Synthesis, characterization, and properties of amphiphilic chitosan copolymers with mixed side chains by click chemistry

Article abstract of DOI:10.1002/pola.24136

Novel amphiphilic chitosan copolymers with mixed side chains of poly(ε-caprolactone) and poly(ethylene oxide) (CS-g-PCL/PEO) were successfully synthesized by "graft to" approach via click chemistry. The melting and crystallization behaviors and crystalline morphology of CS-g-PCL/PEO copolymers can be adjusted by the alteration of the feed ratio of PCL and PEO segments. CS-g-PCL/PEO copolymers revealed crystalline morphology different from that of linear alkynyl PCL and alkynyl PEO due to the influence of brush structure of copolymers and the mutual influence of PCL and PEO segments. The hydrophilicity of the CS copolymers can be improved and adjusted by the alteration of the composition of PCL and PEO segments. Moreover, the CS copolymers can self-assemble Into spherical micelles in aqueous solution. Investigation shows that the size of the CS copolymer micelles increased with the increase of the content of hydrophobic PCL segments in copolymers, which indicated that the micellar behavior of the copolymers can be controlled by the adjustment of the ratio of PCL and PEO segments in copolymer.

Full text of DOI:10.1002/pola.24136

Synthesis, Characterization, and Properties of Amphiphilic Chitosan
Copolymers with Mixed Side Chains by Click Chemistry
WEIZHONG YUAN, ZHENGDA ZHAO, SHUYING GU, JIE REN
School of Materials Science and Engineering, Institute of Nano and Bio-polymeric Materials, Tongji University, Shanghai 20092,
People’s Republic of China
Received 24 March 2010; accepted 17 May 2010
DOI: 10.1002/pola.24136
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel amphiphilic chitosan copolymers with mixed
side chains of poly(e-caprolactone) and poly(ethylene oxide)
(CS-g-PCL/PEO) were successfully synthesized by ‘‘graft to’’
approach via click chemistry. The melting and crystallization
behaviors and crystalline morphology of CS-g-PCL/PEO copoly-
mers can be adjusted by the alteration of the feed ratio of PCL
and PEO segments. CS-g-PCL/PEO copolymers revealed crys-
talline morphology different from that of linear alkynyl PCL
and alkynyl PEO due to the influence of brush structure of
copolymers and the mutual influence of PCL and PEO seg-
ments. The hydrophilicity of the CS copolymers can be
improved and adjusted by the alteration of the composition of
PCL and PEO segments. Moreover, the CS copolymers can
self-assemble into spherical micelles in aqueous solution.
Investigation shows that the size of the CS copolymer micelles
increased with the increase of the content of hydrophobic
PCL segments in copolymers, which indicated that the
micellar behavior of the copolymers can be controlled by the
adjustment of the ratio of PCL and PEO segments in copoly-
C
V
mer. 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 48: 3476–3486, 2010
KEYWORDS: chitosan; click chemistry; crystalline morphology;
micelles; mixed side chain; self-assembly
INTRODUCTION Chitosan (CS) is an abundant natural bioma-
terial that has attracted much interest due to its good bio-
compatibility, biodegradability, nontoxicity, and bioactivity,
and therefore wide applications in biomedical field.1–6 The
graft modification of chitosan has been explored as a conven-
ient method to overcome the insolubility in common solvents
and develop multifunctional novel CS copolymers.7–13 Dong
et al. reported the preparation of chitosan-graft-poly(L-lac-
tide) (CS-g-PLLA) with ‘‘graft to’’ and ‘‘graft from’’
approaches, respectively.14,15 Chitosan-graft-poly(e-caprolac-
tone) (CS-g-PCL) can be prepared by ring-opening polymer-
ization (ROP) with graft from approach.16–18 Jing et al.
reported the preparation of chitosan-graft-poly(L-lysine) (CS-
g-PLL) for gene transfection via ROP with graft to
approach.19 Moreover, Bai et al. reported the preparation of
CS beads grafted by polyacrylamide via surface-initiated
atom transfer radical polymerization (ATRP).20 However,
using the above methods to prepare CS copolymers exists
some defects, such as low graft efficiency, complex preparing
process, difficulty to synthesize copolymers with complex
structure and so on.
near-perfect fidelity in the presence of most functional
groups.24–46 Therefore, it is promising to use click chemistry
to synthesize the graft copolymers with complex structure.
Especially, a type of click chemistry, namely, Cu(I)-catalyzed
Huisgen 1,3-dipolar cycloaddition of azides and alkynes has
been developed and utilized elegantly in recent years.
Matyjaszewski et al. reported the synthesis of poly(2-hydrox-
yethyl methacrylate)-graft-poly(ethylene oxide) (PHEMA-g-
PEO) molecular brush by the graft to method via combina-
tion of ATRP and click chemistry.47 Liu et al. reported the
synthesis of well-defined amphiphilic copolymer brushes
possessing alternating poly(methyl methacrylate) and
poly(N-isopropylacrylamide), poly(PMMA-alt-NIPAM), via a
combination of ATRP and click chemistry.48 The polymer
brush can self-assemble into stable aggregates. Huang et al.
reported the synthesis of the amphiphilic graft copolymer
with well-defined star-shaped side chain by the graft to
method via combination of anionic polymerization and click
chemistry.49 Emrick et al. reported the preparation of PEG-
or peptide-graft aliphatic polyesters by click chemistry.50
The work presented here demonstrated the synthesis of
novel amphiphilic CS graft copolymers with mixed side
chains (PCL and PEO) by the graft to method via click chem-
istry. PCL has been extensively used for drug delivery system
and biodegradable sutures because of its biodegradability,
Recently, click chemistry invented by Sharpless and co-
workers21–23 has been introduced into the synthesis of novel
polymeric materials with well-defined structures and unique
properties due to its high specificity, quantitative yields, and
Correspondence to: W. Yuan (E-mail: yuanwz@tongji.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3476–3486 (2010) 2010 Wiley Periodicals, Inc.
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biocompatibility, and permeability.51–56 However, PCL suffers
from the difficulty of controlled degradation based on its
high crystallinity and hydrophobicity.57–59 PEO possesses
good biocompatibility, hydrophilicity, and low protein
absorption.60–65 CS copolymers with mixed side chains of
PCL and PEO deserve to be investigated because they are
expected to display unique physical and chemical properties.
CS reacted with phthalic anhydride to improve the solubility
in solvent and then carried out esterification with 2-bromoi-
sobutyryl bromide to obtain bromine-containing side groups,
and further reacted with sodium azide to form CS with azide
groups as pending functional groups. Alkynyl PCL was pre-
pared by two steps: monohydroxyl terminated PCL (PCL-OH)
was synthesized by ROP of e-caprolactone (CL) with benzyl
alcohol used as initiator and the alkynyl PCL was obtained
by the DCC reaction of PCL-OH with propargyl 3-carboxylic-
propanoate. Alkynyl PEO was synthesized by the DCC reac-
tion of methoxy poly(ethylene oxide) (mPEO-OH) with pro-
pargyl 3-carboxylic-propanoate. Amphiphilic CS copolymers
with mixed side chains of PCL and PEO can be easily pre-
pared by click chemistry of the azide groups on CS chain
with alkynyl group at PCL and PEO chain end. The ratio of
PCL and PEO was varied to alter the hydrophilicity, crystal-
line properties and self-assembly behavior of CS copolymers.
The crystalline properties of these CS copolymers were
investigated by differential scanning calorimetric analysis
(DSC) and wide-angle X-ray diffraction (WAXD). The crystal-
line morphologies of CS copolymers were observed by polar-
ized optical microscopy (POM). The hydrophilicity of CS
copolymers was evaluated with static water contact angles.
The morphology of CS copolymer micelles was observed
with transmission electron micrographs (TEM). The size of
the micelles was investigated by dynamic laser light scatter-
ing (DLS) technique.
The chemical shifts were relative to tetramethylsilane. The
molecular weight and molecular weight distribution were
measured on a Viscotek TDA 302 gel permeation chromatog-
raphy equipped with two columns (GMHHR-H, M Mixed
Bed). THF was used as eluent at a flow rate of 1 mL/min at
30 ꢀC. DSC was carried on a DSC TA-60WS thermal analysis
system (Shimadzu, Japan). Samples were first heated from
ꢁ20 ꢀC to 100 ꢀC at a heating rate of 10 ꢀC/min under
nitrogen atmosphere, followed by cooling to ꢁ20 ꢀC after
stopping at 100 ꢀC for 3 min, and finally heating to 100 ꢀC
at 10 ꢀC/min. WAXD patterns of powder samples were
obtained at room temperature on a Cu Ka radiation source
using a Bruker AXS D8 Advanced X-ray diffractionmeter
(Bruker, Germany). The supplied voltage and current were
set on 40 kV and 120 mA. Samples were exposed at a scan-
ning rate of 2y ¼ 4ꢀ/min between 2y values from 1.5 to 60ꢀ.
DLS was performed on a Malven Autosizer 4700 DLS spec-
trometer at a scattering angle 90ꢀ. The morphology of copol-
ymer micelles was observed with a JEOL JEM-2010 TEM at
an accelerating voltage of 120 kV. A small drop from the
aqueous copolymers solution was deposited onto carbon-
coated copper TEM grid. The excess of copolymer solution
was wiped off with a filter paper, and the grid was dried
under ambient atmosphere for 1 h. The crystalline morphol-
ogy of CS copolymers was observed using a XS-402 polarized
optical microscope (Shanghai Microimage Technology, China).
The samples were prepared by spin-coating of copolymer so-
lution in DMF (15 mg/mL) on a glass plate at room tempera-
ture and then placed at room temperature for 24 h for com-
plete evaporation of the solvent. The measurements of static
contact angles were carried out by commercial instruments
(OCA 20, DataPhysics Instruments GmbH, Filderstadt). A dis-
tilled water droplet of 2 lL was used as the indicator.
Synthesis of Propargyl 3-Carboxylic-Propanoate
The synthetic procedure is according to the literature.66
Briefly, succinic anhydride (10 g, 0.1 mol), propargyl alcohol
(11.2 g, 0.2 mol), and THF (20 mL) were added into a 100-
mL round-bottom flask equipped with a magnetic stirrer.
Then, the mixture was refluxed for 24 h. After removing of
THF under reduced pressure, dichloromethane (60 mL) was
added and the solution was washed with aqueous NaCl solu-
tion. The organic layer was collected and dried over anhy-
drous MgSO4. The resulting product was obtained after
removing of the solvent under reduced pressure (12.6 g,
yield: 81%).
EXPERIMENTAL
Materials
CS (viscocity ¼ 200 CPs, degree of deacetylation ¼ 80%) was
ꢀ
purchased from Aldrich and dried in vacuo at 60 C for 12 h
before use. Phthalic anhydride (PA) was used as received.
CuBr was purified by stirring in acetic acid and washing with
ethanol and then dried in vacuum. N,N,N0,N,‘‘N’’-pentamethyl-
diethylenetriamine (PMDETA) (Acros Organic) was stirred
overnight with CaH2 and distilled under reduced pressure.
2-Bromoisobutyryl bromide (Aldrich) was distilled under
reduced pressure. CL (Acros Organic) was purified with CaH2
by vacuum distillation. Tin 2-ethylhexanoate (Sn(Oct)2;
Aldrich) was distilled under reduced pressure before use.
Methoxy poly(ethylene oxide) with Mn ¼ 5K denoted as
mPEO-OH (Fluka) was dried by azeotropic distillation in the
presence of toluene. Dicyclohexylcarbodiimide (DCC; GL Bio-
chem, Shanghai), 4-dimethylaminopyridine (DMAP; Fluka), so-
dium azide (NaN3, Alfa Aesar), succinic anhydride (Fluka), and
propargyl alcohol (Alfa Aesar) were used as received.
1H-NMR (CDCl3, d, ppm): 9.40 (COOH), 4.80 (OCH2C:CH),
2.76 (OOCCH2CH2COOH), 2.60 (OOCCH2CH2COOH), 2.50
(OCH2C:CH).
Synthesis of Alkynyl PCL
Alkynyl PCL was synthesized by two steps. Firstly, PCL-OH
was synthesized by ROP of CL with benzyl alcohol as initia-
tor. Briefly, CL (10 g, 87.6 mmol), benzyl alcohol (177.3 mg,
1.64 mmol), and a catalytic amount of Sn(Oct)2 were added
to a fire-dried polymerization tube quickly. The tube was
then connected to a Schlenk line, where exhausting-refilling
processes were repeated for three times. The tube was put
Characterization
1H-NMR spectra were obtained from a Bruker DMX 500
NMR spectrometer with CDCl3 and DMSO-d6 as the solvents.
AMPHIPHILIC CS COPOLYMERS WITH MIXED SIDE CHAINS, YUAN ET AL.
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ꢀ
into an oil bath at 115 C under argon atmosphere with stir-
water. The powder of CS-Br was obtained by filtration and
dried in vacuum (5 g, yield: 91%).
ring and cooled to room temperature after polymerization
for 24 h. The resulting product was dissolved in chloroform
and precipitated two times in methanol. The purified poly-
mer was dried in vacuum at room temperature (9.5 g, yield:
93%). Secondly, PCL-OH (8 g, 1.33 mmol), propargyl 3-car-
boxylic-propanoate (1.04 g, 6.67 mmol), DCC (619 mg,
3 mmol) and DMAP (184 mg, 1.5 mmol) were dissolved in
40 mL of anhydrous dichloromethane, and the reaction was
performed at room temperature for 48 h under argon atmos-
phere. The reaction byproduct dicyclohexylcarbodiurea was
removed by filtration, and the solution was washed with
aqueous NaHCO3 solution and deionized water. The organic
layer was dried over anhydrous MgSO4 overnight. The puri-
fied product was obtained after removing of solvent and pre-
cipitating in methanol (6.5 g, yield: 80%).
1H-NMR (DMSO-d6, d, ppm): 7.39-7.91 (aromatic protons),
2.56-5.62 (protons of CS chain), 1.86 (C(CH3)2Br).
Synthesis of CS with Azide Groups (CS-N3)
CS-Br (1.5 g, containing 0.94 mmol of C-Br), NaN3 (0.61 g,
9.4 mmol), and DMF (50 mL) were added into a 100-mL
round-bottom flask equipped with a magnetic stirrer, and
the reaction was carried out at 45 ꢀC for 24 h. After filtra-
tion of excess NaN3, the solution was dialyzed against deion-
ized water. After freeze drying, the resulting product was
obtained (1.23 g, yield: 84%).
1H-NMR (DMSO-d6, d, ppm): 7.47-7.93 (aromatic protons),
2.56-5.62 (protons of CS chain), 1.18 (C(CH3)2N3).
Synthesis of Amphiphilic CS Copolymers with
Mixed Side Chains (CS-g-PCL/PEO)
1
PCL-OH: Mn,GPC ¼ 5800, Mw/Mn ¼ 1.21, Mn,NMR ¼ 6000, H-
NMR (CDCl3, d, ppm): 7.35 (aromatic protons), 5.12
(CH2OOC), 4.05 (OOCCH2CH2CH2CH2CH2O), 3.63 (OOCCH2
CH2CH2CH2CH2OH), 2.30 (OOCCH2CH2CH2CH2CH2O), 1.66
(OOCCH2CH2CH2CH2CH2O), 1.39 (OOCCH2CH2CH2CH2CH2O).
CS copolymers with mixed side chains were prepared via
click chemistry. A typical procedure was as follows. Alkynyl
PCL (0.6 g, 0.1 mmol), alkynyl PEO (0.5 g, 0.1 mmol) and
CS-N3 (142 mg, containing 0.1 mmol of N3) were dissolved
in DMF (50 mL). Then CuBr (29 mg, 0.2 mmol) and PMDETA
(35 mg, 0.2 mmol) were added into the above solution. After
degassed via three freeze-evacuate-thaw cycles, the reaction
was carried out at 85 ꢀC for 36 h. The resulting copolymer
was obtained by dialysis with chloroform to remove
unreacted alkynyl PCL and alkynyl PEO. The resulting prod-
uct was obtained by precipitation from cold ethyl ether.
Alkynyl PCL: 1H-NMR (CDCl3, d, ppm): 7.35 (aromatic pro-
tons), 5.12 (CH2OOC), 4.70 (COOCH2C:CH), 4.06 (OOC
CH2CH2CH2CH2CH2O), 2.65 (OOCCH2CH2COO), 2.50 (COO
CH2C:CH), 2.30 (OOCCH2CH2CH2CH2CH2O), 1.66 (OOCCH2
CH2CH2CH2CH2O), 1.39 (OOCCH2CH2CH2CH2CH2O).
Synthesis of Alkynyl PEO
mPEO-OH (10 g, 2 mmol), propargyl 3-carboxylic-propanoate
(1.56 g, 10 mmol), DCC (1.24 g, 6 mmol), and DMAP (366
mg, 3 mmol) were dissolved in 50 mL of anhydrous chloro-
form, and the reaction was performed at room temperature
for 40 h under argon atmosphere. The reaction byproduct
dicyclohexylcarbodiurea was removed by filtration, and the
solution was washed with aqueous NaHCO3 solution and
deionized water. The organic layer was dried over anhydrous
MgSO4 overnight. The purified product was obtained after
removing of solvent and precipitating in ethyl ether (7.8 g,
yield: 76%).
1H-NMR (DMSO-d6, d, ppm): 7.95 (proton in 1,2,3-triazole
group), 7.72-7.87 (aromatic protons), 7.36 (aromatic
protons), 5.08 (COOCH2C(N) ¼ C(N)), 3.99 (OOCCH2CH2
CH2CH2CH2O), 3.50 (OCH2CH2O), 3.24 (CH3O), 2.26 (OOCC
H2CH2 CH2CH2CH2O), 1.54 (OOCCH2CH2CH2CH2CH2O), 1.28
(OOCCH2CH2CH2CH2CH2O), 0.85 (C(CH3)2).
Preparation of Self-Assembled Micelles of CS Copolymers
Samples for DLS and TEM were prepared as follows: CS
copolymers (50 mg) was dissolved in DMF (10 mL) and sub-
sequently dialyzed against deionized water for 48 h (dialysis
membrane, MW cut: 14, 000 Da). During the dialysis process,
CS copolymers self-assembled into micelles with CS and PCL
core and the PEO corona. After diluted by deionized water
and equilibrated at room temperature for 48 h, the resulting
micelles solution had a concentration of 0.5 mg/mL for DLS
and TEM measurements.
1H-NMR (CDCl3, d, ppm): 4.70 (COOCH2C:CH), 4.24
(CH2OOC), 3.65 (OCH2CH2O), 3.38 (CH3O), 2.69 (OOCCH2CH2-
COO), 2.50 (COOCH2C:CH).
Synthesis of CS with Bromine-Containing Side
Groups (CS-Br)
CS was heated with excess phthalic anhydride (PA) (PA/GlcN
units ¼ 3/1 mol/mol) in anhydrous DMF to give phthaloyl
CS according to previously reported procedure.15 Then, the
dried phthaloyl CS (5 g, 17.6 mmol of GlcN units) was dis-
solved in anhydrous DMF (100 mL) under stirring. To this
solution was added triethylamine (2.19 g, 22 mmol) under
nitrogen at room temperature. The mixture was stirred and
cooled to 0 ꢀC with ice bath. 2-Bromoisobutyryl bromide
(5 g, 22 mmol) in anhydrous DMF (20 mL) was added drop-
wise to the mixture within 40 min. The reaction mixture was
stirred for 12 h at room temperature before it was washed
with saturated NaHCO3 aqueous solution and deionized
RESULTS AND DISCUSSION
Synthesis of CS-g-PCL/PEO Polymers
with Mixed Side Chains
The synthetic route employed for the preparation of CS-g-
PCL/PEO polymers with mixed side chains was shown in
Scheme 1. Amphiphilic CS copolymers with mixed side
chains of PCL and PEO prepared by click chemistry of the az-
ide groups on CS chain with alkynyl group at PCL and PEO
chain end. To adjust the hydrophilicity of CS copolymers, the
feed ratio of PCL and PEO was varied (mol %/mol %: 100/
0, 75/25, 50/50, 25/75, and 0/100). To investigate the graft
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SCHEME 1 Synthesis of CS copolymers with mixed side chains (CS-g-PCL/PEO) by click chemistry.
ratio, CS-g-PCL/PEO samples were weighted before and after
the graft polymerization with PCL and PEO. The graft ratio
(G, wt %) was calculated according to eq 1.
trum of alkynyl PEO was shown in Figure 1(b). The signal at
3.65 ppm (b) should be ascribed to the methylene protons
of PEO main chain, whereas signals at 2.69 ppm (c), 4.70
ppm (d), and 2.50 ppm (e) were attributed to methylene
protons next to carbonyl, methylene protons and alkynyl
protons of the terminal propargyl group, respectively. By
comparing integral ratios of peaks d and e to that of peak b,
the degree of end group functionalization was calculated to
be 100%; namely, all the hydroxyl groups have been trans-
formed to alkynyl groups.
W2 ꢁ W1
G ¼
ꢂ 100
(1)
W1
where W1(g) is the weight of CS-N3 and W2 (g) is the weight
of the CS-g-PCL/PEO samples. Table 1 summarizes the
results of CS-g-PCL/PEO copolymers prepared by click
chemistry.
CS can be easily converted to phthaloyl CS by the phthaloyla-
tion of amino groups of CS. Introduction of phthaloyl groups
can reduce the inter- or intramolecular hydrogen bonds,
which results in the solubility of CS in organic solvents such
as DMSO and DMF. Thus, this will facilitate the hydroxyl
groups of CS to react with 2-bromoisobutyryl bromide under
homogeneous conditions. CS-Br was obtained by the reaction
Alkynyl PCL was prepared by two steps. PCL-OH was synthe-
sized by ROP of CL with benzyl alcohol used as initiator. The
molecular weight of PCL-OH can be determined by the molar
ratio of CL and benzyl alcohol (Mn,NMR ¼ 6000). Alkynyl PCL
was obtained by the DCC reaction of PCL-OH with propargyl
3-carboxylic-propanoate. To ensure complete end group
transformation, an excess amount of propargyl 3-carboxylic-
propanoate was used. 1H-NMR spectrum of alkynyl PCL was
shown in Figure 1(a). The major resonance signals (a-f) are
attributed to PCL initiated by benzyl alcohol. The signals at
2.65 ppm (g), 4.70 ppm (h), and 2.50 ppm (i) can be
ascribed to methylene protons next to carbonyl, methylene
protons and alkynyl protons of the terminal propargyl group,
respectively. The signal at 3.63 ppm (the terminal methylene
protons of PCL-OH) disappeared completely. This observation
confirms the complete transformation of hydroxyl to alkynyl
groups.
TABLE 1 Synthesis of CS-g-PCL/PEO Copolymers by Click
Chemistry
PCL/PEO (mol %/mol %)
Sample
Feed Ratio Actual Ratioa Graft Ratio (%)b
CS-g-PCL
100/0
100/0
67/33
49/51
18/82
0/100
537
338
268
306
493
CS-g-PCL/PEO(1) 75/25
CS-g-PCL/PEO(2) 50/50
CS-g-PCL/PEO(3) 25/75
Alkynyl PEO was synthesized by the DCC reaction of mPEO-
OH and propargyl 3-carboxylic-propanoate. An excess
amount of propargyl 3-carboxylic-propanoate was used to
ensure complete end group transformation. 1H-NMR spec-
CS-g-PEO
0/100
a
The actual ratio was determined by 1H NMR spectroscopy.
Graft ratio was calculated according to eq 1.
b
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FIGURE 1 1H-NMR spectra of (a) alkynyl PCL, (b) alkynyl PEO, (c) CS-N3, and (d) CS-g-PCL/PEO(2).
of phthaloyl CS with 2-bromoisobutyryl bromide under ho-
mogeneous conditions. CS-N3 was obtained by the reaction
of CS-Br with NaN3. The nucleophilic substitution reaction of
terminal Br with excess NaN3 in DMF was very rapid. The
1
chemical structure of CS-N3 was confirmed by H-NMR spec-
trum [Fig. 1(c)]. The signal at 1.86 ppm corresponding to
the methyl protons next to the terminal bromine disap-
peared completely, and the signal at 1.18 ppm (b) appeared,
which confirmed the introduction of azide group.
The synthesis of CS copolymers with mixed side chains, CS-
g-PCL/PEO, were accomplished by the click reaction between
1
alkynyl PCL, alkynyl PEO and CS-N3. A typical H-NMR spec-
trum of CS-g-PCL/PEO is shown in Figure 1(d). The charac-
teristic signals of CS, PCL and PEO were observed clearly. In
comparison with that of alkynyl PCL and alkynyl PEO, the
signals at 4.70 ppm and 2.50 ppm corresponding to the
methylene protons next to alkynyl group and methine proton
in alkynyl group disappear completely, and the new peak at
5.08 ppm that belongs to metnylene protons next to 1,2,3-
triazole group can be seen clearly. A new peak at 7.95 ppm
FIGURE 2 DSC curves of (a) alkynyl PCL, (b) CS-g-PCL, (c) CS-
g-PCL/PEO(1), (d) CS-g-PCL/PEO(2), (e) CS-g-PCL/PEO(3), (f) CS-
g-PEO, and (g) alkynyl PEO in the cooling run and the second
heating run, respectively.
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TABLE 2 Melting and Crystallization Behavior of CS-g-PCL/PEO Copolymers
TC (ꢀC)a
Tm (ꢀC)b
DHm (J/g)c
Xc (%)d
Sample
TC,PCL
TC,PEO
Tm,PCL
Tm,PEO
DHm,PCL
DHm,PEO
Xc,PCL
Xc,PEO
Alkynyl PCL
CS-g-PCL
34.3
29.6
25.8
15.5
53.3
53.1
51.9
71.5
52.8
44.3
52.5
38.8
32.5
CS-g-PCL/PEO(1)
CS-g-PCL/PEO(2)
CS-g-PCL/PEO(3)
CS-g-PEO
26.4
19.3
38.4
39.1
52.5
66.5
53.1
57.1
58.2
74.1
122.9
172.5
34.7
57.5
80.7
Alkynyl PEO
a
TC,PCL and TC,PEO denote the crystallization temperature of PCL and PEO segments in the cooling run, respectively.
Tm,PCL and Tm,PEO are the melting points of PCL and PEO segments in the second heating run, respectively.
DHm,PCL and DHm,PEO denote the fusion enthalpies of PCL and PEO segments in the second heating run, respectively.
Xc,PCL ¼ DHm,PCL/DH0m;PCL and Xc,PEO ¼ DHm,PEO/DHm0 ;PEO, where DHm0 ;PCL is 136.1 J/g and DH0m;PEO is 213.7 J/g.
b
c
d
can be ascribed to methine proton in 1, 2, 3-triazole group.
contrary, in CS-g-PCL/PEO(3), the crystallizability of PCL was
completely hindered by the constrained geometry of
the brush copolymer and the PEO crystallinity. Meanwhile,
the crystallizability of the PEO segment was also affected by
the macromolecular structure and the PCL segment, and its
Tc, Tm, and Xc are lower than those of linear alkynyl PEO. As
shown in the DSC curve of CS-g-PCL/PEO(2) in Figure 2, in
which the contents of PCL and PEO segments are almost the
same, the PCL and PEO segments possessed crystallizability.
However, the crystallizability of PCL and PEO segments were
hindered by the brush structure of the copolymer and the
mutual influence between the PCL and PEO segments. In
addition, the chain conformation of brush copolymer was dif-
ferent from that of linear polymer. The flexibility of the
chains of brush copolymer was poorer than that of the
chains of linear polymer. Therefore, the chain conformation
of brush copolymer results in less intermolecular interaction,
which led to decreasing of the enthalpy of fusion. Therefore,
All these indicate the occurrence of click cyclization. The
actual ration of PCL and PEO in CS copolymers can be calcu-
lated by the integral ratio of methylene protons (e) in PCL
1
and methylene protons (j) in PEO according to H-NMR spec-
tra (Table 1).
DSC and WAXD Analyses
The melting and crystallization behaviors of CS-g-PCL/PEO
copolymers were investigated by DSC as shown in Figure 2
and Table 2. The crystallization temperature (Tc) was
obtained from the cooling run, and the melting temperature
(Tm) and the degree of crystallinity (Xc) were obtained from
the second heating run. The results listed in Table 2 show
that Tc, Tm, and Xc of CS-g-PCL and CS-g-PEO are lower than
those of linear PCL and linear PEO, respectively, which
should be attributed to the brush structure of copolymer
and CS main chain which make contributions to the crystal-
line imperfection of PCL or PEO side chains. For CS copoly-
mers with mixed side chains, CS-g-PCL/PEO, the melting and
crystallization behaviors are relatively complicated. Only Tc
of PCL segments can be obviously observed in CS-g-PCL/
PEO(1) and the value of Tc,PCL was lower than that of PCL
segments in CS-g-PCL. However, there two Tc’s (Tc,PCL and
Tc,PEO) in CS-g-PCL/PEO(2), and only a single crystallization
peak assigned to PEO segments can be observed in CS-g-
PCL/PEO(3). In the second heating run of CS-g-PCL/PEO(1),
Tm of PCL can be detected and was lower than that PCL in
CS-g-PCL. From the DSC curve of CS-g-PCL/PEO(2), the indi-
vidual fusion enthalpies of PCL and PEO segments can not
be calculated separately due to the overlapping of the melt-
ing endotherms of PCL and PEO segments. As for CS-g-PCL/
PEO(3), only a single melting peak assigned to PEO segment
was observed. In CS-g-PCL/PEO(1), the content of PEO is
much less than that of PCL, and the crystallizability of PEO
was completely hindered by the constrained geometry of the
brush copolymer and the PCL crystallinity. Meanwhile, the
crystallizability of the PCL segment was also affected by the
macromolecular structure and the PEO segment, and its Tc,
Tm, and Xc are lower than those of linear alkynyl PCL. On the
FIGURE 3 WAXD patterns of (a) CS-g-PCL, (b) CS-g-PCL/
PEO(1), (c) CS-g-PCL/PEO(2), (d) CS-g-PCL/PEO(3), and (e) CS-g-
PEO. All measurements were carried out at room temperature.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE
4 Polarized
optical
micrographs of (a) alkynyl PCL,
(b) CS-g-PCL, (c) CS-g-PCL/
PEO(1), (d) CS-g-PCL/PEO(2), (e)
CS-g-PCL/PEO(3), (f) CS-g-PEO,
and (g) alkynyl PEO.
the crystallizability of PCL and PEO segments were
restrained.
ing to the (110) and (200) planes of the orthorhombic crys-
tal form. CS-g-PEO shows two intensive peaks at 19.1 and
23.3ꢀ. In CS-g-PCL/PEO(1), a weak diffraction peak at 19.1ꢀ
can be observed in addition to the obvious diffraction peaks
of PCL crystal, which demonstrated that PEO can show crys-
tallizability to some extent. As for CS-g-PCL/PEO(2), the
WAXD pattern shows reflections at 19.1, 21.6, and 23.3ꢀ, typ-
ical of PCL and PEO crystalline phases.61,67 The reflections of
WAXD is another useful method to investigate the crystal
structure of CS-g-PCL/PEO copolymers. Figure 3 shows
WAXD patterns of CS-g-PCL, CS-g-PCL/PEO(1), CS-g-PCL/
PEO(2), CS-g-PCL/PEO(3) and CS-g-PEO. All of the samples
were measured at room temperature without annealing. CS-
g-PCL shows intensive peaks at 21.6 and 23.9ꢀ, correspond-
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ARTICLE
FIGURE 5 Photograph of the
water contact angles of (a)
alkynyl PCL, (b) CS-g-PCL, (c) CS-
g-PCL/PEO(1),
(d)
CS-g-PCL/
PEO(2), (e) CS-g-PCL/PEO(3), (f)
CS-g-PEO, and (g) alkynyl PEO.
CS-g-PCL/PEO(3) WAXD pattern are similar to those of the
CS-g-PCL/PEO(2) WAXD pattern except the intensity of dif-
fraction peaks of PEO crystal is stronger than that of PCL
crystal. The WAXD patterns of CS-g-PCL/PEO(2) and CS-g-
PCL/PEO(3) are almost the summation of those for the PEO
and PCL homopolymers, suggesting that the PEO and PCL
segments formed separate crystal microdomains before
annealing.
The size of spherulities of CS-g-PEO was much smaller than
that of linear alkynyl PEO. The crystalline morphologies of
CS-g-PCL/PEO(1), CS-g-PCL/PEO(2) and CS-g-PCL/PEO(3)
were similar, but these crystalline morphologies are different
from those of alkynyl PCL and alkynyl PEO. Imperfect spher-
ulities can be observed. Because of the brush structure of
the copolymer and the mutual influence between the PEO
and PCL segments, the mobility of the chains of PCL or PEO
segments decreased and the nucleation ability increased.
Therefore, the formation of large and perfect spherulities
was impossible.
Crystalline Morphologies of Polymers
The crystalline morphologies of alkynyl PCL, CS-g-PCL, CS-g-
PCL/PEO(1), CS-g-PCL/PEO(2), CS-g-PCL/PEO(3), CS-g-PEO
and alkynyl PEO were investigated by polarized optical
micrographs. According to Figure 4, linear alkynyl PCL shows
a typical spherulitic morphology. As for CS-g-PCL, small crys-
tals were observed, which can be attributed to the brush
structure of copolymer and the presence of CS main chain in
the copolymer interfering with the natural growth of the
crystals. Linear alkynyl PEO showed a typical large spheru-
litic morphology, and Maltese cross patterns were observed.
Hydrophilicity Investigation of Polymers
The hydrophilicity of the films prepared from alkynyl PCL,
CS-g-PCL, CS-g-PCL/PEO(1), CS-g-PCL/PEO(2), CS-g-PCL/
PEO(3), CS-g-PEO and alkynyl PEO was measured with water
contact angles. As shown in Figure 5, the contact angle of
the linear alkynyl PCL is similar to that of CS-g-PCL, but
with the increase of content of PEO in CS-g-PCL/PEO, the
contact angles decreased from 69.5ꢀ to 44.8ꢀ. This should be
AMPHIPHILIC CS COPOLYMERS WITH MIXED SIDE CHAINS, YUAN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 6 Micelles size distribution of (a) CS-g-PEO, (b) CS-g-PCL/PEO(3), (c) CS-g-PCL/PEO(2), (d) CS-g-PCL/PEO(1). (c ¼ 0.5 mg/mL)
attributed to the presence of PEO segments with good
hydrophilicity. Therefore, the hydrophilicity of copolymers
can be improved and adjusted by the alternation of the com-
position of the PEO and PCL segments in copolymers.
increase of the content of PCL in CS copolymer, which is con-
sistent with the results of DLS. As a result, it can be seen
that the size of micelles and the aggregation behavior can be
adjusted by the alteration of ratio of PCL and PEO segments.
Micellization Properties of CS Copolymers
The obtained CS copolymers, including CS-g-PEO, CS-g-PCL/
PEO(1), CS-g-PCL/PEO(2), and CS-g-PCL/PEO(3) contain
hydrophobic CS or CS/PCL segments, and a hydrophilic PEO
segment. Using a cosolvent approach, stable micelles/aggre-
gates were obtained in aqueous solution. Based on chemical
intuition, the formed micelles should possess CS or CS/PCL
cores and PEO coronas. The self-assembly micelles were
characterized by DLS and TEM. Figure 6 shows the DLS anal-
ysis of these micelles. The average diameter of the micelles
was in the range of 37.3–81.8 nm for all the samples. This
indicated that the average diameter of the micelles appa-
rently increased with the increase of the content of PCL seg-
ment. Obviously, with the increase of the content of PCL, the
hydrophobicity of CS copolymers, and the micelles tend to
aggregate into larger aggregates. The morphology of the
micelles was further observed by TEM, as shown in Figure 7.
The micelles of all the samples present spherical morphology
and tend to cluster into aggregates with larger size with the
CONCLUSIONS
CS-g-PCL/PEO amphiphilic copolymers with mixed side
chains (PCL and PEO) were successfully prepared via graft
to method by click chemistry of CS-N3, alkynyl PCL and
alkynyl PEO. The graft efficiency of PCL and PEO is compara-
tively high and the graft ratio of PCL and PEO can be accu-
rately adjusted via alteration of the feed ratio of alkynyl PCL
and alkynyl PEO due to the unique properties of click chem-
istry. The melting and crystallization behaviors and crystal-
line morphology of CS-g-PCL/PEO copolymers investigated
with DSC, WAXD and POM can be adjusted by the alteration
of the feed ratio of PCL and PEO segments. CS-g-PCL/PEO
copolymers revealed crystalline morphology different from
that of linear alkynyl PCL and alkynyl PEO. The hydrophilic-
ity of CS copolymers can be improved and adjusted by the
alteration of the composition of PCL and PEO segments. CS
copolymers can self-assemble into micelles in aqueous
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ARTICLE
FIGURE 7 TEM photographs of (a)
CS-g-PEO, (b) CS-g-PCL/PEO(3), (c)
CS-g-PCL/PEO(2), (d) CS-g-PCL/
PEO(1). (c ¼ 0.5 mg/mL)
10 Sun, S. J.; Liu, W. G.; Cheng, N.; Zhang, B. Q.; Cao, Z. Q.;
Yao, K. D.; Liang, C.; Zuo, A. J; Guo, G; Zhang, J. Y. Bioconju-
gate Chem 2005, 16, 972–980.
solution and the micelles were characterized by DLS and
TEM. The size of micelles increased with the increase of the
content of PCL segments in copolymers. The potential appli-
cations of CS-g-PCL/PEO amphiphilic copolymers with mixed
side chains in biomedical field will be investigate further.
11 Skotak, M.; Leonov, A. P.; Larsen, G.; Noriega, S.; Subrama-
nian, A. Biomacromolecules 2008, 9, 1902–1908.
12 Zhang, J. P.; Wang, L.; Wang, A. Q. Ind Eng Chem Res
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (no. 20804029).
2007, 46, 2497–2502.
13 Li, J.; Yun, H.; Gong, Y. D.; Zhao, N. M.; Zhang, X. F. Bioma-
cromolecules 2006, 7, 1112–1123.
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