Preparation of hydrolyzable polyrotaxane containing ester linkages and its degradation behavior

Article abstract of DOI:10.1246/cl.2008.988

A hydrolyzable polyrotaxane composed of an ester-containing poly(ethylene glycol) chain and α-cyclodextrins showed gradual degradation into its water-soluble components in aqueous conditions, based on the dissociation of the polyrotaxane triggered by the hydrolysis of the ester groups. Copyright

Full text of DOI:10.1246/cl.2008.988

988
Chemistry Letters Vol.37, No.9 (2008)
Preparation of Hydrolyzable Polyrotaxane Containing Ester Linkages
and Its Degradation Behavior
Ryo Katoono, Shin-ichiroh Fukuda, Hojoon Shin, and Nobuhiko Yuiꢀ
Japan Advanced Institute of Science and Technology, Asahidai, Nomi 923-1292
(Received June 5, 2008; CL-080568; E-mail: yui@jaist.ac.jp)
A hydrolyzable polyrotaxane composed of an ester-contain-
ing poly(ethylene glycol) chain and ꢀ-cyclodextrins showed
gradual degradation into its water-soluble components in aque-
ous conditions, based on the dissociation of the polyrotaxane
triggered by the hydrolysis of the ester groups.
In the last several decades, biodegradable polymers have
been studied as implantable materials for cell growth and tissue
engineering.1 These materials must satisfy various requirements
such as sufficient mechanical strength, non-toxicity, and bio-
inertness before and after degradation for clinical use in a living
body. In order to design and construct biodegradable materials,2
some attention should be paid to the fact that the material has to
have the potential to degrade perfectly in appropriate conditions,
and that undesirable decomposition has to be avoided during the
preparation and purification. It is of course necessary to purify
the materials aimed at a clinical use in a living body. In this con-
text, our approach for the design of biodegradable materials
based on the structural features of polyrotaxanes, and especially
on their dissociation, is promising. Stimuli-responsive biode-
gradable polyrotaxanes as shown in Scheme 1 would provide
quite a new model as a mode of biodegradation.3 Degradation
based on the dissociation of polyrotaxanes, that is, the transfor-
mation of the supramolecular materials with a high molecular
weight into water-soluble and bioinert components would be
favorable in terms of effective degradation, low toxicity, and
biocompatibility in a living body.
Scheme 2. Preparation of hydrolyzable polyrotaxane 1 and
chemical structure of ester-free polyrotaxane 4: Reagents and
conditions; a) ꢀ-CD, water; b) succinic anhydride, DMAP,
pyridine (74%); c) N-hydroxysuccinimide, DCC, THF; d)
DMF (15%).
in DMF as shown in Scheme 2. The pseudopolyrotaxane 2 was
obtained by mixing a PEG chain 5 attached to amino groups at
both ends with ꢀ-CD in water, followed by lyophilization
according to a method reported by Harada et al.5 The bulky
capping molecule 3 was derived from a commercially available
Z-phenylalaninol by treatment with succinic anhydride in
pyridine containing a catalytic dimethylaminopyridine, and then
employed as a succinimidyl succinate just before the capping
reaction by condensation with 2. Reprecipitation and dialysis us-
ing DMSO and water allowed 1 to be isolated without decompo-
sition, which was confirmed by 1H NMR and GPC measure-
ments. The ester-free polyrotaxane 4 was also prepared as a ref-
erence by capping the pseudopolyrotaxane 2 with Z-phenylala-
nine in a similar manner to that used for 1.
According to Figure 1a, notable broadening signals were
observed for CD protons in 1, which is characteristic of CD-
based polyrotaxanes in DMSO-d6.5 Aromatic protons assigned
to capping moieties in 1 were also detected. Figure 1b shows
sharp signals as is observed for common pseudopolyrotaxanes
without any terminal bulky groups implying the dissociation of
2 in DMSO-d6 into the respective components ꢀ-CD and
PEG. These observations clearly indicate that the polyrotaxane
1 was successfully prepared and isolated in pure form.6 This
was also confirmed by GPC measurements for 1, 2, and 4 eluted
with DMSO, in which a shorter retention time (40 min) for the
polyrotaxanes 1 and 4 was observed than for the ꢀ-CD
(50 min) accompanied with the dissociation of pseudopolyrotax-
ane 2 in DMSO (Figure S1).9
A polyrotaxane composed of a PEG chain containing ester
groups at both ends and ꢀ-cyclodextrins (ꢀ-CDs) was designed
and prepared as a candidate for biodegradable polymers, in
which the ester group(s) would be expected to hydrolyze to
trigger the following dissociation in response to pH. Thus, a
successful preparation of the ester-containing polyrotaxane in
spite of the potential of degradation4 and its hydrolysis behavior
were demonstrated.
The hydrolyzable polyrotaxane 1 was prepared by capping a
pseudopolyrotaxane 2 with an ester-containing bulky N-benzyl-
oxycarbonyl (Z-) phenylalanine-based succinic acid derivative 3
Scheme 1. Biodegradation based on dissociation of polyrotax-
ane triggered by stimuli-responsive cleavage of biodegradable
linkages.
The hydrolysis property of the ester-containing polyrotax-
ane 1 was investigated by monitoring the time change in the
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.9 (2008)
989
also examined through transmittance measurement (Figure 2b).
It then took much more time for the ester-containing polyrotax-
ane 1 to initiate hydrolysis than under basic conditions (pH 9.18),
to reach only 10% even after 13 days.
In conclusion, we demonstrated the design and preparation
of the hydrolyzable polyrotaxane 1 composed of an ester-
containing PEG chain and ꢀ-CDs. The hydrolysis of 1 into its
water-soluble components under aqueous conditions adjusted
to both a basic and almost neutral pH was confirmed by monitor-
ing continuous changes in transmittance and by GPC measure-
ments of clear upper portions collected from suspensions. We
are now studying the design of hydrogels based on hydrolyzable
polyrotaxanes. It is easily conceivable that the dissociation
of polyrotaxanes in hydrogel form into its components upon
degradation of labile linkage(s) such as the ester group will be
suitable for biocompatible materials such as scaffolds for tissue
engineering.
Figure 1. 1H NMR spectra (300 MHz) of (a) polyrotaxane 1
and (b) pseudopolyrotaxane 2 in DMSO-d6 at room temperature.
References and Notes
1
a) R. Langer, D. A. Tirrel, Nature 2004, 428, 487. b) K.
Rezwan, Q. Z. Chen, J. J. Blaker, A. R. Boccaccini, Bio-
materials 2006, 27, 3413. c) M. E. Furth, A. Atala,
M. E. V. Dyke, Biomaterials 2007, 28, 5068.
2
Aliphatic polyesters such as poly(lactic acid), and poly-
(glycolic acid) have been widely investigated to develop
biomaterials which undergo hydrolysis for degradation.
Recent reviews: a) Y. Tokiwa, A. Jarerat, Biotechnol. Lett.
2004, 26, 771. b) H. Tsuji, Macromol. Biosci. 2005, 5,
569. c) Y. Tokiwa, B. P. Calabia, Appl. Microbiol. Biotech-
nol. 2006, 72, 244. d) D. Chitkara, A. Shikanov, N. Kumar,
A. J. Domb, Macromol. Biosci. 2006, 6, 977.
Figure 2. Continuous changes in transmittance of suspensions
for 1 (thin line), 2 (bold line), and 4 (dashed line) at (a)
pH 9.18 and 1 (thin line), and 2 (bold line) at (b) pH 6.86 at room
temperature.
transmittance by comparison with that for the pseudopolyrotax-
ane 2 and the ester-free polyrotaxane 4. It is difficult to estimate
exactly the amount of cleaved ester linkages from the change in
transmittance because the cleavage of both ester linkages is not
required for triggering the dissociation. Each sample was sus-
pended in a tetraborate pH standard solution adjusted to a pH
of 9.18. A gradual increase in the transmittance at 500 nm from
the baseline for 1 indicates qualitatively7 that the water-insoluble
1 was gradually reduced by hydrolysis to produce water-soluble
component(s), whereas an instantaneous change from a turbid
suspension to a clear solution for 2 showed the dissociation of
the inclusion complex. Any change in the transmittance for the
ester-free polyrotaxane 4 was not found during the observation
(Figure 2a).
In order to obtain further information on water-soluble
component(s) in each suspension, several clear upper portions
were collected at an arbitrary time for GPC measurements.
The signals detected by RI for each portion collected from the
suspension of 1 and 2 had the same retention time, indicating
that at least one of the water-soluble components is ꢀ-CD
(Figure S2).9 The concentration of the PEG component in the
portion was too low to be detected by RI on GPC measure-
ments.8 On the other hand, the portions collected from the sus-
pension of 4 did not contain any water-soluble components,
which means that it was intact under the conditions in accord-
ance with the result of the transmittance measurements. These
observations demonstrate that the ester-containing polyrotaxane
1 was hydrolyzed to produce its water-soluble components.
Hydrolysis under almost neutral conditions (pH 6.86) was
3
4
N. Yui, T. Ooya, Chem.—Eur. J. 2006, 12, 6730.
A labile bond such as ester linkage was introduced prospec-
tively into a capping agent not a CD-based pseudopolyrotax-
ane5 which is commonly prepared in water. Another ester-
containing polyrotaxane 109 had been designed in previous
reports such as the following a) and b), however, the
preparation and isolation of desired polyrotaxane 10 could
not be achieved sufficiently due to the lack of attention to
the potential of degradation. a) J. Watanabe, T. Ooya, N.
Yui, Chem. Lett. 1998, 1031. b) J. Watanabe, T. Ooya, N.
Yui, J. Biomater. Sci. Polym. Ed. 1999, 10, 1275.
A. Harada, J. Li, M. Kamachi, Nature 1992, 356, 325.
The number of threading ꢀ-CD molecules in the obtained
polyrotaxane 1 was calculated from the ratio of peak integra-
tions for both C(1) protons in CD and methylene protons
in PEG in the NMR spectrum measured in D2O containing
approximately 1 wt % NaOD to be ca. 18 (Figure S3a).9
A quantitative understanding for the dissociation of polyro-
taxane is difficult because some inclusion complexes can
be soluble in water when the number of threading ꢀ-CD
molecules is small.
5
6
7
8
A PEG component in the clear upper portion was detectable
by TLC on silica gel significantly. Also, after evaporation
of the portion, the remaining solid was suspended in CH2Cl2.
It was confirmed by 1H NMR that the CH2Cl2 layer
contained the PEG component.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
9
Published on the web (Advance View) August 23, 2008; doi:10.1246/cl.2008.988