Poly(carbonate ester)s Based on Units of
6-Hydroxyhexanoic Acid and Glycerol
Jesse B. Wolinsky,†William C. Ray III,†
Yolonda L. Colson,‡and Mark W. Grinstaff*,†
Departments of Chemistry and Biomedical Engineering,
Boston UniVersity, Boston, Massachusetts 02215,
and DiVision of Thoracic Surgery, Department of
Surgery, Brigham and Women’s Hospital,
Boston, Massachusetts 02115
ReceiVed June 8, 2007
ReVised Manuscript ReceiVed August 13, 2007
Biodegradable polymers such as polyesters and polycarbon-
ates formed by ring-opening polymerizations, including poly-
(?-caprolactone), poly(p-dioxanone), poly(trimethylene carbon-
ate), and most notably poly(glycolic acid) and poly(lactic acid),
have met wide acceptance for medical uses as a consequence
of their low toxicity, degradation properties, and ease of
synthesis.1-4As medical device materials, biodegradable poly-
mers do not require removal after implantation, thereby
eliminating a second surgical procedure; furthermore, the chronic
immune response often associated with permanently implanted
synthetic materials is reduced or eliminated as the macromol-
ecules degrade after performing their intended function. These
polymers are used in medical applications in various roles
including sutures, staples, and stent coatings, as orthopedic cell
scaffolds, and as micro- and nanoparticles for drug delivery
applications.5-8When a polymer does not meet the requirements
for an intended application, two monomers are often copolymer-
ized or two or more polymers are blended with each other to
alter properties such as degradation rate, flexibility, and strength.
A representative example is Vicryl, a commercially available
poly(lactide-co-glycolide) suture produced by Ethicon. In
general, these aforementioned polymers are distinctly limited
by the range of properties attainable and lack of chemical side
groups for further functionalization, potentially hindering the
development and synthesis of more tailored materials.
In recent years, a number of new polymers have been
introduced to address the need for functionalizable materials.
These include linear polyesters based on amino acids,9,10
sugars,11,12or modified hydroxy acids,13,14linear polycarbonates
based on sugars,12glycerol,15and dihydroxyacetone,16modified
trimethylene carbonate monomers,17,18and others.19Polyester
dendrimers composed of glycerol and lactic or succinic acids
have also been reported.20
Poly(?-caprolactone) is one polymer that has been used widely
in a variety of medical devices. Currently, poly(?-caprolactone)
is incorporated in materials for tissue scaffolding including
bone,21blood vessels,22and nerves,23as well as drug delivery
systems24and suture materials,25but it is limited by a lack of
functional side groups. An advantage of using poly(?-capro-
lactone) is because of its slow degradation rate which does not
create acidic microenvironments like poly(glycolic acid) and
poly(lactic acid).26Herein we report new copolymers based on
6-hydroxyhexanoic acid and glycerol which contain pendant side
chains with varying common reactive groups. The utility of these
functionalizable polymers was further demonstrated through the
covalent attachment of coumarin, a fluorescent dye molecule.
Nanoparticles can be formed from the poly(carbonate ester)s
and these particles are internalized by cells.
?-Caprolactone was copolymerized with 5-benzyloxy-1,3-
dioxan-2-one to form a copolymer, which, after the removal of
the benzyl side chains from the glycerol carbonate units via a
mild hydrogenolysis reaction afforded a secondary hydroxyl
pendant side groups with the capacity for subsequent function-
alization. The carbonate of glycerol monomer, 5-benzyloxy-
1,3-dioxan-2-one (1), was prepared in three steps, starting from
2-phenyl-1,3-dioxan-5-ol (see synthesis in Supporting Informa-
tion).15Scheme 1 depicts the copolymerization of the carbonate
monomer, 1, with ?-caprolactone, 2, via a ring-opening polym-
erization using tin(II) 2-ethylhexanoate (Sn(oct)2). Deprotection
of the secondary hydroxyl group on the glycerol was achieved
using a Pd/H2catalyzed reaction to remove the benzyl-protecting
Copolymers were synthesized from varying mole ratios of
the two monomers, ?-caprolactone and 5-benzyloxy-1,3-dioxan-
2-one; molecular weight measurements were obtained using size
exclusion chromatography and thermal data were recorded using
differential scanning calorimetry. Copolymer composition, mo-
lecular weights, and thermal data are summarized in Table 1. It
should be noted that the molecular weights of aliphatic
polyesters and polycarbonates tend to be overestimated by up
to 50% by polystyrene-calibrated SEC measurements.27-29
Both benzyl-protected and -deprotected copolymers demon-
strated similar solubility. All copolymers were soluble in
dichloromethane, dimethylformamide, and toluene, with limited
solubility in tetrahydrofuran. Conversely, all copolymers were
insoluble in more polar solvents such as methanol, ethanol,
dimethyl sulfoxide, water, and ethyl acetate. Copolymers with
melting temperatures above room temperaturesprotected co-
polymers with g70 mol % CG and deprotected copolymers with
g80 mol % CGsformed tough, opaque films upon drying.
All other copolymers were viscous liquids at room temper-
The stannous octoate (Sn(oct)2) catalyst was chosen for the
ring-opening polymerization in part due to its ubiquity for
catalyzing numerous other copolymerization reactions between
biodegradable cyclic monomers such as ?-caprolactone, trim-
ethylene carbonate, glycolide, and lactide. This catalyst is
currently used to synthesize materials intended for use in vivo.30
Sn(oct)2has a high polymerization activity for both ?-capro-
lactone and cyclic carbonates, leading to linear chains via a
The relative mole fractions of monomers in the resulting
copolymers were determined using1H NMR analysis. Compar-
ing the integrations of unique CH2species in each monomer
unit of the protected copolymersthe benzyl CH2peak at δ )
4.65 for glycerol and CH2peak alpha to the carbonyl at δ )
2.29 for 6-hydroxyhexanoic acidsshows that the polymerization
proceeds with expected mole fractions of each monomer
incorporated into the copolymer. Also, the methylene R to the
carbonyl of the 6-hydroxyhexanoic acid unit splits into two
multiplets with integrations indicative of the monomer ratio in
the deprotected polymer (Figure 1). A linear trend of relative
mole percent in the polymer feed vs mole percent in the
copolymer was observed (see figure in Supporting Information).
†Departments of Chemistry and Biomedical Engineering, Boston
‡Division of Thoracic Surgery, Department of Surgery, Brigham and
Macromolecules 2007, 40, 7065-7068
10.1021/ma071276v CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/07/2007
Next,13C NMR spectra of the poly(carbonate ester)s were
analyzed to determine the monomer sequence in the copolymer.
For the case of a copolymer with two constituent monomers,
23triads can theoretically be formed, and their relative intensities
in the spectra can suggest a blocky, random, or statistically
random chemical structure. There are eight possible triads for
the ester-carbonate copolymer system (see Supporting Informa-
tion for details). Triad peaks that do not overlap the equivalent
homopolymer peaks from triads LLL (?-caprolactone) and CCC
(5-benzyloxytrimethylene carbonate) indicate a degree of ran-
domness in the sequence. Comparison of relative intensities
shows a propensity for randomness as carbonate monomers tend
to precede ?-caprolactone additions.
Thermal analysis via differential scanning calorimetry (DSC)
revealed several property trends as a function of glycerol
carbonate mol % in the copolymer. Table 1 shows the glass
transition temperature (Tg) of the copolymers increasing, from
-64 °C (pure poly(?-caprolactone)) to -10 °C (pure poly(5-
benzyloxy-1,3-dioxan-2-one)), as the ratio of 5-benzyloxy-1,3-
dioxan-2-one to ?-caprolactone monomer increased. Conversely,
the melting temperature (Tm) of the copolymer decreased with
increasing 5-benzyloxy-1,3-dioxan-2-one monomer, before dis-
appearing completely for CL-CG-60-40-Bn (40 mol % carbon-
ate monomer). Relative amounts of crystallinity were determined
from the heat of fusion, ∆Hf. For each additional 10 mol %
5-benzyloxy-1,3-dioxan-2-one monomer in the reaction mixture,
the percent crystallinity was approximately halved. When the
reaction consisted of less than 70 mol % ?-caprolactone, the
copolymer became completely amorphous, as determined by
the absence of a crystallinity temperature and melting temper-
Primary hydroxyl-, amine-, and carboxylic acid-derivatized
copolymers were synthesized via the addition of 6-benzyloxy-
hexanoic acid, hexanedioic acid monobenzyl ester, or fmoc-6-
aminohexanoic acid, respectively, to poly(carbonate-co-ester),
CL-CG-80-20-OH (Scheme 2). Side chain deprotection was
subsequently performed by Pd-catalyzed hydrogenation to
remove the benzyl-protecting group, or a 40% piperidine/
dimethylformamide mixture to remove the fmoc-protecting
group, and verified by1H NMR. The three copolymers including
CL-CG-80-20-C6-OH, 5, CL-CG-80-20-C5-COOH, 6, and
CL-CG-80-20-C6-NH2, 7, demonstrated similar thermal trends
as CL-CG-80-20-Bn with appropriate changes in molecular
weights, with the exception of small increases in melting
temperatures (Table 1). Differences in melting temperature are
likely due to increased hydrogen bonding from the side chains.
The modification of a functionalizable copolymer was demon-
strated by conjugating the fluorescent chromophore coumarin
to the free secondary hydroxyl units on CL-CG-80-20-OH.
Poly(?-caprolactone) was identified for this study in part due
to the wide use of this polymer in drug delivery systems, where
an agent, for example, is encapsulated within the interior
Scheme 1. Synthesis of Poly(E-caprolactone-co-glycerol Carbonate)a
aKey: (a) Sn(Oct)2, 140 °C; (b) H2, Pd/C, Pd(OH)2/C, DCM.
Table 1. Composition, Molecular Weight, and Thermal Data of Copolymersa
mol wt thermal properties
afcg) mole percent carbonate monomer in polymerization feed; Fcg) mole percent carbonate monomer in copolymer; Mn) number average molecular
weight; Mw/Mn) polydispersity index; Tg) glass transition temperature; Tc) crystallization temperature; Tm) melting temperature; Hf) heat of fusion.
The synthesis and characterization of poly(carbonate-ester)s prepared form 5-benzyloxy-1,3-dioxan-2-one and ?-caprolactone are reported. These copolymers
contain a hydrolyzable backbone and functionalizable heteroatom pendant groups.
Communications to the Editor
Macromolecules, Vol. 40, No. 20, 2007
structure of microparticles or nanoparticles.24Nanoparticles were
formed from CL-CG-80-20-OH or CL-CG-80-20-coumarin
copolymers using an emulsion/solvent evaporation method.32
Size distribution measurements obtained by dynamic light
scattering indicate a mean particle diameter of 125 nm with a
narrow polydispersity index (0.128). To determine the potential
applicability of these nanospheres as drug delivery vehicles, we
performed an initial cell uptake using fluorescent CL-CG-80-
20-coumarin nanoparticles and A549 human lung carcinoma
cells. As shown in Figure 2, particles were internalized by the
cells. The cytotoxicity of nanoparticles (CL-CG-80-20-OH)
was then assessed with A549 cells. Cell viability was determined
1H NMR spectra of the CL-CG-80-20-OH copolymer.
Scheme 2. Functionalized Poly(E-caprolactone-co-glycerol carbonate)s, with Hydroxyl (5), Carboxylic Acid (6), Amine (7), and
Coumarin-Conjugated (8) Copolymers Synthesized from CL-CG-80-20-OH Copolymer
Macromolecules, Vol. 40, No. 20, 2007
Communications to the Editor
using a standard MTT cell proliferation assay. No cytotoxicity Download full-text
was observed with the nanoparticles, as the data was similar to
the positive controlsuntreated cells (see Supporting Informa-
New poly(ester-carbonate)s comprised of glycerol and 6-hy-
droxycaproic acid repeating units have been synthesized via
ring-opening polymerization of 5-benzyloxy-1,3-dioxan-2-one
and ?-caprolactone, followed by catalytic hydrogenolysis.
Copolymerizing 5-benzyloxy-1,3-dioxan-2-one with biodegrad-
able aliphatic polyesters such as poly(?-caprolactone) introduces
side chains for subsequent modification by incorporating a
biocompatible monomer unit of glycerol. Further functional-
ization via hydroxyl, carboxylic acid, and/or amine side chains
will facilitate the attachment of chemically diverse molecules
to the polymer chain. The utility of side-group modification was
demonstrated through the covalent attachment of the chro-
mophore, coumarin, to the secondary hydroxyl groups of the
polymer. Furthermore, the polymer can be processed to give
nanoparticles. Application-specific tailoring of the chemical,
physical, and mechanical properties of the polymer for medical
uses by varying monomer units, composition, degradable
linkages, and side group moieties, is highly advantageous given
the varied design requirements of a specific application, be it
controlled drug delivery or degradable scaffolding. Continued
research in this area will lead to increasingly specialized
materials that are multifunctional (e.g., delivery, targeting, and
imaging), responsive to stimuli from their local environment
(e.g., pH), or processable into unique structures (e.g., fibers,
particles, and 3D constructs).
Acknowledgment. This work was supported by a grant from
the Whitaker Foundation, and we thank Ann C. Gaffey for
assistance with the cell experiments.
Supporting Information Available:
procedures, NMR analysis, TEM data, figures showing a plot of
Text giving synthetic
relative mole percentages,13C NMR anaylsis, and cytotoxicity of
the nanoparticles. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 2. Fluorescence microscopy images of coumarin-conjugated
CL-CG-80-20-OH nanoparticles after cellular uptake by A549 non-
small cell lung cancer cells (phase contrast, left; fluorescence, right;
10×, scale bar ) 100 µm).
Communications to the Editor
Macromolecules, Vol. 40, No. 20, 2007