Poly(carbonate-ester)s of dihydroxyacetone and lactic acid as potential biomaterials

Article abstract of DOI:10.1021/bm101342p

The synthesis of new polymeric biomaterials using biocompatible building blocks is important for the advancement of the biomedical field. We report the synthesis of statistically random poly(carbonate-ester)s derived from lactic acid and dihydroxyacetone by ring-opening polymerization. The monomer mole feed ratio and initiator concentration were adjusted to create various copolymer ratios and molecular weights. A dimethoxy acetal protecting group was used to stabilize the dihydroxyacetone and was removed using elemental iodine and acetone at reflux to produce the final poly(lactide-co-dihydroxyacetone) copolymers. The characteristics of the copolymers in their protected and deprotected forms were characterized by ¹H NMR, ¹³C NMR, GPC, TGA, and DSC. Hydrolytic degradation of the deprotected copolymers was tracked over an 8-week time frame. The results show that faster degradation occurred with increased carbonate content in the copolymer backbone. The degradation pattern of the copolymers was visualized using SEM and revealed a trend toward surface erosion as the primary mode of degradation.

Full text of DOI:10.1021/bm101342p

ARTICLE
pubs.acs.org/Biomac
Poly(carbonate-ester)s of Dihydroxyacetone and Lactic Acid
as Potential Biomaterials
Jennifer R. Weiser,† Peter N. Zawaneh,‡,§ and David Putnam*,†,‡
†Department of Biomedical Engineering and ‡School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York
14853, United States
§Millennium: The Takeda Oncology Company, Cambridge, Massachusetts 02139, United States
ABSTRACT: The synthesis of new polymeric biomaterials
using biocompatible building blocks is important for the
advancement of the biomedical field. We report the synthesis
of statistically random poly(carbonate-ester)s derived from
lactic acid and dihydroxyacetone by ring-opening polymeriza-
tion. The monomer mole feed ratio and initiator concentration
were adjusted to create various copolymer ratios and molecular
weights. A dimethoxy acetal protecting group was used to
stabilize the dihydroxyacetone and was removed using elemen-
tal iodine and acetone at reflux to produce the final poly(lactide-co-dihydroxyacetone) copolymers. The characteristics of the
copolymers in their protected and deprotected forms were characterized by 1H NMR, 13C NMR, GPC, TGA, and DSC. Hydrolytic
degradation of the deprotected copolymers was tracked over an 8-week time frame. The results show that faster degradation
occurred with increased carbonate content in the copolymer backbone. The degradation pattern of the copolymers was visualized
using SEM and revealed a trend toward surface erosion as the primary mode of degradation.
’ INTRODUCTION
degradation rates, and ability to be cleared via normal metabolic
pathways.16-20 Previous work by Grinstaff et al. shows the value
of polymerizing LA with other cyclic carbonates, such as glycerol-
derived monomers, to increase the hydrolytic degradation
rate. The characteristics of these new LA-based copolymers
for use as scaffolds for delivering drugs has been reported.21
From this initial work, other poly(carbonate-ester)s have been
explored and developed as new biomaterials for various medical
applications.22-24
The properties imparted to the copolymer from each mono-
mer are the main driving force in exploring the creation of
random copolymers formed via ring-opening polymerization of
DL-lactide and a protected cyclic carbonate of DHA. Previous
studies show that the degradation characteristics enable both LA-
and DHA-based polymers to hydrolyze into safe products and
are a central motivation to maximize the biocompatibility of the
materials.12,25
It is the intent of this work to report the creation of poly-
(carbonate-ester)s derived from DHA and LA and to demon-
strate the ease of synthesis for various molar ratios of the two
monomers. Additionally, we report a new method through which
to deprotect the DHA in a way that is more compatible with
polyesters. Previous work using DHA for the synthesis of
copolymers exemplified the need to create a protected monomer
due to DHA’s vulnerability toward nucelophilic addition at the
The creation of new devices and materials with desirable
biomedical characteristics, such as biocompatibility and easily
tunable physicochemical parameters, has played a key role in the
advancement of the biomedical industry. In recent years, the
combination of classical engineering principles with polymer
chemistry has led to a wide range of materials that influence the
manner in which drugs are delivered, tissues are engineered, and
surgery is performed.1-7 The combination of engineering and
chemistry, aided by the use of naturally occurring biocompatible
monomeric building blocks, has yielded a number of advances in
the field of biomedical engineering.
Examples of two naturally occurring monomeric building
blocks that have shown to be valuable in the area of polymeric
biomaterials are dihydroxyacetone and lactic acid. Dihydroxya-
cetone (DHA), a glycolytic metabolite, has only in recent years
been explored for its use as a polymeric biomaterial. In its natural
form, DHA is an intermediate in the conversion of glucose to
pyruvate, is used as the active ingredient in sunless tanners, and
has been used as a building block for a number of different types
of polymers.8-12
In contrast with the more recently developed DHA monomer,
lactide, a cyclic dilactone of lactic acid (LA), has been used for
decades as a robust building block for the creation of a variety of
tunable LA-containing polymers via a coordination-insertion
controlled ring-opening polymerization.13-15 One traditional
example, poly(lactic acid-co-glycolic acid) (PLGA) copolymers,
has been one of the most well-characterized and widely used
biomaterials because of its low toxicity, easily controllable
Received: November 10, 2010
Revised:
February 4, 2011
Published: March 14, 2011
r
2011 American Chemical Society
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C2 carbonyl and its capacity for forming a dimer in solution.10,26
However, we considered that the previously reported method of
acetal deprotection via acid-catalyzed cleavage in the presence of
water, while proven to be efficacious for the case of diblock
copolymers based on a polycarbonate of DHA and ethylene
glycol, may be unsuitable for copolymers containing the more
hydrolytically susceptible ester-based copolymer backbone.27
This motivation led to an alternative deprotection method for
the polymeric DHA’s protecting group (a dimethoxy acetal)
using elemental iodine in the presence of acetone and heat.28
Additionally, the in vitro degradation characteristics of this new
material are reported. Contrary to the behavior normally seen in
compressed polyester tablets, specifically those composed of
poly(lactic acid), the copolymers of DHA and LA appear to have
a primarily surface erosion pattern of degradation rather than a
bulk erosion pattern. The tablets also show no sign of a liquid
core that would be the result of a faster degradation inside the
copolymer than outside, which is the case for tablets composed of
poly(lactic acid).25,29,30 Lastly, contrary to most polycarbonate
degradation studies, increasing the carbonate group content in
the copolymer backbone increases the rate of degradation.31-33
were dissolved with magnetic stirring in methanol (700 mL). After 24 h,
sodium carbonate (750 mg) was added, and the reaction mixture was
allowed to stir for an additional 24 h, after which the mixture was filtered,
the solvent removed in vacuo, and the remaining viscous, brown liquid
recrystallized repeatedly from diethyl ether to afford 2,2-dimethoxy-
propane-1,3-diol (II) as a white, crystalline material. This compound was
then cyclized by a 15 min dropwise addition of triethylamine (27.8 mL,
0.2 mol) in THF (50 mL) to a stirring flask of 2,2-dimethoxy-propane-
1,3-diol (14.3 g, 105 mmol) and ethyl chloroformate (19 mL, 0.2 mol) in
THF (200 mL) at 0 °C. After the addition of triethylamine was
complete, the reaction was allowed to stir at room temperature for an
additional 3 h, after which the mixture was filtered and the THF was
removed in vacuo. The viscous, yellow product was recrystallized from
ethyl ether three times to yield, 2,2-dimethoxypropylene carbonate (III),
a white, crystalline powder. Yield and characterization are reported in the
aforementioned references.
Synthesis of Poly(DL-lactide-co-2,2-dimethoxy-1,3-propy-
lene carbonate) (IV) (Protected pLAx-co-DHAy). Prior to each
reaction, a 5 mL pear-shaped flask with magnetic stir bar was flame-dried
and evacuated for 10 min. Varying feed mole ratios of LA and 2,2-
dimethoxypropylene carbonate (III), for a combined total of 3 mmol
(pLA100, protected pLA85-co-DHA15, protected pLA75-co-DHA25, pro-
tected pLA50-co-DHA50, protected pLA25-co-DHA75, protected pLA15-
co-DHA85, pDHA100), were added to the reaction vessel and allowed to
evacuate for an additional 5 min. The reaction vessel was then sealed and
partially immersed in a 130 °C paraffin oil bath for approximately 1-3
min until a melt had formed, at which time the reaction was allowed to
stir magnetically. Following this step, 8 μl of Sn(Oct)2 was added
immediately using a 20 μL micropipet, and the vessel was evacuated for
10 s to 50 mmHg exactly, then immediately sealed under vacuum. The
reaction was allowed to proceed at 130 °C for 1 h with stirring, after
which the reaction was cooled to room temperature. The solid,
semitransparent copolymer was dissolved in 1 mL of dichloromethane
and precipitated by dropwise addition to 60 mL of stirring methanol
whereupon the material congealed into a sticky mass which was then
isolated by decanting the solvent and dried under vacuum. Reprecipita-
tion was performed twice in the same manner and each time yielded a
solid, white copolymer. The yield for each feed ratio ranged from 50 to
80%, and each ratio was synthesized in triplicate. For the in vitro
degradation study, the amount of initiator was altered to create
consistent molecular weights (Mw ∼70 kDa). For the degradation
study, the amount varied from 5.1, 6.4, or 12 μL of Sn(Oct)2 for the
protected pLA50-co-DHA50, protected pLA25-co-DHA75, protected
pLA15-co-DHA85, reactions, respectively. These three ratios were chosen
because of their physical characteristics during deprotection. Upon
deprotection, the ratios containing g50% DHA precipitated out of
solution as a fine white powder and thereby were amenable to direct
compression into uniform discs. Those with <50% DHA in the back-
bone remained in solution until precipitation in cold diethyl ether,
whereby they became a very tough, solid white mass. Molecular weights
obtained by GPC and thermal data are provided in Tables 1 and 2. The
1H and 13C NMR spectra are similar for each copolymer and are
exemplified by the spectrum found for protected pLA50-co-DHA50
’ EXPERIMENTAL SECTION
Materials. DHA dimer, p-toluene sulfonic acid, stannous octoate
(Sn(Oct)2), ethyl chloroformate, iodine, and acetone (CHROMASOLV,
for HPLC, g99.9%) were purchased from Sigma-Aldrich (St. Louis, MO)
and used as received. Triethylamine, tetrahydrofuran (THF), sodium
carbonate (Na2CO3), dichloromethane (CH2Cl2), diethyl ether, and
methanol were purchased from VWR (West Chester, PA) and used as
received. Trimethyl orthoformate was purchased from Alfa Aesar (Ward
Hill, MA) and used as received. DL-Lactide was purchased from VWR
(West Chester, PA) and recrystallized in methanol. Phosphate-buffered
saline (PBS) without calcium or magnesium was purchased from Bio-
Whittaker (Lancaster, MA).
Characterization Methods. 1H and 13C NMR spectra were
recorded on Mercury 300 MHz and Inova 400 MHz spectrometers,
respectively. 1H NMR traces were normalized to the residual DMSO-d6
solvent peak at δ = 2.50 and 13C NMR was normalized to the DMSO-d6
residual peak at δ = 39.52. Gel permeation chromatography (GPC) was
carried out using SDV columns (Polymer Standard Service) 500A, 50A,
and linear M (in series) with a THF mobile phase (1 mL/min) and 20
polystyrene standards ranging from 1000 to 1 000 000 Da (Polymer
Standard Service Win GPC software fit to a fifth order polynomial
equation) as a reference for the number-average and weight-average
molecular weight calculations and UV (Waters 486) and RI (Waters
2410) detection. Thermal gravimetric analysis (TGA) experiments were
conducted on a TGA Q500 (TA Instruments) with a heating rate of
10 °C/min and nitrogen flow rate of 50 mL/min. Differential scanning
calorimetry (DSC) measurements were performed on DSC Q1000 (TA
Instruments) equipment at a heating/cooling rate of 10 °C/min and
nitrogen flow rate of 50 mL/min. SEM images were taken with a Leica
440 (5 kV, 600 pA, Leica Microsystems) apparatus on samples sputter-
coated with gold-palladium (60-40) using a Denton Vacuum Sputter
Coater Desk II. Static contact angle measurements were performed
under ambient conditions with a Ramꢀe-Hart contact angle goniometer
equipped with a syringe and flat-tipped needle allowing for a minimum
droplet size of 10 μL. The probe fluid used was purified water and
applied directly to the surface of dry copolymer tablets.
1
copolymer in Figures 1 and 2, respectively. H NMR (DMSO-d6) δ:
5.05-5.21 (broad m; LA 1H), 4.12 (s; DHA 2H), 3.18 (s; DHA 3H),
1.43-1.48 (broad; LA 3H). 13C NMR (DMSO-d6): δ 169 (LA, CO,
multiplet), 153 (DHA, O-CO-O, multiplet), 99 (DHA, C), 68-69
and 71 (LA, CH, multiplet), 62 and 60 (DHA, CH2, multiplet), 49
(DHA, OCH3), 16 (LA, CH3). Multiplets and ranges are designated as
shown in Figure 2.
Synthesis of 2,2-Dimethoxypropylene Carbonate (III). This
Synthesis of Poly(DL-lactide-co-2-oxypropylene carbonate)
(V) (pLAx-co-DHAy). V was prepared via conversion of the dimethoxy
acetal to a carbonyl using molecular iodine in acetone. Deacetalization of
IV with molecular iodine (1:1 w/w) was carried out in acetone
compound was synthesized following
a previously published
method.26,27 In brief, DHA dimer (62.5 g, 0.348 mol), trimethylortho-
formate (76.1 mL, 0.695 mol), and p-toluenesulfonic acid (250 mg)
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Table 1. Molecular Weight and Thermal Data of pLAx-co-DHAy Copolymers
molecular weight (GPC)
protected pLAx-co-DHAy
deprotected pLAx-co-DHAy
mole feed
confirmed
LA/DHA ratio LA/DHA
Mw (g/mol)
Mn (g/mol)
56400
Mw/Mn
1.44
Td (°C) 50 wt % Tg (°C) % deprotected Td (°C) 50 wt %
Tg (°C)
68
0:100
15:85
25:75
50:50
75:25
85:15
100:0
100:0a
0:100
16:84
27:73
52:48
78:22
89:11
100:0
100:0
81000
246
45
80
273
75100 ( 700
62400 ( 900
52500 ( 1 000 1.43 ( 0.05
45500 ( 1 900 1.37 ( 0.04
240 ( 11.40
239 ( 1.30
240 ( 3.70
236 ( 2.40
228 ( 5.30
236 ( 5.40
51 ( 0.90
50 ( 1.00
51 ( 0.30
53 ( 0.40
52 ( 1.30
53 ( 0.10
86
252 ( 1.30
246 ( 4.00
261 ( 5.50
330 ( 2.50
316 ( 27.30
65 ( 1.10
64 ( 1.60
58 ( 0.40
55 ( 0.60
54 ( 0.20
90
51400 ( 4 100 35700 ( 7 200 1.47 ( 0.20
50400 ( 3 800 37800 ( 3 200 1.33 ( 0.03
47500 ( 7 100 35400 ( 6 900 1.35 ( 0.07
37900 ( 1 200 26800 ( 2 700 1.42 ( 0.15
>95
>95
>95
309 ( 37.00
53
a This value is based on a pLA100 sample subjected to the acetone/I2 deprotection conditions.
Table 2. Molecular Weight and Thermal Data for the Copolymers Used in the in Vitro Degradation Study
molecular weight (GPC)
deprotected pLAx-co-DHAy
mole feed LA/DHA
Mw (g/mol)
Mn (g/mol)
Mw/Mn
contact angle (deg)
Td (°C) 50 wt %
Tg (°C)
15:85
25:75
50:50
69900 ( 1 200
71200 ( 2 400
70000 ( 2 300
40500 ( 3 800
39800 ( 2 600
43500 ( 2 000
1.77 ( 0.10
1.79 ( 0.10
1.66 ( 0.10
60.5 ( 0.7
61.5 ( 0.7
61.5 ( 0.7
233
234
260
58
56
56
iodine (250 mg) in acetone (7 mL) and allowed to stir for 2 h partially
submerged in a 52 °C paraffin oil bath. For the copolymers containing
50% DHA or more, the deprotected copolymer was too insoluble to
remain in solution for the entirety of the set time period. For these
copolymers, the insoluble, partially deprotected copolymer was allowed to
settle to the bottom of the flask after the 2 h reaction, whereupon the
acetone was decanted. The fine powder product was then washed
repeatedly with fresh diethyl ether to remove any visible signs of residual
iodine before being transferred to a 20 mL scintillation vial. Fresh ether
(15 mL) was added, the vial was capped, and the suspension was allowed
to stir for an additional 3 days. During this time, the ether was decanted
twice daily, and fresh ether was added as needed to remove excess iodine
before being dried under vacuum. For the copolymers containing <50%
DHA, the deprotected copolymer remained in solution for the set time
period. For these copolymers, all but 2 mL of solvent was removed in
vacuo, and the copolymer was precipitated by the slow dropwise addition
of cold ether to the vigorously swirling flask containing the copolymer/
iodine solution. The precipitated copolymer was washed with diethyl
ether before dissolution in a minimal amount of dichloromethane and
precipitated in stirring methanol (60 mL) three times. The copolymers
were washed a final time with diethyl ether, isolated by filtration, and dried
under vacuum. The yield for each deprotected copolymer was ∼75%.
Thermal data are shown in Tables 1 and 2. Elemental analysis (Intertek
QTI Laboratory) of the material shows minimal residual iodine (<0.1%).
The 1H NMR spectra are similar for each copolymer and are represented
by Figure 4 for the pLA50-co-DHA50 copolymer. Because of low solubility,
Figure 1. 1H NMR spectrum of protected poly(lactic acid-co-2,2-di-
methoxy-1,3-propylene carbonate) protected (IV) (pLA50-co-DHA50) in
DMSO-d6.
a
13C NMR analysis of the deprotected copolymer was not possible. 1H
NMR (DMSO-d6) δ: 5.05-5.21 (broad m; LA 1H), 4.99 (broad; DHA
2H), 1.43-1.48 (broad; LA 3H).
Figure 2. 13C NMR spectrum of protected poly(lactic acid-co-2,2-
dimethoxy-1,3-propylene carbonate) (IV) protected (pLA50-co-
DHA50) in DMSO-d6. The peaks assigned to a0, b0, c0, and d0 are peak
shifts due to the heterogeneous placement of monomers throughout the
backbone, for example, a DHA next to an LA or a DHA next to
another DHA.
Monomer Distribution Analysis. 13C NMR was performed on
the protected copolymers using an Inova 400 MHz spectrometer in
DMSO-d6 for a period of 12 h, and the results are provided in Figure 3.
In Vitro Degradation Analysis. The in vitro degradation study
was designed to run for 56 days and followed three copolymer monomer
ratios, pLA50-co-DHA50, pLA25-co-DHA75, and pLA15-co-DHA85 all
with Mw ∼70 000 g/mol. Each ratio was synthesized at least five times,
(CHROMASOLV, for HPLC, g99.9%). A 25 mL round-bottomed flask
was flamed under vacuum before being charged with IV (250 mg) and
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Figure 3. 13C NMR spectra for protected copolymers of pDHA100, pLA15-co-DHA85, pLA25-co-DHA75, pLA50-co-DHA50, pLA75-co-DHA25, pLA85-co-
DHA15, and pLA100 in DMSO-d6. The chemical structures of the eight possible triads along with their respective carbonyls noted have been included.
and the fine, white powders were combined before being sized through a
250 μm sieve. Tablets were formed using direct mechanical compression
to 1000 psi for 10 s on 20 mg of copolymer at room temperature. The
final cylindrical tablets had dimensions of approximately 5.32 ( 0.02
mm ꢀ 0.84 ( 0.02 mm (diameter ꢀ thickness), as measured by digital
calipers. Each tablet was placed in 1ꢀ PBS buffer solution (1.5 mL, pH
7.4) and incubated with rotation (N = 3) at 37 °C. Every 3 days the
buffer solutions were exchanged, and a control was used to ensure a
constant pH. At predetermined time points, the samples were removed
from the buffer solution, rinsed with DI water, dried by KimWipe, and
lyophilized for 3 days. The mass loss and new tablet geometry were then
recorded. The percent mass loss was calculated using the following
expression
was cut in half with a razor blade, immersed in liquid nitrogen and
fractured in half again by hand before sputter coating.
Surface Characterization. Static contact angle measurements
were used to explore the surface of the three copolymer ratios used in the
hydrolytic degradation study. Manual addition of a droplet of water to
the top surface of each copolymer tablet allowed the contact angle to be
measured. This experiment was repeated for each ratio to confirm the
validity of the reading.
’ RESULTS AND DISCUSSION
Synthesis of Poly(DL-lactide-co-2-oxypropylene carbo-
nate). The synthetic strategy employed to create copolymers
of lactide and DHA is shown in Scheme 1. The DHA monomer
was prepared based on earlier work using a dimethoxy acetal
protecting group to prevent the DHA monomer from forming a
dihemiacetal dimer.26,27 The protected DHA was then converted
to a six-membered, cyclic carbonate ring to be used for ring-
opening polymerization (III).26 Stannous octoate (Sn(Oct)2)
was used as the coordination catalyst because of its efficacy to
promote ring-opening polymerizations of both the cyclic lactide
and DHA monomers.12,14,26,27
ꢀ
ꢁ
m0 - mf
m0
Mlossð%Þ ¼
ꢀ 100
ð1Þ
where m0 refers to the initial mass and mf refers to the final mass at a
specific time point.
Morphological Characterization. Scanning electron micro-
scopy (SEM) was used to determine the morphology of the surface
and the cross-section of each tablet at each time point (Leica 440). In
brief, each tablet was placed on a small metal stub and sputter coated
with gold-palladium (60-40) to a thickness of 10 nm using a Denton
Vacuum Sputter Coater Desk II. For the cross-sectional view, each pellet
Five monomer ratios of the pLAx-co-DHAy (pLA85-co-DHA15,
pLA75-co-DHA25, pLA50-co-DHA50, pLA25-co-DHA75, pLA15-co-
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Scheme 1. Synthetic Route to Poly(DL-lactide-co-2-oxypro-
To elucidate further the distribution of the monomers in the
copolymers, 13C NMR was employed to first verify the structure,
seen in Figure 2, and then to determine the monomer sequence
based on the carbon resonances, which can be seen in Figure 3.
13C NMR spectra provided the means to determine whether the
copolymer is random, block, or alternating depending on how
each monomer interacts with its neighboring species.21,34-37 In
the case of a statistically random copolymer, 23 triads can be
theoretically formed, and these can be used to understand the
molecular architecture of the copolymer. For the DL-lactide (l)
and the protected DHA (d) copolymers, the eight possible triads
that can be formed are as follows: LLL, LLD, DLL, DLD, LDL,
LDD, DDL, and DDD. 13C spectra of the homopolymers
provided the basis for the LLL and DDD homopolymer reso-
nances and thus the position of the subunits throughout the
backbone. The pLA100 spectrum indicated that the resonance for
the central carbonyl carbon in LLL is a trio of peaks that lies
between 169.0 and 169.21 ppm. The multiple resonances seen
for pLA100 were attributed to the creation of a mixture of atactic
and isotactic regions along the polymer backbone and are due to
a number of influencing factors. These factors include, but are
not limited to, the polymerization kinetics, lactide feed composi-
tion, the favoring of meso dyads in L,L or D,D dimeric repeat units
when D,L lactide is used, the extent of conversion, and the
possibility of transesterification when the reaction conditions
have a temperature set point >120 °C.35-38 The protected
pDHA100 spectrum showed the DDD resonance at 153.52
ppm for the carbon associated with its carbonate bond. The
determination of the two extra shifts associated with the carbo-
nate bond’s carbon seen in the copolymer spectra was deter-
mined by the gradual superposition of the spectra toward the
homopolymers as the molar feed ratios changed, as shown in
Figure 3. Using this information and preliminary predictive 13C
NMR modeling software, the new peaks observed were assigned
to the two XLD triads (LLD, DLD) and two XDL triads (DDL,
LDL) with signals located at 169.47 and 153.18 ppm,
respectively.39 This information supports the conclusion that
the synthesized polymers are random copolymers of the LA and
DHA monomers.
pylene carbonate) (V)a
a (a) Trimethyl orthoformate, p-toluene sulfonic acid, methanol, 24 h;
sodium carbonate, 24 h. (b) Ethyl chloroformate, triethylamine, tetra-
hydrofuran, 3 h. (c) Stannous octoate, vacuum, 130 °C, 1 h. (d) Iodine,
acetone, reflux, 2 h.
DHA85) were synthesized by adjusting the feed ratio of the two
monomers while maintaining a constant injection volume of 8 μl
of Sn(Oct)2. As a basis for head-to-head comparisons, LA and
DHA homopolymers (pLA100 and pDHA100) were also synthe-
sized. For the three ratios of copolymers that were investigated in
the in vitro degradation study (pLA50-co-DHA50, pLA25-co-
DHA75, pLA15-co-DHA85), a constant molecular weight was
maintained by varying the amount of Sn(Oct)2, as described in
the Experimental section. Because of the higher melting tem-
perature of the LA monomer as compared with the protected
DHA monomer, all molar ratios were synthesized by a melt
injection of the Sn(Oct)2 at 130 °C to ensure the creation of a
homogeneous monomer mixture. The mole fractions of each
monomer in the copolymers were confirmed via 1H NMR analysis
of the -CH3 species present in the LA (δ = 1.43-1.48) and the
protected DHA (δ = 3.18), which is shown in Figure 1, and the
average values reported in Table 1. GPC analysis of the protected
copolymers with different feed ratios was performed, and the
relative molecular weight, shown in Table 1, shows that a direct
relationship exists between increasing DHA content and Mw for
each copolymer. In addition, the GPC traces, which show a single
peak, helped to substantiate the creation of the copolymers rather
than two populations of homopolymers.
The removal of the dimethoxy acetal group to obtain the final
copolymer, (V), was conducted utilizing a method different from
the one previously reported by our group for the protected DHA
derived monomer.10,12,27 To eliminate the possibility of ester
hydrolysis from the reaction conditions present in a trifluoroa-
cetic acid-water deprotection, a deprotection catalyzed by
molecular iodine in acetone was investigated.28 With this meth-
od, the degree of deprotection was directly correlated to the
molar ratio of LA and DHA within the copolymer. This relation-
ship was found to be caused by the poor solubility of the
deprotected DHA monomer in acetone. At higher concentra-
tions of DHA in the copolymer (g50%), the deprotection
reached a solubility limit prior to complete deprotection. When
a significant amount of the protected DHA within the copolymer
backbone had been deprotected, the LA and remaining protected
DHA were no longer able to keep the copolymer soluble in
solution, leading to its precipitation. The degree of deprotection
for these ratios, including pDHA100, ranged from 80 to >95% and
1
was determined via H NMR. An example of the deprotected
1
copolymer H NMR spectrum is shown in Figure 4. Interest-
ingly, it was noticed that even though the pLA50-co-DHA50 ratio
copolymers came out of solution prior to the 2 h time point, their
degrees of deprotection were all >95%. In contrast, at lower
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Figure 5. Mass loss as a function of degradation time for LA and DHA
copolymers: ([) pLA50-co-DHA50, (9) pLA25-co-DHA75, and (b)
pLA15-co-DHA85.
Figure 4. 1H NMR spectrum of deprotected poly(lactic acid-co-2-
oxypropylene carbonate) (V) (pLA50-co-DHA50) in DMSO-d6. (*)
denotes residual protected DHA peaks.
observed for the deprotected DHA homopolymer was also
observed for each deprotected copolymer. A trend was observed
toward an increasing Tg as the DHA content within the
copolymers increased, as shown in Table 1. The trend of an
increased Tg as the molecular weight increased was expected
because of the reported relationship that exists between Tg and
molecular weight.41
An interesting feature of the pLAx-co-DHAy copolymers is
their variable solubility as the DHA content within the copoly-
mers changes. Previous work on DHA homopolymers showed
that characterization was a challenge.27 The introduction of 50%
or more LA monomer within the random copolymer allowed
more facile characterization of the copolymers. Copolymers with
higher LA monomer ratios were soluble in common organic
solvents such as acetone, dimethyl sulfoxide, and dichloro-
methane. This solubility allowed the I2/acetone deprotection
method to be much more reliable and effective than the
previously reported TFA-based methods because it avoids the
potential acid-catalyzed hydrolysis of the copolymer backbone,
which can lead to a reduced molecular weight.
In Vitro Degradation of Poly(DL-lactide-co-2-oxypropy-
lene carbonate). To begin to evaluate hydrolysis of the copo-
lymer, an 8 week in vitro degradation study was performed.
Because of the physical form of the >50% LA deprotected
copolymers when precipitated, the pLA75-co-DHA25 and the
pLA85-co-DHA15 copolymers were unsuitable candidates for the
degradation study. Following deprotection, the precipitated
pLA75-co-DHA25 and the pLA85-co-DHA15 became very tough,
solid materials that were not amenable to grinding into a powder.
However, the ratios of pLAx-co-DHAy with e50% LA had a fine
powdery physical form upon deprotection, making them much
more suitable for a study of this nature. Three copolymers with
monomer ratios were synthesized: pLA50-co-DHA50, pLA25-co-
DHA75, and pLA15-co-DHA85 by varying the amount of initiator
to obtain copolymers with a Mw ∼70 kDa and slightly higher
molecular weight distribution of ∼1.7 to 1.8 than the previous
materials (Table 2). Because of the insolubility of the depro-
tected copolymers in common GPC mobile phase solvents,
molecular weight values to track the degradation of the copoly-
mer backbone versus time were unable to be obtained. Thermal
characterization of these higher molecular weight copolymers
showed slightly altered values for Td and Tg due to the change in
molecular weight and polydispersity. The glass-transition tem-
perature data observed for the 70 kDa in vitro degradation
concentrations of DHA (<50%), all of the copolymers remained
soluble in the acetone, and the degrees of deprotection were all
found to be >95%. We anticipate that total deprotection was not
achieved over this 2 h time scale because of the presence of trace
amounts of water in the acetone, which has been reported to
affect the rate and degree of deprotection.28
Thermal analysis of the materials was conducted to character-
ize the influence of each ratio of monomers on the copolymer
characteristics. TGA revealed that protected pDHA100 had a Td
of ∼246 °C, whereas LA100 showed a Td of ∼237 °C. The Td for
all of the protected copolymers was consistent within this
temperature range, and the data are presented in Table 1. Also
shown in Table 1 are the Td values for the deprotected
copolymers. Upon deprotection, the Td of pDHA100 was ele-
vated to 273 °C, and a new trend was observed. As the ratio of
LA increased within the deprotected copolymers, the observed
Td also increased to values greater than pLA100. This phenom-
enon may be attributed to the solubility difference of the
copolymers with varying pLAx-co-DHAy ratios. Because of the
insolubility of the copolymer with g50% DHA, these copoly-
mers precipitate out of solution upon deprotection as a fine
powder. In contrast, the copolymers that contain more LA
remain soluble throughout the deprotection and precipitate in
cold ether as a sticky mass. Using pLA100 as a control under the
same deprotection conditions, elemental analysis found a very
small residual amount of iodine in all of the deprotected
copolymers (<0.1%), which may be the cause for the increased
Td foundin the deprotected pLA100, pLA75-co-DHA25 and pLA85-
co-DHA15 samples.
DSC was conducted to distinguish further the thermal char-
acteristics of each copolymer. A glass-transition temperature, Tg,
was observed for each copolymer, whereas a melting tempera-
ture, Tm, was not observed for any of the copolymers within the
set temperature range of -60 to 140 °C. The lack of a Tm also
supports the conclusion that the LA in the copolymer backbone
has little stereospecificity due to the random distribution of the D-
and L-chiral carbons from the DL-lactide monomer and the
inclusion of DHA in the backbone that may disrupt the config-
uration of chain segments needed to form the crystalline lattice.40
The Tg of the protected pDHA100 was ∼45 °C, whereas the Tg of
pLA100 was ∼53 °C. All of the Tg values for the protected
copolymers were between 49 and 53 °C. However, deprotected
pDHA100 showed an increased Tg at ∼68 °C. The increase in Tg
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Figure 6. SEM of the top surface of pLAx-co-DHAy copolymers taken at select time points (magnification = 5000ꢀ). The day 56 image for pLA15-co-
DHA85 does not exist because of complete degradation prior to this time point.
copolymers are shown in Table 2. The decrease in Tg as
compared with the Tg of the copolymers shown in Table 1 is most
likely attributed to the increase in the polydispersity, where the
molecular weight range increases the number of lower molecular
weight polymers in the final material that can act as plasticizers.42
Static contact angle measurements revealed that the surface of each
copolymer ratio, while tending to be more hydrophilic in nature, did
not vary significantly among the three different ratios.
Visual inspection of the compressed tablets from each time
point, post lyophilization, showed a white exterior and cylindrical
shape, with each tablet maintaining its geometry and color over
the course of the erosion study. Upon fracture, the interior was
the same white color as the exterior, and each sample appeared to
have uniform structural integrity throughout the tablet. This
result is significant because monomeric DHA can degrade into a
dark brown product, suggesting that chemical degradation does
not occur within the pellet. Depending on the copolymer
composition, the longer time points of 4 and 8 weeks contained
samples that were more brittle. However, comparing the interior
and exterior of each sample, there were no visual signs of any
large difference in mechanical properties, color changes, or
morphology. This result is particularly interesting because a
number of studies have tracked the in vitro degradation of LA-
derived polymers and copolymers and have found significant
morphological changes over the course of the experiment. To
compare directly with relevant literature, Li et al. tracked the
degradation of DL-LA-based polymer tablets formed using direct
compression molding at a temperature below its Tg. They
observed that amorphous PLA polymers underwent heteroge-
neous degradation with the rate of degradation on the inside
faster than that of the outside.43,44 This “inside-out” degradation
pattern leads to the formation of a hard, white outer shell
surrounding a viscous, transparent core that could be seen
starting at 3 weeks for 100% LA (Mw = 65 000 kDa). Other
studies have investigated similar morphological changes and the
autocatalytic effect that causes these polyesters to degrade in this
particular pattern.25,29,30,44
Compared with the in vitro degradation of polyesters, speci-
fically LA homopolymers, the in vitro degradation of polycarbo-
nates and poly(carbonate-ester)s is quite different. Long-
term in vitro degradation studies using high molecular weight
polycarbonate homopolymers derived of 1,3-trimethylene car-
bonate (TMC) show no sign of degradation after 2 years (Mn =
320 kDa), whereas pTMC50-co-LA50 copolymers (Mn = 220 kDa)
show mass loss starting around 10 weeks with the onset of degra-
dation occurring later with increasing TMC content.31 Another
study with lower molecular weight homopolymers of TMC (Mn
= 114 kDa) showed no mass loss during a 53 week in vitro
degradation study, whereas pTMC67-co-LA33 (Mn = 88 kDa) and
pTMC50-co-LA50 (Mn = 104 kDa) displayed mass loss at 9
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Figure 7. SEM of the cross-sectional surface near the interior of LAx-co-DHAy copolymers taken at select time points (magnification = 5000ꢀ). The day
56 image for pLA15-co-DHA85 does not exist because of complete degradation prior to this time point.
weeks.32 Both studies exemplify the trend that TMC- and LA-
based poly(carbonate-ester)s have an increasing degradation rate
with increasing ester content in their backbone, opposite of the
results with pLAx-co-DHAy copolymers seen in Figure 5. In
general, the poly(carbonate-ester) degradation pattern can be
attributed to carbonate bonds being less labile to hydrolysis than
ester bonds, but for pLAx-co-DHAy copolymers, we see the
opposite.33 These results, however, are consistent with previous
reports of DHA polycarbonates.26,27 Also of note is that unlike
copolymers containing only ester bonds, studies using other
poly(carbonate-ester)s composed of glycerol and L-LA showed
no appreciable change in physical appearance throughout a 2
week study time period, even though there was a molecular
weight decrease of 35%.21
To explore further the mechanism of degradation, SEM was
used to visualize the erosion pattern on a microscopic level.
Images were taken from the top, side, and cross-sectional surfaces
to get a better understanding of the copolymer erosion mechan-
ism. The images show that those copolymers with more LA in
their backbone took longer to erode during the experimental
time frame than those richer in DHA. This visual finding
correlates to the mass loss data seen in Figure 5, which also
shows that copolymers with more LA in the backbone erode at a
much slower rate than those with more DHA in the backbone.
The top and side views show a similar erosion pattern for the
three ratios, although the erosion was much more accelerated in
pLA15-co-DHA85 than in pLA50-co-DHA50 and can be seen in
Figure 6. Similar to the top and side views, upon comparison of
the cross-sectional views, a different degradation pattern was
seen among the three ratios and these results are shown in
Figures 7 and 8. The materials made of pLA50-co-DHA50 showed
very little morphological change in the center of their cores
until 4 weeks, whereupon small pores had begun to form.
However, the edges closest to the outside surfaces of the
cross-sectional view appear to erode much more rapidly at earlier
time points. At the 1 day time point, subtle morphological
changes and small pores seem to form near the edge. This area
of erosion increased in size with the length of time the tablets
were allowed to rotate in the buffer solution, and by day 56,
noticeable regions of erosion near the edges can be seen in
Figure 9, as compared with the relatively unaltered interior.
Tablets made of pLA25-co-DHA75 showed a similar erosion
pattern to those made of pLA50-co-DHA50, where the very center
of the material did not show signs of degradation until the 4 week
time point. At this time, a change in morphology and pores was
once again observed. Unlike pLA50-co-DHA50, the interior edge of
pLA25-co-DHA75 showed a larger, more distinct region of erosion,
and by week 8, the entirety of the material’s cross-section showed
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Figure 8. SEM of the cross-sectional surface at the edge of pLAx-co-DHAy copolymers taken at select time points (magnification = 1000ꢀ, tilt = 61°).
The day 56 image for pLA15-co-DHA85 does not exist due to complete degradation prior to this time point.
patterns seen with other LA or LA-derived copolymers. The
SEM images show that there is an erosion zone near the edges of
the sample, and the area of this zone increases during later time
points. However, at later times during the study, the very center
of the interior does begin to show morphological changes as well.
The erosion rate for all three samples does display some linearity,
but there is not a constant value throughout the study that would
indicate that this material erodes by only surface erosion. The
data also do not indicate that this material erodes only by bulk
erosion because the geometry of the materials stays consistent
and there is no distinct time at which a drastic change in erosion
rate occurs, a phenomenon normally seen with LA or LA
copolymers. Li et al. reported previously that pure DL-LA
polymers (Mw = 65 kDa) showed this extreme rate change at 7
weeks, whereas for the pLA50-co-DHA50 copolymer there is no
extreme change of erosion rate throughout the 8 week study.43,44
This is a surprising result because extensive work by the
G€opferich group to model erosion behavior shows that there is
a critical thickness required before a sample can undergo surface
erosion.45,46 This thickness has been theorized to be 7.4 cm for
poly(R-hydroxy-acids). Other studies that track the erosion of
TMC and LA poly(carbonate-ester)s reported a shift from
surface erosion to bulk erosion with a LA content of > 30% in
the backbone.33
Figure 9. SEM of the entire cross-sectional surface of pLA50-co-DHA50
copolymers taken at 56 days (magnification = 315ꢀ).
morphological changes. In comparison, the copolymers composed
of pLA15-co-DHA85 began to show morphological changes to the
center of the interior by 2 weeks. These copolymers also had the
largest eroded region near the edges of the interior as compared
with the copolymers containing more LA. Also of note is that the
pLA15-co-DHA85 copolymers eroded to completion between
weeks 6 and 7, and by week 2, the copolymer’s cross-section
showed morphological changes throughout the tablet.
The findings indicate that these poly(carbonate-ester)s display
both surface and bulk erosion patterns that are unlike the erosion
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’ CONCLUSIONS
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Random copolymers of cyclic DL-lactide and a 6-membered
ring of carbonyl-protected DHA can be synthesized via a ring-
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using elemental iodine in acetone. By successfully employing this
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hydrolysis) than previously studied.27 It was found that by
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LA and DHA copolymers revealed degradation temperatures and
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The copolymer also showed unique degradation and erosion
characteristics. It was found that even with LA amounts of 50%
within the copolymer backbone, there appeared to be a more
surface-like erosion pattern. It was also observed that contrary to
previously reported polycarbonate behavior, increasing the
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the increased degradation rate with copolymers more rich in
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’ AUTHOR INFORMATION
(27) Zawaneh, P. N.; Doody, A. M.; Zelikin, A. N.; Putnam, D.
Biomacromolecules 2006, 7, 3245–3251.
(28) Sun, J. W.; Dong, Y. M.; Cao, L. Y.; Wang, X. Y.; Wang, S. Z.;
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Corresponding Author
*Tel: (607) 255-4352. Fax: (607) 255-7330. E-mail: dap43@
cornell.edu.
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’ ACKNOWLEDGMENT
(32) Yang, J.; Liu, F.; Yang, L.; Li, S. M. Eur. Polym. J. 2010, 46,
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We thank Anthony Condo, Dr. Ivan Keresztes, John Hunt,
and the Cornell Center for Materials Research for their expertise
and assistance in polymer characterization methods. We would
also like to thank Dr. Jeisa Pelet, Dr. Sara Yazdi, Dr. Anne Doody,
Laura Austen, Bennett Wechsler, and Lindsay Blackwell for their
expertise and advice. This research was supported in part by the
National Science Foundation (CAREER Award), the Wallace H.
Coulter Foundation Early Career Award, and the National
Science Foundation GK-12 Fellowship.
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