Regioregular and regioirregular oligoether carbonates: A 13C{1H} NMR investigation

Article abstract of DOI:10.1021/ma049850f

Oligoether carbonates R(PO)_nOCO₂(PO)_nR, where R = Me, Et, or H, PO = propylene oxide ring-opened unit, and n = 1, 2, 3, 4, ～10, and ～30, have been prepared and characterized by ESI/MS or MALDI/MS and ¹³C{¹H} NMR spectroscopy in addition to ¹H NMR, DEPT, COSY, and HMQC. The propylene oxide (PO) units have been derived from S-PO and rac-PO. The compounds have been examined as potential models for polyether carbonate units in poly(propylene carbonate). For HH junctions, assignments of isotactic (i) and syndiotactic (s) diads and iii, iis/sii, sis, isi, ssi/iss, and sss tetrads are unequivocal. Assignments at the hexad level are limited. For higher oligoether carbonates, i.e., n ～ 10 or ～30, only the i and s diad sensitivity is possible at 150 MHz ¹³C{¹H} NMR. Calculations on the compounds MeOCH ₂CHMeOCO₂CHMeCH₂OMe (RR (i) and SR (s)) and MeOCO₂CH₂CHMeOCO₂CHMeCH₂OCO ₂-Me were carried out employing density functional theory (DFT) at the B3LYP/6-31G(d) level for geometry optimization and the B3LYP/6-311+G(2d,p) level for NMR calculations. These results are compared with the experimental work and structures of dimethyl carbonate.

Full text of DOI:10.1021/ma049850f

Macromolecules 2004, 37, 4139-4145
4139
Regioregular and Regioirregular Oligoether Carbonates: A 13C{1H}
NMR Investigation
Ma tth ew J . Byr n es, Ma lcolm H. Ch ish olm ,* Ch r istop h er M. Ha d a d ,* a n d
Zh ip in g Zh ou
Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210
Received J anuary 22, 2004; Revised Manuscript Received March 5, 2004
ABSTRACT: Oligoether carbonates R(PO)nOCO2(PO)nR, where R ) Me, Et, or H, PO ) propylene oxide
ring-opened unit, and n ) 1, 2, 3, 4, ∼10, and ∼30, have been prepared and characterized by ESI/MS or
MALDI/MS and 13C{1H} NMR spectroscopy in addition to 1H NMR, DEPT, COSY, and HMQC. The
propylene oxide (PO) units have been derived from S-PO and rac-PO. The compounds have been examined
as potential models for polyether carbonate units in poly(propylene carbonate). For HH junctions,
assignments of isotactic (i) and syndiotactic (s) diads and iii, iis/ sii, sis, isi, ssi/ iss, and sss tetrads are
unequivocal. Assignments at the hexad level are limited. For higher oligoether carbonates, i.e., n ∼ 10 or
∼30, only the i and s diad sensitivity is possible at 150 MHz 13C{1H} NMR. Calculations on the compounds
MeOCH2CHMeOCO2CHMeCH2OMe (RR (i) and SR (s)) and MeOCO2CH2CHMeOCO2CHMeCH2OCO2-
Me were carried out employing density functional theory (DFT) at the B3LYP/6-31G(d) level for geometry
optimization and the B3LYP/6-311+G(2d,p) level for NMR calculations. These results are compared with
the experimental work and structures of dimethyl carbonate.
In tr od u ction
There is an increasing interest in the synthesis of
polycarbonates by environmentally friendly routes, such
as in the copolymerization of epoxides (oxiranes) and
carbon dioxide.1-3 This might provide an alternative to
condensation polymerization involving phosgene and
diols.4 The copolymerization of propylene oxide and
carbon dioxide by certain metal catalysts has been
known for some time as shown below,5 though these
processes are not very efficient and, as such, do not offer
commercial attraction.6 However, recent studies have
suggested that these problems may be overcome by
appropriate catalyst modification.7-10 In our studies of
poly(propylene carbonate), PPC, we have found that
different catalyst systems yield markedly different
polymers as deduced by high field NMR studies. A
common impurity in certain samples of PPC is the
occurrence of ether-rich segments. As shown in Figure
1, the 13C{1H} signals in the carbonate region reveal
the existence of carbonate carbons flanked by more than
one ether unit.11,12 Inasmuch as the regio- and stereo-
chemistry of a polymer bears a memory of the reaction
pathway leading to its formation, we were struck by the
fact that we had little or no basis upon which to assign
these carbonate signals. We were thus prompted to
synthesize some model compounds and report herein the
findings of these studies.
F igu r e 1. 13C{1H} (150 MHz, CDCl3) NMR spectra of carbon-
ate carbon region of PPC made from rac-PO employing the
(TPP)AlCl/EtPh3PBr catalytic system. HH′, HT′, and TT′ are
ether-rich carbonate signals. The i and s assignment in the
HH′ region is proposed on the basis of the work reported
herein.
Sch em e 1
Resu lts a n d Discu ssion
selection of rac-PO and S-PO, we can determine the
stereochemical sequences in the etherols. Then in the
reaction with triphosgene by combinations of etherols
derived from S-PO and rac-PO, we obtained carbonates
containing statistical (i + s) and enriched (i) stereo-
sequences.
Syn th esis. The general synthetic route to these ether
carbonates is shown in Scheme 1.13-15 The etherols
Et(OCH2CHMe)nOH, where n ) 1, 2, 3, and 4, were
separated by spinning band column distillation. For n
) 2, 3, and 4, the distillations were carried out under
reduced pressure. The ring-opening of propylene oxide
(PO) in Scheme 1 involves backside attack, and by
NMR Sp ectr a a n d Assign m en ts. The 13C{1H}
signals of the carbonate carbon for n ) 1 and 2 are
10.1021/ma049850f CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/27/2004

4140 Byrnes et al.
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 3. 13C{1H} (150 MHz, CDCl3) NMR spectra of carbon-
ate carbon region of regioirregular oligoether carbonate com-
pounds (n ) 1, 2).
n ) ∼10 and ∼30, are shown in Figure S3. These spectra
further demonstrate that only diad sensitivity is seen.
From the use of commercial sources of regioirregular
etherols, CH3(PO)n(OCH2CHMe)OH, where n ) 1 and
2, we have made ether carbonates that have HH
junctions at the carbonate carbon but are otherwise
regioirregular. The 13C{1H} spectra of the carbonate
carbon are shown in Figure 3. For n ) 1, we observed
tetrad sensitivity and can identify the stereosequences
associated with the regioregular component (see Sup-
porting Information for details). Once again, however,
as the chain length increases on each side of the
carbonate, the sensitivity is reduced so that only i and
s regions can be unequivocally identified. Finally, we
should note that the chemical shift difference between
the unique resonances is quite small, typically less than
0.1 ppm.
F igu r e 2. 13C{1H} (150 MHz, CDCl3) NMR spectra of carbon-
ate carbon region of regioregular oligoether carbonate com-
pounds (n ) 1, 2). Carbonate compounds in (b) were made from
etherol derived from rac-PO, while the ones in (c) were
prepared from etherol mixtures (1:1) derived separately from
rac-PO and S-PO.
Str u ctu r a l Con sid er a tion s. As an aid to the inter-
pretation of the NMR data, we have employed calcula-
tions using density functional theory (DFT)16 and the
Gaussian 98 suite of programs17 to examine the energies
of various conformers. As a starting point, we examined
dimethyl carbonate, MeOCO2Me, which can exist in
three local minima, namely cis-cis, trans-trans, and
cis-trans conformers as shown in A, B, and C below,
respectively. Experimentally, it is known that cis-cis
is thermodynamically preferred, and a related carbonate
molecule has been structurally characterized by single-
crystal crystallography in this cis-cis geometry.18 Pre-
vious DFT calculations, analogous to the ones reported
here, also have revealed the preference for the cis-cis
shown in Figure 2. For n ) 2, it is clear that the 13C
carbonate signal shows tetrad sensitivity, and further-
more, since this molecule has a regiosequence (TH)(TH)‚
(HT)(HT) (H ) methine group; T ) methylene group),
the signal for iis and sii cannot be distinguished.
Similarly, the ssi and iss sequence is the same.
The spectra for n ) 3 and 4 are shown in Figure S2
of the Supporting Information. For n ) 3, there are 20
possible stereosequences, but no more than 12 signals
are partially resolved. Thus, for n g 4 we can only assign
the signal at the diad level, i or s. The spectra obtained
for polyether carbonates Et(PO)nOCO2(PO)nEt, where

Macromolecules, Vol. 37, No. 11, 2004
Oligoether Carbonates 4141
F igu r e 5. Six local minimum conformers in the gas-phase
calculations for SR isomers of MeOCH2CHMeOCO2CHMeCH2-
OMe.
F igu r e 4. Six local minimum conformers in the gas-phase
calculations for RR isomers of MeOCH2CHMeOCO2CHMeCH2-
OMe.
The SR isomers are compared in Figure 5 for the cis-
cis geometry about the carbonate. Again we show the
calculated local minima, and here for the conformers
[1,1], [2,2], and [3,3], there is Cs symmetry but for all
others only C1. In the Newman projections, we show
only the S half of the molecule. Once again the MeO
groups are anti to the carbonate O-CHMe bond. For
the SR case, the gas-phase structures of the [1,1], [1,3],
and [3,3] conformers are very close in energy, differing
by less than 0.15 kcal/mol, indicating that these would
be the dominant species in equilibrium at 298 K.
We also investigated the solvent effects on the pre-
ferred conformations of the methoxy-terminated oligo-
mers using the polarizable continuum model (PCM) in
our calculations,20 and their respective energies in
chloroform and benzene differed little from those cal-
culated for the gas phase. These energies are compared
in Table 1. This suggests that the dominant species
present in the gas phase also exist in chloroform and
benzene predominantly.
The molecules of MeOCO2CH2CHMeOCO2CHMeCH2-
OCO2Me, considered as a small segment of the PPC
chain, were also investigated in our calculations. The
results of the calculations for RR isomers in both the
gas phase and chloroform are summarized in Figure 6.
Because of the huge number of all possible conforma-
tions, we simplified the calculations by only dealing with
the ones having C2 symmetry derived from [1,1] in
Figure 4 and keeping all three carbonate groups in the
cis-cis conformation. The Newman projections depicted
in Figure 7 are viewed down the bonds of CH(Me)-O
(1), CH2-CH(Me) (2), and O-CH2 (3). The nomenclature
for the calculated six minima in Figure 6 indicates a
combination of those three views. For example, [1,3′,2′′]
represents that a half of this isomer has conformation
1 for the CH(Me)-O bond, 3′ for the CH2-CH(Me), and
2′′ for the O-CH2, and the entire isomer has a C2
symmetry. The bond angles and lengths are fully
optimized. The relative energies of the six isomers are
in the range 0-1.1 kcal/mol as shown in Figure 6, which
geometry, and our results closely parallel these earlier
calculations.19 The cis-trans isomer is calculated to be
higher in energy by 3.0 kcal/mol while the trans-trans
is higher by 21.1 kcal/mol. On the basis of these energy
differences, the cis-cis isomer will constitute more than
99% of the mixture at room temperature, 298 K.
Consequently, we started our calculations of the com-
pounds MeOCH2CHMeOCO2CHMeCH2OMe with the
cis-cis geometry.
The results of the gas-phase calculations for the RR
isomer, the molecule with the isotactic carbonate HH
junction, are summarized in Figure 4. The conforma-
tions depicted in Figure 4 are viewed down the C-O
bond of the carbonate, and all of these conformers
represent local minima as confirmed by vibrational
frequency analyses. The nomenclature for the calculated
six minima indicates whether or not the molecule has
C2 or C1 symmetry. Thus, structures [1,1], [2,2], and
[3,3] have C2 symmetry whereas the combinations [2,3],
[1,3], and [1,2] contain two different halves on either
side of the molecule and have C1 symmetry. In the latter
cases, only the gross geometry of the 1, 2, or 3 fragment
is present in the respective [1,1], [2,2], or [3,3] isomer.
The bond angles and bond lengths are fully optimized
for each structure. On the basis of these gas-phase
structures, we can estimate that [1,1] and [1,3] isomers
constitute >95% of the species present at 298 K, and
the [3,3] and [1,2] isomers are less than 2% each. The
structures depicted in Figure 4 all have an anti confor-
mation of the ether oxygen with respect to the carbonate
O-CHMe bond as shown. The gauche conformer is
higher in energy by 0.94 kcal/mol in the case of the [1,1]
isomer.
Ta ble 1. Ca lcu la ted P op u la tion s for th e Isom er s of MeOCH2CHMeOCO2CHMeCH2OMe in th e Ga s P h a se, Ch lor ofor m ,
a n d Ben zen e
gas phase
chloroform
benzene
conformers
∆E (kcal/mol)
population ratio
∆E (kcal/mol)
population ratio
∆E (kcal/mol)
population ratio
RR
[1,1]
[1,3]
[1,1]
[3,3]
[1,3]
0
1
0
1
0
1
0.050
0.010
0.131
0.027
0.918
1
0.815
0.971
0.077
0.004
0.080
0.005
0.879
1
0.880
0.998
0.056
0.012
0.099
0.014
0.909
1
0.863
0.997
SR

4142 Byrnes et al.
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 6. Six local minimum conformers in both the gas
phase and chloroform calculations for RR isomers having C2
symmetry of MeOCO2CH2CHMeOCO2CHMeCH2OCO2Me. The
conformers’ structures are according to Figure 7: 1, [1,1′,1′′];
2, [1,2′,2′′]; 3, [1,3′,1′′]; 4, [1,1′,2′′]; 5, [1,2′,2′′]; 6, [1,3′,2′′].
F igu r e 8. Eight local minimum conformers in both the gas
phase and chloroform calculations for SR isomers having Cs
symmetry of MeOCO2CH2CHMeOCO2CHMeCH2OCO2Me. The
structures of conformers are as follows: 1, [1,1′,1′′]; 2, [1,2′,2′′];
3, [1,3′,1′′]; 4, [1,1′,2′′]; 5, [1,3′,2′′]; 6, [3,1′,1′′]; 7, [3,3′,1′′]; 8,
[3,1′,2′′].
Ta ble 2. Ca lcu la ted NMR Ch em ica l Sh ifts for th e Isom er s
of MeOCH2CHMeOCO2CHMeCH2OMe in th e Ga s P h a se
a n d Ch lor ofor m
conformers
gas phase (ppm)
chloroform (ppm)
RR
[1,1]
[1,3]
avea
[1,1]
[3,3]
[1,3]
ave
162.529
162.820
162.668
162.429
163.352
162.903
162.864
-0.196
162.345
162.675
162.499
162.377
163.062
162.662
162.685
-0.186
SR
RR-SRb
a
b
Statistical average chemical shift. Chemical shift difference
between RR and SR.
(GIAO) method available in Gaussian.21 For the mol-
ecules of MeOCH2CHMeOCO2CHMeCH2OMe, the re-
sults are tabulated in Table 2. For the RR isomer, two
carbonate carbon signals are predicted at 162.345 and
162.675 ppm in chloroform for the predominant local
minima. Given that at room temperature rotations
about single bonds will be rapid on the NMR time scale,
this predicts a single carbonate carbon resonance at
162.499 ppm. For the SR isomer where three local
minima are predicted to contribute to the average
carbon resonance of the carbonate carbon in chloroform,
we find 162.377, 163.062, and 162.662 ppm. Given the
relative population of these conformers at 298 K (1:
0.880:0.998), the predicted carbonate carbon signal is
162.685 ppm. The calculated chemical shift separation
for the carbonate carbon signals of the two isomers
representing i and s junctions is 0.186 ppm, which may
be compared to 0.042 ppm in CDCl3 solution. Moreover,
the calculations predict that the RR isomer should have
a chemical shift upfield relative to the SR isomers,
which is contrary to our experimental assignment in the
model compounds (with the ethoxy end groups) for i and
s. It is probably unreasonable to expect our current level
of computation of NMR chemical shifts to simulate with
accuracy such small experimental variations. For in-
stance, the concentration of the solution (0.2 M CDCl3
solution used in NMR experiments), not simulated in
the calculations, may have significant effects on the
intermolecular chain interactions, which changes the
relative energies and populations of the conformations
and results in deviations in the calculated NMR chemi-
cal shifts from the experimental ones. However, the
Figu r e 7. Newman projections for the RR isomers of MeOCO2-
CH2CHMeOCO2CHMeCH2OCO2Me, viewing down the bonds
of CH(Me)-O, CH2-CH(Me), and O-CH2.
indicates that all of them have contributions more or
less to the Boltzmann average at 298 K. The relative
energies for some isomers in chloroform differ signifi-
cantly from the gas phase, in contrast to the molecules
of MeOCH2CHMeOCO2CHMeCH2OMe, which suggests
that solvent effects will play a more important role on
the conformations of this molecule.
The SR isomers of MeOCO2CH2CHMeOCO2CHMe-
CH2OCO2Me are compared in Figure 8. Again, we only
show the results of symmetric conformers (Cs) derived
from [1,1] and [3,3] in Figure 5. We began with many
initial starting geometries, but some potential struc-
tures optimized to a reduced number of final geometries.
Indeed, we obtained eight minima in the relative energy
range 0-1.7 kcal/mol. Similarly to RR isomers, the
relative energies for some SR isomers in chloroform
were much different from the gas phase, as shown in
Figure 8.
NMR Ca lcu la tion s. With an attempt to simulate the
13C NMR chemical shifts of central carbonate carbons
of the model compounds, we carried out NMR calcula-
tions employing the gauge-independent atomic orbital

Macromolecules, Vol. 37, No. 11, 2004
Oligoether Carbonates 4143
Ta ble 3. Ca lcu la ted NMR Ch em ica l Sh ifts for th e Isom er s
of MeOCO2CH2CHMeOCO2CHMeCH2OCO2Me in th e Ga s
P h a se a n d Ch lor ofor m
Exp er im en ta l Section
All syntheses and solvent manipulations were carried out
under a nitrogen atmosphere using standard Schlenk-line and
drybox techniques. Solvents were distilled from sodium ben-
zophenone ketyl. Racemic propylene oxide (Fisher) and S-
propylene oxide (Alfa Aesar) were distilled from calcium
hydride. Potassium ethoxide (Aldrich), triphosgene (Aldrich),
and anhydrous 1,4-dioxane (Aldrich) were used as received.
Di- and tri(propylene glycol) methyl ethers (Aldrich), poly-
(propylene glycol) (typical Mn ) 425) (Aldrich), and deuterated
solvents were stored over 4 Å molecular sieves for 24 h prior
to use.
NMR Exp er im en ts. 1H and 13C{1H} NMR experiments
were carried out with a Bruker DRX-500 (5 mm broad band
probe) and a Bruker DRX-600 (5 mm broad band probe)
spectrometers, operating at proton Larmor frequencies of 500
and 600 MHz, respectively. The parameters used in 13C{1H}
NMR experiments on a Bruker DRX-600 spectrometer were
as follows: number of data point, TD ) 65 536; sweep width,
SWH ) 1502 Hz; relaxation time, D1 ) 2 s; and chemical shift
range 0-200 or 150-160 ppm. Typically 0.2 M sample
solutions in chloroform-d were used in the analyses. Their peak
frequencies were referenced against the solvent, chloroform-d
at 7.24 ppm for 1H and 77.0 ppm for 13C{1H} NMR.
conformers
gas phase (ppm)
chloroform (ppm)
RR
[1,1′,1′′]
[1,2′,1′′]
[1,3′,1′′]
[1,1′,2′′]
[1,2′,2′′]
[1,3′,2′′]
avea
[1,1′,1′′]
[1,2′,1′′]
[1,3′,1′′]
[1,1′2′′]
[1,3′,2′′]
[3,1′,1′′]
[3,3′,1′′]
[3,1′,2′′]
ave
162.307
163.052
163.257
161.967
163.436
163.291
162.761
162.468
162.995
163.233
161.941
163.167
163.093
163.508
163.252
162.766
-0.005
162.127
162.879
162.968
160.488
163.665
162.695
162.741
161.976
161.951
162.645
161.510
162.336
162.664
162.341
SR
162.167
0.574
RR-SRb
a
b
Statistical average chemical shift. Chemical shift difference
between RR and SR.
calculations do indicate that the conformations of
molecules and the configurations of stereocenters can
make differences in chemical shifts. Consistently, it is
experimentally evident from Figure 2 that the carbonate
carbon is sensitive to the stereocenters of the ring-
opened PO units in a chain of the type R(PO)nOCO2-
(PO)nR, where n ) 2. For n > 2, the rapidly increasing
number of stereoisomers leads to overlapping reso-
nances, and only diad sensitivity can be claimed by 13C
NMR spectroscopy. In the case of longer chain mol-
ecules, such as R(PO)10OCO2(PO)10R and R(PO)30OCO2-
(PO)30R, only single broad resonances assignable to i
and s are observed (see Supporting Information, Figure
S3.) For the isotactic oligomer prepared from S-PO, the
half-width of the signal for i is less than 0.01 ppm for n
∼ 10. This is not significantly broader than the half-
width seen for the iii tetrad signal where n ) 2, which
indicates that the time averaging of various conformers
in the long chain oligomer remains fast on the NMR
time scale.
Ma ss Sp ectr om etr y. The polymer samples obtained in the
reactions of polypropylene glycols and triphosgene were ana-
lyzed by matrix-assisted laser desorption ionization time-of-
flight mass spectrometry (MALDI-TOF/MS). MALDI-TOF
was performed on a Bruker Reflex III (Bruker, Breman,
Germany) mass spectrometer operated in a linear, positive ion
mode with a N2 laser. Laser power was used at the threshold
level required to generate signal. The accelerating voltage was
set to 28 kV. The instrument was calibrated with protein
standards bracketing the molecular weights of the samples.
The 2,5-dihydroxybenzoic acid (DHBA) matrix was prepared
as saturated solutions in THF. Allotments of 10 mL of matrix
and 2 mL of a solution of the sample were thoroughly mixed
together; 0.5 mL of this was spotted on the target plate and
allowed to dry.
Small molecular weight di-, tri-, and tetra(propylene glycol)
ethyl ethers and the carbonates produced from them were
analyzed on a Bruker Esquire ion trap mass spectrometer
(Bremen, Germany) equipped with an orthogonal electrospray
source operated in positive ion mode. Samples were prepared
in a solution containing methanol/acetic acid (99:1) and infused
into the electrospray source at a rate of 5-10 mL/min. Optimal
ESI conditions were as follows: capillary voltage, 3500 V;
source temperature, 250 °C; the ESI drying gas, nitrogen. The
ion trap was set to pass ions from m/z 50 to 2000 amu. Data
were acquired in continuum mode until acceptable averaged
data were obtained.
Ch r om a togr a p h y. Gel permeation chromatographic (GPC)
analysis was performed at 35 °C on a Waters Breeze system
equipped with a Waters 410 refractive index detector and a
set of two columns, Waters Styragel HR-2 and HR-4 (Milford,
MA). THF (HPLC grade) was used as the mobile phase at 1.0
mL/min. The sample concentration was 0.1%, and the injection
volume was 100 µL. The samples were centrifuged and filtered
before analysis. The calibration curve was made with six
polystyrene standards covering the molecular weight range
from 580 to 460 000 Da. High-performance liquid chromato-
graphic (HPLC) analysis was performed on the same instru-
ment as GPC. Instead, a Symmetry C18 (4.6 × 150 mm) column
(Milford, MA) and methanol/water (50:50) mobile phase at 1.0
mL/min were used.
Ca lcu la tion s. The calculations were performed with den-
sity functional theory and the Gaussian 98 suite of pro-
grams.16,17 The geometry optimizations and vibrational fre-
quency calculations were carried out at the B3LYP/6-31G(d)
level.22 The calculations for NMR chemical shift prediction
were done with the GIAO method and at the B3LYP/6-311+G-
(2d,p) level in both the gas phase and with the PCM method
for chloroform, using the optimized geometries of each local
minimum.20,21
The NMR chemical shifts for MeOCO2CH2CHMeOCO2-
CHMeCH2OCO2Me were also calculated, and the results
are tabulated in Table 3. In the gas phase, for RR
isomers, the average central carbonate carbon signal is
predicted at 162.761 ppm, while for the SR isomers, it
is 162.766 ppm. In chloroform, the predicted chemical
shifts for the central carbonate carbon are 162.741 ppm
for RR and 162.167 ppm for SR.
Con clu sion s
The model compounds, R(PO)nOCO2(PO)nR, where R
) Me, Et, or H, and n ) 1, 2, 3, 4, ∼10, and ∼30, were
synthesized for PPC microstructural assignments in
NMR studies. The 13C{1H} NMR investigations of those
compounds showed that the carbonate carbon signals
have both regio- and stereosensitivity at the diad and
the tetrad levels to its adjacent PO ring-opened units.
The calculations on the model compounds indicated that
the carbonate groups exist predominantly at cis-cis
geometries with more than one stable conformation for
each molecule. In addition, the 13C chemical shifts
predicted in the calculations were sensitive to the
conformations of the molecules and the configurations
of the stereocenters in PO ring-opened units.

4144 Byrnes et al.
Macromolecules, Vol. 37, No. 11, 2004
Syn th esis of Oligo(p r op ylen e glycol) Eth yl Eth er s. In
a typical reaction, potassium ethoxide (2.0 g, 23.8 mmol) was
allowed to react with rac-PO (or S-PO) (5 mL, 71.6 mmol) in
40 mL of 1,4-dioxane at 90 °C for 20 h. After cooling to room
temperature, the product was allowed to react with excess HCl
solution. The solvents and any other volatile species were
evaporated under vacuum. The residue was extracted with
hexane. After hexane was distilled out, a yellow liquid was
obtained, which was a mixture of oligo(propylene glycol) ethyl
ethers. This mixture was separated using spinning band
column distillation under vacuum at elevated temperatures
to obtain pure (>98%) di-, tri-, and tetra(propylene glycol) ethyl
ethers (colorless liquids). The final products were analyzed by
HPLC, ESI/MS, and NMR.
Di(rac-propylene glycol) Ethyl Ether. ESI/MS: peak at m/z
) 185 for Et(PO)2OH‚Na+. 1H NMR (CDCl3, δ, ppm): 0.97 (m,
2CH3-CH), 1.06 (t, CH3-CH2), 3.78 (m, CH-OH), 3.00-3.60
(m, CH2, CH, OH). 13C {1H} NMR (CDCl3, δ, ppm): 14.76,
14.78, 16.22, 16.94, 18.00, 18.26 (CH3), 66.42, 66.44, 74.16,
74.26, 75.53 (CH2), 65.44, 66.88, 74.19, 75.94 (CH).
3.83 (m, CH-OH), 3.00-3.60 (m, CH2, CH, OH). 13C {1H} NMR
(CDCl3, δ, ppm): 17.71, 17.83 (CH3), 72.73, 72.78, 72.86, 72.99,
73.23, 73.26 (CH2), 74.99, 75.18, 75.22, 75.37 (CH).
Syn th esis of Mod el Ca r bon a te Com p ou n d s. R-(PO)n-
OCOO-(PO)n-R, R ) CH3, C2H5, or H, were synthesized as
following. R-(PO)n-OH (0.5 mL) was reacted with 1/6 molar
equivalent of triphosgene in 2 mL of benzene, stirring.15 After
3 h, 2 mL of pyridine was added into the flask, and the
temperature was increased to 60 °C for another 12 h. After
the removal of solvent and volatile species, the product was
extracted with hexane. The final products were analyzed with
MS and NMR.
CH3(PO)2OH/ Triphosgene. ESI/MS: peak at m/z ) 373 for
Me(PO)2OCO2(PO)2Me‚Na+. 1H NMR: 1.00-1.30 (m, CH3-
CH), 3.10-3.60 (m, CH3-O, CH2, CH), 4.86 (m, CH-OCO2).
13C {1H} NMR: 16.83-17.43 (CH3-CH), 57.18, 59.52 (CH3-
O), 71.67, 71.69, 71.75, 72.05, 72.10, 73.95, 74.01, 74.06, 75.46,
75.49, 77.00, 77.13 (CH2), 73.51-73.90, 74.09, 74.15, 75.66,
75.68, 76.28 (CH), 154.10-154.25 (CdO).
CH3(PO)3OH/ Triphosgene. ESI/MS: peak at m/z ) 489 for
Me(PO)3OCO2(PO)3Me‚Na+. 1H NMR: 1.05-1.30 (m, CH3-
CH), 3.20-3.70 (m, CH3-O, CH2, CH), 4.87 (m, CH-OCO2).
13C {1H} NMR: 16.48-18.30 (CH3-CH), 59.46 (CH3-O),
71.50-73.85, 74.74, 74.83, 76.37-77.08 (CH2), 73.87, 74.05,
74.15, 74.93-76.31 (CH), 154.10-154.25 (CdO).
CH3CH2(rac-PO)1OH/ Triphosgene. ESI/MS: peak at m/z )
257 for Et(PO)1OCO2(PO)1Et‚Na+. 1H NMR: 1.08 (t, CH3-
CH2), 1.19 (m, CH3-CH), 3.00-4.00 (m, CH2, CH), 4.82 (m,
CH-OCO2). 13C {1H} NMR: 14.88, 16.49, 16.51 (CH3), 66.53,
66.54, 72.54, 72.60 (CH2), 73.13, 73.18 (CH), 154.13-154.18
(CdO).
CH3CH2(rac-PO)2OH/ Triphosgene. ESI/MS: peak at m/z )
373 for Et(PO)2OCO2(PO)2Et‚Na+. 1H NMR: 1.00-1.30 (m,
CH3), 3.00-3.60 (m, CH2, CH), 4.80 (m, CH-OCO2). 13C {1H}
NMR: 15.00-17.20 (CH3), 66.54 (CH3-CH2-O), 71.20, 71.26,
71.59, 71.62, 74.33, 74.37, 74.50 (CH2), 73.34, 73.38, 73.51,
73.59, 75.10, 75.12, 75.34, 75.35 (CH), 154.09-154.20 (CdO).
CH3CH2(rac-PO)3OH/ Triphosgene. ESI/MS: peak at m/z )
489 for Et(PO)3OCO2(PO)3Et‚Na+. 1H NMR: 1.00-1.30 (m,
CH3), 3.00-3.60 (m, CH2, CH), 4.78 (m, CH-OCO2). 13C {1H}
NMR: 15.00-18.30 (CH3), 66.49 (CH3-CH2-O), 71.10-73.30,
74.20-74.40, 75.83 (CH2), 73.31, 73.50, 73.58, 73.60, 74.42-
75.62, 76.43 (CH), 154.10-154.21 (CdO).
CH3CH2(rac-PO)4OH/ Triphosgene. ESI/MS: peak at m/z )
605 for Et(PO)4OCO2(PO)4Et‚Na+. 1H NMR: 1.00-1.30 (m,
CH3), 3.10-3.60 (m, CH2, CH), 4.78 (m, CH-OCO2). 13C {1H}
NMR: 15.00-17.30 (CH3), 66.48 (CH3-CH2-O), 71.09-71.70,
72.80-73.25, 73.34, 73.38, 74.30-74.77 (CH2), 73.27, 73.31,
73.49, 73.57, 73.59, 74.86-75.64 (CH), 154.08-154.20 (CdO).
50% CH3CH2(rac-PO)2OH/ 50% CH3CH2(S-PO)2OH/ Tri-
phosgene. ESI/MS: peak at m/z ) 373 for Et(PO)2OCO2(PO)2-
Et‚Na+. It has similar chemical shifts for the signals in 1H
and 13C {1H} NMR spectra to the product of CH3CH2(rac-PO)2-
OH/triphosgene, except that the relative intensities are dif-
ferent.
50% CH3CH2(rac-PO)3OH/ 50% CH3CH2(S-PO)3OH/ Tri-
phosgene. ESI/MS: peak at m/z ) 489 for Et(PO)3OCO2(PO)3-
Et‚Na+. It has similar chemical shifts for the signals in 1H
and 13C {1H} NMR spectra to the product of CH3CH2(rac-PO)3-
OH/triphosgene, except that the relative intensities are dif-
ferent.
CH3CH2(rac-PO)nOH (Average n ) 10)/Triphosgene. MALDI/
MS: major series, Et(PO)nOCO2(PO)mEt‚Na+.1H NMR: 1.00-
1.30 (m, CH3), 3.00-3.60 (m, CH2, CH), 4.80 (m, CH-OCO2).
13C {1H} NMR: 16.60-18.40 (CH3), 66.50 (CH3-CH2-O),
70.20-76.00 (CH2, CH), 154.08-154.20 (CdO).
Tri(rac-propylene glycol) Ethyl Ether. ESI/MS: peak at m/z
1
) 243 for Et(PO)3OH‚Na+. H NMR (CDCl3, δ, ppm): 0.90-
1.10 (m, 4CH3), 3.77 (m, CH-OH), 3.00-3.60 (m, CH2, CH,
OH). 13C {1H} NMR (CDCl3, δ, ppm): 14.84, 14.87, 14.88,
14.91, 16.68, 16.70, 16.78, 16.81, 16.86, 16.88, 16.94, 16.99,
18.00, 18.03, 18.30 (CH3), 66.42, 66.43, 72.92, 73.04, 73.06,
73.13, 74.14, 74.23, 74.31, 74.34, 75.67 (CH2), 65.31, 65.38,
66.87, 66.91, 74.47, 74.63, 74.90, 74.91, 74.98, 75.00, 76.24,
76.50 (CH).
Tetra(rac-propylene glycol) Ethyl Ether. ESI/MS: peak at
m/z ) 301 for Et(PO)4OH‚Na+. 1H NMR (CDCl3, δ, ppm):
0.90-1.10 (m, 4CH3), 3.77 (m, CH-OH), 3.00-3.60 (m, CH2,
CH, OH). 13C {1H} NMR (CDCl3, δ, ppm): 14.78, 14.81, 14.82,
14.85, 16.62, 16.64, 16.67, 16.69, 16.72, 16.74, 16.79, 16.81,
16.84, 16.87, 17.97, 17.99, 18.00, 18.02 (CH3), 66.32, 66.36,
72.63, 72.67, 72.83, 72.85, 72.87, 72.92, 72.96, 72.99, 73.05,
73.19, 73.21, 74.08, 74.12, 74.14, 74.16, 74.18, 74.25, 74.28,
75.54, 75.59, 75.61 (CH2), 65.26, 65.31, 65.34, 66.77, 66.82,
74.41, 74.45, 74.57, 74.60, 74.75, 74.77, 74.83, 74.84, 74.89,
74.91, 74.93, 74.97, 75.03, 75.10, 75.16, 75.18, 76.14, 76.24,
76.26, 76.29, 76.30, 76.39 (CH).
Di(S-propylene glycol) Ethyl Ether. ESI/MS: peak at m/z )
185 for Et(PO)2OH‚Na+. 1H NMR (CDCl3, δ, ppm): 0.97 (m,
2CH3-CH), 1.06 (t, CH3-CH2), 3.78 (m, CH-OH), 3.00-3.60
(m, CH2, CH, OH). 13C {1H} NMR (CDCl3, δ, ppm): 14.89,
16.76, 18.29 (CH3), 66.53, 74.31, 74.38 (CH2), 65.50, 74.29 (CH).
Tri(S-propylene glycol) Ethyl Ether. ESI/MS: peak at m/z
1
) 243 for Et(PO)3OH‚Na+. H NMR (CDCl3, δ, ppm): 0.90-
1.10 (m, 4CH3), 3.77 (m, CH-OH), 3.00-3.60 (m, CH2, CH,
OH). 13C {1H} NMR (CDCl3, δ, ppm): 15.01, 16.79, 17.01, 18.32
(CH3), 66.57, 73.17, 74.31, 74.44 (CH2), 65.48, 74.55, 75.12
(CH).
C2H5-[OCH2CH(CH3)]n-OH (Average n ) 10). Potassium
ethoxide (0.24 g, 2.86 mmol) was reacted with rac-PO (or S-PO)
(2 mL, 28.6 mmol) in 20 mL of 1,4-dioxane at 90 °C for 20 h.
After the addition of HCl solution, removal of solvents and
extraction with hexane, a yellow liquid was obtained (90%
1
yield). Using rac-PO: GPC, Mn ) 740, PDI ) 1.28. H NMR
(CDCl3, δ, ppm): 0.90-1.10 (m, CH3), 3.83 (m, CH-OH), 3.00-
3.60 (m, CH2, CH, OH). 13C {1H} NMR (CDCl3, δ, ppm):
15.02-18.63 (m, CH3), 67.01, 73.30-74.90, 76.24, 76.32 (CH2),
65.95, 65.98, 67.51, 67.57, 75.08-75.90, 77.46 (CH). Using
S-PO: GPC, Mn ) 780, PDI ) 1.25. 1H NMR (CDCl3, δ, ppm):
0.90-1.10 (m, CH3), 3.83 (m, CH-OH), 3.00-3.60 (m, CH2,
CH, OH). 13C {1H} NMR (CDCl3, δ, ppm): 15.46-19.20 (m,
CH3), 73.00, 73.08, 73.13, 74.17, 74.18 (CH2), 65.27, 66.30,
74.38, 74.84, 75.13, 75.16, 75.20 (CH).
CH3CH2(S-PO)nOH (Average n ) 10)/Triphosgene. MALDI/
MS: major series, Et(PO)nOCO2(PO)mEt‚Na+.1H NMR: 1.00-
1.30 (m, CH3), 3.10-3.60 (m, CH2, CH), 4.78 (m, CH-OCO2).
13C {1H} NMR: 16.40-17.40 (CH3), 66.45 (CH3-CH2-O),
71.50-75.80 (CH2, CH), 154.21 (CdO).
C2H5-[OCH2CH(CH3)]n-OH (Average n ) 50). This large
MW polymer sample was synthesized to investigate the regio-
and stereoselectivity of PO ring-opening in the above reactions
(see Supporting Information). Potassium ethoxide (24 mg, 0.29
mmol) was allowed to react with rac-PO (2 mL, 28.6 mmol) in
the same condition and following the same procedure as above.
A yellow viscous liquid was finally yielded. GPC: Mn ) 5040,
PDI ) 1.32. 1H NMR (CDCl3, δ, ppm): 0.90-1.10 (m, CH3),
Poly(propylene glycol) (Mn ) 2000)/ Triphosgene. MALDI/
MS: two major series, HO(PO)nOCO2(PO)mOH‚Na+, and
1
HO(PO)nOCO2(PO)mOCO2(PO)lOH‚Na+. H NMR: 1.00-1.30
(m, CH3), 3.10-3.60 (m, CH2, CH), 4.78 (m, CH-OCO2). 13C

Macromolecules, Vol. 37, No. 11, 2004
Oligoether Carbonates 4145
{1H} NMR: 17.00-17.50 (CH3), 72.50-73.40 (CH2), 74.70-
75.50 (CH), 154.10-154.22 (CdO).
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Ack n ow led gm en t. We thank the Department of
Energy, Office of Basic Sciences, Chemistry Division,
for financial support of this work. We gratefully ac-
knowledge generous computational resources from the
Ohio Supercomputer Center where these calculations
were performed. The mass spectrometers were pur-
chased by a grant from the Hayes Investment Fund of
the Ohio Board of Regents. We thank the staff of the
Campus Chemical Instrument Center (CCIC) for run-
ning the samples by both NMR and MS techniques. We
also thank Dr. Edward M. Dexheimer at BASF Corp.,
Dr. Wei Chen, and Dr. Meng-Lin Tsao at The Ohio State
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Su p p or tin g In for m a tion Ava ila ble: 13C{1H} NMR (150
MHz, CDCl3) spectrum of PPO obtained from PO polymeriza-
tion with KOEt (50:1), showing the regio- and stereoselectivity
involved in propylene oxide ring-opening polymerization; 13C-
{1H} (150 MHz, CDCl3) NMR spectra of carbonate carbon
region of regioregular oligoether carbonate compounds (n )
3, 4); statistical calculations for stereosequences of carbonate
carbons in regioregular Et(PO)2OCO2(PO)2Et and Et(PO)3-
OCO2(PO)3Et made from etherol derived from rac-PO, as well
as from etherol mixtures (1:1) derived separately from rac-
PO and S-PO; statistical calculations for regio- and stereo-
sequences of carbonate carbons in regioirregular Me(PO)2OCO2-
(PO)2Me made from regioirregular di(propylene glycol) methyl
ethers; and 13C{1H} (150 MHz, CDCl3) NMR spectra of
carbonate carbon region of regioregular oligoether carbonate
compounds (n ) ∼10, ∼30); energetic and coordinate informa-
tion on the optimized geometries obtained in the calculations
for dimethyl carbonate, MeOCH2CHMeOCO2CHMeCH2OMe,
and MeOCO2CH2CHMeOCO2CHMeCH2OCO2Me. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
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