Isospecific polymerization of racemic epoxides: A catalyst system for the synthesis of highly isotactic polyethers

Article abstract of DOI:10.1039/b926888j

A highly active bimetallic cobalt catalyst system is reported for the polymerization of racemic terminal epoxides to yield isotactic polyethers. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b926888j

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Isospecific polymerization of racemic epoxides: a catalyst system
for the synthesis of highly isotactic polyethersw
Peter C. B. Widger, Syud M. Ahmed, Wataru Hirahata, Renee M. Thomas,
Emil B. Lobkovsky and Geoffrey W. Coates*
Received (in Berkeley, CA, USA) 21st December 2009, Accepted 15th February 2010
First published as an Advance Article on the web 22nd March 2010
DOI: 10.1039/b926888j
A highly active bimetallic cobalt catalyst system is reported
for the polymerization of racemic terminal epoxides to yield
isotactic polyethers.
Stereoregular polymers often display superior mechanical and
thermal properties compared to their atactic analogues.1
Polyethers are an important class of polymers with industrial
and biological applications.2 Much progress has been made
in controlling the regiochemistry and molecular weight of
polyethers,3 but controlling the stereochemistry of polyethers
synthesized from racemic epoxides remains a challenge.4
Previous isoselective polymerization systems for racemic
epoxides produced mixtures of atactic and isotactic materials,
or generated only moderately isotactic polymers.5 We
previously reported a highly active and isoselective cobalt
catalyst for the polymerization of racemic propylene oxide
(rac-PO).6 Using the information gained by studying the
mechanism of this catalyst,7 we have recently designed a highly
active and enantioselective polymerization catalyst system that
exhibits greater substrate scope. It consists of a bimetallic
cobalt complex (1) and a bis(triphenylphosphine)iminium
(PPN) acetate cocatalyst (4, Scheme 1). This system selectively
polymerizes one enantiomer of racemic terminal epoxides to
produce highly isotactic polyethers and enantiopure epoxides.8
Unfortunately, this limits the theoretical maximum yield of
polymer at ca. 50% unless the racemic form of 1 is used.
Scheme 1 Enantioselective and isospecific polymerization of racemic
Attempts to synthesize rac-1 (equimolar mixture of (R4S)-1
epoxides using bimetallic cobalt catalysts.
and (S4R)-1) from racemic starting materials produced
inseparable diastereomers, which led us to develop a new
monomer is enchained, possibly allowing simplification of 1
and 2 by removal of the chiral diamine linker.
catalyst that is active and isoselective while being derived from
inexpensive racemic and/or achiral starting materials.
Having determined that the axial chirality of the binaphthol
linker is responsible for the stereoselectivity of the poly-
merization, we synthesized complex 3, which incorporated
ethylene diamine and a rac-binaphthol linker.w All attempts
Insight into the origin of enantioselectivity of 1 was gained
through the reactivity of the diastereomeric cobalt complex 2,
which possesses all S stereochemistry. When activated with
PPN compound 4, complex 2 displayed lower activity than 1
for the polymerization of neat rac-PO (Trxn = 0 1C), but
preferred the same enantiomer (S) with a kS/kR selectivity
factor of 210.9 This result confirmed that the stereochemistry
of the binaphthol linker determines which enantiomer of
at crystallizing
3 from non-coordinating solvents were
unsuccessful, however crystals were grown from pyridine
and toluene giving black needles that were analyzed by single
crystal X-ray diffraction. Fig. 1 shows the structure of 3 with
hydrogens omitted and pyridine ligands truncated for clarity.
Two pyridines are bound to each cobalt center displacing the
chlorides giving a pseudo C2 symmetric complex.10 The
Co–Co distance is 6.774 A and the endo naphthol–naphthol
dihedral angle is 84.31.
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell University, Ithaca, NY 14853-1301, USA.
E-mail: gc39@cornell.edu; Fax: +1 607-255-4137;
Tel: +1 607-255-5447
w Electronic supplementary information (ESI) available: Experimental
procedures and crystallographic details. CCDC 759359. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b926888j
Initial polymerizations of neat rac-PO using catalyst 3 and
cocatalyst 4 (3/4) (Table 1, entry 1) were less active in comparison
with system 1/4, yielding regioregular poly(propylene oxide)
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Fig.
1
Molecular structure of the tetrapyridine adduct of 3
crystallized from pyridine and toluene (hydrogen atoms omitted and
pyridine ligands truncated for clarity; carbon atoms are unlabelled).
Thermal ellipsoids are at the 30% probability level.
Fig. 2 Methine regions of the 13C NMR spectra of PPO produced
using (a) catalyst 3 and cocatalyst 4; (b) catalyst 3 and cocatalyst 6.
(PPO) with moderate isotacticity (67% mm triad content).
Analysis of the polymer tacticity using 13C NMR spectroscopy
showed stereoerrors11 indicative of enantiomorphic site
control.w
In an effort to increase the activity of 3, we modified the
anions of PPN salts, which have been found to have
dramatic effects on the activity and selectivity of PO/CO2
copolymerizations.12 We investigated the use of cocatalyst 5,
and found that when combined with 3 the rate of polymerization
dramatically increased. When run neat, the reaction exothermed
vigorously; thus, further polymerizations were conducted in
toluene at 0 1C for 5 min. Cocatalyst 5 also dramatically
increased the selectivity, producing highly isotactic PPO with
96% mm triad content (Table 1, entry 2).
Scheme 2 Proposed mechanism of polymerization (P = polymer
chain; X = Cl or carboxylate).
create the polymer chain. Our working hypothesis is that the
increased bulk of 6 compared to 4 prevents its carboxylate
from entering the catalyst cleft, favoring its position anti to the
epoxide/polymer alkoxide.14
The new catalyst system (3/6) was screened for the poly-
merization of a number of terminal epoxides possessing
aliphatic, ether, vinyl, aromatic and fluorinated side chains
(Table 2). Catalyst system (3/6) produced racemic, highly
isotactic polyethers at rapid rates. Nearly all polymers had
molecular weights significantly higher than the theoretical
molecular weights calculated from one polymer chain per
metal complex. This is consistent with 3 being a mixture of
chloride diastereomers, of which only a fraction are catalytically
active.8 The molecular weight distributions (Mw/Mn) are ca. 2
consistent with a single site catalyst.
Two bulky achiral cocatalysts 6 and 7 were synthesized to
determine if the enhanced selectivity resulting from 5 originated
from its chirality or increased sterics. Both 6 and 7 were very
active for the polymerization of PO in the presence of 3 and
produced highly isotactic polymer (Table 1, entries 3 and 4).13
Cocatalyst 6 produced the highest rates and isoselectivity
(Fig. 2) when combined with 3. Thus 6 was used in all
subsequent polymerizations. The role of the cocatalyst anion
in the polymerization is currently unclear. We propose that the
cocatalyst anion coordinates to one cobalt center. An epoxide
molecule then coordinates to the adjacent cobalt center inside
the catalyst cleft, and is ring-opened by a nucleophile (Cl or
carboxylate, depending on initial position of Cl ligands) to
initiate polymerization (Scheme 2). Subsequent ring-opening
events occur alternatively at the adjacent cobalt centers to
Propylene oxide and 1-butene oxide were both polymerized
at rapid rates with high isoselectivities to polymers with high
molecular weight (Table 2, entries 1 and 2). Phenyl glycidyl
ether was polymerized very rapidly and displayed very high
isotacticity (entry 3). Butadiene monoepoxide was polymerized
rapidly, but had moderate isotacticity (entry 4). Poly(butadiene
Table 1 Polymerization of racemic propylene oxide with 3 and various co-catalystsa
Conv.b (%)
Mnc/kg molÀ1
Mw/Mn
[mm]d (%)
c
Entry
[PPN]X
trxn/min
Trxn/1C
1e
2
3
4
4
5
6
7
1440
20
0
0
29
43
55
7
57
153
134
47
2.2
1.7
1.8
3.6
67
96
97
96
5
5
5
0
a
b
General conditions: [3] : [PPN]X = 1 : 2, [3] : [PO] = 1 : 4000, and [PO] = 2 M in toluene. Determined by 1H NMR spectroscopy.
c
d
Determined by gel-permeation chromatography calibrated with polystyrene standards in 1,2,4-Cl3C6H3 at 140 1C. [mm] triad content
determined by 13C NMR spectroscopy. Reaction run in neat PO, [3] : [PO] = 1 : 2000.
e
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Table 2 Screening of racemic epoxides for isoselective polymeriza-
tion catalyzed by system (3/6)a
We have also synthesized a new cocatalyst (6) that increased
the rate and selectivity of 3. The catalyst system (3/6) displays
high activity, surpassing that of system (1/4). Future work will
focus on understanding the mechanism of this polymerization
system and the origin of the effect of the cocatalyst on activity
and selectivity.
c
Epoxide
subs. (R)
trxn
min
/
Conv.b
(%)
Mn
/
Mw/
Mn
[mm]d
(%)
c
Entry
kg molÀ1
1
2
3
4
5
6
Me
Et
CH2OPh
CHQCH2
Ph
5
5
1
10
45
90
55
60
89
46
23
32
134
239
328
212
77
1.8
1.5
1.4
1.5
1.9
13e
97
97
Z 97
92
Z 97
Z 97
We acknowledge the NSF (CHE-0809778) and the King
Abdullah University of Science and Technology (KAUST;
Award No. KUS-C1-018-02) for support of this research. The
authors are grateful for the help of T. Mourey at Kodak for
polymer molecular weight determination.
CF3
20e
a
General conditions: [3] : [6] = 1 : 2, [3] : [epoxide] = 1 : 4000,
b
Trxn = 0 1C and [epoxide] = 2 M in toluene. Determined by
1H NMR spectroscopy. Determined by gel-permeation chromato-
graphy calibrated with polystyrene standards in 1,2,4-Cl3C6H3 at
c
Notes and references
140 1C. [mm] triad content determined by 13C NMR spectroscopy.
Determined by gel-permeation chromatography calibrated with
d
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e
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0.01 M lithium nitrate and 1% formic acid at 35 1C.
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, S. Carlotti and
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merization, see: (a) M. Gervais, A. Labbe
´
A. Deffieux, Macromolecules, 2009,
c
Epoxide
Entry subs. (R)
Conv.b
(%)
Mn
/
Mw/ [mm]d
trxn/h
kg molÀ1
Mn
(%)
c
1
2
3
4
5
6
Me
Et
CH2OPh
CHQCH2
Ph
1
1
0.08
1
15
6
499
499
499e
499
499
499
107
163
278
135
59
1.8
1.6
1.3
2.1
2.0
6.9f
97
97
Z 97
92
Z 97
Z 97
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´
A. Deffieux,
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CF3
85f
a
General conditions: [3] : [6] = 1 : 2, [3] : [epoxide] = 1 : 1000,
b
Trxn = 0 1C and [epoxide] = 0.6 M in toluene. Determined by
molecules, 2003, 36, 5470–5481; (i) O. Rexin and R. Mulhaupt,
¨
1H NMR spectroscopy. Determined by gel-permeation chromato-
graphy calibrated with polystyrene standards in 1,2,4-Cl3C6H3 at
c
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¨
140 1C. [mm] triad content determined by 13C NMR spectroscopy.
Determined gravimetrically. Determined by gel-permeation
d
e
f
4 W. Kuran, Prog. Polym. Sci., 1998, 23, 919–992.
5 For a review, see: H. Ajiro, S. D. Allen and G. W. Coates, Discrete
Catalysts for Stereoselective Epoxide Polymerization, in ref. 1,
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6 K. L. Peretti, H. Ajiro, C. T. Cohen, E. B. Lobkovsky and
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9 This is in contrast to previous catalytic systems using (S,S)-
(salcy)CoOAc which have shown selectivity for the hydrolysis
of R-PO: M. Tokunaga, J. F. Larrow, F. Kakiuchi and
E. N. Jacobsen, Science, 1997, 277, 936–938.
chromatography calibrated with poly(methyl methacrylate) in N,N-
dimethylformamide containing 0.01 M lithium nitrate and 1% formic
acid at 35 1C.
monoepoxide) posses a pendant olefin potentially allowing for
functionalization. Styrene oxide was polymerized at a reason-
able rate with high tacticity (entry 5). Trifluoropropylene
oxide was polymerized slowly but the resulting polymer
displayed high isotacticity (entry 6). The polymer was insoluble
in traditional GPC solvents and had to be chromatographed
using DMF, making molecular weight comparisons difficult.
Racemic epoxides were quantitatively polymerized to racemic
isotactic polyether at slightly higher loading of catalyst and
higher dilution as shown in Table 3. The rates of polymerization
were quite rapid, and all polymers displayed high isotacticity.
In conclusion, we have shown that the axial chirality of the
binaphthol linker in our bimetallic cobalt catalysts is
responsible for enantioselectivity of monomer enchainment
in epoxide polymerization. We synthesized a new racemic
bimetallic cobalt complex (3) that can quantitatively poly-
merize racemic terminal epoxides to highly isotactic polyethers.
10 Pyridine was also used for NMR analyses of cobalt(III) complexes
giving diamagnetic spectra: S. Kemper, P. Hrobarik, M. Kaupp
´
and N. E. Schlorer, J. Am. Chem. Soc., 2009, 31, 6641.
¨
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13 A sample of enantiopure 3 synthesized from S-binaphtholen-
chained S-PO with a kS/kR selectivity factor of 160 when combined
with 6 at 0 1C.
14 L. P. C. Nielsen, C. P. Stevenson, D. G. Blackmond and
E. N. Jacobsen, J. Am. Chem. Soc., 2004, 126, 1360–1362.
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Chem. Commun., 2010, 46, 2935–2937 | 2937