Bimetallic zirconium amine bis(phenolate) polymerization catalysts: Enhanced activity and tacticity control for polyolefin synthesis

Article abstract of DOI:10.1021/om500608j

Binucleating multidentate amine bis(phenolate) ligands with rigid terphenyl backbones were designed to support two zirconium centers locked in close proximity. Polymerizations of propylene or 1-hexene with the synthesized bimetallic precatalysts resulted in polymers with significantly higher isotacticity (up to 79% mmmm) in comparison to the stereoirregular polymers produced with previously reported C_s-symmetric monometallic analogues. The bimetallic precatalysts also display higher activity (up to 124 kg of poly(1-hexene) (mmol of Zr)^-1 h^-1), in comparison to the monometallic analogues, and among the highest activities reported for nonmetallocene catalysts. The stereocontrol is consistent with a bimetallic mechanism involving remote steric interactions with the ligand sphere of the second metal center.

Full text of DOI:10.1021/om500608j

Communication
pubs.acs.org/Organometallics
Bimetallic Zirconium Amine Bis(phenolate) Polymerization Catalysts:
Enhanced Activity and Tacticity Control for Polyolefin Synthesis
Madalyn R. Radlauer and Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, MC 127-72,
Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Binucleating multidentate amine bis(phenolate) ligands with
rigid terphenyl backbones were designed to support two zirconium centers
locked in close proximity. Polymerizations of propylene or 1-hexene with
the synthesized bimetallic precatalysts resulted in polymers with
significantly higher isotacticity (up to 79% mmmm) in comparison to the
stereoirregular polymers produced with previously reported Cs-symmetric
monometallic analogues. The bimetallic precatalysts also display higher
activity (up to 124 kg of poly(1-hexene) (mmol of Zr)−1 h−1), in
comparison to the monometallic analogues, and among the highest activities reported for nonmetallocene catalysts. The
stereocontrol is consistent with a bimetallic mechanism involving remote steric interactions with the ligand sphere of the second
metal center.
he mechanisms responsible for stereoregulation in the
homogeneous polymerization of propylene and α-olefins
workers.10 This ligand design was selected for its versatility, ease
of synthesis, and interesting catalytic properties of related
monometallic Zr complexes. Highly active Cs-symmetric
tetradentate amine bis(phenolate) Zr complexes produce up to
102 kg of poly(1-hexene)) (mmol of Zr)−1 h−1 or 101 kg of
polypropylene (mmol of Zr)−1 h−1 of stereoirregular poly-
mer.4b,10b C2-symmetric complexes supported by related ligands
show increased isospecificity, albeit with significantly lower
activity.11 C1-symmetric versions displayed enhanced activity and
selectivity by exploiting both steric and electronic effects.4e,h,12
Only the syn atropisomers of the binucleating ligand
precursors (H4R2R′2-L) were studied, with the focus on
examining the effect of a proximal metal center (Figure 1; see
the Supporting Information (SI) for detailed ligand synthesis). A
series of compounds was prepared with variation of donor L
(methoxy or dimethylamino) and substituents R and R′ for steric
and electronic tuning (chloride, bromide, methyl, or tert-butyl).
T
have been studied extensively.1 ansa-Zirconocene catalysts are
the most successful systems for pairing high activity and
stereoselectivity, though the development of new classes of
catalysts with different selectivity and stability profiles is
desirable.1b,c,g,2 Thus, the design of stable and inexpensive
nonmetallocene catalysts with high activity, regioselectivity,
stereoselectivity, and comonomer incorporation is an active area
of study.1b,g,3 Numerous nonmetallocene catalysts have been
reported that exhibit either high activity or stereoselectivity, but
rarely both.1b,4 Although bimetallic catalysts for olefin polymer-
ization display a variety of beneficial effects, they have received
only limited attention as a strategy for tacticity control.5
Improved syndioselectivity in styrene polymerization has been
reported with linked dititanium systems relative to their
monometallic analogues.6 Dipalladium catalysts have been
shown to produce moderately isotactic CO/styrene copolymers,
whereas the monometallic Pd system yielded stereoirregular
polymers.7 Dizirconium bis-propagators, in the presence of
ZnEt2 as a chain transfer agent, retained the stereoselectivity
generally observed with the analogous monometallic catalysts in
the absence of ZnEt2, thereby overcoming a limitation of chain
transfer polymerization with these systems.8
The dizirconium complexes (Zr2R2R′2-L) were synthesized by the
addition of 1 equiv of the tetraphenol H4R2R′2-L to 2 equiv of
ZrBn4. In all cases, two major species (in a ratio of 1:1) were
observed in the reaction mixture by 1H NMR spectroscopy, and
isolation of one of these species in analytically pure form was
1
achieved via recrystallization. The H NMR spectrum of this
Recently we reported dinickel bisphenoxyiminato polymer-
ization catalysts based on a rigid terphenyl ligand framework with
a permethylated central arene.9 The locked conformation of
these complexes allowed the isolation and purification of syn and
anti atropisomers.9a Early-metal-based dinuclear catalysts are of
interest for higher activity, α-olefin incorporation, and tacticity
control. For the present work, the terphenyl backbone was
utilized with altered donor sets to target the synthesis of
bis[amine bis(phenolate)] dizirconium systems analogous to Cs-
symmetric monometallic systems reported by Kol and co-
isolated species displays eight doublets corresponding to benzylic
protons, consistent with either a pseudo-C2-symmetric species in
which the C2 axis runs through the center of the central arene of
the terphenyl backbone or a pseudo-Cs-symmetric species in
which the mirror plane cuts perpendicularly through the central
arene and the terphenyl vector (Figure 1). X-ray-quality crystals
Received: June 9, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om500608j | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Figure 1. Synthesis of Zr2R2R′2-L (left) and representations of the possible metalation isomers of Zr2R2R′2-L (right).
were obtained for Zr2Cl4-OMe and Zr2Cl4-NMe2. Solid-state
characterization by single crystal X-ray diffraction (XRD)
showed structures of pseudo-C2 symmetry for the isolated
isomer of Zr2Cl4-OMe (Figure 2) and Zr2Cl4-NMe2 (Figure S82
(SI)). Isolation of the second major product was possible only for
compound Zr2Me4-OMe by successive recrystallizations. An
XRD study revealed a solid-state structure of pseudo-Cs
symmetry (Figure 2). Crystal structures were correlated with
1H NMR spectra to identify diagnostic benzyl peaks for each
metalation isomer in the 1H NMR spectra between 3 and 4 ppm.
No isomerization between the two isomers was observed by 1H
NMR spectroscopy in solution at room temperature with the
isolated pseudo-C2-symmetric species or with different ratios of
the two isomers. As the racemic mixtures of the pseudo-C2-
symmetric complexes were isolable in all cases, these species
were utilized in polymerization trials, except when stated
otherwise.
polymerization activities reported for nonmetallocene cata-
lysts.1b,g,4 At low temperatures (−30 °C), poly(1-hexene) with
79% mmmm (Figure S79 (SI)) was obtained at an activity of 2.7
kg of poly(1-hexene) (mmol of Zr)−1 h−1. This polymer also
exhibited a high molecular weight of 1.2 × 106 Da and low
polydispersity (PDI = 1.1), as determined by size exclusion
chromatography. Under the same conditions, the best of the
monometallic catalysts produced 0.054 kg of poly(1-hexene)
(mmol of Zr)−1 h−1 with 33% mmmm. Although propylene
polymerizations with the bimetallic complexes showed lower
activity and isoselectivity (Table S4 (SI)) in comparison to 1-
hexene, similar trends were observed. The most active catalyst
was again Zr2Cl4-NMe2 with 47 kg of polypropylene (mmol of
Zr)−1 h−1 and 43% mmmm. In contrast, the monometallic catalyst
Zr1Cl2tBuAr-NMe2 (vide infra), showed significantly lower activity
(2.9 kg of polypropylene (mmol of Zr)−1 h−1) and polymer
tacticity (31% mmmm). The present results indicate that the
bimetallic catalysts display considerably enhanced activity and
tacticity control in comparison to the monometallic analogues.
The expected stereoerror distribution for enantiomorphic site
control primarily includes the following pentads: mmmr, mmrr,
and mrrm. The bimetallic zirconium precatalysts produce
polymers with these stereoerrors as the most common, but the
distribution includes the remaining possible stereoerrors as well.
A model displaying a combination of enantiomorphic site control
and chain end is most consistent with the data.14 Comparative
studies between the bimetallic and the monometallic systems
were performed to gain insight into the structural features
responsible for stereocontrol. A series of new monometallic C1-
symmetric complexes was synthesized (Chart 1; see the SI for
detailed synthetic procedures and characterization). XRD studies
of Zr1Cl2tBu2-OMe revealed a structure very similar to those of the
Cs-symmetric complexes in the literature (Figure S85 (SI)).10a,c
These monometallic complexes produce poly(1-hexene) of
varied isotacticity, from stereoirregular with Zr1tBu4-L to 10−35%
mmmm with Zr1Cl2tBu2-L and Zr1Cl2tBuAr-L. The tacticity control
increases from the Cs-symmetric to the C1-symmetric precata-
lysts.
Monometallic (Chart 1) and bimetallic precatalysts were
highly active for 1-hexene polymerization, causing significant
heating upon stoichiometric activation with a solution of
[CPh3][B(C6F5)4] in chlorobenzene (PhCl). Polymerizations
run for 10 min led to greater than 80% conversion of 1-hexene to
poly(1-hexene).13 As expected, the literature complexes Zr1
-
tBu4
L10c produced stereoirregular polymers with <10% mmmm. In
contrast, the bimetallic complexes produced polymers with
increased isotacticity (17−50% mmmm). The polymer iso-
tacticity varied in the bimetallic systems, with the systems bearing
amino ethers (L = OMe) being more selective than those with
Cl4
diamines (L = NMe2): Zr2Cl4-OMe > Zr2Br4-OMe > Zr2
-
NMe2 > Zr2Br4-NMe2 > Zr2Me4-OMe > Zr2H2tBu2-OMe. For the
same L, precatalysts with the smallest substituent in the position
ortho to the phenoxide moiety, R = Cl, resulted in the highest
tacticity, while the large tBu substituent led to the lowest tacticity.
Precatalysts bearing diamine ligands were more active. Using
mixtures of the two metalation isomers (Cs and C2 symmetric)
did not alter either the activity or the stereoselectivity of the
complexes (Table S1 (SI), entries 9 and 12). Similarly, for
Zr2Me4-OMe, which is the only ligand system where the Cs-
symmetric isomer was isolated and purified, polymerizations
with each isomer could be compared and were found to be very
similar in activity and stereoselectivity (Table S1, entry 16).
These results suggest that, although the presence of the second
metal site significantly affects tacticity, its relative conformation is
less consequential.
The increase in isotacticity is improved further by the
incorporation of a second metal site. Each metal center in the
bimetallic systems has local C1 symmetry (Scheme 1). The two
sites available for polymeryl coordination are very different in the
bimetallic vs the monometallic species. The site pointing toward
the second metal is considerably more hindered by the distal
steric bulk in both Cs-symmetric and C2-symmetric structures
(Figure 2). This steric effect is proposed to favor a polymeryl
location away from this position (species A vs C), lowering the
degrees of freedom available for the polymeryl chain and the
monomer compared to the monometallic species. Although the
site pointing toward the second metal may also disfavor olefin
binding (D), this effect is expected to be lower, due to the lower
steric profile of the olefin with the substituent residing in the
The complexes Zr2Cl4-L, Zr1Cl2tBu2-L, and Zr1Cl2tBuAr-L were
tested with temperature control (Table 1). The bimetallic
catalysts are more active than the monometallic analogues by up
to several orders of magnitude. Zr2Cl4-NMe2 generated a catalyst
that is 1 order of magnitude more active than Zr2Cl4-OMe. Upon
optimization of polymerization conditions, activities of up to 124
kg of poly(1-hexene) (mmol of Zr)−1 h−1 (ambient temperature)
were achieved for Zr2Cl4-NMe2. This is among the highest olefin
B
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Organometallics
Communication
a
Table 1. 1-Hexene Polymerizations
time
yield
(g)
mmmm
b
c
complex
Zr2Cl4-OMe
T (°C)
(min)
activity
2.3
(%)
1
room
temp
10
0.75
50
2
3
4
5
6
Zr2Cl4-OMe
Zr2Cl4-OMe
Zr2Cl4-OMe
Zr2Cl4-OMe
60
25
0
10
10
0.67
0.68
0.78
0.01
1.17
2.0
37
47
57
76
38
2.0
480
480
1
0.05
−30
0.0007
35
Zr2Cl4-NMe2 room
temp
7
Zr2Cl4-NMe2 60
Zr2Cl4-NMe2 25
1
2
2
5
1
0.65
1.28
1.42
1.51
0.82
19
19
21
33
33
38
58
41
8
9
Zr2Cl4-NMe2
0
10
11
Zr2Cl4-NMe2 −20
9.0
d
Zr2Cl4-NMe2 room
124
temp
d
d
d
d
12
13
14
15
16
Zr2Cl4-NMe2 60
Zr2Cl4-NMe2 25
2
2
0.58
0.62
1.39
0.18
0.11
43
46
26
34
41
48
79
9
Zr2Cl4-NMe2
0
8
Zr2Cl4-NMe2 −30
10
480
2.7
Cl2tBu2
Zr1
Zr1
Zr1
-
0
0.007
0.003
0.09
3.0
OMe
Cl2tBuAr
17
18
19
20
-
0
480
60
0.04
0.03
0.61
0.17
23
11
35
33
OMe
Cl2tBu2
d
d
d
-
0
NMe2
Cl2tBuAr
Zr1
-
0
30
NMe2
Cl2tBuAr
Zr1
-
−30
480
0.01
NMe2
a
Polymerizations were run with 2.5 mL of 1-hexene in 2.5 mL of PhCl
with 2 μmol of [Zr], 1 equiv of [CPh3][B(C6F5)4], and 5 equiv of
AliBu3. Room temperature polymerizations were run without temper-
b
ature control and varied in the strength of their exotherms. Activity in
c
kg of poly(1-hexene) (mmol of Zr)−1 h−1. Determined from 13C
d
NMR spectra.13 Polymerizations were run with 2.5 mL of 1-hexene in
2.5 mL of PhCl with 0.4 μmol of [Zr], 3 equiv of [CPh3][B(C6F5)4],
and 15 equiv of AliBu3.
Scheme 1. Proposed Steric Interactions Due to the Bimetallic
Nature of the Catalysts
Figure 2. Side-on and top-down views of the solid-state structures of
Zr2Cl4-NMe2 (pseudo-C2-symmetric, top) and Zr2Me4-OMe (pseudo-
Cs-symmetric, bottom) with thermal ellipsoids at the 50% probability
level. Solvent molecules and H atoms have been omitted for clarity.
Chart 1. Monometallic Zirconium Complexes
plane perpendicular to the Zr−olefin bond. The highest level of
tacticity control is observed for Zr2Cl4-L, which has substituents
C
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ortho to the phenoxide moiety for each metal site (R = Cl vs
C6Me4Ar) that have significantly different steric profiles. The
reduced isotacticity observed for the polymerization of
propylene relative to that for 1-hexene may be caused by the
smaller size of the polymer chain, leading to lower levels of steric
repulsion and orientation preference.
The steric profiles of the bimetallic catalysts outlined here
suggest that tacticity could be controlled by a distal steric
interaction unique to these rigid systems. This effect is more
pronounced with the larger 1-hexene monomer, which leads to a
more sterically demanding polymer chain. A similar distal steric
interaction is proposed to engender high catalytic activities by
weakening metal−anion interactions (B). A related mechanism
was evidenced for the copolymerization of ethylene with amino
olefins by nickel phenoxyimine bimetallic catalysts; in the
absence of the bimetallic effect, little activity and polar monomer
incorporation was observed.9b,c Without a strong interaction
with the counterion, the metal center is more electrophilic and
reactive in olefin polymerization catalysis.
In summary, we have synthesized a series of dizirconium
bis[amine bis(phenolate)] precatalysts that are effective for the
polymerization of propylene and 1-hexene with very high
activities and significant stereocontrol. 1-Hexene polymers with
>75% mmmm content and remarkable activities exceeding 120 kg
of poly(1-hexene) (mmol of Zr)−1 h−1 were obtained. Studies of
related monometallic systems show that the bimetallic character
is required for the increased activity and isoselectivity. The distal
steric interactions caused by the presence of the second metal site
are proposed to lead to increased activity and stereocontrol.
Ongoing efforts are focused on expanding the applicability of the
distal steric effect in bimetallic and monometallic catalysts toward
the synthesis of stereoregular materials with other nonpolar as
well as functionalized monomers.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental
procedures, characterization data, crystallographic details for
complexes Zr2Cl4-NMe2, Zr2Me4-OMe, Zr2Cl4-OMe, and
Zr1Cl2tBu2-OMe, and additional polymerization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(11) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000,
122, 10706. (b) Busico, V.; Cipullo, R.; Ronca, S.; Budzelaar, P. H. M.
Macromol. Rapid Commun. 2001, 22, 1405. (c) Segal, S.; Goldberg, I.;
Kol, M. Organometallics 2005, 24, 200. (d) Yeori, A.; Goldberg, I.;
Shuster, M.; Kol, M. J. Am. Chem. Soc. 2006, 128, 13062.
(12) (a) Cohen, A.; Yeori, A.; Kopilov, J.; Goldberg, I.; Kol, M. Chem.
Commun. 2008, 2149. (b) Cohen, A.; Coates, G. W.; Kol, M. J. Polym.
Sci., Part A: Polym. Chem. 2013, 51, 593.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.A.: agapie@caltech.edu.
Notes
■
The authors declare no competing financial interest.
(13) Asakura, T.; Demura, M.; Nishiyama, Y. Macromolecules 1991, 24,
2334.
(14) Inoue, Y.; Itabashi, Y.; Chujo, R.; Doi, Y. Polymer 1984, 25, 1640.
̂
̂
ACKNOWLEDGMENTS
■
We are grateful to Dow Chemical, Caltech, and the ACS PRF for
funding. We thank Dr. Mike Takase, Lawrence M. Henling, and
Emily Tsui for crystallographic assistance. The Bruker KAPPA
APECII X-ray diffractometer was purchased via an NSF
CRIF:MU award to Caltech (CHE-063-9094). The 400 MHz
NMR spectrometer was purchased via an NIH award
(RR027690).
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dx.doi.org/10.1021/om500608j | Organometallics XXXX, XXX, XXX−XXX