Controlled synthesis of amphiphilic rod-coil biodegradable maltoheptaose-graft-poly(ε-caprolactone) copolymers

Article abstract of DOI:10.1016/j.carbpol.2010.10.034

The controlled synthesis of novel amphiphilic biodegradable maltoheptaose-graft-poly(ε-caprolactone) copolymers was achieved through a three-step method. The first step consisted of partial silylation of the maltoheptaose hydroxyl groups. This protection step was followed by ring-opening polymerization of ε-caprolactone initiated from the remaining OH functional group of the partially silylated oligosaccharide. The third step involved the deprotection of the silylether group under mild conditions. The effects of varying the experimental conditions on grafting efficiency and graft length were investigated to ensure controlled polymerization of ε-caprolactone. The protection and deprotection of the TMS group during the entire procedure were carefully monitored with Fourier transform infrared (FTIR) and ¹H NMR. The final graft copolymers were characterized by FTIR, ¹H NMR, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC).

Full text of DOI:10.1016/j.carbpol.2010.10.034

Carbohydrate Polymers 83 (2011) 1723–1729
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Controlled synthesis of amphiphilic rod-coil biodegradable
maltoheptaose-graft-poly(␧-caprolactone) copolymers
Qiu Xiongying, Wang Caiqi∗, Shen Juan, Jiang Mingwei
College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 March 2010
Received in revised form 2 October 2010
Accepted 19 October 2010
Available online 23 October 2010
The controlled synthesis of novel amphiphilic biodegradable maltoheptaose-graft-poly(␧-caprolactone)
copolymers was achieved through a three-step method. The first step consisted of partial silylation of the
maltoheptaose hydroxyl groups. This protection step was followed by ring-opening polymerization of ␧-
caprolactone initiated from the remaining OH functional group of the partially silylated oligosaccharide.
The third step involved the deprotection of the silylether group under mild conditions. The effects of
varying the experimental conditions on grafting efficiency and graft length were investigated to ensure
controlled polymerization of ␧-caprolactone. The protection and deprotection of the TMS group during
the entire procedure were carefully monitored with Fourier transform infrared (FTIR) and 1H NMR. The
final graft copolymers were characterized by FTIR, 1H NMR, gel permeation chromatography (GPC), and
differential scanning calorimetry (DSC).
Keywords:
Graft copolymer
Maltoheptaose
Biodegradable
Amphiphilic
Rod-coil
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
(TMS) groups have been successfully used in the preparation
of polysaccharide-based graft copolymers with controlled struc-
Interest in the study of amphiphilic biodegradable copolymers
has increased due to their controlled biodegradation rates and
potential applications in drug delivery and separation technolo-
gies (Nie, Zhao, Xie, & Wu, 2003; Ouchi, Kontani, & Ohya, 2003;
Rosler, Vandermeulen, & Klok, 2001). At present, amphiphilic lin-
ear copolymers have been extensively studied (Caillol et al., 2003;
Chang, Bender, Phelps, & Allcock, 2002; Luo et al., 2004; Nie,
Zhao, Xie, & Wu, 2003; Xiong, Tam, & Gan, 2003). In contrast,
relatively few works have been done on amphiphllic non-linear
copolymers such as graft copolymers (Jeong, Kang, Yang, & Kim,
2002; Li, Zhu, Sunintaboon, & Harris, 2002) despite the numerous
advantages of providing integrations of considerable functionality
onto the polymer backbone (Breitenkamp, & Emrick, 2003; Sato
et al., 2005).
Recently, saccharide-based graft copolymers have attracted
much attention not only because of their potential in separation
technologies and controlled drug release but also because they
serve as possible alternatives to existing non-biodegradable for-
mulated systems (Hua, Jianga, Dinga, Gea, Yuanb, Yanga, 2002; Liu,
Tian, & Hu, 2004; Nouvel, Frochot, Sadtler, Dubois, Dellacherie & Six,
2004a; Tatsuro, Tomohiro, & Yuichi, 2003). Especially, the protec-
tion and deprotection of partial hydroxyl groups via trimethylsilyl
tures in a homogeneous system (Nouvel et al., 2004a; Nouvel,
Dubois, Dellacherie, & Six, 2004b; Ohya, Maruhashi, & Ouchi, 1998;
Ouchi et al., 2003; Ydens, Rutot, Degee, Six, Dellacherie, & Dubois,
2000). For example, Ohya and Nouvel synthesized pollulan-g-
poly(lactic acid) and dextran-g-poly(lactic acid) via ring-opening
graft polymerization of lactide onto the corresponding trimethylsi-
lyl (TMS) protected polysaccharides followed by the removal of
TMS protection groups. In this regard, we have also synthesized
hydroxylpropyl cellulose-g-poly(␧-caprolactone) with controlled
structures (Wang, Dong, & Tan, 2003).
Most saccharide-based surfactants are based on polysaccha-
rides, so studies on the synthesis of saccharide-based graft
copolymers mostly focus on the traditional comb-shaped or brush-
shaped graft copolymers based on longer main chains as backbone
(Mn ≈ 10,000 ∼ several ten thousands) and shorter side chains as
graft segments (Nouvel et al., 2004a,b; Ydens et al., 2000; Ohya
et al., 1998; Ouchi et al., 2003; Liu et al., 2004; Yao et al., 2003).
Only a few studies mention the use of oligosaccharides as pre-
cursors of macromolecular amphiphilic architectures. Recently, we
reported the controlled synthesis of a new kind of amphiphilic graft
copolymer based on a chitooligosaccharide (COS, Mn = 1500–2000,
the degree of polymerization is about 11) as the hydrophilic short
rigid backbone and poly(␧-caprolactone) (PCL) as the hydropho-
bic long side chain (Wang, Li, & Guo, 2005). The structure of
this graft copolymer is different from the conventional comb-
shaped or brush-shaped graft copolymers, and these special graft
∗
Corresponding author. Tel.: +86 10 88256677; fax: +86 10 88256092.
E-mail address: wang-caiqi@gucas.ac.cn (C. Wang).
0144-8617/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbpol.2010.10.034

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X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
copolymers COS-g-PCL can form multiple self-assembled mor-
phologies.
totally dissolved, a predetermined amount of HMDS was added
under nitrogen flow using previously dried syringes. The reaction
medium was kept at the desired temperature for 3 h at a suitable
stirring rate. The mixture was precipitated in deionized cool water
after cooling slowly, and the product was washed thrice with cool
water to remove unreacted maltoheptaose. The crude product was
purified by repeated dissolution in acetone and precipitation in
water, filtrated, and then dried for 72 h at 50 ◦C under vacuum.
The trimethylsilyl substitution (DTMS) of maltoheptaose was deter-
mined using the following equation:
In this study, maltoheptaose (MH, with a degree of poly-
merization of 7) which has a shorter saccharide chain and a
narrower molecular distribution compared with chitooligosac-
charide is chosen as the hydrophilic short rigid backbone and
poly(␧-caprolactone) (PCL) as the hydrophobic long side chain.
A new amphiphilic poly(␧-caprolactone) (PCL)-grafted maltohep-
taose copolymer is synthesized by the protection and deprotection
of partial hydroxyl groups via trimethylsilyl (TMS) groups. Further
studies on the self-assembled morphologies of MH-g-PCL are still
ongoing.
7ISi(Me)
3
DTMS
=
× 100%
(1)
23 × 9IH
1
where ISi(Me) and IH are the integral areas of the signals for the
2. Experimental
1
3
methyl protons of the TMS groups at around ı = 0.1 ppm and that
for the methine protons (H-1) of the monosaccharide residue at
around ı = 5.0 ppm, respectively (Table 1).
2.1. Materials
IR (KBr, power, cm−1): 3450 (O–H), 2960 (C–H), 2896 (C–H),
␤-cyclodextrin (␤-CD) purchased from Beijing Solarbio Sci-
ence & Technology Co., Ltd. (PR China) was recrystallized twice
from water and vacuum-dried at 60 ◦C under vacuum before
use. ␧-caprolactone (Acros Organics, 99%) was dried over CaH2
for 48 h, distilled under reduced pressure with the fraction col-
lected at 96–98 ◦C (5 mmHg), and stored under inert atmosphere.
Stannous octoate (Sn(Oct)2) was purchased from Alfa-Aesar and
used without any further purification. p-Xylene and THF were
dried by refluxing over CaH2 and Na/benzophenone complex
1250 (Si–CH3), 1150–1000 (pyranose), 877, 843 (Si–CH3), 752. 1
H
NMR (600 MHz, D2O or CDCl3, ı, ppm): 5.3 (H-1 of pyranose), 5.1
(H-1ꢀ of pyranose), 3.0–4.0 (H-2, -3, -4, -5, and -6 of pyranose), 0.1
(–O–Si(CH3)3)
2.3.2. ROP of CL from silylated maltoheptaose
The ROP of CL was performed in a previously dried two-necked
round-bottom flask equipped with a stopcock and a rubber septum,
purged with nitrogen. TMS-maltoheptaose (1.0 g) was dissolved
in fresh, purified p-xylene (concentration ∼20%), and a desired
amount of the ␧-caprolactone monomer and a drop of Sn(Oct)2
were added under N2. The mixture in a capped vial under N2
was placed in a preheated oil bath at 110 ◦C and stirred for 24 h.
Finally, TMS-maltoheptaose-g-PCL was recovered by precipitation
in methanol, filtration, and drying under vacuum.
˚
and distilled just before use. DMSO was dried over 4 A molec-
ular sieves, distilled under reduced pressure, and stored under
nitrogen atmosphere. Hydrochloric acid (36–38%), 1,1,1,3,3,3- hex-
amethyldisilazane (HMDS), and other regents were purchased from
commercial sources and used as received without further purifica-
tion.
2.2. Maltoheptaose from ˇ-cyclodextrin
IR (KBr, power, cm−1): 3440 (O–H), 2950 (C–H), 2896 (C–H),
1730 (C O), 1251, 1150–1000 (pyranose), 877, 841, 753. 1H NMR
(600 MHz, CDCl3, ı, ppm): 4.2–4.9 (H-1 of pyranose), 3.0–4.0
(H-2, -3, -4, -5, and -6 of pyranose), 3.9 (–CH2–O–C (O)–), 2.5
(–O–C (O)–CH2–), 1.5 (–O–C(O)–C–CH2– and –CH2–C–O–C (O)–),
1.2 (–C(O)–C–C–CH2–C–C–O–), 0.1 (–O–Si(CH3)3).
A total of 100 g of ␤-CD was dissolved in 600 ml of 0.01 M
hydrochloric acid and kept under reflux for exactly 2 h (Volker,
Gerd, & Reimund, 1995). The solution was neutralized with a
calculated amount of 1 M NaOH (6 ml) and buffered to pH = 7.0
with Na2HPO4/NaH2 PO4. The color changed to a pale yellow. The
solution was stored at 4 ◦C for 12 h. A total of 85% of the educt
crystallized from the solution. The regenerated cyclodextrin was
filtered off and collected for the next run. The remaining solution
was warmed up to 60 ◦C, saturated with p-xylene (approximately
1 ml), and stirred for several hours. Crystallization of cyclodex-
trin was completed by placing it in the refrigerator overnight.
p-Xylene/cyclodextrin complex was removed from the solution by
filtration, and water was removed under reduced pressure to l/10
of the initial volume. After further saturation with p-xylene, final
traces of the cyclical educt were crystallized overnight at 4 ◦C and
removed by filtration. Evaporation of the water under reduced pres-
sure and drying in a vacuum resulted in crude maltoheptaose that
was contaminated only by traces of oligoglucans (Glcz6 up to Glcs2)
and sodium chloride/phosphate. The crop was dissolved in 40 ml of
water and added dropwise to 1500 ml of ethanol. Pure precipitated
maltoheptaose was removed by filtration and dried in a vacuum for
several days with a yield of 10% maltoheptaose.
2.3.3. Deprotection of TMS maltoheptaose-g-PCL
The protected graft copolymers were dissolved in THF (10 wt%
PCL-grafted silylated maltoheptaose) along with the addition of a
slight excess of HCl aqueous solution (0.1 M) with respect to the
number of “–O–Si(CH3)3” functions. The deprotected copolymers
were recovered by precipitation in cold water, filtration, and vac-
uum drying.
The average polymerization degree of ␧-caprolactone grafted on
every glucose unit of MH backbone (DP) was calculated from the
ratio of the integral areas of the methylene protons signal of PCL
at 2.2 ppm to the methine proton signal (H-1) of MH at 4.3 ppm
(Table 2).
2.4. Measurements
1H NMR analyses were carried out using a JOEL JNM-ECA600
spectrometer in CDCl3, in DMSO-d6, or in D2O at room tempera-
ture (TMS-free solvents). FT-IR measurements were carried out on
an AVATAR 360 FT-IR spectrometer (Thermo Nicolet). The samples
for FT-IR analysis were prepared by dispersing the powder in KBr
and compressing the mixtures to form disks. Molecular weight (Mn)
and molecular weight distribution (Mw/Mn) were measured with
a Viscotek TDA 302 GPC instrument with tetrahydrofuran (THF)
as the mobile phase and polystyrene as calibration standard. The
melting point (Tm) was measured by employing a differential scan-
ning calorimeter (DSC-60 SHIMADZA, Japan). The samples were
2.3. Preparation of maltoheptaose-g-PCL copolymers
2.3.1. Trimethylsilylation of maltoheptaose
Maltoheptaose was placed in a previously dried and nitrogen-
purged round-bottom two-necked flask with a stopcock and
connected to an oil valve for ammoniac evolution. After flush-
ing with high-purity N2 for 20 min, the desired amount of DMSO
(10 wt % dried maltoheptaose) was added. Once maltoheptaose was

X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
1725
Table 1
Characterization of ␤-CD, MH, and trimethylsilyed maltoheptaose.
Sample
DSa (%)
Solubility
DMSOb
Chloroform
Acetone
Petroleum ether
p-Xylene
Water
␤-CD
MH
0
0
+
+
+
+
−
−
−
−
−
−
−
+
+
+
−
−
−
+
+
+
−
−
−
−
+
+
+
+
−
−
−
+
+
+
−/+
+
+
TMSMH-1
TMSMH-2
TMSMH-3
TMSMH-4
TMSMH-5
TMSMH-6
4.78
9.83
48.77
58.81
68.24
77.31
+
−
−
−
−
+
+
+
+
+
+
+: Almost soluble; −: Insoluble.
a
Degree of substitution (DS) of maltoheptaose was measured as the average number of hydroxymethyl groups –Si(CH3)3 groups per all the –OH groups in the maltoheptaose,
calculated for the intensity of IH1 and I–Si(CH3)3 by 1H NMR. DS = 7ISi(Me /(23 ∗ 9IH1 ) × 100%.
)
3
3
b
DMSO, dimethyl sulfoxide.
first heated from 25 to 100 ◦C at a rate of 10 ◦C/min (first scan),
cooled to room temperature at a cooling rate of 10 ◦C/min, and then
heated again to 100 ◦C at a rate of 10 ◦C/min (second scan). Wide-
angle X-ray diffraction (WAXD) measurements were carried out on
an X-ray diffractometer (MSAL XD-2, China) operating at 40 kV and
200 mA. Nickel-filtered Cu K␣ radiation (␭ = 0.154 nm) was used.
(a)
(b)
β
-CD
2930
2930
(c)
MH
3410
3410
1030
(d)
(e)
TMSMH-2
2960
TMSMH-4
TMSMH-5
3. Results and discussion
3410
3450
3450
2960
2960
3.1. Maltoheptaose from ˇ-cyclodextrin
752
As shown in Scheme 1, the ␣-1,4 glucose bond of ␤-cyclodextrin
(␤-CD) is cleaved readily in an appropriate acidic solution, and the
maltoheptaose (MH) contaminated only by traces of oligoglucans
(Glcz6 up to Glcz2) can be achieved. Pure precipitated maltohep-
taose is obtained by reprecipitation with water as solvent and
ethanol as precipitant. ␤-cyclodextrin has poor solubility in water,
but maltoheptaose (MH) is soluble in water. With the opening of the
␤-cyclodextrin (␤-CD) ring, the cyclo-structure of ␤-cyclodextrin
(␤-CD) changed into a line chain, and two other –OHs can be
observed. Obvious differences in the IR spectrum between mal-
toheptaose (MH) and ␤-cyclodextrin (␤-CD) cannot be observed
in Fig. 1. However, with the ␤-cyclodextrin (␤-CD) ring opening,
the cis- and trans-formation of glucose can be observed, and a new
peak at 5.1 ppm can be found in the 1H NMR spectrum (Fig. 2). Dif-
ferences are also shown in the X-ray diffraction profiles (Fig. 3).
␤-cyclodextrin (␤-CD) formed crystals, and the narrow peak at
2ꢀ = 9.14, 12.89, 14.92, 18.97 was readily observed. However, mal-
toheptaose (MH) precipitated from ethanol is non-crystalline, and
only the broad diffraction peck at 2ꢀ = 17.80 can be found.
1250
843
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm-1)
Fig. 1. FTIR spectra of ␤-CD, MH, and TMSMH with different DTMS. (a) ␤-CD, (b) MH,
(c) TMSMH-2, (d) TMSMH-4, and (e) TMSMH-5.
the –OH group substituted by trimethylsilyl groups (defined as
DTMS) was controlled by the adjustment of the molar ratio of MH
and HMDS.
Comparing the spectra of TMSMH with maltoheptaose, a new
peak in the region 1250 cm−1 and three new peaks in the region
891–748 cm−1, all corresponding to the Si–CH3 group, can be
observed (Fig. 1). The signal at 1000–1100 cm−1 is due to both
the CH3–Si–O-maltoheptaose bond and the C–O–C bond of the
maltoheptaose chain. An increase in DTMS increases the relative
stretching intensity of the Si–CH3 group at 1250 cm−1. The remain-
ing peak around 3410–3450 cm−1 is attributed to the stretching
vibration of the associated –OH group, indicating that malto-
heptaose is not completely substituted. As shown in Fig. 1, the
associated –OH frequency of TMSMH shifts upwards compared
with that of MH, and the relative stretching intensity decreases
with an increase in DTMS, suggesting that the intermolecular and
3.2. Maltoheptaose silylation reaction
The preparation of the MH-g-PCL copolymer was carried out
according to the procedure shown in Scheme 1. Trimethylsilyl mal-
toheptaose (TMSMH) was synthesized from maltoheptaose and
hexamethyldisilazane (HMDS) in DMSO solvent at 80 ◦C, in which
Table 2
Characterization and thermal properties of MH-g-PCL copolymers.
Sample
Characterization
Thermal properties
b
b
DP of PCLa
Mn (Mw/Mn
)
First heating (10 ◦C/min) Tm1 (◦C)
First cooling (10 ◦C/min) Tc (◦C)
Second heating (10 ◦C/min) Tm2 (◦C)
g1 (MH7-g-PCL2)
2
10
60
–
61.73
63.79
63.86
39.21
39.43
40.05
55.40
58.43
58.75
g2(MH7-g-PCL10
g3(MH7-g-PCL60
)
)
15,689 (1.64)
30,903 (1.54)
MH-g-PCL with different length of PCL branches were denoted MH7-g-PCLx, wherein the suffix of 7 means the number of glucose unit of maltohepatoase, and the suffix of x
means the polymerization degree of a single PCL branch attached to pyranose unit. DS of the TMSMH employed was 68.24%.
a
Calculated by the result of 1H NMR.
Estimated by GPC in THF.
b

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X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
OH
O
OH
OH
O
O
O
HO
OH
HO
O
HO
O
OH
OH
HO
+
O
H
O
O
HO
HO
OH
OH
HO
OH
OH
OH
O
reflu
O
HO
O
OH
O
O
HO
OH
O
OH
HO
OH
O
OH
OH
OH
5
OH
OH
O
O
O
OH
HO
β-Cyclodextrin
Maltoheptaose
O
C
O
OR
O
catalyst
OR
HMDS
silytation
RO
RO
OR
O
O
RO
OR
RO
OR
O
OR
O
OR
R. OP. of CL
OR
O
RO
O
O
OR
O
RO
O
OR
O
OR
OR
RO
O
5
OR
OR
5
Where R=Si(CH3)3 or H
O
n
H
TMSMH
TMSMH-g-PCL
OH
H+
O
HO
HO
OH
OH
OH
O
O
O
O
O
O
OH
HO
OH
OH
5
O
n
H
MH7-g-PCLx
Scheme 1. Graft copolymerization of ␧-caprolactone onto maltoheptaose.
intramolecular hydrogen bonding in TMSMH weakens with the loss
of –OH groups.
indicating that the hydroxyl groups of MH were only partly substi-
tuted by the TMS groups. Table 1 shows the degree of substitution
which were calculated by 1H NMR spectrum.
The introduction of the TMS group was also confirmed from
the methyl proton signal at ı = 0.1 ppm in addition to the methy-
lene protons (H-1) signal of the monosaccharide residue groups
at ı = 5.0 ppm and the broad methine and methylene proton sig-
nals of the parent maltoheptaose at ı = 3.0–4.0 ppm in the 1H NMR
spectrum in CDCl3 (Fig. 2). The DTMS of TMSMH was determined
from the ratio of the integral areas of the signals for the methyl
protons of the TMS groups at ı = 0.1 ppm to those for the methyl
protons of the methylene protons (H-1) signal of the monosaccha-
ride residue groups at ı = 5.0 ppm. The DTMS were all less than 3,
As shown in Fig. 3, the broad peaks of MH and TMSMH reveal
amorphous structures. The peaks around 2ꢀ = 8–9◦ and 2ꢀ = 15–17◦
indicate that the structural characteristics of linear oligosaccharid−e
are preserved, with two peaks corresponding to the 101 and 10
1
planes. However, comparisons of the pattern of TMSMH with that
of the parent MH indicate broader peaks and smaller diffraction
12.89
9.14
18.97
14.92
19.71
24.5127.39
β-CD
17.8
MH
16.35
9.18
TMSMH-5
30
5
10
15
20
25
35
2θ
Fig. 2. 1H NMR spectrum of (A) ␤-CD (in DMSO-d6), (B) MH (in D2O), and (C)TMSMH-
1 (in D2O).
Fig. 3. X-ray diffraction profiles of (a) ␤-CD, (b) maltoheptaose (MH), and (c)
TMSMH-5.

X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
1727
Mw Polydispersitya
a
sample
DS(%)
Mnc
Mna
TMSMH-4
TMSMH-5
58.81 2127 2140 2259 1.05
68.24 2283 2249 2381 1.03
TMSMH-5
a
CCcalculated by the results of 1H-NMR.
aEstimated by GPC in THF.
2960
3450
b
c
TMSMH-g-PCL
MH-g-PCL
1250
1050
843
843
1050
3440
3440
1420
2950
2940
a
TMSMH-4
TMSMH-5
1730
d
1420
PCL
1050
1190
b
1730
1727
2943
3000
1190
1000
4000
3500
2500
2000
1500
500
15
20
25
30
Wavenumber(cm-1)
elute time(min)
Fig. 4. GPC traces of the resultant silylated maltoheptaose with a different DS. (a)
Fig. 5. IR spectra of (a) TMSMH-5, (b) TMSMH-g-PCL, (c) MH-g-PCL and (d) PCL.
TMSMH-4 (DS = 58.81%) and (b) TMSMH-5 (DS = 68.24%).
deprotection of the TMS groups was confirmed by the disappear-
ance of methyl proton signals from TMS at 0.10 ppm. The methylene
proton signals of PCL can be observed at 4.1 ppm (three peaks),
2.3 ppm (three peaks), 1.7 ppm (multipeaks), and 1.4 ppm (multi-
peaks). The methine proton signal was at 4.3 ppm, and methylene
proton signals were at 3.0–4.0 ppm.
angles corresponding to larger d101 and d101 values calculated by
the Bragg equation (Fig. 3). Taking into account the previous dis-
cussion on the IR results, this can be attributed to not only the loss
of the hydrogen bond but also the steric hindrance effect of the TMS
groups after the hydroxyl group in the plane of 101 and 101 was
partially substituted by the TMS group.
¯
The incorporation of TMS groups ensures the control of the PCL
graft number and the position on the MH backbone. The average
degree of ␧-caprolactone grafted on every glucose unit of MH back-
bone (DP) was calculated from the ratio of the integrated area of
the methylene signal of PCL at 2.3 ppm to the methine proton sig-
nal (H-1) of MH at 4.3 ppm (Table 2). The average length of every
PCL number can be calculated by considering that the number of
the remaining hydroxyl groups in every glucose unit is about 1 and
assuming that the free remaining hydroxyl group of the partially
silylated maltoheptaose (MH) can effectively initiate the ROP of CL.
As shown in Fig. 7, the contamination of the PCL homopoly-
mer resulting from transesterification was not detected in the GPC
curves. Furthermore, the molecular weight distribution is narrow
(1.54 ≤ Mw/Mn ≤ 1.64).
As shown in Table 1, maltoheptaose (MH) dissolved only in
water and some strong polar solvent (for example, DMSO and DMF).
The obtained TMSMH achieved solubility in a variety of organic sol-
vents, such as chloroform, acetone, THF and p-xylene. Insolubility
in DMF and DMSO increased with the substitution of trimethylsi-
lyl groups (DTMS > 48.77%), indicating that the polarity of TMSMH
decreases with an increase in DTMS. Together with the previous
discussion on the IR and WAXD results, the intermolecular and
intramolecular hydrogen bond in the TMSMH weakens with the
loss of –OH groups, and the structure of TMSMH becomes more
expanded and disorganized compared with MH, all contributing
to its easier dissolution in common organic solvent. As shown in
Fig. 4, the molecular distributions (Mw/Mn) of TMSMH-4 (58.81%)
and TMSMH-5 (68.24%) were around 1.0, proving that the malto-
heptaose from ␤-cyclodextrin was monodispersed.
In this work, MH has a molecular weight of 1152, which means
that the amount of glucose units per MH chain is 7. The structure
3.3. ROP of CL and deprotection of TMS-maltoheptaose-g-PCL
In the ROP of CL, the DS of TMSMH employed was approximately
68.24%. For this highly silylated MH, the glucose units were nearly
disilylated with a majority of the remaining free hydroxyl groups
in the third position (OH3) due to the relative weak reactivity of
the OH3 group in each glucose unit toward silylation (Nouvel et al.,
2004b). About 7 OH3 groups per 7 glucose units remained, and these
remaining free hydroxyl groups can serve as the initiating points
for the subsequent ROP reaction of ␧-caprolactone.
The obtained TMSMH achieved solubility in
a variety of
organic solvents. Therefore, the homogeneous ring-opening graft-
ing copolymerization of ␧-caprolactone onto the modified MH was
successfully carried out in a non-polar p-xylene solvent.
Finally, the TMS protection groups of TMSMH-g-PCL were
removed by the incubation of the polymer samples into an iso-
propyl alcohol/H2O/HCl mixture. The disappearance of absorption
bands related to the trimethylsilyl groups at 750, 843, 874, 1050,
1190, and 1250 cm−1 can be shown by FT-IR spectroscopy (Fig. 5).
The 1H NMR spectrum of TMS-deprotected MH-g-PCL was in good
agreement with the expected structure as shown in Fig. 6. The
Fig. 6. 1H NMR spectrum of (A) TMSMH-5 (in CDCl3), (B) TMS-protected graft
copolymer TMSMH-g-PCL (in CDCl3), and (C)TMS-deprotected graft copolymer MH-
g-PCL (in DMSO-d6).

1728
X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
a
b
b
55.4043
c
a
58.4267
58.749
60
10
15
20
25
elute time (min)
40
50
70
80
90
100
Tem. (°C)
Fig. 7. GPC traces of poly(␧-caprolactone)-grafted maltoheptaose with different PCL
lengths. (a) MH7-g-PCL10, and (b) MH7-g-PCL60
.
Fig. 9. DSC thermograms of (a) MH7-g-PCL2, (b) MH7-g-PCL10, and (c) MH7-g-PCL60
with a rate of 10 ◦C/min at second heating run.
of the obtained graft copolymer is comb shaped when the short
oligo(␧-caprolactone) is grafted onto MH backbone. Increasing the
length of ␧-caprolactone segments transforms the structure of the
obtained graft copolymer into brush shaped. It becomes tassel
shaped with an asymmetric and unconventional graft copolymer
structure consisting of a short main chain as backbone and long
side chains as grafting segments. The schematic representations of
their architecture are shown in Fig. 8.
21.22
23.53
d
PCL
20.74
23.06
20.71
22.97
3.4. Thermal analysis of MH-g-PCL copolymers
c
MH7-g-PCL60
MH7-g-PCL2
The crystalline property of amphiphilic copolymer influences
its biodegradation and the hierarchical structure of its formed
nanoparticles (Lin, & Gast, 1996; Portinha, Bouteiller, Pensec, &
Richez, 2004; Richter, Schneiders, Monkenbusch, & Willner, 1997;
Ohya et al., 1998). Thus, the thermal behavior of the synthe-
sized new graft copolymers with a different Dp was investigated
using DSC. PCL homopolymer is a semicrystalline polymer. The
detected thermal phenomenon in DSC can only be attributed to
PCL segments in MH-g-PCL because MH did not show any melt-
ing transition. The melting points of crystalline polymers may also
depend on the thermal history of the sample. To erase the ther-
mal history of the samples, the DSC thermograms were recorded
during the second heating at 10 ◦C/min (Fig. 9). The values of
the melting temperature (Tm) of all samples at first heating, first
cooling, and second heating are listed in Table 2. The melting
temperature(Tm) shifted from 55.4 ◦C to 58.4 ◦C with the increase
b
17.8
MH
a
5
10
15
20
2θ
25
30
35
Fig. 10. X-ray diffraction profiles of (a) maltoheptaose (MH), (b) MH7-g-PCL2, (c)
MH7-g-PCL60, and (d) PCL.
in side chain lengths of the copolymer from 2 to 10. However, the
influence of the maltoheptaoses is weak that the continual increase
in side chain length did not alter the crystallization and melting
temperatures.
The results of the XRD analysis of the grafted copolymers are
shown in Fig. 10. Compared with 10-a, a peak of the PCL segment
in the grafting copolymers is observed, and the peak intensity of
the PCL segments is increased with an increase in side chain length
(Fig. 10b and c).
4. Conclusion
Amphiphilic graft copolymers (MH-g-PCL) with controlled
structures were synthesized by protection and deprotection of
the partial hydroxyl groups of MH via TMS groups and homoge-
nous ring-opening polymerization of ␧-caprolactone with stannous
octoate as catalyst. A new kind of graft copolymers with controlled
structure was obtained by adjusting the molar ratio of the CL to
Fig. 8. Schematic illustration of comb-shaped, brush-shaped, and tassel-shaped
copolymers with different branch lengths.

X. Qiu et al. / Carbohydrate Polymers 83 (2011) 1723–1729
1729
MH glucose unit. Crystalline properties indicate that an increase
in PCL content in the graft copolymer results in a higher Tm. The
synthesis method can be applied to other oligosaccharides and
biodegradable aliphatic polyesters, such as poly(lactone, lactide,
and glycolide). Furthermore, such copolymers are expected to be
applied as potential drug carrier materials.
Nouvel, C., Frochot, C., Sadtler, V., Dubois, P., Dellacherie, E., & Six, J.-L. (2004).
Polylactide-grafted dextrans: Synthesis and properties at interfaces and in solu-
tion. Macromolecules, 37(13), 4981–4988.
Nouvel, C., Dubois, P., Dellacherie, E., & Six, J.-L. (2004). Controlled synthesis of
amphiphilic biodegradable polylactide-grafted dextran copolymers. Journal of
Polymer Science Part A: Polymer Chemistry, 42(11), 2577–2588.
Ouchi, Tatsuro, Kontani, Tomohiro, & Ohya, Yuichi. (2003). Mechanical property and
biodegradability of solution-cast films prepared from amphiphilic polylactide-
grafted dextran. Journal of Polymer Science Part A: Polymer Chemistry, 41(16),
2462–2468.
Ohya, Yuichi, Maruhashi, Shotaro, & Ouchi, Tatsuro. (1998). Graft polymerization of
L-lactide on pullulan through the trimethylsilyl protection method and degra-
dation of the graft copolymers. Macromolecules, 31(14), 4662–4665.
Portinha, D., Bouteiller, L., Pensec, S., & Richez, A. (2004). Influence of preparation
conditions on the self-assembly by stereocomplexation of polylactide contain-
ing diblock copolymers. Macromolecules, 37(9), 3401–3406.
Acknowledgment
This research was financially supported by the Natural Science
Foundation of China (Grant No. 20604033)
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