Preparation of supramolecular extenders with precise chain lengths via iterative synthesis and their applications in polyurethane elastomers

Article abstract of DOI:10.1021/ma300815q

In this study, we synthesized a dual-functional building intermediate, 4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoyl chloride (DEDA-BC), from readily available starting materials, including 4-isocyanatobenzoyl chloride and p-tolyl isocyanate. In its iterative syntheses of hard segments, we first treated the highly reactive acid chloride of DEDA-BC with the monoamine (aniline) or the diamine (4,4′-methylenedianiline, 4,4′-MDA) to form first-generation azetidien-2,4-dione intermediates. We then reacted these derivatives with 4-aminobenzylamine at the more-selective azetidine-2,4-dione group of DEDA-BC to form the first-generation of benzyl amine extenders. Using this alternating method, we obtained high yields of supramolecular extenders of various chain lengths (n = 1-3) in a systematic manner, without the need for tedious purification steps, under catalyst-free conditions. The mono- and diamine extenders with numbers of repeating units ranging from one to three were synthesized precisely through this new iterative synthetic approach. The molar mass increases between each generation were 365 g mol^-1 for the monoamine series and 730 g mol^-1 for the diamine series. The three generations of supramolecular extenders possessed the distinctive characteristics of multiple hydrogen bonding moieties and narrow molecular weight distributions. Their gelation phenomena in THF revealed that these amine extenders underwent supramolecular assembly, through intermolecular hydrogen bonding, to form organogels. We used these well-defined extenders with various chain lengths in the preparation of polyurethane (PU) elastomers. Small-angle X-ray scattering revealed well-defined microdomains in the morphologies of the PU elastomers presenting multiply hydrogen-bonded terminal groups. The tensile and thermal properties of the prepared PUs were dependent on the effects of the content of hard segments, the chain length, and the strength of hydrogen bonding.

Full text of DOI:10.1021/ma300815q

Article
pubs.acs.org/Macromolecules
Preparation of Supramolecular Extenders with Precise Chain Lengths
via Iterative Synthesis and Their Applications in Polyurethane
Elastomers
Ming-Chieh Kuo,† Shi-Min Shau,† Je-Min Su,† Ru-Jong Jeng,*,‡ Tzong-Yuan Juang,*,§
and Shenghong A. Dai*,†
†Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
‡Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
§Department of Applied Chemistry, National Chiayi University, Chiayi, 60004, Taiwan
S
* Supporting Information
ABSTRACT: In this study, we synthesized a dual-functional building
intermediate, 4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoyl chloride (DEDA-
BC), from readily available starting materials, including 4-isocyanatobenzoyl
chloride and p-tolyl isocyanate. In its iterative syntheses of hard segments, we
first treated the highly reactive acid chloride of DEDA-BC with the
monoamine (aniline) or the diamine (4,4′-methylenedianiline, 4,4′-MDA) to
form first-generation azetidien-2,4-dione intermediates. We then reacted
these derivatives with 4-aminobenzylamine at the more-selective azetidine-
2,4-dione group of DEDA-BC to form the first-generation of benzyl amine
extenders. Using this alternating method, we obtained high yields of
supramolecular extenders of various chain lengths (n = 1−3) in a systematic manner, without the need for tedious purification
steps, under catalyst-free conditions. The mono- and diamine extenders with numbers of repeating units ranging from one to
three were synthesized precisely through this new iterative synthetic approach. The molar mass increases between each
generation were 365 g mol−1 for the monoamine series and 730 g mol−1 for the diamine series. The three generations of
supramolecular extenders possessed the distinctive characteristics of multiple hydrogen bonding moieties and narrow molecular
weight distributions. Their gelation phenomena in THF revealed that these amine extenders underwent supramolecular
assembly, through intermolecular hydrogen bonding, to form organogels. We used these well-defined extenders with various
chain lengths in the preparation of polyurethane (PU) elastomers. Small-angle X-ray scattering revealed well-defined
microdomains in the morphologies of the PU elastomers presenting multiply hydrogen-bonded terminal groups. The tensile and
thermal properties of the prepared PUs were dependent on the effects of the content of hard segments, the chain length, and the
strength of hydrogen bonding.
extent and pattern of hydrogen bonding. In general, thermo-
reversible PU elastomers consisting of alternating soft and rigid
(e.g., urethane, urea) segments are classified as thermoplastic
PUs. The hard domains act as thermo-reversible physical cross-
links, providing the characteristic thermoplastic and elastomeric
behavior of the polymer. External thermal stimuli can alter the
thermodynamic equilibrium, offering a route toward functional
supramolecular networks that can be exploited to attain the
thermo-reversible microphase-separated morphologies respon-
sible for the characteristic viscoelastic mechanical properties of
PUs. These supramolecular PUs may enable the production of
new thermoplastic elastomers, adhesives, and tunable polymeric
materials.
INTRODUCTION
■
The self-assembly of macromolecules into microphase patterns
is an interesting topic because relatively low molecular weight
supramolecular arrays are capable of spontaneously forming
ordered nanostructures.1,2 The versatility of supramolecular
assemblies offers the possibility to build a diverse range of
chemical architectures from amphiphiles,1 linear block copoly-
mers,3 and dendritic systems.4 The incorporation of multiple
hydrogen bonding units can improve a polymer’s properties,
including its melt rheology, phase separation behavior, and
thermal stability during melt processing.5 Van der Schuur et al.6
were the first to establish chain-extended polyurethane (PU)
elastomers rich in hydrogen-bonded amide and amine units.
They found that increasing the length of the amide chain
extender improved the thermal stability and tensile properties
of the PU. The Hayes group reported a series of thermo-
responsive microphase-separated supramolecular PUs7−11 in
which the degree of microphase separation, depending on the
nature of the terminal functional groups, was affected by the
Much research6,12−14 has revealed that the molecular weight
polydispersity (PD) and the length of the hard segments both
Received: April 23, 2012
Revised: June 11, 2012
Published: June 19, 2012
© 2012 American Chemical Society
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Scheme 1. Two Synthetic Routes Toward DEDA-BC, from (a) p-Tolyl Isocyanate (Route A) and (b) 4-Isocyanatobenzoyl
Chloride (Route B)
play significant roles in affecting the material properties of PUs,
due to the formation of different morphologies in the bulk
state. Reports of hard-segment intermediates with uniform
chain lengths are rare because of a lack of precise and efficient
methodologies for their synthesis. Harrell15 synthesized
monodisperse hard segments with uniform chain length, but
these compounds did not possess any hydrogen bonding sites
or rigid structures; nevertheless, narrowing the size distribution
of the hard segments did increase the modulus and tensile
strength of the PU. Harthcock and co-workers16 used a
cumbersome process, based on gel permeation chromatog-
raphy, to isolate individual extenders, of uniform chain length,
from a random mixture of products from the reaction of 1,4-
butanediol with 4,4′-methylene-p-diphenyl diisocyanate (MDI)
blocks; the critical hard-segment size required to initiate phase
separation was an MDI unit extended at least with two
butanediol moieties. Recently, Sijbesma et al.17 used a
protection/deprotection approach to prepare segmented poly-
(ether urea)s with hard segments of uniform chain length, but
these systems consisted mainly of aliphatic backbones; that is,
they differed vastly from the majority of PUs prepared herein
from aromatic diisocyanate and malonamide/(urethane urea)
systems. Nakaname and co-workers18,19 also found that the
physical properties of materials reached their optimal perform-
ance when the weight ratio of the hard and soft segments was at
a specific optimal ratio; beyond this ratio, the properties of PU
elastomers do not necessarily rise as their hard-segment
contents increase.
Recently, we used the building block 4-isocyanato-4′-(3,3-
dimethyl-2,4-dioxoacetidino)diphenylmethane (IDD), which
exhibits selective reactivity, to synthesize a series of polyurea/
malonamide dendritic structures without the need for
protection/deprotection chemistry.20 The presence of strong
noncovalent interactions, particularly hydrogen bonds, in the
side-chain and end-capped dendritic PU elastomers improved
their mechanical properties significantly, especially with respect
to the shape memory effect.21−23 Iterative synthesis based on
two alternative addition reactions of the dual-functional
intermediate IDD allowed us to introduce three generations
of supramolecular extenders, with uniform chain lengths, into
thermo-reversible PUs.24 We prepared supramolecular pseudo-
triblock PUs having a hard segment content of approximately
41% by attaching extenders of uniform chain length to both
ends of NCO-terminated prepolymers. The increases in the
number of hydrogen bonding sites and aromatic units of the
chain extenders with narrow molecular weight distributions
resulted in more physical cross-linking in the hard segment
domains. The pseudotriblock system24 appeared to enhance the
mechanical properties of the elastomers more effectively than
did an approach using dendritic extenders.22,23 In addition, we
employed the dual-functional IDD building blocks to prepare a
series of specific polymer intermediates that we applied as
dendritic intercalating agents for clays,25−27 silicate-anchored
macroinitiators,28 multiply hydrogen-bonding epoxy reactive
modifiers,29 and polymalonamide elastomers.30
In this present study, we synthesized a new dual-functional
building intermediate, 4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)-
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benzoyl chloride (DEDA-BC), from readily available starting
materials, including 4-isocyanatobenzoyl chloride and p-tolyl
isocyanate. The synthesized DEDA-BC possesses selective
reactivity, featuring both an azetidine-2,4-dione group and a
highly active acid chloride group in the same molecule. An acid
chloride is more reactive than an isocyanate, converting an
amino group into a stable amide within seconds. Moreover, we
prepared DEDA-BC directly from 4-isocyanatobenzoyl chloride
by diethyl ketene cycloaddition. Alternatively, an efficient
synthetic scheme (see Scheme 1a) based on p-tolyl isocyanate
in three steps involving cycloaddition, oxidation and chlorina-
tion has also been developed. The intermediate DEDA-BC is
superior to IDD for the systematic preparation, through
iterative synthesis, of well-defined amine extenders allowing
multiple hydrogen bonding supramolecular interactions (so-
called “supramolecular extenders”).24 The iterative processes
investigated in this study allow rapid and efficient syntheses
without the need for complicated separations. The resulting
supramolecular amine extenders increase the number of
hydrogen bonding sites and the molecular length while
narrowing the molecular weight distribution (i.e., monodis-
persion). Indeed, we synthesized both mono- and di-functional
amine extenders using an iterative strategy (Figure 1) and then
Through efficient and iterative syntheses, we have established
the structure−property relationships of elastomeric PUs in
terms of the impacts of different hard segments, with precise
chain lengths, on phase separation, physical properties, and
domain sizes.
EXPERIMENTAL SECTION
■
Materials. 4-Isocyanatobenzoyl chloride, 2-ethylbutyryl chloride,
triethylamine, and aniline were obtained from Acros Organics; 4-
aminobenzylamine was obtained from Aldrich. Liquid reagents were
further purified through distillation prior to use. The polyether diol
PTMO 2000, with an average of approximately 27 repeating
tetramethylene ether groups, was obtained commercially from DuPont
(Terathane); it had an average molecular weight of 2058 g mol−1,
calculated based on the hydroxyl number of 54.5 mg of KOH g−1 (as
determined by ASTM D4274 and ASTM E222); it was dried under
vacuum at 80 °C for 6 h prior to use.
Measurements. NMR spectra were recorded using Varian Unity
Inova-600 and Gemini-200 FT-NMR depending on sample’s
solubility. Thermal analyses of the polymers were performed under
a N2 atmosphere through differential scanning calorimeter (DSC)
using a Seiko SII Model SSC/5200 operated at heating and cooling
rates of 10 °C min−1, holding for 5 and 3 min at +180 and −100 °C,
respectively. Thermogravimetric analysis (TGA) was performed using
a Seiko SII Model SSC/5200 operated at a heating rate of 10 °C
min−1. Fourier transform infrared (FTIR) spectra were recorded over
the range from 4000 to 400 cm−1 using a Perkin-Elmer Spectrum One
FTIR spectrometer. Melting points (mp) of the samples were
measured using a Fargo MP-2D melting point apparatus operated at
a heating rate of 3 °C min−1. Gel permeation chromatography (GPC)
of the polymer materials and supramolecular extenders was performed
using either Shodex GPC KD-800 (802, 803, and 806M) columns and
N,N-dimethylformamide (DMF) as the mobile phase or Viscotek
viscoGEL (I-MBLMW-3078 and I-MBHMW-3078) columns and N-
methyl-2-pyrrolidine (NMP) as the mobile phase. The samples were
analyzed using a Shodex RI-101 GPC operated at a flow rate of 1.0 mL
min−1 with polystyrene calibration over the molecular weight range
from 682 to 1 670 000 g mol−1. Elemental CHN analysis was
performed using an Elementar vario EL III system. Fast atom
bombardment (FAB) and electron ionization (EI) mass spectra were
recorded using a Finnigan/Thermo Quest MAT 95XL apparatus. The
mechanical behavior of the materials was measured at room
temperature using a universal testing machine (HT-8504; Hung Ta
Instruments, Taiwan) operated at a crosshead speed of 50 mm min−1.
Dumbbell-shaped film samples were prepared having a gauge section
of 20 mm (L) × 5 mm (W) × 0.4−0.5 mm (H). Small-angle X-ray
scattering (SAXS) and wide-angle X-ray diffraction (WXRD) data
were recorded using Rigaku D/MAX-2500 and D/MAX-2200PC
instruments equipped with a rotating anode X-ray tube, with CuK
radiation (wavelength: 1.54 Å), operated at 40 kV and 30 mA.
Notation. 4-(3,3-Diethyl-2,4-dioxoazetidin-1-yl)benzoyl chloride is
abbreviated herein as “DEDA-BC.” The monoamine extenders and
their azetidine-2,4-dione intermediates are assigned as “MG0n0” and
“MG0(n−1)5,” respectively, where n represents the number of
repeating units of the extender; for example, the extender featuring
just one repeating unit is MG010, while its corresponding azetidine-
2,4-dione intermediate is MG005. Similarly, the difunctional amine
extenders and their intermediates are assigned as “DG0n0” and
“DG0(n−1)5,” respectively, where n represents the number of
repeating units of the extender; for example, the extender featuring
just one repeating unit is DG010 and its corresponding intermediate is
DG005. The “pseudo-triblock” systems for the MHB PUs were
prepared by attaching MG-series monoamine extenders to both ends
of the NCO-terminated prepolymers; they comprise the “M-series”
PUs. Correspondingly, the thermal elastomeric PUs consisting of DG-
series diamine extenders are designated as “D-series” PUs.
Furthermore, 1,4-butanediol was employed in control experiments
for the D-series PUs.
Figure 1. Iterative syntheses of the MHB (a) monofunctional MG
series and (b) difunctional DG series of extenders.
embedded them into PUs featuring precisely sized hard
segments (Figure S1, Supporting Information). We used the
well-defined monoamine extenders as end-capping agents to
react with the isocyanate-terminated prepolymers, forming
supramolecular “pseudo-triblock” A−B−A PU systems. The
design of these PUs terminated with multiple hydrogen
bonding sites, abbreviated herein as “MHB PUs” (Figure S1a,
Supporting Information), has been adopted in previous
studies.7−11,24 On the other hand, we also prepared conven-
tional thermal elastomeric PUs by extending the isocyanate-
terminated prepolymer using diamine extenders (Figure S1b,
Supporting Information). Structurally, these designed extenders
exhibited large numbers of rigid amide units and aromatic rings,
potentially making them more incompatible with the polyether
soft segments of resulting PUs. On the basis of experimental
results obtained from thermal analyses, morphological studies,
and tensile measurements, herein we describe the effects of the
supramolecular extenders on the hydrogen bonding−induced
phase separation and mechanical properties of PU elastomers.
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DEDA-BC. Two synthetic routes of DEDA-BC molecule were
employed, either directly from 4-isocyanatobenzoyl chloride (Scheme
1b; route B) or indirectly from p-tolyl isocyanate (Scheme 1a; route
A). Pure crystalline DEDA-BC was directly synthesized from 4-
isocyanatobenzoyl chloride with diethyl ketene via ketene/isocyanate
cycloaddition (route B).30 A solution of Et3N (10.1 g, 100 mmol) in
dry xylene (40 mL) was added dropwise to a solution of 4-
isocyanatobenzoyl chloride (5.00 g, 27.5 mmol) and 2-ethylbutyryl
chloride (7.50 g, 55.7 mmol) in dry xylene (50 mL) at 110 °C under
dry N2 over a period of 1 h. The resulting mixture was then heated
under reflux for another 2 h. After cooling to room temperature, the
precipitated salts were filtered off and the clear filtrate and washings
were concentrated under vacuum. Fine crystals (3.5 g, 45%; mp 107−
108 °C) were obtained after recrystallization of the residue from dry
cyclohexane (50 mL). Finally, a solution of DEDA-BC (2.48 × 10−7
M) in cyclohexane was prepared for subsequent experiments. FTIR
(cm−1, cyclohexane): 1769 (CO of acyl chloride), 1748 and 1870
1.82 (q, 4H, CH2), 1.94 (q, 4H, CH2), 4.32 (d, CH2), 7.05 (t, 1H,
ArH), 7.25−7.36 (m, 4H, ArH), 7.66−7.86 (m, 8H, ArH), 7.94 (2H,
ArH), 8.07 (2H, ArH), 8.60 (t, 1H, NH), 10.14 (s, 1H, NH), 10.29 (s,
1H, NH), 10.60 (s, 1H, NH). Anal. Calcd for C41H43N5O6: C, 70.17;
H, 6.18; N, 9.98. Found: C, 69.57; H, 6.29; N, 9.98. FABMS: calcd, m/
z 701.32; found, 702.4. GPC (DMF): PD = 1.01; Mn = 6480 g mol−1.
MG020. A solution of 4-aminobenzylamine (0.38 g, 3.1 mmol) in
dry DMF (1 mL) was added slowly at room temperature to a solution
of MG015 (2.0 g, 2.8 mmol) in dry DMF (12 mL) under dry N2. After
1.5 h, all of the MG015 had been consumed [TLC; eluent: EtOAc/n-
hexane, 3:1 (v/v)]. The mixture was poured into water (300 mL) and
stirred for several minutes. The precipitate was purified through two
cycles of dissolving in DMF and precipitating from water. Collection
of the precipitate and drying under vacuum at 60 °C provided a white
powder (2.2 g, 94%). Mp 100−110 °C. FT-IR (cm−1): 3332 (−NH
1
stretch of amides), 1654 (−CO of amide groups). H NMR (400
MHz, DMSO-d6, δ): 0.67 (m, 12H, CH3), 1.91 (m, 8H, CH2), 4.16 (d,
2H, CH2), 4.31 (d, 2H, CH2), 4.92 (s, 2H, NH2), 6.44 (d, 2H, ArH),
6.93 (d, 2H, ArH), 7.04 (t, 1H, ArH), 7.23−7.34 (m, 4H, ArH), 7.66−
7.86 (m, 8H, ArH), 7.91 (t, 4H, ArH), 8.44 (t, 1H, NH), 8.58 (t, 1H,
NH), 10.09 (s, 1H, NH), 10.12 (s, 1H, NH), 10.59 (s, 1H, NH), 10.76
(s, 1H, NH). Anal. Calcd for C48H53N7O6: C, 69.97; H, 6.48; N, 11.90.
Found: C, 68.75; H, 6.78; N, 11.65. FABMS: calcd, m/z 823.41;
found, 825. MALDI−TOF−MS: m/z 846.5 [M + Na]+, 853.6 [M +
Na + Li]+. GPC (DMF): PD = 1.02; Mn = 4140 g mol−1.
1
(CO of azetidine-2,4-dione). H NMR (200 MHz, CDCl3, δ): 1.04
(t, 6H, CH3), 1.84 (q, 4H, CH2), 8.02 and 8.17 (AA′XX′, 4H, ArH).
EIMS: calcd, m/z 279.07; found, 279.2.
4-(3,3-Diethyl-2,4-dioxozazetidin-1-yl)-N-phenylbenzamide
[MG005]. The solution of DEDA-BC (2.48 × 10−7 M, 3.5 g, 12
mmol) in cyclohexane was added dropwise over 30 min to a solution
of aniline (1.2 g, 13 mmol) and Et3N (1.3 g, 13 mmol) in cyclohexane
(50 mL) at room temperature under dry N2. The precipitate was
filtered off; evaporation of the solvent provided a powder that was
partitioned between EtOAc and water (1:1, v/v). Concentration of the
organic phase provided a slightly yellow powder, which was dissolved
in THF (30 mL) and then added dropwise into cyclohexane (600
mL). After repetitive washing and drying in a vacuum oven at 60 °C, a
white powder (4.1 g, 98%) was obtained. Mp: 163−167 °C. FT-IR
(cm−1): 3310 (−NH stretch of amides), 1869 and 1740 (stretching
vibration of −CO of azetidine-2,4-diones), 1650 (-CO of amide
groups). 1H NMR (300 MHz, acetone-d6, δ): 1.03 (t, 6H, CH3), 1.87
(q, 4H, CH2), 7.09 (t, 1H, ArH), 7.33 (t, 2H, ArH), 7.82 (2H, ArH),
7.92 (2H, ArH), 8.12 (2H, ArH), 9.61 (s, 1H, NH). Anal. Calcd for
C20H20N2O3: C, 71.41; H, 5.99; N, 8.33. Found: C, 70.73; H, 5.90; N,
8.95. EIMS: calcd, m/z 336.15; found, 336.4. GPC (DMF): PD =
1.01; Mn = 1680 g mol−1.
MG025 and MG030 were prepared using similar procedures, but
with dry DMF instead of THF to improve the solubility.
MG025. Yield = 77%. Mp: 273−277 °C. FT-IR (cm−1): 3315
(−NH stretch of amides), 1870 and 1737 (stretching vibration of
−CO of azetidine-2,4- diones), 1654 (−CO of amide groups).
1H NMR (600 MHz, DMSO-d6, δ): 0.73 (tt, 12H, CH3), 0.97 (t, 6H,
CH3), 1.84 (q, 4H, CH2), 1.96 (qq, 8H, CH2), 4.34 (d, 4H, CH2), 7.07
(t, 1H, ArH), 7.26 (t, 4H, ArH), 7.33 (t, 2H, ArH), 7.68−8.10 (m,
20H, ArH), 8.59 (q, 2H, NH), 10.10 (s, 1H, NH), 10.12 (s, 1H, NH),
10.27 (s, 1H, NH), 10.61 (s, 1H, NH), 10.62 (s, 1H, NH). 13C NMR
(150 MHz, DMSO-d6, δ): 8.87, 8.92, 22.74, 27.26, 42.17, 58.62, 71.35,
118.72, 119.40, 120.20, 120.33, 123.47, 127.50, 127.56, 128.40, 128.43,
128.54, 129.14, 129.43, 129.47, 132.92, 134.66, 135.03, 135.33, 137.61,
137.91, 139.25, 141.59, 164.29, 164.69, 164.79, 171.26, 171.61, 172.56.
Anal. Calcd for C62H66N8O9: C, 69.77; H, 6.23; N, 10.50. Found: C,
69.34; H, 6.24; N, 10.37. FABMS: calcd, m/z 1066.50; found, 1067.
GPC (DMF): PD = 1.02; Mn = 6560 g mol−1.
N1-(4-Aminobenzyl)-2,2-diethyl-N3-(4-(phenylcarbamoyl)-
phenyl)malonamide [MG010]. A solution of 4-aminobenzylamine
(380 mg, 3.1 mmol) in dry DMF (1 mL) was added slowly at room
temperature to a solution of MG005 (1.0 g, 2.9 mmol) in dry DMF (6
mL) under dry N2. After 1 h, all of the MG005 had been consumed
[TLC; eluent = EtOAc/n-hexane, 3:2 (v/v)], so the mixture was
poured into water (150 mL) and stirred for several minutes. The
precipitate was purified through two cycles of dissolving in DMF and
precipitating from water. Collection and drying under vacuum at 60
°C provided a white powder (1.2 g, 91%). Mp 212−218 °C. FT-
IR(cm−1): 3330 (−NH stretch of amides), 1652 (-CO of amide
MG030. Yield = 92%. Mp: 151−158 °C. FT-IR (cm−1): 3325
1
(−NH stretch of amides), 1653 (-CO of amide groups). H NMR
(600 MHz, DMSO-d6, δ, Figure S2a, Supporting Information): 0.71
(m, 18H, CH3), 1.92 (m, 12H, CH2), 4.19 (d, 2H, CH2), 4.34 (d, 4H,
CH2), 4.92 (s, 2H, NH2), 6.47 (dd, 2H, ArH), 6.94 (d, 2H, ArH), 7.07
(t, 1H, ArH), 7.26 (d, 4H, ArH), 7.33 (t, 2H, ArH), 7.69 (dd, 4H,
ArH), 7.76−7.97 (m, 14H, ArH), 8.44 (t, 1H, NH), 8.59 (m, 1H,
NH), 10.10 (s, 1H, NH), 10.12 (s, 1H, NH), 10.62 (s, 1H, NH), 10.64
(s, 1H, NH), 10.79 (s, 1H, NH). 13C NMR (150 MHz, DMSO-d6, δ):
8.93, 8.99, 27.25, 27.33, 27.71, 42.19, 42.23, 58.48, 58.62, 113.64,
119.34, 119.41, 120.21, 120.34, 123.47, 126.38, 127.50, 128.21, 128.42,
128.54, 129.43, 129.47, 134.66, 137.91, 139.25, 141.52, 141.57, 141.59,
147.43, 164.69, 164.80, 171.27, 171.36, 172.45, 172.56, 172.58. Anal.
Calcd for C69H76N10O9: C, 69.68; H, 6.44; N, 11.78. Found: C, 68.11;
H, 7.44; N, 11.49. FABMS: calcd, m/z 1188.58; found, 1189.
MALDI−TOF−MS: m/z 1211.6 [M + Na]+ (Figure S3a, Supporting
Information). GPC (DMF): PD = 1.02; Mn = 8580 g mol−1.
1
groups). H NMR (300 MHz, DMSO-d6, δ): 0.68 (t, 6H, CH3), 1.93
(q, 4H, CH2), 4.17 (d, 2H, CH2), 4.94 (s, 2H, NH2), 6.45 (d, 2H,
ArH), 6.92 (d, 2H, ArH), 7.05 (t, 1H, ArH), 7.31 (t, 2H, ArH), 7.76
(4H, ArH), 7.93 (2H, ArH), 8.44 (t, 1H, NH), 10.13(s, 1H, NH),
10.78 (s, 1H, NH). Anal. Calcd for C27H30N4O3: C, 70.72; H, 6.59; N,
12.22. Found: C, 70.81; H, 6.57; N, 12.58. EIMS: calcd, m/z 458.23;
found, 458.6. GPC (DMF): PD = 1.01; Mn = 4020 g mol−1.
MG015. The solution of DEDA-BC (2.48 × 10−7 M; 3.5 g or 12
mmol) in cyclohexane was added dropwise at room temperature to a
solution of MG010 (5.0 g, 11 mmol) and Et3N (1.2 g, 12 mmol) in
dry THF (90 mL) under dry N2 over 30 min. The precipitate was
filtered off and the solvents evaporated. The residue was partitioned
between EtOAc and brine and then the organic phase was washed
sequentially with 0.5 N NaOH(aq) and 0.5 N HCl(aq). Evaporation of
the solvent provided a powder that was dissolved in DMF (30 mL)
and poured into H2O (1.200 L). The precipitate was collected to
provide a fine powder (7.3 g, 96%). Mp: 258−263 °C. FT-IR (cm−1):
3330 (−NH stretch of amides), 1870 and 1742 (stretching vibration of
−CO of azetidine-2,4-diones), 1650 (−CO of amide groups). 1H
NMR (300 MHz, DMSO-d6, δ): 0.70 (t, 6H, CH3), 0.95 (t, 6H, CH3),
3,3-Diethyl-1-p-tolylazetidine-2,4-dione (Compound I). An-
other synthesis of DEDA-BC molecule based on p-tolyl isocyanate in
three steps involving cycloaddition, oxidation and chlorination has
been prepared as follows (Scheme 1a; route A). A solution of Et3N
(60.8 g, 600 mmol in dry xylene (150 mL) was added dropwise over 4
h to a solution of p-tolyl isocyanate (20.0 g, 150 mmol) and 2-
ethylbutyryl chloride (40.4 g, 300 mmol) in xylene (170 mL) at 115
°C. The resulting mixture was then heated at 120 °C for another 49 h,
at which point FTIR spectroscopy revealed the complete disappear-
ance of the signal for the CNO groups (2270 cm−1). The reaction
mixture was cooled to −10 °C and filtered to remove the precipitated
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salt. The clear filtrate and washings were concentrated under vacuum.
Distillation under high vacuum at 78−80 °C provided 3,3-diethyl-1-p-
tolylazetidine-2,4-dione (28.16 g, 81%). FTIR (cm−1): 1851 and 1742
(symmetric and asymmetric stretching of azetidine-2,4-dione CO
units). 1H NMR (200 MHz, δ, DMSO-d6): 0.92 (t, 6H, CH3), 1.79 (q,
DG015. SOCl2 (9.11 g, 76.6 mmol) was added to a suspension of
4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoic acid (2.00 g, 7.70 mmol)
in cyclohexane (15 mL) at 60 °C under N2. After 2 h, a homogeneous
solution had formed, with all of the starting material having
disappeared [TLC; EtOAc/n-hexane, 1:4 (v/v)]. The solvents were
evaporated to provide a solid product (2.14 g), which was collected
and dissolved in dry methyl ethyl ketone (35 mL). A solution of
DG010 (2.77 g, 3.00 mmol) and Et3N (1.55 g, 15.3 mmol) in dry
methyl ethyl ketone (40 mL) was added slowly (overnight) into the
former solution at 0 °C. The precipitated salts were filtered off and the
solvent evaporated; the residue (4.31 g) was added into EtOH (200
mL) to remove impurities. The solid was collected and dried at 60 °C
under vacuum for 6 h to provide DG015 (3.83 g, 91%). Mp 184−185
°C. FT-IR (cm−1): 3330 (−NH stretch of amides), 1870 and 1742
(stretching vibration of −CO of azetidine-2,4-diones), 1650 (−C
O of amide groups). 1H NMR (300 MHz, DMSO-d6, δ): 0.73 (t, 12H,
CH3), 0.98 (t, 12H, CH3), 1.84 (q, 8H, CH2), 1.99 (q, CH2), 3.88 (s,
2H, ArCH2Ar), 4.34 (d, 4H, CH2), 7.20 (d, 4H, ArH), 7.28 (d, 4H,
ArH), 7.67 (4H, ArH), 7.70 (4H, ArH), 7.79 (4H, ArH), 7.86 (4H,
ArH), 7.96 (4H, ArH), 8.10 (4H, ArH), 8.60 (t, 2H, NH), 10.08 (s,
2H, NHCO), 10.27 (s, 2H, ArNH), 10.58 (s, 2H, NHCO). Anal.
Calcd for C83H86N10O12: C, 70.42; H, 6.12; N, 9.89. Found: C, 70.05;
H, 5.63; N, 9.98. EIMS: calcd, m/z 1414.5; found, 1415. GPC (NMP):
PD = 1.11; Mn = 2490 g mol−1.
4H, CH2), 2.31 (s, 3H, ArCH3), 7.27 and 7.56 (AA′XX′, 4H, ArH). 13
C
NMR (150 MHz, CDCl3, δ): 8.34, 20.70, 39.50, 71.11, 119.30, 129.88,
130.51, 136.63, 171.80. Anal. Calcd for C14H17NO2: C, 72.70; H, 7.41;
N, 6.06. Found: C, 72.19; H, 7.31; N, 5.61. EIMS: calcd, m/z 231.13;
found, 231.1.
4-(3,3-Diethyl-2,4-dioxoazetidin-1-yl)benzoic Acid (Com-
pound II). The autoxidation30 of 3,3-diethyl-1-p-tolylazetidine-2,4-
dione was conducted in a three-neck, round-bottom, Pyrex reaction
flask equipped with a magnetic stirrer bar, a gas dispersion fritted disk,
a snake-shaped condenser, and a thermometer under an atmosphere of
dry O2. 3,3-Diethyl-1-p-tolylazetidine-2,4-dione (10.0 g, 43 mmol) was
mixed with NaBr, Co(OAc)2, and Mn(OAc)2 (200 mg, 2 wt % each)
and dicumyl peroxide (400 mg, 4 wt %) in AcOH (110 mL). O2 was
introduced into the solution through a fritted disk. The solution was
stirred at 115 °C. After 8 h, 3,3-diethyl-1-p-tolylazetidine-2,4-dione
had been consumed completely [TLC; EtOAc/n-hexane, 1:4 (v/v)]
and FTIR spectroscopy revealed the appearance of new absorptions at
1730 and 2700−3700 cm−1 (CO2H). The mixture was poured into
water (800 mL) and then the solid was collected and dried to provide
4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoic acid (9.94 g, 88%). Mp:
144−145 °C. FTIR (cm−1): 1730 and 2700−3700 (CO and O−H
groups of carboxylic acid), 1851 and 1742 (symmetric and asymmetric
stretching of azetidine-2,4-dione CO groups). 1H NMR (200 MHz,
δ, DMSO-d6): 0.92 (t, 6H, CH3), 1.79 (q, 4H, CH2), 7.82 and 8.04
(AA′XX′, 4H, ArH), 13.18 (broad s, 1H, COOH). 13C NMR (150
MHz, CDCl3, δ): 8.81, 39.50, 71.36, 118.79.3, 128.65, 130.92, 136.56,
167.95, 171.54. Anal. Calcd for C14H15NO4: C, 64.36; H, 5.79; N, 5.36.
Found: C, 64.00; H, 5.28; N, 5.05. EIMS: calcd, m/z 261.1; found,
261.1.
More information on preparation and characterizations for
difunctional extenders from DG020 to DG030 were presented in
the Supporting Information.
MHB PUs. MHB PUs were prepared from NCO prepolymers that
had been end-capped with monoamine extenders (MG series) at both
ends. PUs of the same molar ratio were prepared from the same batch
of prepolymers to obtain consistent molecular weights. A general
procedure for preparing NCO prepolymers is described below. The
reaction was conducted under a dry N2 atmosphere in a three-neck,
round-bottom flask equipped with a magnetic stirrer, a snake-shaped
condenser, and a thermometer. The NCO prepolymer (20 wt % THF
solution) was prepared by reacting 4,4′-MDI (9.7 g, 39 mmol) with
PTMO 2000 (40 g, 19 mmol) in the presence of di-n-butyltin dilaurate
(T-12) at 60 °C for 2 h. The reactions were monitored for the
disappearance of the diol OH groups (3333 cm−1). In each case, a
stoichiometric amount of extenders dissolved in DMF (5 mL) was
added into the NCO prepolymer solution (9.3 g) and vigorously
stirred at room temperature for at least 30 min to ensure complete
consumption of the isocyanate. The resulting solution was cooled and
then poured into a Teflon mold to form polymer films for testing.
NCO prepolymers that had been capped with stoichiometric amounts
of aniline were prepared for comparison. Table 1 lists the molar ratios
used in each case, with detailed reaction conditions.
DG005. SOCl2 (22.7 g, 191 mmol) was added to a suspension of 4-
(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoic acid (5.00 g, 19.1 mmol) in
cyclohexane (40 mL) at 60 °C under N2. After 2 h, a homogeneous
solution had formed, with all of the starting material having been
consumed [TLC; EtOAc/n-hexane, 1:4 (v/v)]. Evaporation of the
solvents provided a solid (5.36 g), which was dissolved in dry methyl
ethyl ketone (50 mL). A solution of 4,4′-methylenedianiline (4,4′-
MDA) (1.94 g, 9.8 mmol) and Et3N (3.88 g, 38.3 mmol) in dry
methyl ethyl ketone (40 mL) was added slowly into the former
solution at 0 °C over 10 min. The precipitated salts were filtered off
and the solvent evaporated. The residue (5.73 g) was purified with
EtOH (80 mL) to remove impurities. Drying at 60 °C under vacuum
provided DG005 (5.50 g, 83%). Mp: 162−163 °C. FT-IR (cm−1):
3310 (−NH stretch of amides), 1869 and 1740 (stretching vibration of
1
−CO of azetidine-2,4-diones), 1650 (-CO of amide groups). H
NMR (200 MHz, DMSO-d6, δ): 0.95 (t, 12H, CH3), 1.82 (q, 8H,
CH2), 3.89 (s, 2H, ArCH2Ar), 7.19 (d, 4H, ArH), 7.67 (4H, ArH),
7.84 (4H, ArH), 8.07 (4H, ArH), 10.26 (s, 2H, NH). Anal. Calcd for
C41H40N4O6: C, 71.91; H, 5.89; N, 8.18. Found: C, 71.77; H, 5.83; N,
7.92. EIMS: calcd, m/z 684.3; found, 684.5. GPC (NMP): PD = 1.06;
Mn = 920 g mol−1.
Table 1. Compositions of MHB PUs with MG Series
Extenders as End-Capping Agents
4,4′-MDI/PTMO/MG
molar
ratio
hard-segment content
(%)
DG010. 4-Aminobenzylamine (2.16 g, 17.7 mmol) was added into
a solution of G005 (5.5 g, 8 mmol) in dry DMF (40 mL) at room
temperature (20−25 °C) under N2. After 30 min, DG005 had been
consumed [TLC; EtOAc/n-hexane, 2:3 (v/v)]. The solution was
poured slowly into water (900 mL) and then the precipitate was
filtered off and dried at 80 °C under vacuum to give G010 (6.87 g,
92%). Mp: 203−204 °C. FT-IR(cm−1): 3330 (−NH stretch of
amides), 1652 (−CO of amide groups). 1H NMR (200 MHz,
DMSO-d6, δ): 0.67 (t, 12H, CH3), 1.93 (q, 8H, CH2), 3.88 (s, 2H,
ArCH2Ar), 4.17 (d, 4H, CH2), 4.92 (s, 4H, NH2), 6.45 (d, 4H, ArH),
6.92 (d, 4H, ArH), 7.17 (d, 4H, ArH), 7.67 (4H, ArH), 7.74 (4H,
ArH), 7.92 (4H, ArH), 8.45 (t, 2H, NH), 10.09 (s, 2H, NH), 10.77 (s,
2H, NH). Anal. Calcd for C55H60N8O6: C, 71.10; H, 6.51; N, 12.06.
Found: C, 71.06; H, 5.96; N, 11.93. EIMS: calcd, m/z 928.5; found,
929. GPC (NMP): PD = 1.10; Mn = 2070 g mol−1.
PU
extender
aniline
feed (g)
M1
M2
2/1/2
3/2/2
4/3/2
2/1/2
3/2/2
4/3/2
2/1/2
3/2/2
4/3/2
2/1/2
3/2/2
4/3/2
0.36/1.5/0.136
0.27/1.5/0.068
0.24/1.5/0.045
0.36/1.5/0.668
0.27/1.5/0.334
0.24/1.5/0.223
0.36/1.5/1.20
0.27/1.5/0.60
0.24/1.5/0.40
0.36/1.5/1.73
0.27/1.5/0.866
0.24/1.5/0.578
25
18
16
41
29
24
51
37
30
58
43
35
M3
M4
MG010
MG020
MG030
M5
M6
M7
M8
M9
M10
M11
M12
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Elastomeric PUs. Thermal elastomeric PUs were prepared from
NCO prepolymers that had been extended with diamine extenders
(DG series). A general procedure for preparing the NCO prepolymers
is described below. The reaction was conducted under dry N2 in a
three-neck, round-bottom flask equipped with a mechanical stirrer, a
snake-shaped condenser, and a thermometer. The NCO prepolymer
(20 wt % in toluene) was prepared by reacting 4,4′-MDI (12.97 g)
with PTMO 2000 (70.01 g) in the presence of T-12 in dry toluene
(34.48 g) at 80 °C for 2.5 h. The reaction was monitored for the
disappearance of the diol OH groups. The isocyanate-equivalent
weights of the NCO prepolymers (2441.86) was determined through
isocyanate titration. A calculated amount of 1,4-butanediol, DG010,
DG020, or DG030 dissolved in DMAc (10 g) was added into the
prepolymer mixture at 80 °C. When rod-climbing occurred, the
mechanical stirring was stopped and postcuring was performed at 100
°C for 3 h. PUs having the same molar ratio were prepared from the
same batch of prepolymers to obtain consistent molecular weights.
To obtain a hard-segment content at 35 wt %, NCO prepolymers
were prepared at four NCO/OH molar ratios and reacted with the DG
extenders; Table 2 lists their recipes. The NCO prepolymers that had
been reacted with stoichiometric amounts of 1,4-butanediol were
prepared for comparison.
acid chloride group was highly sensitive to moisture, a portion
of the DEDA-BC molecules formed anhydride derivatives as
soon as they had been prepared. The best procedure to avoid
byproduct formation was for the crude, freshly prepared
DEDA-BC to be dissolved in cyclohexane and reacted directly
with aniline to form the amide derivative, named herein as
MG005, in 98% yield.
In an alternative synthetic approach, we prepared DEDA-BC
in three steps from readily available p-tolyl isocyanate (Scheme
1a, route A). In this alternative, but a longer synthesis, p-tolyl
isocyanate was first converted into 3,3-diethyl-1-p-tolylazeti-
dine-2,4-dione through cycloaddition of diethyl ketene. When
compared with the reactivity of the isocyanate unit of 4-
isocyanatobenzoyl chloride, that of p-tolyl isocyanate is much
lower, due to the electron donating effect of the methyl group
in the para-position. The yield of the cycloadduct could be
enhanced to 81% by using xylene as the solvent and raising the
reaction temperature to 130 °C. In the subsequent reaction, the
autoxidation of the methyl group in 3,3-diethyl-1-p-tolylazeti-
dine-2,4-dione was performed for 8 h at 115 °C in an AcOH
solution under O2 at atmospheric pressure, with Co(OAc)2 and
Mn(OAc)2 as cocatalysts. The oxidation product, 4-(3,3-
diethyl-2,4-dioxoazetidin-1-yl)benzoic acid, was readily isolated
from the solution in 83% yield after recrystallization. This
autoxidation procedure has been practiced in our group
previously as a means of obtaining aromatic acids, ketones,
and anhydrides.30
Table 2. Compositions of the Thermal Elastomeric PUs with
DG Series Extenders
4,4′-MDI/PTMO/extender (DG)
hard-segment
PU
D1
extender
molar ratio
1.52/1/0.44
feed (g)
content (%)
1,4-
butanediol
3.70/20.00/0.53
17
The carboxylic acid unit in 4-(3,3-diethyl-2,4-dioxoazetidin-
1-yl)benzoic acid was further converted into an acid chloride
moiety through treatment with SOCl2 in cyclohexane under
reflux for 2 h. After evaporation of the solvent and excess
SOCl2, we dissolved the crude DEDA-BC in dry methyl ethyl
ketone to avoid contact with moisture from the air. The
resulting solution containing DEDA-BC was added rapidly into
an ice bath−cooled methyl ethyl ketone solution containing
4,4′-MDA and Et3N, to form the amide derivative DG005. The
yield of DG005 for the two-step reaction was generally high
(>83%) after purification. In summary, the building block
DEDA-BC synthesis through route A involved more steps than
that through route B, but it provided a higher total yield (ca.
57%; cf. ca. 45%).
D2
D3
D4
D5
DG010
DG020
DG030
0.37/2.00/0.43
0.37/2.00/0.77
0.37/2.00/1.10
8.42/20.03/2.47
28
35
41
34
1,4-
butanediol
3.45/1/2.29
D6
D7
DG010
DG030
1.81/1/0.72
1.40/1/0.33
0.45/2.06/0.66
0.35/2.01/0.79
35
35
RESULTS AND DISCUSSION
■
Synthesis of the Building Block DEDA-BC. Pure
crystalline DEDA-BC was synthesized from 4-isocyanatoben-
zoyl chloride and diethyl ketene, through a ketene/isocyanate
cycloaddition, in 45% yield (Scheme 1b, route B). Because the
Table 3. Characterization of the MG (Monoamine) and DG (Diamine) Series of Extenders
a
b
c
d
yield (%)
melting point (°C)
Tm,onset (°C)
ΔHm (mJ mg−1
)
Tg (°C)
Td (°C)
Mn (g mol−1
)
Mn (g mol−1
)
PD
Monoamine Extenders
MG005
MG010
MG015
MG020
MG025
MG030
98
91
96
94
77
92
163−167
212−218
258−263
100−111
273−277
165
216
260
−
115.3
−
71
275
338
311
341
326
345
336
458
1680
4020
6480
4140
6560
8580
1.01
1.01
1.01
1.02
1.02
1.02
148.3
117.6
121
112
150
137
701
e
−
98.4
823
259
1067
1189
e
151−158
−
−
Diamine Extenders
DG005
DG010
DG015
DG020
DG025
DG030
83
92
91
90
84
91
163−163
203−204
184−185
169−177
207−209
181−182
167
−
176
50.5
98
89
321
336
325
353
331
356
684
929
920
2070
2490
3830
4250
5470
1.06
1.10
1.11
1.12
1.12
1.12
e
−
14.9
−
143
135
156
158
1415
1659
2145
2389
e
−
184
17.8
e
−
c
−
a
b
d
DSC, first heating run. DSC, second heating run. Theoretical molecular weights. GPC: MG005−MG030, DMF; DG005−DG030, NMP.
Obscured mp.
e
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Synthesis of MHB Monoamine Extenders with Precise
Hard-Segments. We synthesized well-defined mono- and di-
functional amine extenders, the MG and DG series, through
iterative syntheses using DEDA-BC and 4-aminobenzylamine
as the building blocks. The main difference was in the first step,
where the monoamine extenders employed aniline as the
starting material and the diamine extenders used 4,4′-MDA
(Figure 1). In the iterative syntheses, the highly reactive acid
chloride unit of DEDA-BC was first reacted with aniline or 4,4′-
MDA to form the first generation of azetidien-2,4-dione
intermediates, which were then reacted with 4-aminobenzyl-
amine, at the more-selective azetidine-2,4-dione groups, to form
the first generation of benzylamine extenders. With this
alternating method, we systematically prepared supramolecular
extenders with various chain lengths (n = 1−3) in high yields,
without the need for tedious purification steps, and under
catalyst-free conditions. Because all of the reactions were simple
repetitive addition reactions, these syntheses were straightfor-
ward and efficient. Notably, this strategy could be performed
without the need for protection and deprotection steps. We
purified the extenders prepared using our iterative-synthesis
strategy simply through precipitation or recrystallization,
without resorting to column chromatography. We characterized
all off the extenders and intermediates in terms of their melting
points, their FTIR, NMR, and mass spectra, and their
elementary analyses (see the Experimental Section and
Supporting Data). For the purpose of illustration, we analyzed
the chemical structure of the MG030 extender from its 600
1
MHz H NMR spectrum (DMSO-d6), which is presented in
Figure S2a (Supporting Information) along with proton-by-
proton assignments. Table 3 lists the yields and physical
properties of these compounds. Theoretically, the molar mass
increases between each generation would be 365 and 730 g
mol−1 for the monoamine and diamine extender series,
respectively.
Figure 2. GPC traces of the prepared monoamine MG series of
extenders and of the diamine extender DG030.
We used GPC to confirm the consistent chain lengths of the
amine extenders and the generations of azetidine-2,4-dione
intermediates. The three monoamine extenders MG010−
MG030 and their corresponding azetidine-2,4-dione inter-
mediates MG005−MG025 all exhibited well-defined mono-
disperse structures in their GPC traces. The monodisperse
peaks, each with a low PD of 1.01−1.02, indicated that these
monoamine extenders possessed narrow molecular weight
distributions (Figure 2a). In addition, the mass spectral data for
the extenders matched their structural assignments. Therefore,
we have successfully developed a basic strategy of iterative
synthesis for the systematic preparation of monodisperse
supramolecular extenders. It is important to note that similar
retention times of MG010 and MG020 were observed in GPC
results (Figure 2a). One possible explanation for this elution
behavior is that two repeating units of MG020 might
spontaneously fold up via strong intramolecular H-bonding
interaction, resulting in same hydrodynamic volume shape
when MG020 was dissolved in DMF.24,30 Similar phenomenon
was also observed for the monoamine extenders of MG015 and
MG025 (Figure 2a). The key feature of our iterative synthesis
lies in its dual-functional intermediates, possessing a highly
reactive group and a highly selective group, that can allow the
stepwise and alternating assembly of the molecular structure.
The iterative synthesis is rapid and efficient, with simple
purification of both the intermediates and final products, and
little byproduct formation. Furthermore, we found that the
building block DEDA-BC is superior to IDD, which we had
used previously (PDs: 1.07−1.29)24 for the systematic
preparation of well-defined monoamine extenders through a
similar iterative synthesis approach.
Properties of MHB Hard-Segment Monoamine Ex-
tenders. TGA data revealed that all of the prepared extenders
and intermediates possess remarkable thermal stability. As
listed in Table 3, the decomposition (5% weight loss)
temperatures (Td) of all of the intermediates were greater
than 311 °C. The value of Td of MG030, the longest
monoamine extender, was 345 °C, the highest among the
MG series. A clear trend existed in that the thermal stabilities of
the monoamine extenders MG010−MG030 were greater than
those of the corresponding azetidine-2,4-dione intermediates
MG005−MG025. Furthermore, a similar trend existed among
intermediates from the same series: the value of Td increased
upon increasing the chain length. These results suggest that the
thermal properties of the extenders were enhanced significantly
upon increasing the chain length of the hard segment,
presumably through increased supramolecular interactions.
In Table 3, the melting points measured using DSC match
perfectly with those observed from the melting point apparatus,
except for those of MG020 and MG030. Interestingly, we could
not observe clear melting points for MG020 and MG030 when
using the melting point apparatus, consistent with the broad
endothermal signals observed below 120 °C in their DSC
traces. These unusual endothermal phenomena for MG020 and
MG030 were presumably due to gradual dissociation of the
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inter- and intramolecular hydrogen bondsresulting from the
amine and amide groups along their respective long chainsin
these temperature ranges.31,32 In addition, the crystallinity and
enthalpy observed through DSC analyses appeared to decrease
upon increasing the chain length. As one might expect, longer
molecular chains have greater difficulty at packing in an orderly
and consistent manner, as would be the case for MG020 and
MG030.
Gelation of MHB Hard-Segment Monoamine Extend-
ers. We employed WAXS to investigate the crystalline
morphology of the prepared monoamine extenders and
azetidine-2,4-dione intermediates. Figure 3 reveals no obvious
Figure 4. Photographs of the organogels and WAXS patterns of the
MG030 extender (a) before and (b, c) after gelation at (b) 3 and (c)
6.4 wt %.
bonding sites in MG010, MG020, and MG030 were 6, 12, and
18, respectively. Increasing the degree of hydrogen bonding
would presumably have a major effect on the gelation
capabilities of these species. Because there are also many
aromatic rings in these molecular chains, the secondary effects
of π−π stacking should also be taken into account, albeit as a
minor factor.
We used WXRD spectroscopy to further characterize the
self-assembled structures formed by the powders obtained after
slow evaporation of THF gels (Figure 4). Dramatic differences
appeared in the WXRD patterns before and after gelation. We
observed crystalline patterns for the MG030 gel powder,
indicating that the gelation procedure enhanced the self-
organization of MG030 to form ordered structures. Similar
results have been reported previously for hydrogen bond−rich
urea, amide, and carboxylic acid derivatives; their XRD patterns
also provided evidence for regular 2D molecular packing in the
organogel state.33−36 The DSC first-heating endothermal
signals of the organogel powders were narrower than those of
their original samples, becoming two glass transitions in their
second-heating curves. This finding confirms that well-ordered
structures had assembled after gelation. We suspect that time-
and concentration-dependent structural arrangements formed
after gelation of the amorphous monoamine extenders.
Preparation of MHB-Terminated PUs. We prepared the
targeted MHB PU samples by attaching monoamine extenders
with precise chain lengths to both ends of the NCO
prepolymers. Table 1 lists their resulting formulations; Table
4 provides their characterization data. All the members of the
PU M series had close molecular weights, in the range 24000−
43000 g mol−1; because they were produced from prepolymers
with the same molecular weight, their different physical
properties could have resulted only from differences in the
lengths of their hard segments. Because these hard segments
had precise chain lengths, but different numbers of polar amide
sites, their polarities and degrees of noncovalent interactions
would differ among the series and from those of the soft
segments, which consisted of long-chain polyethers. Thus, the
PU M-series constructed from hard and soft segments could
essentially be considered as psudo-segmented PU elastomers,
except that their molecular weights and some amide-linkages
were not equal to those of traditional PUs. The PUs M1−M6,
end-capped with aniline and MG010, were tacky-liquid
polymers; similar to their low molecular weight preopolymers,
they did not form films and we did not examine their thermal
and mechanical properties.37 Notably, however, the incorpo-
Figure 3. WAXS patterns of the MG series of extenders.
signals for MG020 and MG030 (amorphous), consistent with
the data collected from the DSC and melting point analyses. In
contrast, clear WAXS patterns existed for all of the other
monoamine extenders and azetidine-2,4-dione intermediates.
Similar to our findings in a previous study,24 these low-
molecular-weight extenders displayed distinct glass transitions
during their second scans (Table 3). The glass transition
behavior of these extenders appeared to resemble those of long-
chain polymers, suggesting that the prepared extenders
possessed supramolecular performance. Thus, we can consider
the monoamine extenders to be “pseudo-polymers.” The
variations in the glass transition temperature might also have
been due to different degrees of hydrogen bonding, which
would have differed from the short amine chain in MG010 to
the long amine chains in MG020 and MG030.
To obtain evidence for the association capability of the
prepared monoamine extenders through their MHB sites, we
performed a simple gelation test using THF as the solvent.
Here, we dissolved the monoamine extenders and azetidine-2,4-
dione intermediates in THF at room temperature and adjusted
the concentration to less than 15 wt %. The gelation behavior
of the extenders MG020 and MG030 in THF was particularly
noteworthy. The solution of MG030 in THF at a dilute
concentration of 3 wt % gelled after 60 min (inset to Figure 4,
right-hand image), whereas it took just 10 min to gel at a
concentration of 6.4 wt %. Gelation also occurred, after 2 days,
for the solution of MG020 in THF at 5.9 wt %. The gelation
ability increased upon increasing the chain length. The gels
presumably formed as a result of trapping of the solvent in the
3D hydrogen-bonded networks formed by the amine extenders.
The gelation phenomena, which could be switched through
variations in temperature, appeared to be related to structural
effects, particularly to the number of the hydrogen bonding
sites resulting from the presence of amide and malonamide
groups in the main chains. The numbers of such hydrogen
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a
Table 4. Characterization of the Prepared MHB PUs (M Series)
PU
tensile strength (MPa)
elongation (%)
TgS (°C)
TgH (°C)
Tm (°C)
ΔHm (mJ mg−1
)
Td (°C)
Mn (g mol−1
)
PD
M7
5
0.7
0.8
11
9
22
211
430
88
−72
−71
−
−61
−72
−
−
−
−
130
139
157
14
17
20
16
14
20
10.5
29.8
42.9
14.5
19.6
42.1
302
318
316
297
321
318
24 350
29 230
43 650
31 170
34 920
38 600
1.29
1.26
1.31
1.24
1.22
1.29
M8
M9
M10
M11
M12
418
851
16
a
PUs M1−M6, as tacky liquids, did not form films and were not subjected to DSC analysis. Molecular weights and PDs: M1, 14,765 and 1.67; M2,
22,715 and 1.48; M3, 31,228 and 1.46; M4, 20,210 and 1.46; M5, 25,489 and 1.38; M6, 31,830 and 1.42.
ration of MG020 and MG030 hard-segment end groups had a
dramatic effect on the physical properties of the bulk materials
M7−M12, transforming the viscous PUs into elastomers that
exhibited highly temperature-dependent characteristics.
Among the stress−strain curves (Figure 5), PU M12
displayed particularly soft and tough characteristics, with PU
bonding sites in the hard-segment portions of the monoamine
extenders from 24 in PUs M7−M9 to 36 in PUs M10−M12.
Furthermore, these hydrogen bonding sites formed physical
interchain 3D networks within the supramolecular structures.
Thus, the degree of physical cross-linking in PUs M10−M12
was also higher than that in PUs M7−M9. As a result, the hard
segment having a molecular weight of 1188 g mol−1 (MG030,
third generation) provided PUs with optimized mechanical
properties. In a prior study, we found that the optimal
composition, with respect to the phase-segregation morphology
and mechanical strength, was that of a second-generation
supramolecular extender (having a molecular weight of 1047 g
mol−1) coupled with an MDI/PTMO prepolymer having a
molecular weight of 30,000 g mol−1.24 Recent reports have
indicated that functionalization of the ends of PU oligomers
with MHB groups can greatly enhance their physical proper-
ties.5,8,21,24
Thermal Properties and Thermoreversible Behavior
of MHB-Terminated PUs. The TGA data of the MHB-
terminated PUs (Table 4) revealed that they could be
distinguished from conventional PUs in terms of their
remarkable thermal stabilities. The values of Td of the measured
PUs were approximately 300 °C. The value of Td of PU M11
(321 °C) was the highest in this series. In general, PUs are
unstable at temperatures greater than 230 °C because of
urethane bond decomposition and trans-urethane reaction,
whereas amide-interchange reactions are rare at such temper-
atures. Thus, our amide-modified MHB PUs enhanced the
thermal stability of the PUs. By introducing MHB-rich chain
extenders, the value of ΔHm of the PU decreased depending on
decreasing the hard-segment content. With increasing gen-
erations and longer hard segments, the measured values of TgH
became clear. The values of TgH increased from 130 to 157 °C
Figure 5. Stress−strain curves of the end-capped PUs M7−M12.
M10 exhibiting hard and strong characteristics (e.g., a Young’s
modulus of 1 MPa). The mechanical properties of PU M12
were clearly superior to those of other members of this series.
The mechanical properties of the prepared PUs M10−M12
were also superior to those of PUs M7−M9. Thus, the
mechanical properties of these PUs were greatly influenced by
the composition, especially the length of the hard segment and
the overall chain length. We attribute the superior performance
of PU M12 to the increase in the total number of hydrogen
Figure 6. Variable-temperature FTIR spectra of the PU M12 after (a, c) heating and (b, d) cooling processes.
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(from M10 to M12) upon increasing the molecular weight
from 31170 to 38600 g mol−1. Thus, a longer MHB hard
segment resulted in greater segregation of the phases of the
hard and soft domains in the PUs.
nm. These values appear to be related to the precise chain
lengths of the hard segments used to prepare the end-capped
PUs, and are consistent with our previous findings.21,22,24
Incidentally, the intensities of the signals in SAXS patterns are
often related to the degrees of ordering of the structures in the
materials. Thus, the intensities of the signals for PUs M10−
M12 were greater than those for PUs M8−M9, except for PU
M7; again, this result implies that the hard segment having a
molecular weight of 1188 g mol−1 had the critical length
required for a well-ordered morphology in the prepared PUs.
These results suggest that weak noncovalent interactions can be
utilized to afford a range of self-assembled PU-based materials
with various morphologies.
Synthesis of Diamine Extenders With Precision Chain
Length. We prepared extenders DG010−DG030 with difunc-
tional groups, and their respective thermal elastomeric PUs, to
address the more-practical aspects of PU formulation. The
diamine extenders with precise chain lengths were also
prepared from DEDA-BC and 4,4′-MDA as starting materials,
again using an iterative approach. We used these diamine
extenders to form thermal elastomeric PUs, for which we then
determined structure−property relationships, especially those
related to the chain length of the hard segments.
Using NMP as the eluting solvent, GPC analysis of the
diamine extenders revealed a narrow molecular weight
distribution, within the range 1.06−1.12 (Figure 2b). The
molecular weight distributions of the diamine extenders were
slightly broader than those for the corresponding monoamines,
most likely because of the different eluents that we used and the
large differences in molecular mass. Nevertheless, the precision
and consistency of the chain lengths in the DG series was
confirmed through elemental analysis and MALDI−TOF mass
spectrometry. For instance, a signal appeared at m/z 2412 [M +
Na]+ in the MALDI−TOF mass spectrum of the extender
DG030, close to the theoretical value of m/z 2389 (Figure S3b,
Supporting Information).
Properties of Diamine Extenders with Precise Chain
Lengths. In Table 3, both of monoamine MG and diamine
DG series of extenders exhibit similar trends in their thermal
stabilities: with increasing chain length, the decomposition
temperatures increased. The value of Td of DG030 (356 °C)
was the highest among the DG series; it was also higher than
that of the corresponding monoamine MG030 extender (345
°C). Thus, the thermal stability of the MG- and DG-series of
extenders was dependent on their chain lengths as well as the
number of malonamide and amide units, with DG030 having
the highest molecular weight (2389 g mol−1) and greatest
number of hydrogen bonding sites (36). The DSC first-heating
curves for DG010, DG020, and DG030 were similar to those of
the corresponding monoamine extenders, with the endothermal
peaks for DG005, DG015, and DG025 broadening upon
increasing the chain length. As was the case with the M-series,
we could not detect clear melting points for DG010, DG020,
and DG030 in the melting point apparatus, whereas those of
DG005 (167 °C), DG015 (177 °C), and DG025 (184 °C)
exhibited an increasing trend. Because of the gradual
dissociation of hydrogen bonds, the crystallinity and enthalpy,
observed through DSC analyses, appeared to decrease upon
increasing the chain length. The values of ΔHm of the DG
series were lower than those of the MG series. A comparison of
the properties of the DG and MG series of extenders revealed
that the magnitude of the generation-to-generation variation in
the DG series was less dramatic than that in the MG series.
We studied the thermo-reversibility behavior of the MHB
PUs in terms of changes in the intensity and shifts in
wavelength of the signals in their variable-temperature FTIR
spectra. The signals for the N−H and CO bonds of the
malonamide and urethane groups underwent the most obvious
changes upon changing the temperature. The formation and
breaking of inter- and intrapolymer hydrogen bonds could be
monitored during the thermal heating and cooling cycles. For
example, upon increasing the sample temperature from room
temperature to 190 °C, the characteristic signal for the N−H
bonds, initially located at 3300 cm−1, shifted gradually to 3340
cm−1 (see arrow in Figure 6a). Conversely, the signal at 3340
cm−1 for the N−H bonds at 190 °C gradually returned to 3300
cm−1 (Figure 6b) upon cooling of the sample to ambient
temperature. Thus, extensive hydrogen bonding between
polymer chains was evident at room temperature, but it
gradually weakened upon elevating the temperature. Similarly,
the characteristic absorptions of the CO groups of the
urethane and malonamide units in the prepared PUs also
exhibited reversible phenomena, as indicated by the arrows in
Figure 6, parts c and d. From the behavior of the signal at 1710
cm−1 for the hydrogen-bonded CO groups, we hypothesize
that physical cross-linking network of hydrogen bonds
disappeared upon heating to approximately 90 °C, but it
reappeared after cooling to below 70 °C. As evidenced from the
variable-temperature FTIR spectra, our PUs displayed optimal
mechanical and physical properties at ambient temperature
because they featured a high density of physically cross-linked
networks formed through hydrogen bonding.
Morphologies of MHB-Terminated PUs. To better
differentiate the morphologies and performances of the PUs
with respect to their monoamine extenders, we used SAXS to
analyze their well-defined phase-segregated microdomains. The
SAXS patterns in Figure 7 reveal that well-ordered hard
domains existed in the prepared PUs. Using the equation
d = 2π/q
for lamellar structures as our basis, the average interdomain
spacings in all of our prepared PUs were in the range 38−48
Figure 7. SAXS patterns of the end-capped PUs M7−M12 at room
temperature.
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Table 5. Characterization of the Prepared Thermal Elastomeric PUs (D Series)
tensile strength
(MPa)
elongation
(%)
TgS
(°C)
TgH
(°C)
Tc
(°C)
ΔHc
Tm
(°C)
ΔHm
Td
(°C)
PU
(mJ mg−1
)
(mJ mg−1
)
Mn (g mol−1
)
PD
a
D1
D2
D3
D4
D5
D6
D7
−
29
25
9
−
734
378
231
350
411
342
−66
−61
−62
−63
−58
−52
−61
−
112
128
170
−
−
−9
−4
−20
−12
−9
−
18
18
19
19
21
20
19
17.2
47.5
45.1
32.7
26.8
42.4
39.6
279
303
284
297
298
322
316
24 860
21 860
23 880
29 400
15 880
31 550
17 440
2.11
2.86
2.53
2.36
2.43
1.95
1.81
−48.2
−44.1
−22.3
−28.5
−45.6
−38.9
6
59
8
149
165
−11
a
PU D1 was a tacky liquid that did not form films and was not subjected to mechanical testing.
Thus, the behavior of the DG and MG series of extenders was
dependent on their molecular chain lengths as well as their
numbers of malonamide and amide units. Moreover, the values
of Tg and Td for the MG and DG series of extenders did not
increase limitlessly upon lengthening their chain lengths. It
appears that the highest values of Tg (158 °C) and Td (356 °C)
for the extender DG030 were almost the maximum possible
values for such malonamide/amide chain materials.
Preparation of Thermal Elastomeric PUs. Using a
strategy and procedures similar to those described for the
monoamines, we prepared conventional thermal elastomeric
PUs from the diamine extenders. The NCO prepolymer was
prepared as a general batch and then it was reacted individually
with the extenders DG010, DG020, and DG030. Table 2
reveals that the reactions of a fixed NCO prepolymer with
extenders of different chain lengths provided systems D1−D4
with a great variation in hard-segment contents from 17 to 41
wt %. For comparison, we prepared PUs with a hard-segment
content controlled at 35 wt % using the three DG series of
extenders and 1,4-butanediol (D3 and D5−D7).
Thermal Properties of Thermal Elastomeric PUs. Table
5 reveals that the PUs D1−D4 prepared from the same batch of
NCO prepolymer had closely matching melting points of
approximately 18 °C, with thermo-stabilities lying closely
between 279 and 303 °C. There were no dramatic differences
in the glass transition temperatures for their PTMO segments
(TgS), which were observed near −62 °C. The glass transitions
for their hard segments (TgH), however, varied considerably,
with values of 112, 128, and 170 °C for D2, D3, and D4,
respectively. The values of TgH of the control polymers
prepared using 1,4-butanediol as an extender were not clear for
D1 and D5. Furthermore, the values of ΔHm of these PUs
chain-extended with diamines of precise chain length were
larger than that of the system chain-extended with 1,4-
butanediol. Thus, the great increase in chain length in each
generation significantly increased the value of TgH of the PU.
Among the systems with a constant hard-segment content of
35% (D3 and D5−D7), the decomposition temperature of PU
D6 was the greatest (322 °C), with the glass transition for the
hard segments appearing at 149 °C. Thus, the thermostability
of the MHB-modified PUs, such as PU D6 and even D7, were
superior to those of the 1,4-butanediol−based PU D5 by
approximately 20 °C.
Figure 8. Stress−strain curves of the PU elastomers (a) D2−D4,
prepared from the same prepolymer, and (b) D3 and D5−D7, with a
hard-segment content of 35%.
content (41%), D4 displayed the same initial tensile strength,
but lower elongation (230%). In the stress−strain curves, the
PUs D2 and D6 chain-extended with DG010 displayed good
elongation; the PUs D3, D4, and D7 displayed high stiff yield
points, with Young’s moduli of 1 MPa. Upon increasing the
length of the hard segment, the mechanical behavior of the PU
transformed from that of a ductile elastomer to that of a stiff
plastic. Among all of the samples featuring a hard-segment
content of 35%, PU D6 had the highest tensile strength (59
MPa), presumably because it featured the most suitable hard-
segment length and was the PU with the highest molecular
weight. We conclude that DG010, having a molecular weight of
928 g mol−1, was the optimal extender for the D series. Thus,
Mechanical Properties of Thermal Elastomeric PUs. In
Table 5 and Figure 8, for the systems based on the same-length
prepolymer, PU D2 exhibited the best elongation (734%) and
PU D3 displayed the greatest tensile strength before 400%
elongation. Hence, the mechanical properties of the PU D
series were greatly influenced by the hard-segment length. With
its longer chain extender (DG030) and greater hard-segment
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the critical hard-segment length in the PU chain appears to be
approximately 1428 g mol−1, a result of the combination of two
4,4′-MDI units and one DG010 moiety. A longer chain length
did not improve the overall mechanical properties of the PU. In
general, crystalline and rigid hard segments provide PUs with
greater stiffness. The PUs extended with DG010 (M2, M4)
displayed the largest enthalpies (ΔHc and ΔHm in Table 5),
implying that DG010 possessed the optimal crystalline
characteristics. The crystalline structures of the hard segments
were not readily formed in the PUs when the hard segments
became too long. Conversely, when the length of the hard
segment was too short, the total number of available hydrogen
bonding sites was too low for noncovalent association.15
Therefore, this study revealed the optimal chain length of the
hard segment in PU elastomers.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures of difunctional extenders from
DG020 to DG030, PU structures, and 1H NMR and
MALDI−TOF mass spectra for extenders MG030 and
DG030. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Telephone: +886-4-22840510 ext 412. Fax: +886-4-22874159.
E-mail: shdai@dragon.nchu.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSION
■
■
We thank the National Science Council (NSC) of Taiwan and
the Ministry of Education, Taiwan, under the ATU plan, for
financial support. Partial grants for this research also have been
provided by GECO of Taichung, Taiwan.
We have synthesized DEDA-BC, a dual-functional building
block that possesses a highly reactive acid chloride unit and a
highly selective azetidine-2,4-dione group in the same molecule,
both (i) indirectly from p-tolyl isocyanate and (ii) directly from
4-isocyanatobenzoyl chloride. By starting with the monoamine
aniline or the diamine 4,4′-MDA, we used an alternating step-
by-step approach to build, at room temperature under catalyst-
free conditions, six new supramolecular extenders of various
chain lengths (n = 1−3), without the need for tedious
purification. We prepared these supramolecular extenders with
mono- and di-functional benzylamine end groups in high yield
(77−98%). These prepared extenders, which could take part in
multiple hydrogen bonds, displayed narrow monodispersities
(1.01−1.12), as determined using GPC. The thermal stabilities,
transition temperatures, and numbers of hydrogen bonding
sites all improved upon increasing the chain lengths of the
amine extenders. The gelation behavior of the supramolecular
extenders in organic solution, induced by intermolecular
hydrogen bonding, was also dependent on the chain lengths.
PU elastomers incorporating these extenders exhibited thermo-
reversible properties, as observed using variable-temperature
FTIR spectroscopy. The physically cross-linked networks
existing in these PUs disappeared at temperatures above 90
°C, but reappeared upon cooling to below 70 °C. Because they
incorporated hard-segment extenders with precise chain
lengths, both the MHB-terminated PUs and thermal elasto-
meric PUs (Mn = 17 000−43 000 g mol−1) prepared in this
study possessed excellent thermal stability at temperatures
greater than 300 °C. Morphologically, SAXS analysis revealed
that the prepared MHB PUs featured well-defined micro-
domains having an average period of 39−48 nm. The optimal
molecular masses of the extenders, confirmed through
mechanical testing, were 1188 g mol−1 (MG030; n = 3) for
the MHB-terminated PUs and 928 g mol−1 (DG010; n = 1) for
the thermal plastic PUs. Using an efficient and iterative
synthetic approach, we have systematically prepared supra-
molecular extenders with variable, yet precise, hard-segment
contents, and then used them to synthesize elastomeric PUs to
gain insight into their structure−property relationships,
directed by reversible noncovalent interactions. Our study
also found that triblock A−B−A PU elastomer systems appears
to have a greater propensity to achieve phase-segregation in
polymers than those of traditional PU elastomers where hard
segments are en-restricted in both ends.
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