Modulation of reversible self-assembling of dumbbell-shaped poly(ethylene glycol)s and β-cyclodextrins: precipitation and heat-induced supramolecular crosslinking
Polymer Journal (2011) 43, 893–900
& The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/11 $32.00
www.nature.com/pj
ORIGINAL ARTICLE
Modulation of reversible self-assembling of
dumbbell-shaped poly(ethylene glycol)s and
b-cyclodextrins: precipitation and heat-induced
supramolecular crosslinking
Yuichiro Kobayashi1, Ryo Katoono1,2, Masayuki Yamaguchi1 and Nobuhiko Yui1,2
A series of dumbbell-shaped poly(ethylene glycol) (PEG) chains 1 attached to bulky end groups were prepared, and some of
the chains formed supramolecular assemblies with b-cyclodextrin (b-CD) and its multiple, ditopic and tetratopic, derivatives.
The chains with proper end groups successfully allowed b-CD to be trapped onto PEG through formation of hydrogen bonds at
room temperature and higher. Mixing of the PEG chain and the ditopic supramolecular crosslinker in water at 40 1C led to a
change in solution property from viscous to elastic, accompanied by a significant increase in viscosity, whereas this change was
not induced at room temperature. A supramolecular network formed only when the PEG chain was mixed with the tetratopic
supramolecular crosslinker at 40 1C. Once formed, the supramolecular crosslinking was maintained even after the system cooled
down. Instead, dilution and shaking at room temperature resulted in a return to a solution with low viscosity. These assemblies
and dissociations were affected by the end groups of 1.
Polymer Journal (2011) 43, 893–900; doi:10.1038/pj.2011.71; published online 3 August 2011
Keywords: b-cyclodextrin; poly(ethylene glycol); pseudopolyrotaxane; supramolecular crosslinking; viscoelastic properties; viscosity
INTRODUCTION
One of the greatest findings in the 1990s was the self-assembly of cyclic
molecules onto a linear polymeric chain, represented by a-cyclodextrin (a-CD) onto poly(ethylene glycol) (PEG)1,2 and b-cyclodextrin
(b-CD) onto poly(propylene glycol),3,4 leading to the development of
supramolecular materials based on cyclodextrins5–8 and others.9,10
In the assembly, CD molecules formed intermolecular hydrogen
bonds2,4,6,7 with each other to stay on the chain while threading and
dethreading were competing, and finally a supramolecular assembly
called pseudopolyrotaxane was obtained as a kinetic product. As a
matter of course, some thermodynamic advantages accompanied this
process, such as enthalpic gain on forming hydrogen bonds,6 and total
entropic changes on assembly of the components, as well as desolvation. It seems that PEG and poly(propylene glycol) chains provided a
suitable guide for a-CD and b-CD to fill the size-matched cavity and
align cooperatively through formation of hydrogen bonds. In recent
years, Takata et al. reported an excellent synthetic approach for
yielding pseudopolyrotaxanes based on self-assembly of modified
CD molecules onto a linear polymeric chain without relying on
forming hydrogen bonds, but using heterogeneous systems in which
permethylated a-CD and poly(tetrahydrofuran) or PEG were used in
hydrocarbon solvents11 as well as in water.12 Even though the initial
Scheme 1 (a) Pseudopolyrotaxane formation between size-matched cyclic
and linear components, (b) pseudopolyrotaxane formation between sizemismatched cyclic and dumbbell-shaped linear components and
(c) supramolecular crosslinking based on pseudopolyrotaxane formation between
size-mismatched crosslinked cyclic and dumbbell-shaped linear components.
1School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan and 2JST, CREST, Chiyoda-ku, Tokyo, Japan
Correspondence: Dr R Katoono, School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
E-mail:
Received 23 March 2011; revised 11 May 2011; accepted 10 June 2011; published online 3 August 2011
Polymer Journal
OH
MeO
MeO
MeO
MeO
O
R6
2
O
O
OH
6
O
O
HO
O
H
N
O
1
O
n
N
H
R
O
O
OMe
N
H
OH
O
6
O
4
O
MeO
l
O
H
N
MeO
MeO
O
NH2
6
O
a: n = 22, R2 = OPr, R4 = OBn, R6 = H
b: n = 22, R2 = OEt, R4 = OBn, R6 = H
c: n = 22, R2 = OPr, R4 = OPr, R6 = H
d: n = 22, R2 = OPr, R4 = H, R6 = H
e: n = 21, R2 = OPr, R4 = H, R6 = OPr
O
R2
3
O
O
O
O
6
OMe
OMe
OMe
OMe
OMe
6
O
4
R
OMe
O
MeO
O
R2
MeO
MeO
MeO
MeO
O
O
6
O
O
N
OMe
N
N
O
O
m-1
O
N
N
N
MeO
O
N
O
N
d
O
5
N
O
MeO
O
O
OMe
m-1
N
O
MeO
O
m-1
b
O
N
a
N
N
O
N
N
O
OMe
O
N
O
N
N
O
O
MeO
6
OMe
O
OMe
c
O
N
OMe
N
N
6
O
O
m-1
O
N
N
MeO
N
Figure 1 Chemical structures of dumbbell-shaped poly(ethylene glycol) chains 1a–e, b-cyclodextrin 2, permethylated b-cyclodextrin 3, ditopic crosslinker 4 and tetratopic crosslinker 5.
HO
OH
R4
O
MeO
O
O
O
O
6
OMe
OMe
OMe
OMe
Reversible self-assembling of PEG and b-CD
Y Kobayashi et al
894
Reversible self-assembling of PEG and b-CD
Y Kobayashi et al
895
R6
R6
O
Cl PEGBA, Et3N
R4
6
O
N
H
CH2Cl2
91-95%
R2
R2
O
R4
O
R4
H
N
n
R6
O
R2
1
a: R2 = OPr, R4 = OBn, R6 = H
b: R2 = OEt, R4 = OBn, R6 = H
c: R2 = OPr, R4 = OPr, R6 = H
d: R2 = OPr, R4 = H, R6 = H
e: R2 = OPr, R4 = H, R6 = OPr
Scheme 2 Preparation of dumbbell-shaped poly(ethylene glycol) chains 1. PEGBA, poly(ethylene glycol) bis(2-aminoethyl) ether
O
N3
OMe
O
O
O
O
m-1
MeO
MeO
CuSO4
(+)-ascorbic acid
O
O
+
O
MeO
6
MeO
OMe
N
O
8
MeO
OMe
O
DMF
17%
O
7
OMe
O
OMe
O
OMe
O
N
6
OMe
O
N
m-1
OMe
9
OMe
O
O
6
O
MeO
N
N
N
O
N3
O
O
d
O
N3
O
a
m-1
O
c N3
O
N
O
N
N
b
MeO
10
OMe
O
CuSO4
(+)-ascorbic acid
DMF
52%
OMe
O
N3
O
d
MeO
N
O
MeO
N
O
N
O
N
O
m-1
N
N
O
O
a
c
N
N
N
O
O
N
O
O
m-1
N
N
O
6
OMe
OMe
O
MeO
OMe
O
MeO
N
N
b
N
O
O
6
O
m-1
O
N
N
N
OMe
O
O
O
MeO
MeO
MeO
O
6
OMe
5
Scheme 3 Preparation of tetratopic crosslinker 5.
Polymer Journal
Reversible self-assembling of PEG and b-CD
Y Kobayashi et al
896
r.t. or Δ
dilution
1a or 1b + 2
precipitation
R6
R2
O
N
H
R4
O
O
R4
+
H
N
2
n
O
R2
R6
1
1c, 1d or 1e + 2
no precipitation
Scheme 4 Mixing of 1 with 2 in water leading to precipitation (1a, 1b) at room temperature (r.t.) or elevated temperature, or no precipitation (1c, 1d and
1e), and dissociation by dilution.
Table 1 Conditions (concentration and temperature) for complexation of 1a, 1b or PEGBA with 2, 3 or a-CD,a and results (time for
precipitation, ratiob of PEG to CD in an isolated solid and yieldc)
Concentration (mM)
PEG
CD
PEG
1a
2
1.3
4.8
64
64
3
CD
Ratiob (PEG:CD)
Yieldc (%)
Temperature (1C)
Time (day)
15
rt
10
1:10
29
57
60
4
1:10
28d
7.1102
rt
–
–
0
7.1102
60
–
–
0
15
rt
5
1:10
23
20d
1b
2
1.3
4.8
55
60
2
1:10
PEGBA
2
1.3
15
rt
–
–
0
a-CD
13
1.4102
rt
o1
–(1:10)
0 (92)
Abbreviations: a-CD, a-cyclodextrin; PEG, poly(ethylene glycol); PEGBA, poly(ethylene glycol) bis(2-aminoethyl) ether; rt, room temperature.
aPEG was added t (...truncated)
