Š) between two nitrogen atoms in
2 is also in favor of such interactions. The broad molecular-
weight distribution as well as the presence of polymer
fractions with different tacticities associated with 2/MAO
imply that IS is in thermal equilibrium with the other species
shown in the upper part of Scheme 2. A similar temperature-
dependent Al···N interaction has been employed in alkyl-
amine-functionalized zirconocene/MAO to modulate the
molecular-weight distribution of polyethylene.[24] Clearly,
1
Variable-temperature H NMR spectroscopy studies of 3
and 3/MAO (Al/Zr= 200) solutions in [D8]toluene
(Figure 4)[16] provide further evidence for the nature of the
Figure 4. Cyclopentadienyl (Cp) proton region of the variable-temper-
ature NMR (VT-NMR) spectra of 3, 3/TMA, 3/TOA(Al/Zr=3), and
3/MAO(Al/Zr=200). TMA=trimethylaluminum.
isospecific catalytic species. The singlet appearance of the two
magnetically different Cp protons of 3 at temperatures as low
as 203 K suggests that the ligand in 3 rotates rapidly in
solution. Waymouth-type neutral precatalysts undergo very
rapid ligand rotation in solution,[19] and the bulky precatalyst
bis(2-(3,5-di-tert-butylphenyl)indenyl)dimethylzirconium
rotates at a rate of 94700 sÀ1 even at 196 K, as measured by
the longitudinal relaxation time.[20] Therefore, the ligand
rotation rate of compound 3 should be at least of the order of
104 sÀ1 at the low-temperature limit.
The addition of MAO to 3 causes the appearance of one
set of two Cp proton signals, with equal intensities, at around
d = 5.7 ppm and an upfield shift of the Me2N signal,[21] thus
indicating the presence of an active species in which the
Scheme 2. Suggested thermal equilibrium of possible active species upon MAO
activation (top) and generation of the aspecific active species AS by the addition
of [Ph3C][B(C6F5)4] to the 3/MAO system (bottom).
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Communications
MAO, in the form of [Me-MAO]À, acts as an in situ ansa-
line-shape-analysis expression (k = p((W1/2)eÀ(W1/2)o) where
W1/2, e, and o stand for the peak-width at half height, with
exchange, and no exchange, respectively, for the rates in the
slow-exchange regime below the coalescence point cannot be
used. Thus, the rotation rate at the coalescence point was
bridge and plays the unprecedented role of endowing
aspecific class Iunbridged-metallocene precatalysts with
isospecificity, although cocatalysts are also known to affect
the stereoselectivity of unbridged-metallocene catalysts to
some extent.[25–27]
calculated and taken as a comparison value: k = pDn0/21/2
=
p(2307–2252 Hz)/21/2 = 122 sÀ1
.
In conclusion, we have devised a novel synthetic strategy
for the synthesis of isospecific, unbridged-metallocene cata-
lytic systems for propylene polymerization. The class I
unbridged metallocene with E = NEt2, OMe, SMe, etc.
behaves similarly and efforts are continuing to establish the
scope of the synthetic strategy.[28]
[23] D. E. Babushkin, H. H. Brintzinger, J. Am. Chem. Soc. 2002, 124,
12869.
[24] C. Müller, D. Lilge, M. O. Kristen, P. Jutzi, Angew. Chem. 2000,
112, 800; Angew. Chem. Int. Ed. 2000, 39, 789.
[25] G. M. Wilmes, J. L. Polse, R. H. Waymouth, Macromolecules
2002, 35, 6766.
[26] V. Busico, V. V. A. Castelli, P. Aprea, R. Cipullo, A. Segre, G.
Talarico, M. Vacatello, J. Am. Chem. Soc. 2003, 125, 5451.
[27] A. Macchioni, Chem. Rev. 2005, 105, 2039.
[28] CCDC 299059 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Received: February 20, 2006
Revised: June 9, 2006
Published online: August 22, 2006
Keywords: isotactic polypropylene · Lewis acids · metallocenes ·
.
polymerization · zirconium
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[21] The intensity of the upfield peak at d = 5.5 ppm is dependent on
the [Al]/[Zr] ratio. This peak disappears at ratios higher than
200. Its origin is considered to be due to a 3···TMA species since
a mixture of 3 and TMA gives a Cp signal at the same position.
[22] A [D8]toluene solution of 3/MAO at 273 K is extremely viscous
and no proton signal was seen at the lower temperatures. Thus,
273 K was taken as the no-exchange limit. In addition, owing to
the possible influence of viscosity on the line width at 273 K, the
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