Rotating benzyl substituent in ansa -bis(indenyl)zirconocenes controls propene polymerization

Article abstract of DOI:10.1021/om9009262

A series of benzyl-substituted, dual-site ansa-metallocenes were synthesized and characterized. Their isolated rac- and meso-diastereomers were studied in propene polymerization after methylaluminoxane or borate activation. Catalysts polymerization behavior were investigated in various polymerization conditions, and produced polypropenes (PPs) were characterized with NMR, GPC, and DSC. The rac- and meso-diastereomers of these unsymmetric catalysts bearing a SiMe₂ bridge produced PP with similar activity, tacticity, and molar mass. According to quantum chemical calculations, the benzyl group in the catalysts can rotate, having significant energy minima. The reason that the diastereomers produce PP with similar molar mass is linked to these local energy minima and is further discussed.

Full text of DOI:10.1021/om9009262

4018 Organometallics 2010, 29, 4018–4024
DOI: 10.1021/om9009262
Rotating Benzyl Substituent in ansa-Bis(indenyl)zirconocenes Controls
Propene Polymerization
Arto Puranen,†,^ Mikko Linnolahti,‡ Tanja Piel,§,4 Pertti Elo,†, Ilpo Mutikainen,†
‡
Tapani Pakkanen, Barbro Lofgren, Erkki Aitola, Jukka Seppala,
§
†
€
Markku Leskela, and Timo Repo*
€ € §
†
,†
€
†Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, FIN-00014, University of
Helsinki, Finland, ‡Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu,
Finland, and §Laboratory of Polymer Technology, Helsinki University of Technology, P.O. Box 6100,
FIN-02015 HUT, Finland. ^ Current address: Software Point, Valkjarventie 1, 02130 Espoo, Finland.
€
4 Current address: Borealis AB, SE-44486. Stenungsund, Sweden. Current address: Borealis
Polymers Oy, P.O Box 330, FI-06101 Porvoo, Finland
Received October 22, 2009
A series of benzyl-substituted, dual-site ansa-metallocenes were synthesized and characterized.
Their isolated rac- and meso-diastereomers were studied in propene polymerization after methyl-
aluminoxane or borate activation. Catalysts’ polymerization behavior were investigated in various
polymerization conditions, and produced polypropenes (PPs) were characterized with NMR, GPC,
and DSC. The rac- and meso-diastereomers of these unsymmetric catalysts bearing a SiMe2 bridge
produced PP with similar activity, tacticity, and molar mass. According to quantum chemical
calculations, the benzyl group in the catalysts can rotate, having significant energy minima. The
reason that the diastereomers produce PP with similar molar mass is linked to these local energy
minima and is further discussed.
Introduction
(PPs) have been successfully produced. Topics concerning
the influence of ligand structures on the polymer tacticities
as well as the polymerization mechanisms are thoroughly
reviewed.6
Since the discovery of methylaluminoxane (MAO) as an
efficient activator of homogeneous metallocene catalysts in
the mid 1970s1,2 and isospecific polymerization of propene in
1985, massive research efforts in both academia and the
chemical industry were directed toward metallocene-based
polymerization catalysts. During the last 20 years funda-
mental concepts concerning the influence of the chiral ligand
framework on polymer tacticity, including enantiomorphic
site control and chain back-skipping mechanisms, were deve-
loped.3-5 Because stereospecific propene polymerization
requires by definition a chiral metal center, the ligand frame-
works are designed to be rigid. Even a flexible ethylene
bridge between the cyclopentadienyl moieties is often chan-
ged to a more rigid dimethylsilyl bridge. By tuning the
catalyst structure, highly iso- and syndiotactic polypropenes
In the area of metallocene catalysts unsymmetric, dual-site
metallocenes are especially intriguing, as they can be used for
controlled stereoerror formation during the propene polym-
erization. With these catalysts structurally defined hemi-
isotactic polypropene and various syndio- and isotactic
polypropenes with variable amounts of stereoerrors in
their microstructure were introduced.4,7-9 When the mo-
lar mass and amount of stereoerrors are balanced, the
polypropenes can possess significant elastic properties. It
appears that in most unsymmetric, dual-site catalysts
reported so far, the origin for the stereoselectivity is a
consecutive chain back-skipping prior to the monomer
coordination to the metal.10-12
In general, the meso-isomers of bis(indenyl)zirconocene
catalysts are known to produce atactic PP with low molar
mass. We reported recently the catalytic properties of the
*To whom correspondence should be addressed. E-mail: timo.
repo@helsinki.fi.
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r
pubs.acs.org/Organometallics
Published on Web 08/26/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 18, 2010 4019
Scheme 1
Figure 1. Diastereomers of SiMe2(3-benzylindenyl)(indenyl)ZrCl2
(meso-1 and rac-1) produce isotactic polypropenes with similar
microstructures and molar masses under equivalent polymerization
conditions.
meso- and rac-diastereomers of benzyl-substituted metallo-
cene SiMe2(3-benzylindenyl)(indenyl)ZrCl2 (1) in propene
polymerization after MAO activation; unexpectedly these
diastereomers produce polypropene with similar tacticities
and molar masses under equivalent polymerization condi-
tions (Figure 1).13 This previously unprecedented similarity
in polymer characteristics indicates that the coordination
sites of the unsymmetric diastereomers have equivalent steric
surroundings caused by the ligand framework. Furthermore,
the rotating benzyl substituent in the 3-position seems to
provide an interesting and novel way to control the stereo-
specificity and especially molar mass of the polypropene. In
this respect, we herein report on a set of new benzyl-sub-
stituted complexes and computational studies together with
polymerization experiments to understand how the benzyl
group influences the molar mass and selectivity in the
enantiofacial coordination of propene.
3-[2-(1H-inden-1-yl)ethyl]-1H-indene (10.34 g, 35.8 mmol) (2a) in
200 mL of diethyl ether at 0 ꢀC. The reaction mixture was stirred at
room temperature for 4 h. Benzyl bromide (2b) (15.3 g, 89.5 mmol)
in 40 mL of diethyl ether was added dropwise to the cooled (0 ꢀC) Li
salt solution. After overnight stirring at RT the reaction mixture was
quenched with 150 mL of a saturated solution of NH4Cl. Water and
the organic layer were separated, and the water phase was extracted
three times with 30 mL of diethyl ether. Combined organic layers
were dried with NaSO4(s) and evaporated to constant weight. The
product (yellow viscous oil) was purified by column chromatogra-
phy using a toluene/hexane (1:4) mixture as an eluent. Yield: 2.52 g
(20%). 13C NMR could not be interpreted due to overlapping peaks
of the diastereomers. 1H NMR (CHCl3): δ 7.39-7.04 (m, 13H, Ar,
CpInd), 6.75 (m, 1H, CpInd), 6.42 (m, 1H, CpInd), 6.03 (m, 1H,
CpInd), 3.77 (d, 2H, CH2), 3.32 (m, 2H, Cpaliphatic), 1.94-1.35 (m,
4H, Etbridge). MS(EIþ) m/z (%): 348 (47), 219 (53), 129 (100).
(3-Benzyl-1H-inden-1-yl)(2-methylbenz[e]-1H-inden-1-yl)-
dimethylsilane (3c/3c*). n-Butyllithium (31 mL, 50 mmol, 1.6 M
in hexane) was added to 2-methylbenzindene (3b) (9.0 mL,
50 mmol) in 150 mL of diethyl ether at 0 ꢀC. The reaction mix-
ture was stirred at room temperature for 2 h. (3-Benzylindenyl)-
(chloro)dimethylsilane (3a) (14.8 g, 50 mmol) in 40 mL of diethyl
ether was added to the cooled, 0 ꢀC, Li salt solution. After
overnight stirring at RT the reaction mixture was quenched with
150 mL of a saturated solution of NH4Cl. The organic layer was
separated, dried with NaSO4(s), and evaporated to constant
weight. The product (yellow viscous oil) was purified by column
chromatography using a toluene/hexane (1:4) mixture as an eluent.
Yield: 10 g (48%). 13C NMR could not be interpreted due to
overlapping peaks of the diastereomers. 1H NMR (CHCl3): δ 8.02
(d, 1H, Ar), 7.79 (d, 1H, Ar), 7.52-7.03 (m, 12H, Ar and 1H,
CpInd), 6.05/6.00 (s, 1H, CpInd/s, 1H, CpInd), 3.82 (d, 2H, CH2),
3.67 (s, 1H, Cpaliphatic), 3.56/3.47 (s, 1H, Cpaliphatic/s, 1H,
Cpaliphatic), 2.28/2.22 (s, 3H, CH3/s, 3H, CH3), -0.21, -0.27,
-0.49, -0.51 (s, 3H, SiCH3 and s, 3H, SiCH3/s, 3H, SiCH3 and
s, 3H, SiCH3). MS(EIþ) m/z(%): 442 (35), 263 (55), and 237 (100).
Ethyl(3-benzylindenyl)(2-methylbenz[e]indenyl)zirconium Dichlo-
ride (meso-2 and rac-2). n-Butyllithium (8.25 mL, 13.2 mmol, 1.6 M
in hexane) was added dropwise to a precooled (0 ꢀC) solution
of 1-benzyl-3-[2-(1H-inden-1-yl)ethyl]-1H-indene (2c) (2.3 g, 6.6
mmol) in 150 mL of diethyl ether. Stirring was continued for 2 h
at room temperature. Diethyl ether was removed. Precooled
(-78 ꢀC) precipitate and 150 mL (-78 ꢀC) of dichloromethane
were combined. When the temperature of the mixture had stabi-
lized to -78 ꢀC, zirconium tetrachloride (1.54 g, 6.6 mmol) was
added and the reaction mixture was allowed to warm to room
Experimental Section
General Information and Materials. All syntheses and mani-
pulations were performed under an inert argon atmosphere by using
standard Schlenk techniques. All starting materials were standard
reagent grade and used as received. Diethyl ether, pentane, hexane,
and toluene were distilled over sodium, and benzophenone was used
as an indicator, if not mentioned otherwise. Dichloromethane was
distilled over calcium dihydride.
1
Instrumentation and Measurements. H NMR spectra were
recorded on a Varian Gemini 200 spectrometer operating at 200
MHz, and 13C NMR spectra of polypropenes were obtained on
a Varian Gemini 2000XL (75 MHz, 10 mm probe in TCB/C6D6
(9:1) at 120 ꢀC with 45ꢀ pulse angle, 7 s delay, and 2000 scans).
The [mmmm] methyl signal of polypropene with chemical shift
δ = 21.80 ppm was used as an internal standard. GC/MS
analysis was made with a Varian 3800 GC and a Varian Saturn
2000 GC/MS/MS. Mass spectra and high-resolution mass spec-
tra were recorded on a JEOL JMS-SX102 mass spectrometer,
operating at 70 eV.
Synthesis and Characterization. The addition of 2-methyl-4,5-
benzindene (3b)14 to (benzylindenyl)chlorodimethylsilane13,15
(3a) gave the desired ligand precursor (3c) (Scheme 1). The yield
was only 48% compared to nearly quantitative reaction with
unsubstituted indene. Successful synthesis of the ethyl-bridged
preligand analogue (2c) was achieved in a reaction between
benzyl bromide (2b) and ethylbisindenyllithium (2a). Com-
plexation of the silyl- and ethyl-bridged preligands in dichloro-
methane and toluene with ZrCl4 gave the desired products,
respectively.
1-Benzyl-3-[2-(1H-inden-1-yl)ethyl]-1H-indene (2c/2c*). n-Butyl-
lithium (22.5 mL, 35.8 mmol, 1.6 M in hexane) was added to
€
(13) Puranen, A. J.; Klinga, M.; Leskela, M.; Repo, T. Organome-
tallics 2004, 23, 3759–3762.
(14) Obtained from Prof. Rieger’s group. Roll, W.; Karl, E.; Brint-
€
zinger, H. H.; Stehling, U.; Rieger, B. EP 0519237, priority 18.06.1991.
(15) Alt, H. G.; Reb, A.; Kundu, K. J. Organomet. Chem. 2001, 628,
211.

4020 Organometallics, Vol. 29, No. 18, 2010
Puranen et al.
temperature. After stirring overnight at room temperature the
reaction mixture was filtered and evaporated in vacuo. Diastereo-
mers were separated by recrystallization from toluene/hexane
mixtures. Meso- and rac-isomers were yellow powders, and yields
were 1.1 g (30%) and 0.74 g (22%), respectively. meso-2 1H NMR
(CHCl3): δ 7.56-6.99 (m, 13H) 6.76 (d, 2JHH = 3.29 Hz, 1H) 6.41
(d, 2JHH = 3.30 Hz, 1H) 6.20 (s, 1H), 4.18 (s, 2H) 3.98 (m, 2H) 3.59
(m, 2H). MS(EIþ): m/z (%) 508 (90), 473 (45), 382 (100). HRMS-
(EIþ) m/z for C27H22Cl2Zr: observed 506.0123; error -4.5 ppm.
rac-2 1H NMR (CHCl3): δ 7.66-7.02 (m, 13H) 6.57 (d, 3JHH = 3.3
Solid-State Structure. A crystal suitable for X-ray measure-
ments was selected and mounted on a glass fiber using the oil
drop method,16 and the data were collected at 173 K using a
Nonius KappaCCD diffractometer. The intensity data were
corrected for Lorentz and polarization effects and for absorp-
tion by the multiscan method.17 The crystal structures were
solved and refined with SHELX97.18 Graphics were done using
the SHELXTL19 program package. The hydrogen atoms were
introduced into their calculated positions and refined with fixed
geometry with respect to their carrier atoms or were picked from
the residual electron density map without further refinement.
Computational Studies. Cationic isobutyl-substituted com-
plexes of meso-3 and rac-3 were used in the computational
studies. Isobutyl was used to mimic a growing polymer chain.
Benzyl rotation calculations were done while propene was
coordinated on the benzyl side of the catalyst and isobutyl on
the other side. Benzyl rotated in 20-degree steps around its bond
to an indenyl group.
3
Hz, 1H), 6.23 (d, JHH = 3.3 Hz 1H), 5.95 (s, 1H), 4.03 (AB,
2JHH = 21.62 Hz, 2H), 3.71 (m, 4H) . MS(EIþ) m/z(%):508(100),
473 (70), 382 (95). HRMS(EIþ) m/z for C27H22Cl2Zr: observed
506.0121; error -5.0 ppm.
Dimethylsilyl(3-benzylindenyl)(2-methylbenz[e]indenyl)zirconium
Dichloride (meso-3 and rac-3). n-Butyllithium (15 mL, 23.9 mmol,
1.6 M in hexane) was added dropwise to a precooled (0 ꢀC)
solution of (3-benzylindenyl)(2-methylbenzindenyl)dimethylsilane
(3c) (5.3 g, 12 mmol) in 150 mL of diethyl ether. Stirring was
continued for 2 h at room temperature. Diethyl ether was removed.
Precooled (-78 ꢀC) precipitate and 150 mL (-78 ꢀC) of dichlor-
omethane were combined. When the temperature of the mixture
had stabilized to -78 ꢀC, zirconium tetrachloride (2.8 g, 12 mmol)
was added and the reaction mixture was allowed to warm to room
temperature. After stirring overnight at room temperature the
reaction mixture was filtered and evaporated in vacuo. Diastere-
omers were separated by recrystallization from toluene/hexane
mixtures. Meso- and rac-isomers were yellow powders, and yields
were 1.1 g (15%) and 0.6 g (8%), respectively. meso-3 1H NMR
Calculations were performed by the hybrid density functional
B3LYP method. Huzinaga’s all-electron basis set (Zr, 433321/
433/421)20 was used for zirconium and the standard 6-31G*
basis set for other atoms. All complexes were verified as either a
minimum or a transition state by vibrational frequency calcula-
tions. All calculations were carried out by Gaussian 03 quantum
chemistry software.21
Results and Discussion
Ligands and Complexes. To gain more understanding
about the influence of the benzyl group on the catalyst
stereoselectivity and chain growth, the ligand framework in
meso-1 and rac-1 was modified by attaching a methyl group
to the 2(R)-position and a benzo substituent to the 4- and
5-positions (meso-3 and rac-3 in Scheme 2). The former is
supposed to increase the molar mass of the polymer, while
the latter is known to improve metallocene catalysts’ activity
and stereoselectivity.22 In addition, ethyl-bridged analogues
of 1 (meso-2 and rac-2) were prepared to increase the flex-
ibility of the ligand framework. The selected ligands and
complexes were prepared by using straightforward synthesis
3
(CHCl3): δ 7.95 (d, JHH = 7.33 Hz, 1H), 7.59-7.23 (m, 7H),
=
7.16-7.01 (m, 7H), 6.80 (m, 1H), 5.76 (s, 1H), 4.12 (AB, 2JHH
19.05 Hz, 2H), 2.26 (s, 3H), 1.34 (s, 3H,), 0.97 (s, 3H). MS(EIþ):
m/z (%) 602 (100), 567 (90), 237 (40). HRMS(EIþ) m/z for
C32H28Cl2SiZr: observed 600.0370; error -2.4 ppm. rac-3 1H
NMR (CHCl3): δ 7.98 (m, 1H), 7.71 (m, 1H), 7.60-7.00 (m,
13H), 6.90 (m, 1H), 5.68 (s, 1H), 3.85 (AB, 2JHH = 16.12 Hz, 2H),
2.26 (s, 3H), 1.21 (s, 3H), 1.08 (s, 3H). MS(EIþ) m/z (%): 602 (90),
567 (100), 237 (45). HRMS(EIþ) m/z for C32H28Cl2SiZr: observed
600.0442; error þ9.6 ppm.
Polymerization Studies. Propene polymerizations with meso-3
€
and rac-3 were carried out in a 1 L Buchi steel autoclave. The
monomer consumption and pressure as well as the inside
temperature were controlled by computerized real-time mon-
itoring. First MAO (Al:Zr=2000) and 280 mL of toluene
(distilled over sodium after 1.5 h refluxing) were loaded into
the autoclave, and the temperature was set. After the tempera-
ture was set the propene pressure was adjusted to the desired
level and the zirconium dichloride complex (meso-3 or rac-3) in
20 mL of toluene was injected into the autoclave. The polym-
erization reaction was quenched in 400 mL of acidic methanol.
After stirring an hour the precipitate was washed carefully with
methanol for 2 h and dried overnight at 60 ꢀC.
(16) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615.
(17) Nonius COLLECT; Nonius BV: Delft, The Netherlands, 2002.
(18) Sheldrick, G. M. SHELX97, Program for the Solution and
€
Refinement of Crystal Structures; University of Gottingen: Germany, 1997.
(19) Sheldrick, G. M. SHELXTL Version 5.10; Bruker AXS Inc.:
Madison, WI, 1997.
(20) Huzinaga, S. Gaussian Basis Sets for Molecular Calculations,
Elsevier: Amsterdam, 1984.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.
Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT,
2004.
Polymerizations with meso-2 and rac-2 were carried out in
a 0.5 dm3 Buchi stainless steel autoclave equipped with a
€
propeller-like stirrer. Toluene (300 mL, pro analysis, supplied
by Merck) was used as solvent. Toluene and propene (3.5,
Messer) were purified by passing through three columns con-
taining molecular sieves, CuO, and Al2O3. MAO (10% solution
in toluene, Crompton Corporation) and TIBA (Crompton
Corporation) were used as delivered. Cocatalyst, [HNPhMe2]-
[B(C6F5)4], was purchased from Akzo Nobel.
The progress of the polymerization was followed using a
mass-flow meter to monitor the propene consumption. Weight
average molecular weights (Mw) and molecular weight distribu-
tions (MWD) were measured at 140 ꢀC with a Waters Alliance
2000 size exclusion chromatography instrument (SEC) equipped
with Waters HT3, HT4, HT5, and HT6 columns. 1,2,4 -Trichloro-
benzene was used as solvent, and the flow rate was 1.0 cm3/min.
The columns were universally calibrated with narrow molecular
weight polystyrene standards.
€
(22) (a) Stehling, U.; Diebold, J.; Kirsten, R.; Roll, W.; Brintzinger,
€
€
H. H.; Jungling, S.; Mulhaupt, R.; Langhauser, F. Organometallics
1994, 13, 964. (b) Spaleck, W.; Antberg, M.; Aulbach, M.; Bachmann, B.;
Dolle, V.; Haftka, S.; K€uber, F.; Rohrmann, J.; Winter, A. In Ziegler
Catalysts; Fink, G., M€ulhaupt, R., Brintzinger, H.-H., Eds.; Springer-Verlag:
Berlin, 1995; p 83.

Article
Organometallics, Vol. 29, No. 18, 2010 4021
Scheme 2
routes, and the rac- and meso-diastereomers were separated
by fractional recrystallization. It should be noted that meso
and rac abbreviations are used in this article intentionally to
symbolize the orientation of the indenyl ligand in the com-
plex, although both are actually racemic mixtures of two
enantiomers.
A solid-state structure was measured for meso-[(3-benzyl-
1-η5-indenyl)(2-methylbenz[e]-1-η5-indenyl)dimethylsilane]-
zirconium dichloride (meso-3) (Figure 2). A comparison of
the solid-state structure of meso-3 with the previously pub-
lished structure of meso-113 does not show any significant
differences in angles or distances between zirconium and Cp
rings. The angle of the Cp planes in the structure is nearly the
same as in the unsubstituted meso-1, being 1.15 degrees smaller.
In both crystal structures the benzyl group points away from
the metal center. Accordingly, as the coordination spheres of
meso-1 and meso-3 are similar, possible differences in propene
polymerization arises from the R-methyl and 4,5-benzo sub-
stituents of meso-3.
Polymerization. Diastereomers of MAO-activated metal-
locene complexes 1-3 were studied in propene polymeriza-
tion, and classical rac-Et(Ind)ZrCl2 (EBI) was used for a
comparison. Under similar polymerization conditions (50 ꢀC
and 0.71 M propene) EBI/MAO gave 6.6 times higher,
benzo-substituted rac-3/MAO gave 4.1 times higher, and
the ethyl-bridged rac-2/MAO gave 1.6 times higher activities
than rac-1. When compared to rac-1, the influence of the
R-methyl and the 4,5-benzo substituents in rac-3 is clearly
seen, besides in the higher activity, in improved isotacticity
and molar mass of the PP (Table 1). As for rac-1 and meso-1,
detailed pentad analysis reveals that also the diastereomer
pair rac-3 and meso-3 produced nearly identical PP, even if
the polymerization conditions were varied (Figure 3). Pro-
duced polypropenes have a pentad ratio of [mmmr]:[mmrr]:
[mrrm] ≈ 2:2:1, and this indicates the presence of enantio-
morphic site control. Furthermore, the facts that catalysts
lose their stereocontrol with increasing temperature and
catalysts improve the stereocontrol with increasing mono-
mer concentrations point to a chain migratory insertion
mechanism rather than a chain back-skipping mechanism
(Table 1 and Figure 3).23
Figure 2. Solid-state structure of meso-dimethylsilyl(3-benzyl-
η5-indenyl)(2-methyl[4,5]benz-η5-indenyl)zirconium dichloride
(meso-3). Atoms are represented by anisotropic displacement
˚
ellipsoids at the 50% probability level. Selected bond distances (A)
and angles (deg): Cpplane-Cpplane, 60.95; Cpcenter-Zr-Cpcenter
,
128.46; Cpcenter(lower)-Zr, 2.222; Cpcenter(upper)-Zr, 2.250.
In general, the polymerization activities of the dimethylsi-
lyl-bridged rac-1/MAO and the ethyl-bridged rac-2/MAO
do not differ markedly from each other, but the latter
produces PP with slightly lower isotacticity and molar mass
(Table 1). As with rac-3/MAO, isospecificity of the catalysts
reduces rapidly with the increasing reaction temperature,
while decreasing monomer pressure has only a modest
influence (Figure 3). At very low propene concentrations
(here 0.48 M) the chain end epimerization strengthens and
reduces significantly the activity as well as the isotacticity.
This is typical for catalysts working under the enantio-
morphic site control mechanism.24
Opposite the diastereomers pairs of 1 and 3, rac-2/MAO
turned out to be more active than meso-2/MAO, and more
surprisingly the latter produces PP with 7% lower [mmmm]
pentad content at 50 ꢀC and 0.71 M propene. The change of
MAO with borate cocatalyst did not introduce any signifi-
cant differences in polymer pentad distribution, although a
slight decrease in PP stereoselectivity and molar mass is
observed. Also, the counterion does not influence the ratio
of stereoselectivity between rac-2/MAO and meso-2/MAO
diastereomer pairs (Table 1).
Due to the flexible ethylene bridge, the indenyl ligands have
a chance to move between the forward and backward con-
formations. By blocking this fast flip between the conformers
with a bridge substituent, Rieger et al. have shown that the
conformers have a marked difference in stereoslectivity; the
forward conformer has significantly higher stereocontrol than
the backward conformer.25 The presence of these conformers
(23) (a) See ref 4. (b) Busico, V.; Cipullo, R. Prog. Polym. Sci. 2001,
26, 443. (c) See ref 3. (d) Coates, G, W. J. Chem. Soc., Dalton Trans. 2002,
467. (e) Coates, G. W. Chem. Rev. 2000, 100, 1223. (f) Brintzinger, H.-H.;
Fischer, D.; M€ulhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1143.
(24) Resconi, L.; Fait, A.; Piemontesi, F.; Colonnesi, M.; Rychlicki,
H.; Ziegler, R. Macromolecules 1995, 28, 6667–6676.
(25) Rieger, B.; Jany, G.; Fwazi, R.; Steimann, M. Organometallics
1994, 13, 647–653.

4022 Organometallics, Vol. 29, No. 18, 2010
Table 1. Propene Polymerization Results of the MAO-Activated Complexesa
Puranen et al.
b
d
f
catalyst
run id
temp
(ꢀC)
[C3]
(M)
tp
(min)
yield
(g)
[mmmm]
(%)
[mmmr]
(%)
[mmrr]
(%)
[mrrm]
(%)
Mw
(kg/mol)
Tm
(ꢀC)
activityc
PDe
rac-113
rac-1
rac-1
rac-1
rac-1
rac-1
rac-1
rac-1
rac-2
rac-2
rac-2
rac-2
rac-2
rac-2
30
50
50
50
50
50
70
85
30
50
50
50
50
70
85
50
30
50
50
50
70
85
50
30
50
50
50
50
50
70
85
30
50
50
50
50
50
70
85
0.71
0.48
0.71
1.16
1.76
3.38
0.71
0.71
0.71
0.48
0.71
1.16
1.76
0.71
0.71
0.71
0.71
0.71
1.16
1.76
0.71
0.71
0.71
0.71
0.48
0.71
1.16
1.76
3.38
0.71
0.71
0.71
0.48
0.71
1.16
1.76
3.38
0.71
0.71
60
33
31
44
33
34
35
41
30
30
30
30
30
30
30
50
30
30
30
30
30
30
50
33
31
30
30
31
31
31
33
36
30
30
31
31
31
33
32
2.9
0.9
4.6
11.3
14.2
24.4
8.6
14.2
2.5
0.8
1.4
2.5
2.7
3.0
2.6
4.2
5.9
1.4
1.1
4.1
4.6
5.8
9.3
12.8
1.7
1.4
2.8
3.5
3.4
6.8
7.0
2.5
3.0
45.3
37.1
40.2
41.9
44.1
45.0
26.4
15.9
52.3
33.5
45.9
50.1
53.5
33.3
20.2
42.9
45.4
39.0
43.0
45.7
27.8
17.3
36.8
64.5
53.5
57.2
60.3
61.9
62.5
39.1
20.6
67.9
51.4
55.4
59.8
63.6
65.5
38.5
21.3
15.5
16.1
16.0
15.9
15.6
15.4
15.9
13.2
14.7
16.5
15.6
15.1
14.4
16.4
15.3
15.4
16.2
15.8
15.5
15.3
15.1
16.7
14.8
13.7
16.7
15.1
14.2
13.6
16.9
16.6
7.7
8.3
8.1
8.0
7.7
7.6
8.4
7.7
6.8
8.3
7.5
7.1
6.8
8.5
8.5
38
16
25
29
33
38
12
4.8
30
11
18
24
29
9.7
5.6
14
23
13
17
21
7.3
4.5
9.0
133
26
37
41
51
68
16
5.8
131
41
49
54
66
79
17
6.8
2.0
2.0
1.9
1.9
2.1
2.0
2.4
2.2
2.0
1.8
1.8
1.9
2.0
1.8
1.8
1.8
2.3
2.1
2.1
2.2
1.8
1.8
1.8
2.38
2.13
2.11
2.09
2.02
1.99
2.76
2.38
2.4
2.05
2.03
2.06
1.96
2.07
2.35
2.34
74
62
67
69
73
76
n.d.h
n.d.
92
72
80
88
94
1.3
7.3
13.2
25.6
16.5
22.7
2.9
n.d.
n.d.
rac-2
rac-2g
meso-2
meso-2
meso-2
meso-2
meso-2
meso-2
meso-2g
rac-3
2.4
5.1
15.0
16.0
15.1
15.0
15.7
14.1
14.0
14.9
14.6
14.0
16.2
15.4
6.8
7.7
7.0
7.0
7.7
7.8
n.d.
n.d.
82
89
n.d.
n.d.
10.2
14.8
12.0
12.5
4.5
3.0
4.7
9.2
16.5
29.8
52.7
16.9
14.4
2.4
3.7
6.8
10.8
16.2
18.4
9.4
12.1
14.5
13.7
13.1
12.6
12.5
15.8
14.8
11.7
15.0
14.1
13.2
12.4
11.8
16.2
15.2
11.7
14.2
13.4
12.8
12.2
12.1
16.1
16.5
11.0
15.1
14.0
12.8
11.8
11.3
16.7
17.0
5.8
7.0
6.6
6.3
6.0
5.9
8.0
8.3
5.3
7.3
6.9
6.3
5.8
5.5
8.1
8.3
108
91
96
100
104
106
n.d.
n.d.
111
84
92
100
106
110
67
rac-3
rac-3
rac-3
rac-3
rac-3
rac-3
rac-3
meso-3
meso-3
meso-3
meso-3
meso-3
meso-3
meso-3
meso-3
7.6
10.2
11.2
13.1
12.2
18.3
14.7
4.5
12.4
15.3
14.2
14.4
8.51
19.0
16.9
7.9
n.d.
a In toluene and Al:Zr = 2000 See Supporting Information for detailed pentad analysis. b Polymerization time. c Activity expressed as kg
polymer/[(mmol Zr)[C3] h]. d Weight average molar mass. e Polydispersity. f Polypropene melting temperature. g Borate cocatalyst. h Not detected.
Figure 3. Isotactic pentads as a function of polymerization temperature (left) and propene concentration (right).
in ethyl-bridged 2explains in general the reduced stereoregularity
as well as molar mass when compared those of 1.Apparently,the
different stereoselectivity of rac-2/MAO and meso-2/MAO is
also connected to the presence of the bridge conformers and
illustrates the different steric shielding caused by the rotating
benzyl group and aromatic indenyl ring at the β-positions.
Computational studies were performed to understand in
more detail the influence of the benzyl substituent on pro-
pene polymerization, especially on the origin of the stereo-
specificity and the high molar mass of the polymers produced
by the meso-diastereomers. In meso-3 one of the coordina-
tion sites is shielded only by the benzyl substituent (A-site),
while the other (B-site) is heavily shielded by the indenyl and
benz[e]indenyl groups (Figure 4). When the growing PP
chain lies on the A-site, the C(R)-C(β) chain segment of
the polymer tends to orientate down toward the benz-
[e]indenyl ligand, and thus the propene coordinated on the
B-site is re-coordinated; that is, the methyl group points
toward the benzylindenyl (Figure 5). The B-site is clearly
stereoselective for propene coordination. After insertion the

Article
Organometallics, Vol. 29, No. 18, 2010 4023
Cationic complexes of meso- and rac-3 bearing a coordi-
nated propene and an isobutyl ligand as a model for the
polymer chain were selected for starting points to investi-
gate the role of the benzyl substituent in the activated
metallocene (Figure 5). Accordingly, when propene coordi-
nates to the A-site and the polymer chain (isobutyl ligand)
occupies the B-site, the benzyl substituent can rotate quite freely
around its Cp-CH2 bond. The energy barriers of the rotations
are nearly independent of the used diastereomer and propene
enantioface (Figure 6). In both diastereomers the Cp-CH2-Ph
angle varies between 111ꢀ and 123ꢀ during a 360-deg rotation.
The most stable rotamer for all of these complexes is
achieved when the dihedral angle (CPh-CH2-CCp-CCp) is
60ꢀ; that is, benzyl is pointing upward, away from the
cationic metal center (compare to the orientation of the
benzyl group in the solid state, Figure 2). Another significant
minimum is located around 165ꢀ. Here the benzyl substituent
is approximately in the plane of the indenyl ligand and occupies
the place where a methyl substituent in R-position to the bridge
head atom would be if present. These R-substituents are known
to be important, as they diminish the rate for the chain
termination via β-H elimination, and as a result, polymers with
increased molar mass are obtained.3,23,26 This effect is also seen
here when the molar mass of PPs from 1/MAO and 3/MAO are
compared (Table 1). As benzyl-substituted meso-2 produces PP
with clearly higher molar mass than the unsubstituted meso-
Et(Ind)ZrCl2, this rotamer seems to have, indeed, a role in
polymerization (determining the molar mass). This mechanistic
insight provides also an explanation for the dramatic influence
of the reaction temperature on molar mass of the polymers. An
increase in thermal energy allows benzyl to rotate more freely,
thus increasing the tendency for β-H elimination and causes
short-chain PP at high temperatures. This can be detected from
Table 1.
Figure 4. Structures of the complexes used in the calculations.
The dimethylsilyl bridges between the Cp moieties and hydrogen
atoms are omitted for clarity.
As reported previously, a benzyl substituent in the cyclo-
pentadienyl ligand has a tendency to coordinate with the
cationic metal center.27 Benzyl coordination is favored over
propene coordination by 11-23 kJ/mol. The energy differ-
ences are not high enough to block the polymerization, but
certainly reduce the catalytic activity in comparison to
metallocenes without the benzyl group (activity of EBI is
6.6 times higher than with rac-1). It is worth noticing that the
rotation energy barrier of the benzyl group is in the same
range as the energy required to substitute coordinated benzyl
with propene (Figure 6). According to the modeling, the
benzyl group may also turn away from the coordination site
to give enough space for propene coordination and propaga-
tion without completely losing its interaction with the metal
center.
Figure 5. Optimized structures of meso- and rac-3 bearing a
coordinated propene at the A-site and an isobutyl ligand as a
model for the polymer chain at the B-site illustrate two impor-
tant energy minima of the benzyl rotation. The dimethylsilyl
bridges between the Cp moieties and the hydrogen atoms are
omitted for clarity.
polymer chain moves to the B-site, and now propene can
coordinate to the A-site next to the rotating benzyl group. As
the B-site is shielded by indenyl and benz[e]indenyl groups,
the growing chain slightly prefers the orientation toward the
indenyl. The propene coordination on the A-site leads to
random stereoerror formation. As a result, meso-3 produces
isotactic PP with variable amounts of stereoerrors.
In rac-3, the B-site is surrounded by the indenyl group,
while the A-site is shielded by the benzyl substituent and the
benz[e]indenyl ligand (Figure 4). When the polymer chain is
located on the B-site, the C(R)-C(β) chain segment has a
clear preference to point down to the benz[e]indenyl ligand
and the propene coordination on the A-site occurs selectively
from the si-side (i.e., the methyl group of propene points up
toward the benzyl group) (Figure 5). The chain at the more
shielded A-site has no clear preference for the orientation,
and this causes the formation of stereoerrors. It is plausible
that rac-3 and meso-3 produce PP with similar tacticity.
To provide an estimate of the overall capability of the
benzyl substituent to block the active metal center, we have
calculated the transition state for si-propene insertion to
benzyl-coordinated meso-3 complex. The height of the acti-
vation barrier is 53.4 kJ/mol, suggesting that the coordinated
benzyl group is effective in blocking the active site, but the
(26) Resconi, L.; Jones, R. L.; Rheingold, A. L.; Yap, G. P. A.
Organometallics 1996, 15, 998.
€
(27) (a) Buhl, M.; Sassmannshausen, J. J. Chem., Dalton Trans. 2001,
79–84. (b) Bochmann, M.; Green, M. L. H.; Powell, A. K.; Sassmannshausen,
J.; Triller, M. U.; Wocadlo, S. J. Chem., Dalton Trans. 1999, 43–49. (c)
€
Doerrer, L. H.; Green, M. L. H.;Haussinger, D.; Sassmannshausen, J. J. Chem.
Soc., Dalton Trans. 1999, 2111–2118. (d) Aitola, E.; Surakka, M.; Repo, T.;
Linnolahti, M.; Lappalainen, K.; Kervinen, K.; Klinga, M.; Pakkanen, T.;
€
Leskela, M. J. Organomet. Chem. 2005, 690, 773–783.

4024 Organometallics, Vol. 29, No. 18, 2010
Puranen et al.
Figure 6. Energy curves of meso- and rac-3 bearing a coordinated propene at the A-site and an isobutyl ligand at the B-site during the
benzyl rotation against the Cp-Bn dihedral angle. At 60ꢀ benzyl is pointing away from the zirconium, and at 165ꢀ the phenyl ring is
residing in the position of the R-substituent of the indenyl ligand (see Figure 5).
energy is not that high that it prevents the withdrawal of the
benzyl group to allow the propene coordination and chain
propagation to initiate. Likely, the overall result is the loss of
activity, also observed experimentally.
swings to the indenyl plane. This causes the high molar mass
of the polymer. As the catalysts work under the enantio-
morphic site control mechanism, a low temperature is needed
for high stereospecificity. With increasing reaction tempe-
rature, the energy for the benzyl ring rotation increases and
this causes a substantial decrease in stereospecificity and
in PP molar mass, whereas benzyl spends less time around
the active center and thus the activity substantially increases.
The benzyl-substituted catalysts respond as classical C2-sym-
metric metallocenes toward changes in monomer concentra-
tion. The stereoselectivities of the catalysts and molar masses
of the PPs increase steadily with elevated propene concentra-
tions. At low propene concentrations (at 0.48 M) polymeri-
zation activity declines due to the chain end epimerization.
In general, this study underlines that a rotating substituent
can indeed be applied in catalyst design and a catalyst with
high response under the reaction conditions can be designed.
It is an intriguing idea to apply similar concepts in different
fields of homogeneous catalysis and to improve by this
manner catalysts’ response to changes in reaction conditions.
Conclusions
Unlike typical metallocene catalysts, the diastereomer pairs
of the studied unsymmetric catalysts produce PP with similar
activity, tacticity, and molar mass. While similar stereoselec-
tivity in the diastereomer pairs is mainly explainable by
β-substituents in the studied ligand frameworks, the intriguing
similarity in molar mass of the polymers arises from the
rotating benzyl group. Quantum chemical calculations show
that the benzyl group in the diastereomeric pairs where both
propene and polymer chain are coordinated on the zirconium
has two preferable energy minima while it rotates around the
CH2-Cp axis. In this case the phenyl ring tends to point away
from the active center or to locate in the indenyl plane. The
former rotamer has no direct influence on the chain growth,
whereas in the latter rotamer the benzyl group acts as a
2-methyl substituent and therefore reduces the rate of chain
terminations. Due to this rotamer, the indenyl ligand has two
preferred energy minima and resembles occasionally a classical
fluorenyl ligand and, therefore, causes the similarity in poly-
propene molar mass regardless of the applied diastereomer.
Polymerization conditions have a strong influence on
catalytic activity, stereoselectivity, and polymer molar mass.
At low polymerization temperature the benzyl group has
limited mobility and it occupies the coordination place for an
incoming monomer. This is the reason for the low activity.
When occasionally replaced by propene, the benzyl group
Acknowledgment. We would like to thank the Academy
of Finland (Bio- and Nanopolymers Research Group: grant
no. 77317) for the funding. M. Linnolahti gratefully acknow-
ledges funding from the Academy of Finland (project
130815). We are grateful to Dr. Jorma Matikainen,
University of Helsinki, for the mass spectra measurements.
Supporting Information Available: Crystallographic data for
meso-3 are available as a CIF file. Detailed pentad analysis of
the PPs is provided. This material is available free of charge via
the Internet at http://pubs.acs.org.