Graft polyrotaxanes: A new class of graft copolymers with mobile graft chains

Article abstract of DOI:10.1002/anie.201103869

The synthesis and structure of graft polyrotaxane, a novel class of graft copolymer possessing mobile graft chains, is described. Poly(tetrahydrofuran) was bound to the axle components of pseudo[2]rotaxane as the graft chain by a grafting-onto method to a

Full text of DOI:10.1002/anie.201103869

DOI: 10.1002/anie.201103869
Supramolecular Polymers
Graft Polyrotaxanes: A New Class of Graft Copolymers with Mobile
Graft Chains**
Yasuhiro Kohsaka, Yasuhito Koyama, and Toshikazu Takata*
Polyrotaxanes (PRs) have been at the forefront of polymer
and related materials-science research throughout this
decade.[1] In their pioneering work, Gibson and co-workers
reported various structural possibilities regarding PR molec-
ular architectures.[1b] In addition to main-chain-type PRs,[2–4]
poly[n]rotaxanes,[5] daisy-chain-type PRs,[6] and dendritic
PRs[7,8] have attracted much attention as novel classes of
polymers with flexible main chains the repeating units of
which are mechanically connected. Cross-linked PRs[9] are
also simple but quite attractive classes of PRs for applications
in functional materials.[3a,10,11] On the other hand, we pro-
posed a novel class of graft copolymers the graft chains of
which are mechanically bound to the main chain by rotaxane
skeletons and we referred to them as “graft polyrotaxanes
(GPR).”[12] Depending on the function of their graft chains,
there are two types (Figure 1): 1) GPR-A has graft chains as
the axle components that translate through cavities in the
main chain, thus change in the length of the graft chains is
coupled with their movement. 2) GPR-B has graft chains
linked to the wheel components that translate along and
circumrotate around the axle polymer, thus delocalizing their
positions and varying the density of the graft chains on the
axle polymer.
However, GPR-A has not yet been reported, unlike GPR-B,
which several groups, including our own, have synthe-
sized.[12,13] We describe herein the synthesis of GPR-A by
applying the grafting-onto protocol to a poly(pseudorotax-
ane) and analyze its structural characteristics and dynamic
behavior.
We designed GPRH2PF6 as a GPR-A that possesses both a
poly(crown ether) main chain and a poly(tetrahydrofuran)
(poly(THF)) graft chain (Scheme 1). Prior to the synthesis of
Figure 1. Structures of two graft polyrotaxanes (GPRs): GPR-A with
varying graft chain length and GPR-B with varying graft chain density.
We are intrigued by these unique structures, because graft
length and density should significantly affect the physical and
mechanical properties of graft polymers, including their
solution properties, surface properties, and microstructure.
[*] Dr. Y. Kohsaka, Dr. Y. Koyama, Prof. Dr. T. Takata
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
Scheme 1. A) Synthesis of graft chain. Reagents and conditions:
a) AgOTf (1.0 equiv), THF, 08C, 3 min then H2O, 77% yield; b) m-
phenylene diisocyanate (5.0 equiv), CH2Cl2, 258C, 12 h, quantitative;
B) Synthesis of graft polyrotaxane; reagents and conditions: c) CH2Cl2,
258C, 12 h; d) 3 (2.0 equiv), Bu2Sn(OCOC11H23)2 (3 mol%), CH2Cl2,
258C, 12 h; e) Ac2O (3.0 equiv), Et3N (6.0 equiv), DMF, 608C, 72 h,
94%; C) Structures of model [2]rotaxanes. OTf=trifluoromethanesulfo-
nate, THF=tetrahydrofuran, DMF=N,N-dimethylformamide.
2-12-1 (H-126), Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
E-mail: ttakata@polymer.titech.ac.jp
[**] We acknowledge JSPS Fellowships for Young Scientists (Y.K.). This
work was financially supported by a Grant-in-Aid for Scientific
Research (No. 18205014) from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103869.
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Communications
GPRH2PF6, a model [2]rotaxane (MRH2PF6) corresponding
to the repeating unit of GPRH2PF6 was synthesized by the
typical threading-end-capping method[14] in order to inves-
tigate the relative position and mobility of the components.
Figure 2 shows the 1H NMR spectra of the axle
component 5 and MRH2PF6. Signals corresponding to N-
benzylic protons (s) were shifted downfield from d = 4.19
to 4.58 ppm by rotaxanation, while the tBu signal was
shifted upfield from d = 1.31 ppm to 1.23 ppm because of
the deshielding effect of the aromatic ring of DB24C8. The
results indicated that DB24C8 was located on the ammo-
nium station owing to the strong hydrogen-bonding
interaction between the components.[11b,15] We previously
reported the effective removal of hydrogen bonding by N-
acylation.[16] Thus, the ammonium moiety was N-acety-
lated with acetic anhydride to afford MRAc. In the case of
MRAc, the 1H NMR chemical shift of the tBu protons
shifted back to that observed in 5, while most signals of the
tris-THF moiety (c, f, g, and l) were shifted in comparison
with those of MRH2PF6. In addition, NOESY correlations
between DB24C8 and the tris-THF moiety of MRAc were
clearly observed.[17] Therefore, it was concluded that as a
Figure 2. Partial 1H NMR spectra of A) 5, B) MRH2PF6, and C) MRAc
result of N-acetylation, DB24C8 could freely move over (400 MHz, 298 K, CDCl3).
the whole axle component including the tris-THF moiety.
Consequently, this MR system was identified as a suitable
Table 1: Synthesis and thermal property of GPRH2PF6 with controlled
grafting ratio.[a]
unit for the synthesis of GPR-A with mobile graft chains.
GPRH2PF6 was synthesized according to Scheme 1. Mono-
functional poly(THF) 2 (Mn 1.3 ꢀ 103, Mw/Mn 1.53) was
treated with excess m-phenylene diisocyanate to form graft-
ing agent 3.[18] Mixing poly(crown ether) 4[11b,15] (Mn 4.0 ꢀ 103,
Mw/Mn1.35) with an equimolar amount of 5[11b,15] in CH2Cl2
initially formed poly(pseudorotaxane) 6 in situ. The subse-
quent addition of 3 afforded GPRH2PF6-62 as an Et2O-
Run
[5] [m][b]
Product
x [%][c]
Yield [%]
Tg [8C]
1
2
3
0.60
1.00
2.00
GPRH2PF6-25
GPRH2PF6-62
GPRH2PF6-100
25
62
100
57[d]
47[d]
37[e]
11.3
12.8
À60.5
63.2
poly(THF)
poly(crown ether)
2
4
À74.7
11.3
insoluble polymer (47% yield).
[a] Reaction was performed by the initial mixing of 4 (0.50 unit-molLÀ1
and 5 and subsequent addition of 3 (2.0 equiv to 5). [b] Concentration of
5. [c] Grafting ratio. [d] Isolated as MeOH-insoluble polymer. [e] Isolated
by preparative SEC.
)
1
Detailed analysis of the H NMR spectrum of GPRH2PF6
-
62 and comparison with the spectra of the individual polymer
components strongly supports the formation of GPRH2PF6
-
62.[17] For example, N-benzylic proton signals of the axle
component were shifted downfield from d = 4.19 to 4.58 ppm
and each of the methylene proton signals a and g of the
DB24C8 moiety were split into two peaks by rotaxanation.
These spectral changes were consistent with those observed
for the main-chain-type PRs[11b,15] and the above mentioned
MRH2PF6. Further evidence to confirm the structure of
GPRH2PF6-62 was obtained by FTIR spectroscopy and
MALDI-TOF mass spectrometry.[17] The rotaxanation ratio
the axle graft chain (x = 100%), was soluble in most organic
solvents, including CH3OH and Et2O. Therefore, GPRH2PF6
-
100 was purified by preparative size exclusion chromatog-
raphy (SEC) instead of fractional precipitation (Table 1,
run 3), resulting in a considerable decrease in yield (37%). In
addition to their solubility properties, the glass transition
temperatures (Tg) of GPRH2PF6 polymers were also found to
be dependent on the grafting ratio. GPRH2PF6-25 and
GPRH2PF6-62 exhibited single Tg at 11.38C and 12.88C,
respectively, and were similar to that of the trunk polymer 4
(11.38C). In contrast, GPRH2PF6-100 showed two Tg at
À60.58C and 63.28C. The former Tg originates from the
graft chains that consistof poly(THF), while that of the latter
is assigned to the trunk polymer that contains completely
penetrated DB24C8 moieties, which is fully consistent with
1
of 6 was determined to be 62% from the H NMR integral
ratio. The observed graft ratio of 62% for GPRH2PF6-62
precisely correlated with the rotaxanation ratio, which
suggested that all axle components incorporated into 6 were
used in the graft chains of the final product GPRH2PF6-62.
It was found that the percent grafting ratio (x) of
GPRH2PF6 could be controlled by the feed ratio of 5
(Table 1). When 0.60 equiv of 5 were employed, x was
found to be 25% (Table 1, run 1). The obtained polymer
(GPRH2PF6-25) was soluble in CHCl3, acetone, and dimethyl
sulfoxide (DMSO), but insoluble in CH3OH and Et2O. In
contrast, with 2.0 equiv of 5, the product GPRH2PF6-100, the
DB24C8 moieties of which were completely penetrated by
the structural characteristics of GPRH2PF6
.
To achieve high mobility of the graft chain of GPRH2PF6
,
N-acetylation of the polymer was performed to produce
GPRAc in the same manner as that of MRH2PF6. Treatment of
GPRH2PF6-62 with acetic anhydride (Ac2O) and triethylamine
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at 608C for 3 days gave the N-acetylated product GPRAc-62 in
94% yield. Based on the integral ratio obtained from
1H NMR, the percentage conversion to the N-acetylation
product reached 80%.[17] The downfield shift of the tBu signal
from d = 1.20 to 1.31 ppm confirmed the movement of the
DB24C8 wheel to the poly(THF) moiety, which concurred
with the results obtained for MRAc. Thus, the results indicated
As a result, the main chain acquires a somewhat folded
conformation (Figure 3b). In contrast, the graft chain length
of GPRAc-62 apparently increases because of the enhanced
mobility or free translation of the graft chain as shown in
Figure 3d. The unconstrained movement of the graft chain
drives the extension of the folded structure (Figure 3d), thus
resulting in the observed increase in RH. Further, the differ-
ence in D distribution observed in Figure 3 (a and c) seems to
correspond well with the higher size uniformity of GPRAc-62
than GPRH2PF6-62. The present macromolecular shape change
well reveals the characteristics of mobile connections served
by rotaxane skeletons.
In conclusion, we have performed the first synthesis of
GPR-A using the grafting-onto method and with a control-
lable grafting ratio. The mobility of the graft chains was
observed to increase as a result of N-acetylation of GPRH2PF6
to GPRAc. This observation caused the extension of the
GPRAc graft chain, and in accordance with the results of the
model study, lead to an enhancement in its macromolecular
size. Although GPR is similar to conventional graft copoly-
mers with respect to its grafted structure, the nature of its
structure is dynamic because of the mobile rotaxane con-
nection. Hence, the graft copolymer structure resembles that
of a polymer blend, in which each of the component polymers
can move somewhat independently. Further study of the
relationship between the macroscopic properties and
dynamic structure of GPR is currently in progress.
the delocalization of the graft chains; the structure of GPRAc
-
62 was also confirmed by IR, SEC, and DSC.[17]
To evaluate macromolecular structural changes after N-
acetylation, the diffusion ordered spectroscopy (DOSY)
NMR spectra[19] of GPRH2PF6-62 and GPRAc-62 were mea-
sured (Figure 3). The broad distributions of each peak are
attributed to the broad molecular weight distribution of each
component. Because all peaks show a unimodal distribution
of the diffusion coefficient (D), it was confirmed that graft
polyrotaxanes GPRH2PF6-62 and GPRAc-62 contained no
impurities such as unreacted grafting agent or unthreaded
axle component. From the results, we estimated the hydro-
dynamic radii (RH) of GPRs by the Einstein–Stokes equa-
tion.[20] Because D for GPRH2PF6-62 and GPRAc-62 had a
certain distribution, we used the diffusion coefficient at the
point of maximum correlation (Dp, 5.28 ꢀ 10À11 and 4.15 ꢀ
10À11 msÀ1 for GPRH2PF6-62 and GPRAc-62, respectively).
Thus, the calculations give the following values: RH =
5.76 nm for GPRH2PF6-62 and RH = 7.33 nm for GPRAc-62.
The clear increase in RH as a result of N-acetylation can be
attributed to a change in molecular shape. GPRH2PF6-62 has
polyionic structure; thus, the graft chain terminal is fixed at
the crown ether cavity of the main chain. The graft chain
length is sufficient to allow aggregation around the backbone.
Received: June 7, 2011
Published online: September 9, 2011
Keywords: graft polyrotaxanes · host–guest systems ·
.
mobile graft chain · rotaxanes · supramolecular chemistry
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