Versatile synthesis of functional biodegradable polymers by combining ring-opening polymerization and postpolymerization modification via michael-type addition reaction

Article abstract of DOI:10.1021/ma901897y

Various functional biodegradable polymers were readily prepared based on novel cyclic carbonate monomers, acryloyl carbonate (AC) and methacryloyl carbonate (MAC), by combining ring-opening polymerization (ROP) and Michael-type conjugate addition. AC and MAC monomers were synthesized in four straightforward steps from 1,1,1-tris(hydroxymethyl)ethane with good overall yields (ca. 40%). AC and MAC were able to copolymerize with ε-caprolactone (ε-CL) and D,L-lactide (LA) in toluene at 110°C using stannous octoate as a catalyst, yielding biodegradable copolymers with controlled (meth)acryloyl functional groups and molecular weights. The acryloyl groups were amenable to the Michaeltype conjugate addition with varying thiol-containing molecules such as 2-mercaptoethanol, 3-mercapto-propanoic acid, cysteamine, cysteine, and arginine-glycine-aspartic acid-cysteine (RGDC) peptide under mild conditions, to provide biodegradable materials with vastly different functionalities (e.g., hydroxyl, carboxyl, amine, amino acid, and peptides) and properties (e.g., hydrophilicity, cell adhesion). Notably, 100% functionalization was achieved with 2-mercaptoethanol, cysteamine and cysteine. Initial cell culture studies demonstrated enhanced cell adhesion and growth on films containing functional RGDC peptides as compared to those of the parent copolymer. Therefore, combination of ROP and Michael-type conjugate addition provides a versatile access to diverse types of functional biodegradable materials.

Full text of DOI:10.1021/ma901897y

Macromolecules 2010, 43, 201–207 201
DOI: 10.1021/ma901897y
Versatile Synthesis of Functional Biodegradable Polymers by Combining
Ring-Opening Polymerization and Postpolymerization Modification via
Michael-Type Addition Reaction
Wei Chen,† Huicui Yang,‡ Rong Wang,† Ru Cheng,† Fenghua Meng,*,†
Wenxiang Wei,‡ and Zhiyuan Zhong*,†
†Biomedical Polymers Laboratory and Jiangsu Key Laboratory of Organic Chemistry, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China and
‡Department of Cellular and Molecular Biology, School of Medicine, Soochow University, Suzhou 215123,
P. R. China
Received August 26, 2009; Revised Manuscript Received November 24, 2009
ABSTRACT: Various functional biodegradable polymers were readily prepared based on novel cyclic
carbonate monomers, acryloyl carbonate (AC) and methacryloyl carbonate (MAC), by combining ring-
opening polymerization (ROP) and Michael-type conjugate addition. AC and MAC monomers were
synthesized in four straightforward steps from 1,1,1-tris(hydroxymethyl)ethane with good overall yields
(ca. 40%). AC and MAC were able to copolymerize with ε-caprolactone (ε-CL) and D,L-lactide (LA) in
toluene at 110 °C using stannous octoate as a catalyst, yielding biodegradable copolymers with controlled
(meth)acryloyl functional groups and molecular weights. The acryloyl groups were amenable to the Michael-
type conjugate addition with varying thiol-containing molecules such as 2-mercaptoethanol, 3-mercapto-
propanoic acid, cysteamine, cysteine, and arginine-glycine-aspartic acid-cysteine (RGDC) peptide under
mild conditions, to provide biodegradable materials with vastly different functionalities (e.g., hydroxyl,
carboxyl, amine, amino acid, and peptides) and properties (e.g., hydrophilicity, cell adhesion). Notably,
100% functionalization was achieved with 2-mercaptoethanol, cysteamine and cysteine. Initial cell culture
studies demonstrated enhanced cell adhesion and growth on films containing functional RGDC peptides as
compared to those of the parent copolymer. Therefore, combination of ROP and Michael-type conjugate
addition provides a versatile access to diverse types of functional biodegradable materials.
Introduction
properties such as enhanced hydrophilicity and biodegradability,
and on the other hand facilitate drug conjugation or further
Aliphatic polyesters and polycarbonates, such as poly(ε-carpo-
lactone) (PCL), polylactide (PLA), poly(lactide-co-glycolide)
(PLGA), and poly(trimethylene carbonate) (PTMC), are among
the most important synthetic biodegradable materials.1-7 These
polymers are biocompatible, mechanically viable, degradable in
vivo into nontoxic products, and readily processable to fibers,
films, rods, microparticles, nanoparticles, and porous three-
dimensional constructs.8-10 In addition to applications as resorb-
able sutures and in various medical devices, degradable polyesters
and polycarbonates are one of the key biomaterials used and/or
currently investigated for controlled drug delivery, tissue engi-
neering and regenerative medicine.11-16
These degradable polymers are, nevertheless, not ideal. In
practice, very often they can not meet the requirements of
particular applications, due to their high hydrophobicity, im-
proper degradation profile, and/or lack of reactive centers in the
polymer chain for the covalent immobilization of bioactive
molecules such as drugs, peptides and proteins.17,18 While, the
ever advancing biomedical technology demands development of
complex biologically active biomaterials.19,20 In the past decade,
various functional aliphatic polyesters and polycarbonates con-
taining, e.g., hydroxyl,21-30 carboxyl,31-36 amine,27,32,37,38 and
cyclohexene39 pendant groups have been reported. These func-
tional polymers on one hand show improved physiochemical
derivatization. Their synthesis is, however, usually a multistep
process involving protection and deprotection of the functional
groups before and after the polymerization, which may result in
low overall yields as well as degradation.3
Notably, there are several reports on degradable polymers
presenting acryloyl,40-42 allyl,43-45 or alkyne/azide46,47 func-
tional groups, in which no protection/deprotection steps is
needed and they can readily be transformed into diverse func-
tionalities through postpolymerization modification.48 In parti-
cular, functional PCL containing acryloyl groups reported by
Jerome and co-workers is of immense interest in that (1) further
derivatization through the Michael-type conjugate addition with
thiol-containing molecules is highly selective and is tolerant to a
variety of functional groups including hydroxyl, carboxyl, and
amine; (2) reaction takes place under very mild conditions, so
degradation is minimized; and (3) no catalyst is needed and no
side products is produced, thereby precluding possible contam-
inations.49 It has been reported, nevertheless, that polymerization
of γ-acryloyl-ε-caprolactone (ACL) is associated with significant
side reactions.50,51 The other drawback is that ACL is not able to
copolymerize with lactide (LA) monomers.
In this paper, we report on synthesis of a variety of functional
biodegradable polymers based on two novel cyclic carbonate
monomers, acryloyl carbonate (AC) and methacryloyl carbonate
(MAC), by combining ROP and Michael-type conjugate addi-
tion. AC and MAC monomers were readily synthesized and co-
polymerized with ε-CL or LA. The versatile postpolymerization
*To whom correspondence should be addressed. Telephone/
Fax: þ86-512-65880098. E-mail: (F.M.)fhmeng@suda.edu.cn; (Z.Z.)
zyzhong@suda.edu.cn.
r 2009 American Chemical Society
Published on Web 12/09/2009
pubs.acs.org/Macromolecules

202 Macromolecules, Vol. 43, No. 1, 2010
Chen et al.
modification by Michael-type conjugate addition yielded a range
of functional biodegradable materials. The combination of ROP
and Michael-type conjugate addition has appeared to be an
efficient and practical approach to developing biologically active
biomaterials.
Scheme 1. Synthetic Pathway to AC (4a) and MAC (4b) Monomersa
Experimental Section
Materials. 1,1,1-Tris(hydroxyl methyl)ethane (THME, Alfa
Aesar, 99%), triethylamine (Et3N, Alfa Aesar, 99%), acryloyl
chloride (Alfa Aesar, 96%), and stannous octoate (Sn(Oct)2,
95%, Sigma), 3-mercaptopropanoic acid (Alfa Aesar, 99%), 2-
mercaptoethanol (Amresco, >99%), 2-mercaptoethylamine
hydrochloride (Alfa Aesar, 99%), and L-cysteine (Alfa Aesar,
>99%) were used as received. Benzaldehyde (>99%) and
p-toluenesulfonic acid monohydrate (TsOH, >99%) were pur-
chased from Sinopharm Chemical Reagent Co., Ltd. (SCRC)
and used as received. Methacryloyl chloride was purchased from
Wuxi Chemical Reagent Co., Ltd. (Jiangsu, China). RGDC was
purchased from Suzhou ChinaTech Peptide Co., Ltd. ε-Capro-
lactone (ε-CL, Alfa Aesar, 99%) was dried over CaH2 and
distilled under reduced pressure prior to use. D,L-lactide (LA,
Purac, >99%) was recrystallized from dried toluene. Tetra-
hydrofuran (THF) and toluene were dried by refluxing over
sodium wire under an argon atmosphere prior to distillation.
Dichloromethane (CH2Cl2) and isopropanol (SCRC, >99%)
were dried by refluxing over CaH2 under an argon atmosphere.
Ethyl chloroformate (SCRC, >96%) was freshly distilled be-
fore use.
a Conditions: (i) benzaldehyde, TsOH, THF, room temperature; (ii)
acryloyl chloride (2a) or methacryloyl chloride (2b), Et3N, CH2Cl2, 0 °C;
(iii) HCl (1.0 M), methanol, room temperature; (iv) ethyl chloroformate,
Et3N, THF, 0 °C.
Ring-Opening Copolymerization. The polymerization was
carried out in toluene at 110 °C using isopropanol as an initiator
and Sn(Oct)2 as a catalyst. The following is a typical example of
synthesis of P(CL-co-AC) 8.9%. In a glovebox under a nitrogen
atmosphere, to a stirred solution of ε-CL (1.539 g, 13.5 mmol)
and AC (0.3 g, 1.5 mmol) in toluene (20 mL) was quickly added
isopropanol stock solution (0.26 mL, 0.75 M) and Sn(Oct)2
stock solution (1 mL, 0.1 M). The reaction vessel was sealed and
placed in an oil-bath thermostated at 110 °C. After 24 h
polymerization, the reaction was terminated by two drops of
acetic acid. A sample was taken for the determination of
1
monomer conversion using H NMR. The resulting polymer
P(CL-co-AC) 8.9% was isolated by precipitation in cold diethyl
ether and dried in vacuo at room temperature.
Preparation of Acryloyl Carbonate (AC, 4a) and Methacryloyl
Carbonate (MAC, 4b). AC and MAC monomers were synthe-
sized in four steps. The following is a typical example of synthesis
of 4a. To a stirred solution of THME (24 g, 0.2 mol) and TsOH
(1.2 g, 6.3 mmol) in THF (375 mL) at room temperature was
added dropwise benzaldehyde (21.4 mL, 0.21 mol). After 16 h of
reaction, the reaction mixture was neutralized with aqueous
ammonia, the solvent was evaporated under reduced pressure,
and the residues were dissolved in 150 mL of CH2Cl2 and
extracted twice with 150 mL of phosphate buffer (pH 7.4). The
organic phase was concentrated to yield 39 g (94%) of a colorless
powder (1). 1H NMR (400 MHz, CDCl3): δ 0.81 (s, 3H), 3.66 (d,
2H), 3.91 (s, 2H), 4.06 (d, 2H), 5.44 (s, 1H), 7.35-7.49 (m, 5H).
To a stirred solution of 1 (10 g, 48 mmol) and Et3N (12 mL,
86.4 mmol) in 150 mL of anhydrous CH2Cl2 at 0 °C was added
dropwise acryloyl chloride (5.8 mL, 72 mmol) dissolved in
CH2Cl2. After 4 h reaction at 0 °C, the reaction mixture was
filtered. The filtrate was washed twice with phosphate buffer
(pH 7.4) and then concentrated to yield crude 2a. 2a was
dissolved in 160 mL of CH3OH/1.0 M HCl (v/v 1/1) and stirred
at room temperature for 2 h. The solution pH was then adjusted
to 7.0 using 2 M NaOH. The solution was concentrated and
extracted with ethyl acetate. The organic phase was concen-
trated to yield crude product 3a, which was purified by column
chromatography (eluent: ethyl acetate/petroleum ether = 1/1,
v/v). Yield: 5.07 g (61%). 1H NMR (400 MHz, CDCl3): δ 086 (s,
3H), 2.96 (s, 2H), 3.56 (q, 2H), 4.26 (s, 2H), 5.88-6.42 (m, 3H).
To a stirred solution of 3a (4 g, 22.9 mmol) and ethyl
chloroformate (4.6 mL, 48.09 mmol) in dried THF (150 mL)
at 0 °C was added dropwise Et3N (7 mL, 50.49 mmol) dissolved
in THF. After 4 h reaction at 0 °C, the reaction mixture was
filtered, and the filtrate was concentrated under reduced pres-
sure. The residues were crystallized in diethyl ether to yield 4a.
Yield: 2.94 g (64%). 1H NMR (400 MHz, CDCl3): δ 1.14 (s, 3H),
4.17 (d, 2H), 4.19 (s, 2H), 4.33 (d, 2H), 5.91-6.45 (m, 3H). Anal.
Calcd for C9H12O5: C, 54.00; H, 6.04. Found: C, 53.87; H, 6.06.
In a similar way, 4b was prepared with an overall yield of
40%. 1H NMR (400 MHz, CDCl3): δ 1.12 (s, 3H), 1.96 (s, 3H),
4.16 (s, 2H), 4.18 (d, 2H), 4.33 (d, 2H), 5.64 (s, 1H), 6.12 (s, 1H).
Anal. Calcd for C10H14O5: C, 56.07; H, 6.59. Found: C, 56.18;
H, 6.52.
Characterization. 1H NMR spectra were recorded on the
Unity Inova 400 and NMRststem (Varian) operating at 400
and 300 MHz, respectively. CDCl3 and DMSO-d6 were used as
solvents and the chemical shifts were calibrated against residual
solvent signals. The molecular weight and polydispersity of the
copolymers were determined by a Waters 1515 gel permeation
chromatograph (GPC) instrument equipped with two linear
PLgel columns (500 A and Mixed-C) following a guard column
and a differential refractive-index detector. The measurements
were performed using THF as the eluent at a flow rate of 1.0 mL/min
at 30 °C and a series of narrow polystyrene standards for the
calibration of the columns. Contact angle was determined by
POWEREACH Instrument (Micaren, JC2000C/X).
Post-Polymerization Modification by a Michael-Type Conju-
gate Addition Reactions. Michael-type conjugate addition reac-
tion was carried out in DMF at room temperature under a
nitrogen atmosphere. The following is a typical example
on modification of P(CL-co-AC) 8.9%. P(CL-co-AC) 8.9%,
a thiol-containing molecule (R-SH: 2-mercaptoethanol, 2-mer-
captoethylamine hydrochloride, 3-mercaptopropanoic acid
or L-cysteine), and pyridine were reacted in DMF at a mole
ratio AC/R-SH/pyridine of 1/10/10 at room temperature
(Scheme 3). The reaction was allowed to proceed for 2 to 3 d.
The resulting functional polymers were isolated by precipitation
from cold diethyl ether/ethanol and dried in vacuo at room
temperature.
For modification with RGDC, P(CL-co-AC) 8.9%, RGDC
and pyridine were reacted in DMF at a mole ratio AC/RGDC/
pyridine of 1/1/10 at room temperature (Scheme 3). The reaction
was allowed to proceed for 7 d. Then, 2-mercaptoethanol (5-fold
relative to AC units) was added to react with the remaining AC
groups. The reaction was continued for another 2 d (Scheme 3).
The resulting RGD functionalized polymer was isolated by
precipitation from cold ethanol and dried in vacuo at room
temperature.
ꢀ
˚
Preparation of Functional Copolymer Films and Contact Angle
Measurements. Thin films were prepared by casting functiona-
lized copolymer solutions in DMF (0.5 wt %) on microscope
slides. The films on the slides were dried by placing in a
desiccator for 18 h followed by vacuum-drying for 3 days to

Article
Macromolecules, Vol. 43, No. 1, 2010 203
Scheme 2. Ring-Opening Copolymerization of (M)AC with ε-CL and LA: (a) ε-CL and (M)AC; (b) LA and AC
Scheme 3. Modifications of P(CL-co-AC) 8.9% Copolymer with Thiol-Containing Molecules by Michael-Type Conjugate Addition
remove DMF thoroughly. The static contact angle of the films
was measured using a POWEREACH instrument.
with pH 7.4 phosphate buffer, produced benzylidene-THME
(1) with high yield. The (meth)acrylation of 1 was subse-
quently ensued by acid hydrolysis of benzylidene acetal
groups, which afforded after column purification monoa-
cryloyl THME (3a) and monomethacryloyl THME (3b). The
structures of 3a and 3b were corroborated by 1H NMR.
Next, cyclization was performed in the presence of ethyl
chloroformate in dilute anhydrous THF solution at 0 °C via
dropwise addition of triethylamine, similar to preparation of
acid-labile cyclic carbonate monomer.52 The resulting cyclic
carbonate monomers, AC and MAC, were purified by
recrystallization from anhydrous diethyl ether. Notably,
moderate overall yields of ca. 40% were obtained for both
monomers. The 1H NMR spectra (CDCl3) of AC and MAC
as well as the signal assignments are shown in Figure 1.
Taking AC monomer as an example, the resonances at δ 1.14
(s), 4.17 and 4.33 (dd), 4.19 (s), and 5.91-6.45 were attribu-
table to the methyl protons, methylene protons next to the
carbonate, methylene protons neighboring to the ester, and
acryloyl protons, respectively (Figure 1A). Importantly, the
integral ratio of signals at δ 1.14 (methyl protons) and
5.91-6.45 (acryloyl protons) was close to the theore-
tical value (1:1), which combined with results of elemental
Cell Culture. The above-prepared functionalized copolymer
coated microscope slides were placed in a 24-well tissue culture
plate. The whole plate was sterilized by radiation prior to use.
L929 fibroblasts were cultured in Dulbecco’s modified Eagle
medium (DMEM), containing 10% FBS at a density of 5 ꢀ 105
cell/well in a humidified 5% CO2 atmosphere at 37 °C. The
culture media was replaced each day. After 3 d culture, the
media was removed, and the cells were rinsed two times with
fresh media prior to the microscope observation. To visualize
cell nuclei by fluorescence microscopy, the cells were washed
three times with PBS, fixed with 4% paraformaldehyde, and
stained with Hoechst 33258 (KeyGEN, China). The cells were
observed under an inverted microscope (Nikon Eclipse 80i
Microscope equipped with a DS camera cable).
Results and Discussion
Synthesis of AC and MAC Monomers. The two functional
cyclic carbonate monomers, AC (4a) and MAC (4b), were
synthesized in four steps (Scheme 1). The reaction of THME
with benzaldehyde in THF at room temperature in the
presence of catalytic amount of TsOH, followed by washing

204 Macromolecules, Vol. 43, No. 1, 2010
Table 1. Synthesis of (Meth)acryloyl Functionalized Biodegradable Copolymers by Ring-Opening Copolymerizationa
Chen et al.
monomer convn,c %
Mn ꢀ 10-3
entry
copolymer
f,b %
(M)AC
CL or LA
F,d %
theore
1H NMRf
GPCg
PDI GPCg
1
2
3
4
5
6
7
P(CL-co-AC) 8.9%
P(LA-co-AC) 6.1%
P(CL-co-MAC) 7.6%
P(LA-co-MAC) 8.6%
P(CL-co-AC) 16%
10
10
10
10
20
40
40
88.7
51.2
87.5
73.1
90.0
75.1
70.7
98.8
87.5
99.2
85.2
100
8.9
6.1
7.6
8.6
16
33.3
32.5
9.5
9.9
9.6
10.1
10.2
10.3
10.3
9.7
9.9
12.2
14.0
18.5
16.3
19.3
14.1
13.4
1.41
1.26
1.27
1.40
1.44
1.54
1.35
11.2
10.2
11.5
10.3
10.2
P(CL-co-AC) 33.3%
P(CL-co-MAC) 32.5%
100
99.8
a The copolymerization was carried out in toluene at 110 °C using isopropanol as an initiator and Sn(Oct)2 as the catalyst at a total monomer-
to-initiator ratio of 80/1. b Molar fraction of (M)AC monomer in feed. c Determined by 1H NMR. d Molar fraction of (M)AC units in the resulting
copolymer determined by 1H NMR. e Theoretical molecular weights calculated according to the following equation: Mn(theor) = MW(M)AC ꢀ 80 ꢀ f ꢀ
convn(M)AC þ MWCLorLA ꢀ 80 ꢀ (1 - f) ꢀ convnCLorLA. f Estimated by 1H NMR end-group analysis. g Determined by GPC (eluent, THF; flow rate,
1.0 mL/min; standards, polystyrene).
Figure 1. 1H NMR spectra (400 MHz, CDCl3) of AC (A) and MAC
(B) monomers.
analysis confirmed successful synthesis of AC monomer. In a
similar way, H NMR (Figure 1B) and elemental analysis
1
also pointed to successful synthesis of MAC monomer.
Synthesis of Biodegradable Copolymers Containing Acry-
loyl or Methacryloyl Functional Groups. The aim of this study
was to provide a versatile approach to functionalizing bio-
degradable polymers such as PCL and PLA. Hence, co-
polymerization behaviors of (M)AC with ε-CL or LA were
investigated. The copolymerization was performed in
toluene at 110 °C using isopropanol as an initiator and
Sn(Oct)2 as a catalyst (Scheme 2). The monomer to initiator
mole ratio was set at 80/1 and (M)AC monomer in feed was
10 mol %. The results showed that copolymerization of
(M)AC with both ε-CL and LA went smoothly affording
copolymers with controlled molecular weights and moderate
polydispersities (PDI = 1.26-1.54) (Table 1).
1
Importantly, H NMR displayed clearly resonances at δ
5.6-6.4 attributable to intact acryloyl protons (Figure 2,
parts A and B) and at δ 5.6-6.1 assignable to intact
methacryloyl protons (Figure 2C) for copolymers of AC
and MAC, respectively. The composition of the copolymers
could be determined by comparing integrals of signals of
(meth)acryloyl protons with those of PCL methylene pro-
tons at δ 2.30 or PLA methine proton at δ 5.16. Notably,
P(CL-co-AC) chains contained 8.9 mol % of AC units, close
to the feed of 10 mol % (Table 1, entry 1). This copolymer is
accordingly denoted as P(CL-co-AC) 8.9%. The copoly-
Figure 2. 1H NMR spectra (300 MHz, CDCl3) of (M)AC copolymers:
(A) P(CL-co-AC); (B) P(CL-co-MAC); (C) P(LA-co-AC).
merization of AC with LA yielded copolymers with 6.1
mol % AC (Table 1, entry 2). This is in contrast to acryloyl

Article
Macromolecules, Vol. 43, No. 1, 2010 205
1
Figure 3. 1H NMR spectra (300 MHz, CDCl3) of P(CL-co-AC) 8.9%
copolymer modified with 2-mercaptoethanol (A), 2-mercaptoethyl-
amine (B), and 3-mercaptopropanoic acid (C).
Figure 4. H NMR spectra (300 MHz, DMSO-d6) of P(CL-co-AC)
8.9% copolymer modified with -cysteine (A) and RGDC peptide (B).
L
Michael-type conjugate addition is appealing for post-
polymerization modification because its reaction condition
is extremely mild and different bioactive molecules including
peptides and proteins may readily be immobilized through
their cysteine moieties.54-56 In the following, modification of
P(CL-co-AC) 8.9% with cysteine (an amino acid) or RGDC
peptide was undertaken (Scheme 3). Interestingly, 100%
functionalization was observed for modification with cy-
steine under the same reaction conditions as described above
to afford P(CL-co-AC)(cys)8.9%, as shown by 1H NMR
(Figure 4A). The modification of P(LA-co-AC) 6.1% with
cysteine also resulted in quantitative functionalization, yield-
ing P(LA-co-AC)(cys)6.1%. The reaction with RGDC peptide
was slightly different, in which AC/RGDC/pyridine mole
ratio was set at 1/1/10 and after 7 d reaction 2-mercaptoetha-
nol (5-fold relative to AC units) was added to consume the
ε-CL monomer reported by Jerome and co-workers,40,49
wherein copolymerization with LA is generally not feasible.
The capability of copolymerization with LA renders these
functional cyclic carbonate monomers highly attractive for
diverse biomedical applications. Similarly, copolymerization
of MAC with ε-CL and LA afforded P(CL-co-MAC) con-
taining 7.6 mol % MAC (Table 1, entry 3) and P(LA-co-
MAC) containing 8.6 mol % MAC (Table 1, entry 4),
respectively. It should be noted that at higher feed ratios of
20 and 40 mol % AC monomer, P(CL-co-AC) copolymers
with 16 mol % and 33.3 mol % AC units were obtained
(Table 1, entries 5 and 6). The copolymerization of MAC and
LA at a high feed ratio of 40 mol % MAC monomer afforded
P(LA-co-MAC) copolymer with 32.5 mol % MAC (Table 1,
entry 7). 1H NMR end-group analysis revealed that all
copolymers had molecular weights close to the theoretical
values (Table 1). The Mn data determined by GPC were in
general higher than those determined by 1H NMR (Table 1),
which is most likely due to use of polystyrene standards for
molecular weight calibration in our GPC measurements. It is
evident, therefore, that both (meth)acryloyl functionality
and molecular weights of these functional copolymers could
be readily controlled.
1
remaining AC groups. Markedly, H NMR showed that
RGDC has successfully been conjugated to the copolymer
with 58% RGDC functionalization (Figure 4B). The remain-
ing AC units were completely derivatized with 2-mercap-
toethanol, yielding P(CL-co-AC)(RGDC)5.2%(OH)3.7%
.
Therefore, combination of ROP and postpolymerization
modification by Michael-type conjugate addition offers
a direct and versatile access to functional biodegradable
materials with vastly different functionalities including hydro-
xyl, amino, carboxyl, amino acid, and peptides.
Modification of Copolymers by Michael-Type Conjugate
Addition. To investigate whether acryloyl functional groups
are amenable to derivatization by Michael-type conjugate
addition, P(CL-co-AC) 8.9% was reacted under a nitrogen
atmosphere with varying thiol-containing molecules such as
2-mercaptoethanol, cysteamine, and 3-mercaptopropanoic
acid. The reaction was performed in DMF at room tempera-
ture for 2 to 3 d at an AC/R-SH/pyridine mole ratio of 1/10/
Films of Functional Biodegradable Copolymers. Thin films
of functionalized copolymers were prepared on microscope
slides using 0.5 wt % copolymer solution in DMF. The static
contact angle measurements demonstrated that P(CL-co-
AC) 8.9% modified with 2-mercaptoethanol, cysteamine,
cysteine and RGDC, except 3-mercaptopropanoic acid, all
displayed increased hydrophilicity as compared to the parent
copolymer (Figure 5). The negligible change of hydrophili-
city of P(CL-co-AC) 8.9% modified with 3-mercaptopropa-
noic acid is most likely due to its low functionalization. In a
similar way, increased hydrophilicity was also observed for
cysteine-modified P(LA-co-AC) 6.1% copolymer (Figure 5),
though decrease of contact angle was to a less extent than
cysteine-modified P(CL-co-AC) 8.9%.
Initial cell culture studies were performed on the thin films
of functional copolymers using L929 fibroblasts. Figure 6
shows images of cells after 3 d of culture. The cell nuclei were
stained with Hoechst 33258 (blue). It is interesting to note
that films of RGD modified P(CL-co-AC) supported better
cell adhesion and growth as compared to those of the parent
copolymer. Importantly, the morphology of the cells as well
as their nuclei on RGD modified copolymer films appeared
1
10 (Scheme 3). Remarkably, H NMR revealed complete
disappearance of peaks assignable to acryloyl groups and
occurrence of new signals corresponding to 2-mercaptoetha-
nol and cysteamine moieties, respectively (Figure 3, parts A
and B), indicating 100% functionalization with 2-mercap-
toethanol and cysteamine (denoted as P(CL-co-AC) (OH)8.9%
and P(CL-co-AC)(NH2)8.9%, respectively). The treatment of
P(CL-co-AC) 8.9%with3-mercaptopropanoic acid, however,
yielded only 30% conversion of acryloyl groups, to give P(CL-
co-AC)(COOH)2.7% (Figure 3C). This is in line with a report
by Hubbell and co-workers that neighboring carboxylic
groups greatly decrease the reactivity of thiol molecules
toward Michael-type conjugate addition modification.53 It
should be noted that the GPC curves of all modified copoly-
mers remain unimodal with similar PDI to the parent co-
polymer, indicating minimal degradation during modification.

206 Macromolecules, Vol. 43, No. 1, 2010
Chen et al.
significantly smaller number of living cells (ca. 63% relative
to tissue culture plastic) were observed on P(CL-co-AC)
films. These results indicated that activities of RGDC pep-
tides maintained over the mild Michael-type conjugate addi-
tion reactions. RGD peptides have been widely applied to
stimulate cell adhesion (e.g., in tissue engineering appli-
cations); however, proper immobilization of RGD peptides
on biodegradable materials remains a huge challenge.57
These (meth)acryloyl cyclic carbonate monomers provide a
new entry to anchor peptides and perhaps proteins on
synthetic biodegradable polymers for diverse biomedical
applications including medical devices, tissue engineering,
and drug delivery systems.17,20,58,59
Conclusions
We have demonstrated that combination of ring-opening
polymerization and Michael-type conjugate addition chemistry
represents an elegant and remarkably versatile approach to
functionalizing biodegradable copolymers including PCL and
PLA. It offers several interesting features: (1) the acryloyl cyclic
carbonate monomers are readily prepared with good overall
yields; (2) these acryloyl cyclic carbonate monomers are able to
copolymerize with both ε-CL and LA, yielding copolymers with
controlled acryloyl functional groups and molecular weights; (3)
the acryloyl groups in the copolymers can be conveniently
modified via Michael-type conjugate addition chemistry to
different functionalities including hydroxyl, amine, carboxyl,
amino acid, and peptides under extremely mild conditions, which
does not involve any metal catalyst, protection-deprotection
steps, and potentially toxic byproduct; (4) functionalization with
many thiol-containing molecules including 2-mercaptoethanol,
cysteamine, and cysteine is quantitative; and (5) RGDC peptides
are readily anchored onto the biodegradable materials, resulting
in enhanced cell adhesion and growth. We are convinced that
combination of ring-opening polymerization and Michael-type
conjugate addition has a great potential in developing multi-
functional bioactive biomaterials for diverse biomedical applica-
tions including medical devices, tissue engineering, regenerative
medicine, and drug delivery systems.
Figure 5. Contact angle measurements of functional copolymer films.
Films of PCL (Mn = 12.4K) and PLA (Mn = 12.4K) were used as
controls.
Acknowledgment. This work was financially supported by
researchgrants fromthe National Natural Science Foundation of
China (NSFC 50703028, 50973078, 20874070 and 20974073), the
Natural Science Foundation of the Jiangsu Higher Education
Institutions of China (08KJB150016), and the Program of
Innovative Research Team of Soochow University.
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