Bimetallic Chromium Catalysts with Chain Transfer Agents: A Route to Isotactic Poly(propylene oxide)s with Narrow Dispersities

Article abstract of DOI:10.1002/anie.201801380

Bimetallic chromium catalysts are investigated for the enantioselective polymerization of propylene oxide. The catalyst is composed of two salen chromium species linked by an alkyl chain, the length of which significantly impacts the rate of polymerization. While the use of a chloride initiator on the catalyst resulted in bimodal molecular weight distributions, switching to a trifluoroacetate initiating group and adding a diol chain transfer agent afforded polymers of controllable molecular weight with low, unimodal dispersities.

Full text of DOI:10.1002/anie.201801380

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Bimetallic Chromium Catalysts with Chain Transfer Agents:
A New Route to Isotactic Poly(propylene oxide)s with Narrow
Dispersities
Authors: Lilliana S. Morris, Matthew Ian Childers, and Geoffrey William
Coates
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801380
Angew. Chem. 10.1002/ange.201801380
Link to VoR: http://dx.doi.org/10.1002/anie.201801380
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10.1002/anie.201801380
Angewandte Chemie International Edition
COMMUNICATION
Bimetallic Chromium Catalysts with Chain Transfer Agents: A New
Route to Isotactic Poly(propylene oxide)s with Narrow Dispersities
Lilliana S. Morris, M. Ian Childers, and Geoffrey W. Coates*
8d]
and high dispersities (Figure 1, 1st Generation).[8b,
In a
Abstract: Bimetallic chromium catalysts are investigated for the
enantioselective polymerization of propylene oxide. The catalyst is
composed of two salen chromium species linked by an alkyl chain,
the length of which significantly impacts the rate of polymerization.
While the use of a chloride initiator on the catalyst resulted in bimodal
molecular weight distributions, switching to a trifluoroacetate initiating
group and adding a diol chain transfer agent afforded polymers of
controllable molecular weight with low, unimodal dispersities.
subsequent effort to better control polymer molecular weight and
develop a more synthetically accessible catalyst, we explored the
use of a bimetallic (salalen)Cr(III) catalyst linked through a
binaphthol moiety. When used in conjunction with chain shuttling
agents, this catalyst affords iPPO of targeted molecular weight
with moderate control of dispersity (Figure 1, 2nd Generation).[2b]
However, the relatively high loading of chain shuttling agent
necessary to control dispersity hinders the synthesis of higher
molecular weight polymers.
In an effort to increase molecular weight control through
catalyst flexibility and modularity, we examined the bimetallic
(salen)Cr(III) catalysts originally prepared by Jacobsen and
Atactic poly(propylene oxide) (aPPO) is an important
commercial polymer that is primarily used as a component in
polyurethane synthesis.[1] Whereas aPPO is amorphous, isotactic
PPO (iPPO) is a semi-crystalline analogue that has the promise
lead to improved properties and therefore new applications.
However, there are few catalysts that display sufficient
stereocontrol to produce semi-crystalline polymers from racemic
epoxides. Furthermore, these systems generally suffer from poor
control of molecular weight and high dispersity.[2] Yet, industrially,
use of telechelic polyethers in polyurethanes requires low
dispersities. We therefore sought a new class of catalysts that
would enable the synthesis of isotactic PPO with low dispersities
from racemic propylene oxide.
P
R
O
M
O
M
~6 Å
Optimal M-M Separation
tBu
P
t
R
Recent advances in catalyst development for the synthesis
of atactic polyethers with precise control of molecular weight[3]
inspired us to develop a catalyst that would give isotactic
polyethers with similar control. To achieve a higher level of
Bu
O
N
N
O
Co
O
tBu
N
O
O
Co
N
O
isotacticity while maintaining
a
low dispersity, anionic
tBu
polymerizations using enantiopure epoxides have been used.[4]
However, the high cost of enantiopure epoxides limits the
economic feasibility of these methods for large scale applications.
By contrast, polymerizing racemic epoxides with enantioselective
or isoselective catalysts avoids the additional kinetic resolution
step.[2a] Several catalysts have been reported to produce partially
isotactic polyether from a chain-end control mechanism.[5] Some
uranyl complexes produce semi-crystalline PPO with controlled
molecular weight, though the resulting polymer has relatively low
isotacticity and Tm (40 °C,[6] compared to 67 °C for the highly
isotactic polymer). Recently, dibutyl magnesium was used to
synthesize isotactic PPO of targeted molecular weight with
dispersities as low as 1.4, although increasing to 2.3 at higher
molecular weights.[7]
Our group has developed several catalyst systems for the
iso- and enantioselective polymerization of epoxides, ranging
from monometallic (salen)Co(III) catalysts to bimetallic
(salen)Co(III) and (salalen)Cr(III) catalysts that rapidly polymerize
epoxides with excellent stereocontrol, with selectivity factors (krel)
exceeding 40 (Figure 1).[2b, 8] While the (salen)Co(III) complex
enabled access to a wide array of isotactic poly(epoxide)
architectures, it produced polymers with uncontrolled Mn values
1st Generation
• rigid binaphthol linker
• highly enantioselective
• poor MW control
tBu
P
t
R
Bu
Me
N
O
N
O
Cr
O
tBu
N
O
O
Cr
Me
O
tBu
N
2nd Generation
• rigid binaphthol linker
• highly isoselective
• moderate MW control
tBu
R
P
N
N
O
O
Cr
N
tBu
tBu
Cr
O
O
O
O
N
tBu
tBu
tBu
3rd Generation (this work)
• flexible alkyl linker
• excellent enantioselectivity
• excellent MW control
[*] L. S. Morris, Dr. M. I. Childers, Prof. G. W. Coates
Department of Chemistry and Chemical Biology
Baker Laboratory, Cornell University
Ithaca, NY 14853-1301 (USA)
E-mail: coates@cornell.edu
Figure 1. Stereoselective epoxide ring-opening polymerization with three
generations of bimetallic catalysts.
Supporting information for this article is given via a link at the end of the
document
This article is protected by copyright. All rights reserved.

10.1002/anie.201801380
Angewandte Chemie International Edition
COMMUNICATION
coworkers for the asymmetric ring opening of cyclopentene oxide
with Me3Si-N3 (Table 1).[9] Chen and co-workers used the pimelic
acid linked variant of these catalysts (5-Cl) to study gas phase
oligomerization of propylene oxide using mass spectrometry, as
well as the solution polymerization. In the gas phase, the authors
proposed a cationic oligomerization mechanism, and in neat
monomer at 80 °C, they reported the formation of partially
isotactic polymer, although presumably with low molecular
weight as end groups were observed in the 13C NMR
spectrum.[10] In the presence of a 4-(dimethylamino)pyridine
(DMAP) cocatalyst, the average turnover frequency of the
catalyst increased from 49 h-1 to 63 h-1. More recently, Nozaki
and Nakano have investigated the use of a cobalt(III) variant of
5-Cl for the copolymerization of epoxides and carbon dioxide.[11]
We chose to further examine the polymerization of
propylene oxide using bimetallic complex 5-Cl. In contrast to
Chen’s study, we found negligible polymer formation when
diluting the monomer to 4.8 M and cooling the reaction
temperature to 22 °C, even when adding DMAP as a potential
cocatalyst (Table 1, entries 1,2). Given the significant activity
improvement of our first and second generation epoxide
Table 1. Mono- and bimetallic (salen)CrCl complexes used in this study,
and screening of catalyst/cocatalyst systems for propylene oxide
polymerization.
(R)
(R)
N
N
O
Cr
Cl
O
O
O
nBu
tBu
tBu
mono-Cl
tBu
(R)
(R)
(R)
(R)
N
N
O
N
O
N
O
O
Cr
Cl
Cr
Cl
tBu
O
O
tBu
O
O
n
tBu
tBu
tBu
tBu
n-Cl (n = 4-7)
5-Cl or mono-Cl
cocatalyst
O
O
O
O
+
+
22 °C
m
polymerization catalysts by adding [PPN]Cl ([PPN]+ = [Ph3P–
Entry[a]
Catalyst
Cocatalyst
Conv.[b]
(%)
TOF[c]
(h-1)
N=PPh3]+),[2b,
we examined the addition of this potential
8b-8f]
cocatalyst. Notably, this resulted in rapid polymerization, with an
average catalyst turnover rate of 520 h-1 (Table 1, entry 3) to give
highly isotactic PPO, warranting further examination. The
monometallic model (mono-Cl) for 5-Cl was ineffective for
propylene epoxide under all conditions tested (Table 1, entries
4–6), strongly suggesting the necessity of a bimetallic complex.
We hypothesized that the flexible alkyl linkers in this
catalyst allow for more conformational freedom of the active
salen units compared to the relatively rigid binaphthol linker used
in our 1st and 2nd generation catalysts (Figure 1). This increased
freedom allows the salen units to adopt the ideal conformation
for cooperative ring opening; it is therefore necessary to optimize
the linker length to facilitate this geometry. Similarly, when
Jacobsen and co-workers employed these catalysts for the ring
opening of epoxides, the number of methylene units between
metal centers affected the activity. Optimal linker lengths were
between five (5-Cl) and six (6-Cl) methylene units; catalysts with
other lengths were significantly less active.[9] As shown in Table
2, for polymerization of PO, 4-Cl generates little conversion
[d]
1
2
3
4
5
6
5-Cl
5-Cl
5-Cl
None
DMAP
[PPN]Cl
None
<2
<2
39
<2
<2
<2
-
[d]
-
520
[d]
mono-Cl
mono-Cl
mono-Cl
-
[d]
DMAP
[PPN]Cl
-
[d]
-
[a] Polymerization conditions: 1 mL PO; [PO] = 4.8 M in dimethoxyethane
(DME); [PO]:[Cr]:[cocatalyst] = 8,000:2:1; trxn = 6 h. [b] Determined
gravimetrically. [c] (mol PO consumed) • (mol catalyst)-1 • trxn-1. [d] Not
determined. DMAP = 4-(dimethylamino)pyridine. [PPN] = [Ph3P–N=PPh3].
Table 2. Effect of linker length of bimetallic (salen)CrCl catalysts on propylene oxide polymerization.
22 °C
[PPN]Cl
(R)
(R)
(R)
(R)
N
N
N
N
O
O
O
O
O
O
+
+
Cr
Cl
Cr
Cl
tBu
tBu
O
O
O
O
O
O
m
n
tBu
tBu
tBu
tBu
n-Cl (n = 4-7)
[g]
Entry[a]
Catalyst
Conv.[b]
(%)
TOF [c]
(h-1)
Mn theo.[d]
(kDa)
Mn GPC[e]
(kDa)
Ð
[mm][f]
(%)
[mr]+[rm][f]
[rr][f]
(%)
krel
(%)
[h]
[h]
[h]
[h]
[h]
[h]
1
2
3
4
4-Cl
5-Cl
6-Cl
7-Cl
4
39
47
21
53
520
627
280
16.0
185.9
220.8
104.7
-
-
-
-
-
-
67.8
48.2
35.8
1.38
1.29
1.33
88.2
87.9
91.8
8.0
8.2
5.8
3.8
3.9
2.4
42
59
40
[a] Polymerization conditions: 1 mL PO; [PO] = 4.8 M in DME; [PO]:[catalyst]:[PPNCl] = 8,000:1:1; trxn = 6 h. [b] Determined gravimetrically. [c] (mol PO
consumed) • (mol catalyst)-1 · trxn-1. [d] (mg polymer) • (mmol catalyst)-1. [e] Determined by GPC relative to polystyrene standards at 30 °C in THF. [f] Determined
by 13C NMR spectroscopy. [g] See Supplementary Information for calculation. [h] Not determined.
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10.1002/anie.201801380
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COMMUNICATION
Table 3. Polymerization of PO with bimetallic (salen)CrOAcF3 catalysts of varied linker length, and effect of amount of 1,6-hexanediol CTA.
22 °C
[PPN][OAcF3
]
(R)
(R)
(R)
(R)
N
N
N
N
O
O
O
O
O
H
O
O
+
+
Cr
Cr
O
O
H
3
m
tBu
O
O
tBu
O
O
O
O
OAcF3
n
m
OAcF3
HO
n-OAcF3 (n = 4-7)
OH
tBu
tBu
tBu
tBu
3
Entry[a]
Catalyst
[CTA]:
[cat]
Conv.[b]
(%)
TOF [c]
(h-1)
Mn [d] theo.
(kDa)
Mn [e] GPC
(kDa)
Ð
[mm][f]
(%)
[mr]+[rm][f]
[rr][f]
(%)
(%)
1
2
3
4
5
6
7
8
6-OAcF3
6-OAcF3
6-OAcF3
6-OAcF3
6-OAcF3
4-OAcF3
5-OAcF3
7-OAcF3
0
5
48
47
46
38
40
<2
35
12
640
627
613
507
533
220.8
27.2
19.4
11.1
9.5
47.3
31.0
22.0
15.6
13.6
1.42
1.14
1.08
1.05
1.05
86.1
87.8
88.3
91.2
90.4
9.7
8.4
8.0
6.0
6.6
4.2
3.8
3.7
2.8
3.0
10
15
20
10
10
10
[h]
[h]
[h]
[h]
[h]
[h]
[h]
-
-
-
-
-
-
-
467
160
15.3
4.9
18.3
7.9
1.08
1.06
89.7
92.2
7.0
5.5
3.3
2.3
[a] Polymerization conditions: 1 mL PO; [PO] = 4.8 M in DME; [PO]:[1,6-hexanediol]:[catalyst]:[PPNOAcF3] = 8,000:10:1:1; 22 °C; trxn = 6 h. [b] Determined
gravimetrically. [c] (mol PO consumed) • (mol catalyst)-1 · trxn-1. [d] (mg polymer) • (mmol catalyst + mmol alcohol)-1. [e] Determined by GPC relative to polystyrene
standards at 30 °C in THF. [f] Determined by 13C NMR spectroscopy. [g] See Supplementary Information for calculation. [h] Not determined.
(Table 2, entry 1), whereas 5-Cl is much more active with a krel of
42 (Table 2, entry 2). While increasing the linker length to six
methylene linkages (6-Cl) decreased activity in Jacobsen’s small
molecule system, this catalyst exhibits the highest activity and krel
(59) for PO polymerization (Table 2, entry 3). However, further
increase in the linker length to seven methylene units (7-Cl)
decreases both turnover frequency and enantioselectivity (Table
2, entry 4). None of the catalysts show activity for terminal
epoxides with larger substituents.
In addition to the excellent activity and enantioselectivity of
these catalysts for the kinetic resolution of PO, the corresponding
polymers display low dispersities. Catalysts 5-Cl, 6-Cl and 7-Cl
offer a remarkable improvement over our previous systems,
producing dispersities as low as 1.3–1.4. We propose that the
more flexible linking unit and the attenuated rate of polymerization
relative to initiation lower the observed dispersities. Further
examination of the gel-permeation chromatography (GPC)
chromatograms revealed a bimodal molecular weight distribution,
where one peak was approximately twice the molecular weight of
the other. The high molecular weight peak was attributed to
polymer chain initiation from adventitious water,[12] while the low
molecular weight peak arose from polymer chains initiated by
chloride anions. Previous work by Darensbourg and Williams
demonstrated the use of a trifluoroacetate anion initiators and
additional water as a chain transfer agent (CTA) to decrease
dispersities of the produced polymers.[12a-d]
To avoid the complexities of introducing additional water
into our polymerization system, we used 1,6-hexanediol as a
difunctional CTA when exploring trifluoroacetate as an initiating
group. As expected for an enantioselective polymerization, the
isotacticity of the polymers remained high in the presence of
increasing CTA, with [mm] above 86% for all cases (Table 3,
entries 1–5). Increasing CTA loading led to lower molecular
weights and dispersities (Figure 2). While adding 20 equivalents
of CTA afforded the lowest dispersity (1.05, Table 3, Entry 5), 10
equivalents of CTA yielded a polymer of nearly comparable
dispersity (1.08, Table 3, Entry 3) but notably higher molecular
weight (19.4 kDa compared to 9.5 kDa). Therefore these
conditions were determined to be the ideal loading of CTA for
polymerizations targeting moderate molecular weight polymers.
The dispersities of <1.10 are comparable to other controlled
radical or living anionic polymerization systems.
Table 3, Entry 1
Table 3, Entry 2
Table 3, Entry 3
Table 3, Entry 4
Table 3, Entry 5
14
16
18
20
22
Retention Time (min)
Figure 2. The effect of increasing equivalents of 1,6-hexanediol CTA on
polymer molecular weight and dispersity as shown by normalized GPC
chromatography.
With a the new system in hand, we then reevaluated the
activity of the different linker lengths under the optimal conditions
(Table 3, entries 6–8). Analogous to the chloride derivatives, 4-
OAcF3 exhibited no conversion. 5-OAcF3 achieves 35
%
conversion compared to the 46 % observed for 6-OAcF3 and the
12 % observed for 7-OAcF3. Activity and enantioselectivity were
again related, as catalysts 5-OAcF3, 6-OAcF3, and 7-OAcF3 have
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10.1002/anie.201801380
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COMMUNICATION
krel values of 45, 53, and 47, respectively. Therefore, 6-OAcF3 with
a 1,6-hexanediol CTA was confirmed as the most active and
enantioselective system. To validate the living nature of the
polymerization, we monitored Mn and Ð as a function of
conversion and found that Mn increased linearly while Ð remained
less than 1.1 (Figure 3).
References
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25
20
15
10
5
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0
0
10
20
30
40
50
Conversion (%)
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Figure 3. Mn and Ð as a function of conversion for the polymerization of PO by
6-OAcF3 using the conditions in Table 3, Entry 3.
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In conclusion, the polymerization of PO with a bimetallic
complex comprised of two (salen)CrOAcF3 centers linked by six
methylene units (6-OAcF3), optimized [PPN][OAcF3] cocatalyst,
and 1,6-hexanediol as a chain transfer agent produces low
dispersity, isotactic, and semi-crystalline PPO with moderate
molecular weight. We expect that these new materials will be
important to numerous fields in which well-defined polymers are
desired. We continue to explore similarly linked (salen)Cr(III)
complexes as catalysts for polymerization of PO and other
epoxides.
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Experimental Section
For information regarding experimental procedures and materials,
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Acknowledgements
We thank the Office of Naval Research (N00014-14-1-0551) for
financial support of this work. This work made use of the Cornell
University NMR Facility, which is supported, in part, by the NSF
under Award Number CHE-1531632. We thank Mr. Anthony
Condo for HRMS analysis.
Keywords: Polymerization • Polyether • Homogeneous Catalyst
• Chain Transfer • Bimetallic
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10.1002/anie.201801380
Angewandte Chemie International Edition
COMMUNICATION
tBu
O
Bimetallic catalysts composed of two
chromium salen species linked by an
alkyl chain were explored for epoxide
polymerization. Complexes tethered
by five or six methylene linkers were
highly active and enantioselective for
propylene oxide polymerization. Using
Lilliana S. Morris, M. Ian Childers,
Geoffrey W. Coates *
O
tBu
N
X
Me
Cr
N
O
tBu
Page No. – Page No.
O
O
O
O
O
tBu
O
Bimetallic Chromium Catalysts with
Chain Transfer Agents: A New Route
to Isotactic Poly(propylene oxide)s
with Narrow Dispersities
N
Cr
Me
X
O
P
N
tBu
tBu
a
diol chain transfer agent and
O
trifluoroacetate initiating groups,
Bimetallic Epoxide Polymerization
polymers of controllable molecular
weight with low, unimodal dispersities
were formed.
• excellent enantioselectivity (krel = 59)
• excellent MW control (< 1.1)
• excellent activity (> 500 TO/h)
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