Synthesis and characterization of segmented copoly(ether urea)s with uniform hard segments

Article abstract of DOI:10.1021/ma0478207

Employing protective group strategy and novel isocyanate chemistry, segmented copoly-(ether urea)s with uniform hard segments were prepared. Amine-terminated poly(tetrahydrofuran) served as the soft segment of these polymers. The size of the hard segments and the number of urea groups it contains were varied systematically, and their influence on the properties was investigated. The strength of hydrogen bonding between the urea groups in monodisperse hard segments containing exactly 1 to exactly 4 urea groups was studied by infrared spectroscopy and compared with materials containing polydisperse hard segments. The strength of hydrogen bonding in polymers possessing exactly two urea groups per hard segment resulted in an optimal balance between excellent mechanical properties and good processability and solubility.

Full text of DOI:10.1021/ma0478207

3176
Macromolecules 2005, 38, 3176-3184
Synthesis and Characterization of Segmented Copoly(ether urea)s with
Uniform Hard Segments
Ron M. Versteegen, Rint P. Sijbesma,* and E. W. Meijer
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received October 21, 2004; Revised Manuscript Received February 11, 2005
ABSTRACT: Employing protective group strategy and novel isocyanate chemistry, segmented copoly-
(ether urea)s with uniform hard segments were prepared. Amine-terminated poly(tetrahydrofuran) served
as the soft segment of these polymers. The size of the hard segments and the number of urea groups it
contains were varied systematically, and their influence on the properties was investigated. The strength
of hydrogen bonding between the urea groups in monodisperse hard segments containing exactly 1 to
exactly 4 urea groups was studied by infrared spectroscopy and compared with materials containing
polydisperse hard segments. The strength of hydrogen bonding in polymers possessing exactly two urea
groups per hard segment resulted in an optimal balance between excellent mechanical properties and
good processability and solubility.
1. Introduction
Thermoplastic elastomers (TPEs) are polymers that
combine advantages of both thermoplastics and elas-
tomers.1 In general, their specific properties are a result
of their morphology. At ambient temperature, physical
cross-links in the amorphous matrix give the material
its elastomeric, rubberlike properties. At higher tem-
peratures, these physical cross-links are broken due to
their reversibility, and the material can be processed
easily. In many thermoplastic elastomers, the reversible
cross-links originate from crystallization of one of the
blocks of the segmented copolymer. These “hard” blocks
are generally polyester,1,2 polyamide,3 or polyurethane4
segments. At low temperatures, crystalline domains
account for the mechanical stability of the material;
however, above the melting point of the hard block, a
polymer melt is obtained.
Among segmented copolymers, thermoplastic elasto-
meric polyurethanes (TPUs) are the most widely used.
The TPU consists basically of three building blocks: a
long-chain diol, normally with a polyether or polyester
backbone; a diisocyanate, and a chain extender, e.g.,
water, a short-chain diol, or a diamine. TPU’s are often
prepared via a one-pot procedure, in which the long-
chain diol is first reacted with an excess of the diiso-
cyanate to form an isocyanate-functionalized prepoly-
mer, which is subsequently reacted with the chain
extender, resulting in the formation of the high molec-
ular weight polyurethane. This synthetic procedure to
prepare TPUs has the intrinsic disadvantage that it
leads to a statistical distribution in the lengths of the
hard segments.5 As a result, the phase separation of
these block copolymers is incomplete. Part of the hard
blocks, in particular the shorter segments, are dissolved
in the soft phase, causing an increase in the glass
transition temperature, which is undesirable for the
low-temperature flexibility of the material. The poly-
dispersity of the hard segment is manifested in a broad
melting range and a rubbery plateau that is dependent
on temperature. To improve these properties, and to get
Figure 1. Bifurcated hydrogen-bonding motif of urea deriva-
tives.
more insight into the structure-properties relationship,
block copolymers containing hard segments of uniform
length have been prepared in the past.6,7 These studies
showed that uniform hard blocks leads to polymers with
better properties, which are less temperature depend-
ent.
Several types of hard blocks have been used to reach
this objective, such as non-hydrogen-bonding poly-
urethanes,6a,b normal polyurethanes,6c,d,f and poly(ure-
thane urea)s.6e Niesten et al. used hard blocks based
on uniform aramid units7 and have shown that these
units form perfect crystals which melt in a narrow
temperature region. Most TPEs containing uniform
hard blocks were prepared by fractionation of a mixture
of hard block oligomers and subsequent copolymeriza-
tion of the uniform hard oligomer of a specific length
with the prepolymer. This procedure is hampered by the
low solubility ofsespecially the longersoligomers and
the difficulty in fractionating oligomers that are differ-
ent in length but similar in chemical structure.
Urea groups are excellent functionalities for use in
hard blocks of thermoplastic elastomers because they
are known to associate via bifurcated hydrogen bonds
(Figure 1).8 The hydrogen bond strength exceeds that
of amides and urethanes. Many researchers utilized the
strong association between urea groups to obtain gelling
agents.9
Thermoplastic elastomers containing urea groups
have been prepared before,10 but few examples are
known of TPEs possessing hard blocks comprising solely
urea groups.11 Yilgo¨r et al. described segmented poly-
(dimethylsiloxane) and poly(ethylene oxide) based urea
copolymers and studied their behavior in detail.11a-e
These polymers were prepared by chain extension of
amine-terminated prepolymers with diisocyanates. How-
ever, their synthetic strategy does not allow the prepa-
* Corresponding author. E-mail r.p.sijbesma@tue.nl.
10.1021/ma0478207 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/24/2005

Macromolecules, Vol. 38, No. 8, 2005
Segmented Copoly(ether urea)s 3177
Bis(3-aminopropyl)poly(tetrahydrofuran), 3. To a solu-
tion of borane-tetrahydrofuran complex (80 mL of 1 M THF,
80 mmol) in dry THF (240 mL) was added slowly bis(2-
cyanoethyl)poly(tetrahydrofuran), 2 (20.00 g, 9.5 mmol), dis-
solved in dry THF (160 mL) at 0 °C. The solution was stirred
for 30 min at 0 °C, after which it was heated to reflux for 4 h.
The reaction mixture was cooled to 0 °C, and methanol (80
mL) was added dropwise (be careful: hydrogen gas evolution).
Hydrochloric acid (4 mL, 37% in water) was added slowly, and
the reaction mixture was stirred for 1 h and subsequently
evaporated to dryness under reduced pressure. Trimethyl
borate was removed by three coevaporations with methanol
(3 times 100 mL). To the viscous liquid was added sodium
hydroxide solution (150 mL, 1 M in water), and this was
extracted with diethyl ether (3 times 300 mL). The combined
organic layers were dried with sodium sulfate and filtered, and
the solvent was evaporated on a rotary evaporator without
putting the flask in the water bath. During the evaporation,
the polymer precipitated from the cold solution and was
ration of uniform hard blocks possessing more than two
urea groups.
In this paper, we present the synthesis of segmented
block copolymers possessing uniform (monodisperse)
hard blocks based on urea groups, making use of a
protective group strategy and employing di-tert-butyl
tricarbonate, a mild and selective reagent for the
formation of isocyanates that was recently developed by
us.12 Poly(tetrahydrofuran) (pTHF) was used as the soft
segment since it is commercially available in a variety
of molecular weights and may be prepared on a labora-
tory scale via a cationic ring-opening polymerization of
tetrahydrofuran.13
2. Experimental Section
General Methods and Instrumentation. NMR spectra
were recorded on a Varian Inova 500 MHz spectrometer, a
Bruker 400 MHz spectrometer, and a Varian Gemini 300 MHz
spectrometer. Infrared spectra were measured on a Perkin-
Elmer 1600 FT-IR. Size exclusion chromatography (SEC) was
performed on a Shimadzu LC10-AT, using a Polymer Labo-
ratories Plgel 5 µm Mixed-D column, a Shimadzu RID-6A
detector, and N-methylpyrrolidone as eluent. Molecular weights
were calculated relative to polystyrene standards. MALDI-
TOF spectra were obtained at a Perseptive Biosystems Voy-
ager DE-Pro MALDI-TOF mass spectrometer (accelerating
voltage, 20 kV; grid voltage, 74.0%; guide wire voltage, 0.030%;
delay, 200 ms; low mass gate, 900 amu). Samples for MALDI-
TOF were prepared by adding a solution of the polymers in
THF (20 µL, c ) 1 mg/mL) to a solution of R-cyano-4-
hydroxycinnamic acid in THF (10 µL, c ) 20 mg/mL) and
subsequent thoroughly mixing. This mixture (0.3 µL) was
brought on a sample plate, and the solvent was evaporated.
Size exclusion chromatography (SEC) was performed on a
Shimadzu LC10-AT, using a Polymer Laboratories Plgel 500
A column, a Shimadzu SPD-10AV UV-vis detector, and
chloroform as eluent. Polystyrene standards were used for
calibration.
Materials. Di-tert-butyl tricarbonate, 4, was prepared ac-
cording to literature procedure.12 Bis(3-aminopropyl)poly-
(tetrahydrofuran) with molecular weights 350 and 1100 g/mol,
poly(tetrahydrofuran) with molecular weights 2000 and 2900
g/mol, and poly(ethylene oxide) with molecular weight 2000
were purchased from Aldrich. PTHF/EO4000, a random copoly-
mer of THF and EO in a ratio of 90:10, was kindly provided
by Prof. Dr. Doetze Sikkema (Akzo-Nobel). 1,4-Diisocyana-
tobutane, borane-tetrahydrofuran complex (1 M in THF), and
sodium hydride (60% dispersion in mineral oil) were purchased
from Aldrich, 1,3-diaminopropane was from Acros, 1,2-ethyl-
enediamine was from Janssen, and 1,5-diaminopentane and
1,6-diisocyanatohexane were from Fluka.
Bis(2-cyanoethyl)poly(tetrahydrofuran), 2. Poly(tet-
rahydrofuran)diol, Mn ) 2000 g/mol (20.00 g, 10.0 mmol), and
15-crown-5 (44 mg, 0.2 mmol) were dissolved in acrylonitrile
(40 mL) and cooled in an ice bath. Sodium hydride (8 mg 60%
dispersion in mineral oil, 0.2 mmol) was added to the solution,
and the reaction mixture was stirred at 0 °C for about 15 min,
after which the reaction mixture turned slightly yellow. At this
point, the reaction was quenched by addition of a drop of
concentrated hydrochloric acid. The solution was concentrated,
and the residue was taken up in dichloromethane (100 mL)
and centrifuged at 4500 rpm. The mixture was decanted,
filtered, and concentrated in vacuo. The product was obtained
as a slightly yellow, viscous liquid, which slowly crystallized
(20.13 g, 96%). 1H NMR (CDCl3): δ 3.62 (t, 4H, OCH2CH2-
CN), 3.51 (t, 4H, CH2OCH2CH2CN), 3.40 (br t, 106H, OCH2-
CH2CH2CH2O main chain), 2.59 (t, 4H, CH2CN), 1.60 ppm (br
m, 110H, OCH2CH2CH2CH2O main chain). 13C NMR (CDCl3):
δ 117.7 (CN), 71.0 (CH2OCH2CH2CN), 70.4 (OCH2CH2-
CH2CH2O main chain), 65.1 (OCH2CH2CN), 26.3 (OCH2-
CH2CH2CH2O main chain), 18.7 ppm (CH2CN). FT-IR (ATR):
ν 2939, 2855, 2161 (w, CtN stretching), 1367, 1103 cm-1 (C-O
stretching).
1
obtained as a white powder (18.74 g, 93%). H NMR (CDCl3):
δ 3.49 (t, 4H, OCH2CH2CH2NH2), 3.41 (br. t, 138H, OCH2CH2-
CH2CH2O main chain), 2.79 (t, 4H, CH2NH2), 1.71 (t, 4H,
OCH2CH2CH2NH2), 1.62 (br m, 142H, OCH2CH2CH2CH2O
main chain), 1.1 ppm (br s, 4H, NH2). 13C NMR (CDCl3): δ
70.5 (OCH2CH2CH2CH2O main chain), 68.8 (OCH2CH2CH2-
NH2), 39.7 (CH2NH2), 33.6 (OCH2CH2CH2NH2), 26.4 ppm
(OCH2CH2CH2CH2O main chain). FT-IR (ATR): ν 3564, 3539,
2941, 2862, 1492, 1372, 1107, 996 cm-1. MALDI-TOF [M +
Na+] ) Calcd 155.1 + n × 72.0 Da. Obsd: 155.9 + n × 72.0
Da. SEC (phenyl urea derivative): Mn ) 3769 g/mol, PDI )
1.5.
pTHF1100-U.
Bis(3-aminopropyl)poly(tetrahydrofuran),
Mn ) 1100 g/mol (2.00 g, 1.82 mmol), was dissolved in
chloroform (20 mL), and a solution of di-tert-butyl tricarbonate
(0.48 g, 1.82 mmol) in chloroform (4 mL) was injected into this
solution at 20 °C. The reaction mixture was stirred for 2 h at
20 °C and subsequently evaporated to dryness under reduced
pressure. The product was obtained as a very viscous colorless
1
oil. H NMR (DMSO): δ 3.33 (60H, CH2O), 2.95 (4H, CH2N),
1.50 ppm (65H, CH2CH2CH2). FT-IR (ATR): ν 3350 (N-H
stretching), 2938, 2853, 1637 (CdO stretching), 1568, 1366,
1103 cm-1 (C-O stretching). SEC (NMP, rel to PS): Mn ) 68
× 103 g/mol.
pTHF1100-U-C2H4-U. Via Route A (Scheme 3). Bis(3-ami-
nopropyl)poly(tetrahydrofuran), Mn ) 1100 g/mol (0.50 g, 0.45
mmol), was dissolved in chloroform (10 mL), and a solution of
di-tert-butyl tricarbonate (0.235 g, 0.91 mmol) in chloroform
(1 mL) was injected into this solution at 20 °C. The reaction
mixture was stirred for 30 min. 1,2-Ethylenediamine (0.0269
g, 0.45 mmol) in chloroform (3 mL) was added dropwise at 20
°C, and the solution was stirred for 1 h, and subsequently
partly concentrated, and methanol (1 mL) was added. The
product was precipitated in pentane (50 mL), filtered, and
dried in vacuo. It was obtained as white, fluffy, elastic fibers
(0.47 g, 86%). 1H NMR (DMSO): δ 5.91 (4H, NH), 3.34 (59H,
CH2O), 2.99 (8H, CH2N), 1.51 ppm (56H, CH2CH2CH2). FT-
IR (ATR): ν 3329 (N-H stretching), 2937, 2854, 1615 (CdO
stretching), 1589 1366, 1105 cm-1 (C-O stretching). SEC
(NMP, rel to PS): Mn ) 41 × 103 g/mol.
pTHF1100-U-C4H8-U () pTHF1100-U2). Via Route B (Scheme
3). Bis(3-aminopropyl)poly(tetrahydrofuran), Mn ) 1100 g/mol
(10.00 g, 9.09 mmol), was dissolved in chloroform (100 mL)
and a solution of 1,4-diisocyanatobutane (1.40 g, 9.99 mmol)
in chloroform (40 mL). About 75% of the latter solution was
added to the former solution at once, and the rest was added
dropwise at 20 °C. The solution was stirred for 1 h, and
subsequently partly concentrated, and methanol (5 mL) was
added. The product was precipitated in hexane (500 mL),
filtered, and dried in vacuo. It was obtained as white, fluffy,
elastic fibers (10.62 g, 93%). 1H NMR (CHCl3): δ 5.4-4.8 (4H,
NH), 3.41 (58H, CH2O), 3.25 (4H, OCH2CH2CH2N), 3.17 (4H,
NCH2CH2CH2CH2N), 1.74 (4H, OCH2CH2CH2N), 1.62 (58H,
OCH2CH2CH2CH2O), 1.50 ppm (4H, NCH2CH2CH2CH2N).
FT-IR (ATR): ν 3324 (N-H stretching), 2940, 2854, 1615
(CdO stretching), 1580 1365, 1104 cm-1 (C-O stretching).

3178 Versteegen et al.
Macromolecules, Vol. 38, No. 8, 2005
Scheme 1. Synthesis of Amine-Terminated Prepolymers 3
SEC (NMP, rel to PS): Mn ) 42 × 103 g/mol, F ) 0.98
g/cm3.
pTHF1100-U3. O,O′-Bis(4-aminobutylureido)poly(tetrahy-
drofuran), 8 (0.50 g, 0.368 mmol), was dissolved in chloroform
(5 mL), and di-tert-butyl tricarbonate (0.19 g, 0.735 mmol) in
chloroform (1 mL) was injected into this solution at 20 °C. The
reaction mixture was stirred for 30 min, and methanol (3 mL)
was added, immediately followed by O,O′-bis(4-aminobutyl-
ureido)poly(tetrahydrofuran), 8 (0.50 g, 0.368 mmol), in chlo-
roform (3 mL). The reaction mixture was stirred for 1 h at 20
°C and subsequently precipitated in pentane (100 mL). The
product was obtained as a white fluffy powder (0.89 g, 88%).
1H NMR (10% TFA in CDCl3): δ 3.71 (64H, CH2O), 3.45 (4H,
OCH2CH2CH2N), 3.32 (8H, NCH2CH2CH2CH2N), 1.99 (4H,
OCH2CH2CH2N), 1.73 ppm (71H, OCH2CH2CH2CH2O +
NCH2CH2CH2CH2N). FT-IR (ATR): ν 3324, 2940, 2854, 1615,
4-(tert-Butoxycarbonylamino)-1-butylamine, 5. Di-tert-
butyl dicarbonate (14.40 g, 50 mmol) was dissolved in chloro-
form (100 mL) and added dropwise to a solution of 1,4-
diaminobutane (17.60 g, 200 mmol) in chloroform (150 mL) at
0 °C. The suspension was stirred overnight at 20 °C. The
reaction mixture was washed three times with water (100 mL),
and the product was extracted by hydrochloric acid solution
(100 mL 1 M in water). The aqueous layer was isolated,
basified by addition of sodium hydroxide solution (10 mL 10
M in water), and subsequently extracted 3 times with dichlo-
romethane (3 times 100 mL). The combined organic layers
were dried with sodium sulfate and filtered, and the dichlo-
romethane was removed under reduced pressure. Recrystal-
lization of the product from diisopropyl ether gave the pure
product as colorless crystals (7.80 g, 80%, with respect to di-
1576 1366, 1105 cm-1
pTHF1100-U4. O,O′-Bis(4-aminobutylureido)poly(tetra-
.
hydrofuran), 8 (0.50 g, 0.368 mmol), was dissolved in chloro-
form (7 mL) and methanol (3 mL). 1,4-Diisocyanatobutane
(0.0567 g, 0.405 mmol) was dissolved in chloroform and added
dropwise to the former solution at 20 °C. Subsequently, the
reaction mixture was stirred for 1 h. The viscous solution was
poured into heptane, and the suspension was stirred for 3 h.
Then it was allowed to settle, the liquid was decanted, and
the product was dried under reduced pressure. It was obtained
as a rubbery solid (0.40 g, 73%). 1H NMR (10% TFA in
CDCl3): δ 3.71 (66H, CH2O), 3.45 (4H, OCH2CH2CH2N), 3.31
(12H, NCH2CH2CH2CH2N), 1.98 (4H, OCH2CH2CH2N), 1.72
ppm (73H, OCH2CH2CH2CH2O + NCH2CH2CH2CH2N). FT-
1
tert-butyl dicarbonate); Tm ) 84 °C. H NMR (CDCl3): δ 4.71
(s, 1H, NHBoc), 3.13 (q, 2H, BocNH-CH2, J ) 6.2 Hz), 2.71
(t, 2H, H2NCH2, J ) 6.6 Hz), 1.57-1.46 (m, 4H, CH2CH2CH2),
1.44 (s, 9H, CH3), 1.28 ppm (s, 2H, NH2). 13C NMR (CDCl3):
δ 155.8 (CdO), 79.0 ((CH3)3C), 41.9 (CH2NHBoc), 40.5 (CH2-
NH2), 31.0 (CH2CH2NHBoc), 28.5 (CH3), 27.6 ppm (CH2CH2-
NH2). FT-IR (ATR): ν 3359 (N-H stretching), 1689 (CdO
stretching), 1521, 1167 cm-1. Anal. Calcd (%) for C9H20N2O2
(188.27): C, 57.42; H, 10.71; N, 14.88. Found (%): C, 56.65;
H, 10.62; N, 14.58.
O,O′-Bis(4-(tert-butoxycarbonylamino)butylureido)-
poly(tetrahydrofuran), 7. Di-tert-butyl tricarbonate 4 (1.03
g, 3.94 mmol) was dissolved in chloroform (20 mL), and 4-(tert-
butoxycarbonylamino)-1-butylamine 5 (0.74 g, 3.94 mmol) in
chloroform (4 mL) was injected in this solution at 20 °C. The
reaction mixture was stirred for 30 min to yield isocyanate 6.
Subsequently, bis(3-aminopropyl)poly(tetrahydrofuran), Mn )
1100 g/mol (2.00 g, 1.82 mmol), in chloroform (8 mL) was
added, and the solution was stirred for 1 h at 20 °C, after which
IR (ATR): ν 3323, 2940, 2855, 1616, 1574 1368, 1104 cm-1
.
pTHF1100-U2PD. Bis(3-aminopropyl)poly(tetrahydrofuran),
Mn ) 1100 g/mol (2.00 g, 1.79 mmol), and 1,4-diaminobutane
(0.158 g, 1.79 mmol) were dissolved in chloroform (20 mL). A
solution of di-tert-buty tricarbonate (0.94 g, 3.58 mmol) in
chloroform (3 mL) was injected into this solution at 20 °C. The
reaction mixture was stirred for 1 h at room temperature,
during which the solution formed a gel. Methanol (5 mL) was
added, and the solution was partly concentrated and then
precipitated in pentane (150 mL). The product was obtained
1
it was concentrated in vacuo (2.75 g, 99%). H NMR (CDCl3):
δ 5.4 (br s, 2H, NHBoc), 4.8 (br s, 4H, NHCdONH), 3.53 (t,
4H, OCH2CH2CH2N), 3.42 (br m, 60H, OCH2CH2CH2CH2O),
3.31 (t, 4H, OCH2CH2CH2N), 3.16 (m, 8H, CH2CH2CH2CH2N),
1.80 (qui, 4H, OCH2CH2CH2N), 1.62 (br m, 70H, OCH2CH2CH2-
CH2O), 1.50 (m, 4H, NCH2CH2CH2CH2N), 1.44 ppm (s, 18H,
CH3). FT-IR (ATR): ν 3357, 1680, 1626, 1579, 1520, 1365, 1102
1
as white, fluffy, elastic fibers (2.02 g, 90%). H NMR (CDCl3/
methanol-d4): δ 3.34 (80H, CH2O), 3.21 (4H,), 3.13 (4H, NCH2-
CH2CH2CH2N), 1.75 (4H, OCH2CH2CH2N), 1.64 (77H, OCH2-
CH2CH2CH2O), 1.49 ppm (4H, NCH2CH2CH2CH2N). FT-IR
(ATR): ν 3326 (N-H stretching), 2939, 2853, 1620 (CdO
stretching), 1579 1366, 1104 cm-1 (C-O stretching). SEC
(NMP, rel to PS): Mn ) n.d.
cm-1
.
O,O′-Bis(4-aminobutylureido)poly(tetrahydrofuran),
8. O,O′-Bis(4-(tert-butoxycarbonylamino)butylureido)poly(tetra-
hydrofuran), 7 (6.80 g, 8.95 mmol), was dissolved in dichlo-
romethane (10 mL), and trifluoroacetic acid (5 mL) was added.
The reaction mixture was stirred at 20 °C overnight. Dichloro-
methane and trifluoroacetic acid were removed under reduced
pressure, and the residue was redissolved in dichloromethane
(50 mL) and washed with sodium hydroxide solution (50 mL
1 M in water). The aqueous layer was extracted two more
times with dichloromethane (two times 50 mL). The combined
organic layers were dried with sodium sulfate and filtered, and
the dichloromethane was removed under reduced pressure.
3. Results
3.1. Synthesis. For the preparation of pTHF-based
thermoplastic elastomers with hard blocks comprising
solely urea groups, amine-terminated pTHF was re-
quired. A route to obtain these prepolymers by end
group modification was optimized (Scheme 1). Cyano-
ethylation14 of hydroxy end groups of pTHF 1 by
reaction with acrylonitrile yielded the prepolymers with
nitrile functionalities 2. The reaction was catalyzed by
either potassium hydroxide or sodium hydride in an
amount of 1 mol %. Next, the nitrile groups of 2 were
hydrogenated to yield the amine-terminated prepoly-
mers 3.15 Since catalytic hydrogenation and reduction
with LiAlH4 were unsuccessful, this hydrogenation was
carried out with borane in THF. After several coevapo-
rations with methanol and hydrochloric acid to remove
boron-nitrogen complexes, the product was obtained as
1
The product was obtained as a waxy solid (4.90 g, 85%). H
NMR (CDCl3): δ 4.9-4.7 (d br s, 4H, NHCdONH), 3.50 (t,
4H, NCH2CH2CH2O, J ) 5.9 Hz), 3.41 (br m, 70H, OCH2CH2-
CH2CH2O), 3.27 (q, 4H, OCH2CH2CH2N, J ) 6.2 Hz), 3.27 (q,
4H, (CdO)NHCH2CH2CH2CH2N, J ) 5.9 Hz), 2.71 (t, 4H,
NH2CH2, J ) 6.8 Hz), 1.75 (qui, 4H, NCH2CH2CH2O, J ) 6.3
Hz), 1.62 (br m, 70H, OCH2CH2CH2CH2O), 1.50 (m, 8H,
NHCH2CH2CH2CH2N), 1.03 ppm (s, 4H, NH2). FT-IR (ATR):
ν 3327, 2938, 2859, 1616, 1587, 1365, 1106 cm-1
.

Macromolecules, Vol. 38, No. 8, 2005
Scheme 2. Synthesis of PTHF1100-U
Segmented Copoly(ether urea)s 3179
fibers. Solution casting of the polymers from chloroform
solution gave completely transparent elastic films, in
contrast to pTHF1000-U with one urea group, which was
obtained as a viscous liquid at room temperature.
To study the influence of the molecular weight of the
prepolymer (soft block), amine-terminated prepolymers
with varying molecular weights were chain extended by
reaction with 1,4-diisocyanatobutane. These polymers
are schematically denoted as e.g. pTHF1100-U2. After
precipitation, the polymers were obtained as white,
elastic, fluffy fibers, except for pTHF350-U2, which was
obtained as a powder. The molecular weight of this
polymer was also considerable lower than that of the
other polymers, probably caused by the low functionality
of the starting amine-terminated prepolymer.
Scheme 3. Two Routes toward PTHF1100-U-R-U
Although the copoly(ether urea)s with two urea
groups in the hard block possess already highly elastic
properties, we also aimed at synthesizing a polymer
comprising three urea groups in the hard block,
pTHF1100-U3, since it will have a higher melting tem-
perature of the hard block. The synthetic strategy to
achieve this goal is outlined in Scheme 4. The first step
was the monoprotection of 1,4-diaminobutane by reac-
tion with di-tert-butyl dicarbonate. After several extrac-
tions and a recrystallization 4-(tert-butoxycarbonylamino)-
1-butylamine (5) was obtained in a yield of 80%.
Reaction of the remaining amine group with di-tert-
butyl tricarbonate gave isocyanate 6 within 15 min in
a quantitative yield.
Isocyanate 6 was reacted with the amine-terminated
pTHF1100, and in the next step the protective group was
removed by treatment with trifluoroacetic acid (TFA),
yielding the amine-terminated prepolymer 8 with one
urea group at both end groups. Reaction of 8 with 2
equiv of di-tert-butyl tricarbonate (4) gave the corre-
sponding isocyanate-terminated prepolymer, and poly-
condensation with amine-terminated prepolymer 8 re-
sulted in the segmented copolymer with exactly three
urea groups in the hard block. During this step some
methanol was added to avoid gelation. After precipita-
tion of the product in pentane, the polymer was obtained
as a white fluffy powder. Solution-casting of the product
gave a transparent flexible film. The molecular weight
could not be determined by SEC, since the polymer was
insoluble in the eluent NMP.
The synthesis of a segmented copoly(ether urea)
having a hard block containing exactly four urea groups
in the hard block, pTHF1100-U4, is a straightforward
continuation of the synthesis of the one with three urea
groups. Amine-terminated prepolymer 8 was reacted
with 1,4-diisocyanatobutane in a mixture of methanol
and chloroform (Scheme 5). Because of the very strong
interaction between this hard block comprising four
urea groups, the solution became highly viscous, and
this hampered the precipitation of the product in
heptane. The polymer was obtained as a rubbery solid.
The molecular weight could not be determined by SEC
since the polymer was insoluble in the eluent NMP.
Finally, a segmented copoly(ether urea) with a poly-
disperse hard block with on average two urea groups,
pTHF1100-U2PD, was synhesized to study the influence
of the uniformity of the hard block. This polymer was
prepared by adding di-tert-butyl tricarbonate 4 (half an
equivalent of per amino group) to an equimolar mixture
of amino-terminated pTHF1100 and 1,4-diaminobutane
(Scheme 6). Assuming equal reactivity of the amino
groups of the prepolymer and diaminobutane, a random
a white powder. Characterization by 1H NMR, 13C NMR,
FT-IR, SEC, and MALDI-TOF MS proved the proposed
structure of 3 and showed a degree of functionalization
of more than 97%.
Prepolymers with a range of molecular weights were
prepared and were used for the synthesis of thermo-
plastic elastomeric polyurethanes.
Thermoplastic elastomers with one to four urea
groups in the hard segment were prepared using the
di-tert-butyl tricarbonate reagent,12 in combination with
a protective group strategy for the polymers with 3 or
4 urea groups. These polymers are schematically de-
noted as pTHFy-Ux, in which x is the number of urea
groups in the hard block and y the number-averaged
molecular weight of the pTHF soft block.
The synthesis of pTHF1100-U, a polymer with exactly
one urea group in the hard block, is shown in Scheme
2. Amine-terminated prepolymer, pTHF, with molecular
weight of 1100 g/mol was reacted with 1 equiv of di-
tert-butyl tricarbonate (4) per two amine groups. The
formed isocyanate groups react immediately with an-
other amine group, resulting in a single urea group.
After workup, the product was obtained as a viscous
liquid in a yield of 97% and with Mw ) 68 × 103 g/mol
based on SEC.
Polymers pTHF1100-U2 having a hard block compris-
ing exactly two urea groups separated by 2-6 methyl-
ene groups were synthesized by chain extension of the
amine-terminated prepolymer with the appropriate
diisocyanate (method A) or by reacting the isocyanate-
terminated prepolymer with a diamine (method B)
(Scheme 3), depending on the availability of the diiso-
cyanate. The chain extender was added dropwise to a
solution of the prepolymer in CHCl3, to ensure the exact
1 to 1 ratio of both functional groups in the equivalence
point of the reaction, thus obtaining a maximum degree
of polymerization. Monitoring the disappearance of the
isocyanate band with infrared spectroscopy followed the
extent of the reaction.
The polymers were obtained in good yields with
molecular weights ranging from 20 × 103 to 55 × 103
g/mol. All polymers were obtained as highly elastic fluffy

3180 Versteegen et al.
Macromolecules, Vol. 38, No. 8, 2005
Scheme 4. Synthesis of a Polymer Comprising Three Urea Groups per Hard Block, PTHF1100-U3
Scheme 5. Synthesis of a Polymer Comprising Four Urea Groups per Hard Block, PTHF1100-U4
Scheme 6. Synthesis of Polymer with Polydisperse Hard Block, PTHF1100-U2PD
distribution of the hard block length will be obtained.
The polymer was obtained as white, elastic, fluffy fibers
by precipitation in pentane. The material was moder-
ately soluble in chloroform-methanol mixtures but
insoluble in NMP, hampering the determination of its
molecular weight.
3.2. Hydrogen Bonding within Block Copoly-
(ether urea)s. Infrared spectroscopy is a helpful tool
for studying the extent of hydrogen bonding of several
hydrogen-bonding groups.11f,16 Coleman and Painter
reported on a temperature-dependent infrared study of
a polyurea and model ureas.16a They concluded that the
frequency of both the N-H and the CdO (amide I)
vibration depend strongly on the hydrogen-bonding
nature of the urea groups (Figure 2).
If the hydrogen-bonding strength between urea groups
increases, the frequency of both the N-H and CdO
vibrations decreases, in contrast to the amide II band
(a combined N-H bending and C-N stretching vibra-
tion) at approximately 1575 cm-1, which increases in

Macromolecules, Vol. 38, No. 8, 2005
Segmented Copoly(ether urea)s 3181
Figure 2. Characteristic infrared bands for urea groups.
Table 1. Synthesis of Amine-Terminated Prepolymers 3
prepolymer
Mn (g mol-1
)
Mw/Mn
yield (%)
pTHF350
350
1100
2500
5200
4500
1.8
1.7
1.5
1.5
1.4
pTHF1100
pTHF2000
pTHF2900
PTHF/EO4000
86
90
89
a
a Random copolymer of tetrahydrofuran and ethylene oxide.
Table 2. Synthesis of PTHF-U2 with Varying Size of Soft
and Hard Segments
spacer R
soft block
method yield (%) Mw (103 g/mol)a
n-C2H4
n-C3H6
n-C4H8
n-C4H8
n-C4H8
n-C4H8
n-C4H8
pTHF1100
pTHF1100
pTHF350
pTHF1100
pTHF2000
pTHF2900
PTHF/EO4000
B
B
A
A
A
A
A
B
A
80
86
83
89
85
84
93
84
89
41
47
8.5
42
53
32
58
32
43
Figure 3. Carbonyl region of infrared spectra of copoly(ether
urea)s.
n-C5H10 pTHF1100
n-C6H12 pTHF1100
a Measured by SEC, with NMP as solvent, relative to polysty-
rene standards.
frequency. Thus, the position of these bands is a direct
indication of the strength of hydrogen bonding. Fur-
thermore, competitive hydrogen bonding between urea
and ether groups in the soft block, so-called mixing of
hard and soft phases, was quantified. It is more practi-
cal to use the carbonyl stretching vibration since this
band is narrower than the N-H stretching vibration,
and it shifts more upon increase of the hydrogen-
bonding strength. In our copolymers, the absence of any
carbonyl groups other than from the urea groups, such
as urethanes, amides, or esters, is advantageous, al-
lowing an unambiguous analysis of the hydrogen bonds.
The block copoly(ether urea)s show a remarkable
difference in their physical properties. The polymer with
only one urea group in the hard block, pTHF1100-U,
possesses hardly any mechanical properties; it was
obtained as a viscous liquid. This is an indication of the
absence of stable cross-links at room temperature. In
the infrared spectra this is evidenced by the position of
the carbonyl stretching vibration at 1637 cm-1 (Figure
3 top), corresponding to weakly hydrogen-bonded urea
groups. Apparently, one single urea group is not suf-
ficient to form stable cross-links by strong hydrogen
bonds. Hence, this unit cannot form a hard block.
However, for the sake of conformity, we like to use this
term here.
free urea groups were observed. Thus, two urea groups
in the hard blocks induce strong association and stable
cross-links at room temperature. The length of the
spacer between the urea groups (Table 2) has no effect
on the nature of hydrogen bonding. In all these poly-
mers, strong hydrogen bonding is present according to
infrared spectroscopy.
Table 2 shows the different soft blocks that were used
in the segmented polymers with two urea groups in the
hard block. Upon comparison of the FT-IR spectra of
the polymers with different soft block length, no differ-
ence is observed for the position of the carbonyl stretch-
ing vibration. However, the intensity of this peak varies
considerably since the concentration of urea groups
depends on the molecular weight of the soft block.
The copoly(ether urea)s having three and four urea
groups in the hard block show comparable FT-IR spectra
as the polymers with two urea groups (Figure 3). The
intensity of the bands corresponding to urea groups
increases with increasing number of urea groups. The
position of the amide I band does not change upon
introducing more than two urea groups, indicating no
change in hydrogen-bonding strength. The amide II
band shifts slightly and broadens; the reason for this is
unclear.
To compare the hydrogen bonding in the uniform hard
blocks with the polydisperse ones, part of the infrared
spectra of polymers pTHF1100-U2 and pTHF1100-U2PD is
shown in Figure 4. Both polymers have the same overall
composition, which is reflected by the equal intensity
The segmented copolymers with two urea groups in
the hard block (Table 2) possess very intriguing elastic
properties. The fibers are shape-persistent and not
tacky. The infrared spectrum of pTHF1100-U2 (Figure 3)
shows a strong, sharp peak at 1615 cm-1, indicative of
strong hydrogen bonding between urea groups. No other
peaks corresponding to weakly hydrogen bonded and

3182 Versteegen et al.
Macromolecules, Vol. 38, No. 8, 2005
the aliphatic, linear spacer between the urea groups.
The polymers with an even number of methylenes
systematically have a higher flow temperature com-
pared to those with an odd number. This is often
observed in such a homologous series, including the
[n]-nylons and [n]-polyurethanes.18 The polymer with
a butylene spacer (n ) 4) has the highest flow temper-
ature, and this is one of the reasons for us to study this
hard segment in more detail.
In Figure 5b, the influence of the number of urea
groups in the hard block is depicted. As discussed before,
the polymer with only one urea group shows viscous flow
even below room temperature. Incorporation of one
additional urea group makes the material elastomeric
and increases the flow temperature to about 140 °C. The
melting of the hard blocks is completely reversible.
Increasing the number of urea groups in the hard block
to three or four results in polymers that do not flow
below 200 °C. Above this temperature, degradation
starts to occur. This was evidenced by discoloration and
fuming of the sample. Although both materials started
to flow when heated slightly above 200 °C for prolonged
times, this process was not reversible, which is also an
indication of degradation. The polymer with a poly-
disperse hard block (on average two urea groups) shows
a flow temperature only slightly lower than the one with
exactly two urea groups.
In Figure 5c, the flow temperature as a function of
soft block length is shown for the polymers with two
urea groups separated by a butylene spacer in the hard
block. The flow temperature decreases with increasing
soft block length. This is a general trend observed for
segmented copolymers and is explained by the solvent
effect proposed by Flory.19 Upon dilution of the crystal-
lizable hard segment, the size of the crystals will become
smaller and hence its melting point decreases. Thus,
varying the soft block length enables us to alter the flow
temperature of the material.
Figure 4. Carbonyl region of infrared spectra of copoly(ether
urea)s: uniform hard block (top) and polydisperse hard block
(bottom).
of the different functional groups in the polymer.
However, the polymer with the polydisperse hard block
shows a broader peak corresponding to the carbonyl of
the urea group compared to the uniform hard block. This
indicates a distribution in hydrogen-bonding strengths
within the polydisperse hard blocks. The position of this
peak is also at higher frequency (1620 cm-1 vs 1615
cm-1, respectively), which denotes that, on average, the
hydrogen bonding in the polydisperse hard blocks is
weaker than in the uniform hard blocks. This confirms
our hypothesis that uniform hard blocks associate
stronger than polydisperse hard blocks.
3.3. Flow Temperatures. Thermoplastic elastomers
possess elastic properties at ambient temperatures and
thermoplastic properties at temperatures above the
melting point of the thermoreversible cross-links. The
flow temperatures of the different block copoly(ether
urea)s were determined by optical microscopy. The
temperatures at which the material lost its dimensional
stability, the flow temperature, are depicted in Figure
5. Since we are interested in the material properties at
and above ambient temperature, the lower limit of the
desired flow temperature range is room temperature,
and the upper limit is 200 °C, which is the decomposi-
tion temperature of the urea groups.17 This window is
indicated by the two dotted horizontal lines.
4. Conclusions
Using an optimized two-step synthesis, consisting of
cyanoethylation followed by hydrogenation with borane,
R,ω-diamino-poly(tetrahydrofuran)s are readily obtained
in excellent yields on a 50 g scale from commercially
available hydroxy-terminated prepolymers. The amine-
terminated pTHF prepolymers are convenient materials
for use as the soft block in block copoly(ether urea)s with
hard blocks consisting of exactly 1-4 urea groups. The
Figure 5a shows the flow temperature of the material
as a function of the number of methylene units within
Figure 5. Flow temperature of block copoly(ether urea)s: (a) Dependence of flow temperature on number of methylene units
between urea groups in hard block. (b) Dependence on number of urea groups in hard block. Urea groups are separated from eac
other by four methylene units (2PD denotes a polymer with a polydisperse hard block with on average two urea groups). (c)
Dependence of flow temperature on molecular weight of soft block.

Macromolecules, Vol. 38, No. 8, 2005
Segmented Copoly(ether urea)s 3183
Prepr. 1999, 40, 1012. (c) Niesten, M. C. E. J.; Gaymans, R.
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number of urea groups in the hard blocks can be
controlled exactly by employing novel isocyanate chem-
istry, using di-tert-butyl tricarbonate, and protective
group strategy. The polymer having only one urea group
in the hard block is a viscous liquid. Polymers having
two urea groups in the hard block possess an optimal
balance between mechanical properties and process-
ability, as they are highly elastic and very soluble. Their
synthesis is straightforward, as no elaborate protective
group chemistry has to be employed. A number of
polymers with varying spacers between the two urea
groups and with different soft block lengths were
prepared. The flow temperature of these materials
ranged from 100 to 165 °C and decreased with increas-
ing soft block length. Increasing the number of urea
groups in the hard block to three or four gave insoluble
gel-forming polymers that are hard to process. Appar-
ently, the association between the hard blocks in these
polymers is too strong.
The strength of hydrogen bonding of the urea groups
in the polymers was monitored by infrared spectroscopy.
This confirmed our observation that hydrogen bonding
between hard blocks having only one urea group is
weak. By increasing the number of urea groups to two
or more, the association of the hard blocks increases,
and strong hydrogen bonding is observed by FT-IR.
Measurements on materials with two urea groups in the
hard block shows that the length of the spacer has little
influence on urea hydrogen bonding.
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Comparison of the FT-IR spectra of a polymer pos-
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polydisperse hard block indicates a distribution of
hydrogen-bonding strengths and weaker hydrogen bond-
ing between the polydisperse hard blocks, illustrating
the advantages of the well-defined character of these
polymers. A more detailed study of the morphology,
thermal behavior, and mechanical properties of these
polymers will be discussed in a forthcoming paper.20
Acknowledgment. We acknowledge J. L. J. van
Dongen, X. Lou, and R. A. A. Bovee for their experi-
mental help and analyses. Support from the Council for
Chemical Sciences of the Netherlands Organization for
Scientific Research (CW-NWO) with financial aid from
the Netherlands Technology Foundation (STW) is grate-
fully acknowledged.
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