Construction of Versatile and Functional Nanostructures Derived from CO2-based Polycarbonates

Article abstract of DOI:10.1002/anie.201505076

The construction of amphiphilic polycarbonates through epoxides/CO₂ coupling is a challenging aim to provide more diverse CO₂-based functional materials. In this report, we demonstrate the facile preparation of diverse and functional nanoparticles derived from a CO₂-based triblock polycarbonate system. By the judicious use of water as chain-transfer reagent in the propylene oxide/CO₂ polymerization, poly(propylene carbonate (PPC) diols are successfully produced and serve as macroinitiators in the subsequent allyl glycidyl ether/CO₂ coupling reaction. The resulting ABA triblock polycarbonate can be further functionalized with various thiols by radical mediated thiol-ene click chemistry, followed by self-assembly in deionized water to construct a versatile and functional nanostructure system. This class of amphiphilic polycarbonates could embody a powerful platform for biomedical applications.

Full text of DOI:10.1002/anie.201505076

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Angewandte
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International Edition: DOI: 10.1002/anie.201505076
German Edition: DOI: 10.1002/ange.201505076
CO2-Based Materials
Hot Paper
Construction of Versatile and Functional Nanostructures Derived from
CO2-based Polycarbonates**
Yanyan Wang, Jingwei Fan, and Donald J. Darensbourg*
Abstract: The construction of amphiphilic polycarbonates
through epoxides/CO2 coupling is a challenging aim to provide
more diverse CO2-based functional materials. In this report, we
demonstrate the facile preparation of diverse and functional
functional aliphatic polycarbonates from CO2 containing
hydroxy, furfuryl, and oligoethylene glycol (OEG) groups.[4]
In these examples, preparation of monomers is usually
required and only one type of functional group can be
prepared at a time. More importantly, restrained by the
reactivity of catalysts available, the type of functionality is
usually limited by direct coupling of functional monomers and
CO2. An alternative methodology is to incorporate orthogo-
nal, “click” chemistry into the material design. We and others
have employed “thiol–ene” click reactions to successfully
anchor various functionalities onto polycarbonates with
a vinyl pendant group.[5]
In the last few decades, amphiphilic block polymers have
been extensively studied due to their potential applications in
material science and biomedicine. Owing to their unique
biodegradability and biocompatibility, aliphatic poly-
carbonates have received considerable attention in the
construction of amphiphilic polymers. To date, most of the
amphiphilic block polymers consisting of polycarbonate are
based on the ring-opening polymerization of functionalized
six-membered cyclic carbonate monomers.[6] Generally,
poly(ethylene glycol) (PEG) is used as a macroinitiator and
is the hydrophilic component in the resulting block polymers.
To the best of our knowledge, amphiphilic polymers with
hydrophobic and hydrophilic components both derived from
CO2-based polycarbonates have not been reported. Com-
pared to the ring-opening polymerization, this alternative
route from directive CO2/epoxides copolymerization elimi-
nates the need for the separate preparation of cyclic
carbonates.
nanoparticles derived from
a CO2-based triblock poly-
carbonate system. By the judicious use of water as chain-
transfer reagent in the propylene oxide/CO2 polymerization,
poly(propylene carbonate (PPC) diols are successfully pro-
duced and serve as macroinitiators in the subsequent allyl
glycidyl ether/CO2 coupling reaction. The resulting ABA
triblock polycarbonate can be further functionalized with
various thiols by radical mediated thiol–ene click chemistry,
followed by self-assembly in deionized water to construct
a versatile and functional nanostructure system. This class of
amphiphilic polycarbonates could embody a powerful plat-
form for biomedical applications.
U
sing the abundant, nontoxic and inexpensive CO2 as
a renewable C1 feedstock, the coupling of CO2 and epoxides
provides an attractive method for preparing polycarbonates.
This environmentally more benign approach for poly-
carbonate synthesis has attracted a lot of attention in both
academic and industrial research.[1] With the recent develop-
ment of catalytic systems, both heterogeneous and homoge-
neous, CO2/epoxides coupling has been commercialized by
many companies throughout the world.[2] A new trend on
CO2-based polycarbonates is the production of poly(propy-
lene carbonate) diols which can undergo condensation
reaction with diisocyanate to afford polyurethane.[3] Never-
theless, the hydrophobic nature and lack of functionalities of
the commonly studied CO2-based polycarbonate have pre-
vented their use in functional materials, especially for
biomedical applications.
Here, we demonstrate the facile preparation of CO2-based
amphiphilic block polycarbonates with different functional-
ities and charges. To achieve this goal, the first challenge to
address is the construction of polycarbonate block polymers.
Due to the living nature[7] of the coupling reaction of
epoxides/CO2 catalyzed by (salen)Co system, we hypothe-
sized that block polymers can be synthesized by a “two step,
one-pot” strategy by sequentially adding different monomers.
For polymerization of epoxides and CO2 catalyzed by the
(salen)CoX/PPNX (XÀ = ClÀ, AcOÀ, BrÀ, 2,4-dinitrophen-
oxide (DNPÀ), etc.) binary catalyst system, it is inevitable for
chain-transfer reaction to occur due to trace water impurity.[8]
Thus, the coupling of epoxides and CO2 gives polymers with
differing end groups, OH-PC-OH and OH-PC-X. Upon
addition of the second epoxide, an undesired mixture of
both ABA and AB block polymers will be produced. To
circumvent this problem, a certain amount of water is
intentionally added to the system. The resulting hydroxy
end-capped polymers can serve as macroinitiators in the
In order to expand the use of CO2-based polycarbonates
towards improved material performances, it is necessary to
synthesize more diverse CO2-based polymers with function-
alities. There are a few reports on the development of
[*] Y. Wang, J. Fan, Prof. D. J. Darensbourg
Department of Chemistry, Texas A&M University
3255 TAMU, College Station, TX 77843 (USA)
E-mail: djdarens@chem.tamu.edu
[**] We gratefully acknowledge the financial support of the National
Science Foundation (CHE-1057743) and the Robert A. Welch
Foundation (A-0923). In addition, grant support to Prof. Wooley
(J.F.) through the National Science Foundation (CHE-1410272,
DMR-1309724) and the Robert A. Welch Foundation A-0001 is
greatly appreciated. The microscopy & imaging center (MIC) at
Texas A&M University is also gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505076.
subsequent copolymerization reaction.[9] A (salen)CoTFA[10]
/
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(to the catalyst). The polymerization reaction was carried out
at ambient temperature for 48 h to ensure complete con-
version of PO in order to minimize tapering when the second
monomer (AGE) is added. Consistent with the result we
reported before where a higher catalyst loading was used,[10a]
the resulting polymer shows an extremely narrow polydis-
persity index (PDI; 1.01) in gel permeation chromatography
(GPC) and the MALDI-TOF mass spectrometry showed only
one series of signals assigned to two hydroxy end groups
(Figure S1 in the Supporting Information). The results
indicate that water not only acts as a chain-transfer agent to
protonate the anion of the growing polymer chain but also
completely hydrolyzes the initiating trifluoroacetate groups at
the chain-end.[10a,11]
Figure 1. Catalyst systems utilized in copolymerization of epoxides and
CO2. TFA=trifluoroacetic acid, DNP=2,4-dinitrophenoxide.
Subsequently, after careful release of CO2, various
amounts of AGE was added into the reactor followed by
recharging with CO2. Triblock polycarbonates with different
molecular weight and composition were produced. The GPC
traces of the resultant copolymers are presented in Figure 3,
PPNTFA (catalyst 1, Figure 1) catalyst system will be
employed due to the fact that trifluoroacetate can undergo
hydrolysis more easily and rapidly than other end-
groups.[10a,11]
Since common CO2-based aliphatic polycarbonates, such
as poly(propylene carbonate) and poly(cyclohexene carbon-
ate), are known to be highly hydrophobic, another challenge
is to find a candidate for the hydrophilic component of the
block polymers. A strategy which can be employed to
overcome the hydrophobic nature of the polycarbonate
backbone is the introduction of water-soluble functional
groups by postpolymerization modification. In our previous
report, we demonstrated that water-soluble CO2-based poly-
carbonates can be produced employing thiol–ene click
chemistry in the postpolymerization functionalization of
CO2/2-vinyloxirane copolymers.[5a] However, polymerization
of 2-vinyloxirane and CO2 requires the use of a bifunctional
catalyst (catalyst 2, Figure 1) in order to achieve high polymer
selectivity. The synthesis of catalyst 2 is quite tedious with low
overall yield.[5a,12] From a practical view, we choose another
epoxide, allyl glycidyl ether (AGE), which can couple with
CO2 using easily accessible catalyst 1. The overall synthetic
route for amphiphilic CO2-based polycarbonate is shown in
Figure 2. Two epoxides are employed in the sequential
Figure 3. GPC traces of triblock polycarbonates with different composi-
tion (Table 1).
which shows an increase in the copolymerꢀs molecular weight
with an increase in AGE loading. The fact that the measured
Mw is close to the theoretical values (Table 1) together with
the narrow PDI of these triblock copolymers confirm that the
reaction system maintained its living character during the
course of chain extension. Differential scanning calorimetry
(DSC) revealed decreasing Tg (glass transition temperature)
with a larger poly(allylglycidylether carbonate) (PAGEC)
Figure 2. Synthesis of CO2-based amphiphilic polycarbonates.
Table 1: Results from “two-step, one-pot” strategy to prepare ABA
triblock polycarbonate.
polymerization reaction, one is propylene oxide (PO) that
leads to the hydrophobic segment, the other is allyl glycidyl
ether that carries an alkene functionality for copolymeriza-
tion followed by modification to the hydrophilic blocks.
Related chemistry has been utilized in AGE based poly-
ethers.[13]
To prepare the triblock polycarbonate, we first examined
the copolymerization of PO and CO2 using catalyst
1 (0.1 mol% loading) in the presence of 20 equiv of water
[b]
Mw(in theory)
Mw(GPC)
PDI[b]
Tg[c] [8C]
[gmolÀ1
]
[gmolÀ1
]
PPC diol
–
4600
7000
8100
10300
1.01
1.01
1.01
1.01
23
7
4
À1
m/n=1:2
m/n=2:3
m/n=1:1
8100
9300
11700
[a] See the Experimental Section for the exact procedure. [b] Determined
by GPC. PDI=Mw/Mn. [c] Determined by DSC.
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spectrum in D2O in which
the proton signal from the
hydrophobic PPC was
only slightly shielded (Fig-
ure S13). This result indi-
cates that no shell–core
structure was formed. In
contrast, the DLS studies
of anionic block polymer 4
and cationic polymer 6
showed that they under-
went assembly to form
nanoparticles with high
uniformity, giving a similar
intensity-averaged hydro-
dynamic diameters of 26 Æ
15 nm (Figure 4a,b). The
formation of micelles can
also be confirmed by
1H NMR spectra of poly-
mers 4 and 6 in D2O (Fig-
ure S11 and S15). TEM
images revealed that the
morphologies of both
nanoparticles formed by
4 and 6 are spherical (Fig-
ure 4c,d). The surface
Scheme 1. Synthesis of amphiphilic block polycarbonates with different charges. Boc=tert-butoxycarbonyl.
ratio. This is attributed to the long flexible pendant group of
the PAGEC blocks.
charge densities of the resulting nanoparticles in DI water
were characterized by zeta potential analysis. Zeta potential
values of À59 mV for polymer 4 and + 28 mV for polymer 6
indicated the anionic and cationic surface characteristics of
these nanoparticles. It is noted that an attempt to make
zwitterionic triblock polycarbonate by removal of HCl from
polymer 6 resulted in a non-soluble polymer, even in DMSO
(dimethylsulfoxide) and DMF (dimethylformamide). This is
due to the strong interchain charge interaction between
-CO2 and -NH3 which is unbreakable through solvation.
Both polymers 4 and 6 displayed a similar critical micelle
concentration (CMC) of 66 mgmLÀ1 in DI water, determined
by pyrene fluorescence measurements at room temperature
(Figure S2 and S3).
The functionalization of PAGEC-b-PPC-b-PAGEC (m/
n = 1) triblock polycarbonates was achieved by radical-
mediated thiol–ene click reaction with various thiols includ-
ing mercaptoacetic acid, 2-(Boc-amino)ethanethiol and boc-
l-cysteine, to give polymers 1, 2, and 3, respectively
(Scheme 1). In order to avoid possible chain–chain coupling
reactions, 20 equivalents of thiols to alkene groups were used
in the thiol–ene click reaction under UV irradiation with
dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator.
The resulting polymers were readily purified and their
structures were confirmed. The disappearance of the terminal
À
+
1
alkene protons in the H NMR spectra of the three resulting
polymers confirmed the complete conversion of the alkene
groups. GPC analysis of each of these polymers shows
a narrow single peak without any tailing at the high-molecular
weight portion, confirming the successful suppression of
crosslinking side reactions.
In order to provide high hydrophilicity of the end
blocks, the deprotonation of the pendant carboxylic acid of
polymer 1 and the Boc deprotection of polymer 2/3 were
conducted in THF, giving the corresponding negatively
charged 4 and positively charged 5/6 triblock amphiphilic
polycarbonates.
All three amphiphilic polycarbonates were dissolved in
deionized (DI) water by sonication for 10 min at room
temperature. The morphology of the resulting nanostructures
were characterized by dynamic light scattering (DLS) and
transmission electron microscopy (TEM). DLS analysis
shows no hydrodynamic diameter distribution for the aque-
ous solution of polymer 5, consistent with its 1H NMR
In summary, we have demonstrated the facile construction
of CO2-based versatile and functional amphiphilic polymer
with negatively and positively charged functionalities from
a sequential copolymerization and chemical transformation
strategy. By the judicious use of water as a chain-transfer
reagent, well-defined ABA triblock polycarbonates were
prepared by a “two-step, one-pot” strategy. The living nature
of the (salen)CoX catalyst systems for epoxides/CO2 coupling
enabled the preparation of multiblock polymers with precise
control of block chain lengths. Furthermore, the clickable
alkene groups were then modified to install different func-
tionalities and charges onto the polymer backbones, yielding
amphiphilic CO2-based polycarbonates. This emerging class
of polycarbonates derived from CO2 could provide a powerful
platform for biomedical applications.
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Figure 4. a) DLS results of anionic nanoparticles of 4: Dh (intensity)=26Æ15 nm, Dh (volume)=13Æ6 nm, Dh (number)=9Æ3 nm. b) DLS
results of cationic nanoparticles of 6: Dh (intensity)=26Æ15 nm, Dh (volume)=12Æ6 nm, Dh (number)=9Æ2 nm. TEM images for c) anionic
and d) cationic nanoparticles. The scale bars in both TEM images are 100 nm.
General procedure of thiol–ene reactions of PAGEC-b-PPC-b-
Experimental Section
PAGEC triblock polycarbonate with functional thiols: A solution of
Materials: All manipulations involving air- or/and moisture-sensitive
ABA triblock polymer (0.30 g, Mn = 10300 gmolÀ1
, 1.15 mmol
compounds were carried out in a glovebox or with standard Schlenk
technique under an Ar atmosphere. Allyl glycidyl ether (97%, Alfa)
and propylene oxide (98%, Alfa) were distilled over CaH2 under
reduced pressure prior to use. 2-aminoethanethiol and dimethoxy-2-
phenylacetophenone (DMPA) were purchased from TCI. l-cysteine
hydrochloride anhydrous and mercaptoacetic acid were acquired
from Amresco and Alfa Aesar, respectively. Di-tert-butyl dicarbonate
was purchased from Chem Impex Intꢀl Inc. Tetrahydrofuran (THF),
dichloromethane, and toluene were purified using an MBraun manual
solvent purification system packed with Alcoa F200 activated
alumina desiccant. Bone-dry carbon dioxide supplied in a high-
pressure cylinder and equipped with a liquid dip tube was purchased
from Scott Specialty Gases.
alkenes), functional thiol (23 mmol) in 25.0 mL of THF was degased
for 15 min and then refilled with Ar. DMPA (0.1 mmol) was added
into the solution followed by UV irradiation (365 nm) for 2 h. The
reaction mixtures were precipitated from THF into diethyl ether or
hexane to remove excess functional thiols and photoinitiator by-
products to give the product polymers.
Keywords: carbon dioxide · epoxide · micelles · nanoparticles ·
polycarbonates
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10206–10210
Angew. Chem. 2015, 127, 10344–10348
Representative procedure for the synthesis of PAGEC-b-PPC-b-
PAGEC triblock polycarbonates: (Salen)cobalt(III)X/PPNX (X =
trifluoroacetate) (0.0125 mmol), propylene oxide (0.87 mL,
12.5 mmol) and 0.7 mL toluene/CH2Cl2 (1/1 volume ratio) with
20 equiv water were added into a 15 mL autoclave, and pressurized to
2.5 MPa. After 48 h, the CO2 pressure was slowly released and allyl
glycidyl ether was added. The reactor was recharged with CO2 to
2.5 MPa. The pressure was released after 48 h. The crude polymer was
dissolved in CH2Cl2 and precipitated from methanol. This process was
repeated three times to completely remove the catalyst. The obtained
polymer was dried under high vacuum at room temperature.
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Received: June 3, 2015
Published online: July 14, 2015
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