Healable network polymers bearing flexible poly(lauryl methacrylate) chains via thermo-reversible furan-maleimide diels–alder reaction

Article abstract of DOI:10.1002/pola.28677

A new ATRP initiator containing two furyl rings, namely, bis(furan-2-ylmethyl) 2-bromopentanedioate was synthesized starting from commercially available l-glutamic acid as a precursor. Well-defined bisfuryl-terminated poly(lauryl methacrylate) macromonomers with molecular weight and dispersity in the range 5000–12,000 g mol^?1 and 1.30–1.37, respectively, were synthesized employing the initiator by atom transfer radical polymerization (ATRP). Independently, 1,1′,1″-(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-2,5-dione) was synthesized as a tris-maleimide counterpart for furan-maleimide click reaction. Thermo-reversible network polymer bearing flexible poly(lauryl methacrylate; (PLMA) chains was obtained by furan-maleimide Diels–Alder click reaction of bisfuryl-terminated PLMA with 1,1′,1″-(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-2,5-dione). The prepared network polymer showed retro-Diels–Alder reaction in the temperature range 110–170 °C as determined from DSC analysis. The presence of low Tg (–40 °C) PLMA chains induced chain mobility to the network structure which led to the complete scratch healing of the coating at 60 °C in five days due to furan-maleimide adduct formation. The storage modulus of the network polymer was found to be 3.7 × 10⁴ Pa at the constant angular frequency of 5 rad/sec and strain of 0.5%. The regular reversal of storage (G′) and loss modulus (G″) was observed with repeated heating (40 to 110 °C) and cooling cycles (110 to 40 °C) at constant angular frequency and strain.

Full text of DOI:10.1002/pola.28677

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Healable Network Polymers Bearing Flexible Poly(Lauryl Methacrylate)
Chains via Thermo-Reversible Furan-Maleimide Diels–Alder Reaction
Sachin S. Patil, Arun Torris A. T., Prakash P. Wadgaonkar
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune,
Maharashtra 411008, India
Correspondence to: P. P. Wadgaonkar (E-mail: pp.wadgaonkar@ncl.res.in)
Received 26 March 2017; accepted 18 May 2017; published online 00 Month 2017
DOI: 10.1002/pola.28677
ABSTRACT: A new ATRP initiator containing two furyl rings,
namely, bis(furan-2-ylmethyl) 2-bromopentanedioate was syn-
thesized starting from commercially available L-glutamic acid
as determined from DSC analysis. The presence of low Tg (–40
8C) PLMA chains induced chain mobility to the network struc-
ture which led to the complete scratch healing of the coating
at 60 8C in five days due to furan-maleimide adduct formation.
The storage modulus of the network polymer was found to be
3.7 3 104 Pa at the constant angular frequency of 5 rad/sec and
strain of 0.5%. The regular reversal of storage (G0) and loss
modulus (G00) was observed with repeated heating (40 to 110
as
a precursor. Well-defined bisfuryl-terminated poly(lauryl
methacrylate) macromonomers with molecular weight and dis-
persity in the range 5000–12,000 g mol21 and 1.30–1.37, respec-
tively, were synthesized employing the initiator by atom
transfer radical polymerization (ATRP). Independently, 1,10,100-
(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-2,5-dione) was syn-
thesized as a tris-maleimide counterpart for furan-maleimide
click reaction. Thermo-reversible network polymer bearing flex-
ible poly(lauryl methacrylate; (PLMA) chains was obtained by
furan-maleimide Diels–Alder click reaction of bisfuryl-
terminated PLMA with 1,10,100-(nitrilotris(ethane-2,1-diyl))tris(1H-
pyrrole-2,5-dione). The prepared network polymer showed
retro-Diels–Alder reaction in the temperature range 110–170 8C
8C) and cooling cycles (110 to 40 8C) at constant angular fre-
C
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quency and strain. 2017 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: atom transfer radical polymerization; chain mobil-
ity; Diels–Alder polymers; furan-maleimide click reaction; mac-
romonomers; thermo-reversible network polymers
crosslinked counterparts. For example, the storage modulus
(G0) of a supramolecular network polymer based on metal-
coordination interactions between PNIPAAm with terpyridine
side groups and Mn12 ions has been reported to be lower
than 1000 Pa at a frequency of 5 rad/sec.25 Furthermore, a
supramolecular system based on urea hydrogen bonding
interactions exhibited the storage modulus of around 104 Pa
at 5 rad/sec.26
INTRODUCTION Vulcanization and peroxide curing are the
main industrial techniques for crosslinking of elastomers.
The irreversible nature of these crosslinks prevents their
reprocessability.1 The cleavage of polymer backbone is one
of the problems in thermal reprocessing of vulcanized and
peroxide cured rubbers.2 Considerable efforts have been
devoted by scientific community for obtaining polymeric
materials with reversible chemical bonding which can even-
tually be easily processed and recycled.3 Smart materials
based on healable polymers4,5 have contributed significantly
to the field of coatings6–8 and dielectrics.9 The response of
healable smart materials based on linear and crosslinked/
network polymers to temperature has been extensively stud-
ied.10–18 A range of smart materials based on supramolecular
chemistry involving hydrogen bonding, k–k stacking,
electrostatic-interactions, host–guest interactions, metal-
coordiation and hydrophobic interactions have been stud-
ied.19–24 However, most of the supramolecular systems have
been found to be mechanically weaker than the chemically
The combination of controlled polymerization methods such
as NMP, ATRP, RAFT, and ROP with different click reactions
such as thiol-ene, alkyne-azide, anthracene-maleimide, furan-
maleimide, and aldehyde-aminooxy have been widely
explored to obtain chemically crosslinked smart materi-
als.27,28 Of these, smart materials based on macromolecular
architectures obtained by the combination of controlled radi-
cal polymerization methods with furan-maleimide click reac-
tion have been considered to be of practical interest due to
their ease of accessability.29–31 The thermo-reversible nature
of furan-maleimide click reaction allows the material to heal
Additional Supporting Information may be found in the online version of this article.
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repeatedly without addition of any chemical reagent or cata-
lyst. Furthermore, this reaction is initiated by temperature
and the equilibrium can be changed by varying the tempera-
ture and is, therefore, the most frequently used reaction for
obtaining healable materials.32
equipped with Viscotek VE-1122 pump, two Viscotek
(T6000N, General Mixed) columns and Viscotek VE-3580 RI
detector and tetrahydrofuran as an eluent at a flow rate of
1 mL/min. Polymer concentration for GPC analysis was
5 mg/mL and narrow dispersity polystyrene(s) were used as
calibration standards. The detector and column temperatures
were set to 30 8C and the sample injection volume for GPC
analysis was fixed at 20 lL.
A range of linear and crosslinked/network polymeric systems
containing thermo-reversible furan-maleimide Diels–Alder
adducts have been reported so far.33–45 However, a limited
number of reports are available in the literature concerning
polymeric systems with network/crosslinked structure bearing
flexible chains.46,47 Such systems are expected to show healing
ability at relatively moderate temperatures due to the induced
chain mobility to the resulting structure which is an important
requirement for self-healing property of materials.6 Flexible
chains such as butyl, hexyl, and lauryl have been introduced in
the network/crosslinked polymers to impart better healing
ability.6,48,49 Of these, lauryl chains are the most flexible due to
their low Tg (< 0 8C) and are, therefore, of great interest for
the preparation of self-healing materials.47
Matrix assisted laser desorption ionisation—time of flight
(MALDI-TOF) spectra were recorded on AB-SCIEX TOF/TOF
5800 instrument with linear/reflector positive operating
method in tetrahydrofuran as a solvent using trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)
(10 mg/mL, 5 volume equivalent with polymer) as a matrix at
26 8C. The end functionality of polymers was determined by
spotting the mixture of 4 lL polymer sample (1 mg/mL) with
20 lL of DCTB (10 mg/mL) as matrix and 1 lL of sodium iodide
as an ionising agent (0.2 g/mL) on a metal target plate. Mass
range was selected on the basis of molecular weights previously
determined from 1H-NMR spectroscopy. Thermogravimetric
analysis (TGA) and differential scanning calorimetric (DSC)
analysis of polymer samples were carried out on STA 6000 (Per-
kinElmer) and DSC Q10 or Q100 TA instruments with refriger-
ated cooling system, respectively. The extent of crystallinity of
polymer samples was determined from MicroMax-007HF
(Rigaku) powder XRD machine with rotating anode X-ray gener-
ator. The progress of Diels–Alder and retro Diels–Alder reac-
tions was determined on Specord 210 Plus (analyticjena) UV-
Visible spectrophotometer using 0.1 cm path length. Scratch
healing experiments were performed using OLYMPUS BX60
microscope with OLYMPUS SC30 camera. The polymer films
were prepared with thickness of ꢀ 50 lm. The precursors were
dissolved in TCE, their films were cast on glass plate and heated
for 20 days. The thickness of films was determined from optical
profilometer. The visco-elastic behavior of network polymer
was studied on MCR-301 (Anton Paar) Rheometer using 25 mm
parallel plate geometry. Swelling measurements were carried
out by swelling 30 mg of crosslinked/network polymer in tolu-
ene at room temperature for 4 days to attain equilibrium.30 The
percentage degree of swelling (Q) of the network polymer was
calculated by the equation:50,51
We wish to report herein a network polymeric smart material
based on combination of ATRP and thermo-reversible furan-
maleimide Diels–Alder click reaction. A new ATRP initiator
containing two furyl rings viz. bis(furan-2-ylmethyl) 2-
bromopentanedioate was synthesized based on L-glutamic
acid as a starting material and was utilized to obtain bisfuryl-
terminated PLMA macromonomers. Thermo-reversible net-
work polymer bearing flexible PLMA chains was obtained by
furan-maleimide click reaction of bisfuryl-terminated PLMA
macromonomer with 1,10,100-(nitrilotris(ethane-2,1-diyl))tr-
is(1H-pyrrole-2,5-dione). The scratch healing study was per-
formed on the films prepared from network polymer which
showed the complete healing at 60 8C in five days, thanks to
the presence of flexible PLMA chains which induced the chain
mobility in these systems due to their low Tg (–40 8C).
EXPERIMENTAL
Materials
L-Glutamic acid (Thomas Baker, 99%), sodium nitrite (Loba
Chemie, 97%), sodium bromide (Thomas Baker, 99.5%), N, N0-
dicyclohexylcarbodiimide (DCC; Aldrich, 99%), 4-dimethylami-
nopyridine (DMAP) (Lancaster, 99%), copper bromide
(Aldrich, 99.99%), N,N,N0,N00,N"-pentamethyldiethylenetriamine
(Aldrich, 99%), tris(2-aminoethyl)amine (Aldrich, 96%) and
maleic anhydride (Aldrich, 99%) were used as received. Fur-
furyl alcohol were distilled before use. Lauryl methacrylate
(LMA) was stirred over calcium hydride and distilled under
reduced pressure just before use. Toluene and dichloromethane
were stirred over calcium hydride overnight and distilled.
ðWs2WdÞ
Q5
3100:
Wd
where Ws5weight of equilibrium swollen network polymer
and Wd5initial weight of dry polymer. The measurements
were carried out in triplicate and average value is reported.
Synthesis
Synthesis of 2-Bromopentanedioic Acid (1)
Characterisation and Measurements
Into a 500 mL round bottom flask equipped with a magnetic
stir bar and an addition funnel were charged L-glutamic acid
(20 g, 135.9 mmol), sodim bromide (27.97 g, 271.8 mmol),
and 2.5 M H2SO4 (177 mL). The reaction mixture was cooled
to 0 8C in an ice bath and the solution of sodium nitrite
(11.72 g, 169.9 mmol) in water (50 mL) was added drop-
wise while stirring over the period of 20 min. The reaction
FT-IR spectra were recorded on Perkin Elmer Spectrum GX
spectrophotometer using chloroform as a solvent. NMR spec-
tra were recorded on Bruker- 400 or 500 MHz spectrometer
in CDCl3 or tetrachloroethane-d2 as a solvent. Molecular
weight and dispersity values of PLMA macromonomers were
determined by gel permeation chromatography (GPC)
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TABLE 1 Reaction Parameters and Results of Synthesis of Bisfuryl-Terminated PLMA Macromonomers
d
e
Sr.
Time
(min)
Conv.
(%)b
Mn,NMR
Mn,GPC
(g mol21
)
(g mol21
)
Mw/Mn
Ieff f
a
c
No.
[M]0/[I]0
Mn,theo
1
2
3
18
25
50
30
50
65
86
94
74
4300
6300
9800
5000
7500
1.3
0.86
0.94
0.82
6700
9300
1.34
1.37
12,000
15,000
d
e
Temperature: 50 8C, solvent: toluene (1:1 w/w, w. r. t. monomer).
M
M
n,NMR 5 Determined by 1H NMR spectroscopy.
n,GPC 5 Determined by GPC; Polystyrene standard; Tetrahydrofuran
a
[M]0/[I]0: [Monomer]0/[Initiator]0 feed ratio.
Conversions were determined by 1H NMR spectroscopy.
b
as an eluent.
f
c
M
n,theo 5 {[M]0/[I]0 3 (% conv.)/100 3 mol. weight of monomer} 1 mol.
Ieff 5 Mn,theo/Mn,NMR
.
weight of initiator.
mixture was stirred at 0 8C until evolution of nitrogen gas
ceased. The progress of reaction was monitored by TLC. The
aqueous solution of urea (0.82 g, 13.6 mmol) was added to ter-
minate the reaction. The reaction mixture was extracted with
diethyl ether (3 3 100 mL). The combined organic layer was
evaporated and the crude product was recrystallized from eth-
anol to afford 11.47 g (40%) of 2-bromopentanedioic acid (1)
as a white solid (Melting Point- 40 8C).
Synthesis of Bisfuryl-Terminated PLMA Macromonomers
by ATRP
Into a 20 mL flame dried Schlenk tube fitted with a rubber
septum and an argon inlet were charged bis(furan-2-
ylmethyl) 2-bromopentanedioate (0.437 g, 1.179 mmol),
lauryl methacrylate (6 g, 23.58 mmol), PMDETA (0.204 g,
1.179 mmol), and toluene (6 g). The reaction mixture was
purged with argon for 10 min and then copper bromide
(0.169 g, 1.179 mmol) was added under the positive pres-
sure of argon. After three vaccum freeze pump-thaw cycles,
the Schlenk tube was immersed in an oil bath preheated at
50 8C and stirred for appropriate time under argon. The
polymerization reaction was quenched by sudden cooling
the Schlenk tube in liquid nitrogen with exposure to air.
The solvent was removed under reduced pressure. The per-
centage conversion of reaction was determined by 1H NMR
spectroscopy. Polymer was purified by dissolving in
dichloromethane and passing the solution through a small
bed of neutral alumina column to remove the copper salts,
washed with aqueous solution of disodium salt of EDTA,
concentrated and precipitated into cold methanol, filtered
and dried at 40 8C under reduced pressure to afford a thick
liquid.
À
Á
IR CHCl3; cm21 : 1714 ðC5OÞ:
1H-NMR (200 MHz, CDCl3, d/ppm): 9.38 (broad s, 2H), 4.42
(q, 1H), 2.64 (td, 2H), 2.20–2.55 (m, 2H).
13C-NMR (50 MHz, CDCl3, d/ppm): 178.52, 174.95, 43.97,
31.15, 29.11.
Synthesis of Bis(furan-2-ylmethyl)
2-bromopentanedioate (2)
Into a 250 mL round bottom flask equipped with a magnetic
stir bar, an argon inlet and an addition funnel were charged
1 (8 g, 37.91 mmol), furfuryl alcohol (8.93 g, 90.99 mmol),
DMAP (1.85 g, 15.64 mmol), and DMF (40 mL). The reaction
mixture was cooled to 0 8C in an ice bath and the solution of
DCC (18.77 g, 90.99 mmol) in DMF (30 mL) was added
dropwise while stirring over the period of 20 min. The reac-
tion mixture was stirred additionally for 1 h at 0 8C and at
room temperature for 15 h. The reaction mixture was fil-
tered and solvent was evaporated under reduced pressure.
The crude product was dissolved in dichloromehane and
washed with saturated NaHCO3 solution (2 3 100 mL) and
water (2 3 100 mL). The combined organic layer was evapo-
rated and the crude product was purified by silica gel column
chromatography using pet ether:ethyl acetate (80:20, v/v) as
an eluent to afford 11.25 g (80%) of bis(furan-2-ylmethyl) 2-
bromopentanedioate (2) as a colorless liquid.
À
Á
IR CHCl3; cm21 : 1730 ðC5OÞ:
1H-NMR (200 MHz, CDCl3, d/ppm): 7.36–7.40 (m, 2H), 6.27–
6.42 (m, 4H), 5.01 (s, 2H), 4.98 (broad s, 2H), 4.08 (t, 1H),
3.90 (broad s, CH2AO from PLMA), 2.32–2.6 (m, 2H), 2.12–
2.3 (m, 2H), 1.67–2.05 (m, ACH2 from PLMA backbone),
1.59 (broad peak, ACH2 from lauryl chain), 1.26 (broad
peak, (ACH2A)9 from lauryl chain and ACH3 from PLMA
backbone) and 0.87 (t, ACH3 from lauryl chain).
Synthesis of Thermo-Reversible Network Polymer with
Flexible PLMA Chains via Furan-Maleimide Diels–Alder
Click Reaction
À
Á
IR CHCl3; cm21 : 1738 ðC5OÞ and 1504 ðC5CÞ:
A flame dried 10 mL Schlenk tube fitted with a rubber sep-
tum and equipped with a magnetic stir bar was thoroughly
evacuated and to the Schlenk tube were charged bisfuryl-
terminated PLMA macromonomer (0.4 g, 0.1 molar solution
in TCE, Table 1, Entry-1, Mn,NMR-5000 g mol21, Dispersity-
1.3) and 1,10,100-(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-
2,5-dione) (30.9 mg, 0.1 molar solution in TCE). The reaction
mixture was purged with argon for 20 min, heated initially
1H-NMR (200 MHz, CDCl3, d/ppm): 7.40–7.44 (m, 2H), 6.33–
6.46 (m, 4H), 5.15 (s, 2H), 5.07 (s, 2H), 4.37 (q, 1H), 2.48–
2.59 (td, 2H), 2.15–2.46 (m, 2H).
13C-NMR (100 MHz, CDCl3, d/ppm): 171.62, 168.93, 149.15,
148.46, 143.54, 143.34, 111.27, 110.77, 110.63, 110.57,
59.43, 58.26, 44.47, 31.26, 29.55.
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SCHEME 1 Synthesis of bis(furan-2-ylmethyl) 2-bromopentanedioate (2).
RESULTS AND DISCUSSION
at 110 8C for 30 min and stirred for appropriate time at
60 8C under argon. The solvent was evaporated after comple-
tion of reaction and the insoluble product was dried at 50 8C
to obtain network polymer as a sticky solid. The progress of
furan-maleimide Diels–Alder click reaction was determined
from 1H-NMR spectroscopy and the final spectrum was
obtained just before the polymer sample started to precipi-
tate out from the reaction medium.
Design and Synthesis of Bis(furan-2-ylmethyl)
2-bromopentanedioate (2) ATRP Initiator
A new ATRP initiator containing two clickable furyl rings viz.
bis(furan-2-ylmethyl)
2-bromopentanedioate
(2)
was
designed and synthesized starting from L-glutamic acid-a
commercially available and an inexpensive starting material.
À
Á
The nucleophilic substitution of diazonium salt prepared
form L-glutamic acid with sodium bromide afforded 2-
bromopentanedioic acid (1).52 The complete shift of a quar-
IR CHCl3; cm21 : 1730 ðC5OÞ:
1H-NMR (500 MHz, TCE-d2, d/ppm): 7.3–7.5 (m, furyl ring),
6.67 (s, maleimide), 6.47–6.6 (m, 2H), 6.25–6.47 (m, 2H),
5.19 (broad peak, 2H), 5.0 (broad s, 4H), 4.06 (broad t, 1H),
3.90 (broad s, ACH2AO from PLMA), 3.48 (t, ACH2 adjacent
to maleimide), 3.44 (broad peak, 6H), 3.15–3.43 (m, 4H,
endo), 2.82–3.1 (m, 4H, exo), 2.66 (t, ACH2AN), 2.62 (broad
peak, 6H, ACH2AN), 2.32–2.5 (m, 2H), 2.15–2.32 (broad
peak, 2H), 1.68–2.05 (m, ACH2 from PLMA backbone), 1.59
(broad peak, ACH2A from lauryl chain), 1.25 (broad peak,
(ACH2)9 from lauryl chain and ACH3 from PLMA backbone),
0.87 (t, ACH3 from lauryl chain).
1
tet at 4.42 ppm in H NMR spectrum and the appearance of
a peak at 748 cm21 corresponding to C-Br stretching in FT-
IR spectrum revealed the formation of 1. The esterification
reaction of 1 with furfuryl alcohol in the presence of DCC
and DMAP resulted into the formation of bis(furan-2-
ylmethyl) 2-bromopentanedioate (2) (Scheme 1).
The formation of bis(furan-2-ylmethyl) 2-bromopentanedioate
was confirmed by FT-IR, NMR, and HR-MS spectroscopy. The
appearance of peaks at 1738 cm21 and 1504 cm21 corre-
sponding to carbonyl stretching of ester and C@C stretching of
FIGURE 1 1H NMR spectrum (in CDCl3) of bis(furan-2-ylmethyl) 2-bromopentanedioate (2).
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presence of copper bromide/PMDETA as the catalyt system
(Scheme 2).
The appearance of a peak at 1730 cm21 attributed to car-
bonyl group of PLMA in FT-IR spectrum (Fig. S4, Supporting
Information) and presence of furyl proton resonances at
their respective positions (multiplets in the range 7.36–7.40
ppm and 6.27–6.42 ppm) in 1H-NMR spectrum (Fig. 2) of
PLMA macromonomers attested the stability of furyl ring
under ATRP conditions.
SCHEME 2 Synthesis of bisfuryl-terminated PLMA macromo-
nomers by ATRP.
furyl rings, respectively, in FT-IR spectrum (Fig. S1, Supporting
Information) supported the formation of 2.
Well-defined bisfuryl-terminated PLMA macromonomers
with molecular weights and dispersity in the range 5000–
12,000 g mol21 and 1.3–1.37, respectively, were prepared by
varying monomer to initiator feed ratios (Table 1). Molecular
weights and dispersities of synthesized PLMA macromono-
mers were determined from 1H NMR spectroscopy and GPC.
Molecular weights of PLMA macromonomers were found to
be controlled and dispersities were found to be narrow
(Table 1).
The appearance of multiplets in the range 7.40–7.44 ppm
and 6.33–6.46 ppm corresponding to furyl ring protons in
1H NMR spectrum (Fig. 1) confirmed the formation of bis(-
furan-2-ylmethyl) 2-bromopentanedioate.
1
The remaining sets of protons in H-NMR spectrum and sets
of carbons in 13C-NMR spectrum (Fig. S2, Supporting Infor-
mation) were found to be in good agreement with the pro-
posed structure. Furthermore, the mass spectrum of 2
showed M/Z of 392.994, which exactly matched with the
theoretically calculated molecular weight (Fig. S3, Supporting
Information).
A typical GPC chromatogram of PLMA macromonomer (Table
1, Entry 1) (Fig. 3) was found to be symmetric and monomo-
dal which further supported the controlled polymerization
behavior.
Synthesis of Bisfuryl-Terminated PLMA Macromonomers
by ATRP and their Characterization
Bisfuryl-terminated PLMA macromonomers were synthesized
by ATRP of lauryl methacrylate employing bis(furan-2-
ylmethyl) 2-bromopentanedioate (2) as the initiator in the
The presence of furyl rings at the initiation end of polymer
chain was also determined by MALDI-TOF analysis (Fig. 4).
The observed molecular weight of line series of PLMA (Table
1, Entry 1) was found to be in good agreement with the
FIGURE 2 1H NMR spectrum (in CDCl3) of bisfuryl-terminated PLMA macromonomer (Table 1, Entry 1).
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SCHEME 3 Synthesis of thermo-reversible network polymer
bearing flexible PLMA chains via furan-maleimide click
reaction.
FIGURE 3 GPC trace of bisfuryl-terminated PLMA macromono-
mer (Table 1, Entry 1).
FIGURE 4 MALDI-TOF spectrum of bisfuryl-terminated PLMA macromonomer (Table 1, Entry 1). [Color figure can be viewed at
wileyonlinelibrary.com]
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FIGURE 5 The progress of Diels–Alder reaction (in TCE-d2, 0.1 M, 60 8C) as monitored by 1H-NMR spectroscopy. [Color figure can
be viewed at wileyonlinelibrary.com]
molecular weight calculated by taking into consideration the
molecular weight of initiator fragment.
macromonomer (Mn,NMR-5000 g mol21, Dispersity-1.3) and
1,10,100-(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-2,5-dione)
in TCE (Scheme 3). These reactions were performed at 60 8C
due to the faster rate of formation of furan-maleimide
adducts at this temperature.
Synthesis of Network Polymer Bearing Flexible PLMA
Chains via Furan-Maleimide Click Reaction and the
Progress of Furan-Maleimide Diels–Alder and
Retro-Diels–Alder Reaction
The network polymer bearing flexible PLMA chains was syn-
thesized by furan-maleimide Diels–Alder click reaction of
equimolar (0.1 M) amounts of bisfuryl-terminated PLMA
The progress of Diels–Alder and retro-Diels–Alder reaction
was determined by 1H-NMR spectroscopy. The equimolar
(0.1 M) amounts of bisfuryl-terminated PLMA and 1,10,100-
(nitrilotris(ethane-2,1-diyl))tris(1H-pyrrole-2,5-dione) in TCE-
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FIGURE 6 The progress of retro Diels–Alder reaction (in TCE-d2, 0.1 M, 110 8C) as monitored by 1H-NMR spectroscopy. [Color fig-
ure can be viewed at wileyonlinelibrary.com]
d2 were allowed to react at 60 8C in NMR tube. The progressive
decrease in integration of multiplets in the range 7.36–7.40
ppm and 6.27–6.42 ppm corresponding to furyl ring protons
and a singlet at 6.67 ppm corresponding to maleimide ring
protons was observed in 1H-NMR spectrum. The successive
increase in integration of multiplets in the range of 6.47–6.60
ppm and 6.25–6.47 ppm corresponding to double bonds of
Diels–Alder adduct (Fig. 5) confirmed the formation of furan-
maleimide Diels–Alder adduct.
The reaction mixture in NMR tube was heated at 110 8C to
determine the progress of retro-Diels–Alder reaction. The
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FIGURE 9 The reversible appearance (at 60 8C) and dissolution
(at 110 8C) of network polymer in TCE. [Color figure can be
viewed at wileyonlinelibrary.com]
296 nm corresponding to maleimide ring revealed the for-
mation of Diels–Alder adducts (Fig. 7).
FIGURE 7 The progress of Diels–Alder reaction (in TCE,
0.01 M, 60 8C) as monitored by UV spectroscopy. [Color figure
can be viewed at wileyonlinelibrary.com]
The same reaction mixture was heated at 110 8C and the
value of absorbance at 296 nm with time was obtained to
determine the regeneration of respective precursors by
retro-Diels–Alder reaction. The progressive increase in the
value of absorbance corresponding to maleimide rings with
time revealed the regeneration of respective precursors by
retro-Diels–Alder reaction (Fig. 8).
progressive increase in integration of multiplets and singlet cor-
responding to protons of furyl and maleimide rings, respec-
tively, (Fig. 6) with time attested the regeneration of bisfuryl-
terminated PLMA macromonomer and trismaleimide precursor.
The successive decrease in integration of multiplets correspond-
ing to olefinic protons of Diels–Alder adducts confirmed the
retro-Diels–Alder reaction. It should be noted that, the stereo-
chemistry of formed furan-maleimide adducts (endo and exo)53
may not affect the healing processes and hence determination
of their proportion was not of interest in the present studies.
The formation of network polymer on heating at 60 8C due to
the formation of furan-maleimide adduct and regeneration of
precursors on heating at 110 8C due to retro- Diels–Alder reac-
tion was also confirmed by simple visualization of solubility
behavior of polymer in the reaction medium, that is, TCE (Fig.
9). The swelling measurements on network polymer in toluene
under equilibrium conditions were carried out and the per-
centage degree of swelling was found to be 725.
The progress of formation of these Diels–Alder adducts and
regeneration of respective precursors by retro-Diels–Alder
reaction was also determined by UV spectroscopy. The reac-
tion of equimolar (0.01 M) amounts of these reactants in
TCE at 60 8C was performed and the absorbance at 296 nm
was obtained at different time intervals from UV spectros-
copy. The progressive decrease in the value of absorbance at
Thermal Properties of Bisfuryl-Terminated PLMA
Macromonomer and Network Polymer Bearing Flexible
PLMA Chains
The presence of furan-maleimide adducts confer the thermo-
reversible characteristics to the formed network polymer.
FIGURE 8 The progress of retro-Diels–Alder reaction (in TCE,
0.01 M, 110 8C) as monitored by UV spectroscopy. [Color figure
can be viewed at wileyonlinelibrary.com]
FIGURE 10 DSC trace of bisfuryl-terminated PLMA macromo-
nomer (Table 1, Entry 1). [Color figure can be viewed at
wileyonlinelibrary.com]
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FIGURE 14 The angular frequency dependence of storage and
FIGURE 11 DSC traces of repeated heating-cooling cycles of
network polymer bearing flexible PLMA chains. [Color figure
can be viewed at wileyonlinelibrary.com]
loss modulus of network polymer.
DSC analysis of PLMA macromonomer showed the glass
transition temperature (Tg) of 240 8C (Fig. 10) whereas that
of network polymer showed Tg of 244 8C (Fig. 11). The
decrease in Tg of network polymer by 4 8C was ascribed to
the introduction of tris-maleimide counterpart in the net-
work structure. In the repeated heating-cooling cycles in
DSC, endothermic transition from 100 to 170 8C was
observed which corresponds to retro-Diels–Alder reaction of
furan-maleimide adducts to regenerate the precursors.
TG curves of bisfuryl-terminated PLMA and network polymer
showed stability upto 280 8C and 295 8C, respectively, as
determined from T10 values. In both TG curves, weight loss
was observed in two stages. The initial weight loss was
observed due to decomposition of dodecyl chains followed
by decomposition of polymer backbone (Fig. 12).47
FIGURE 12 TG curves of A) bisfuryl-terminated PLMA macro-
monomer and B) network polymer. [Color figure can be viewed
at wileyonlinelibrary.com]
FIGURE 13 The percent strain dependence of storage and loss
modulus of network polymer. [Color figure can be viewed at
wileyonlinelibrary.com]
FIGURE 15 Thermo-reversibility of network polymer with
repeated heating and cooling cycles from 40 to 110 8C and 110
to 40 8C. [Color figure can be viewed at wileyonlinelibrary.com]
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indicated that the rheological behavior in network polymer
was dominated by an elastic rather than viscous property.
The angular frequency dependence of storage and loss mod-
ulus was also studied by performing the frequency sweep
experiment at constant strain (0.5%). The network polymer
showed the reversal of G0 and G00 at angular frequency of
around 125 rad/sec at 25 8C (Fig. 14).
From the experiments of strain and frequency sweep, the
values of angular frequency and strain were selected as 5
rad/sec and 0.5%, respectively, for temperature sweep
experiment with varying temperatures from 40 to 110 8C.
The thermo-reversibility of the prepared network polymer
bearing PLMA chains was determined by temperature sweep
experiments. The temperature sweep experiment was per-
formed with varying temperature from 40 to 110 8C and 110
to 40 8C. The repeated heating and cooling cycles were per-
formed on the network polymer. In these repeated cycles,
the regular reversal of storage (G0) and loss modulus (G00)
was observed with varying temperature from 40 to 110 8C
and 110 to 40 8C at constant angular frequency and strain
(Fig. 15). The initial values of storage (G0) and loss modulus
(G00) were regained after the completion of each cycle which
revealed the thermo-reversibility of network polymers. The
thermo-reversibility of the prepared network polymer was
demonstrated upto nine repeated cycles which confirmed its
recyclability and healability.
FIGURE 16 X-Ray diffraction patterns of A) bisfuryl-terminated
PLMA macromonomer and B) network polymer. [Color figure
can be viewed at wileyonlinelibrary.com]
The Visco-Elastic Behavior and Thermo-Reversibility of
Network Polymer Bearing Flexible PLMA Chains with
Varying Temperature
The rheological properties of network polymer were studied
to investigate the visco-elastic behavior. The linear region of
storage and loss modulus was selected by performing the
strain sweep experiment on the network polymer with
1 mm thickness (25 mm parallel plate geometry, 400 mg
sample) at 25 8C. The oscillations at constant angular fre-
quency (5 rad/sec) were applied with increasing strain (Fig.
13). It was observed that upto 3% strain the values of stor-
age and loss modulus were constant. The higher value of
storage modulus (G0) (3.7 3 104 Pa) than loss modulus (G00)
Crystallinity of Network Polymer Bearing PLMA Chains
The crystallinity of the network polymer was found to be
lower compared to that of the starting bisfuryl-terminated
PLMA macromonomer as indicated by X-ray diffractograms
FIGURE 17 The optical microscopy images of network polymer A) before scratch, B) after scratch, C) scratch after 4 days at 60 8C
and D) scratch after 5 days at 60 8C. [Color figure can be viewed at wileyonlinelibrary.com]
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REFERENCES AND NOTES
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