Bimetallic effects in homopolymerization of styrene and copolymerization of ethylene and styrenic comonomers: Scope, kinetics, and mechanism

Article abstract of DOI:10.1021/ja076407m

This contribution describes the homopolymerization of styrene and the copolymerization of ethylene and styrenic comonomers mediated by the single-site bimetallic "constrained geometry catalysts" (CGCs), (μ-CH ₂CH₂-3,3′){(η⁵

Full text of DOI:10.1021/ja076407m

Published on Web 01/26/2008
Bimetallic Effects in Homopolymerization of Styrene and
Copolymerization of Ethylene and Styrenic Comonomers:
Scope, Kinetics, and Mechanism
Neng Guo, Charlotte L. Stern, and Tobin J. Marks*
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
Received August 25, 2007; E-mail: t-marks@northwestern.edu
Abstract: This contribution describes the homopolymerization of styrene and the copolymerization of
ethylene and styrenic comonomers mediated by the single-site bimetallic “constrained geometry catalysts”
(CGCs), (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2; Ti2], (µ-CH2CH2-3,3′){(η5-
indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2], (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2
[MBICGC(TiMe2)2; C1-Ti2], and (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [MBICGC(ZrMe2)2; C1-
Zr2], in combination with the borate activator/cocatalyst Ph3C+B(C6F5)4 (B1). Under identical styrene
-
homopolymerization conditions, C1-Ti2 + B1 and Ti2 + B1 exhibit ∼65 and ∼35 times greater polymerization
activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + B1. C1-Zr2 +
B1 and Zr2 + B1 exhibit ∼8 and ∼4 times greater polymerization activities, respectively, than does the
monometallic control [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) + B1. NMR analyses show that the bimetallic
catalysts suppress the regiochemical insertion selectivity exhibited by the monometallic analogues. In
ethylene copolymerization, Ti2 + B1 enchains 15.4% more styrene (B), 28.9% more 4-methylstyrene (C),
45.4% more 4-fluorostyrene (D), 41.2% more 4-chlorostyrene (E), and 31.0% more 4-bromostyrene (F)
than does Ti1 + B1. This observed bimetallic chemoselectivity effect follows the same general trend as the
π-electron density on the styrenic ipso carbon (D > E > F > C > B). Kinetic studies reveal that both Ti2
+ B1 and Ti1 + B1-mediated ethylene-styrene copolymerizations follow second-order Markovian statistics
and tend to be alternating. Moreover, calculated reactivity ratios indicate that Ti2 + B1 favors styrene insertion
more than does Ti1 + B1. All the organozirconium complexes (C1-Zr2, Zr2, and Zr1) are found to be
incompetent for ethylene-styrene copolymerization, yielding only mixtures of polyethylene and polystyrene.
Model compound (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)][Ti(CH2Ph)2]}2 {EBICGC[Ti(CH2Ph)2]2; Ti2-
(CH2Ph)4} was designed, synthesized, and structurally characterized. In situ activation studies with cocatalyst
B(C6F5)3 suggest an η1-coordination mode for the benzyl groups, thus supporting the proposed polymerization
mechanism. For ethylene-styrene copolymerization, polar solvents are found to increase copolymerization
activities and coproduce atactic polystyrene impurities in addition to ethylene-co-styrene, without diminishing
the comonomer incorporation selectivity. Both homopolymerization and copolymerization results argue that
substantial cooperative effects between catalytic sites are operative.
Introduction
tions and special, conformationally advantaged active site-
substrate interactions.2 For single-site olefin polymerization
Intensive recent research efforts have focused on discovering
unique or more efficient homogeneous catalytic processes which
benefit from cooperative effects between adjacent active centers
in multinuclear metal complexes.1 In ideal cases, these mimic
enzymatic capabilities in creating high local reagent concentra-
catalysts,3 we recently reported that -CH2CH2- (Ti2, Zr2) and
-CH2- (C1-Ti2, C1-Zr2) linked bimetallic “constrained ge-
ometry catalysts” (CGCs)4 exhibit remarkable nuclearity effects
in terms of chain branch formation, R-olefin comonomer
enchainment selectivity, and molecular weight enhancement
compared to their mononuclear counterparts (Ti1, Zr1) (Chart
1).5 For ethylene copolymerizations, we speculated that when
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10.1021/ja076407m CCC: $40.75 © 2008 American Chemical Society

Ethylene and Styrenic Comonomers
A R T I C L E S
Chart 1
Scheme 1. Proposed Mechanistic Scenario for Enhanced Comonomer Enchainment by Bimetallic Catalysts
the double bond of the alkene comonomer binds to the first
metal center, the second d0, highly electrophilic metal center
can engage in secondary, possibly agostic interactions6 with sp3
sites, leading to enhanced comonomer binding affinity and
activating capacity (Scheme 1). Density functional theory (DFT/
B3LYP) calculations reveal that this agostic interaction con-
tributes ∼2 kcal/mol stabilization to the coordinated bimetallic
R-olefin complex.7 The next intriguing question is whether these
binuclear cooperative enchainment effects are more likely to
mediate unusual polymerization patterns involving monomers,
such as styrenes and dienes, with more basic secondary
coordinating moieties.
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A R T I C L E S
Guo et al.
cessful, typically yielding homopolymer mixtures or copolymers
with styrene incorporation <1 mol %.9 The development of
homogeneous single-site polymerization catalysts has led to a
resurgence of interest in this field; however, challenges
remain.10-12 For Cp′TiXYZ-type catalysts10 (Cp′ ) substituted
or unsubstituted η5-cyclopentadienyl, indenyl, fluorenyl; X, Y,
Z ) halogen, alkyl, alkoxy, aryloxy, ketimide, etc. ligand),
substantial quantities of homopolymer contaminants are copro-
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to multiple active species, and in certain cases, the presence of
excess cocatalyst. CGCTi catalysts represent another major
advance in this field, producing ethylene-styrene copolymers
exclusively; however styrene incorporation is invariably <50
mol %, regardless of the styrene:ethylene feed ratio. The
copolymer obtained is described as “pseudo-random”, because
no head-to-tail styrene coupling is detected, even at relatively
high levels of styrene incorporation.12
As a common, indispensable commodity plastic, polystyrene
has also attracted extensive research efforts. Isotactic polystyrene
was first synthesized by heterogeneous Ziegler-Natta catalysis13
and was recently synthesized by homogeneous catalysis.14,15
Cp′TiXYZ-type metallocene catalysts16 and some other metallo-
cene11b,17 and nonmetallocene18 catalysts are known to afford
syndiotactic polystyrene; however, the nature of the catalytically
active species and the mechanism of stereocontrol have not been
unambiguously established. Mononuclear CGCTi catalysts
exhibit marginal activity in styrene homopolymerization,5e,12h
which is thought to be due to catalyst deactivation via arene
“back-coordination”19 in the 2,1-insertion product (A). It would
therefore be desirable to have a generalizable catalyst type,
which, by tuning the symmetry of the ancillary ligand structure,
could afford polystyrene products with efficient productivity
and predetermined stereochemistry.
In a preliminary investigation,5e we briefly communicated that
Ti2 not only exhibits far greater activity for styrene homopo-
lymerizations than does Ti1, and installs unusual 1,2-insertion
regiochemistry20 (up to ∼50%) in the initiation steps, but affords
broad-range controllable styrene incorporation in ethylene-
styrene copolymerizations, arguing that multinuclear cooperative
catalysis indeed mediates unusual styrene polymerization pat-
terns, although neither the scope, kinetics, nor mechanism were
defined. In the present contribution, we investigate ethylene and
styrene reactivity ratios for both Ti2- and Ti1-mediated copo-
lymerizations, and extend comparative copolymerization studies
to a selected variety of substituted styrenic comonomers (Chart
2) and to CGCZr catalysts (C1-Zr2, Zr2, and Zr1) to fully
characterize the scope and mechanism of this bimetallic effect.
We also investigate the influence of metal-metal distance on
these bimetallic cooperative effects in styrene homopolymeri-
zation and ethylene-styrene copolymerizations by comparing
the properties of the methylene-bridged bimetallic catalysts C1-
Ti2 and C1-Zr2 to the ethylene-bridged bimetallic variants Ti2
and Zr2. In addition, model compound (µ-CH2CH2-3,3′){(η5-
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2003, 125, 4964-4965.
9
2248 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
Chart 2
1
Unity- or Mercury-400 (FT, 400 MHz, H; 100 MHz, 13C), or Inova-
500 (FT, 500 MHz, 1H; 125 MHz, 13C) spectrometer. Chemical shifts
(δ) for 1H and 13C spectra were referenced using internal solvent
resonances and are reported relative to tetramethylsilane. NMR experi-
ments on air-sensitive samples were conducted in Teflon valve-sealed
sample tubes (J. Young). 13C NMR assays of polymer microstructure
were conducted in 1,1,2,2-tetrachloroethane-d2 containing 0.05 M Cr-
(acac)3 (as a relaxation reagent) at 130 °C. Resonances were assigned
according to the literature for polystyrene and ethylene-styrene
copolymers. Elemental analyses were performed by Midwest Microlabs,
LLC, Indianapolis, Indiana.
indenyl)[1-Me2Si(tBuN)][Ti(CH2Ph)2]}2 [Ti2(CH2Ph)4] was
designed, synthesized, characterized, and activated with the
cocatalyst/activator B(C6F5)3 to probe structural aspects of the
proposed mechanism for the observed bimetallic enchainment
effects.
Previously, it was reported that a polar solvent can depress
bimetallic effects in ethylene copolymerization by weakening/
supplanting mechanistically important agostic interactions.5a,b
In the present study, we carry out detailed ethylene-styrene
copolymerizations in the same polar solvent to determine
whether such medium effects can weaken/displace the metal-
arene interactions. It will be seen that, by manipulating the
achievable metal-metal distances, styrenic comonomer sub-
stituents (B-F), and polymerization medium, the observed
bimetallic effects can be varied dramatically.
Gel permeation chromatographic (GPC) analysis was carried out on
a Waters Alliance GPCV 2000 high-temperature instrument equipped
with three Polymer Laboratories 10 µm mixed B columns (three
columns: Waters Styragel HT 6E, HT 4, HT 2) operating at 150 °C
and a refractive index detector. A flow rate of 1.0 mL/min was used,
and HPLC grade 1,2,4-trichlorobenzene was employed as the eluent.
Typically, ca. 5 mg of the sample was dissolved in 7.0 mL of TCB.
The hot solutions were filtered using a 0.5 µm stainless steel filter. A
polystyrene relative calibration was carried out using narrow molecular
weight distribution polystyrene standards from Polymer Laboratories
with Ionol (4-(2,6,6-trimethyl-2-cyclohexen-1-yl)-3-buten-2-ol) added
as the flow marker.22 Alternatively, GPC measurements were performed
on a Polymer Laboratories PL-GPC 220 instrument using 1,2,4-
trichlorobenzene solvent (stabilized with 125 ppm BHT) at 150 °C. A
set of three PLgel 10 µm mixed columns was used. Samples were
prepared at 160 °C. Molecular weights were determined by GPC using
narrow polystyrene standards and are uncorrected.
Experimental Section
Materials and Methods. All manipulations of air-sensitive materials
were performed with rigorous exclusion of oxygen and moisture in
flamed Schlenk-type glassware on a dual-manifold Schlenk line or
interfaced to a high-vacuum line (10-5 Torr), or in a dinitrogen-filled
MBraun and Vacuum Atmospheres glove box with a high capacity
recirculator (<1 ppm O2). Argon (Matheson or Airgas, prepurified)
and ethylene (Matheson or Airgas, polymerization grade) were purified
by passage through a supported MnO oxygen-removal column and an
activated Davison 4A molecular sieve column. Hydrocarbon solvents
(toluene and pentane) were dried using an activated alumina column
and Q-5 columns according to the method described by Grubbs,21 and
were additionally vacuum transferred from Na/K alloy and stored in
Teflon-valve sealed bulbs for polymerization experiments. Ether
solvents (THF and Et2O) were distilled under nitrogen from sodium
benzophenone ketyl. The solvent 1,2-difluorobenzene was distilled from
CaH2 and stored over freshly activated Davison 4A molecular sieves.
Deuterated solvents were purchased from Cambridge Isotope Labora-
tories (all g99 atom % D). Methylene chloride-d2 was dried over CaH2
and vacuum-transferred into J. Young NMR tubes. The solvent for
polymer NMR characterization, 1,1,2,2-tetrachloroethane-d2, was used
as received. Other deuterated solvents were distilled from Na/K alloy
and stored in vacuum-tight storage tubes over freshly activated Davison
4A molecular sieves. Chlorobenzene, styrene, 4-methylstyrene, 4-fluo-
rostyrene, 4-chlorostyrene, and 4-bromostyrene (Aldrich) were dried
sequentially for a week over CaH2 and then triisobutylaluminum and
were freshly vacuum-transferred prior to polymerization experiments.
The reagent TMSCl was purchased from Aldrich and redistilled. The
reagent PhCH2MgCl‚0.66Et2O was prepared by removing all the
volatiles from PhCH2MgCl (1.0 M in Et2O) (Aldrich). The reagent (µ-
CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)][Ti(NMe2)2]}2 {EBICGC-
[Ti(NMe2)2]2; Ti2(NMe2)4}, and the catalysts Ti1, Ti2, C1-Ti2, Zr1, Zr2,
and C1-Zr2 were prepared and purified according to literature
procedures.5a,b
Polymer glass transition temperatures and melting temperatures were
measured on a TA Instruments 2920 Modulated Differential Scanning
Calorimeter. Typically, ca. 10 mg samples were examined, and a ramp
rate of 10 °C/min was used to measure the polymer glass transition
points and melting points. To erase thermal history effects, all samples
were run through at least two melt-freeze cycles. The data from the
second melt-freeze cycle are presented here.
Styrene Homopolymerization Experiments in Toluene. In the
glovebox, a 250 mL round-bottom three-neck Morton flask, which had
been dried at 160 °C overnight and equipped with a large magnetic
stirring bar and a thermocouple probe, was charged with 25 mL of dry
toluene and 5 mL of dry styrene. The flask was then attached to a
high-vacuum line (10-5 Torr) and equilibrated at the desired reaction
temperature using an external bath. The catalytically active species was
freshly generated in 1.5 mL of dry 1,2-difluorobezene in the nitrogen-
filled glovebox. Under 1.0 atm of rigorously purified argon (pressure
control using a mercury bubbler), the catalyst solution was quickly
injected into the rapidly stirred flask using a gastight syringe equipped
with a flattened spraying needle. After a measured time interval, the
polymerization was quenched by the addition of 5 mL of methanol,
and the reaction mixture was then poured into 800 mL of methanol.
The polymer was allowed to fully precipitate overnight and then
collected by filtration, washed with fresh methanol, and dried on a high-
vacuum line overnight at 80 °C to constant weight.
Physical and Analytical Measurements. NMR spectra were
Ethylene Copolymerization Experiments in Toluene. In the
glovebox, a 250 mL round-bottom three-neck Morton flask, which had
been dried at 160 °C overnight and equipped with a large magnetic
stirring bar and a thermocouple probe, was charged with 50 mL of dry
1
recorded on a Varian Innova 400 (FT 400 MHz, H; 100 MHz, 13C),
(21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518-1520.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2249

A R T I C L E S
Guo et al.
toluene and 10 mL of dry styrene. The flask was then attached to a
high-vacuum line (10-5 Torr), freeze-pump-thaw degassed, presatu-
rated with 1.0 atm (pressure control using a mercury bubbler) of
rigorously purified ethylene, and equilibrated at the desired reaction
temperature using an external bath. The catalytically active species was
freshly generated in 1.5 mL of dry 1,2-difluorobezene in the nitrogen-
filled glovebox. The catalyst solution was then quickly injected into
the rapidly stirred flask using a gastight syringe equipped with a
flattened spraying needle. After a measured time interval, the polym-
erization was quenched by the addition of 5 mL of methanol, and the
reaction mixture was then poured into 800 mL of methanol. The
polymer was allowed to fully precipitate overnight and then collected
by filtration, washed with fresh methanol, and dried on a high vacuum
line overnight at 80 °C to constant weight.
Ethylene Copolymerization Experiments in Chlorobenzene. In
the glovebox, a 250 mL round-bottom three-neck Morton flask, which
had been dried at 160 °C overnight and equipped with a large magnetic
stirring bar and a thermocouple probe, was charged with 10 mL of dry
styrene. The flask was then attached to a high-vacuum line (10-5 Torr),
freeze-pump-thaw degassed, and then 50 mL of chlorobenzene was
vacuum transferred in. The flask was presaturated with 1.0 atm (pressure
control using a mercury bubbler) of rigorously purified ethylene and
equilibrated at the desired reaction temperature using an external bath.
The catalytically active species was freshly generated in 1.5 mL of
dry 1,2-difluorobezene in the nitrogen-filled glovebox. The catalyst
solution was then quickly injected into the rapidly stirred flask using
a gastight syringe equipped with a flattened spraying needle. After a
measured time interval, the polymerization was quenched by the
addition of 5 mL of methanol, and the reaction mixture was then poured
into 800 mL of methanol. The polymer was allowed to fully precipitate
overnight and then collected by filtration, washed with fresh methanol,
and dried on a high-vacuum line overnight at 80 °C to constant weight.
Determination of Comonomer Content by 1H NMR. The solvent
1,1,2,2-tetrachloroethane-d2 (C2D2Cl4) was used as the deuterated
solvent for polymer NMR analysis because its NMR spectral features
do not overlap with any of the polymer resonances. Delay times of 20
s were used to ensure the accuracy of NMR peak integration. The
comonomer contents were calculated based on the integral of the
aromatic region (Aaromatic) and the aliphatic region (Aaliphatic) according
to the following equations:
were obtained with a fine focus, sealed tube Mo KR radiation source
(λ ) 0.71073 Å) and a graphite monochromator. Twenty frames (20 s
exposures, 0.3° slices) were collected in three areas of space to
determine the orientation matrix. The parameters for data collection
were determined by the peak intensities and widths from the 60 frames
used to determine the orientation matrix. The faces of the crystal were
then indexed and data collection was begun. After data collection, the
frames were integrated, the initial crystal structure was solved by direct
methods, the structure solution was refined through successive least-
squares cycles and subjected to a face-indexed absorption correction.
Crystal data, data collection, and refinement parameters are summarized
in Table 4 and in the Crystallographic Information File (CIF, see
Supporting Information).
Synthesis of Bimetallic Metallocene Complex Ti2Cl4. The reagent
Ti2(NMe2)4 (500 mg, 0.638 mmol) was partially dissolved in 75 mL
of dry toluene in a 100 mL Schlenk flask and the solution was cooled
to -78 °C. Next, Me3SiCl (3.0 mL, 23.64 mmol) was added dropwise
by syringe with stirring. The solution was then allowed to slowly warm
to room temperature and to stir for 48 h. Large quantities of wine-red
solid precipitated, which was separated by filtration, washed with fresh
pentane, and subsequently dried on the high-vacuum line. Yield: 370
mg (77%). This product was used for the next reaction without further
1
purification. H NMR (C6D6, 23 °C, 499.748 MHz): δ 7.58 (d, 2H,
3
3JH-H ) 8.5 Hz, Ind, C6H4), 7.36 (d, 2H, JH-H ) 8.5 Hz, Ind, C6H4),
3
3
7.04 (t, 2H, JH-H ) 7.5 Hz, Ind, C6H4), 6.94 (t, 2H, JH-H ) 8.0 Hz,
2
Ind, C6H4), 6.25 (s, 2H, Ind, C5H), 3.59 (dd, 2H, JH-H ) 14.5 Hz,
3JH-H ) 8.5 Hz, CH2CH2), 3.25 (dd, 2H, 2JH-H ) 14.0 Hz, 3JH-H ) 8.0
Hz, CH2CH2), 1.31 (s, 18H, NCMe3), 0.53 (s, 6H, SiMe2), 0.28 (s, 6H,
SiMe2). Anal. Calcd for C32H44N2Si2Cl4Ti2: C, 51.21; H, 5.92; N, 3.73.
Found: C, 51.14; H, 5.84; N, 3.98.
Synthesis of Bimetallic Metallocene Complex Ti2(CH2Ph)4. Ti2Cl4
(150 mg, 0.200 mmol) was suspended in 75 mL of dry toluene in a
100 mL Schlenk flask, and after the mixture was cooled to -78 °C,
PhCH2MgCl‚0.66Et2O (153 mg, 0.764 mmol) dissolved in 10 mL of
dry toluene was added dropwise by syringe with stirring. The reaction
mixture was then allowed to warm to room temperature and stirring
continued for 24 h. Next, the MgCl2 precipitate was filtered off, and
the red filtrate was condensed to saturation and slowly cooled to -40
°C to afford red crystals, which were isolated by filtration, washed
with cold pentane twice, and subsequently dried on the high-vacuum
line. Yield: 80 mg (41%). Spectropic and analytical data are: 1H NMR
4Aaromatic
3
(C6D6, 23 °C, 499.748 MHz): δ 7.66 (d, 2H, JH-H ) 8.5 Hz, Ind,
poly(ethylene-co-styrene) S% )
C6H4), 7.54 (d, 2H, 3JH-H ) 8.5 Hz, Ind, C6H4), 7.40-6.70 (m, Ind +
Aaromatic + 5Aaliphatic
2
TiCH2Ph), 5.75 (s, 2H, Ind, C5H), 3.49 (dd, 2H, JH-H ) 14.0 Hz,
4Aaromatic
3JH-H ) 6.2 Hz, CH2CH2), 3.34 (dd, 2H, 2JH-H ) 14.0 Hz, 3JH-H ) 6.2
poly(ethylene-co-4-halostyrene) S% )
2
Aaromatic + 4Aaliphatic
Hz, CH2CH2),, 3.18 (d, 2H, JH-H ) 10.0 Hz, TiCH2Ph), 2.05 (d, 2H,
2JH-H ) 10.0 Hz, TiCH2Ph), 1.83 (d, 2H, 2JH-H ) 10.5 Hz, TiCH2Ph),
1.49 (s, 18H, NCMe3), 0.61 (s, 6H, SiMe2), 0.27 (s, 6H, SiMe2), 0.20
(d, 2H, 2JH-H ) 10.0 Hz, TiCH2Ph). 1H NMR (CD2Cl2, 23 °C, 499.748
2Aaromatic
poly(ethylene-co-4-methylstyrene) S% )
2Aaliphatic - Aaromatic
3
3
MHz): δ 7.83 (d, 2H, JH-H ) 8.5 Hz, Ind, C6H4), 7.60 (d, 2H, JH-H
) 8.0 Hz, Ind, C6H4), 7.52 (t, 2H, 3JH-H ) 7.0 Hz, Ind, C6H4), 7.18 (t,
2H, 3JH-H ) 7.5 Hz, Ind, C6H4), 7.025 (t, 4H, 3JH-H ) 7.8 Hz, m-Ph),
Solvent Fractionation of Ethylene-co-styrene and Polystyrene
Mixtures. A known amount of polymer mixture was loaded into a
cellulose fiber thimble placed inside a Soxhlet extractor. Methyl ethyl
ketone (MEK) was used as the solvent to extract the polystyrene. After
24 h of refluxing, the remaining insoluble copolymer was carefully
collected and dried on a high vacuum line overnight at 80 °C to constant
weight.
X-Ray Crystal Structure Determination of Ti2(CH2Ph)4. Crystals
of the title complex suitable for X-ray diffraction were obtained by
slow diffusion of pentane into a saturated toluene solution at room
temperature. Inside the glovebox, the crystals were placed on a glass
slide and covered with dry Infineum V8512 oil. The crystals were then
removed from the box, and a suitable crystal was selected under a
microscope using plane-polarized light. The crystal was mounted on a
glass fiber and transferred to a Bruker SMART 1000 CCD area detector
diffractometer in a nitrogen cold stream at 153 (2) K. Diffraction data
3
3
7.022 (t, 4H, JH-H ) 7.5 Hz, m-Ph), 6.79 (t, 4H, JH-H ) 7.5 Hz,
3
3
p-Ph), 6.76 (t, 4H, JH-H ) 7.8 Hz, p-Ph), 6.71 (d, 2H, JH-H ) 7.0
Hz, o-Ph), 6.62 (d, 2H, 3JH-H ) 7.0 Hz, o-Ph), 5.50 (s, 2H, Ind, C5H),
2
3
3.33 (dd, 2H, JH-H ) 8.0 Hz, JH-H ) 3.5 Hz, CH2CH2), 3.26 (dd,
2H, JH-H ) 7.5 Hz, JH-H ) 4.0 Hz, CH2CH2), 2.80 (d, 2H, JH-H
10.5 Hz, TiCH2Ph), 1.75 (d, 2H, JH-H ) 10.0 Hz, TiCH2Ph), 1.64
(2H, TiCH2Ph, overlapping with NCMe3), 1.62 (s, 18H, NCMe3), 0.80
(s, 6H, SiMe2), 0.18 (s, 6H, SiMe2), -0.23 (d, 2H, JH-H ) 10.5 Hz,
2
3
2
)
2
2
TiCH2Ph). 13C{1H} NMR (CD2Cl2, 23 °C, 125.674 MHz): δ 150.09
(ipso-TiCH2Ph), 146.58 (ipso-TiCH2Ph), 134.56 (Ind), 133.25 (Ind),
131.27 (Ind), 129.51 (Ind), 129.02 (ortho-TiCH2Ph), 128.70 (ortho-
TiCH2Ph), 128.27 (meta-TiCH2Ph), 128.11 (Ind), 127.03 (meta-
TiCH2Ph), 126.72 (Ind), 125.90 (Ind), 123.99 (Ind), 122.76 (para-
TiCH2Ph), 122.06 (para-TiCH2Ph), 94.34 (Ind), 84.90 (TiCH2Ph), 80.23
9
2250 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
Scheme 2. Propagation Patterns in Ethylene-Styrene
Copolymerization for a First-Order Markovian Statistical Model
and Ti1-mediated ethylene-styrene copolymerization, we dis-
cuss the effects of styrene substituents on the comonomer
incorporation difference between Ti2 and Ti1. Next, styrene
homopolymerization will be addressed in terms of polymeri-
zation activity and insertion regiochemistry. Finally, the effect
of polar solvation on the bimetallic cooperative effects is
discussed.
Figure 1. Fineman-Ross plot for Ti2 + B1-mediated ethylene-styrene
copolymerization, F ) ethylene/styrene feed ratio, f ) ethylene content in
copolymer in mol %/styrene content in copolymer in mol %.
I. KineticAnalyses of Ethylene-Styrene Copolymerization
Mediated by Ti2 + B1 and Ti1 + B1. Previously, we reported
that under identical copolymerization conditions, the catalytic
system Ti2 + B1 incorporates significantly more styrene into
the polyethylene backbone than does Ti1 + B1. To understand
this bimetallic effect on the selectivity of monomer enchainment,
kinetic analyses were carried out to determine the reactivity
ratios for both monomers. A series of ethylene-styrene
copolymerizations were carried out with increasing styrene:
ethylene feed ratios for both Ti2 + B1 and Ti1 + B1-mediated
copolymerizations. All the copolymerization experiments were
terminated at low styrene conversions (<10%) to ensure a
constant feed ratio.
Figure 2. Fineman-Ross plot for Ti1 + B1-mediated ethylene-styrene
copolymerization, F ) ethylene/styrene feed ratio, f ) ethylene content in
copolymer in mol %/styrene content in copolymer in mol %.
First-Order Markovian Analysis. In a first-order Markovian
model for ethylene-styrene copolymerization statistics, which
takes into account only the influence of the last inserted
monomer during chain propagation,23a the reactivity ratios rE
and rS are defined as the ratios of homopropagation rate
constants to crosspropagation rate constants of ethylene and
styrene, respectively (rE ) kEE/kES, rS ) kSS/kSE, Scheme 2).
With styrene content in the monomer feed and in the copolymer
known, the reactivity ratios can be obtained from Fineman-Ross
plots (Figures 1 and 2).23a For Ti2 + B1-mediated ethylene-
styrene copolymerization, the slope and the intercept of the
Fineman-Ross plot yield the values rE ) 13.2 ( 0.8 and rS )
0.039 ( 0.003, respectively, whereas for Ti1 + B1, the Fineman-
Ross plot gives the values rE ) 14.5 ( 1.0 and rS ) 0.014 (
0.003. It can be seen that Ti2 + B1 possesses a significantly
larger rS and slightly smaller rE than Ti1 + B1, in agreement
with the NMR analytical observations that Ti2 always incor-
porates more styrene than Ti1 under identical reaction conditions.
Second-Order Markovian Analysis. The second-order
Markovian assumption takes into account the influence of the
second-to-the-last inserted monomer unit on incoming monomer
enchainment selectivity.23a As shown in Scheme 3, when the
last inserted monomer is ethylene, four different propagation
equations can be written (eqs 1-4). Dividing eq 1 by eq 2 and
eq 3 by eq 4 yields eqs 5 and 6, respectively, where XE is the
ethylene:styrene feed ratio. Two reactivity ratios are defined to
(TiCH2Ph), 61.28 (NCMe3), 34.46 (NCMe3), 30.12 (CH2CH2), 4.46
(SiMe2), 1.26 (SiMe2). Anal. Calcd for C60H72N2Si2Ti2: C, 74.04; H,
7.47; N, 2.88. Found: C, 74.80; H, 7.47; N, 2.90.
In Situ NMR Study of the Bimetallic Metallocene Ion Pair
[Ti2(CH2Ph)2]2+[PhCH2B(C6F5)3-]2. In the glove box, Ti2(CH2Ph)4
and B(C6F5)3 in a 1:2 molar ratio were loaded into a J. Young NMR
tube. The sealed tube was then removed from the glovebox, attached
to the vacuum line, and cooled to -78 °C, and CD2Cl2 was immediately
transferred in. The sample was shaken vigorously and transferred
directly to the NMR spectrometer. Upon activation, the solution color
changed from red to dark brown. 1H NMR (C6D6, 23 °C, 499.748
MHz): δ 7.60-6.20 (m, Ind + Ti+CH2Ph + B-CH2Ph), 5.68 (s, 2H,
2
Ind, C5H), 3.62 (br, 4H, B-CH2Ph), 2.91 (d, 2H, JH-H ) 8.5 Hz,
2
Ti+CH2Ph), 2.43 (d, 2H, JH-H ) 7.5 Hz, Ti+CH2Ph), 1.05 (s, 18H,
NCMe3), 0.26 (s, 6H, SiMe2), 0.10 (s, 6H, SiMe2).
Results
The goal of this study was to investigate the scope, kinetics,
and mechanism of bimetallic enchainment cooperative effects
in styrene homopolymerization and ethylene-styrene copoly-
merizations. Previously, we briefly communicated evidence for
such bimetallic effects in the case of Ti2, manifested by
significantly greater activity in styrene homopolymerization and
enhanced comonomer incorporation in ethylene-styrene copo-
lymerization. In this contribution, we extend the study to include
organozirconium catalysts and broaden the copolymerization
scope to include a variety of informative styrenic comonomers.
We also design, synthesize, and characterize a bimetallic model
compound to probe the coordination mode of inserted styrene
to the coordinatively open and highly electrophilic single-site
catalytic center. After a brief discussion of the kinetics of Ti2
(22) Protivova, J.; Pospisil, J.; Zikmund, L. J. Polym. Sci. Polym Symposia 1973,
40, 233-243.
(23) (a) Fink, G.; Richter, W. J. In Polymer Handbook, 4th ed.; Brandup, J.,
Immergut, E. H., Grulke, E. A., Eds.; John Wiley & Sons: New York,
1999; Vol. II, pp 329-337. (b) Oliva, L.; Longo, P.; Izzo, L.; Di Serio, M.
Macromolecules 1997, 30, 5616-5619.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2251

A R T I C L E S
Guo et al.
Scheme 3. Propagation Patterns in Ethylene-Styrene
Copolymerizations for a Second-Order Markovian Statistical Model
quantify the preference of ethylene over styrene to be inserted
into the Ti-polymeryl bond when the second-to-the-last inserted
monomer is ethylene (rE) or styrene (r′E).23b If rE is inequivalent
to r′E, then the penultimate unit specifically exerts an effect on
the incoming monomer enchainment selectivity.
d[EEE]
) kEEE[Ti-EE-P] [E]
) kEES[Ti-EE-P] [S]
) kSEE[Ti-ES-P] [E]
) kSES[Ti-ES-P] [S]
(1)
(2)
(3)
(4)
(5)
dt
Figure 3. 13C NMR spectrum (100 MHz, C2D2Cl4, 130 °C) of poly-
(ethylene-co-styrene) showing spectral assignments in the backbone region.
d[EES]
dt
d[SEE]
dt
d[SES]
dt
k
EEE [E]
[EEE]
)
) rE XE
) r′E XE
[EES] kEES [S]
k
SEE [E]
[SEE]
)
(6)
[SES] kSES [S]
Figure 3 shows a typical 13C NMR spectrum of an ethylene-
styrene copolymer and its assignments (see more below), from
which the triad distribution can be extracted (eqs 7-9). As
illustrated in Figure 4, for Ti2 + B1-mediated ethylene-styrene
copolymerization, plotting [EEE]/[EES] and [SEE]/[SES] vs XE
yields straight lines, the slopes of which afford the reactivity
ratios defined above (eqs 7 and 8). The fact that rE
[EEE] 0.5(Sγ+γ+ - 0.5Sâγ+
)
(7)
(8)
(9)
[SES] Sââ
[SEE] ) [EES] 0.5Sâγ+
Figure 4. Triad distribution analysis plots for Ti2 + B1-mediated ethylene-
styrene copolymerization.
is larger than r′E (rE ) 7.79 ( 1.02 > r′E ) 3.26 ( 0.11)
indicates that the aforementioned bimetallic catalyst Ti2-
mediated ethylene-styrene copolymerization follows second-
order Markovian statistics (penultimate model), and more
interestingly, when the second-to-the-last inserted monomer is
styrene and the last inserted one is ethylene, the incoming
styrene is preferred over ethylene for insertion into the Ti-
polymeryl bond, thereby generating SES triads. This alternating
copolymerization trend is also evidenced by the product of the
two reactivity ratios defined above by the first-order Markovian
statistics (rE × rS ) 0.51).
lows second-order Markovian statistics as well. In addition, the
values of both reactivity ratios for Ti2 are invariably smaller
than the corresponding ones for Ti1, demonstrating that Ti2
favors styrene insertion more than Ti1, or in other words, Ti1
favors ethylene insertion more than Ti2, which is in agreement
with the observed bimetallic enchainment selectivity effects that
under identical ethylene-styrene copolymerization conditions,
bimetallic catalyst Ti2 incorporates styrene more efficiently than
does monometallic catalyst Ti1.
II. Copolymerization of Ethylene and Styrenic Comono-
mers. Previously, we communicated that under identical co-
polymerization conditions, Ti2 incorporates significantly more
styrene than does Ti1 in ethylene-styrene copolymerizations.
To test the generality of this observed bimetallic effect, a variety
of substituted styrenic comonomers with either electron-donating
For Ti1 + B1 mediated ethylene-styrene copolymerization,
reactivity ratios can also be obtained by plotting [EEE]/[EES]
and [SEE]/[SES] vs XE (Figure 5). Again, rE )13.49 ( 0.78 is
larger than r′E ) 6.45 ( 0.14, suggesting that monometallic
catalyst Ti1-mediated ethylene-styrene copolymerization fol-
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2252 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
g 1) sequences, respectively.10g The signals observed at δ )
146.4 and 125.7 ppm in the aromatic region are assigned to the
ipso carbon and para carbon of the phenyl ring attached to the
copolymer backbone, respectively.
Regarding nuclearity effects, it is found that under strictly
identical copolymerization conditions the Ti2 + B1 combination
incorporates 15.4% more styrene than the mononuclear analogue
Ti1 + B1 (Table 1, entry 2 versus entry 3). To further explore
the correlation between catalyst structure and polymerization
behavior, methylene-bridged C1-Ti2 was also employed in
catalytic studies. It can be seen from entry 1 versus entry 2 in
Table 1 that C1-Ti2 + B1 incorporates 14.9% more styrene than
does Ti2 + B1, presumably due to the enhanced cooperative
effects arising from the diminished achievable Ti-Ti distance
(see more below).
All of the present ethylene-styrene copolymers are amor-
phous and exhibit a single glass transition temperature (Tg),
suggesting the resultant copolymers have approximately homo-
geneous styrene distributions. Moreover, it is found that Tg
increases from 5.8 to 16.6 °C and then to 22.3 °C as the styrene
incorporation level increases from 28.0 to 32.3 mol % and then
to 37.1 mol %, in agreement with reported Tg values for
copolymers with similar styrene contents.8c
Copolymerization of Ethylene and 4-Methylstyrene. The
13C NMR spectra (Figure 7) of the ethylene + 4-methylstyrene
copolymers share an almost identical pattern to the ethylene-
styrene copolymers in the aliphatic region except for the
additional resonance at δ ) 20.9 ppm, which can be assigned
to the phenyl ring methyl substituent. In the aromatic region,
the chemical shifts of the ipso carbon and para carbon are
displaced to δ ) 143.4 and 134.7 ppm, respectively. As for the
comonomer incorporation level, 1H NMR spectra show that Ti2
+ B1 enchains 29.9 mol % 4-methylstyrene, which is 28.9%
more than does Ti1 + B1 (23.2 mol %), and the copolymer Tg
also increases from 8.0 to 19.0 °C.
Copolymerization of Ethylene and 4-Fluorostyrene. Figure
8 shows the 13C NMR spectra of representative ethylene +
4-fluorostyrene copolymers. Compared to the ethylene-styrene
copolymers, the methylene and ethylene region exhibits an
almost identical pattern. In the aromatic region, the ispo carbon
shifts to δ ) 142.1 ppm. The phenyl ring para carbon appears
as a doublet centered at δ ) 161.4 ppm, due to the coupling to
the fluoro substituent (1JC-F ) 243.4 Hz). As for bimetallic
effects in terms of comonomer incorporation level, the 1H NMR
Figure 5. Triad distribution analysis plots for Ti1 + B1-mediated ethylene-
styrene copolymerization.
or electron-withdrawing substituents at the para positions were
examined in copolymerization experiments with ethylene and
the aforementioned organotitanium catalysts. For all of these
styrenic comonomers, it will be seen that the bimetallic catalysts
exhibit significantly enhanced comonomer enchainment selec-
tivity versus the monometallic analogue under identical reaction
conditions.
Copolymerization of Ethylene and Styrene. Figure 6 shows
13C NMR spectra of the poly(ethylene-co-styrene) samples of
Table 1, entries 1-3, and assignments made according to
literature.10 The resonances at δ ) 34.4 and 34.9 ppm are
attributed to SRâ, which represents either a tail-to-tail coupled
styrene dyad or an ethylene unit bridged head-to-head coupled
styrene dyad. Other polymer resonances centered at δ ) 25.5,
27.7, 29.8, 36.9, and 46.0 ppm in the aliphatic region can be
assigned to Sââ, Sâγ+, Sγ+γ+, SRγ+, and Tδ+δ+, respectively,
corresponding to SES, SEE, SEEnS, SES + SEE, and EnSEn (n
Figure 6. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of the poly(ethylene-co-styrene) samples from Table 1, entries 1-3 in which catalyst nuclearity
and connectivity is varied.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2253

A R T I C L E S
Guo et al.
a
Table 1. Ethylene-Styrene Copolymerization Results for Catalysts Ti2 and Ti1 with Cocatalyst B1
c
c
d
comonomer
activityb
Tg
C)
Tm
C)
Mw
105)
comonomer%e
entry
catalyst
comonomer
concentration (M)
(×
106)
(
°
(
°
(
×
Mw/Mn
(mol %)
d
1
2
3
4
5
6
7
8
9
C1-Ti2
Ti2
Ti1
Ti2
Ti1
Ti2
Ti1
Ti2
Ti1
B
B
B
C
C
D
D
E
E
F
1.45
1.45
1.45
1.26
1.26
1.40
1.40
1.39
1.39
1.27
1.27
3.7
5.8
2.2
28.8
24.7
5.9
13.1
3.1
3.1
22.3
21.7
5.8
19.0
8.0
47.2
44.8
52.7
43.1
48.4
36.0
n. o.f
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
10.34
3.67
0.76
1.23
2.14
2.26
3.21
8.55
0.77
6.65
1.50
2.2
2.1
1.4
3.3
1.2
1.7
2.1
1.7
1.9
1.7
4.0
37.1
32.3
28.0
29.9
23.2
29.8
20.5
20.9
14.8
16.9
12.9
10
11
Ti2
Ti1
5.2
4.4
F
a [Ti] ) 10 µmol + [B] ) 10 µmol at 20 °C, under 1.0 atm ethylene pressure. b Units: g polymer/(mol Ti‚atm ethylene‚h). c By DSC. d By GPC relative
1
to polystyrene standards. e Calculated from H NMR. f Not observed.
Figure 7. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of the poly(ethylene-co-4-methylstyrene) samples from Table 1, entries 4-5 in which catalyst
nuclearity is varied.
ethylene-styrene copolymers in the aliphatic region. In the
aromatic region, the chemical shifts of the ipso and para carbons
are displaced to δ ) 145.0 and 131.5 ppm, respectively.
Concerning the comonomer enchainment level, 1H NMR spectra
indicate that Ti2 + B1 enchains 20.9 mol % 4-chlorostyrene,
which is 41.2% greater than does Ti1 + B1 (14.8 mol %). As a
result, the Tg of the copolymers also increases from 43.1 to
52.7 °C.
Copolymerization of Ethylene and 4-Bromostyrene. The
13C NMR spectra (Figure 10) of the ethylene + 4-bromostyrene
copolymers share an almost identical pattern to the ethylene-
Figure 8. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of the poly-
(ethylene-co-4-fluorostyrene) samples from Table 1, entries 6-7, in which
the catalyst nuclearity is varied.
styrene copolymers in the aliphatic region. In the aromatic
region, the chemical shifts of the ipso and para carbons shift
to δ ) 145.3 and 119.7 ppm, respectively. As for the
1
comonomer incorporation level, H NMR spectra reveal that
Ti2 + B1 enchains 16.9 mol % 4-bromostyrene, which is 31.0%
more than does Ti1 + B1 (12.9 mol %). As a result, the Tg of
the copolymers increases from 36.0 to 48.4 °C.
III. Copolymerization of Ethylene and Styrene by Mono-
nuclear and Binuclear Organozirconium Catalysts. Copo-
lymerization of ethylene and styrene in the presence of
organozirconium catalysts was also investigated. Although
CGCZr catalysts (Zr1, Zr2, and C1-Zr2) are competent for both
ethylene5b,h and styrene homopolymerizations (see more below),
Figure 9. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of the poly-
attempts to effect ethylene-styrene copolymerization were
(ethylene-co-4-chlorostyrene) samples from Table 1, entries 8-9, in which
unsuccessful, yielding only heterogeneous polyethylene and
the catalyst nuclearity is varied.
polystyrene mixtures (Figure 11). In addition, with increasing
styrene:ethylene feed ratios, the percentage of polystyrene in
the obtained polymeric product increases accordingly. Copo-
data reveal that Ti2 + B1 enchains 45.4% more 4-fluorostyrene
than does Ti1 + B1 (29.8 vs 20.5 mol %). As a result, the
lymerization of ethylene and styrene at both elevated and
decreased polymerization temperatures, trying to depress the
homopropagation selectivity, also failed, again producing
mixtures of homopolymers. This is in agreement with previous
copolymer Tg also increases from 44.8 to 47.2 °C.
Copolymerization of Ethylene and 4-Chlorostyrene. The
13C NMR spectra (Figure 9) of the ethylene + 4-chlorostyrene
copolymers also share an almost identical pattern to the
9
2254 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
Figure 10. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of the poly(ethylene-co-4-bromostyrene) samples from Table 1, entries 10-11, in which the
catalyst nuclearity is varied.
Table 2. Styrene Homopolymerization Resultsa
c
c
e
e
entry
cat.
+
cocat.
time (h)
yield (g)
activityb
(×
105)
Tg
(°
C)
Tm
(
°C)
Mw
(×
104)
Mw/Mn
1
2
3
4
5
6
7
8
Ti1 + B1
3
3
2
3
3
2
2
1
0.08
3.13
3.93
0.06
3.36
1.09
3.91
4.07
0.03
1.04
1.96
0.02
1.12
0.54
1.96
4.07
104.6
96.4
94.6
100.5
89.2
101.2
94.2
n.o.d
n.o.d
n.o.d
n.o.d
n.o.d
n.o.d
n.o.d
n.o.d
1.96
1.04
0.64
1.21
0.80
1.30
1.02
0.70
1.9
1.6
2.6
1.7
1.5
1.6
1.6
2.7
Ti2 + B1
C1-Ti2 + B1
Ti1 + B2
Ti2 + B2
Zr1 + B1
Zr2 + B1
C1-Zr2 + B1
93.1
a
[M] ) 10 µmol + [B] ) 10 µmol, 5 mL styrene + 25 mL toluene at 20 °C. b Units: g polymer/(mol metal‚h). c DSC. d Not observed. e GPC relative
to polystyrene standards.
observations that CGCZr catalysts are not efficient for ethylene
+ methylenecycloalkane copolymerization.5b
Scheme 4. Proposed Mechanism for Bimetallic Styrene Insertion
Regiochemistry
IV. Homopolymerization of Styrene and End Group
Analysis. Previously, we communicated that under identical
styrene homopolymerization conditions, bimetallic catalyst Ti2
exhibits ∼50 times greater activity than the analogous mono-
metallic catalyst Ti1, and end group analysis suggests that
unusual 1,2-regiochemistry is installed in ∼50% of the initiation
steps. Here, we extended our studies to the methylene-bridged
bimetallic catalyst C1-Ti2 and the organozirconium analogues
C1-Zr2, Zr2, and Zr1 to study the effects of metal-metal
proximity on the cooperative bimetallic effect. As illustrated in
Table 2, under identical styrene polymerization conditions, C1-
Ti2 + B1 and Ti2 + B1 exhibit ∼65 and ∼50 times greater
homopolymerization activities than does monometallic Ti1 +
B1, respectively. Furthermore, C1-Zr2 + B1 and Zr2 + B1
exhibit ∼8 and ∼4 times greater activities, respectively, than
does monometallic Zr1 + B1. The monomodal GPC traces
together with polydispersities ∼2 and end group analyses (see
more below) suggest that all of the polystyrene homopolymers
are produced exclusively via a coordinative/insertive single-
site pathway. The marginal activity of mononuclear catalyst Ti1
is thought to arise from the “back-coordination” of the last
inserted styrene (A), which prevents incoming monomer
coordination and enchainment, while mononuclear Zr1 exhibits
respectable styrene polymerization activity. This disparity in
styrene homopolymerization activity argues that the more open
coordination sphere of Zr here is better able to overcome the
“back-coordination”. The aforementioned trends in styrene
homopolymerization activities for both organotitanium and
organozirconium catalysts (C1-M2 > M2 > M1) most likely
reflect enhanced intramolecular cooperative effects with in-
creased metal-metal proximity as the second metal center is
poised to disrupt styrene “back-coordination” to the first metal
center (Scheme 4). Interestingly, the molecular weights and glass
transition temperatures of the product polystyrenes exhibit the
opposite trend from activities: C1-M2 < M2 < M1, suggesting
functionally different propagation/termination kinetics (Table
2). Furthermore, organozirconium catalysts always exhibit
greater activities than the corresponding organotitanium catalysts
of the same nuclearity and afford comparable molecular weight
polystyrene, in sharp contrast to ethylene homopolymerizations,
where organotitanium catalysts exhibit far greater activities and
afford much higher molecular weight polyethylenes than the
corresponding organozirconium catalysts.
Regarding styrene insertion regiochemistry,20 Scheme 5
depicts all possible end groups produced during the initiation
steps in styrene homopolymerization. As can be seen from
Figure 12,26 C1-Ti2 and Ti2 share very similar end group
(24) For theoretical studies on styrene homopolymerization, see: (a) Yang, S.
H.; Huh, J.; Jo, W. H. Organometallics 2006, 25, 1144-1150. (b) Yang,
S. H.; Huh, J.; Yang, J. S.; Jo, W. H. Macromolecules 2004, 37, 5741-
5751. (c) Nifant’ev, I. E.; Ustynyuk, L. Y.; Besedin, D. V. Organometallics
2003, 22, 2619-2629. (d) Minieri, G.; Corradini, P.; Guerra, G.; Zambelli,
A.; Cavallo, L. Macromolecules 2001, 34, 5379-5385. (e) Minieri, G.;
Corradini, P.; Zambelli, A.; Guerra, G.; Cavallo, L. Macromolecules 2001,
34, 2459-2468.
(25) For studies of styrene coordination with cationic metal center, see: (a)
Kaminsky, W.; Lenk, S.; Scholtz, V.; Roesky, H. W.; Herzog, A.
Macromolecules 1997, 30, 7647-7650. (b) Flores, J. C.; Wood, J. S.; Chien,
J. C. W.; Rausch, M. D. Organometallics 1996, 15, 4944-4950. (c) Grassi,
A.; Zambelli, A.; Laschi, F. Organometallics 1996, 15, 480-482.
(26) For polystyrene end group analysis, see: (a) Caporaso, L.; Izzo, L.; Sisti,
I.; Oliva, L. Macromolecules 2002, 35, 4866-4870. (b) Zambelli, A.;
Longo, P.; Pellecchia, C.; Grassi, A. Macromolecules, 1987, 20, 2035-
2037. (c) Sato, H.; Tanaka, Y. Macromolecules 1984, 17, 1964-1966.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2255

A R T I C L E S
Guo et al.
Figure 11. Representative 13C NMR spectrum (100 MHz, C2D2Cl4, 130 °C) of the polymeric product in ethylene-styrene copolymerization mediated by
CGCZr catalysts.
Scheme 5. Possible Styrene Insertion Pathways during Chain Initiation
distributions, installing 1,2-insertion regiochemistry in ∼50%
of all initiations. Interestingly, when the CGCZr-based catalysts
are changed from Zr1 to Zr2 and C1-Zr2, an increasing
percentage of 2,1-insertion regiochemistry is installed (Figure
13). This is in agreement with the proposed mechanism, that
is, although mononuclear catalysts have strongly preferred
styrene insertion regiochemistries (2,1-insertion for organo-
titanium,20a,d,e 1,2-insertion for organozirconium26a), the bimetal-
lic catalytic environments tend to moderate the opposite Ti vs
Zr selectivities in insertion regiochemistry.
proposed agostic interactions can be suppressed by polar C6H5-
Cl (ꢀr ) 5.68 vs ꢀr ) 2.38 for toluene),29 as evidenced by the
diminished comonomer enchainment efficiency of Ti2. However,
such polar solvent effects might not be suppressed in the present
styrene polymerizations since the metal-arene interactions in
the proposed mechanistic scenario may be stronger than the
C-H agostic interactions in R-olefin polymerizations. As shown
in Table 3, the data on ethylene-styrene copolymerizations
mediated by both Ti2 and Ti1 in C6H5Cl reveal that substantial
amounts of atactic polystyrene are coproduced in addition to
V. Polar Solvent Effects. Previously reported Ti1- and Ti2-
mediated ethylene + R-olefin copolymerization results5a,b in a
more polar, ion pair weakening medium suggested that the
Figure 12. 13C NMR end group analysis (100 MHz, C2D2Cl4, 130 °C) of
the polystyrenes from Table 2, entries 2-3.
Figure 13. 13C NMR end group analysis (100 MHz, C2D2Cl4, 130 °C) of
the polystyrenes from Table 2, entries 6-8.
9
2256 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
a
Table 3. Copolymerization of Ethylene and Styrene in C6H5Cl with Cocatalyst B1
d
d
f
f
entry
cat.
time (min)
yield (g)
PS wt %b
activityc
(
×
106)
Tg
(°
C)
Tm
(°
C)
Mw
(
×
105)
Mw/Mn
styrene%g (mol %)
1
2
Ti2
Ti1
40
18
4.85
3.06
79.7
55.4
1.4
2.0
9.2
-3.9
n.o.e
n.o.
3.49
4.62
2.4
1.8
28.5
21.7
a [Ti] ) 5 µmol + [B] ) 5 µmol, 10 mL styrene + 50 mL chlorobenzene at 20 °C. b Determined from solvent fractionation. c Units: g polymer/(mol
1
Ti‚atm ethylene‚h). d By DSC. e Not observed. f By GPC relative to polystyrene standards. g Calculated from H NMR.
Figure 14. 13C NMR spectra (100 MHz, C2D2Cl4, 130 °C) of two poly(ethylene-co-styrene) samples from Table 4.
poly(ethylene-co-styrene), in sharp contrast to copolymerization
results carried out in toluene where only copolymers are
obtained. After removing the atactic polystyrene via solvent
fractionation with methyl ethyl ketone (MEK), 13C NMR data
(Figure 14) reveal that the styrene incorporation levels for both
Ti2 and Ti1 are significantly depressed in comparison to the
copolymerization results in toluene, largely due to the decreased
styrene:ethylene feed ratio arising from the coproduction of
atactic polystyrene (depletion of styrene). Note that there are
negligible solubility differences for ethylene in toluene and
chlorobenzene under the present polymerization conditions.30
Interestingly, although the effective styrene:ethylene feed ratio
for Ti2 is lower than that for Ti1, because the former produces
more atactic polystyrene, Ti2 still incorporates 31.3% more
styrene than Ti1, while in toluene Ti2 only enchains 15.4% more
styrene than Ti1. It can therefore be seen that this bimetallic
selectivity effect is actually enhanced in the polar solvent,
arguing that any C6H5Cl coordination cannot compete with the
metal-arene interaction.
VI. Synthesis, Characterization, and Activation Studies
of the Model Compound Ti2(CH2Ph)4. To further probe the
proposed enchainment mechanism in C1-Ti2 and Ti2-mediated
styrene homopolymerization involving one cationic metal center
interacting with the phenyl ring of the last inserted styrene on
the other cationic metal center, thus preventing “back-coordina-
tion” and deactivation of the electrophilic metal center, model
compound Ti2(CH2Ph)4 was designed to simulate bimetallic
Ti-polymeryl species after cocatalyst alkyl abstraction. Initial
attempts to synthesize the title complex via direct protonolytic
alkane elimination31 were unsuccessful, presumably due to the
sterically demanding environment of the bimetallic CGC ligand
structure (Scheme 6). Thus, more conventional methodology
was employed.32,33 As illustrated in Scheme 6, reaction of known
Ti2(NMe2)45a with excess Me3SiCl at room temperature cleanly
affords the tetrachloro complex Ti2Cl4. Subsequent reaction with
PhCH2MgCl affords the tetrabenzyl complex Ti2(CH2Ph)4,
which was characterized by standard spectroscopic and analyti-
cal techniques, as well as by X-ray diffraction (vide infra).
1
In the H NMR spectrum of Ti2(CH2Ph)4, the methylene
(27) For theoretical studies on monomer insertion in ethylene-styrene copo-
lymerizations, see: (a) Ramos, J.; Mun˜oz-Escalona, A.; Mart´ınez, S.;
Mart´ınez-Salazar, J.; Cruz, V. J. Chem. Phys. 2005, 122, 074901/1-074901/
4. (b) Yokota, K.; Kohsaka, T.; Ito, K.; Ishihara, N. J. Polym. Sci., Part A:
Polym. Chem. 2005, 43, 5041-5048. (c) Mart´ınez, S.; Exposito, M. T.;
Ramos, J.; Cruz, V.; Mart´ınez, M. C.; Lo´pez, M.; Mun˜oz-Escalona, A.;
Mart´ınez-Salazar, J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 711-
725. (d) Mart´ınez, S.; Cruz, V.; Mun˜oz-Escalona, A.; Mart´ınez-Salazar, J.
Polymer 2003, 44, 295-306. (e) Yang, S. H.; Jo, W. H.; Noh, S. K. J.
Chem. Phys. 2003, 119, 1824-1837. (f) Mun˜oz-Escalona, A.; Cruz, V.;
Mena, N.; Mart´ınez, S.; Mart´ınez-Salazar, J. Polymer 2002, 43, 7017-
7026. (g) Oliva, L.; Caporaso, L.; Pellecchia, C.; Zambelli, A. Macromol-
ecules 1995, 28, 4665-4667.
protons of the two magnetically nonequivalent diastereotopic
benzyl proton pairs appear as two AB spin patterns with 2JH-H
coupling constants ∼10.5 Hz. The ortho protons of the benzyl
groups exhibit normal resonances at δ ) 6.71 and 6.62 ppm,
respectively.19d,33a,c The 13C{1H} NMR spectrum reveals two
distinct signals at δ ) 84.90 and 80.23 ppm, respectively,
corresponding to the two nonequivalent methylene carbons of
the magnetically nonequivalent benzyl groups. Furthermore, the
ipso carbons of the benzyl groups are found to exhibit normal
chemical shifts at δ ) 150.09 and 146.58 ppm, respectively.
Unlike those reported for some neutral multihapto metal-benzyl
complexes,33 the aforementioned NMR spectroscopic evidence
as well as the solid-state structural data (see more below) suggest
(28) (a) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673-686. (b) Gagne,
M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275-294.
(29) (a) Bouwkamp, M. W.; de Wolf, J.; del Hierro Morales, I.; Gercama, J.;
Meetsma, A.; Troyanov, S. I.; Hessen, B.; Teuben, J. H. J. Am. Chem.
Soc. 2002, 124, 12956-12957. (b) Kawabe, M.; Murata, M. Macromol.
Chem. Phys. 2001, 202, 2440-2446. (c) Rybtchinski, B.; Konstantinovsky,
L.; Shimon, L. J. W.; Vigalok, A.; Milstein, D. Chem.-Eur. J. 2000, 6,
3287-3292. (d) Carr, N.; Mole, L.; Orpena, A. G.; Spencer, G. L. J. Chem.
Soc., Dalton Trans. 1992, 18, 2653-2662. (e) Peng, T. S.; Gladysz, J. A.
J. Am. Chem. Soc. 1992, 114, 4174-4181. (f) Agbossou, S. K.; Bodner,
G. S.; Patton, A. T.; Gladysz, J. A. Organometallics 1990, 9, 1184-1191.
(g) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443-
1444.
(31) Chen, Y. X.; Marks, T. J. Organometallics 1997, 16, 3649-3657.
(32) (a) Amor, F.; Butt, A.; du Plooy, K. E.; Spaniol, T. P.; Okuda, J.
Organometallics 1998, 17, 5836-5849. (b) Amor, F.; Okuda, J. J.
Organomet. Chem. 1996, 520, 245-248.
(33) (a) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.;
Huffman, J. C. Organometallics 1985, 4, 902-908. (b) Lubben, T. V.;
Wolczanski, P. T.; Van Duyne, G. D. Oganometallics 1984, 3, 977-983.
(c) Mintz, E. A.; Moloy, K. G.; Marks, T. J.; Day, V. W. J. Am. Chem.
Soc. 1982, 104, 4692-4695. (d) Wolczanski, P. T.; Bercaw, J. E.
Organometallics 1982, 1, 793-799.
(30) The solubility of ethylene is reported to be 0.117 mol/L in toluene30a and
0.118 mol/L in chlorobenzene30b under the present polymerization conditions
(25 °C, 1.0 atm): (a) Atiqullah, M.; Hammawa, H.; Hamid, H. Eur. Polym.
J. 1998, 34, 1511-1520. (b) Sahgal, A.; La, H. M.; Hayduk, W. Can. J.
Chem. Eng. 1978, 56, 354-357.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2257

A R T I C L E S
Guo et al.
Scheme 6. Synthetic Route to Model Compound Ti2(CH2Ph)4
that for Ti2(CH2Ph)4, all of the benzyl groups engage in an
η1-coordination mode both in solution and in the solid state.
Although mixing Ti2(CH2Ph)4 and 2 equiv of B(C6F5)3 in
benzene-d6 immediately results in the formation of dark-red
solids, and 1H NMR spectroscopy indicates clean double-benzyl
abstraction by the cocatalyst/activator, attempts to isolate the
pure crystalline bimetallic ion pair complex were unsuccessful,
most likely due to rapid thermal decomposition within hours,
C-H bond, which is thought to stabilize cationic metal centers
during olefin polymerization.6c,34 More importantly, this 2JH-H
reduction as well as normal Ti+CH2Ph ortho proton chemical
shifts19d,33a,c also argue against the possibility of dominant ηn-
PhCH2Ti+ coordination, which is expected to increase the sp2
character of the benzylic carbon and thus increase 2JH-H
.
33 The
1
fact that only one broad H resonance at δ ) 3.62 ppm is
observed for the BCH2Ph protons and that no signal in the range
of δ ) 5-6 ppm is observed for the BCH2Ph ortho protons
1
as monitored by H NMR spectroscopy. Interestingly, among
-
the three possible diastereomeric double benzyl abstraction
products, only one of the two Ci-symmetric ion pair complexes
could be identified, because only a single set of upfield-shifted
ligand resonances was observed. This preferential reactivity of
B(C6F5)3 with one of the two inequivalent benzyl groups is in
sharp contrast to parallel activation studies of Ti1, where both
possible ion pair complex isomers are observed in solution.5a
The diastereotopic benzylic proton pairs of the two isomeric
cationic Ti centers still exhibit AB spin patterns, evidenced by
two sets of doublets centered at δ ) 2.91 and 2.43 ppm with
an average 2JH-H ≈ 8.0 Hz. This decrease in the chemical shift
difference between the two benzylic hydrogens (∆δ ) 0.48 after
activation vs ∆δ ) 1.63 before activation) provides evidence
for an η1-PhCH2Ti+ coordination mode, because ηn-PhCH2Ti+
bonding should plausibly make the two benzylic protons more
magnetically inequivalent and therefore increase the chemical
shift difference.19d The decrease in coupling constant (cf., Ti2-
argues that η1-PhCH2B(C6F5)3 bonding also predominates.31
VII. Molecular Structure of Model Compound Ti2-
(CH2Ph)4. A summary of crystal structure data for the complex
Ti2(CH2Ph)4 is presented in Table 4, and selected bond
distances and angles for Ti2(CH2Ph)4 are summarized in Table
5. Similar to the previously reported molecular structure of Ti2-
(NMe2)4, the crystal structure of Ti2(CH2Ph)4 (Figure 15)
reveals an inversion center with a CGCTi moiety located on
either side of the ethylenebis(indenyl) fragment and with the
two π-coordinated indenyl rings in a diastereomeric relationship.
As can be seen from Figure 15, the crystal consists of a single
diastereomer (SR, RS). The bond angles C(15)-C(14)-Ti(1)
) 126.12(18)° and C(8)-C(7)-Ti(1) ) 117.79(18)° suggest
that all of the benzyl groups exhibit an η1-coordination mode,
since ηn-PhCH2Ti coordination would bend the phenyl moiety
close to the metal center and consequently afford substantially
(34) (a) Mashima, K.; Nakamura, A. J. Organomet. Chem. 1992, 428, 49-58.
2
(CH2Ph)4: JH-H ) 10.5 Hz) most likely reflects an R-agostic
(b) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395-
interaction between the electrophilic Ti center and a benzylic
408.
9
2258 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
a
symmetrical Cp ligand in [(η5-C5Me4)SiMe2(tBuN)]TiCl2, which
is 2.436 - 2.305 ) 0.131 Å,35 indicating a substantially more
“slipped” coordination of the Cp ligand in Ti2(CH2Ph)4.
Table 4. Summary of Crystal Structure Data for Ti2(CH2Ph)4
empirical formula
formula weight
crystal color, habit
crystal dimensions (mm)
crystal system
space group
a, Å
C62.50H76N2Si2Ti2
1007.23
red, block
0.520 × 0.424 × 0.374
triclinic
P1h
Discussion
I. Bimetallic Proximity Effects in Polymerization. From
the polymerization results outlined above, the enhanced styrene
incorporation in C1-Ti2- vs Ti2-mediated ethylene-styrene
copolymerizations, and the significantly enhanced activities in
C1-M2- vs M2-mediated styrene homopolymerizations (M )
Ti and Zr), suggest that C1-M2 structures exhibit enhanced
metal-metal cooperativity effects compared to M2. It is known
that in the single-crystal structure of C1-symmetric C1-Zr2,5b
the two indenyl rings are locked (estimated rotation barrier >63
kcal/mol, Spartan 2002, MP3 level) into a twisted conformation
by the methylene bridge, constraining the two metal centers to
the same side of the molecule,5b whereas in the solid-state
structure of Ci-symmetric Zr2, the two Zr atoms reside on
opposite sides of the molecule but with minimal estimated
barriers to accessing other conformations. As a result, the
minimum accessible Zr‚‚‚Zr distance in C1-Zr2 (7.392 Å) is
∼1.28 Å shorter than that in Zr2 (8.671 Å).5b Therefore, the
locked and shorter accessible metal-metal distance in the case
of C1-M2 would enable more efficient binuclear metal-styrene
binding (Scheme 4), hence affording more efficient comonomer
enchainment and greater styrene homopolymerization activity
than M2. A similar trend has been reported for ethylene +
1-hexene copolymerizations, where C1-Zr2 incorporates more
1-hexene than does Zr2 under identical reaction conditions.5b
12.299(2)
16.117(3)
16.873(3)
97.363(3)
101.480(3)
112.119(2)
2959.5(8)
2
1.130
0.348
0.8481-0.8912
15176
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
d (calcd), g/cm3
µ, mm-1
Tmin - Tmax
measured reflections
independent reflections
reflections > 2σ (I)
Rint
15176
12744
0.0000
0.0655
R[F2 > 2σ (F2)]
wR (F2)
0.1894
S
1.038
no. of parameters
653
a Conditions: CCD area detector diffractometer; ψ and ω scans;
temperature for data collection 153(2) K; Mo KR radiation; λ ) 0.71073
Å.
Table 5. Selected Bond Distances (Å) and Bond Angles (deg) for
Ti2(CH2Ph)4 Bond Distances
Bond Distances
Ti(1)-C(21)
Ti(1)-C(22)
Ti(1)-C(23)
Ti(1)-C(24)
Ti(1)-C(29)
Ti(1)-C(7)
Ti(1)-C(14)
C(7)-C(8)
2.498(3) C(14)-C(15)
2.341(2) Ti(1)-N(1)
2.271(2) Si(1)-N(1)
2.413(2) Si(1)-C(1)
2.564(3) Si(1)-C(2)
2.154(3) Si(1)-C(23)
2.132(3) N(1)-C(3)
1.480(4)
1.493(4)
1.931(2)
1.753(2)
1.870(3)
1.853(3)
1.854(3)
1.486(3)
II. Comparison of CGCTi and CGCZr Catalytic Proper-
ties. In the aforementioned styrene homopolymerization studies,
CGCZr-based catalysts exhibit far greater activities than do
CGCTi-based catalysts having the same nuclearity and afford
polymeric products with comparable molecular weights. In
contrast, for polymerizations involving ethylene, organotitanium
catalysts generally exhibit far greater activities and afford much
higher molecular weight polyethylenes than do analogous
organozirconium catalysts. It is known that for ethylene
polymerizations, coordination of ethylene to the cationic metal
center in the presence of the counteranion is usually the rate-
determining step for each ethylene insertion,36 while for styrene
polymerizations, insertion of the coordinated styrene into the
metal-polymeryl bond is thought to be the rate-determining
step.24b Theoretical studies regarding ethylene-styrene
copolymerizations27a,c,e as well as experimental results23b also
reveal that ethylene has a lower complexation energy than does
styrene while the latter has a significantly higher insertion barrier
than the former. DFT calculations also suggest that solvent
molecules are much more likely to compete with ethylene for
coordination to CGCZr than to CGCTi,37 and therefore, CGCTi
is expected to have more efficient ethylene coordination and
subsequent insertion. Moreover, tighter ion pairing in CGCZr
versus CGCTi38 also makes ethylene coordination to CGCZr,
Bond Angles
N(1)-Ti(1)-C(14)
N(1)-Ti(1)-C(7)
C(14)-Ti(1)-C(7)
106.42(10) N(1)-Si(1)-C(23)
117.39(10) C(2)-Si(1)-C(1)
99.55(11) C(1)-Si(1)-C(23)
93.54(10)
108.26(15)
112.45(13)
110.22(13)
124.59(17)
133.74(17)
101.58(10)
117.79(18)
C(15)-C(14)-Ti(1) 126.12(18) C(2)-Si(1)-C(23)
C(8)-C(7)-Ti(1)
N(1)-Si(1)-C(1)
N(1)-Si(1)-C(2)
117.79(18) C(3)-N(1)-Si(1)
116.89(13) C(3)-N(1)-Ti(1)
114.77(12) Si(1)-N(1)-Ti(1)
C(15)-C(14)-Ti(1) 126.12(18) C(8)-C(7)-Ti(1)
smaller bond angles (<90°).19a,b,c,f,i,j,k The sum of the bond
angles around nitrogen atom N(1) is 359.91°, indicating that
the formal hybridization of nitrogen atom N(1) is sp2. The -
t
BuN-Ti bond distance (Ti(1)-N(1)) is 1.931(2) Å, substantially
shorter than the one reported for Ti2(NMe2)4 (1.994(4) Å),
largely due to increased π donation from the N formal lone
pair electrons to the empty Ti4+ d orbitals since no additional
nitrogen atoms are engaged in π donation. The sum of bond
angles around indenyl ring carbon atom C(23) is 351.2°,
indicating that the C(23)-Si(1) bond deviates appreciably from
the indenyl ring plane because of the constrained geometry. The
carbon atoms of the Cp ring do not exhibit equal bonding
distances to the Ti center. The average bond lengths of Ti(1)-
C(22)/Ti(1)-C(23) and Ti(1)-C(21)/Ti(1)-C(24)/Ti(1)-C(29)
are 2.306(2) and 2.492(3) Å, respectively. The difference is
0.186(5) Å, 0.018(14) Å greater than the value reported for the
Cp ligand in Ti2(NMe2)4 [2.542(5) - 2.374(4) ) 0.168(9) Å],5a
and 0.055 Å greater than that value found for the more
(35) Carpenetti, D. W.; Kloppenburg, L.; Kupec, J. T.; Petersen, J. L.
Organometallics 1996, 15, 1572-1581.
(36) (a) Motta, A.; Fragala`, I. L.; Marks, T. J. J. Am. Chem. Soc. 2007, 129,
7327-7338. (b) Xu, Z.; Vanka, K.; Ziegler, T. Organometallics 2004, 23,
104-116. (c) Nifant’ev, I. E.; Ustynyuk, L. Y.; laikov, D. N. Organome-
tallics 2001, 20, 5375-5393.
(37) Chan, M. S. W.; Vanka, K.; Pye, C. C.; Ziegler, T. Organometallics 1999,
18, 4624-4636.
(38) Luo, L.; Marks, T. J. Top. Catal. 1999, 7, 97-106.
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2259

A R T I C L E S
Guo et al.
Figure 15. The molecular structure and atom numbering scheme for the model compound Ti2(CH2Ph)4. Thermal ellipsoids are drawn at the 30% probability
level.
Scheme 7. Pathways for Styrenic Comonomer Enchainment in Bimetallic Catalyst-Mediated Ethylene Copolymerization
by displacing the counteranion, more energetically demanding.
On the other hand, insertion of styrene into the metal-polymeryl
bond is a sterically more sensitive process, and thus CGCZr
with a larger ionic radius should promote more rapid enchain-
ment, all other factors being equal. This trend is similar to that
observed for organolanthanide-catalyzed intramolecular ami-
noalkene hydroamination/cyclization, where the cyclization rate
increases with increasing lanthanide radius since olefin insertion
is turnover-limiting.28
Unlike their CGCTi counterparts which are competent for
efficient ethylene-styrene copolymerization, under the reaction
conditions investigated, all of the three CGCZr catalysts (C1-
Zr2, C2-Zr2, and Zr1) fail to produce ethylene-styrene copoly-
mers although both monomers undergo homopolymerization at
these Zr centers. As a matter of fact, very few Zr-based
catalysts9j,21b are reported in the literature to efficiently copo-
lymerize ethylene and styrene. This difference in the catalytic
comonomer incorporation selectivity was previously observed
in our work on isobutene, methylenecyclopentane, and
methylenecyclohexane copolymerizations with ethylene: CGC-
Ti catalysts incorporate significant quantities of sterically
encumbered comonomers into polyethylene backbones, whereas
CGCZr catalysts do not. One plausible explanation is that tighter
ion pairing in CGCZr versus CGCTi structures leads to lower
reactivity in terms of comonomer enchainment.38
analogue, demonstrating the generality of the bimetallic effect
previously observed only with styrene.5e It is proposed that
coordination of the styrenic comonomer to a cationic metal
center is stabilized by interactions between the π-system of the
substituted/unsubstituted phenyl ring with the proximate cationic
metal center, which may facilitate/stabilize comonomer capture/
binding and enhance the subsequent enchainment probability
(Schemes 4 and 7). The bimetallic effect, which is defined here
as the relative comonomer enchainment selectivity difference
between Ti2 and Ti1 (eq 10), is found to depend strongly on
the arene substituent and, all other factors being equal
% Styrenic(Ti2) - %Styrenic(Ti1)
Bimetallic Effect )
× 100%
%Styrenic(Ti1)
(10)
in the proposed model, would be expected to increase as the
interaction between the arene π-system and the electrophilic
metal center increased. Because all of the styrenic comonomers
possess almost identical steric characteristics, it is reasonable
to assume that the π-electron density on the phenyl ring would
dominate the metal-arene interaction.
It is known that such metal-benzyl arene interactions
primarily involve the benzylic ipso carbon,19,33 and therefore,
the bimetallic effect should track the ipso carbon π-electron
density of the styrenic comonomer. As shown in Table 6, the
observed bimetallic effect parallels the same trend as the electron
density on the styrenic comonomer ipso carbon atom as
qualitatively assayed by the 13C NMR chemical shifts.39
Although reactivity ratio data are not available for direct
III. Bimetallic Effects on Comonomer Enchainment. For
all five styrenic comonomers investigated having various para
substituents, under identical copolymerization conditions, bi-
nuclear catalyst Ti2 + B1 invariably incorporates far greater
levels of comonomer than does the mononuclear Ti1 + B1
9
2260 J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008

Ethylene and Styrenic Comonomers
A R T I C L E S
Table 6. Correlation between the Bimetallic Effect (Ti2 vs Ti1
Comonomer Enchainment Selectivity) and ipso Carbon Chemical
Shift of the Styrenic Comonomers
coordination” of the last inserted styrene to the parent cationic
metal center occurs.
substituent
F
Cl
Br
Me
H
Conclusions
bimetallic effect
ipso (ppm)
45.4%
136.34
41.2%
136.62
31.0%
136.99
28.9%
137.93
15.4%
138.29
δ
The results of the present study significantly expand the scope
of applicable comonomers in binuclear CGC olefin polymeri-
zation catalysis. In styrene homopolymerizations, bimetallic
organotitanium catalysts (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si-
(tBuN)](TiMe2)}2 [MBICGC(TiMe2)2; C1-Ti2] + Ph3C+B(C6F5)4-
(B1) and (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2
[EBICGC(TiMe2)2; Ti2] + B1 exhibit ∼65 and ∼35 times
greater activities, respectively, than does monometallic [1-Me2-
Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + B1. Bimetallic orga-
nozirconium catalysts (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)]-
(ZrMe2)}2 [MBICGC(ZrMe2)2; C1-Zr2] + B1 and (µ-CH2CH2-
3,3′){(η5-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2;
Zr2] + B1 exhibit ∼8 and ∼4 times higher activities, respec-
tively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]-
ZrMe2 (Zr1) + B1. The binuclear catalysts exhibit significantly
greater activities than the corresponding mononuclear catalysts,
and more interestingly, as the minimum accessible distance
between the adjacent metal centers decreases, this observed
cooperative nuclearity effect increases in the following order:
C1-M2 > M2 > M1 (M ) Ti and Zr). In situ activation studies
of the model compound Ti2(CH2Ph)4 suggest that under
polymerization conditions, minimal monometallic “back-
coordination” of the last inserted styrene to the parent cationic
metal center occurs.
comparison due to the lack of ethylene solubility data in some
comonomers, the general observed trend is informative, and the
results are consistent with the mechanistic picture proposed: the
stronger the metal-arene interaction, the more efficiently Ti2
enchains the styrenic comonomer vs Ti1.
IV. Solvent Polarity Effects on Polymerization. It was
reported previously5a,b that for ethylene + R-olefin copolymer-
izations carried out in polar C6H5Cl as the solvent, the
comonomer enchainment level difference between bimetallic
M2 catalysts and monometallic M1 catalysts (M ) Ti for
ethylene + methylenecyclopentane copolymerization;5a M )
Zr for ethylene + 1-hexene copolymerization5b) diminishes since
the polar solvent can compete for/coordinate to the electrophilic
metal centers and weaken/replace agostic interactions,29 which
were proposed to be mechanistically central to the observed
binuclear enchainment effects. The present copolymerization
results in the same polar solvent argue that, operationally, the
metal-arene interaction remains largely intact in C6H5Cl since
the significant disparity of styrene incorporation levels between
Ti2 and Ti1 remain almost unchanged. Indeed, it has been
reported that the η2-bonding mode of d0 metal-benzyl species
remains largely undisrupted in polar solvents such as 1,1,2,2-
tetrachloroethane19h (ꢀr ) 8.2) and methylene chloride19i (ꢀr )
9.1).
Increases in styrenic comonomer enchainment selectivity into
the polyethylene microstructure for variously para-substituted
styrenes are observed with binuclear catalyst Ti2 + B1 versus
the corresponding mononuclear catalyst Ti1 + B1 under identical
polymerization conditions. The relative magnitude of this
bimetallic effect approximately mirrors the π-electron density
at the ipso carbon: 4-fluorostyrene >4-chlorostyrene > 4-bro-
mostyrene > 4-methylstyrene > styrene. Polar solvation is found
to play a significant role in binuclear ion pairing, affording
different polymeric products while not diminishing the bimetallic
effect.
V. Activation Studies of Model Compound Ti2(CH2Ph)4.
The design of the model compound Ti2(CH2Ph)4 is to simulate
the growing polystyrene chain with the last styrene installed in
a 2,1 fashion. Upon alkyl abstraction by the cocatalyst/activator
B(C6F5)3, the resulting bimetallic ion pair [Ti2(CH2Ph)2]2+
-
[PhCH2B(C6F5)3-]2 should closely resemble the bimetallic Ti-
polymeryl propagating species. Although some neutral metal-
benzyl complexes are reported to exhibit an ηn-coordination
mode (n > 1) of the benzyl groups,33 the benzyl groups of the
model compound Ti2(CH2Ph)4, however, engage in an η1-
coordination mode both in solution and in the solid state, largely
due to lack of coordinative unsaturation around the metal
center. The significant styrene homopolymerization activity
disparity between Ti2 and Ti1 most likely arises from, as
proposed above, the preferential coordination of the phenyl
ring of the last inserted styrene to the second metal center.
Although definitive solution structural data (chemical shifts of
The results of this study indicate that multinuclear single-
site polymerization catalysts can effect unusual cooperative
enchainment processes involving comonomers which possess
additional coordinating moieties and hence offer the potential
of creating new macromolecular architectures which conven-
tional monometallic catalysts cannot offer.
1
the benzylic ipso carbons and JC-H) as well as solid-state
Acknowledgment. Financial support from NSF (Grant No.
CHE-0415407) and DOE (Grant No. 86ER13511) is gratefully
acknowledged. We thank Dr. E. Szuromi and Prof. R. F. Jordan
of University of Chicago for generous help with the GPC
measurements. N.G. thanks Dr. L. Li, Dr. H. Li, and Prof. J. B.
Lambert for helpful discussions.
structure for the bimetallic ion pair are not available to confirm
the interaction of the benzylic arene to the second metal center,
the present polymerization results as well as spectroscopic
evidence argue at least that, in the presence of a second
electrophilic metal center, minimal monometallic “back-
(39) (a) Casabianca, L. B.; De Dios, A. C. J. Phys. Chem. A 2006, 110, 7787-
7792. (b) Termaten, A.; van der Sluis, M.; Bickelhaupt, F. Euro J. Org.
Chem. 2003, 11, 2049-2055. (c) Stothers, J. B. In Carbon-13 NMR
Spectroscopy; Academic Press: New York, 1972; pp 90-91. (d) Nelson,
G. L.; Levy, G. C.; Cargioli, J. D. J. Am. Chem. Soc. 1972, 94, 3089-
3094. (e) Spiesecke, H.; Schneider, W. G. J. Chem. Phys. 1961, 35, 731-
738.
Supporting Information Available: Crystallographic data for
Ti2(CH2Ph)4. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA076407M
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2261