Dramatic behavioral differences of the copolymerization reactions of 1,4-cyclohexadiene and 1,3-cyclohexadiene oxides with carbon dioxide

Article abstract of DOI:10.1021/acs.macromol.5b00172

The copolymerization of 1,3-cyclohexadiene oxide (1,2-epoxy-3-cyclohexene) with CO₂ in the presence of the binary catalyst (salen)CoX or (salen)CrX and onium salts was shown to selectively afford the completely alternating copolymer poly(1,3-cy

Full text of DOI:10.1021/acs.macromol.5b00172

Article
pubs.acs.org/Macromolecules
Dramatic Behavioral Differences of the Copolymerization Reactions
of 1,4-Cyclohexadiene and 1,3-Cyclohexadiene Oxides with Carbon
Dioxide
Donald J. Darensbourg,* Wan-Chun Chung, Andrew D. Yeung, and Mireya Luna
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: The copolymerization of 1,3-cyclohexadiene oxide (1,2-epoxy-3-
cyclohexene) with CO2 in the presence of the binary catalyst (salen)CoX or
(salen)CrX and onium salts was shown to selectively afford the completely
alternating copolymer poly(1,3-cyclohexadiene carbonate) in good yield. In the
process catalyzed by the cobalt(III) system, the reaction was 100% selective for
copolymer, whereas employing the higher temperature chromium(III) catalyst, the
reaction yielded in addition to copolymer a significant quantity of the cis-1,3-
cyclohexadiene carbonate. Importantly, no corresponding trans-1,3-cyclohexadiene
carbonate was produced. The reactivity of 1,3-cyclohexadiene oxide in coupling
reactions with CO2 was strikingly greater than that of 1,4-cyclohexadiene oxide
under either catalytic conditions. Authentic samples of the cis-cyclic carbonate and
trans-cyclic carbonate were synthesized from epoxide and CO2 using ZnCl2/PPNI catalyzed and trans-diol/ethyl chloroformate
routes, respectively. trans-1,3-Cyclohexadiene carbonate was fully characterized by X-ray crystallography. Unlike the copolymer
derived from the symmetrical 1,4-cyclohexadiene oxide (1,2-epoxy-4-cyclohexene), deprotonation of poly(1,3-cyclohexadiene
carbonate) by a strong base did not lead to depolymerization with formation of the trans-cyclic carbonate. Computational studies
revealed the trans-cyclic carbonate was thermodynamically unstable relative to the polycarbonate, with the enthalpy of reaction
being +12.8 kcal/mol. The enhanced reactivity of the 1,3-isomeric epoxide versus that of its 1,4-isomer was further demonstrated
by the facile terpolymerization reaction of 1,3-cyclohexadiene oxide with propylene oxide and CO2. This latter process is useful
for the preparation of cross-linked or functionalized polycarbonates via thiol−ene chemistry.
these three epoxides, CHO, 1, and 2, are all very similar.
Similarly, the pKb’s of the three epoxides are not very different,
with CHO being slightly more basic toward a proton than
epoxides 1 and 2.5 That is, the νOD vibrational frequencies as
compared to that of MeOD in benzene (2667.4 cm−1) are
2600.0, 2605.8, and 2603.8 cm−1 for CHO, 1, and 2,
respectively. Therefore, it is expected that all three epoxides
have similar binding abilities to the metal centers of the cobalt
or chromium catalysts, and hence any differences in reactivity
can mainly be ascribed to the kinetics of the ring-opening step.6
INTRODUCTION
■
Several recent contributions have been published on the
copolymerization reactions of 1,2-epoxy-4-cyclohexene (1)
with carbon dioxide (eq 1).1−3 This epoxide monomer with
carbons 4 and 5 unsaturated reacts with CO2 to provide
copolymer more sluggishly under the same catalytic conditions
than its saturated counterpart, cyclohexene oxide (CHO).2,3 As
expected, the physical and thermal properties of the two
copolymer are similar, with the added feature that the
copolymer derived from epoxide 1 has the ability to be
postmodified and the epoxide can be obtained from renewable
resources.4
We were interested in whether the location of the double
bond in the six-membered carbon ring system would alter the
reactivity of the epoxide monomer. Hence, we report herein
an examination of the analogous process in eq 1, instead using
1,2-epoxy-3-cyclohexene (2). As might be anticipated, the
structural parameters, and therefore the steric requirements of
In this report, we present the synthesis and characterization
of the copolymer derived from epoxide 2 and carbon dioxide,
along with the corresponding cyclic carbonates. Further, the
terpolymerization reactions of epoxide 2 with propylene oxide
and CO2 were investigated as well as the depolymerization of
Received: January 26, 2015
Revised: February 24, 2015
© XXXX American Chemical Society
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Synthesis of trans-1,3-Cyclohexadiene Carbonate. Poly(1,3-cyclo-
hexadiene carbonate) (0.203 g, 1.45 mmol repeating unit, 1 equiv),
NaOH (0.117 g, 2.92 mmol, 2 equiv), and 10 mL of methanol were
added to a 50 mL round-bottom flask and heated to 57 °C for 3 h. The
reaction mixture was neutralized by adding 0.6 mL of 6 M HCl and then
dried with MgSO4. After removing MgSO4 by filtration, the solvent
was evaporated under reduced pressure affording 0.162 g (1.42 mmol,
97.8% yield) of a brown powder as the desired trans-diol product.
Subsequently, trans-diol (0.162 g, 1.42 mmol, 1 equiv) was converted to
trans-cyclic carbonate based on the literature procedure2 using ethyl
chloroformate (0.3 mL, 3.15 mmol, 2.2 equiv) and triethylamine
(0.4 mL, 2.87 mmol, 2 equiv) in THF. The crude reaction mixture was
filtered, and the solvent was removed under reduced pressure, followed
by being redissolved in hexane/ethyl acetate (3/1) solvent and filtered
through a silica pad to purify. The eluent was concentrated and dried to
afford a yellow powder (84.5 mg, 0.603 mmol, 42.5% yield). Elemental
analysis calculated for C7H8O3 (found): C, 60.00 (60.83); H, 5.75
(6.08). 1H NMR (300 MHz, CDCl3): δ 6.07 (dd, J = 3, 9 Hz, 1H), 5.69
(m, 1H), 4.75 (d, J = 9 Hz, 1H), 4.31 (dd, J = 3, 9, 12 Hz, 1H), 2.30−
2.51 (m, 3H) and 2.10−1.90 (m, 1H) ppm. 13C NMR (75 MHz,
CDCl3): δ 155.4, 129.6, 122.6, 81.3, 80.5, 25.2, and 23.8 ppm. Infrared
(CH2Cl2): 1809.2, 1834.3, and 1867.0 cm−1.
the copolymer derived from epoxide 2 and CO2, for com-
parison with analogous studies involving epoxide 1.2,3 This
study has provided some striking differences in reactivity
patterns for copolymers produced from the two isomeric forms
of cyclohexadiene oxide, epoxides 1 and 2.
EXPERIMENTAL SECTION
■
General Information. All manipulations involving air- and/or
water-sensitive compounds were carried out in a glovebox under an
argon atmosphere. 1,2-Epoxy-3-cyclohexene (Sigma-Aldrich) was
stirred over CaH2 distilled and stored in an argon-filled glovebox.
Research grade 99.999% carbon dioxide supplied in a high-pressure
cylinder and equipped with a liquid dip tube was purchased from
Airgas. The CO2 was further purified by passing through two steel
columns packed with 4 Å molecular sieves that had been dried under
vacuum at ≥200 °C. The (salen)CrCl complex was purchased from
Strem Chemicals. [PPN]N3 was prepared according to the published
literature.7 Catalyst 1 was prepared following the procedure of Lu and
co-workers.8 The 15 mL high-pressure stainless steel reactors used in
the copolymerization and cycloaddition reactions were previous dried
at 170 °C for 6 h prior to their use.
Ring-Opening Polymerization of trans-1,3-Cyclohexadiene Car-
bonate. trans-1,3-Cyclohexadiene carbonate (51.8 mg, 0.37 mmol,
50 equiv) was added to a round-bottom flask which was charged with
0.10 mL of a stock solution of 73 mM TBD (7.3 μmol, 1 equiv) and
77 mM benzyl alcohol (7.7 μmol, 1 equiv) in toluene. Toluene
(0.1 mL) was subsequently added to the above solution, resulting in a
trans-1,3-cyclohexadiene carbonate solution with a concentration of
1.85 M. After stirring the solution at 60 °C for 64 h, an NMR spec-
trum of the crude reaction mixture was taken to determine the mono-
mer conversion (87.4%). The copolymer was isolated from methylene
chloride upon addition of methanol. The molecular weight (Mn) of the
copolymer was determined to be 2100 Da with a PDI of 1.12.
Measurements. Molecular weight determinations (Mn and Mw)
were carried out with a Malvern Modular GPC apparatus equipped
with ViscoGEL I-series columns (H+L) and Model 270 dual detector
composed of RI and light scattering detectors. Samples (∼10 mg)
were weighed into a 2 mL volumetric cylinder, dissolved in THF, and
filtered with 0.2 μm syringe filter before injection. Glass transition
temperatures (Tg) were measured using a Mettler Toledo polymer
DSC. Samples (∼6 mg) were weighed into 40 μL aluminum pans and
subjected to two heating cycles. The first cycle covered the range from
25 to 150 °C at 10 °C/min heating rate and was cooled down to 0 °C
at −10 °C/min cooling rate. The second cycle ranged from 0 to
150 °C at 5 °C/min heating rate and was where Tg was measured.
Representative Coupling Reaction of 1,2-Epoxy-3-cyclohexene
and CO2. (S,S)-(salen)CoDNP (4.5 mg, 5.7 μmol, 1 equiv), PPNDNP
(4.1 mg, 5.7 μmol, 1 equiv), and 1,2-epoxy-3-cyclohexene (0.5 mL,
5.7 mmol, 1000 equiv) were charged in a 15 mL stainless steel
autoclave reactor. The reactor was pressurized to slightly less than
2.0 MPa and heated to 40 °C in an oil bath with magnetic stirring. After
5 h, the reactor was cooled to 0 °C and depressurized, and a 1H NMR
spectrum of the crude reaction mixture was taken immediately. The
crude reaction mixture was dissolved in CH2Cl2 and added to about 1 M
HCl/methanol solution to quench the reaction and precipitate any
copolymer formed. The supernatant HCl/methanol solution was
removed, and the polymer precipitate was redissolved in CH2Cl2 and
reprecipitated from methanol. The resulting copolymer was obtained by
removing the supernatant and subsequently drying in vacuo at 40 °C for
further analysis by GPC and DSC. 1H NMR (300 MHz, CDCl3): δ 5.95
(br, 1H), 5.68 (br, 1H), 5.19 (d, J = 18 Hz, 1H), 4.91 (br, 1H), 2.20
(s, 2H), 2.08 (s, 1H) and 1.87 (s, 1H) ppm. 13C NMR (125 MHz,
CDCl3): δ 153.6, 132.4, 122.8, 72.8−75.06, 24.4, and 22.6 ppm. Infrared
(CH2Cl2): 1749.4 cm−1.
Thiol−Ene Click Reaction of Poly(1,3-cyclohexadiene carbonate)
and Thioglycolic Acid. This was done in a similar manner as was
previously reported in the literature. Poly(1,3-cyclohexadiene carbo-
nate) (Mn = 10.0 kDa, 0.254 g, 1.81 mmol of CC groups, 1 equiv)
and thioglycolic acid (5.2 mL, 71.4 mmol, 40 equiv) were added to
10 mL of THF solution of AIBN (0.098 g, 0.589 mmol, 0.33 equiv) in
a 50 mL Schlenk flask under an argon atmosphere. The reaction
mixture was stirred for 24 h at 70 °C and subsequently concentrated
under reduced pressure. The crude product was redissolved in THF
and precipitated from diethyl ether three times in order to purify the
1
material. After the first thiol−ene coupling, the H NMR spectrum of
product showed nonreacted olefin, so the secondary thiol−ene
coupling was conducted. After the second time thiol−ene coupling,
1
no olefin was observed by the H NMR spectrum, indicating 100%
conversion of olefin to thioether. 0.182 g (0.783 mmol repeating unit,
43.3% isolated yield) of white product was obtained with Mn 15.7 kDa
and a Tg value of 89 °C. The functionalized polymer dissolves in polar
solvents like methanol, DMSO, acetone, and THF but not in
acetonitrile, ethyl acetate, dichloromethane, or chloroform.
X-ray Crystal Structure Analyses. Single crystals of trans-1,3-
CHDC (CHDC = cyclohexadiene carbonate) were obtained by slow
evaporation of a diethyl ether solution at −18 °C. A Leica MZ7.5
stereomicroscope was used to identify suitable crystals of the same
habit. Each crystal was coated in paratone, affixed to a Nylon loop and
placed under streaming nitrogen (150 K) in a SMART Apex CCD
diffractometer (see details in Supporting Information .cif files). The
space group was determined on the basis of systematic absences and
intensity statistics. The structure was solved by direct methods and
refined by full-matrix least-squares on F2. Anisotropic displacement
parameters were determined for all non-hydrogen atoms. Hydrogen
atoms were placed at idealized positions and refined with fixed
isotropic displacement parameters. The following is a list of programs
used: data collection and cell refinement, APEX2;9 data reductions,
SAINTPLUS Version 6.63;10 absorption correction, SADABS;11 struc-
tural solutions, SHELXS-97;12 structural refinement, SHELXL-97;13
graphics and publication materials, Mercury version 3.0.14
Synthesis of cis-1,3-Cyclohexadiene Carbonate. ZnCl2 (5.7 mg,
0.042 mmol, 1 equiv), PPNI (107.5 mg, 0.162 mmol, 4 equiv), and
1,2-epoxy-3-cyclohexene (0.35 mL, 4.0 mmol, 100 equiv) were charged
in a 15 mL stainless steel autoclave reactor. The reactor was pressurized
to slightly less than 3.0 MPa and heated to 70 °C in an oil bath with
magnetic stirring. After 43 h, the reactor was cooled to 0 °C and
1
depressurized, and a H NMR spectrum of the crude reaction mixture
was taken immediately, which showed 100% conversion to cis-cyclic
carbonate product. Ether was added to the reaction mixture in order to
isolate the product. The ether solution was filtered through a Celite pad
to remove insoluble ZnCl2 and PPNI, and the filtrate was dried in vacuo
to provide 0.463 g (3.30 mmol, 82.6% isolated yield) of a yellow oil as
desired product. Elemental analysis calculated for C7H8O3 (found): C,
1
60.00 (60.20); H, 5.75 (5.68). H NMR (300 MHz, CDCl3): δ 6.22
(m, 1H), 5.78 (d, J = 4.5 Hz, 1H), 5.01 (m, 1H), 4.90 (m, 1H) and
1.85−2.38 (m, 4H) ppm. 13C NMR (75 MHz, CDCl3): δ 154.9, 135.2,
121.3, 74.8, 72.1, 24.1, and 19.5 ppm. Infrared (CH2Cl2): 1799.5 cm−1.
B
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Computational Methods. All calculations were performed using
the Gaussian 09 suite.15 All local minima and saddle points were
confirmed by their calculated vibrational frequencies (zero and one
imaginary frequency, respectively). The saddle points found were
confirmed to be the correct ones by visualizing the imaginary
vibrational modes with AGUI16 and Avogadro.17
catalyst 1 exhibited similar reactivity while affording higher
molecular weight copolymer due to reduced chain transfer
processes (entries 2 and 4). When the CO2/1,3-CHDO
coupling reaction was catalyzed using the chromium salen
complex, catalyst 2, at 90 °C both copolymer and cis-1,3-
cyclohexadiene carbonate were produced (eq 3). Of importance,
Consistent with previous work,6,18,19 gas phase enthalpies of
polymerization were obtained by the CBS-4 M composite method
(1-mer to 2-mer).20,21 In the same way, changes in energy for the
exchange and epoxide ring-opening reactions (representing the slow
step in the enchainment reaction) were calculated using the M0622
and M06L functionals,23 in conjunction with the BS2 and BS2+ basis
sets.6 These basis sets comprise the Stuttgart/Dresden effective core
potential and basis sets (SDD)24 for the cobalt and chromium atoms,
and the all-electron 6-31G(d′,p′) of Petersson and co-workers were
used for remaining atoms.25,26 Basis set BS2+ was similar to BS2,
except that diffuse functions were added (i.e., 6-31+G(d′,p′) instead of
6-31G(d′,p′)). Free energies of carbonate and alkoxide backbiting
reactions were calculated using the CBS-QB320 and CBS-QB3(+)27,28
composite methods, respectively.
no trans-cyclic carbonate was observed (vide infra). Although, the
conversion to products increased with reaction time, there was
no increase in the molecular weights of the copolymer
produced (entries 5−7). Presumably, as the monomer is
consumed, the rate of enchainment decreases relative to the
backbiting process depicted in eq 4.
Except for determining enthalpies of polymerization and cyclic
carbonate formation, solvation was applied. Tetrahydrofuran was the
prototypical solvent, and the integral equation formalism polarization
continuum model (IEFPCM) calculation with radii and nonelectrostatic
terms for Truhlar and co-workers’ SMD solvation model was used.29
RESULTS AND DISCUSSION
■
Initially, we synthesized the epoxide monomer, 1,2-epoxy-3-
cyclohexene (2), via the commonly employed route of
epoxidation of 1,3-cyclohexadiene with m-CPBA, however in
very low yield. Alternatively, the required epoxide was
synthesized by reacting the diene with NBS (N-bromosuccini-
mide) to afford bromohydrin followed by ring closure (eq 2).
This procedure also provided a yield of only 20%.30 Hence, we
resorted to obtaining the epoxide from commercial sources.
Compared to 1,4-cyclohexadiene oxide and carbon dioxide
copolymerization reactions catalyzed by catalyst 1, 1,3-cyclo-
hexadiene oxide is observed to be more reactive. For example,
under the same reaction conditions as in entry 3, the 1,4-
cyclohexadiene oxide/CO2 coupling reaction exhibited 33.9%
conversion, or about one-half the reactivity of the 1,3-isomeric
form. Similarly, in the presence of catalyst 2 in entry 7, 1,3-
CHDO provided 40.8% polymer selectivity with 100%
conversion, whereas 1,4-CHDO under identical conditions
afforded 36.6% polymer selectivity with 57% conversion.
Noteworthy, in the latter process trans-cyclic carbonate was
produced. On the other hand, using cyclohexene oxide (CHO)
as epoxide monomer where all carbon bonds are saturated, in
the presence of the cobalt catalyst 1 1,3-CHDO was slightly less
reactive (33.3% conversion, TOF = 66.5 h−1) in entry 1
compared to CHO (42.2% conversion, TOF = 84.4 h−1).
However, under the reaction conditions in entry 7 where the
chromium catalyst 2 is used, CHO was less reactive with a 63%
conversion to copolymer.
Coupling of epoxide 2 (1,3-CHDO) and CO2 using cobalt
salen catalyst 1 at 40 °C under solventless conditions afforded
poly(1,3-cyclohexadiene carbonate) exclusively with a decent
TOF of 30−70 h−1. The epoxide/CO2 coupling reactions are
summarized in Table 1.
As to be anticipated, upon increasing reaction times the
TOFs decreased while higher conversions and molecular
weights were observed (entries 1−3). At lower temperature,
As indicated in the Introduction, based on a relative basicity
study of epoxides using the νOD shift of MeOD in epoxides
from the corresponding shift in benzene, CHO is slightly more
basic (Δν = −67.4 cm−1) than 1,3-CHDO (Δν = −63.6 cm−1)
a
Table 1. Coupling of 1,3-CHDO and CO2
b
b
b
entry
catalyst
temp (°C)
time (h)
conv (%)
TOF (h−1
)
polymer selectivity (%)
Mn (kDa)
PDI
Tg (°C)
1
2
3
4
5
6
7
1
1
1
1
2
2
2
40
40
40
RT
90
90
90
5
10
20
10
1
33.3
53.3
66.9
58.2
55.2
90.0
100.0
66.5
53.3
33.5
58.2
552
100
100
100
100
69.2
55.6
40.8
8.7
16.5
22.0
24.6
11.4
10.8
8.9
1.09
1.06
1.07
1.05
1.10
1.14
1.25
105
104
108
104
n.d.
107
104
2.5
5
360
200
a
b
Reaction condition: 1,3-CHDO/Co/PPNDNP = 1000/1/1, 1,3-CHDO/Cr/PPNN3 = 1000/1/2, CO2 pressure 2.0 MPa. Determined by 1H NMR.
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Figure 1. Binary cobalt−salen catalyst 1 and chromium−salen catalyst 2.
Figure 2. 13C NMR spectra of poly(1,3-cyclohexadiene carbonate) (blue) and poly(1,4-cyclohexadiene carbonate) (red) in the carbonate region
(left) and olefin region (right).
and 1,4-CHDO (Δν = −61.6 cm−1). That is, the sp2 carbons of
the double bond in both cyclohexadiene oxides act as weak
electron-withdrawing groups. Hence, 1,3-CHDO, which bears a
double bond next to the epoxy carbon, should be the easiest to
ring-open because the π-electrons can stabilize the ring-opening
transition state (vide infra).
The glass transition temperature (Tg) of poly(1,3-cyclo-
hexadiene carbonate) is lower (104−108 °C) than its 1,4-
counterpart (123 °C)2 and as well as poly(cyclohexene
carbonate) (116 °C).31 This is likely due to the unsymmetric
nature of the double bond in 1,3-cyclohexadiene carbonate.
The 13C NMR spectrum of poly(1,3-cyclohexadiene carbonate)
is similar to the 1,4-isomer, except for exhibiting an additional
set of peaks in the methylene and olefinic regions. The two 13C
NMR spectra are shown in Figure 2 for comparisons.
Attempts to depolymerize poly(1,3-cyclohexadiene carbo-
nate) by deprotonation of the hydroxyl polymer end-group,
which normally leads to an unzipping of the polymer chain to
provide the cyclic carbonate, were unsuccessful.32,33 That is,
poly(1,3-cyclohexadiene carbonate) was stable in toluene in the
presence of the strong base NaHMDS at 110 °C, with no
degradation to trans-1,3-cyclohexadiene carbonate (eq 5). This
is in stark contrast to poly(1,4-cyclohexadiene carbonate) which,
under similar conditions, readily unzips quantitatively to trans-1,4-
cyclohexadiene carbonate.2 As anticipated based on the process
depicted in eq 4 where the thermodynamically more stable cis-
1,3-cyclohexadiene carbonate is produced, if the depolymeriza-
tion reaction is carried out in CO2, slow formation of the
cis-cyclic carbonate is afforded. That is, in the presence of
catalyst 2, CO2 addition occurs at the polymeric anionic alkoxy
end group and backbiting proceeds via unzipping of the
carbonate intermediate.
However, the ring-opening polymerization (ROP) of trans-
1,3-cyclohexadiene carbonate was found to occur in the
presence of the organo-based catalyst system TBD (1,5,7-
triazabicyclo[4.4.0]dec-5-ene) and benzyl alcohol (see Exper-
imental Section). The conversion of trans-1,3-cyclohexadiene
carbonate was observed to proceed at a significantly faster rate
under similar reaction conditions than reported for trans-1,4-
cyclohexadiene carbonate.34 The higher reactivity for the ROP
of the 1,3-isomer is consistent with computational results which
predicts the driving force for this process to be greater than that
for the 1,4-isomer (vide infra). Similar to the polymer derived
from the ROP of the trans-1,4-isomeric form, the 13C NMR
spectrum in the carbonate region of the polymer resulting from
the ROP of trans-1,3-cyclohexadiene carbonate differs in
tacticity from that obtained from the corresponding copoly-
merization of CO2 and epoxide.
Nevertheless, hydrolysis of poly(1,3-cyclohexadiene carbo-
nate) with NaOH was successful to provide trans-diol.35
Subsequent carbonylation of trans-diol with ethyl chlorofor-
mate in the presence of triethylamine provided trans-1,3-
cyclohexadiene carbonate (trans-1,3-CHDC) in 42.5% isolated
yield (eq 6). The trans conformation was confirmed by
X-ray crystallography. The solid state structure is similar to
trans-cyclohexenecarbonate (trans-CHC) but exhibits a more
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Figure 4. Normalized infrared spectra of cis-1,3-CHDC and trans-1,3-
CHDC in the carbonyl region.
it is desirable to incorporate other less rigid epoxide mono-
mers, e.g., propylene oxide (PO). Unfortunately, when 1,4-
cyclohexadiene oxide is terpolymerized with propylene oxide
and CO2, very little 1,4-CHDO is incorporated into the
propylene carbonate backbone. Indeed, it has been shown that
cyclohexene oxide itself is much more reactive than 1,4-CHDO
in terpolymerization reactions with CO2.3 By way of contrast,
the 1,3-CHDO isomer’s reactivity compares favorably with
propylene oxide in terpolymerization processes with CO2
(eq 7). A similar observation was noted by Lu and co-workers
Figure 3. Crystal structures of trans-1,3-CHDC (left) and trans-CHC
(right). The bottom ones are the views along the C1−C6 axis.
Table 2. Dihedral Angles (deg) of trans-1,3-CHDC and
trans-CHC
for the terpolymerization of cyclohexene oxide and PO with
CO2.39 Herein, we have investigated the reactivity ratios (ratio
of self- to cross-propagation) of propylene oxide and 1,3-
CHDO by a Fineman−Ross analysis (Scheme 1).40
trans-1,3-CHDC
trans-CHC
O1−C1−C6−O2
H4B−C4−C5−H5A
C2−C1−C6−C5
39.2(1)
34.6(2)
67.3(2)
23.9(9)
58(2)
A set of terpolymerization reactions with different epoxide
feed ratios were conducted and the components of the resulting
72(1)
1
terpolymers were analyzed by H NMR spectroscopy. The
twisted cyclohexyl ring (Figure 3).36,37 trans-1,3-CHDC has a
larger O1−C1−C6−O2 dihedral angle of 39.2° compared to
that of trans-CHC of 23.9° (Table 2). Also, trans-1,3-CHDC
exhibits a small H4B−C4−C5−H5A dihedral angle of 34.7°,
whereas, trans-CHC has a nearly perfectly staggered con-
formation with a H4B−C4−C5−H5A dihedral angle of 58°.
cis-1,3-Cyclohexadiene carbonate (cis-1,3-CHDC) was prepared
by an established route which involved the coupling of epoxide
2 and CO2 in the presence of ZnCl2 and PPNI at 70 °C and
3.0 MPa pressure.2,38 cis-1,3-CHDC synthesized in this manner
with 100% conversion displayed identical spectroscopic proper-
results of this study are summarized in Table 3 and Figure 5.
The reactivity ratio of propylene oxide for self- vs cross-
propagation was found to be 0.553, whereas for 1,3-cyclo-
hexadiene oxide was determined to be 0.846. The observation
that propylene carbonate incorporates 1,3-CHDO faster than
propylene oxide is ascribed to the π-orbital of the adjacent
olefin stabilizing the transition state in the ring-opening step of
1,3-CHDO. This effect was also observed in the terpolymeriza-
tion reaction of propylene oxide/styrene oxide/CO2, where
styrene oxide was more easily ring-opened.5 On the other hand,
cyclohexadiene carbonates incorporate propylene oxide slightly
faster than 1,3-CHDO as a result of propylene oxide’s better
metal binding and less steric hindrance than 1,3-CHDO.
Because the terpolymerization reactions are carried out to low
conversion as required by the Fineman−Ross kinetic analysis,
the glass transition temperatures of these low molecular
weight polymeric materials are not very meaningful. However,
we have prepared a terpolymers of higher molecular weight
(7.6 kDa) from a 1:1 mixture of PO and 1,3-CHDO with
a PPC/PCHDC ratio of 0.809 and measured its Tg value of
68.9 °C, approximately midway between the Tgs of the
respective copolymers.
ties (νCO and 1H, 13C NMR) as the byproduct produced in eq 3.
3
trans-1,3-CHDC in dichloromethane showed three carbonate
infrared bands at 1867.0, 1834.3, and 1809.2 cm−1, whereas cis-
1,3-CHDC exhibited only one band at a lower frequency
(1799.5 cm−1) (Figure 4). A similar situation was noted in
the isomeric forms of 1,4-CHDC where the trans-isomer
had two carbonate bands at higher frequencies (1824.6 and
1809.2 cm−1) compared to one band at 1797.6 cm−1 for the cis-
isomer.2
In order to enhance the applicability of polymers derived
from carbon dioxide and cyclohexene oxide or its derivatives,
E
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Scheme 1. Self- and Cross-Propagation Pathways and Reactivity Ratios of Both Epoxides in the CO2/1,3-CHDO/PO
Terpolymerization Reaction
a
Table 3. Terpolymerization of CO2/1,3-CHDO/PO
COMPUTATIONAL STUDIES
■
Computational modeling has the potential to provide deeper
insight into experimental observations at a qualitative and
quantitative level, and its application toward the CO2−epoxide
copolymerization has been surveyed.42 We have employed such
studies to quantify the thermodynamics of polymerization vs
cyclic carbonate formation, the kinetics of metal-catalyzed chain
growth, and associated degradation reactions.6,18 Such calcu-
lations indicate that displacement of the growing polymer
strand (terminated with carbonate) with an epoxide, followed
by epoxide ring-opening (Scheme 2), is the rate-limiting step in
PO
conv
(%)
b
1,3-CHDO/
Co
time
PO/Co (min)
1,3-CHDO
b
PPC/PCHDC
b
conv (%)
(m/n)
600
800
1800
1600
1200
800
60
30
30
40
43
24.4
17.9
13.6
8.4
15.8
9.6
9.0
2.5
5.6
2.04
1.43
1200
1600
1800
0.855
0.489
0.330
600
6.9
a
Reaction conditions: CO2 pressure 2.0 MPa, ambient temperature.
Determined by H NMR. m/n is the degree of polymerization of
b
1
PC/PCHDC via eq 7.
Scheme 2. Ring-Opening of a Metal-Bound Epoxide by a
Polymeric Carbonate Nucleophile
Figure 5. Fineman−Ross analysis of CO2/1,3-CHDO/PO terpolyme-
rization. The slope indicates reactivity ratio of 1,3-CHDO, and the
intercept indicates that of PO.
the overall enchainment reaction.6 That is to say, the last step,
carboxylation of the polymeric alkoxide, is rapid for these
systems and does not constrain the catalytic reaction.
As has been previously reported for the copolymer derived
from carbon dioxide and 1,4-cyclohexadiene oxide or
2-vinyloxirane, poly(1,3-cyclohexadiene carbonate) can be
functionalized with thioglycolic acid using thiol−ene chemistry
in the presence of AIBN (eq 8).2,41 All olefinic groups were
Pertinent to the observations noted on the epoxide/CO2
coupling reactions reported herein, the enthalpies and free
energies of the respective processes involving cyclohexene
oxide, 1,4-cyclohexadiene oxide, and 1,3-cyclohexadiene oxide
were calculated and are presented in Figures 6 and 7,
respectively. As is typical for these processes, copolymer
formation was found to be exothermic by 18−22 kcal/mol,
making it the enthalpic product. On the other hand, formation
of cyclic carbonates is much less exothermic, and are the
thermodynamic products of these coupling reaction due to
entropy (Table 4).
While formation of trans-cyclic carbonate from the backbiting
process involving all three polycarbonates are endothermic,
they are exergonic for poly(cyclohexene carbonate) and
poly(1,4-cyclohexadiene carbonate) with ΔG values of −4.3
and −5.9 kcal/mol, respectively. By way of contrast, poly(1,3-
cyclohexadiene carbonate) degradation to trans-1,3-cyclohex-
adiene carbonate was endergonic by 1.3 kcal/mol due to the
enthalpic component of free energy. The computed kinetic
coupled with the thiol bearing acetic acid pendant group, with
the resulting polymer having a Mn value of 15.70 kDa and PDI
of 1.2 (theoretical Mn 16.6 kDa for a parent copolymer with Mn
of 10.0 kDa). The functionalized polycarbonate exhibited a
lower Tg of 89 °C compared to its parent (108 °C).
F
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Figure 8. trans-1,3-Cyclohexadiene carbonate, highlighting the
eclipsed conformation between two adjacent carbon atoms in the ring.
Table 5. Energy Barriers (kcal/mol) for Metal-Free Alkoxide
Backbiting
ΔH‡
ΔG‡
Figure 6. Enthalpies of the reactions between CO2 and cyclohexene,
1,4-cyclohexadiene, and 1,3-cyclohexadiene oxides. See Table 4.
CHC
15.3
16.2
15.9
15.0
14.6
15.7
15.1
14.9
a
trans-13CHDC-up
a
trans-13CHDC-down
trans-14CHDC
a
See Figure 9.
Figure 7. Free energies of the reactions between CO2 and
cyclohexene, 1,4-cyclohexadiene, and 1,3-cyclohexadiene oxides. See
Table 4.
Figure 9. Tetrahedral intermediates involved in the alkoxide
backbiting reaction for trans-13-CHDC-up (left) and trans-13-
CHDC-down (right).
Table 4. Thermodynamic Data (Enthalpies and Free
Energies) for the CO2 Coupling Reactions with CHO,
a
1,4-CHDO, and 1,3-CHDO
highest, these barriers are quite typical and should not, in
themselves, preclude cyclic carbonate formation. Experimen-
tally, attempts to prepare trans-1,3-cyclohexadiene carbonate via
treating the polymer with a strong base were not successful.
These calculations emphasize that the failure is attributed to the
thermodynamics of the overall reaction (ΔH = +12.8 kcal/mol,
ΔG = +1.3 kcal/mol). Indeed, the reverse process of ROP
of trans-1,3-cyclohexadiene carbonate should be favored as
observed experimentally.
ΔH (ΔG)
ΔH (ΔG) cis- ΔH (ΔG) trans-
epoxide
copolymerization cyclic carbonate cyclic carbonate
cyclohexene oxide
−22.6 (+3.4)
−18.6 (+6.0)
−16.7 (−4.6)
−14.6 (−2.6)
−13.0 (−0.9)
−12.3 (+0.1)
1,4-cyclohexadiene
oxide
1,3-cyclohexadiene
oxide
−20.6 (+2.6)
−16.9 (−5.2)
−7.8 (+3.9)
a
Energies provided in kcal/mol with free energies included in parentheses.
The rate-limiting step for the (salen)M(III)Cl-catalyzed
CO2−epoxide copolymerization has previously been estab-
lished to be displacement of the metal-bound polymeric
carbonate with an epoxide molecule, followed by ring-opening
the metal-bound epoxide (Scheme 2). The overall energy
barriers presented here are calculated as
barriers for the alkoxide backbiting reactions for the three
alicyclic epoxide derived copolymers were quite similar (vide
infra); therefore, thermodynamic explains why treatment of
poly(1,3-cyclohexadiene carbonate) with a strong base yields no
trans-1,3-cyclohexadiene carbonate (eq 5).
The ∼4.0 kcal/mol difference in free energy for formation of
trans-cyclic carbonate from the other two epoxides is ascribed
to the small H−C−C−H dihedral angle of 35.1° (eclipsed con-
formation) of trans-1,3-cyclohexadiene carbonate (Figure 8).
Such intramolecular steric repulsion is greater than that for the
corresponding dihedral angles for the 1,4-isomer, whereas this
angle is close to the ideal 60° for cyclohexene carbonate
(gauche conformation).
ΔE‡ = E(transition state) − E([M]−polymeric carbonate)
− E(epoxide)
As a further refinement, the growing polymer chain is
−
represented by cyclohexyl carbonate (C6H11OCO2 ) that
better represents the steric bulk of the incoming nucleophile
−
than methyl carbonate (CH3OCO2 ). Between the bulkier
As alluded to earlier, the kinetic barriers for the copolymers
to undergo alkoxide backbiting to afford trans-cyclic carbonates
were determined to be nonrate limiting (Table 5). On the
other hand, formation of the cis-cyclic carbonates from the
carbonate backbiting process has activation barriers about
10 kcal/mol higher. While the energy barrier leading to trans-
1,3-cyclohexadiene carbonate via alkoxide backbiting is the
nucleophile and a more typical cyclohexylene salen backbone,
we would preclude transition states overstabilized by dipolar
interactions between the carbonate oxygen atoms and the
hydrogen atoms on the salen ligand’s ethylene backbone that
are adjacent to the electron-withdrawing nitrogen atoms.
The overall free energy barriers for these two steps are 22−
27 kcal/mol; carboxylation has trivial barriers in comparison
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Table 6. Overall Energy Barriers (kcal/mol) for Various
Epoxides To Copolymerize with CO2
Table 8. Energy Barriers (kcal/mol) for the Elementary
Epoxide Ring-Opening Reaction, Catalyzed by
(salen)Cr(III)Cl and (salen)Co(III)Cl
[Cr]-bound
[Co]-bound
a
[Cr]-bound
[Co]-bound
epoxide
CHO
ΔH‡
ΔG‡
ΔH‡
ΔG‡
a
epoxide
CHO
ΔH‡
ΔG‡
ΔH‡
ΔG‡
16.8
13.1
16.8
15.1
29.1
24.2
30.0
27.5
16.7
12.2
14.5
13.8
29.1
24.1
26.6
26.4
1,3-CHDO-1
1,3-CHDO-2
1,4-CHDO
9.5
3.6
7.5
8.8
11.4
4.3
9.4
5.4
7.9
9.1
10.5
6.1
1,3-CHDO-1
1,3-CHDO-2
1,4-CHDO
10.2
9.9
9.7
a
11.0
CHDO = cyclohexadiene oxides. Cyclohexyl carbonate nucleophile.
a
Cyclohexyl carbonate nucleophile.
(ΔG‡ = 6−8 kcal/mol).6 The overall energy barriers for
cyclohexene as well as 1,3- and 1,4-cyclohexadiene oxides
to yield polycarbonates are presented in Table 6. In this table,
“−1” refers to the polymeric carbonate (represented by
cyclohexyl carbonate) ring-opening the activated epoxide at
the vinylic carbon for alicyclic epoxides, whereas “−2” refers
to attack at the adjacent methylene position. As before, the
chromium-catalyzed reactions have higher energy barriers than
the cobalt-catalyzed reactions.
Consistent with the case for styrene oxide, attack at the
methine positions of vinylic epoxide (1,3-cyclohexadiene oxide) has
a free energy barrier lower by ca. 2−6 kcal/mol than for methylene
attack.6 This occurs despite the steric hindrance at the more sub-
stituted carbon atom, whereas methine attack has a higher barrier
for propylene oxide. The large difference in methine/vinylic vs
methylene attack suggest that [Cr] may catalyze analogous reaction
with cyclohexadiene oxide in a regioselective head−tail manner.
The calculated results predict that the ease of copolymerization is in
the order: 1,3-CHDO > 1,4-CHDO > CHO for both [Cr]- and
[Co]-catalyzed reactions. These results are in general agreement
with experimental observations. To be specific, such experiments
indicate that 1,3-cyclohexadiene oxide is expected to react more
readily than cyclohexene oxide.
complexes have been assumed to be the same for the metal-
catalyzed epoxide ring-opening reactions at the vinylic (“−1”)
and methylene (“−2”) positions. The relative differences
between these two epoxide ring-opening modes can thus be
fairly compared. We should exercise caution when comparing the
energy barriers between different epoxides or between different
metal-catalyzed systems because the {[M]−epoxide + polymeric
carbonate} van der Waals complexes are poorly defined.
Even so, we find that attack at the vinylic carbon (“−1”) is
consistently more favorable than at the methylene carbon in
general. One might imagine that the vinylic pπ system will
stabilize the pentacoordinate transition state for the reaction.
Such interactions have been seen in the reaction where
poly(styrene carbonate) undergoes the metal-free carbonate
backbiting reaction, but a careful review of the calculated
molecular orbitals did not reveal such a simple answer for the
metal-catalyzed system.18
Even though 1,3-cyclohexadiene oxide has a lower free
energy barrier for epoxide ring-opening than cyclohexene oxide,
this epoxide is less able to displace the polymeric carbonate
from the metal center. The advantage that 1,3-cyclohexadiene
oxide has is thus muted. The importance of considering the li-
gand exchange and the epoxide ring-opening steps are empha-
sized as a result.
How well these epoxide ligands bind to the Lewis acid
catalysts were calculated (Table 7). As was observed previously,
Table 7. Enthalpies and Free Energies (kcal/mol) for
Epoxide Ligands to Bind to the Square-Pyramidal
(salen)4Cr(III)Cl and (salen)1Co(III)Cl Fragments
CONCLUSIONS
■
The copolymerization reaction of 1,3-cyclohexadiene oxide and
carbon dioxide differ strikingly from the corresponding
processes involving its symmetrical or saturated analogues,
1,4-cyclohexadiene oxide and cyclohexene oxide, respectively.
Notably, it is the most reactive of the three epoxides, in general
being slightly more reactive than cyclohexene oxide, with the
1,4-isomer being by far the least reactive. Computational
studies support these experimental observations, i.e., the free
energy barriers for epoxide ring-opening increase in the order
1,3-CHDO < CHO < 1,4-CHDO. This reactivity order is
especially evident in terpolymerization reactions of 1,3-
cyclohexadiene oxide with propylene oxide and CO2, where
the reactivity ratios were determined to be rPO = 0.553 and
r1,3‑CHDO = 0.846 at ambient temperature from a Fineman−
Ross analysis. For reaction processes catalyzed by (salen)CrX
in the presence of onium salts, unlike the other epoxides, the
1,3-cycloheadiene oxide and CO2 produce no trans-cyclic
carbonate. This is ascribed, based on computational studies
to the trans-1,3-cyclohexadiene carbonate being thermodynami-
cally less stable than its polymeric form. This is further
demonstrated when the isolated, pure copolymer is deproto-
nated by a strong base; no depolymerization takes place with
formation of trans-1,3-cyclohexadiene carbonate. That is,
contrary to the other two closely related polycarbonates,
[Cr]-bound
[Co]-bound
L
enthalpy
free energy
enthalpy
free energy
CHO
−19.5
−13.7
−16.0
−15.3
−8.8
−1.9
−5.5
−4.5
−18.8
−11.7
−15.5
−13.0
−4.1
3.3
R-1,3-CHDO
S-1,3-CHDO
1,4-CHDO
−1.4
2.0
R-epoxides bind more weakly to the Lewis acid catalyst than
S-epoxides due to conformational reasons.6 The corresponding
Lewis basicities toward [Cr] and [Co] are determined here to
be in the order CHO > 1,3-CHDO ≈ 1,4-CHDO. These results
substantiate comments previously made about the Bronsted
basicity rankings of these epoxides, determined via infrared
spectroscopy (vide supra) as follows: CHO > 1,3-CHDO > 1,4-
CHDO. These results indicate that 1,3- and 1,4-CHDO are less
able to displace the growing polymer chain than CHO prior to
epoxide ring-opening. The assumption made is that the electronics
of the respective polymeric carbonates are similar.
The energy barriers for the elementary epoxide ring-
opening reaction are presented in Table 8. The structures of
the {[M]−epoxide + polymeric carbonate} van der Waals
H
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L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J.
J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.;
Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.;
Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian Inc., Wallingford, CT, 2009.
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poly(1,3-cyclohexadiene carbonate) does not degrade to cyclic
carbonate in the presence of base.
Currently, we are exploring the ring-opening polymerization
of trans-cyclohexadiene carbonate to afford the corresponding
copolymer, for preliminary observations suggest the copolymer
produced via this route has a different microstructure from
that obtained by the copolymerization reaction of the epoxide
and CO2.
̈
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystal structure analyses of single crystals of trans-1,3-
CHDC. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone 979.845.2983; Fax 979.845.0158; e-mail djdarens@
chem.tamu.edu (D.J.D.).
■
(20) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G.
A. J. Chem. Phys. 1999, 110, 2822−2827.
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Notes
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(23) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101/1−
194101/18.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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We gratefully acknowledge financial support of this research by
the National Science Foundation (CHE-1057743) and the
Robert A. Welch Foundation (A-0923). Computer resources
were provided by the Laboratory for Molecular Simulation and
the Supercomputing Facility at Texas A&M University.
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