Synthesis of poly(indene carbonate) from indene oxide and carbon dioxide - A polycarbonate with a rigid backbone

Article abstract of DOI:10.1021/ja208711c

The catalytic coupling of carbon dioxide with indene oxide utilizing (salen)Co(III)-2,4-dinitrophenoxide in the presence of an onium salt is presented. X-ray structural data for indene oxide monomer as well as cisindene carbonate display near planarity of the fused cyclopentene and benzene rings. Low temperature (0 °C) is required to selectively afford copolymer vs cyclic carbonate from the coupling reactions of CO₂ and indene oxide. The produced poly(indene carbonate) samples have molecular weights of up to 7100 Da, with corresponding glass transition temperatures of up to 134 °C, the highest yet reported for polycarbonates produced from CO₂/epoxides coupling. Poly(indene carbonate) is thermally stable up to 249 °C. The polymerizations are well controlled, with PDI values ≤1.3.

Full text of DOI:10.1021/ja208711c

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pubs.acs.org/JACS
Synthesis of Poly(indene carbonate) from Indene Oxide and Carbon
Dioxide—A Polycarbonate with a Rigid Backbone
Donald J. Darensbourg* and Stephanie J. Wilson
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
S Supporting Information
b
oxide copolymerization. Companies including Bayer, BASF,
ABSTRACT: The catalytic coupling of carbon dioxide with
indene oxide utilizing (salen)Co(III)-2,4-dinitrophenoxide
in the presence of an onium salt is presented. X-ray
structural data for indene oxide monomer as well as cis-
indene carbonate display near planarity of the fused cyclo-
pentene and benzene rings. Low temperature (0 °C) is
required to selectively afford copolymer vs cyclic carbonate
from the coupling reactions of CO2 and indene oxide. The
produced poly(indene carbonate) samples have molecular
weights of up to 7100 Da, with corresponding glass transi-
tion temperatures of up to 134 °C, the highest yet reported
for polycarbonates produced from CO2/epoxides coupling.
Poly(indene carbonate) is thermally stable up to 249 °C.
The polymerizations are well controlled, with PDI values
e1.3.
and Sumitomo are also conducting research into poly(propylene
carbonate) commercialization. These operations represent the
current industrial polycarbonate production processes involving
CO2/epoxide coupling utilizing homogeneous metal catalysts.
Still very important though far less numerous are investiga-
tions into other epoxide monomers. Monomers such as 1-butene
oxide,6 1-hexene oxide,6,7 and [2-(3,4-epoxycyclohexyl)ethyl]-
trimethoxysilane8 have been successfully polymerized with CO2.
Recently, Coates reported the co- and terpolymerization of CO2
with several different cyclohexene oxides that had been func-
tionalized at the monomer’s 4-position.9 Multiblock copolymers
with up to seven distinct cyclohexene oxide-based blocks
were formed. Also notable is the copolymerization of CO2 and
styrene oxide or epichlorohydrin to form polycarbonates with
Tg = 80 and 31 °C, respectively.3a,10,11
Despite the significant progress in this field, the catalytic
coupling of CO2 and epoxides is still lacking in the production
of polycarbonates with glass transition temperatures similar to
that of BPA polycarbonate. In an effort to expand upon this
horizon, our group published a 2004 report investigating the
coupling of 2,3-epoxy-1,2,3,4-tetrahydronaphthalene (1,4-dihydro-
naphthalene oxide) and CO2.12 This monomer was chosen so
that the bulky benzene rings would hinder molecular rotation of
the polymer backbone and therefore increase the Tg. However,
1,4-dihydronaphthalene oxide was found to couple with CO2 to
selectively provide cyclic carbonate. We have recently begun
reexamining these bulky epoxide monomers for polycarbonate
formation. As in the case of 1,4-dihydronaphthalene oxide, we
looked for available bulky olefins that we could convert to the
corresponding epoxide. Indene, consisting of fused cyclopentene
and benzene rings, is a readily available olefin found in 1 wt % of coal
tar.13 Its primary use is in the indeneꢀcoumarone resins (ICRs)
found in paints, printing inks, adhesives, rubber, etc. produced
globally at about 100 000 tonnes/yr.13,14 Following epoxidation to
indene oxide, this monomer represents a sound choice for CO2 cou-
pling with a bulky oxirane. In previous studies, indene,15 indene
oxide,16 trans-2-bromo-1-indanol,16 and cis-1,2-indandiol17 have
been employed as starting materials for the production of cyclic
indene carbonate. Herein we report both the production of cyclic
indene carbonate and the novel production of poly(indene
carbonate) through the catalytic coupling of indene oxide and
carbon dioxide (Scheme 1). To our knowledge, this marks the
first time that an aromatic group has been rigidly incorporated
into the polymer backbone utilizing CO2/epoxides coupling.
olycarbonates are widely used thermoplastic polymers found
P
in nearly every facet of our daily lives. The most common of
these, bisphenol-A (BPA) polycarbonate, has numerous useful
physical and mechanical properties, including high transparency
to visible light and a high glass transition temperature (Tg)
of ∼150 °C.1 Despite controversies regarding the safety of the
material for foodstuff containers, BPA polycarbonate remains a
valuable commodity in today’s world. BPA polycarbonate and
other polycarbonates are industrially produced through the poly-
condensation of a diol and phosgene or phosgene derivative in a
biphasic medium. More recently, a greener and more sustainable
commercial route has been developed by the Asahi Kasei Corp.
which involves a phosgene-free production of BPA polycarbo-
nate from bisphenol-A and diphenylcarbonate.2 Beginning with
Inoue’s discovery in 1969, the catalytic coupling of oxiranes
(epoxides) and carbon dioxide has become a viable alternative for
the production of select polycarbonates.3
Most investigations of CO2/epoxide copolymerization processes
utilize cyclohexene oxide and propylene oxide monomers.4 While
the production of poly(cyclohexene carbonate) has numerous
advantages (i.e., fast reaction time, relatively high Tg of ∼115 °C,
a variety of catalysts capable of production, typically low con-
version to cyclic carbonate), the resulting polymer is brittle in
nature and currently without many practical uses. On the other
hand, poly(propylene carbonate) is an amorphous polymer with
excellent mechanical properties but a rather low Tg (∼41 °C)
that limits its use to applications as ceramic binders, adhesives,
coatings, and packaging materials.5 Presently, Novomer in the
US and SK in Korea have undertaken the industrial production of
poly(propylene carbonate) through catalytic CO2/propylene
Received: September 21, 2011
Published: October 24, 2011
r
2011 American Chemical Society
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The non-commercially available indene oxide is well known to
be both acid-sensitive and labile.18 On the basis of our own
experimental observation of Meinwald rearrangement of indene
oxide to 2-indanone when utilizing mCPBA as an oxidant19
(see Supporting Information), the alternate stepwise synthesis
through formation of the bromohydrin and ring-closing using
NaOH was settled upon (Scheme 2).20,21 Purification by sub-
limation afforded pure rac-indene oxide capable of coupling with
CO2. It is worth noting that indene oxide is slightly hygroscopic
and must be transferred into an inert atmosphere immediately
following sublimation.
Indene oxide crystals suitable for X-ray diffraction were
obtained via sublimation (Figure 1). The fused benzene and
cyclopentene rings are nearly planar, with a C8ꢀC5ꢀC3ꢀC1
torsion angle of 1.591°. This differs from crystals grown of 1,4-
dihydronaphthalene oxide, where a distinct pucker is found
within the cyclohexane ring.12
On the basis of its successes with the difficult copolymeriza-
tion of CO2 and styrene oxide,10a we chose to initially employ N,
N0-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino
cobalt(III)-2,4-dinitrophenoxide (1) as the catalyst along with
PPN-2,4-dinitrophenoxide as cocatalyst. Both resolved [(S,S)- or
(R,R)-] and racemic versions of catalyst 1 were employed in
this study.
Indene oxide melts at ∼32ꢀ33 °C, and thus initial coupling
reactions of neat indene oxide and CO2 were performed at or
above 35 °C (Table 1, entries 1 and 2). Ring-opening of the
epoxide is typically the rate-limiting step at high pressure of
CO2,4a and thus the differences in CO2 pressures for the two
entries can be largely ignored. As expected, the increase in
reaction temperature from 35 to 60 °C is mirrored by an increase
in both conversion to cyclic carbonate from 11.5% to 71.3% and
1
turnover frequency (TOF) from 0.80 to 5.01 hꢀ1. In the H
NMR and FT-IR spectra of the crude reaction products, no
polymer formation was observed in either case.
The production of poly(styrene carbonate) from the same
catalytic system was enhanced and more selective at lower
temperatures.10a Thus, we decided to employ cosolvents and
study the coupling of CO2 and indene oxide both at and below
room temperature. Despite the potential for HCl poison in
dichloromethane, CH2Cl2 was chosen as the solvent due to
PPNX salts’ inability to dissolve in toluene. Similar toluene-
soluble nBu4NX cocatalysts are inherently wet with trace water.
As expected, nBu4NBr with the better leaving group was more
active than PPNY, with respective TOF of 19.0 and 11.4 hꢀ1 at
25 °C (entries 3 and 4).
Scheme 1
Scheme 2
Following removal of residual indene oxide in vacuo, cyclic
indene carbonate was purified by flushing a CH2Cl2 solution
Figure 2. Thermal ellipsoid representation of cis-indene carbonate with
ellipsoids at 50% probability surfaces. All hydrogens have been omitted
for clarity except H1 and H2 to display the cis nature of the product. Inset
image is looking down the C2ꢀC1 bond angle, displaying the torsion in
the five-membered cyclic ring.
Figure 1. Thermal ellipsoid representation of (1S,2R)-indene oxide
with ellipsoids at 50% probability surfaces. Hydrogen atoms have been
omitted for clarity. Bond angles and lengths are identical for the (1R,2S)-
indene oxide enantiomer.
Table 1. Production of Indene Carbonatea
entry
catalyst
t(d)
T (°C)
[IO] (g/mL)
pressure (MPa)
conversion (%)b
TOF (hꢀ1 c
)
1d
2d
3
(S,S)-1/PPNYe
(S,S)-1/PPNY
(R,R)-1/PPNY
(R,R)-1/nBu4NBr
3
3
1
1
60
35
25
25
N/A
N/A
1.0
2.1
3.4
2.1
2.1
71.3
11.5
54.6
91.3
5.0
0.8
11.4
19.0
4
1.0
a The coupling reactions were performed with catalyst/cocatalyst/epoxide = 1/1/500 molar ratio with CH2Cl2 solvent in a 300 mL stainless steel Parr
autoclave. b Determined using H NMR spectroscopy. c Turnover frequency of indene oxide to indene carbonate as determined by H NMR
1
1
spectroscopy. d Experiment run in neat indene oxide. e PPN = bis(triphenylphosphine)iminium, Y = 2,4-dinitrophenoxide anion.
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a
Table 2. Low-Temperature Couplings of Indene Oxide and CO2
Mn (Da)
c
entry
catalyst
t (d)
conversion (%)b
PIC/ICb
TOF (hꢀ1
)
theord
GPCe
PDIe
Tg (°C)f
Td (°C)g
Td50 (°C)h
1i
2
3
4
5
(S,S)-1/PPNY
(S,S)-1/PPNY
rac-1/PPNY
7
6
6
2
2
16.7
59.0
50.1
16.2
50.6
45/55
57/43
59/41
59/41
0/100
1.1
2.1
1.7
1.7
4.0
14600
29600
26000
8300
4200
7000
7100
3500
1.19
1.09
1.16
1.30
114
133
134
109
245
249
245
236
262
264
266
259
rac-1/PPNY
rac-1/nBu4NBr
a The coupling reactions were performed with catalyst/cocatalyst/epoxide = 1/1/500 molar ratio in a 300 mL autoclave with indene oxide
concentrations of 1.0 g/mL in CH2Cl2. Temperature and pressure were held at 0 °C and 3.4 MPa, respectively. Data regarding selectivity for carbonate
linkages can be found in the Supporting Information. b Determined using 1H NMR spectroscopy. c Turnover frequency of indene oxide to products
[poly(indene carbonate) and indene carbonate] as determined by 1H NMR spectroscopy. d Theoretical Mn = (M/I) ꢁ (% conversion) ꢁ (mol. wt. of
indene carbonate). e Monomodal molecular weight distribution determined by gel permeation chromatography in THF; calibrated with narrow
f
polystyrene standards. The midpoint temperature of the major Tg during the second heating using DSC. g Onset decomposition temperature, as
determined by TGA. h Temperature at which 50% of the polymer’s mass has been lost. i 0.2 g/mL indene oxide in CH2Cl2.
through both activated carbon and a silica pad. Clear, colorless
block crystals suitable for X-ray diffraction were grown from a
concentrated CH2Cl2 solution. Two unique yet similar molecules of
cis-indene carbonate are found within the unit cell, one of which is
displayed in Figure 2. Rather than displaying near planarity in the
five-membered cyclic ring as in the case of propylene carbonate22
and cis-1,4-dihydronaphthalene carbonate,12 the two cis-indene
carbonate molecules have an O1ꢀC1ꢀC2ꢀO2 torsion angle of
14.187° and an O4ꢀC11ꢀC12ꢀO5 angle of 8.252°, with the
difference in the two molecules likely due to packing effects.
When the reaction temperature was decreased further to
0 °C, the first indications of polymer formation were observed
(Table 2, entries 1ꢀ3). After 1 week of reaction at 0 °C, we
were gratified to find that a 0.2 g/mL solution of indene oxide
utilizing the (S,S)-1/PPNY system formed a mixture of indene
carbonate and poly(indene carbonate), the latter identified by its
Figure 3. 13C NMR of carbonate [O(CdO)O] stretching region of
characteristic polycarbonate FT-IR stretch at 1750 cmꢀ1 (entry 1).
poly(indene carbonate). Seen from Table 2 are entry 2 (red) and entry 3
(blue). The spectrum of entry 1 is identical to that of entry 2 and is not
In an effort to suppress cyclization to indene carbonate and
induce further polycarbonate production, the concentration of
indene oxide was increased to 1.0 g/mL. After 6 days at 0 °C,
both (S,S)- and rac-1/PPNY afforded an ∼60/40% mixture of
poly(indene carbonate) and cyclic indene carbonate (entries 2
and 3, respectively). The observed low molecular weights are
likely due to slow polymer growth combined with rapid, rever-
sible chain transfer with trace water.
shown in this figure (see Supporting Information).
increased selectivity for specific ring-opening patterns of indene
oxide over rac-1. The peaks centered at 154.5 ppm are enhanced
over those centered at 154.2 ppm for (S,S)-1 (red) relative to
rac-1 (blue). Identical selectivity is observed for the polymer
produced from (S,S)-1 in entry 1. Thermogravimetric analyses of
the polymer samples indicate that poly(indene carbonate) is
thermally stable at up to ∼245 °C, with 50% degradation
occurring by ∼264 °C. The polymer’s molecular weight appears
to play a more significant role in Td determination as compared
to regio- and stereoregularity. A representative TGA decomposi-
tion curve can be found in the Supporting Information.
To conclude, the first samples of poly(indene carbonate) have
been produced via the Co-catalyzed coupling of indene oxide and
CO2. This marks the first time that a benzene ring has been
rigidly linked to the backbone of a polycarbonate produced using
CO2/epoxides coupling, and the resulting polymer has the
highest Tg yet reported for this process, up to 134 °C. While
this is still significantly lower than BPA polycarbonate’s Tg of
150 °C, progress is being made in the right direction. Addition-
ally, the molecular weights of these species are still quite low
(∼7000 Da), and it can thus be inferred that the Tg will be even
higher when larger molecular weight species are obtained. Poly-
(indene carbonate) is thermally stable at temperatures of up to
When purified, poly(indene carbonate) is slightly off-white in
color. Though the color may be due to residual cobalt remaining
in the polymer, it appears to be inherent as it remains after
multiple purifications. Each of the polymer samples has a slightly
different Tg, ranging from 108 to 130 °C. The differences
between entries 1 and 2 (114 vs 133 °C) may be attributed to
molecular weight discrepancies between the more oligomeric
4200 vs the longer-chained 7000 Da sample. Similar jumps in
both Tg (109 vs 134 °C) and molecular weight (3500 vs 7100
Da) are observed between entries 4 and 3. Because the molecular
weight of polymers, especially with lower molar masses, has a
strong influence on the Tg, it is anticipated that copolymers of
molecular weights greater than 10 000 Da will exceed 134 °C.
Distinct differences in the regio- and stereoregularity of the
polymer chains can be observed for polymer samples produced
from different catalysts (Figure 3). Though the peaks have not
been definitively assigned, it can be seen from the carbonate
region of the 13C NMR that the resolved (S,S)-1 does display
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Journal of the American Chemical Society
COMMUNICATION
249 °C. The appropriate choice of catalyst, cocatalyst, and
reaction temperature appears to be very important to induce
polymerization. Ongoing efforts involve finding ways to optimize
polymer production and increase chain length by increasing
reaction time, employing improved catalysts, and reducing the
amount of trace water within the system. Additionally, regio- and
stereoselective ring-opening studies of optically active indene
oxide are planned.
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ed.; Wiley-VCH GmbH & Co. KGaA: Weinheim, 2003.
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Supporting Information. Full experimental details and
b
X-ray structural data. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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1999-195705, 2001.
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Corresponding Author
djdarens@mail.chem.tamu.edu
’ ACKNOWLEDGMENT
We gratefully acknowledge the financial support of the National
Science Foundation (CHE-0543133 and CHE-1057743) and the
Robert A. Welch Foundation (A-0923). This research is supported
in part by the Department of Energy Office of Science Graduate
Fellowship Program (DOE SCGF), made possible in part by the
American Recovery and Reinvestment Act of 2009, administered by
ORISE-ORAU under contract no. DE-AC05-06OR23100. We also
thank Phil Imbesi for assistance with TGA measurements and April
Lovegood for assistance with elemental analysis.
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the similar FT-IR stretching frequencies of 2-indanone (1748 cmꢀ1) and
poly(indene carbonate) (∼1750 cmꢀ1) for kinetic measurements.
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