Polycarbonates derived from glucose via an organocatalytic approach

Article abstract of DOI:10.1021/ja402319m

An organocatalyzed ring-opening polymerization methodology was developed for the preparation of polycarbonates derived from glucose as a natural product starting material. The cyclic 4,6-carbonate monomer of glucose having the 1, 2, and 3 positions methyl-protected was prepared in three steps from a commercially available glucose derivative, and the structure was confirmed by means of NMR and IR spectroscopies, electrospray ionization mass spectrometry (MS), and single-crystal X-ray analysis. Polymerization of the monomer, initiated by 4-methylbenzyl alcohol in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene as the organocatalyst, proceeded effectively in a controlled fashion to afford the polycarbonate with a tunable degree of polymerization, narrow molecular weight distribution, and well-defined end groups, as confirmed by a combination of NMR spectroscopy, gel-permeation chromatography, and MALDI-TOF MS. A distribution of head-to-head, head-to-tail, and tail-to-tail regiochemistries was determined by NMR spectroscopy and tandem MS analysis by electron transfer dissociation. These polycarbonates are of interest as engineering materials because of their origination from renewable resources combined with their amorphous character and relatively high glass transition temperatures as determined by X-ray diffraction and differential scanning calorimetry studies.

Full text of DOI:10.1021/ja402319m

Communication
pubs.acs.org/JACS
Polycarbonates Derived from Glucose via an Organocatalytic
Approach
Koichiro Mikami,† Alexander T. Lonnecker,† Tiffany P. Gustafson,† Nathanael F. Zinnel,‡ Pei-Jing Pai,‡
David H. Russell,‡ and Karen L. Wooley*,†
†Departments of Chemistry and Chemical Engineering, Texas A&M University, College Station, Texas 77842, United States
‡Laboratory for Biological Mass Spectrometry, Department of Chemistry, Texas A&M University, College Station, Texas 77843,
United States
S
* Supporting Information
Moreover, with the advances that have been made in controlled
ABSTRACT: An organocatalyzed ring-opening polymer-
ization methodology was developed for the preparation of
polycarbonates derived from glucose as a natural product
starting material. The cyclic 4,6-carbonate monomer of
glucose having the 1, 2, and 3 positions methyl-protected
was prepared in three steps from a commercially available
glucose derivative, and the structure was confirmed by
means of NMR and IR spectroscopies, electrospray
ionization mass spectrometry (MS), and single-crystal X-
ray analysis. Polymerization of the monomer, initiated by
4-methylbenzyl alcohol in the presence of 1,5,7-
triazabicyclo[4.4.0]dec-5-ene as the organocatalyst, pro-
ceeded effectively in a controlled fashion to afford the
polycarbonate with a tunable degree of polymerization,
narrow molecular weight distribution, and well-defined
end groups, as confirmed by a combination of NMR
spectroscopy, gel-permeation chromatography, and
MALDI-TOF MS. A distribution of head-to-head, head-
to-tail, and tail-to-tail regiochemistries was determined by
NMR spectroscopy and tandem MS analysis by electron
transfer dissociation. These polycarbonates are of interest
as engineering materials because of their origination from
renewable resources combined with their amorphous
character and relatively high glass transition temperatures
as determined by X-ray diffraction and differential scanning
calorimetry studies.
ROP via organocatalysis over the past decade, allowing metal
catalysts to be replaced with less toxic and more environmentally
acceptable alternatives,10 we hypothesized that polycarbonates
could be produced facilely and with potentially broad
applications. ROP for the synthesis of polycarbonates, however,
has been applied mainly with monocyclic six-membered ring
structures, such as trimethylenecarbonate (TMC) derivatives, to
afford aliphatic polycarbonates having relatively low glass
transition temperatures (Tg).10a,11 Although polycarbonates
derived from various monosaccharides have been prepared by
ROP3−5,12 and polycondensation,13 the organocatalyzed poly-
merization of a five-membered cyclic carbonate having a
glucopyranoside structure was reported only recently.6,14 Endo
and co-workers conducted detailed studies of anionic- versus
organobase-initiated and ‑catalyzed ROPs of 4,6-O-benzylidene-
2,3-O-carbonyl-α-D-glucopyranoside, for which they postulated
that a zwitterionic mechanism was involved with the use of an
organobase and observed relatively broad molecular weight
distributions with significant macrocyclic products.14 In contrast,
our interest is in the organocatalyzed ROP of six-membered
bicyclic carbonate monomers derived from glucose by using
initiating species to achieve high degrees of control, with detailed
analysis of the regiochemical outcomes and the physicochemical
properties of the materials.
Herein we report the synthesis of a glucose-based bicyclic
carbonate monomer, 1, and its controlled ROP with an
organocatalyst. The monomer was prepared in three steps, and
the polymerization proceeded in a controlled fashion, yielding
amorphous polycarbonates with interesting physical properties.
As shown in Scheme 1, 1 was synthesized with methyl ether
protection of the hydroxyl groups at the 1, 2, and 3 positions to
suppress undesired side reactions during the polymerization.
Commercially available methyl 4,6-O-benzylidene-α-D-glucopyr-
anoside was methylated by reaction with methyl iodide for 19 h
in the presence of sodium hydride in N,N-dimethylformamide
(DMF) at room temperature to afford 2 in 95% yield.
Hydrogenolysis of 2 using H2, Pd on carbon, and a catalytic
amount of hydrochloric acid in dioxane over several days at room
temperature gave 3 in 93% yield.15 The cyclic carbonylation of 3
was attempted initially by using ethyl or 4-nitrophenyl
ngineering plastics play significant technological roles,1 and
E
in particular, polycarbonate is a material of importance
because of not only its high mechanical strength, transparency,
and impact resistance but also its potential for degradability and
biocompatibility.2 On the other hand, as fossil fuels that are often
used to produce these materials dwindle, much attention has
been paid to the development of novel renewable feedstocks,
such as readily available monosaccharides,3−7 menthides,8 and
terpenes.9 Among these, glucose would potentially be an ideal
feedstock because of its abundance in nature in monomeric and
polymeric forms.9b
We had anticipated that a controlled ring-opening polymer-
ization (ROP) of glucose-based bicyclic carbonate monomers
ideally would lead to renewable polycarbonates having well-
defined structures and amorphous or crystalline phases with high
thermal stabilities based on the cyclic saccharide repeat unit.
Received: March 19, 2013
Published: April 30, 2013
© 2013 American Chemical Society
6826
dx.doi.org/10.1021/ja402319m | J. Am. Chem. Soc. 2013, 135, 6826−6829

Journal of the American Chemical Society
Communication
1, the cyclic carbonylation proceeded using ethyl chloroformate.4
In contrast, the more reactive and symmetrical carbonylation
agent bis(pentafluorophenyl) carbonate was needed to form 1
(vide supra). Recently, Suzuki et al.19 also reported difficulty in
forming the cyclic carbonate of 2′-deoxyadenosine and resorted
to condensation polymerization by a silicon-assisted alkoxy/
carbonylimidazolide coupling reaction. The differences among
these three cyclic carbonylations could arise from the
configuration of the two hydroxyl groups involved with
cyclization: the hydroxyl groups of the xylofuranose have a cis
configuration and those of 2′-deoxyadenosine and 2 have a trans
configuration. The moderate reproducibility of the cyclic
carbonylation method may have contributed to the limited
development of 1 and its lack of study as a monomer for ROP.16
The ROP of 1 was conducted via an initiator/chain-end
activation mechanism under organocatalysis and studied first as a
function of time and conversion at a constant monomer/initiator
stoichiometry and then at various monomer/initiator ratios.
Initially, the ROP kinetics were studied using a monomer/
initiator feed ratio ([M]0/[I]0) of 51 in CH2Cl2 with 4-
methylbenzyl alcohol as the initiator and commercially available
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the organocatalyst
(Table 1, entry 3) to allow for monitoring of the monomer
conversion and the growth of the polymer chain as functions of
time.10a The number-average molecular weights (Mn) were
estimated by gel-permeation chromatography (GPC) and
calculated using 1H NMR spectroscopy by comparing the
resonance for the methyl protons of the initiator with that for the
proton at the 1-position of the repeat unit. The conversion of 1
reached 44% after 1 h and >98% after 6 h (Figure 2A). The GPC
traces showed unimodal peaks during the reaction, which shifted
toward shorter elution times as polymerization progressed while
maintaining narrow polydispersity index (PDI) values (Figure
2A inset). Kinetic plots of ln([M]0/[M]) versus time were linear
(Figure 2B), indicating first-order kinetics, which is characteristic
of ROP.20 Furthermore, when the [M]0/[I]0 feed ratio was
varied to afford the series of polymers 4−6, the value of Mn
increased in proportion to the feed ratio while the PDI remained
narrow (Table 1), indicating that the polymerizations proceeded
in a controlled fashion.
Scheme 1. Synthesis of Bicyclic Glucose-Based Carbonate
Monomer 1
chloroformate, but only monosubstituted compounds were
obtained.16 The cyclization reaction was then performed
successfully using bis(pentafluorophenyl) carbonate and CsF
in tetrahydrofuran (THF)17,18 at 60 °C over 25 h to afford 1 in
67% yield after column chromatography and 36% yield after
subsequent recrystallization in chloroform/hexanes. The struc-
ture was confirmed by 1H NMR (see Figure 3), 13C NMR, and
FT-IR spectroscopies combined with COSY, HMQC, HMBC,
DEPT 90, and DEPT 135 NMR and electrospray ionization mass
spectrometry (ESI-MS) analyses [Figures S1−S6 in the
Supporting Information (SI)]. A single crystal of 1 was analyzed
by X-ray diffraction, which further confirmed the structure
(Figures 1 and S21 and Tables S12−S16). It was expected that
the ROP would proceed efficiently, judging from the strained six-
membered ring revealed by the X-ray analysis.
Figure 1. Single-crystal X-ray structure of 1.
Despite having first been reported in 1970,16 1 had not been
utilized in ROP. Similar bicyclic sugar-based carbonates have also
been synthesized and investigated for ROP, although they
typically did not present difficulties with their preparation as was
observed for 1. For instance, in the case of the synthesis of a
xylofuranose-based monomer having a structure similar to that of
The structure of the polycarbonate was confirmed by IR, 1H
NMR, and 13C NMR spectroscopies. The assignments of the 1H
and 13C NMR spectra were carried out by COSY, HMQC,
HMBC, DEPT 90, and DEPT 135 NMR analyses (Figures S7−
S11). The characteristic carbonyl vibration of the carbonate
linkage was observed at 1751 cm−1 in the IR spectra. As
a
Table 1. ROP of Glucose-Based Monomer 1 (M) via Organocatalysis by TBD with Initiation by 4-Methylbenzyl Alcohol (I)
b
c
d
e
f
g
h
entry
polymer
cat. mol %
[M]
[M]0/[I]0
time (h)
conv. (%)
Mn (g/mol)
Mn (g/mol)
Mn (g/mol)
PDI
1
2
3
4
5
6
2
2
1
1
1
1
15
27
51
2
2
7
>99
>99
>98
3850
6780
2320
5390
4140
7600
1.11
1.16
1.15
12800
14700
13000
a
The polymerization of 1 was carried out with TBD in the presence of 4-methylbenzyl alcohol as the initiator at room temperature in CH2Cl2.
b
c
d
e
1
Concentration of 1 in mol/L. monomer/initiator feed ratio. Estimated by H NMR analysis in benzene-d6. Calculated as (formula weight of
f
initiator) + (formula weight of monomer) × ([M]0/[I]0) × (conversion/100 %). Calculated by GPC (THF eluent) with a static laser detector.
g
h
1
Estimated using H NMR spectra in CD2Cl2. Estimated by GPC (THF eluent) using polystyrene standards.
6827
dx.doi.org/10.1021/ja402319m | J. Am. Chem. Soc. 2013, 135, 6826−6829

Journal of the American Chemical Society
Communication
downfield from ca. 4.0 to 4.6 ppm, whereas the methoxy proton
signals (7−9) were shifted upfield from ca. 3.6 to 3.5 ppm. These
shifts could arise from an electronic effect of the adjacent
carbonyl group and the geometric conformational changes upon
opening of the six-membered cyclic carbonate of the monomer
to give the linear polycarbonate. The value of Mn estimated by 1H
NMR analysis [based on the ratio of the integral for the anomeric
proton at position 1 in the repeat unit (4.8 ppm) to that for the
methyl protons of the initiator at the α-chain end (2.4 ppm)] was
7600 g/mol, which is in agreement with the theoretical value of
6780 g/mol and the GPC-estimated value of 5390 g/mol.
The 13C NMR spectra showed the characteristic resonance for
a carbonate linkage at 155.0−154.4 ppm (Figure S12), with three
sets of observed signals. Multiple carbonyl resonance frequencies
could arise from regiorandom ordering, as Gross and co-workers
observed during their study of ROP of a xylofuranose-based
monomer,4 suggesting that the organocatalytic ROP of glucose-
based monomer 1 propagated nonselectively to give a
distribution of head-to-head (HH), head-to-tail (HT) and tail-
to-tail (TT) regiochemistries.
The regiorandom nature of the polymerization was further
supported through ESI tandem MS analysis by electron transfer
dissociation (ETD) of the trisodiated polymer species of 5. Ion
assignments were made by determining all of the potential cross-
ring cleavages for both the HT and tail-to-head (TH)
orientations of the polymer subunit, and a table associated with
each orientation was formulated (Figure S14a,b). Each
orientation produces unique cross-ring cleavage fragment ions
that were used as diagnostic ions to determine the regiochemistry
for a particular subunit. The product ion spectrum (Figure S14c)
shows loss of full subunits as well as product ions with cross-ring
cleavages between each full subunit loss. The diagnostic ions for
both the HT and TH orientations were observed for each subunit
in the product ion spectrum, which indicates that all three
regiochemistries (HH, HT, and TT) were present in roughly
equivalent quantities (see the SI for a detailed discussion).
Furthermore, the polymer end groups were analyzed by
matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectrometry (Figure S13) to confirm
the controlled nature of the polymerization. The spectrum of 5
contains only one series of peaks corresponding to K+ adducts of
the polymers having chain ends of the 4-methylbenzyl alcohol
initiator and a hydroxyl terminating group. The mass differences
between the peaks (m/z 248.5 1.3) were also in agreement
with the average mass of the polycarbonate repeat units (248.2 g/
mol). For example, the 21-mer of this distribution would be
expected to give a chemical formula of C10H16O7 × 21 (repeat
unit) + C8H9O1 (initiating group) + H + K = C218H346O148K, for
which the calculated average mass is 5374.2 g/mol, and indeed, a
signal was observed at m/z 5373.8. These data confirm not only
the efficient incorporation of the initiator at the α chain end,
■
●
△
Figure 2. (A) Plots of Mn ( , ; left axis) and Mw/Mn ( , right axis) vs
monomer conversion for ROP of 1 using TBD as the catalyst and 4-
methylbenzyl alcohol as the initiator at a 1/initiator/TBD ratio of 51/1/
0.5, as obtained from NMR and GPC analyses of aliquots collected for
evaluation after 1, 3, and 6 h of reaction. The inset shows GPC elution
curves for monitoring of the polymerization of 1. (B) Kinetic plot of
ln([M]0/[M]) vs time using data obtained by GPC [refractive index
(RI) detector].
illustrated by the 1H NMR spectra of 1 and 5 (Figure 3), the most
significant proton resonance differences between the monomer
and the polymer occurred for the pyranoside methine proton
(labeled as 4), the methoxy protons (7−9), and the methylene
protons α to the carbonate linkage (4). Compared with the
monomer, the polycarbonate proton 4 resonance was shifted
1
which is consistent with the chain-end analysis by H NMR
spectroscopy, but also the fact that the MALDI-derived Mn value
is in close agreement with the Mn values estimated by GPC and
1H NMR analysis. These results indicate that the polycarbonates
had well-defined end groups and were prepared by controlled
polymerizations without significant side reactions.
The thermal properties of the glucose polycarbonates were
evaluated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under inert atmosphere (Table S1
and Figures S15 and S16). Thermal degradation proceeded at
relatively low temperatures (250−320 °C for initial to complete
mass loss; Figure S16). One possible mechanism of the thermal
1
Figure 3. H NMR spectra of the monomer 1 and the polymer 5 in
CD2Cl2.
6828
dx.doi.org/10.1021/ja402319m | J. Am. Chem. Soc. 2013, 135, 6826−6829

Journal of the American Chemical Society
Communication
degradation could be abstraction of a proton from C3 or C5 of
the saccharide ring by the carbonyl O atom to afford CO2 (Figure
S17). The mass spectrometer coupled to the TGA detected
peaks in increments of m/z 28 and 44, which could be attributed
(D.H.R., N.F.Z., P.-J.P.)], and the Robert A. Welch Foundation
[A-1176 (D.H.R., N.F.Z., P.-J.P.) and A-0001 (K.M., A.T.L.,
T.P.G., K.L.W.)]. We thank Howard J. Williams for 2D NMR
measurements and Joseph H. Reibenspies for X-ray analysis.
+
to the loss of CO+ and CO2 , respectively. The DSC analysis of
REFERENCES
these polycarbonates (Figure S15) revealed high glass transition
temperatures: even the lowest-molecular-weight polycarbonate,
4, showed Tg = 106 °C, which is significantly higher than those of
common aliphatic polycarbonates such as poly(1,3-trimethylene
carbonate) (Tg ≈ −18 °C).11h The elevated Tg of these glucose-
based polycarbonates over aliphatic equivalents could arise from
the cyclic structure in the main chain, making these materials
attractive for various applications, such as for hard segments of
novel biodegradable elastomers.4
Since DSC analysis did not reveal a melting point below the
thermal degradation temperature, X-ray diffraction (XRD)
measurements were made. Bulk and annealed samples of 6
were subjected to XRD and showed a broad halo rather than a
sharp reflection peak (Figure S18), indicating an amorphous
character. The lack of crystallinity could be explained by the
regiorandom sequence of the polymer, although this is in sharp
contrast to the case of the xylofuranose-based polycarbonate,4
which exhibited a semicrystalline nature even though it also had a
distribution of HH, HT, and TT regiochemistries.
■
(1) Margolis, J. Engineering Plastics Handbook, 1st ed.; McGraw-Hill
Professional: New York, 2005.
(2) Handbook of Polycarbonate Science and Technology, 1st ed.; Legrand,
D. G., Bendler, J. T., Eds.; CRC Press: Boca Raton, FL, 1999.
(3) Chen, X.; Gross, R. A. Macromolecules 1999, 32, 308.
(4) Shen, Y.; Chen, X.; Gross, R. A. Macromolecules 1999, 32, 2799.
(5) Shen, Y.; Chen, X.; Gross, R. A. Macromolecules 1999, 32, 3891.
(6) Haba, O.; Tomizuka, H.; Endo, T. Macromolecules 2005, 38, 3562.
(7) Haba, O.; Furuichi, N.; Akashika, Y. Polym. J. 2009, 41, 702.
(8) (a) Zhang, D.; Hillmyer, M. A.; Tolman, W. B. Biomacromolecules
2005, 6, 2091. (b) Shin, J.; Lee, Y.; Tolman, W. B.; Hillmyer, M. A.
Biomacromolecules 2012, 13, 3833.
(9) (a) Kobayashi, S.; Lu, C.; Hoye, T. R.; Hillmyer, M. A. J. Am. Chem.
Soc. 2009, 131, 7960. (b) Ragauskas, A. J.; Williams, C. K.; Davison, B.
H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett,
J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.;
Tschaplinski, T. Science 2006, 311, 484. (c) Williams, C. K.; Hillmyer, M.
A. Polym. Rev. 2008, 48, 1. (d) Coates, G. W.; Hillmyer, M. A.
Macromolecules 2009, 42, 7987. (e) Besset, C. J.; Lonnecker, A. T.; Streff,
J. M.; Wooley, K. L. Biomacromolecules 2011, 12, 2512.
In conclusion, we have demonstrated the synthesis of a
glucose-based cyclic carbonate monomer and its controlled ROP
via organobase catalysis to afford a polycarbonate having
regiorandom order and well-defined end groups. In contrast to
the recently reported ROP of a five-membered cyclic carbonate,
our monomer contains smaller protecting groups, and the use of
an initiator here affords polymers having narrower molecular
weight distributions. These synthetic methodology develop-
ments are important steps toward the use of glucose as an
effective and innovative feedstock for polycarbonate-based
engineering materials. Furthermore, evaluation by XRD and
thermal analysis revealed that this new glucose polycarbonate
exhibits amorphous character and a significantly higher Tg than
common aliphatic polycarbonates, making it attractive for a
broad range of potential applications. In particular, such materials
may impact numerous fields ranging from degradable plastics to
tissue engineering and nanotherapeutic delivery vehicles.
Moreover, simple modifications to the monomer protection
chemistry that may influence the polymer properties and the
incorporation of a glucose polycarbonate as part of a block-
copolymer system with amphiphilic properties are being
explored to expand beyond this current homopolymer.
(10) (a) Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.;
Choi, J.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules
2007, 8, 153. (b) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R.
C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813.
(c) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M.
Macromolecules 2010, 43, 2093. (d) Tempelaar, S.; Mespouille, L.;
Coulembier, O.; Dubois, P.; Dove, A. P. Chem. Soc. Rev. 2012, 41, 1312.
(11) (a) Rokicki, G. Prog. Polym. Sci. 2000, 25, 259. (b) Dechy-Cabaret,
O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (c) Pratt,
R. C.; Nederberg, F.; Waymouth, R. M.; Hedrick, J. L. Chem. Commun.
2008, 114. (d) Nederberg, F.; Trang, V.; Pratt, R. C.; Kim, S.-H.;
Colson, J.; Nelson, A.; Frank, C. W.; Hedrick, J. L.; Dubois, P.;
Mespouille, L. Soft Matter 2010, 6, 2006. (e) Nederberg, F.; Zhang, Y.;
Tan, J. P. K.; Xu, K.; Wang, H.; Yang, C.; Gao, S.; Guo, X. D.;
Fukushima, K.; Li, L.; Hedrick, J. L.; Yang, Y.-Y. Nat. Chem. 2011, 3, 409.
(f) Tempelaar, S.; Mespouille, L.; Dubois, P.; Dove, A. P. Macromolecules
2011, 44, 2084. (g) Onbulak, S.; Tempelaar, S.; Pounder, R. J.; Gok, O.;
Sanyal, R.; Dove, A. P.; Sanyal, A. Macromolecules 2012, 45, 1715.
̂
(h) Pego, A. P.; Grijpma, D. W.; Feijen, J. Polymer 2003, 44, 6495.
(12) Suriano, F.; Pratt, R.; Tan, J. P. K.; Wiradharma, N.; Nelson, A.;
Yang, Y. Y.; Dubois, P.; Hedrick, J. L. Biomaterials 2010, 31, 2637.
(13) Garcia-Martin, M. G.; Perez, R. R.; Hernandez, E. B.; Espartero, J.
L.; Munoz-Guerra, S.; Galbis, J. A. Macromolecules 2005, 38, 8664.
(14) Azechi, M.; Matsumoto, K.; Endo, T. J. Polym. Sci., Part A: Polym.
Chem. 2013, 51, 1651.
(15) Wood, H. B., Jr.; Diehl, H. W.; Fletcher, H. G., Jr. J. Am. Chem. Soc.
1957, 79, 1986.
(16) Trimnell, D.; Doane, W. M.; Russell, C. R.; Rist, C. E. Carbohyd.
ASSOCIATED CONTENT
* Supporting Information
Experimental section and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
Res. 1970, 13, 301.
(17) Sanders, D. P.; Fukushima, K.; Coady, D. J.; Nelson, A.; Fujiwara,
M.; Yasumoto, M.; Hedrick, J. L. J. Am. Chem. Soc. 2010, 132, 14724.
(18) Engler, A. C.; Chan, J. M. W.; Coady, D. J.; O’Brien, J. M.; Sardon,
H.; Nelson, A.; Sanders, D. P.; Yang, Y. Y.; Hedrick, J. L. Macromolecules
2013, 46, 1283.
AUTHOR INFORMATION
Corresponding Author
wooley@chem.tamu.edu
■
(19) Suzuki, M.; Sekido, T.; Matsuoka, S.-i.; Takagi, K. Biomacromo-
lecules 2011, 12, 1449.
Notes
The authors declare no competing financial interest.
(20) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.;
Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Science Foundation [DMR-1057441 (K.M., A.T.L., T.P.G.,
K.L.W.)], the Department of Energy [BES-DE-FG-04ER-15520
6829
dx.doi.org/10.1021/ja402319m | J. Am. Chem. Soc. 2013, 135, 6826−6829