A Guiding Principle for Strengthening Crosslinked Polymers: Synthesis and Application of Mobility-Controlling Rotaxane Crosslinkers

Article abstract of DOI:10.1002/anie.201813439

Three component mobility controlling vinylic rotaxane crosslinkers with two radically polymerizable vinyl groups (RC_Rs) were synthesized to prove that the mobility of the components of the RC_Rs plays a crucial role in determining the properties of rotaxane-crosslinked polymers (RCPs). RC_Rs (R=H, Me, or Et) were obtained from living ring-opening polymerization. RCP_Et was prepared using RC_Et, which exhibits the lowest component mobility. The low component mobility is reflected in inferior mechanical strength and stretching ability in tensile stress tests compared to components with good (R=Me) and high (R=H) mobility. However, RCP_Et exhibited significantly higher stress and strain values than the corresponding covalently crosslinked polymers (CCP_Rs). These results indicate that a suitable component mobility substantially enhances the mechanical strength of RCPs. This behavior could serve as a guiding principle for the molecular design of advanced RCs.

Full text of DOI:10.1002/anie.201813439

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Accepted Article
Title: A Guiding Principle for Toughening Cross-Linked Polymers:
Synthesis and Application of Mobility-Controlling Rotaxane
Cross-Linkers
Authors: Jun Sawada, Daisuke Aoki, Hideyuki Otsuka, and Toshikazu
Takata
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813439
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A Guiding Principle for Toughening Cross-Linked Polymers:
Synthesis and Application of Mobility-Controlling Rotaxane
Cross-Linkers
Jun Sawada, Daisuke Aoki, Hideyuki Otsuka, and Toshikazu Takata*
Abstract: Three component mobility-controlling vinylic rotaxane
cross-linkers with two radically polymerizable vinyl groups (RC_Rs)
were synthesized to prove that the mobility of the components of the
RC_Rs plays a crucial role in determining the properties of rotaxane-
cross-linked polymers (RCPs). RC_Rs (R = H, Me, or Et) were
obtained from the living ring-opening polymerization of six-
membered cyclic carbonates with two substituents R at the 5
position using pseudorotaxane initiators with a terminal OH group on
the axle. A mixture of n-butyl acrylate and 0.5 mol% RC_R was
exposed to UV irradiation in the presence of a photoinitiator to afford
the corresponding RCPs (RCP_R; R = H, Me, or Et). RCP_Et was
prepared using RC_Et, which exhibits the lowest component mobility.
The low component mobility is reflected in inferior mechanical
strength and stretchability in tensile stress tests compared to
components with good (R = Me) and high (R = H) mobility. However,
RCP_Et exhibited significantly higher stress and strain values than
the corresponding covalently cross-linked polymers (CCP_Rs).
whereby an increasing mobile distance of the RC components
improves the mechanical properties of the RCPs and induces
unique properties such as high swellability, extensibility, and
desirable stress-relaxing ability.[9] As new methods for the
toughening of cross-linked polymers have been developed in
response to the increasing interest in RCPs, the genuine
significance of the component mobility of RCs to endow RCPs
with toughness should be clarified in order to firmly establish this
new concept as a molecular design guideline.
These results indicate that
a
suitable component mobility
substantially enhances the mechanical strength of RCPs. This
behavior could serve as a guiding principle for the molecular design
of advanced RCs.
Rotaxane-cross-linked polymers (RCPs) have attracted great
attention both from a fundamental and a practical perspective
due to the extraordinary functionality based on the mechanically
linked components with high mobility.[1-14] Ito et al. have reported
that cyclodextrin-based polyrotaxane hydrogels with rotaxane
cross-links show excellent swellability and extensibility.[15]
Related studies on such cross-linked cyclodextrin-based
polyrotaxanes also revealed intriguing properties that cannot be
attained by cross-linked polymers using covalent cross-
linkers.[16-21] Meanwhile, our group has reported a synthetic route
to rotaxane-cross-linked vinyl polymers using supramolecular
cross-linkers that consist of cyclodextrins and macromonomers.
This study has delivered very interesting results, especially with
respect to the importance of the use of small amounts of
rotaxane-based materials and the generality of vinyl polymers.[22-
23] We have also studied the synthesis and potential applications
of crown-ether-containing vinylic rotaxane cross-linkers (RCs)
for the synthesis of RCPs by radical polymerization in non-
aqueous systems. RCs endow polymers with high toughness,
Figure 1. Schematic illustration of the structures of rotaxane
cross-linkers (RCs) for the evaluation of the effect of (a) the
mobile distance, and (b) the mobility of the components on the
(c) synthesis of cross-linked polymers toughened by RCs.
We examined the effect of the component mobility of the RCs on
the physical properties of the RCPs in detail and discovered a
guiding principle for the design of RCs and RCPs (Figure 1): not
only the mobile distance, but also the mobility of the components
of the RCs determine the properties of the RCPs (vide infra).
For the evaluation of the effect of the mobility of the components
on the properties of the corresponding RCP_Rs, RC_Rs with
different thickness, which contain two vinyl groups in the two
components,
were
designed
and
synthesized.
Pseudo[2]rotaxane initiator (3), prepared in situ from an hydroxy-
terminated sec-ammonium-type axle (1) and a methacryloyl
group-tethering crown ether wheel (2), was subjected to a DPP-
catalyzed living ring-opening polymerization of six-membered
cyclic carbonates[24-25] that contain two R substituents at the 5
position (R = H, Me, or Et).[26] The polymerization was followed
by the addition of 3,5-dimethylphenyl isocyanate as a bulky end-
capping agent to afford a macromolecular [2]rotaxane that is
comprised of one wheel and a polymeric axle chain (4).
J Sawada, Dr. D. Aoki, Dr. H. Otsuka, and Dr. T. Takata
Department of Chemical Science and Engineering
Tokyo Institute of Technology
2-12-1, O-okayama, Meguro, Tokyo, Japan
ttakata@polymer.titech.ac.jp
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Scheme 1. (a) Synthesis of RCs and (b) structure of the covalent cross-linkers (CCs) synthesized for comparison.
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Treatment of 4 with 2-isocyanatoethyl methacrylate for the
modification of the ammonium group of the axle component
afforded high overall yields of the RC_Rs, which bear two
radically polymerizable vinyl groups. For comparison,
macromolecular covalent cross-linkers (CC_Rs; Scheme 1b)
that possess identical polymer chains, chain length, and two
vinyl groups were synthesized starting from 1,4-
benzenedimethanol instead of 3. The structures of the RC_Rs
and CC_Rs were determined using 1H NMR, GPC, MALDI-
TOF-MS, and DSC measurements (Figures S1-6). The degree
of polymerization (DPn) was determined by 1H NMR
spectroscopy. Table 1 summarizes the synthetic results for
these RC_Rs and CC_Rs. All cross-linkers possess a similar
degree of polymerization (DPn = 25) and molecular weight
(3.1–5.2 kDa; 1H NMR) as well as narrow molecular-weight
distribution (Mw/Mn = 1.06–1.15; GPC), suggesting a negligible
effect of the difference in molecular weight.
Scheme 2. Decomposition of macromolecular pseudo[2]rotaxanes that
contain unprotected terminals (pR_Rs) under N-acetylation conditions.
Cross-linked polymers were obtained from the polymerization
of n-butyl acrylate (BA) as the vinyl monomer in the presence
of a small amount of cross-linker (Scheme 3). Specifically, a
mixture of BA, DMF, RC_R or CC_R (0.5 mol%), and a
photoinitiator (IRGACURE® 500) was exposed for 5 min to UV
irradiation, before being left to stand overnight at room
temperature to afford an insoluble gel. The gel was
subsequently swollen and washed with chloroform and
methanol in order to remove unreacted and soluble materials,
before being dried in vacuo (12 h; 80 °C) to yield RCP_Rs and
CCP_Rs as elastic polymers in moderate yield (45−70%; Table
2, S2).
Table 1. Synthesis of cross-linkers (RC_Rs and CC_Rs)
Mn (GPC)
[kDa] [a]
Mn (NMR)
[kDa] [b]
[a]
[b]
Mw / Mn
DPn
Cross-linker
Yield [%]
RC_H
RC_Me
RC_Et
CC_H
87
86
5.1
4.8
4.2
7.7
7.2
7.1
1.12
1.13
1.15
1.06
1.08
1.07
25
25
25
26
25
24
3.8
4.5
5.2
3.1
3.6
4.2
88
quant.
quant.
89
CC_Me
CC_Et
[a] Determined by GPC (eluent: chloroform; polystyrene standards). [b]
Determined by 1H NMR spectroscopy.
Prior to the synthesis of the RCPs, the mobilities of the
components of the RC_Rs were evaluated based on the
decomposition behavior of the corresponding macromolecular
[2]rotxanes (pR_Rs)(Scheme 2), which contain unprotected
terminals. These were independently prepared by a protocol
similar to that for the formation of precursor 4 (Scheme 1a),
albeit that the end-capping reaction was not carried out.
Specifically, we were interested whether the decomposition of
pR_Rs occurs via an N-acetylation, which would offer valuable
information on whether the wheel component can move on the
axel component.[27] The decomposition of 6 was carried out
under this N-acetylation conditions by treating the ammonium
moiety of the axle with a mixture of acetic anhydride and
trimethylamine (TEA) in THF, which should remove the
attractive interaction between the components. The structures
of 6 and the N-acetylated products (pR_Rs) were characterized
by MALDI-TOF-MS measurements (Figures S7–9). In case of
pR_H and pR_Me, the respective wheel and axle components
were obtained as the decomposition products. In contrast,
pR_Et, did not afford any decomposition products, but only N-
and O-acetylated products. These results clearly suggest that
the wheel component of RC_H and RC_Me, whose R groups
are smaller than the cavity of the crown ether wheel, exhibit
sufficient mobility to freely move along their polymer axles,
while the wheel component of RC_Et exhibits significantly less
mobility, which renders moving along the axle component
impossible.
Scheme 3. Synthesis of cross-linked polymers (RCP_Rs and CCP_Rs) by
photo-induced radical polymerizations of butyl acrylate in the presence of 0.5
mol% of cross-linkers RC_Rs and CC_Rs, respectively.
To clarify the effect of the structure of the cross-linkers on their
mechanical properties, tensile stress tests were conducted
using polymer films of RCP_Rs and CCP_Rs. Figure 2 shows
the corresponding stress-strain curves and the results are
summarized in Table 2. Figure 2 indicates little difference in the
mechanical properties of the three CCP_Rs (for the expanded
curves, see Figure S10). This result seems reasonable, given
that it should be unlikely that the thickness difference in the
polymer backbone of the CCP_Rs, which combine two PBA
polymer chains, affects the mechanical properties. On the other
hand, a clear difference was observed for the three RCP_Rs.
In addition, all RCP_Rs showed much higher fracture energy
(both fracture stress and strain) compared to the CCP_Rs. This
big difference is in good agreement with previously reported
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Table 2. Mechanical properties of RCP_Rs and CCP_Rs.
results, which attributed said high fracture energy to stress
dispersion originating from the movable cross-link points, even
when only small amounts of RCs are used.[9] It was also
confirmed by cycle tensile tests of RCP_Rs, that there was no
hysteresis energy loss up to 300% strain with all RCPs (Figure
S11).
Tensile stress test
Yield [a]
[%]
Cross-linked
polymer
Young’s
modulus
[MPa] [b]
Fracture
energy
Fracture Fracture
strain
[%]
stress
[MPa]
[MJ / m3]
RCP_H
RCP_Me
RCP_Et
CCP_H
45
46
51
58
53
70
0.22 ± 0.04
0.22 ± 0.02
0.15 ± 0.02
0.34 ± 0.01
0.41 ± 0.00
0.40 ± 0.02
1289 ± 51
1351 ± 99
938 ± 9
2.91 ± 0.19
6.54 ± 0.95
1.77 ± 0.06
0.68 ± 0.04
0.63 ± 0.12
0.74 ± 0.07
14.2 ± 1.5
31.9 ± 6.5
5.8 ± 0.2
1.3 ± 0.1
0.9 ± 0.3
1.2 ± 0.2
Meanwhile, the mechanical properties of the RCP_Rs that
depend on the size of the substituent (Figure 2) can be
explained by the difference in mobility of the components of the
RC_Rs (Scheme 2), considering the comparable thermal
properties upon addition of the cross-linkers (Figure S12).
Especially the lower stretchability of RCP_Et compared to
RCP_H and RCP_Me coincides well with the limited
component mobility, i.e., the limited mobility area of the wheel
component on the axle component is caused by the bulky
diethyl substituent in RC_Et. This result agrees with our
previous work, in which RCPs that were prepared using RCs
with shorter mobility lengths showed lower extensibility than
RCPs from RCs with longer mobility lengths.[9] Although we
have no clear answer for the difference in mechanical
properties between RCP_Me and RCP_H at present, the
notion of “molecular friction” upon translation of the wheel
along the axle caused by the extension may explain it. The
movement of the components at the rotaxane-cross-link point
(RC_R) for a thicker axle (RCP_Me) should require more
energy than that for a thinner axle (RCP_H). This explanation
seems feasible, especially when considering that gem-
dimethyl-methylene-moiety-containing polymer chains or t-butyl
groups are complementary in size to the cavity of DB24C8.[4, 8]
357 ± 9
CCP_Me
CCP_Et
248 ± 56
306 ± 28
[a] Calculated based on weight. [b] Determined by the stress between 0 and
10% strain.
In summary, rotaxane cross-linkers (RC_Rs) that exhibit
different mobilities of their components on account of the
presence of axle components with different thicknesses were
synthesized and used for the synthesis of the corresponding
rotaxane-cross-linked polymers (RCP_Rs) to clarify the effect
of the RC_R component mobility on the mechanical properties
of the RCP_Rs. The mechanical properties of the RCP_Rs
depend on the mobility of the components at the cross-link
points, i.e., on the thickness of the axle component. The results
of this study clearly show that the length of the mobile area and
the mobility of the rotaxane components at the cross-link point
of the RCPs play a crucial role for the toughness of the polymer
upon cross-linking with a RC_R. Based on these results, it
should be possible to design and synthesize unprecedented
cross-linked polymers with rotaxane cross-link points.
Acknowledgements
This research was financially supported by a Core Research
for Evolutional Science and Technology (CREST) project from
the Japan Science and Technology Agency (JST) grant
number JPMJCR1522 and by JSPS KAKENHI grants
16K17910 and 16H00754.
Keywords: rotaxane • rotaxane-cross-linked polymer • vinyl
polymer • mechanical property • toughening
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