Star/linear polymer topology transformation facilitated by mechanical linking of polymer Chains

Article abstract of DOI:10.1002/anie.201500578

Topology transformation of a star polymer to a linear polymer is demonstrated for the first time. A three-armed star polymer possessing a mechanical linking of two polymer chains was synthesized by the living ring-opening polymerization of δ-valerolactone

Full text of DOI:10.1002/anie.201500578

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International Edition: DOI: 10.1002/anie.201500578
German Edition: DOI: 10.1002/ange.201500578
Topology Transformation
Star/Linear Polymer Topology Transformation Facilitated by
Mechanical Linking of Polymer Chains**
Daisuke Aoki, Satoshi Uchida, and Toshikazu Takata*
Abstract: Topology transformation of a star polymer to
a linear polymer is demonstrated for the first time. A three-
armed star polymer possessing a mechanical linking of two
polymer chains was synthesized by the living ring-opening
polymerization of d-valerolactone initiated by a pseudo[2]ro-
taxane having three hydroxy groups as the initiator sites on the
wheel component and at both axle termini. The polymerization
was followed by the propagation end-capping reaction with
a bulky isocyanate not only to prevent the wheel component
deslippage but also to introduce the urethane moiety at the axle
terminal. The resulting rotaxane-linked star polymer with
a fixed rotaxane linkage based on the ammonium/crown ether
interaction was subjected to N-acetylation of the ammonium
moiety, which liberated the components from the interaction to
move the wheel component to the urethane terminal as the
interaction site, eventually affording the linear polymer. The
physical property change caused by the present topology
transformation was confirmed by the hydrodynamic volume
and viscosity.
reconstruction using weak and reversible interactions to
change polymer properties.[8] Meanwhile, we have recently
reported[9] the synthesis of mechanically linked block/graft
copolymers as a shape-transformable polymer in which the
two polymer chains are tied by the rotaxane structure; one
polymer chain is threaded into the crown ether wheel to
which the other is connected. Because we have found that the
urethane moiety placed in the rotaxane axle has some
attractive interaction with the crown ether wheel,[9c] which is
consistent with Gibsonꢀs reports,[10] crown ether based rotax-
anes having both ammonium and urethane moieties can be
regarded as two-station-type molecular switches. Namely, the
rotaxane structure plays a key role as a controllable movable
linking unit, thereby being applicable to more sophisticated
transformable systems. This paper describes the synthesis of
a star polymer characterized by the rotaxane linking of the
polymer chains and its conversion to a linear polymer by
a topology transformation (Figure 1).
S
tructure or topology transformation and the resulting
property change often seen in molecular switches or machines
have attracted much attention because of their potential
applicability to the macroscopic change of appearance or
properties.[1] Because such a transformation is mainly limited
to small molecules, a topology-transformable polymer would
be attractive for bulk property control. In comparison with
linear polymers, corresponding topological polymers,[2] such
as star polymers,[3] cyclic polymers,[4] dendrimers,[5] and
hyperbranched polymers,[6] generally provide smaller hydro-
dynamic volume, lower solution viscosity, and less polymer
entanglement in the bulk. These properties are useful in solid
materials that can help not only in reducing solvent use as
sustainable products but also making the molding process
easy because of low melt viscosity.
Figure 1. Polymer topology transformation from star to linear shape
using a mechanically linked polymer system.
The rotaxane-linked star polymer (3-PVL_A) having
a polymer chain fixed at the center of another polymer
chain via the mechanical linkage by using the strong
interaction between the sec-ammonium moiety and crown
ether wheel was designed as shown in Scheme 1. To ensure
the synthesis of the well-defined three-armed star polymer in
which the arm polymer chains have the same degree of
polymerization (DP), a trifunctional pseudo[2]rotaxane ini-
tiator having three primary hydroxy groups as the initiating
sites (3-OH) was designed for living ring-opening polymeri-
zation (ROP) of a lactone. Diphenyl phosphate (DPP)-
catalyzed living ROP of d-valerolactone (d-VL) (ratio of [d-
VL]0/[initiator]0/[DPP]0 = 80/1.0/0.7) was carried out at ambi-
ent temperature. An excess amount of 3,5-dimethylphenyl
isocyanate was added to the polymerization mixture after 2 h
for both the propagation end-capping with the bulky stopper
However, most topological polymers[7] reported to date
are not transformable, because they are constructed by
covalent bonds. There are a few reports on topology
[*] Dr. D. Aoki, Dr. S. Uchida, Prof. Dr. T. Takata
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)
E-mail: ttakata@polymer.titech.ac.jp
[**] This research was financially supported by the ACT-C program of
JST, a Grant-in Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan (No.
26102512, 25410125, 26886006).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500578.
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as shown in Figure 2 and the Support-
ing Information, Figures S3 and S4. A
series of peaks with a clear interval of
100.05 Da corresponding to the molec-
ular weight of d-VL agreed well with
the structure of 3-PVL_A in Fig-
ure 2a. The result supported the prog-
ress of the DPP-catalyzed living ROP
and the subsequent propagation end-
cap to afford 3-PVL_A. Conversely,
a similar series of peaks with the same
m/z interval was recorded for 3-
PVL_U, as shown in Figure 2b. All of
the peaks were shifted to the higher m/
z region with the difference of 65 Da,
the sum of thr acetyl group and
sodium. It is obvious that the acetyla-
tion proceeded successfully to give 3-
PVL_U. The 1H NMR spectra of 3-
PVL_A and 3-PVL_U showed the
signals characteristic of their struc-
tures, including the typical split signals
of the methylene protons next to the
nitrogen group (Supporting Informa-
tion, Figure S4).
Scheme 1. Synthesis of the rotaxane-linked star polymer 3-PVL_A.
To answer the question as to
and the introduction of the urethane moiety at the polymer
end. The resulting mixture was purified by reprecipitation and
fractionation to afford the star polymer 3-PVL_A. The
ammonium moiety of 3-PVL_A was then N-acetylated with
an excess amount of acetic anhydride in the presence of
triethylamine to give the linear polymer 3-PVL_U
(Scheme 2). The formation of 3-PVL_U is supported by the
whether the wheel component having the polymer chain can
really translate from the center to the end of the axle polymer,
the following control experiment was carried out. A rotaxane-
linked star polymer having no bulky end-cap group, 3-PVL-
OH (Scheme 1), could be isolated by the direct precipitation
of the polymerization reaction mixture into an ethanol/
hexane mixed solvent (1:9 vol/vol). The MALDI-TOF-MS
result (Supporting Information,
Figure S5) supports well the
proposed structure of 3-PVL-
OH. The N-acetylation of 3-
PVL-OH under similar condi-
tions to that of 3-PVL_A was
performed.
The reaction mixture after
the acetylation was directly
subjected to MALDI-TOF-MS
1
and H NMR[11] measurements.
Two series of peaks with the
same
interval
of
about
100.05 Da were observed in
the MALDI-TOF-MS (Sup-
porting
Information,
Fig-
ure S5b); one series of peaks
corresponded to PVL with the
crown ether, while the other
corresponded to PVL with the
Scheme 2. Transformation of star polymer 3-PVL_A to linear polymer 3-PVL_U by N-acetylation.
fact that the interaction of the urethane linkage/crown ether is
stronger than that of the ester linkage/crown ether, and the
acetylated moiety no longer interacts with the crown ether
wheel.[10]
axle polymer. These results unambiguously showed that the
wheel component having a PVL chain could travel from the
ammonium moiety to the open end to afford the individual
PVLs by acetylation.
The chemical structures of 3-PVL_A and 3-PVL_U were
Figure 3 shows the GPC profiles of the products from 3-
PVL-OH by N-acetylation, along with 3-PVL_A and 3-
1
characterized by MALDI-TOF-MS and H NMR spectra,[11]
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PVL_U. A broad GPC peak of the products from 3-
PVL-OH observed at a later elution time (Mp
7.0 kDa) compared with those of 3-PVL_A and 3-
PVL_U also supported the dissociation of the 3-PVL-
OH fragments to two polymers, the axle polymer and
the polymer with the wheel. Focusing on the GPC
profiles of linear 3-PVL_U, the unimodal peak (Mp
12 kDa) was slightly shifted to an earlier elution time
region compared with that of star 3-PVL_A (Mp
11 kDa) without change in Mw/Mn, as shown in
Figure 3. It is known that the star polymers generally
show a smaller hydrodynamic volume compared with
the corresponding linear polymer with the same
molecular weight. Therefore, this peak shift indicates
an increase in hydrodynamic volume, that is, the
occurrence of the topology transformation from star
to linear shape. Since a similar result was obtained
with higher molecular weight arm-containing star
polymers 3-PVL_A150 and 3-PVL_U150 (Supporting
Information, Figure S6), it is obvious that the top-
ology transformation from star to linear shape caused
an increase in hydrodynamic volume.
GPC measurement is the most common and facile
method to evaluate the hydrodynamic volume for
various polymers. However, 3-PVL_A possesses the
ammonium moiety, and therefore an electrostatic
interaction with the GPC column or other effect
based on the ionic moiety cannot be ruled out. Thus,
a viscosity measurement was conducted to verify the
difference in hydrodynamic volume between 3-
PVL_A and 3-PVL_U.
Figure 2. MALDI-TOF-MS of a) 3-PVL-A and b) 3-PVL-U formed after N-acetyla-
To undertake the viscosity measurement, two
star- and linear-model polymers having covalently
connected polymer chains with the same molecular
weight were prepared,[11] named the
tion. Black: experimental, blue: theoretical values.
Star-Model (Mn 8.3 kDa by NMR)
and Linear-Model (8.5 kDa by
NMR), respectively (Figure 4).
Figure 4 shows the Huggins plots
of 3-PVL_A, 3-PVL_U, Star-Model,
and Linear-Model. In comparison of
the rotaxane-linked polymers with
the model polymers, the plots of the
star polymers (3-PVL_A and Star-
Model) and the linear polymers (3-
PVL_U and Linear-Model) were in
good agreement with each other,
whereas the intrinsic viscosity value
of the star polymers was lower than
those of the linear polymers, inde-
pendent of the presence of the am-
monium moiety; the results demon-
strated that the presence of the am-
monium group has no extra effect in
this case. Although the slope of the
Huggins plots of 3-PVL_U was
slightly different from that of Linear-
Model, which might be caused by the
effect of the rotaxane linkage, the
Figure 3. GPC profiles of a) acetylated 3-PVL-OH, b) 3-PVL_A, c) 3-PVL_U calibrated with polystyr-
ene standards (eluent CHCl3; flow rate: 0.85 mLminÀ1; detected by RI).
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Figure 4. Huggins plot of a) 3-PVL_A and Star-Model, b) 3-PVL_U and Linear-Model.
intrinsic viscosities [h] of 3-PVL_A and 3-PVL_U were very
similar to those of the model polymers. The g’ value, which is
defined as the ratio of the intrinsic viscosity of branched
polymer to linear polymer with the same molecular weight, is
generally used to theoretically evaluate the extent of branch-
ing. The g’ value of 3-PVL_A to 3-PVL_U was calculated as
g’ = 0.89, which agrees well with the reported value for the
covalently bonded three-armed star polymers.[12] This result
also demonstrates the actual topology transformation of the
rotaxane-linked polymers.
In conclusion, we have successfully achieved the synthesis
of rotaxane-linked star polymer and its topology transforma-
tion to the corresponding linear polymer, which is facilitated
by the movable junction generated by the rotaxane-linking of
the polymer chains. The trifunctional pseudo[2]rotaxane
initiator based on the ammonium/crown ether interaction
and the precise living ROP were the keys for the synthesis of
the rotaxane-linked star polymer. The N-acetylation of the
ammonium moiety caused the movement of the crown ether
wheel having a polymer chain along the axle polymer to reach
the urethane moiety at the w-end, leading to the topology
transformation. The topology transformation was reflected by
the physical property change in not only hydrodynamic
volume but also in viscosity. Thus, it has been demonstrated
that the present rotaxane system provides a useful method for
effective polymer topology transformation. The fusion of
rotaxane chemistry and polymer chemistry opens up further
new possibilities in material science.
Keywords: ring-opening polymerization · rotaxanes ·
star polymers · structure transformation · topology
How to cite: Angew. Chem. Int. Ed. 2015, 54, 6770–6774
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Received: January 21, 2015
Published online: April 20, 2015
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