Synthesis of Self-Healing Polymers by Scandium-Catalyzed Copolymerization of Ethylene and Anisylpropylenes

Article abstract of DOI:10.1021/jacs.8b13316

Self-healing materials are of fundamental interest and practical importance. Herein we report the synthesis of a new class of self-healing materials, formed by the copolymerization of ethylene and anisyl-substituted propylenes using a sterically demanding half-sandwich scandium catalyst. The copolymerization proceeded in a controlled fashion, affording unique multi-block copolymers composed of relatively long alternating ethylene-alt-anisylpropylene sequences and short ethylene-ethylene units. By controlling the molecular weight and varying the anisyl substituents, a series of copolymers that show a wide range of glass-transition temperatures (T_g) and mechanical properties have been obtained. The copolymers with T_g below room temperature showed high elastic modulus, high toughness, and remarkable self-healability, being able to autonomously self-heal upon mechanical damage not only in a dry environment but also in water and aqueous acid and alkaline solutions, while those with T_g around or above room temperature exhibited excellent shape-memory property. The unique mechanical properties may be ascribed to the phase separation of the crystalline ethylene-ethylene nanodomains from the ethylene-alt-anisylpropylene matrix.

Full text of DOI:10.1021/jacs.8b13316

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Synthesis of Self-Healing Polymers by Scandium-Catalyzed
Copolymerization of Ethylene and Anisylpropylenes
Haobing Wang,†,∥ Yang Yang,†,∥ Masayoshi Nishiura,†,‡ Yuji Higaki,§,# Atsushi Takahara,§
and Zhaomin Hou*,†,‡
†Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198,
Japan
‡Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
§Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
S
* Supporting Information
ABSTRACT: Self-healing materials are of fundamental interest and
practical importance. Herein we report the synthesis of a new class of
self-healing materials, formed by the copolymerization of ethylene
and anisyl-substituted propylenes using a sterically demanding half-
sandwich scandium catalyst. The copolymerization proceeded in a
controlled fashion, affording unique multi-block copolymers
composed of relatively long alternating ethylene-alt-anisylpropylene
sequences and short ethylene−ethylene units. By controlling the
molecular weight and varying the anisyl substituents, a series of
copolymers that show a wide range of glass-transition temperatures
(Tg) and mechanical properties have been obtained. The copolymers
with Tg below room temperature showed high elastic modulus, high
toughness, and remarkable self-healability, being able to autono-
mously self-heal upon mechanical damage not only in a dry
environment but also in water and aqueous acid and alkaline solutions, while those with Tg around or above room
temperature exhibited excellent shape-memory property. The unique mechanical properties may be ascribed to the phase
separation of the crystalline ethylene−ethylene nanodomains from the ethylene-alt-anisylpropylene matrix.
polar functional olefins has received much recent attention, as
such copolymerization can, in principle, serve as an atom-
efficient and practical route for the synthesis of functionalized
polyolefins that may show improved beneficial properties.18−21
However, the copolymerization of ethylene and polar func-
tional olefins often suffers from a trade-off between copolymer
molecular weight and polar-monomer incorporation because of
the distinct difference in reactivity between these two types of
olefin monomers.21−36 Despite extensive studies and recent
advances in this area, it is still difficult to synthesize
functionalized polyolefins having both high molecular weight
and high polar-monomer content in a controllable fashion.
This has severely limited the exploration of well-defined,
functionalized polyolefins with potential for a wide range of
practical applications. An ethylene-based autonomous self-
healing polymer has remained unknown to date.37
INTRODUCTION
■
Materials capable of self-healing upon damage have received
tremendous attention over the past decades.1−3 Most of the
self-healing materials reported to date have mainly relied on
sophisticated designs and incorporation of chemical mecha-
nisms into polymer networks, such as irreversible or reversible
covalent bond formation,4−7 hydrogen bonding,7−12 metal−
ligand interaction,13,14 ionic interaction,15 and donor−acceptor
π−π stacking interaction.16 To achieve significant repair, the
input of an external energy or stimulus such as heat,4,5,16
light,14 pressure,8 or solvent15 is usually required, although a
few polymer materials were reported to show autonomous self-
healability with low toughness.6,9−13,15,17 The introduction of
phase-separated morphologies may also facilitate damage
repair,3,10,17 but self-healing materials based on such physical
mechanisms remained scarce. In consideration of practical
applications, it is highly important to develop tough,
autonomous self-healing materials that can be produced in
commercially relevant quantities and function in highly
variable, real-world situations, including acidic and basic
aqueous environments.
We have recently found that heteroatoms (such as oxygen
and sulfur) can serve as an efficient promoter for the
copolymerization of functional α-olefins with ethylene in the
presence of a rare-earth metal catalyst via a heteroatom-
Ethylene is the most widely used olefin monomer in the
Received: December 13, 2018
chemical industry. The copolymerization of ethylene with
© XXXX American Chemical Society
A
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a
Table 1. Scandium-Catalyzed Copolymerization of Ethylene (E) and Anisylpropylene (AP)
[AP]/
b
yield
c
AP conv
activity (g mol-Sc−1
Mn (×103
M /
wd
Tg
T
d
e
f
m f
run [Sc]
[Sc]
time
(g)
(%)
h−1 atm−1
)
g mol−1
5
)
Mn
AP/E
(°C)
(°C)
g
h
1
2
3
4
5
6
1
2
2
2
2
2
200/1
200/1
500/1
1000/1
2000/1
5000/1
10 min
15 min
5 min
15 min
6 h
0.20
0.70
0.91
1.61
3.05
8.35
67
91
95
85
84
92
−
1.65
1.68
1.58
1.94
1.70
1.98
100/0
60
−6
−4
4
5
6
150
124
123
127
125
g
1.4 × 105
1.1 × 106
6.4 × 105
5.1 × 104
3.5 × 104
41 (P1)
90 (P2)
173 (P3)
344 (P4)
552 (P5)
39/61
39/61
41/59
45/55
46/54
24 h
125
b
a
Conditions: [Sc] (0.01 mmol), [Ph3C][B(C6F5)4] (B) (0.01 mmol), ethylene (1 atm), 150 mL toluene, 20 °C, unless otherwise noted. Feed
ratio (in moles) of o-anisylpropylene (AP) and a scandium complex. Grams of the polymer product. Determined by gel permeation
chromatography (GPC) in o-dichlorobenzene at 140 °C against polystyrene standard. Mn = number-average molecular weight, Mw = weight-
average molecular weight. Molar ratio of o-anisylpropylene (AP) and ethylene (E) in the copolymer, determined by H nuclear magnetic
resonance (NMR) analysis. Determined by differential scanning calorimetry (DSC). [Sc] = [B] = 0.02 mmol, 50 mL toluene. Syndiotactic
homopolymer of AP.
c
d
e
1
f
g
h
assisted olefin polymerization (HOP) mechanism.36 Our
previous studies have also demonstrated that the ether group
in an anisole moiety can show an efficient interaction with a
rare-earth metal catalyst to promote otherwise difficult C−H
activation and transformation reactions.38−43 These findings
have promoted us to examine the copolymerization of ethylene
and anisyl-substituted propylenes by rare-earth catalysts.
Herein we report the copolymerization of ethylene and
anisylpropylenes by a sterically demanding half-sandwich
scandium catalyst, which has led to the formation of the
copolymers with both high molecular-weight and high polar-
monomer content in a controllable fashion. By controlling the
molecular weight and varying the anisyl substituents, we have
successfully synthesized a brand-new class of well-defined,
functionalized polyolefins ranging from soft viscoelastic
materials to tough elastomers and rigid plastics. The elastomer
copolymers showed high elastic modulus, high toughness and
remarkable self-healing property, which autonomously self-
healed upon mechanical damage not only in a dry environment
but also in water and aqueous acid and alkaline solutions
without the need for any external energy or stimulus. The
plastic copolymers demonstrated excellent shape-memory
property, approaching to 100% shape fixation and shape
recovery. It has been revealed that the formation of a multi-
block copolymer structure composed of relatively long
alternating ethylene−anisylpropylene sequences and short
ethylene−ethylene segments is critically important to show
the mechanical strength and self-healability. This is probably
due to phase separation to form crystalline nanodomains of the
ethylene−ethylene segments that may serve as the physical
cross-linking points in a flexible alternating ethylene−
anisylpropylene matrix.
2)44 with [Ph3C][B(C6F5)4] as a cocatalyst (Table 1). Under 1
atm of E and the AP monomer/catalyst feed ratio [AP]/[Sc] =
200/1 in toluene at room temperature, the C5H5-ligated
complex 1 did not give a copolymer product, but afforded the
AP homopolymer with high syndiotacticity (Table 1, run 1).
This is probably due to the high activity of 1 for the
homopolymerization of AP (see Table S1). By contrast, when
the sterically demanding C5Me4SiMe3-ligated complex 2 was
used in place of 1, the E−AP copolymer product (P1) was
formed exclusively (Table 1, run 2), although 2 was not
effective for the homopolymerization of AP under the similar
conditions due to steric hindrance (see Table S1). As the
[AP]/[2] feed ratio was raised from 200/1 to 500/1, 1000/1,
2000/1, and 5000/1 under 1 atm of E, the molecular weight
(Mn) of the resulting copolymers showed a pronounced
increase from 41 × 103 (P1) to 90 × 103 (P2), 173 × 103
(P3), 344 × 103 (P4), and 552 × 103 g mol−1 (P5),
respectively, while the incorporation ratio of the AP monomer
in the copolymers gradually increased from 39 mol% to 46 mol
% (Table 1, runs 2−6). In contrast with the efficient AP
incorporation, the copolymerization of ethylene with the
methoxy-free allylbenzene under the same conditions gave a
copolymer with only 2.5 mol% content of allylbenzene (Figure
S60). These results suggest that the methoxy group played a
highly important role in promoting the E−AP copolymeriza-
tion.
The 13C{1H} NMR analyses in C2D2Cl4 revealed that the
E−AP copolymer products possess multi-block microstruc-
tures that mainly consist of the unique alternating E-alt-AP
sequences (67−76%) and a smaller amount of the (E−E)n (n
≥ 1) segments (19−33%), while the AP−AP sequences were
almost negligible (<5%) (see Figures S26, S31, S42, S47, and
S52). The formation of the unique multi-block copolymers
containing relatively long E-alt-AP sequences and short E−E
units in the present copolymerization is probably due to the
unique affinity of the scandium ion in the catalyst for the anisyl
group in AP, which may significantly enhance the coordina-
tion/insertion of the AP monomer, as well as due to the steric
hindrance of the bulky C5Me4SiMe3 ligand in 2, which could
suppress the continuous insertion of the sterically demanding
RESULTS AND DISCUSSION
■
Copolymerization of Ethylene (E) and Anisylpropy-
lene (AP) by Half-Sandwich Scandium Catalysts. The
copolymerization of ethylene (E) with the commercially
available 3-(2-anisyl)-1-propylene (AP) was first examined by
using the half-sandwich scandium bis(aminobenzyl) complexes
bearing cyclopentadienyl ligands of different bulkiness (1 and
B
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a
Scheme 1. A Possible Scenario of Copolymerization of Ethylene (E) and Anisylpropylene (AP) by 2/[Ph3C][B(C6F5)4]
a
The counter anion [B(C6F5)4]− of the cationic scandium species is omitted for clarity.
Figure 1. Mechanical properties of E−AP copolymers P1−P5. (a) Stress−strain curves of P1−P5 measured in a speed of 200 mm min−1. (b)
Tensile strength/hysteresis curves of P5. Ten cycles at successive stretching with 1000% strain were performed. The inset shows the photographs of
a sample after 10 times of 1000% elongation and recovery (top) and its original form (bottom). (c) Tensile strength/hysteresis curves (the first
cycle) of an original sample of P5 (red) and that after being stretched to 1000% and then relaxed for 3 h (blue). (d) Summary of the mechanical
properties of P1−P5 measured at a speed of 200 mm min−1.
AP monomer but may allow the coordination/insertion of the
smaller ethylene monomer (see Scheme 1).36,45
Physical and Mechanical Properties of E−AP Copoly-
mers. The E−AP copolymers P1−P5 exhibited glass-
transition temperatures (Tg) in the range of −6 to 6 °C,
with weak melting points being observed in the range of 124−
127 °C in the differential scanning calorimetry (DSC) analysis
(Table 1). The copolymers can be melt-pressed into colorless,
transparent films with average transmittance in the visible
region up to 85% (Figure S107). The mechanical properties of
C
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Figure 2. Self-healing tests of P2 and P5. (a) Self-healing tests of P2 in air at 25 °C. (b) Self-healing tests of P5 in air at 25 °C. (c) Optical
microscope images of damaged (left) and repaired (right) samples of P5 in air at 25 °C. A film of P5 was cracked by using a razor blade and then
left in air for 5 min for healing. (d) Optical microscope images of damaged (left) and repaired (right) samples of P2 in water at 25 °C. A film of P2
was cracked by using a razor blade and then left in water for 5 min for healing. (e) Self-healing tests of P2 in water at 25 °C. (f) Self-healing tests of
P2 in water at 37 °C. (g) Comparative self-healing tests of P2 for 36 h healing in water (ii), 1 M HCl (iii), and 1 M NaOH (iv) at 25 °C.
the copolymers were significantly influenced by the molecular
weight (Figure 1a,d). The copolymer P1 with a relatively lower
Mn (41 × 103 g mol−1) behaved like a soft viscoelastic material,
showing stress softening after 600% elongation in a speed of
200 mm min−1 (Figure 1a, (i)). By contrast, the longer
polymers P2−P5 (Mn = 90 × 103 to 552 × 103 g mol−1)
exhibited the typical features of elastomers (Figure 1a,b;
Figures S112 and S113). The tensile strength and toughness
increased as the molecular weight became larger, reaching as
high as 10.2 megapascals (MPa) and 68.2 MJ m−3 (MJ =
megajoules), respectively in the case of P5. Similarly, the
Young’s modulus and elastic strain recovery of the copolymers
also increased with increasing molecular weight (Figure 1d). A
film sample of P5 showed 6% residual strain in the first cycle
and 9% residual strain in the tenth cycle of the stress−strain
tests with 1000% elongation (Figure 1b and Figure S113). A
fully recovered stress−strain curve was observed when the
sample was allowed to rest for 3 h after one cycle of 1000%
stretching and release (Figure 1c). These properties are in
sharp contrast with those of the homopolymers, both of which
are semi-crystalline plastics with much lower elasticity.
Self-Healing Properties of E−AP Copolymers P2−P5.
In addition to the excellent mechanical properties, the E−AP
copolymers P2−P5 showed remarkable self-healing properties.
When a film sample of P2 (sample size: 1 mm thickness, 2 mm
width, 12 mm length in the middle) was cut by a razor and
then brought together at room temperature, rapid repairing
was observed (see Movie 1 and Movie 2). After as short as 5
min, the repaired sample can be stretched to 1000% (ca. 50%
of its original value) (Figure 2a, (ii). Longer healing time led to
better repairing. After 120 h, the fractures were completely
repaired as evidenced by observation of a comparable
elongation with that of the pristine sample as well as breaking
at a new place rather than at the repaired place (Figure 2a,
(v)). In most cases, the stress−strain curves of the repaired
samples almost superposed those of the pristine materials with
difference only at the elongation at break (Figure 2a,b). The
higher molecular-weight, tougher polymer P5 needed a
relatively longer healing time, but reached 70% recovery of
its original tensile strength in 120 h at room temperature
(Figure 2b, (v)). The repaired sample P5 exhibited a tensile
strength as high as 6.7 MPa with 1520% elongation
(toughness: 34.6 MJ m−3) (see Table S4). When a film of
P5 was cracked by a razor blade and then left at room
temperature in air or in an N2-filled glovebox, no visible
damages were observed after 5 min (see Figure 2c). More
remarkably, such rapid autonomous self-healing took place
even in water (Figure 2d) and aqueous acid and alkaline
solutions (1 M HCl and 1 M NaOH) (Figure 2g). A repaired
film of P2 can be stretched to 1670%, which is ca. 80% of its
original value, after the completely cut pieces were repaired in
water at 25 °C for 96 h (Figure 2e, (iv)). When the sample was
cut and repaired at 37 °C (human body temperature) in water
for 24 h, a complete recovery was observed (Figure 2f, (iv)).
To gain information on the self-healing mechanism of the
present E−AP copolymers, we carried out several control
experiments. Treatment of P2 by BBr3 followed by hydrolysis
quantitatively converted the OMe group in P2 to OH (Scheme
S1 and Figures S98−S100). The HO-substituted polymer P2-
OH turned to a brittle material, which was insoluble even in
hot 1,2-dichlorobeneze. It showed a Tg at 34 °C, which is
much higher than that of the OMe analogue P2 (−4 °C).
These changes may be due to the presence of strong hydrogen-
bonding interactions in P2-OH. Replacement of the methyl
group of the anisyl unit in P2 with a triethylsilyl group was
quantitatively achieved by reaction with Et3SiH in the presence
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of a catalytic amount of B(C6F5)3 (Scheme S2). The resulting
Et3SiO-substituted polymer product P2-OSiEt3 exhibited a
much lower Tg (−25 °C) without showing a Tm or Tc in the
DSC profile (Figures S101−S103). In contrast to P2, the P2-
OSiEt3 analogue behaved like a stress softening material and
showed a much lower tensile strength (0.07 MPa) (Figure
S114), probably due to the lack of crystallinity (Figure S103).
To see the possible influence of the phenyl unit in P2, the
hydrogenolysis of P2 with H2 (80 atm) in the presence of Rh/
Al2O3 was carried out, which quantitatively afforded the
corresponding methoxycyclohexyl-substituted polymer product
P2-Cy (Scheme S3). The P2-Cy polymer showed a Tg at −7
°C, without a Tm or Tc being observed (Figures S104 and
S105). Similar to P2-OSiEt3 and in contrast to P2, the
cyclohexyl-substituted polymer P2-Cy behaved like a stress
softening material with low tensile strength (0.42 MPa)
(Figure S115). To gain information on the influence of the AP
content and distribution, the E−AP copolymers with lower AP
contents (or shorter alternating E-alt-AP sequences) were
examined. An E−AP copolymer P2′ with an AP content of 24
mol% (40 mol% E-alt-AP), which was synthesized under 15
atm ethylene pressure with the AP monomer/catalyst feed
ratio [AP]/[Sc] = 333/1 (Table S3, run 2), showed a
significant decrease in healing efficiency compared to that of
P2 (39 mol% AP and 67 mol% E-alt-AP) (Figure S116). A
further lower AP content copolymer P2″ (16 mol% AP, 25
mol% E-alt-AP) was synthesized under 18 atm ethylene
pressure with the AP monomer/catalyst feed ratio [AP]/[Sc]
= 167/1 (Table S3, run 3), which led to the complete loss of
healability (Figure S116). These results suggest that the
formation of a multi-block copolymer containing relatively
long E-alt-AP sequences, which could possibly form an
amorphous phase, together with shorter E−E segments
(which could form crystalline nanodomains) is highly
important to exhibit significant self-healability and toughness.
In order to gain information on the phase separation of the
E−AP copolymers, temperature-dependent wide-angle X-ray
diffraction (WAXD) of P5 was performed (Figure 3a). A
diffraction peak assigned to the (110) of the orthorhombic
crystallites was observed at 15.26 nm−1, while a shoulder peak
assigned to the broad (200) reflection appeared at approx-
imately 17 nm−1 in the temperature range of 25−120 °C
(below P5’s Tm = 125 °C). The diffraction peaks disappeared
at 150 °C, indicating the melting of the crystallites that might
result from the E−E segments.46 The crystalline domains of
the copolymer were further characterized by small-angle X-ray
scattering (SAXS). A broad scattering peak with a peak top at
ca. 0.283 nm−1 was observed (Figure 3b). The average domain
distance was estimated to be 22 nm according to the equation
d = 2π/q. An unstained transmission electron microscopy
(TEM) image of P5 was obtained by using an ultrathin film
supported on a 400-mesh carbon-coated copper grid, which
revealed a two-phase morphology with nanoscale crystalline
domains (possibly formed by aggregation of the E−E
segments) being dispersed in a continuous E-alt-AP matrix
(Figure S111). The temperature-dependent viscoelastic
properties were characterized by the dynamic mechanical
analysis (DMA, Figure 3c). A loss modulus (E″) peak at 20 °C
followed by a rubbery plateau was observed. The E−AP
copolymer films lost the rubber elasticity above 150 °C.
The above experimental results suggest that the short E−E
segments in the present E−AP copolymers could form a
number of tiny crystallites (PE), which may serve as physical
Figure 3. Characterization of multiphase morphology of P5. (a)
Wide-angle X-ray scattering (WAXD) profiles at different temper-
atures. (b) Small-angle X-ray scattering (SAXS) profile at 25 °C. (c)
Dynamic mechanical analysis (DMA) of P5. (d) Schematic
representation of micro phase separation.
cross-linking points to connect the flexible E-alt-AP segments,
thus enforcing elasticity and toughness (Figure 3d). Upon
mechanical damage, re-aggregation of the PE crystallites may
facilitate reconstruction of network structure of the flexible E-
alt-AP segments and accelerate damage closure.3,9,15 In the
case of P2-Cy and P2-OSiEt3, the cyclohexyl and ethyl groups
may show interactions with the E−E segments and interfere
with the formation of the crystalline PE domains, thus leading
to the disappearance of Tm and loss of tensile strength.
Synthesis and Mechanical and Self-Healing Proper-
ties of Ethylene (E) and Substituted Anisylpropylene
(ARP) Copolymers (E−ARP). To further examine the
influences of the anisyl moiety on the mechanical and self-
healing properties of the copolymers, we then synthesized a
series of E−ARP copolymers with different substituents (R) at
the phenyl ring (P6−P10) and with a methoxynaphthyl group
in place of the anisyl unit (P11) (Figure 4a and Table S2). The
methyl-substituted E−AMeP copolymer P6 showed much
higher initial modulus (140 MPa), tensile strength (17.7
MPa), and toughness (84.8 MJ m−3) (Figure 4b, i and Figure
4d) than those of the methyl-free analogue P5 (13.5 MPa, 10.2
MPa, and 68.2 MJ m−3, respectively) (Figure 1a (v) and d),
while its elongation at break (1270%) was shorter than that of
P5 (2050%). These results demonstrate that even a methyl
substituent at the phenyl ring can significantly influence the
mechanical properties of the resulting copolymers. A longer
hexyl substituent (P7) resulted in lowering of Tg (−28 °C)
and disappearance of Tm, and the resulting copolymer P7
behaved like a stress softening material (Figure 4b (ii) and
Figure S120), probably because of interactions between the
hexyl groups and the E-E segments. At low temperatures (−10
and 0 °C), P7 showed good elasticity (Figure S122). By
contrast, the tert-butyl-substituted copolymer P8 (Tg = 21 °C)
behaved as a flexible plastic at room temperature, displaying
ductility and strain hardening with the initial modulus as high
as 467.9 MPa (Figure 4c, (i)). At higher temperatures (40−45
°C), P8 also exhibited good elasticity (Figure S123). A
halogen substituent at the anisyl moiety also showed significant
E
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Figure 4. Synthesis and mechanical and self-healing properties of E−ARP copolymers P6−P11. (a) Synthesis and structures of P6−P11. (b)
Stress−strain curves of P6, P7, and P9, which behave as elastomers, measured at a speed of 200 mm min−1. (c) Stress−strain curves of P8 and P10,
which behave as flexible plastic, and P11, which behaves as hard plastic, measured at a speed of 200 mm min−1. (d) Summary of the mechanical
properties of P6−P11. (e) Self-healing tests of P6 in air at 25 °C. (f) Self-healing tests of P7 in air at 25 °C. (g) Self-healing tests of P9 in air at 25
°C.
influence on the mechanical properties of the copolymers. The
fluoro-substituted copolymer P9 (Tg = 4 °C) showed excellent
elasticity with initial modulus of 28.5 MPa, tensile strength of
16.6 MPa, elongation at break of 1450%, and toughness as high
as 72.4 MJ m−3 (Figure 4b, (iii). The chloro-substituted
analogue P10 (Tg = 18 °C) exhibited flexible plastic property,
which showed high initial modulus (218 MPa), moderate
elongation at break (954%), high tensile strength (22.7 MPa),
and high toughness (117.0 MJ m−3) (Figure 4c (ii) and d).
Compared to the anisyl analogue P5, the methoxynaphthyl-
substituted copolymer P11 showed a much higher Tg (47 °C)
and behaved as a rigid plastic at room temperature. Its initial
modulus (1222 MPa) and tensile strength (52.1 MPa) are
much higher than those of P5 along with a dramatic reduction
in strain at the break point (7%) (Figure 4c (iii) and d).
Regarding self-healing property, the hexyl-substituted
polymer P7 showed good self-healability at room temperature
(as well as at 0 °C, see Movie 3) albeit with low tensile
strength (Figure 4f and Figure S120). By contrast, the methyl-
and fluoro-substituted E−ARP copolymers P6 and P9 showed
both strong tensile strength and excellent self-healability
(Figure 4e,g). The completely cut samples of P6 and P9
showed 87% (1100% elongation) and 86% (1250% elonga-
tion) recoveries of their original elongation at break,
respectively, after self-repair at room temperature for 5 days.
The repaired samples showed tensile strength and toughness as
high as 12.6 MPa and 49.7 MJ m−3 in the case of P6 and 11.9
MPa and 47.2 MJ m−3 in the case of P9, respectively (Table
S4). These values are the highest ever reported for an
autonomously repaired material, and are even higher than most
of the pristine autonomous self-healing materials reported to
date.6,9−13,15,17 It is also worth noting that although the tert-
butylanisyl-substituted copolymer P8 behaved as a plastic at
room temperature, it exhibited good elastic property and self-
healability at higher temperatures (40−45 °C) (see Figure
S123 and Movie 4).
Shape-Memory Properties of E−AtBuP and E−ANaph
P
Copolymers. As mentioned above, the tert-butylanisyl-
substituted copolymer P8 is a flexible plastic and the
methoxynaphthyl-substituted copolymer P11 is a rigid plastic
at room temperature, but they both exhibited good elastic
properties at high temperatures. The thermally distinct
F
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plasticity and elasticity of these polymers offered excellent
flexibility in shape manipulations. P8 can be stretched and
deformed at 50 °C. The deformed shape can be fixed upon
cooling to room temperature (see Figure 5a,b). Remarkably,
copolymers can be ascribed to the phase separation of the
crystalline nanodomains of the ethylene−ethylene segments
that may serve as the physical cross-linking points in a flexible
ethylene-alt-anisylpropylene matrix. The unique and highly
tunable properties including high elastic modulus, high
toughness, exceptional self-healability, and excellent shape-
memory along with the ease of synthesis from readily available
staring materials may enable this series of brand-new
copolymers to find a wide range of practical applications in
the future. Moreover, this work may also help design further
new catalysts and new functional olefin monomers for the
synthesis of a variety of desired functional materials by
copolymerization with ethylene or other commodity olefin
monomers.
Figure 5. Shape memory property of P8. (a) An S-shaped sample
prepared by cutting a P8 film painted with black ink (permanent
shape at 20 °C). (b) The S-shaped sample was deformed at 50 °C
(Td) and the deformed temporary shape was fixed by cooling to 20
°C. (c) The deformed temporary shape was placed in a 50 °C water
bath. (d) The permanent (original) shape was recovered within 5 s in
the water bath at 50 °C (Tr). (e) Dual-shape memory cycle at Td = Tr
= 50 °C. Rf = 99.5%, Rr = 99.1%.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b13316.
■
S
Experimental details, methods, NMR spectra, and other
characterization, including Schemes S1−S3, Figures S1−
S126, and Tables S1−S4 (PDF)
Movie 1, fast self-healing of a film of P5 at room
temperature (AVI)
Movie 2, fast self-healing of a 5 mm thick piece of P2 at
room temperature (AVI)
Movie 3, fast self-healing of a film of P7 in ice water
(AVI)
when the fixed sample was returned to its deformed
temperature (50 °C), a full shape-recovery was observed
(see Figure 5c,d). The thermal mechanical analysis revealed
that P8 possesses excellent dual-shape memory property with
shape fixity (Rf) and shape recovery (Rr) as high as 99.5% and
99.1%, respectively, at the deformation and recovery temper-
ature of 50 °C (Figure 5e). Similarly, the methoxynaphthyl-
substituted copolymer P11 showed good shape deformation
and recovery performance at 80 °C (Figure S126).
Movie 4, slow self-healing of a film of P8 in air at room
temperature and fast healing in hot water (AVI)
AUTHOR INFORMATION
Corresponding Author
*houz@riken.jp
ORCID
Haobing Wang: 0000-0002-9104-1726
Yuji Higaki: 0000-0002-1032-4661
Atsushi Takahara: 0000-0002-0584-1525
Zhaomin Hou: 0000-0003-2841-5120
■
CONCLUSION
■
By using a sterically demanding half-sandwich scandium
catalyst such as 2, we have achieved for the first time the
controlled copolymerization of ethylene and anisyl-function-
alized propylenes. The copolymerization has led to the
formation of unique multi-block copolymers composed of
relatively long amorphous ethylene-alt-anisylpropylene sequen-
ces and short crystalline ethylene−ethylene segments. In
contrast to the homopolymers, which are semi-crystalline
plastics, the copolymer products showed high elasticity with
significant dependence on the molecular weight. The larger
molecular-weight copolymers exhibited higher tensile strength
and higher toughness, as demonstrated by P1−P5. Significant
influences of the anisyl substituents on the mechanical
properties have also been observed. By modifying the anisyl
moiety with different substituents, a series of copolymers
showing mechanical properties ranging from soft viscoelastic
materials (such as P7) to tough elastomers (such as P6 and
P9) and flexible and rigid plastics (such as P8, P10, and P11)
have been obtained. In addition to high toughness, the
copolymers with Tg lower than room temperature (such as
P2−P6 and P9) exhibited exceptional self-healing properties,
with the ability to autonomously self-heal from mechanical
damage not only in a dry environment but also in water and
aqueous acid and alkaline solutions without the need for any
external energy or stimulus. The copolymers with Tg around or
higher than room temperature (such as P8 and P11) exhibited
excellent shape-memory property with Rf and Rr approaching
to 100% due to their thermally distinct plasticity and elasticity.
The unique self-healing and shape-memory properties of these
Present Address
#Y.H.: Department of Integrated Science and Technology,
Faculty of Science and Technology, Oita University, 700
Dannoharu, Oita 870-1192, Japan
Author Contributions
∥H.W and Y.Y contributed equally to this work.
Notes
The authors declare the following competing financial
interest(s): H.W., M.N. and Z.H. are authors on a patent
application related to the synthesis of P1 to P5 filed by RIKEN
(application no. 2017-049096, filed on 14 March 2017). Y.Y.,
H.W., M.N. and Z.H. are authors on a patent application
related to the mechanical and self-healing properties of P1 to
P11 filed by RIKEN (application no. 2018-046829, filed on 14
March 2018).
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for
Scientific Research (S) (26220802), a Grant-in-Aid for
Scientific Research (C) (No. 17K05824), and a Grant-in-Aid
for Scientific Research on Innovative Areas (17H06451) from
JSPS and by the ImPACT Program of Council for Science,
G
DOI: 10.1021/jacs.8b13316
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Journal of the American Chemical Society
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