Environmentally Benign CO2-Based Copolymers: Degradable Polycarbonates Derived from Dihydroxybutyric Acid and Their Platinum-Polymer Conjugates

Article abstract of DOI:10.1021/jacs.6b01327

(S)-3,4-Dihydroxybutyric acid ((S)-3,4-DHBA), an endogenous straight chain fatty acid, is a normal human urinary metabolite and can be obtained as a valuable chiral biomass for synthesizing statin-class drugs. Hence, its epoxide derivatives should serve as promising monomers for producing biocompatible polymers via alternating copolymerization with carbon dioxide. In this report, we demonstrate the production of poly(tert-butyl 3,4-dihydroxybutanoate carbonate) from racemic-tert-butyl 3,4-epoxybutanoate (rac-^tBu 3,4-EB) and CO₂ using bifunctional cobalt(III) salen catalysts. The copolymer exhibited greater than 99% carbonate linkages, 100% head-to-tail regioselectivity, and a glass-transition temperature (T_g) of 37 °C. By way of comparison, the similarly derived polycarbonate from the sterically less congested monomer, methyl 3,4-epoxybutanoate, displayed 91.8% head-to-tail content and a lower T_g of 18 °C. The tert-butyl protecting group of the pendant carboxylate group was removed using trifluoroacetic acid to afford poly(3,4-dihydroxybutyric acid carbonate). Depolymerization of poly(tert-butyl 3,4-dihydroxybutanoate carbonate) in the presence of strong base results in a stepwise unzipping of the polymer chain to yield the corresponding cyclic carbonate. Furthermore, the full degradation of the acetyl-capped poly(potassium 3,4-dihydroxybutyrate carbonate) resulted in formation of the biomasses, β-hydroxy-γ-butyrolacetone and 3,4-dihydroxybutyrate, in water (pH = 8) at 37 °C. In addition, water-soluble platinum-polymer conjugates were synthesized with platinum loading of 21.3-29.5%, suggesting poly(3,4-dihydroxybutyric acid carbonate) and related derivatives may serve as platinum drug delivery carriers.

Full text of DOI:10.1021/jacs.6b01327

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Article
Environmentally Benign CO2-Based Copolymers: Degradable Polycarbonates
Derived from Dihydroxybutyric Acid and Their Platinum-Polymer Conjugates
Fu-Te Tsai, Yanyan Wang, and Donald J. Darensbourg
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01327 • Publication Date (Web): 14 Mar 2016
Downloaded from http://pubs.acs.org on March 22, 2016
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Environmentally Benign CO ꢀBased
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Copolymers: Degradable Polycarbonates
Derived from Dihydroxybutyric Acid and
Their PlatinumꢀPolymer Conjugates
FuꢀTe Tsai, Yanyan Wang, and Donald J. Darensbourg*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
djdarens@chem.tamu.edu
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ABSTRACT
S)ꢀ3,4ꢀdihydroxybutyric acid ( (S)ꢀ3,4ꢀDHBA), an endogenous straight chain fatty acid,
(
is a normal human urinary metabolite and can be obtained as a valuable chiral biomass for
synthesizing statinꢀclass drugs. Hence, its epoxide derivatives should serve as promising
monomers for producing biocompatible polymers via alternating copolymerization with carbon
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dioxide. In this report, we demonstrate the production of poly(tertꢀbutyl 3,4ꢀdihydroxybutanoate
t
carbonate) from racemic-tertꢀbutyl 3,4ꢀepoxybutanoate (racꢀ Bu 3,4ꢀEB) and CO using
2
bifunctional cobalt(III) salen catalysts. The copolymer exhibited greater than 99% carbonate
linkages, 100% headꢀtoꢀtail regioselectivity, and a glassꢀtransition temperature (T ) of 37°C. By
g
way of comparison, the similarly derived polycarbonate from the sterically less congested
monomer, methyl 3,4ꢀepoxybutanoate, displayed 91.8% headꢀtoꢀtail content and a lower T of
g
18°C. The tertꢀbutyl protecting group of the pendant carboxylate group was removed using
trifluoroacetic acid to afford poly(3,4ꢀdihydroxybutyric acid carbonate). Depolymerization of
poly(tertꢀbutyl 3,4ꢀdihydroxybutanoate carbonate) in the presence of strong base results in a
stepwise unzipping of the polymer chain to yield the corresponding cyclic carbonate.
Furthermore, the full degradation of the acetylꢀcapped poly(potassium 3,4ꢀdihydroxybutyrate
carbonate) resulted in formation of the biomasses, βꢀhydroxyꢀγꢀbutyrolacetone and 3,4ꢀ
dihydroxybutyrate, in water (pH = 8) at 37°C. In addition, waterꢀsoluble platinumꢀpolymer
conjugates were synthesized with platinum loading of 21.3–29.5%, suggesting poly(3,4ꢀ
dihydroxybutyric acid carbonate) and related derivatives may serve as platinum drug delivery
carriers.
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Introduction
Petroleum derived chemicals currently are by far the most important sources of polymeric
materials. Considering their vast utilization as fuels which lead to their rapid depletion, finding
alternative carbon sources for producing polymers based on renewable monomers has been the
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subject of numerous research programs worldwide. A significant contribution to this area is the
production of biodegradable polycarbonates via the alternating copolymerization of bioꢀderived
2
epoxides and the renewable C1 feedstock, carbon dioxide. The recent development of singleꢀ
site metal catalysts with wellꢀdefined structures have provided active catalytic systems which
operate under mild reaction conditions, and in some instances afford regioꢀand/or stereoꢀ
3
selective copolymers.
Because of their biocompatibility, biodegradability, and approval for use in biomedical
devices by the US Food and Drug Administration (FDA), aliphatic polycarbonates are very
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important synthetic biomaterials. Since it has been reported that aliphatic polycarbonates
5
undergo hydrolytic degradation to give carbon dioxide and the corresponding alcohol, there has
been a significant interest in the preparation of CO ꢀbased polymers that originate from
2
renewable epoxides that undergo degradation to yield nontoxic metabolites in the human body.
t
For this purpose, (S)ꢀtertꢀbutyl 3,4ꢀepoxybutanoate ((S)ꢀ Bu 3,4ꢀEB) is a promising monomer
due to its similarity to (S)ꢀ3,4ꢀdihydroxybutyric acid ((S)ꢀ3,4ꢀDHBA). (S)ꢀ3,4ꢀDHBA, an
endogenous straight chain fatty acid, is a normal human urinary metabolite. It originates from the
degradation of carbohydrates or from the metabolism of γꢀhydroxybutyrate, a naturally occurring
substance found in the human central nervous system. Normal adults excrete (0.37±0.15)
mmoles of 3,4ꢀdihydroxybutyrate per 24 hr. The compound is also detectable in blood (18.0
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(0.0~54.0) mM) and in cerebrospinal fluid (CSF)(15.0±15.0 mM). Moreover, (S)ꢀ3,4ꢀDHBA
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®
is a highly useful chiral intermediate in synthesizing statinꢀclass drugs such as CRESTOR ,
®
LIPITOR and carnitine. In addition, in the microbial (E. coli.) glucose metabolism process, (S)ꢀ
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,4ꢀDHBA is obtained as a valuable chiral biomass, as recently reported by Dhamankar and
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Martin et al.
In chemotherapy based smallꢀmolecule platinumꢀcontaining drugs, cisplatin, carboplatin,
and oxaliplatin have been extensively used in the treatment of solid cancers, such as ovarian,
bladder, testicular and nonꢀsmallꢀcell lung cancers. However, clinical use of platinumꢀcontaining
drugs has been limited due to several major side effects, including nephrotoxicity and
neurotoxicity. Because of the drug’s nonꢀselectivity, normal and transformed cells are equally
affected. Further, poor water solubility leads to suboptimal distribution in the circulation. The
electrophilic nature of platinum in its aquated form, which is partly present when dissolved in the
circulation system, results in its high affinity for amino acids. Because of these limitations,
research efforts in creating carrierꢀbound platinum complexes are aimed at the development of
platinumꢀpolymer conjugates. Hence, it has been proposed to employ polymers for the delivery
of platinum to protect the drugs from nonspecific binding, to increase circulation time and to use
8
the enhanced permeation and retention effect (EPR) of cancer cells.
In efforts aimed at expanding the availability of bioꢀrelated polycarbonates obtained by
the alternating copolymerization of CO and renewable epoxides, the tertꢀbutyl protected poly
2
t
t
(tertꢀbutyl 3,4ꢀdihydroxybutanoate carbonate) (P BuDHBC) was synthesized from racꢀ Bu 3,4ꢀ
t
EB or (S)ꢀ Bu 3,4ꢀEB with CO using the binary [(1R,2Rꢀsalen)Co(III)(DNP)]/[PPN][DNP]
2
ꢀ
(
bnꢀDNP) or bifunctional catalysts [(1R,2Rꢀsalen)M(III)(X) ] (catalyst 1ꢀX: M = Co, X = OH ,
2
ꢀ
ꢀ
ꢀ
ꢀ
Cl , N , DNP (2,4ꢀdinitrophenolate); catalyst 2: M = Cr, X = N ), as shown in Scheme 1.
3
3
Additionally, racꢀmethyl 3,4ꢀepoxybutanoate (racꢀMe 3,4ꢀEB) was used to explore the steric
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effect of the epoxide on the copolymerization process. In order to construct platinumꢀpolymer
conjugates for potential biomedicinal application, postpolymerization functionalization was
conducted by anchoring aspartate/glycineꢀaspartate and (DACH)PtCl (DACH = (1R, 2R)ꢀ
2
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diaminocyclohexane) onto the bioꢀrelated polycarbonate scaffold.
Scheme 1
Results and Discussion
Epoxide synthesis
t
Synthesis of tertꢀbutyl 3,4ꢀepoxybutanoate ( Bu 3,4ꢀEB) was achieved by the twoꢀstep
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process depicted in Scheme 2. This pathway is greener than the threeꢀstep literature procedure.
In the process described here, a catalytic quantity of BF ·OEt triggers the coupling reaction of
3
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ꢀbutenoic acid with tertꢀbutyl acetate to afford tertꢀbutyl 3ꢀbutenoate. Epoxidation of tertꢀbutyl
3
ꢀbutenoate was carried out with mCPBA (metaꢀchloroperoxybenzoic acid) to provide the
1
epoxide in good yield. See SI for the details of the synthetic procedure, as well as H NMR
spectra (Figures S1 and S2).
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Scheme 2
O
O
OH
O
O
3 2
BF · OEt
O
tertꢀbutyl 3ꢀbutenoate
O
mCPBA
O
O
O
CH
2
Cl
2
O
tertꢀbutyl 3,4ꢀepoxybutanoate
t
Bu 3,4ꢀEB
Copolymerization reactions
Initially, the copolymerization reactions of twiceꢀdistilled over CaH (~ 0.043% water
2
t
content) racꢀ Bu 3,4ꢀEB with CO were carried out employing bifunctional catalysts 1ꢀX (X =
2
ꢀ
ꢀ
ꢀ
Cl , N , DNP ). As noted in Table 1, in the absence of added water only catalyst 1ꢀCl afforded
3
any coupling product which was exclusively the corresponding cyclic carbonate. However, upon
t
adding 10 equivalent of water to catalyst 1ꢀDNP, copolymerization of CO /racꢀ Bu 3,4ꢀEB
2
readily occurred at 40°C. A similar reactivity pattern was observed under identical reaction
conditions for the copolymerization process involving the sterically less hindered racꢀmethyl
3,4ꢀepoxybutanoate (racꢀMe 3,4ꢀEB) monomer and CO (Table 2). These results suggest that
2
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small, good nucleophiles with poor leaving group properties, i.e., resulting from chainꢀtransfer
with water or the diol resulting from epoxide hydrolysis, initiates the alternating
1
0
copolymerization process.
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t
a
Table 1. Coupling Reaction of CO and twiceꢀdistilled tert-butyl 3,4ꢀepoxybutanoate ( Bu 3,4ꢀEB)
2
e
e
e
Catalysts
Time
h)
Temp.(°C) Conversion
(%)
TOF
(h )
Selectivity Carbonate M (GPC)
PDI
n
ꢀ
1
(
(polymer
linkages
(%)
(Da/mol)
%
)
b
1
ꢀDNP
18
18
88
18
18
18
40
40
40
40
40
40
0
91.3
81.8
50
0
c
1ꢀDNP
25.4
9.3
13.9
0
98.5
91.8
0
100
100
5600
1.19
1.57
d
1
ꢀDNP
12565
b
1
ꢀCl
b
1
ꢀN3
0
b
2
0
0
a
t
The coupling reaction was conducted in neat racꢀ Bu 3,4ꢀEB in a 25 mL autoclave at 30 bar of
CO pressure. catalyst racꢀ Bu 3,4ꢀEB = 1 : 500. catalyst : H O : racꢀ3,4ꢀE BuB = 1 : 10 : 500.
catalyst : H O : racꢀ Bu 3,4ꢀEB = 1 : 10 : 1000. characterized by H NMR spectrum of crude
b
t
c
t
2
2
d
t
e
1
2
product.
a
Table 2. Coupling Reaction of CO and twiceꢀdistilled methyl 3,4ꢀepoxybutanoate (Me 3,4ꢀEB)
2
e
e
e
Catalysts
Time
(h)
Temp.(°C) Conversion
(%)
TOF
(h )
Selectivity Carbonate M (GPC)
PDI
1.53
n
ꢀ
1
(polymer
%)
linkages
(%)
(Da/mol)
b
1
ꢀDNP
18
18
18
18
18
40
40
40
40
40
0
88.7
69.7
0
0
24.6
19.4
0
c
1
ꢀDNP
97.7
0
100
5375
b
1ꢀCl
b
1
ꢀN3
b
2
0
0
a
The coupling reaction was conducted in neat racꢀMe 3,4ꢀEB in a 25 mL autoclave at 30 bar of
CO pressure. catalyst : racꢀMe 3,4ꢀEB = 1 : 500. catalyst : H O : racꢀMe 3,4ꢀEB = 1 : 10 :
b
c
2
2
d
1
5
00. characterized by H NMR spectrum of crude product.
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t
To further examine this hypothesis, the use of onceꢀdried racꢀ Bu 3,4ꢀEB monomer which
contains traces of adventitious water was subjected to the copolymerization process with CO2.
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As shown in Table 3, the catalyst 1ꢀDNP was found to selectively copolymerize CO and racꢀ
2
t
Bu 3,4ꢀEB to afford the corresponding polycarbonate with greater than 99% carbonate linkages.
This is in contrast to the lower TOF for catalyst 2, and the poor polymer selectivity of the bnꢀ
DNP catalyst (61.7% at 25°C). The isolated copolymers exhibited narrow molecular weight
distributions with PDI values less than 1.10 and displayed bimodal distributions. The mainꢀ
t
chain sequence of the resulting poly(tertꢀbutyl 3,4ꢀdihydroxybutyrate carbonate) (P BuDHBC)
1
13
1
was characterized by H and C NMR spectroscopy. In the H NMR spectrum, the resonance of
the methine CH was observed at 5.24 ppm, and signals at δ = 3.3ꢀ3.6 ppm assignable to ether
linkages were not evident (Figure 1).
By way of contrast with poly(methyl 3,4ꢀ
dihydroxybutyrate carbonate) (PMeDHBC) which exhibited 91.8% headꢀtoꢀtail regionꢀ
1
3
t
selectivity, the C NMR spectrum of the atactic P BuDHBC copolymer shows it to consist of
1
00% headꢀtoꢀtail regionꢀchemistry (Figure 2). This observation implies that the sterically
t
congested tertꢀbutyl group promotes ringꢀopening of racꢀ Bu 3,4ꢀEB to occur exclusively at the
C ꢀO bond during its copolymerization with CO .
β
2
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Table 3. Coupling Reaction of CO and onceꢀdistilled (>0.5% water content) tertꢀbutyl 3,4ꢀ
2
t
epoxybutanoate ( Bu 3,4ꢀEB)
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e
ꢀ
e
e
Catalysts
Time Temp.(°C) Conversion TOF (h Selectivity
Carbonate
linkages
(%)
M (GPC)
(Da/mol)
PDI
n
1
(
h)
(%)
72.5
98.5
96.1
100
100
8.9
)
(polymer
%
)
b
bnꢀDNP
22
22
22
8
25
25
25
40
40
40
16.5
22.4
21.8
62.5
62.5
2.5
61.7
100
100
100
100
100
100
4936
1.17
1.07
1.08
1.04
1.08
b
1
1
1
1
ꢀDNP
ꢀDNP
ꢀDNP
ꢀDNP
100
100
100
100
100
15847
14692
16901
20850
c
b
d
16
18
b
2
a
t
The coupling reaction was conducted in neat Bu 3,4ꢀEB in a 25 mL autoclave at 30 bar of CO
pressure. catalyst : racꢀ Bu 3,4ꢀEB = 1 : 500. catalyst : (S)ꢀ Bu 3,4ꢀEB = 1 : 500. catalyst :
racꢀ Bu 3,4ꢀEB = 1 : 1000. characterized by H NMR spectrum of crude product.
2
b
t
c
t
d
t
e
1
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1
Figure 1. H NMR spectra (CDCl ) of copolymer poly(tertꢀbutyl 3,4ꢀdihydroxybutanoate
3
carbonate) (PtBuDHBC).
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(a)
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0
(
b)
(c)
1
3
Figure 2. C NMR spectra (CDCl ) of carbonate region for copolymers (a) poly(tertꢀbutyl 3,4ꢀ
dihydroxybutanoate carbonate), (b) poly ((S)ꢀtertꢀbutyl 3,4ꢀdihydroxybutanoate carbonate), and
3
(
c) poly(methyl 3,4ꢀdihydroxybutanoate carbonate).
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Provided with this encouraging regioꢀregular result, it was of interest to synthesize a
t
chiral, isotactic version of this copolymer. To this end, a hydrolytic kinetic resolution of racꢀ Bu
3
,4ꢀEB was carried out with Jacobson’s catalyst in a 44% yield (based on the racemic epoxide)
0
1
2
3
4
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9
0
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9
0
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9
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1
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9
0
9b
to afford the Sꢀenantiomer following the published procedure. Subsequent copolymerization of
t
(S)ꢀ Bu 3,4ꢀEB and CO in the presence of catalyst 1ꢀDNP afforded an isotactic poly((S)ꢀtert-
2
t
butyl 3,4ꢀdihydroxybutyrate carbonate) (SꢀP BuDHBC) with a molecular weight of 14.7 kg and
1
3
a PDI of 1.08 (entry 3 in Table 3). In the C NMR spectrum, the isotactic polymer shows 100%
headꢀtoꢀtail selectivity and exhibits a single sharp signal at 153.9 ppm (Figure 2b).
t
t
With the polycarbonates, P BuDHBC and SꢀP BuDHBC, in hand, the tertꢀbutyl
protecting groups of the pendent carboxylate groups were removed using trifluoroacetic acid
(TFA) in dry CH Cl at ambient temperature over 12h. Upon isolation, the resulting
2
2
1
copolymer’s H NMR spectrum (Figure S3) revealed the absence of the tertꢀbutyl resonance at
1
.43 ppm, confirming its removal from the polymer. The poly(3,4ꢀdihyroxybutyric acid
carbonate) (PDHBAC) is not soluble in water or common organic solvents, but was soluble in
methanol and DMSO. However, subsequent deprotonation of PDHBAC by 0.45 equivalents of
K CO in wet MeOH afforded the waterꢀsoluble poly(potassium 3,4ꢀdihydroxybutyrate
2
3
1
carbonate) (PKDHBC) (See Figure S4 for H NMR spectrum). The thermal properties of the
various isolated polycarbonates were examined by conducting DSC analyses. As shown in Table
4, the glass transition temperatures (T ), which are a function of changes in the polymer
g
properties such as modulus, increase as the flexible methyl group is replaced by the sterically
more congested tꢀbutyl group. Because of the polar carboxylic acid pendent groups and the
intrinsic ionic character of the potassium salts, PDHBAC/SꢀPDHBAC and PKDHBC, stronger
interchain interactions lead to these polycarbonates exhibiting higher T values than the others.
g
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Table 4. Glass transition temperature (T ) for polycarbonate
g
Polycarbonates
PMeDHBC
M (g/mol) (GPC (THF))
T (°C) (DSC)
n
g
5375
5600/15847
14692
18
37
40
65
74
10
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57
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59
60
t
P BuDHBC
t
SꢀP BuDHBC
PDHBAC
SꢀPDHBAC
Depolymerization and degradation reactions
It is of interest to investigate depolymerization and degradation reaction pathways of
these polycarbonates under anaerobic conditions. In our previous studies, baseꢀinitiated
depolymerization of polycarbonates derived from CO and epoxides generally occurred via a
2
backbiting process of the deprotonated copolymer chain end, resulting in an unzipping of the
1
1
polymer chain to provide the corresponding cyclic carbonate. As illustrated in Scheme 3, the
depolymerization of poly(tertꢀbutyl 3,4ꢀdihydroxybutyrate carbonate) is found to proceed by
way of a similar endꢀscission pathway following deprotonation of the hydroxyl chain end by the
strong nonꢀnucleophilic base NaHMDS (sodium bis(trimethylsilyl)amide). Upon addition of
1
base, immediate depolymerization was noted as evidenced by the appearance of H NMR
resonances at δ = 3.63 and 3.25 ppm in d ꢀtoluene. These chemical shifts are assigned to the
8
t
methylene hydrogens of the generated cyclic carbonate (Figure S5). At 40°C P BuDHBC
slowly depolymerized cleanly to its cyclic carbonate counterpart, reaching a molar ratio of cyclic
carbonate to copolymer of 1.79 after 12 days (Figure 3).
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Scheme 3
O
O
O
O
O
0
1
2
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9
0
1
2
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8
9
0
O
NaHMDS
O
H
O
O
O
O
O
n
nꢀ1
O
O
O
O
O
O
O
O
O
O
+
O
O
O
O
nꢀ2
O
O
O
O
Figure 3. Depolymerization of poly(tertꢀbutyl 3,4ꢀdihydroxybutanoate carbonate) in d ꢀtoluene
8
o
at 40 C subsequent to deprotonation with NaHMDS.
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On the other hand, for poly(3,4ꢀdihydroxybutyric acid carbonate), neither
1
depolymerization nor degradation was observed by H NMR spectroscopy upon heating the d ꢀ
7
DMF dissolved polymer in the absence of NaHMDS at 37°C for 12 days. This indicates that
intramolecular cyclization of the carboxylic acid group onto the carbonate backbone to afford Oꢀ
carboxyanhydride does not occur. Such an observation is likely a consequence of the lower
nucleophilicity of the carboxylic acid group in PDHBAC as compared to the primary hydroxyl
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2
b
group in poly(1,2ꢀglycerol carbonate) . In contrast, when acetylꢀcapped poly(potassium 3,4ꢀ
dihydroxybutyrate carbonate) was dissolved in water (D O, pH = 8) and heated at 37°C,
2
degradation occurred with formation of cyclic carbonate, βꢀhydroxyꢀγꢀbutyrolacetone and 3,4ꢀ
1
dihyroxybutyrate as determined by H NMR spectroscopy (Figure S6). Over a 3d period,
polycarbonate and cyclic carbonate fully degraded into βꢀhydroxyꢀγꢀbutyrolacetone and 3,4ꢀ
dihyroxybutyrate. To further corroborate cyclic carbonate degradation and βꢀhydroxyꢀγꢀ
butyrolacetone hydrolysis, samples composed of cyclic carbonate or βꢀhydroxyꢀγꢀbutyrolacetone
1
were dissolved in D O (pH = 8) and heated at 37°C while monitoring by H NMR spectroscopy.
2
The complete degradation of cyclic carbonate occurred within 3 days with the formation of βꢀ
hydroxyꢀγꢀbutyrolacetone and 3,4ꢀdihydroxybutyrate (Figure S7). Similarly, the hydrolysis of βꢀ
hydroxyꢀγꢀbutyrolacetone to afford 3,4ꢀdihydroxybutyrate was observed (Figure S8). Based on
these results, the degradation of acetylꢀcapped poly(potassium 3,4ꢀdihyroxybutyrate carbonate)
can be attributed to the more nucleophilic nature of the carboxylate in D O, and it is proposed
2
that the degradation occurs randomly along the carbonate backbone. As illustrated in Scheme 3,
it is presumed that intramolecular cyclization of the carboxylate onto the carbonate backbone
2
e
forms intermediate A containing an Oꢀcarboxyanhydride.
Subsequent hydrolysis of Oꢀ
carboxyanhydride furnishes the polycarbonate containing a hydroxyl chain end. Deprotonation
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of a hydroxyl polymer chain end in weak basic condition triggers endꢀscission depolymerization,
leading to the formation of cyclic carbonate. Presumably, hydrolysis of cyclic carbonate and
concomitant nucleophilic attack of the carboxylate at C ꢀO bond in intermediate B yields βꢀ
α
0
1
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9
0
hydroxyꢀγꢀlactone, with hydrolysis generating 3,4ꢀdihydroxybutyrate. The complete degradation
of acetylꢀcapped poly(potassium 3,4ꢀdihydroxybutyrate carbonate) into biomass, βꢀhydroxyꢀγꢀ
butyrolacetone and 3,4ꢀdihydroxybutyrate, in water (pH = 8) at 37°C indicates that this
polycarbonate is biodegradable, humanꢀfriendly and environmentally benign.
Scheme 4
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Synthesis of platinumꢀpolymer conjugate
Efficient delivery of therapeutics into tumor cells to increase the intracellular drug
concentration is a major challenge for cancer therapy due to drug resistance and inefficient
cellular uptake. The strategy of using delivery vehicles to selectively transport more of
anticancer agents to tumors is clinically attractive. This can be achieved by linking a platinumꢀ
based drug to a waterꢀsoluble, biocompatible, biodegradable polymer in order to exploit the
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8
enhanced permeability and retention (EPR) effect of macromolecules in tumors. The aim of the
current study is to construct waterꢀsoluble polycarbonates which can serve as anticancer drug
carriers. In this regard, the coupling reaction of poly(3,4ꢀdihydroxybutyric acid chloride
carbonate) (PDHBACꢀCl) and diꢀtertꢀbutylꢀLꢀaspartate or glycineꢀdiꢀtertꢀbutylꢀLꢀaspartate was
conducted in dry CH Cl under an argon atmosphere. The subsequent deprotection by TFA and
2
2
dialysis against water afforded waterꢀsoluble polycarbonates PDHBACꢀAspꢀOH and
1
PDHBACꢀGlyꢀAspꢀOH (Scheme 4). The H NMR spectra of the waterꢀsoluble polymers
containing the pendant carboxylic acid groups is shown in the SI Figures S9 and S10, where the
chemical shifts at 8.20 ppm and 4.52 ppm (in DMSO) are assigned to the amide proton
(−C(O)NHCH−) and the methine proton (−C(O)NHCH−) of aspartic acid, respectively. For
PDHBACꢀGlyꢀAspꢀOH, the resonance of methylene protons (−C(O)NHCH C(O)−) of glycine
2
was found at 3.74 ppm (in DMSO). Because of the large number of pendant carboxylic acid
groups in the resulting polymers, both PDHBACꢀAspꢀOH and PDHBACꢀGlyꢀAspꢀOH are not
soluble in CH Cl , but are completely soluble in water and DMSO.
2
2
In an effort to construct platinumꢀpolymer conjugates, PDHBACꢀAspꢀPt and PDHBACꢀ
GlyꢀAspꢀPt, the reaction of PDHBACꢀAspꢀOH or PDHBACꢀGlyꢀAspꢀOH with aqueous
[
(DACH)Pt(OH ) ][NO ] (DACH = (1R, 2R)ꢀdiaminocyclohexane) was conducted. The
2 2 3 2
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carboxylic acid residues from PDHBACꢀAspꢀOH and PDHBACꢀGlyꢀAspꢀOH were
deprotonated at pH 10 using K CO , and subsequently neutralized by cautious addition of
2
3
aqueous 1N HNO . Since the coordination bonds between water ligands and platinum are usually
3
0
1
2
3
4
5
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7
8
9
0
1
2
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0
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9
0
6
labile and subject to ligand substitution reactions, it is presumed that carboxylate and amide
(−C(O)NH−) groups would coordinate to the Pt center, generating a variety of derivatives in the
aqueous environment. After removal of excess [(DACH)Pt(OH ) ][NO ] by dialysis, the light
2
2
3 2
1
yellow solid was characterized by H NMR spectra (Figure S11), and the loaded amounts of
platinum were determined by neutron activation analysis (NAA) spectrometry (Table 5). In the
1
H NMR spectra (D O), chemical shifts ranging from 5.00 to 6.00 ppm are assigned to methine
2
CH protons in polycarbonate backbone. The broadening of methine resonance is due to the
combination of various polymer conformations as Pt is coordinated by carboxylate and amide
side chains. Moreover, the resonances of DACH are found within the range of 1.00−2.50 ppm.
For PDHBACꢀAspꢀPt, the loading was as high as 29.5 % (wt/wt, relative to the polymer), while
a lower loading (21.3 %) was found with PDHBACꢀGlyꢀAspꢀPt. As compared with the
theoretical Pt loading (full Pt loading), the loaded percentage of Pt (86.0 %) onto PDHBACꢀ
AspꢀOH is higher than that (68.3 %) onto PDHBACꢀGlyꢀAspꢀOH. The water solubilities of
PDHBACꢀAspꢀPt and PDHBACꢀGlyꢀAspꢀPt were determined to be 11 mg/mL and 13 mg/mL,
respectively. Dynamic light scattering(DLS) analysis shows no hydrodynamic diameter
distribution observed in PDHBACꢀAspꢀPt and PDHBACꢀGlyꢀAspꢀPt aqueous solutions,
demonstrating platinumꢀpolymer conjugates dissolve completely in water . In combination with
NAA and DLS data, it is presumed that carboxylate, amideꢀchelating (N,Oꢀcoordinate mode) and
amide, amideꢀchelating (N,Nꢀcoordinate mode) are dominant in the platinumꢀpolymer
conjugates. These results suggest that poly(3,4ꢀdihydroxybutyric acid carbonate) and their
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related derivatives have the potential to serve as platinum drug delivery carrier for future
anticancer pharmaceutical applications.
Table 5. Neutron activation analyses for platinum determination.
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Platinumꢀpolymer conjugates
PDHBACꢀAspꢀPt
Pt (wt %)
29.5
Pt (wt %) (full loading)
% of Pt loading
34.3
31.2
86.0
68.3
PDHBACꢀGlyꢀAspꢀPt
21.3
Scheme 5
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Conclusions
Because the endogenous straight chain fatty acid, (S)ꢀ3,4ꢀdihydroxybutyric acid, is a
normal human urinary metabolite and can be obtained as a valuable chiral biomass for the
synthesis of statinꢀclass drugs, its epoxide derivative racemicꢀtertꢀbutyl 3,4ꢀepoxybutanoate
0
1
2
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0
1
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3
4
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9
0
represents a renewable monomer for copolymerization with CO to afford a biodegradable
2
polycarbonate. In summary, we have synthesized new copolymers from this group of epoxides
and carbon dioxide utilizing bifunctional cobalt(III) salen catalysts in the presence of sterically
unhindered nucleophiles as initiators. The isolated polycarbonates contained more than 99%
carbonate linkages and exhibited narrow molecular weight distributions. Poly(tertꢀbutyl 3,ꢀ
dihydroxybutyrate carbonate) was shown to possess 100% headꢀtoꢀtail regioꢀselectivity and a
glass transition temperature of 37°C.
A chiral, isotactic poly((S)ꢀtertꢀbutyl 3,4ꢀ
dihydroxybutyrate carbonate) was synthesized which displayed a slightly higher T of 40°C, and
g
1
3
was characterized by a single C NMR resonance in the carbonate region at 153.9 ppm.
Depolymerization of poly(tertꢀbutyl 3,4ꢀdihydroxybutyrate carbonate) was found to occur via an
endꢀscission pathway following deprotonation of its hydroxyl terminal group by base.
Moreover, full degradation of acetylꢀcapped poly(potassium 3,4ꢀdihydroxybutyrate carbonate)
into the biomass, βꢀhydroxyꢀγꢀbutyrolacetone and 3,4ꢀdihydroxybutyrate, in water (pH = 8) at
37°C indicates that this polycarbonate is biodegradable, humanꢀfriendly, and environmentallyꢀ
benign. Relevant to protecting anticancer drugs from nonspecific binding, increased circulation
time, and taking advantage of the enhanced permeation and retention effect (EPR) of
macromolecules in cancer cells, platinumꢀpolymer conjugates, PDHBACꢀAspꢀPt and
PDHBACꢀGlyꢀAspꢀPt, were synthesized with platinum loading of 29.5% and 21.3%,
respectively. As a result of their high Pt content (>65%) and the absence of a hydrodynamic
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diameter distribution being observed in PDHBACꢀAspꢀPt and PDHBACꢀGlyꢀAspꢀPt aqueous
solutions, it may be concluded that carboxylate, amideꢀchelating (N,Oꢀcoordinate mode) and
amide, amideꢀchelating (N,Nꢀcoordinate mode) are predominant in the platinumꢀpolymer
conjugates. Potentially, these polycarbonates and their platinumꢀpolymer conjugates could
expand the repertoire of biocompatible, biodegradable polymers available for future anticancer
pharmaceutics.
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Acknowledgements. We gratefully acknowledge financial support from the National Science
Foundation (CHEꢀ1057743) and the Robert A. Welch Foundation (Aꢀ0923). We are also
thankful to Professor Karen L. Wooley, YiꢀYun Tsao and Dr. Lu Su for use of their lyophilizer
and dynamic light scattering (DLS) instrument. The NMR facility at Texas A&M University is
also gratefully acknowledged.
Supporting Information available. Figures, experimental details and characterization data for
racemic epoxides, CO ꢀbased polymers, degradation of cyclic carbonate potassium salt and βꢀ
2
hydroxyꢀγꢀbutylrolacetone hydrolysis.
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