Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst

Article abstract of DOI:10.1021/jacs.6b00014

The tungsten alkylidyne [^tBuOCO]W≡C(^tBu) (THF)₂ (1) reacts with CO₂, leading to complete cleavage of one C=O bond, followed by migratory insertion to generate the tungsten-oxo alkylidene 2. Complex 2 is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly cis-syndiotactic cyclic polynorbornene.

Full text of DOI:10.1021/jacs.6b00014

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Communication
Highly Tactic Cyclic Polynorbornene: Stereoselective Ring
Expansion Metathesis Polymerization (REMP) of Norbornene
Catalyzed By a New Tethered Tungsten-alkylidene Catalyst
Stella A. Gonsales, Tomohiro Kubo, Madison K Flint, Khalil A. Abboud, Brent S. Sumerlin, and Adam S. Veige
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00014 • Publication Date (Web): 04 Apr 2016
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Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion
Metathesis Polymerization (REMP) of Norbornene Catalyzed By a
New Tethered Tungsten-alkylidene Catalyst.
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Stella A. Gonsales,1 Tomohiro Kubo,2 Madison K. Flint,1 Khalil A. Abboud,1 Brent S. Sumerlin,2 and
Adam S. Veige1,2,*
1University of Florida, Department of Chemistry, Center for Catalysis, Center for Macromolecular Science & Engineering,
P.O. Box 117200, Gainesville, Florida, 32611
2University of Florida, Department of Chemistry, George & Josephine Butler Polymer Research Laboratory, Center for Macꢀ
romolecular Science & Engineering, P.O. Box 117200, Gainesville, Florida, 32611
Supporting Information Placeholder
ly inert ancillary ligand to promote REMP,9,13 with a tungsten
oxoꢀalkylidene fragment known to promote Zꢀselective ROMP.
(Figure 1).14 Herein, we report the first catalyst to promote stereꢀ
ocontrolled REMP to yield cisꢀsyndiotactic cyclic polyꢀ
norbornene.
Abstract: The tungsten alkylidyne [tBuOCO]W≡C(tBu)(THF)2 (1)
reacts with CO2, leading to complete cleavage of one C=O bond,
followed by migratory insertion to generate the tungstenꢀoxo
alkylidene 2. Complex 2 is the first catalyst to polymerize norꢀ
bornene via Ring Expansion Metathesis Polymerization (REMP)
to yield highly cisꢀsyndiotactic cyclic polynorbornene.
Ringꢀopening metathesis polymerization (ROMP) of norꢀ
bornenes and norbornadienes can lead to a variety of microstrucꢀ
tures depending on the resulting tacticity of the polymer (isotacꢀ
tic, syndiotactic, or atactic) and double bond configuration (cis or
trans).1 Highly tactic polymers often present more wellꢀdefined
properties, and therefore more value, than their atactic analogs.1c,2
However, stereocontrolled ROMP of cyclic monomers still poses
a challenge in polymer chemistry. Wellꢀdefined alkene metathesis
catalysts of tungsten,3 molybdenum,3 and ruthenium1c,4 were only
recently discovered to initiate ROMP of norbornene monomers
with high stereocontrol of tacticity and cis/trans ratio.
Perhaps even more challenging than stereocontrolled ROMP is
the synthesis of cyclic polymers. Cyclic polymers exhibit remarkꢀ
ably different physical properties (e.g., lower intrinsic viscosity,
higher glass transition temperature) compared to their linear counꢀ
terparts of similar molecular weight.5 Despite their interesting
properties, laborious synthesis required for cyclic polymers have
hindered development of this research field. Methods for the synꢀ
thesis of cyclic polymers usually involve intramolecular coupling
between the chain ends of linear precursors;6 however, the inherꢀ
ent limitation of this method is the requirement of dilute condiꢀ
tions and long reaction times.7 Recent discoveries in catalytic
production of cyclic polymers offer exciting new avenues for their
Figure 1. Tethered tungstenꢀoxo alkylidene catalyst 2 design conꢀ
cept.
Catalyst synthesis and characterization.
Previously
reported,
the
tungsten
alkylidyne
[tBuOCO]W≡C(tBu)(THF)2 (1) supported by a trianioninc pincer
ligand15 is the precursor for a highly active catalyst for polymeriꢀ
zation of alkynes to give cyclic polymers.16 Treating 1 with CO2
at 55 °C for 12 h generates the tungsten oxo alkylidene complex
2, and the dinuclear species 3, in a 9:2 ratio, respectively (Scheme
1).
efficient production.8
Grubbs’ ringꢀexpansion metathesis
Scheme 1. Synthesis of complexes 2 and 3.
polymerization (REMP) strategy overcomes some of the chalꢀ
lenges in cyclic polymer synthesis.9 REMP utilizes a cyclometꢀ
alated alkylidene metal complex as the catalyst.9a,10 In this case,
the two ends of the growing polymer remains attached to the metꢀ
al throughout the polymerization process, thus releasing a cyclic
polymer upon intramolecular chain transfer.9 Cyclic dendronized
polymers11 and cyclic brush polymers12 employing norborneneꢀ
based macromonomers were synthesized via REMP using cyclic
ruthenium catalysts;13 however, control over both tacticity and the
cis/trans ratio is either not reported or absent.
Complex 2 crystallizes preferentially in C6D6, resulting in sinꢀ
gle crystals amenable to Xꢀray diffraction. The tungsten ion in
complex 2 (Figure 2) is square pyramidal (τ = 0.12).17 The oxo
Figure 1 outlines the design features of catalyst 2. The catalyst
combines the concept of tethering a M=C bond to a substitutionalꢀ
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group occupies the axial position (W1ꢀO4 = 1.6948(15) Å) and
the alkylidene (W1=C21 = 1.9503(19) Å), a THF ligand, and two
aryloxides reside in the basal plane.
REMP and evidence for stereoselectivity.
1
2
3
4
5
6
7
8
Treating 2 (1 mol %) with norbornene at room temperature
yields cisꢀselective cyclic polynorbornene (>98% by H NMR
1
spectroscopy) after 7 h (Scheme 3). The reaction was quenched
by dropwise addition of the reaction mixture into stirring methaꢀ
nol. The resulting polymer was isolated by filtration and dried
under vacuum. Adding 3 (1 mol %) to norbornene at room temꢀ
perature for 7 h yields polynorbornene in only 42% yield, with no
cisꢀselectivity. Table 1 lists polymerization results as a function of
the ratio of monomer to catalyst.
9
A mixture of 2 and 3 maintains the selectivity of 2 when treated
with norbornene, suggesting initiation with catalyst 2 and propaꢀ
gation of the monomer are much faster than initiation by the more
hindered complex 3. Evidence for fast propagation relative to
initiation comes from a sealed NMR tube polymerization experiꢀ
ment. Combining norbornene and 2 in C6D6 results in polymer
formation but the H NMR spectrum of the reaction mixture exꢀ
hibits signals attributable to unreacted catalyst 2 (see Figure S17),
thus indicating kp>ki..
Table 1. Polymerization of norbornene[a] by catalyst 2 with different
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Figure 2. Left: Solidꢀstate structure of 2 with H atoms, solvent
molecules, and disorder on the coordinated THF omitted for clariꢀ
ty. Right: Solidꢀstate structure of 3 with H atoms are omitted for
clarity. Thermal ellipsoids drawn at 50% probability.
1
Scheme 2 depicts the proposed pathway for the formation
of catalyst 2. Carbonyl addition to tungsten alkylidynes bearꢀ
ing trianionic pincer ligands was recently reported by our
group.18 Thus, the first step involves cycloaddition and
cleavage of CO2 across the alkylidyne to give a tungstenꢀ
oxoꢀketene intermediate. It is noteworthy to mention here
that the cleavage of CO2 across a metalꢀcarbon triple bond is
unprecedented, with only cycloaddition, but not cleavage,
achieved by Fischer.19 In contrast, CO2 cycloaddition and
cleavage across both early20 and late21 metalꢀcarbon double
bonds is more common. Strong support for the ketene interꢀ
mediate comes in the form of an isolable analog employing
an ONO3ꢀ trianionic pincer ligand.22 However, in this OCO3ꢀ
case, the ketene moiety is unstable and instead inserts into
monomer/catalyst ratio
[b]
[d]
[mon/cat]0
25 : 1
[monomer]0
yield (%) %cis[c]
Mn[d](kDa)
126
Mw/Mn
1.24
0.1
0.1
0.1
0.1
97
97
92
60
97
97
98
97
50 : 1
197
1.25
100 : 1
200 : 1
248
1.21
578
1.29
[a]
The appropriate amount of a 1 mg/mL solution of catalyst dissolved in
toluene is added to 40 mg of norbornene dissolved in toluene and stirred for 7
h at room temperature. [b] mol.Lꢀ1. [c] Determined by 1H NMR spectroscopy. [d]
Determined by size exclusion chromatography.
the metalꢀaryl bond of the pincer ligand to give
2.
Scheme 3. Polymerization of norbornene by catalyst 2 to
generate cyclic polynorbornene.
Scheme 2. Proposed pathway for the formation of complex 2
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
O
tBu
O
W
O
O
W
O
O
W
O
O
W
O
C
tBu
C6D6
-THF
O
C
C O
+
CO2
THF
O
O
O
W
O
O
O
THF
THF
THF
THF
toluene
7 h, rt
O
n
+
THF
tBu
tBu
tBu
tBu
1
tBu
2 (1 mol %)
2
(n-2)
2
Complex 3 is independently isolable from complex 2. Further
heating of the initial reaction mixture leads to complete converꢀ
sion of 2 to 3, as monitored by NMR spectroscopy. The initial
formation of complex 2 and 3 in a 9:2 ratio and the eventual conꢀ
version of complex 2 into 3 upon heating, suggests the reaction is
reversible and that complex 3 forms via attack at the αꢀcarbon of
the ketenylide and subsequent loss of CO. Consistent with the
mechanism, a sample of the headspace reveals CO as determined
by GCꢀMS (see ESI). Slow evaporation of a concentrated solution
of 3 in a pentane/Et2O mixture yields single crystals suitable for
Xꢀray diffraction. One of the aryloxides from the trianionic
OCO3ꢀ pincer ligand bridges the two tungsten atoms (W1A and
W1B, Figure 2). Bridges created by trianionic pincer ligands were
noted in two previous structures by our group23 and Bercaw’s.24
Alkylidene W1AꢀC1A and W1BꢀC1B bond lengths of 1.884(4) and
1.885(4) Å are slightly shorter by 0.021(4) than the W=C double
bond in the only related tetraanionic pincer OCO4ꢀ Wꢀ
alkylidene.25 The empirical formula of dimer 3 implies the loss of
CO from 2; however, the mechanism of this transformation is still
under investigation. Addition of a few drops of THF to the reacꢀ
tion mixture of 2 and 3 prevents the conversion of 2 into 3 even
upon heating, implying loss of THF is an integral step in the forꢀ
mation of 3.
Cyclic polynorbornene produced with catalyst 2 is syndiotactic
(>98%), as determined by a comparison to 13C NMR data of preꢀ
viously reported syndiotactic linear polynorbornene.26 Tacticity is
also supported by postꢀfunctionalization of polynorbornene via
bromination, as recently described by Schrock and coworkers.26b
The brominated polymer exhibits two doublets at 3.84 ppm (J =
11.2 Hz) and 3.81 ppm (J = 10.3 Hz) (ESI, Figure S24, top). Conꢀ
sistent with reported cis, syndiotactic polynorbornene, irradiating
the methine protons at 2.61 ppm results in two singlets (ESI, Figꢀ
ure S24, bottom).26b Further evidence for high syndiotacticity
comes from polymerization of the chiral monomer
bis((menthyloxy)carbonyl)norbornadiene (BMCNBD). COSY
NMR is able to distinguish between isotactic and syndiotactic
poly(BMCNBD). In the case of a cis isotactic sample, the olefinic
protons are inequivalent, and therefore couple in a COSY NMR
spectrum. However, a cis syndiotactic poly(BMCNBD) contains
equivalent olefinic protons related by a C2 axis, and thus do not
couple.27 Poly(BMCNBD) produced by 2 does not exhibit any
coupling between the olefinic protons, indicating the polymer is
syndiotactic (Figure S22).
Since the alkylidene in 2 is tethered to the ligand backbone,
REMP is the expected mechanism of polymerization (Scheme 4).
Through REMP, the growing polymer chain remains attached by
both ends to the catalyst throughout the polymerization process
2
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(see intermediate A), and releases a cyclic polynorbornene after
2.25
2.00
1.75
1.50
1.25
1.00
1
2
3
undergoing intramolecular chain transfer via backbiting. Though
less probable, backbiting can occur to regenerate initiator 2, or
instead; backbiting can occur at any of the other C=C bonds to
give intermediate B. The proposed mechanism is consistent with
the observation of slow initiation/fast propagation since metatheꢀ
sis within complex 2 occurs at a disubstituted, sterically hindered
(tBu) alkylidene, whereas; after the initial ring opening event the
new alkylidene is monoꢀsubstituted and relatively unhindered.
cyclic polyNBE
linear polyNBE
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5
6
7
8
9
Scheme 4. Proposed mechanism of REMP of norbornene
to give highly cis and syndiotactic cyclic polynorbornene.
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4.8
5
5.2
5.4
Log (Molar Mass (g/mol))
Figure 4. MarkꢀHouwinkꢀSakurada plot, log [ƞ] versus log M.
A plot of log of molar mass versus elution volume (Figure 3)
shows that the cyclic polynorbornene samples with the same moꢀ
lar mass elute later than their linear counterparts, consistent with
their smaller hydrodynamic volume. A MarkꢀHouwinkꢀSakurada
(MHS) plot (log [ƞ] versus log M, where [ƞ] is the intrinsic visꢀ
cosity and M is the viscosityꢀaverage molar mass (Figure 4) conꢀ
firms the lower intrinsic viscosity of the cyclic polymers relative
to the linear polymers. The experimental ratio [η]cyclic/[η]linear of
0.34 over a range of molecular weights are in good agreement
with the theoretical value of 0.4.9a Additionally, MHS parameters
a of 0.76 and 0.71 for the linear and cyclic samples, respectively,
were determined from the slope of the MHS plots. This result
indicates that both polymers behave as flexible random coils in
solution, meaning the observed differences are caused by different
behavior of the polymers in solution. In addition a plot of mean
Size exclusion chromatography (SEC) equipped with multiꢀangle
light scattering (MALS) and viscosity detectors provide compelꢀ
ling data for a cyclic topology. Cyclic polymers have lower intrinꢀ
sic viscosities and smaller hydrodynamic volumes than their lineꢀ
ar analogs. Catalyst that produce linear polynorbornene with high
cis selectivity (>95%) and syndiotacticity (>95%) are known,28
2
square radius of gyration (<Rg >) versus molar mass (see ESI
Figure S27) obtained for cyclic and linear samples of polyꢀ
norbornene provides a <Rg >cyclic/<Rg >linear ratio of 0.4±0.1,
which is within the experimental error of the theoretical value of
0.5.30
and
a sample was synthesized utilizing Grubbs catalyst
Ru(NHC(Ad)(Mes))(=CH(PhOiPr))(η2ꢀNO3) (4).29
2
2
Table 2. Mn, Mw/Mn, cisꢀselectivity and tacticity of cyclic/linear
poly(NBE).
Catalyst
Mn[a] (kDa)
113
114,
Mw/Mn
1.16
%cis[b]
>98
Tacticity[c]
syndiotactic
syndiotactic
In summary, tethering an alkylidene to a substitutionally inert
ancillary ligand is an effective design for creating catalysts capaꢀ
ble of REMP. Unique to this system, CO2 cleavage across the
metalꢀcarbon triple bond of complex 1 leads to the tethered alꢀ
kylidene catalyst 2. Though other tethered group VI alkylidene
complexes are known,10a complex 2 is the first to function as a
REMP catalyst. Comparing the polymers produced by catalyst 2,
against cis and syndiotactic rich linear analogs permits their conꢀ
clusive assignment as cyclic polymers. To the best of our
knowledge this is the first catalyst to produce cyclic polyꢀ
norbornene with such high stereocontrol.13 The cisꢀselectivity is
presumably due to the steric congestion imposed by the tBuOCO4ꢀ
ligand, and the observed syndiotacticity indicates addition of the
monomer occurs with alternating stereoselectivity. The monomer
adds to one face of the M=C bond first, and in the next addition, it
adds to the opposite face of the alkylidene. This type of “stereoꢀ
genic metal control” occurs when the configuration of the metal
center changes after each monomer addition.1a Complex 2 proꢀ
vides new avenues for exploration of not only cyclic polymers,
but also catalyst design. Considering the extensive history of earꢀ
lyꢀmetal alkylidenes, surely other tethered versions can be deꢀ
signed to initiate REMP.
2 (cyclic)
4 (linear)
2.34
>95
[a]Absolute molecular weights determined by SECꢀMALS. [b] Determined
by 1H NMR spectroscopy. [c] Determined by 13C NMR spectroscopy.
6.5
cyclic polyNBE
6.0
linear polyNBE
5.5
5.0
4.5
4.0
15
16
17
18
Elution Volume (mL)
Figure 3. Plot of log of molar mass versus elution volume.
ASSOCIATED CONTENT
Supporting Information
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Full experimental procedures, NMR spectra, Xꢀray crystalloꢀ
574ꢀ579.(e) Zhang, K.; Tew, G. N. React. Funct. Polym. 2014, 80, 40ꢀ
1
2
graphic and polymer characterization data. This material is availꢀ
able free of charge via the Internet at http://pubs.acs.org.
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Corresponding Author
veige@chem.ufl.edu
ACKNOWLEDGMENT
This material is based upon work supported by the National Sciꢀ
ence Foundation CHEꢀ1265993 (ASV). KAA acknowledges the
NSF (CHEꢀ0821346) for the purchase of Xꢀray equipment.
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