Synthesis of highly cis, syndiotactic polymers via ring-opening metathesis polymerization using ruthenium metathesis catalysts

Article abstract of DOI:10.1021/ja405559y

The first example of ruthenium-mediated ring-opening metathesis polymerization generating highly cis, highly tactic polymers is reported. While the cis content varied from 62 to >95% depending on the monomer structure, many of the polymers synthesized displayed high tacticity (>95%). Polymerization of an enantiomerically pure 2,3-dicarboalkoxynorbornadiene revealed a syndiotactic microstructure.

Full text of DOI:10.1021/ja405559y

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Communication
Synthesis of Highly Cis, Syndiotactic ROMP
Polymers Using Ruthenium Metathesis Catalysts
Lauren Estelle Rosebrugh, Vanessa M. Marx, Benjamin Keith Keitz, and Robert Howard Grubbs
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405559y • Publication Date (Web): 19 Jun 2013
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Synthesis of Highly Cis, Syndiotactic ROMP Polymers Using Ru-
thenium Metathesis Catalysts
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Lauren E. Rosebrugh, Vanessa M. Marx, Benjamin K. Keitz, Robert H. Grubbs*
Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, Cali-
fornia Institute of Technology, Pasadena, California 91125, United States
Supporting Information Placeholder
consequent inability of the Ru=C bond to enforce the
steric pressures necessary to give tacticity, the likeli-
hood of developing a Ru-based metathesis catalyst
able to form polymers exhibiting high tacticity ap-
peared increasingly minimal.5b
ABSTRACT: The first example of ruthenium-mediated
ring-opening metathesis polymerization (ROMP)
generating highly cis, highly tactic polymers is re-
ported. While the cis content varied from 62 to >95%
depending on the monomer structure, many of the
polymers synthesized displayed high tacticity
(>95%). Polymerization of an enantiomerically pure
2,3-dicarboalkoxynorbornadiene revealed a syndio-
tactic microstructure.
We recently reported on the Z-selective ruthenium
metathesis catalyst 1 containing a crucial cyclomet-
alated N-heterocyclic carbene (NHC) ligand (Figure
1), in which the Ru-C bond is formed via C-H activa-
tion induced by the addition of silver pivalate.8 This
catalyst was shown to give on average 80-95% cis
content in the ROMP of norbornene and norbornadi-
ene derivatives,6b thus demonstrating for the first
time the cis-selective ROMP of a wide range of mon-
omers with a single ruthenium-based metathesis cat-
alyst. However, all of the polymers produced by cata-
lyst 1 were atactic.
The precise control of polymer microstructure re-
sulting from the ring-opening metathesis polymeri-
zation (ROMP) of substituted norbornenes, nor-
bornadienes, and other strained, cyclic olefins is crit-
ical for the development of polymers with well-
defined characteristics.1 These microstructures,
which include various tacticities (e.g., syndiotactic,
isotactic, or atactic) and double bond configurations
(cis vs. trans), have a significant impact on the physi-
cal and mechanical properties of the resulted poly-
mer.2 For example, syndiotactic cis-poly(norbornene)
is a crystalline polymer with a high melting point,
while atactic trans-poly(norbornene) is amorphous
and low-melting in comparison.3 Accordingly, the
development of olefin metathesis catalysts capable of
producing highly stereoregular (>95% a single struc-
ture) ROMP polymers is of great interest.
Herein, we report a new series of cyclometalated
catalysts (complexes 2-4) derived from C-H activa-
tion of an N-tert-butyl group. These complexes dis-
play ROMP behavior unprecedented for ruthenium-
based metathesis catalysts: Not only do these cata-
lysts yield polymers with a generally higher cis con-
tent compared to 1 (>95% in many cases), but the
polymers produced are also highly syndiotactic, fur-
ther demonstrating that, similar to their W- and Mo-
based counterparts, Ru-based metathesis catalysts are
capable of producing polymers with a wide range of
specific microstructures without the use of special-
ized monomers or reaction conditions.
ROMP polymers with high cis content have been
synthesized previously using a variety of Re-, Os-, W-
and Mo-based metathesis catalysts,4,5 as well as more
recently with a Ru-derived system.6 However, while
many of these catalysts, particularly W- and Mo-
based systems, have been shown capable of control-
ling both the cis/trans ratio and tacticity of ROMP
polymers,4,5 only limited tacticity control has been
achieved with ruthenium.7 In fact, it was highlighted
in a recent report by Schrock and coworkers that due
to the low barrier of rotation of Ru alkylidenes and
The recent development of a milder method of ef-
fecting the salt metathesis and C-H activation of Ru
metathesis catalysts using sodium pivalate has ena-
bled the synthesis of complexes with significant al-
terations to the chelating N-alkyl group of the NHC
that were previously inaccessible.9 Using this new
approach, we were able to prepare the less sterically
encumbered N-tert-butyl catalysts 2-4 (Figure 1).10
Single-crystal X-ray diffraction of 3 confirmed cy-
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clometallation of the N-tert-butyl substituent, as well more complex
Page 2 of 5
2,3-
monomer
1
2
3
4
5
6
7
8
as bidentate coordination of the pivalate ligand. It
was also revealed that the N-aryl ring is positioned
such that the isopropyl substituent resides on the
same face as the benzylidene. Structural parameters,
including bond lengths and angles, were consistent
with those for 1 and its pivalate derivative.8
dicarbomethoxynorbornadiene (DCMNBD, 6). The
tacticity of poly(DCMNBD) is readily determined by
analyzing the C(7) region of the 13C NMR: multiple
resonances correspond to an atactic polymer, such as
the poly(DCMNBD) produced by 1 (Figure 3a), while
a singularly tactic polymer would be expected to dis-
play only one peak in this region due to symmetry.13
Poly-6 produced by catalyst 2 (>95% cis; Table 1)
was found to be highly tactic, in that the correspond-
ing 13C NMR spectrum contained primarily one car-
bon resonance in the C(7) region, consistent with a
tacticity of >95% for the all-cis triads (Figure 3b).
N
N
Mes
N
N
Mes
9
Ru
O
Ru
O
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O
O
N
tBu
O
iPr
O
iPr
O
1
2
To probe the effect of symmetry of the N-aryl
group on tacticity,14 we evaluated two catalysts (3, 4)
containing a cyclometalated N-tert-butyl group simi-
lar to 2 but with an asymmetric N-aryl group (Figure
1). The geminal dimethyl backbone of 4 was installed
to prevent any rotation of the N-aryl group that
might be occurring in 3. Polymerization of 6 with
catalysts 3 and 4 gave poly-6 that was also >95%
tactic in both cases as determined by 13C NMR (Table
1). Poly-5 produced by these catalysts, was also
highly cis (>95%), albeit slightly less tactic in both
cases than poly-5 generated with 2.
N
N
MIPP
N
N
MIPP
Ru
O
Ru
O
O
O
tBu
O
iPr
tBu
O
iPr
4
3
Figure 1. Catalysts 1-4: Mes = 2,4,6-trimethylphenyl;
MIPP = 2,6-methylisopropylphenyl.
Table 1. Polymerization of Monomers 5-7 with Cata-
lysts 2-4.a
catalyst (1 mol %)
n
THF (0.25 M)
OR* =
CO2Me
CO2Me
CO2R*
CO2R*
O
5
6
7
Figure 2. Solid-state structure of 3, with thermal ellip-
soids drawn at 50% probability. For clarity, hydrogen
atoms have been omitted. Selected bond lengths (Å) for
3: C1-Ru 1.932, C5-Ru 2.071, C18-Ru 1.798, O1-Ru
2.334, O2-Ru 2.202, O3-Ru 2.398.
cis,
%
Mn,
monomer catalyst
yield,%c
PDId
kDad
605e
521e
424e
--f
b
5
5
5
6
6
6
7
7
7
2
3
4
2
3
4
2
3
4
>95
>95
>95
>95
>95
>95
72
79
88
84
78
54
45
42
57
7
1.41
1.49
1.45
--f
We initiated our ROMP studies by adding 2 to a so-
lution of norbornene (5) in THF (0.25 M) at room
temperature, upon which the solution rapidly be-
came viscous.11 The isolated polymer was found to be
--
--
--
--
1
almost exclusively cis (>95%) by H NMR spectros-
--
--
copy (Table 1). Furthermore, the 13C NMR spectrum
62
--
--
of poly-5 prepared with 2 was found to be consistent
with
literature
reports
for
highly
tactic
86
--
--
poly(norbornene).12 In comparison, performing the
same reaction at room temperature with catalyst 1
gives atactic poly(norbornene) that is only 88% cis.6b
aConditions were [monomer]/[initiator] = 100:1 in THF
(0.25M in substrate) at RT. bDetermined by 1H NMR and
13C NMR spectroscopy. Isolated yields. Determined by
gel-permeation chromatography (GPC) with a multian-
gle light scattering (MALS) detector. The specific re-
c
d
In order to test whether the observed tacticity con-
trol was specific to norbornene, we turned to the
e
2
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faster than the rate of carbene epimerization.18 This
fractive index increment (dn/dc) was determined to be
1
2
3
4
5
6
7
8
0.139 ± 0.005 mL/g. efHere and below: Not determined
because insolubility of the isolated polymer in THF pre-
cluded GPC analysis.
is presumably in contrast to catalyst 1, which, likely
due to the slow rate of propagation relative to epi-
merization, generates atactic polymers. This distinc-
tion might be explained by the relative differences in
size and symmetry of the carbon chelate in catalysts
1 and 2. The reduced bulk of the cyclometalated N-
tert-butyl group of 2 (vs. the N-adamantyl chelate in
1) likely results in fewer unfavorable steric interac-
tions between the catalyst and the approaching bulky
norbornene and norbornadiene derivatives, thus in-
creasing the overall rate at which the monomers are
incorporated.19 The local symmetry about the Ru-C
bond in 2-4 is postulated to account for the syndiose-
lectivity.14,18
MeO2C
CO2Me
n
C(7)
a)
b)
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R* R*
R* R*
R* R*
39
38
ppm
37
Ha
Hb
a) cis, isotactic
Ha
Hb
Figure 3. The C(7) 13C NMR region of poly(DCMNBD)
(poly-6) prepared with a) 1 (86% cis, atactic) and b) 2
(>95% cis, >95% tactic).
R* R*
R* R*
As had previously been observed with 1, the exper-
imental number-average molecular weights (Mn) for
poly-5 prepared using catalysts 2-4 were significant-
ly higher than the theoretical values (Table 1). This is
indicative that kp of 5 exceeds ki of 2-4, which would
lead to the broad PDIs observed; this is likely a result
of incomplete catalyst initiation and might be ex-
pected based on the relatively low initiation rate
constants of 2-4.15
R*
Ha
R*
Hb
Ha
Hb
b) cis, syndiotactic
Figure 4. Olefinic protons in the two possible highly-cis,
regular polymers made from an enantiomerically pure
2,3-disubstituted norbornadiene.
To elucidate the absolute tacticities of the nor-
bornene- and norbornadiene-derived polymers, we
employed chiral monomer 7. Due to the lack of mir-
ror planes relating the monomeric units in the result-
ing polymers, it is expected that if the polymers pro-
duced by 2-4 were cis, isotactic, the olefinic protons
would be inequivalent (Figure 4a).16 As such, we
should observe a coupling characteristic of olefinic
protons by NMR spectroscopy. Conversely, in a cis,
syndiotactic polymer, the cis olefinic protons would
be related by a C2 axis passing through the midpoint
of the double bond and would therefore be equiva-
lent and not coupled (Figure 4b). While poly-7 pro-
duced by catalysts 2-4 was only 62-86% cis (Table
1), the tacticity of the all-cis triads remained very
high for all three catalysts (see Figures S14 and S15
in Supporting Information). Furthermore, the cis ole-
finic protons in the isolated polymers were uncou-
pled,17 strongly suggesting that poly-7 is syndiotactic
in all cases (Figure 5).
5.20
5.25
5.30
5.35
5.40
5.45
5.40 5.38 5.36 5.34 5.32 5.30 5.28 5.26 5.24
ppm
Figure 5. COSY spectrum of poly-7 prepared with 2 in
the olefinic proton region. The absence of olefinic cou-
pling suggests that the polymer is syndiotactic.
Finally, we sought to briefly explore the physical
properties of tactic ROMP polymers in comparison to
their atactic counterparts via differential scanning
calorimetry (DSC) and thermogravimetric analysis
(TGA). The Tg of atactic trans-polynorbornene is 37
°C.3a As expected, the Tg of poly-5 was significantly
3
One plausible explanation for the observed tacticity
control is that the rate of monomer incorporation is
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(5) (a) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; Hoveyda, A. H. J.
higher at ca. 70 °C, consistent with a higher packing
order due to the increased stereoregularity of the
polymer. Both the atactic, trans polymer and the
syndiotactic, cis poly-5 decomposed at ca. 430 °C
(see Supporting Information).
Am. Chem. Soc. 2009, 131, 7962. (b) Flook, M. M.; Ng, V. W. L.;
Schrock, R. R. J. Am. Chem. Soc. 2011, 133, 1784. (c) Schrock, R. R.
Dalton Trans. 2011, 40, 7484. (d) Flook, M. M.; Borner, J.; Kilya-
nek, S. M.; Gerber, L. C. H.; Schrock, R. R. Organometallics 2012, 31,
6231. (e) Yuan, J.; Schrock, R. R.; Gerber, L. C. H.; Muller, P.; Smith,
S. Organometallics 2013, 32, 2983.
1
2
3
4
5
6
7
8
In spite of expectations to the contrary, we have
demonstrated the ability of Ru-based metathesis cat-
alysts to yield highly cis, highly tactic polymers.
ROMP of a chiral norbornadiene monomer suggested
that these polymers are syndiotactic. While it ap-
pears that the tacticity of these polymers is derived
from the installation of a comparatively small, sym-
metric N-tert-butyl group, the exact role of these fac-
tors in the control of the tacticity of polymers pro-
duced by cyclometalated Ru-based systems remains
to be determined.
(6) (a) Keitz, B. K.; Fedorov, A.; Grubbs, R. H. J. Am. Chem. Soc.
2012, 134, 2040. (b) For a selection of previously reported spe-
cialized cases, see reference 1 and references therein.
(7) (a) Delaude, L.; Demonceau, A.; Noels, A. F. Macromolecules
1999, 32, 2091. (b) Delaude, L.; Demonceau, A.; Noels, A. F. Mac-
romolecules 2003, 36, 1446. (c) Lee, J. C.; Parker, K. A.; Sampson,
N. S. J. Am. Chem. Soc. 2006, 128, 4578. (d) Lin, W.-Y.; Wang, H.-W.;
Liu, Z.-C.; Xu, J.; Chen, C.-W.; Yang, Y.-C.; Huang, S.-L.; Yang, H.-C.;
Luh, T.-Y. Chem. Asian J. 2007, 2, 764. (e) Song, A. R.; Lee, J. C.;
Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2010, 132, 10513.
(f) Leitgeb, A.; Wappel, J.; Slugovc, C. Polymer 2010, 51, 2927. (g)
Kobayashi, S.; Pitet, L. M.; Hillmyer, M. A. J. Am. Chem. Soc. 2011,
133, 5794. (h) Zhang, J.; Matta, M. E.; Martinez, H.; Hillmyer, M. A.
Macromolecules, 2013, 46, 2535.
(8) (a) Keitz, B. K.; Endo, K.; Patel, P. R.; Herbert, M. B.; Grubbs,
R. H. J. Am. Chem. Soc. 2012, 134, 693. (b) Endo, K.; Grubbs, R. H. J.
Am Chem, Soc. 2011, 133, 8525.
(9) Rosebrugh, L. E.; Herbert, M. H.; Marx, V. M.; Keitz, B. K.;
Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 1276.
(10) While a nitrato ligand afforded increased activity, stability
and selectivity to 1 compared to other X-type ligands, catalysts 2-
4 were significantly more stable in the pivalate form than the
analogous nitrato species. It is also important to note that com-
plex 2 quickly decomposed upon exposure to terminal olefins and
therefore was ineffective at mediating cross metathesis.
(11) For convenience, catalyst solutions were prepared in a
glovebox. However, 2 was determined to be relatively air-stable
in the solid state (minimal decomposition after exposure to air for
3 h).
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ASSOCIATED CONTENT
Experimental details and characterization data for all
compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
rhg@caltech.edu
Notes
The authors declare no competing financial interests.
(12) Al-Samak, H.; Amir-Ebrahimi, V.; Corry, D. G.; Hamilton, J.
G.; Rigby, S.; Rooney, J. J.; Thompson, J. M. J. Mol. Catal. A: Chemical
2000, 160, 13.
(13) McConville, D. H.; Wolf, J. R.; Schrock, R. R. J. Am. Chem. Soc.
1993, 115, 4413.
ACKNOWLEDGMENT
We thank Mr. Zachary Wickens for helpful discussions.
This work was financially supported by the NIH (R01-
GM031332), the NSF (CHE-1212767), and the NSERC of
Canada (fellowship to V.M.M.). Instrumentation on
which this work was carried out was supported by the
NIH (NMR spectrometer, RR027690).
(14) The relationship between symmetry and tacticity is well-
studied for early, metal-based metallocene polymerization cata-
lysts; the stereoregularity of the resulting polymers is a direct
result of the relationship of the stereoselectivities of the two ac-
tive sides of a metallocene initiator (i.e., homotopic, enantiotopic,
or diastereotopic). See: Odian, G. Principles of Polymerization;
John Wiley & Sons, Inc., Hoboken, New Jersey, 2004.
(15) The initiation rate constants (25 °C) of catalysts 2-4 are
2.8×10-3 s-1 (2), 4.1×10-4 s-1 (3), and 1.1×10-4 s-1 (4) (see ESI for
details). For comparison, the initiation rate constant of 1 is 8.4×
10-4 s-1, and >0.2 s-1 for RuCl2(C5H5N)2(IMesH2)(CHPh), which is
the preferred catalyst for ROMP. For a discussion of initiation in
ruthenium metathesis catalysts, see: (a) Sanford, M. S.; Love, J. A.;
Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543. (b) Love, J. A.;
Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem., Int. Ed.
2002, 41, 4035.
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(17) The cis and trans olefinic protons of poly-7 are clearly re-
solved in the 1H NMR.
(18) Detailed mechanistic studies are ongoing and will be re-
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(19) Indeed, catalysts 2-4 are generally less stable than 1,
which might also be a result of decreased steric protection (see
ref. 10).
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N
N Mes
1
Ru
O
2
3
4
O
>95% cis, >95% syndiotactic
tBu
O
iPr
R
R
R
R
R
R
R
5
6
R = CO2Me
R
R
R
R
R
7
8
9
10
11
12
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