Linear poly(amide triazole)s derived from d -glucose

Article abstract of DOI:10.1002/pola.27038

The click reaction between azides and alkynes is been increasingly employed in the preparation of polymers. In this article, we describe the synthesis and click polyaddition reaction of a new A-B-type amide monomer - prepared from d-glucose as renewable r

Full text of DOI:10.1002/pola.27038

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Linear Poly(amide triazole)s Derived from D-Glucose
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Inmaculada Molina-Pinilla, Manuel Bueno-Martınez, Khalid Hakkou, Juan A. Galbis
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Departamento de Quımica Organica y Farmaceutica, Facultad de Farmacia. Universidad de Sevilla, 41012-Sevilla, Spain
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Correspondence to: M. Bueno-Martınez (E-mail: mbueno@us.es)
Received 23 October 2013; accepted 25 November 2013; published online 3 December 2013
DOI: 10.1002/pola.27038
ABSTRACT: The click reaction between azides and alkynes is
been increasingly employed in the preparation of polymers. In
this article, we describe the synthesis and click polyaddition
reaction of a new A-B-type amide monomer—prepared from
D-glucose as renewable resource—containing the alkyne and
azide functions. Both Cu(I)-catalyzed and metal-free click poly-
merization methods were used to prepare glucose-derived
poly(amide triazole)s. The resulting polymers had weight-
average molecular weights in the 45,000–129,000 range and
were characterized by GPC, IR, and NMR spectroscopies. Ther-
mal and X-ray diffraction studies revealed them to be amor-
phous. Their qualitative solubilities in various solvents and
their water sorption have been studied. The poly(amide tria-
zole)s having the alcohol functions protected as methyl ether
were water-soluble. The presence of the amide functions along
the polymer chain made these polytriazoles degradable in the
presence of sodium deuteroxide. The degradation was moni-
tored by NMR analysis, and the degradation product was char-
C
V
acterized by HRMS.
2013 Wiley Periodicals, Inc. J. Polym.
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KEYWORDS: carbohydrate-based polymers; click polymerization;
hydrophilic polymers; degradation; water-soluble polymers
methyl or benzyl groups, which confer different hydrophilic-
ity to the obtained polymers. In some cases, the poly(amide
triazole) was readily water-soluble, a feature which might be
of interest in certain medical applications. In addition, the
presence of the triazole rings in these new polyamides, as
well as the free hydroxyl group present in the repeating
units, could induce the development of biological activity by
association with biological targets through hydrogen bonding
and dipole interactions.11
INTRODUCTION Recently, extensive use has been made of
the highly efficient, orthogonal, and regiospecific copper(I)-
catalyzed azide–alkyne cycloaddition (CuAAC),1
a prime
example of the “click chemistry” concept.2 The attractive fea-
tures of this click reaction have inspired numerous applica-
tions across different scientific disciplines.3 The CuAAC has
been widely used in polymer science, mainly in the postfunc-
tionalization of preformed polymers,4 and for the synthesis
of a wide variety of macromolecular material. However, the
preparations of linear triazole-linked polymers are more
scarce. In this respect, polyaddition of A2 1 B2 or self-
reacting AB small monomers has previously been used for
the preparation of different polytriazoles,5 polyesters,6 poly-
pseudopeptides,7 poly(aroyltriazole)s,8 poly(alkyl aryl
ether)s,9 or poly(glycoamidoamine)s.10
The copper-catalyzed azide–alkyne cycloaddition often leads
to insoluble products in commonly used organic solvents;
moreover, the complete removal of the metallic catalysts
from the resulting polymers can be very difficult. Some
authors have shown the presence of residual copper follow-
ing a CuAAC synthesis, even after extensive washing with
EDTA solution.12 The presence of catalyst residues can be
detrimental to the electronic and optical properties of poly-
mers, and may alter other properties of the resulting mate-
rial—for example its hydrolytic degradation—compromising
the potential applications.13 To avoid these problems related
with the use of copper as a catalyst, a click polymerization
without transition-metal catalyst, that is, metal-free click
polymerization, has been successfully established, and func-
tional linear poly(amide triazole)s have been prepared under
mild conditions.
We have previously studied the CuAAC polymerization of
A2 1 B2 monomers, and the structure and properties of the
resulting linear polytriazoles.6(a) Here, we report the synthe-
sis of two self-reacting AB monomers derived from D-glucose,
a cheap and easily available sugar, making it a very attractive
starting material in the synthesis of polymers based on sus-
tainable chemistry. The polymerizations of these monomers
lead to poly(amide triazole)s containing hydrolyzable amide
functions along the polymer backbone. These carbohydrate-
derived polytriazoles present all of their secondary alcohol
functions protected, except one per repeating unit, with
C
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In this article, the polyaddition of these self-reacting a-azide-
x-alkyne AB monomers by CuAAC in solution or by catalyst-
and solvent-free 1,3-dipolar Huisgen cycloaddition is studied
in detail.
tion. Water sorptions were measured at 100% relative
humidity using the method described by Mori.14
Materials
Chemicals were all used as purchased from Aldrich Chemical.
Solvents were dried and purified when necessary, by appro-
priate standard procedures. 6-Azido-6-deoxy-2,3,4-tri-O-
methyl-D-glucono-1,5-lactone15 (8) and 6-azido-6-deoxy-
2,3,4-tri-O-benzyl-D-glucono-1,5-lactone16 (9) were obtained
from O-methyl-a-D-glucopyranoside in global yields of 45 and
40%, respectively.
EXPERIMENTAL
Measurements
Thin-layer chromatography (TLC) was performed on Silica
Gel 60 F254 (E. Merck) with detection by UV light or char-
ring with H2SO4. Flash column chromatography was per-
formed using E. Merck Silica Gel 60 (230–400 mesh). IR
spectra (films or KBr disks) were recorded with a JASCO FT/
IR-4200 spectrometer. 1H and 13C NMR spectra were
recorded with a Bruker AMX-500 or Bruker AV300 spec-
trometers. Chemical shifts are reported as parts per million
downfield from tetramethylsilane. Two-dimensional 1H–1H
homonuclear and 13C–1H heteronuclear shift correlation
spectra were recorded with the COSY and HETCOR pulse
sequences, respectively. Solid-state NMR measurements were
carried out at room temperature on a Bruker Avance-III
600-MHz spectrometer using broadband X-H cross-polariza-
tion (CP)/magic angle spinning (MAS) probes. The polymers
were first ground into a thin powder, packed down in the
MAS rotors, and analyzed with the CP procedure under MAS.
The samples were spun at 10 kHz, in 4-mm ZrO2 rotor; a
contact time of 2 ms, a repetition time of 5 s, and a spectral
width of 31 kHz were used for accumulation of 800 scans.
Chemical shifts were calibrated indirectly through the glycine
CO signal recorded at 176.0 ppm relative to TMS. Elemental
analyses were determined in the Microanalysis Laboratories
of the CITIUS Service at the University of Seville (Seville,
6-Azido-6-deoxy-2,3,4-tri-O-methyl-N-(prop-2-yn-1-yl)-D-
gluconamide (10)
A solution of 6-azido-6-deoxy-2,3,4-tri-O-methyl-D-glucono-
1,5-lactone15 (8) (3.80 g, 15.49 mmol) and propargylamine
(1.0 mL, 15.6 mmol) in dry methanol (5 mL) was refluxed
for 2 h. The solvent was evaporated under reduced pressure
and the residue was chromatographed on a silica gel column
(eluent 2:1 hexane/ethyl acetate) to give 10 as a syrup
(3.50 g, 75%).
IR: mmax 3600–3100 (OH, NH), 2100 (N3, CBC), 1660 and
1520 cm21 (Amide); NMR data (CDCl3): 1H, d 6.90 (t, 1H,
NH), 4.12 (m, 2H, H-c), 3.94 (d, 1H, J2,3 2.4 Hz, H-2), 3.89
0
(dc, 1H, J4,5 8.4, J5,6 2.8, J5,6 5.4 Hz, H-5), 3.78 (dd, 1H, J3,4
0
4.8 Hz, H-3), 3.54 (dd, 1H, J6,6 12.6 Hz, H-6), 3.50–3.44 (m,
1H, H-4), 3.49–3.46 (m, 9H, 3 OMe), 3.41 (dd, 1H, H-60), 2.26
(t, 1H, Ja,c 2.51 Hz, H-a). 13C, d 170.85 (C-1), 81.55 (C-3),
80.95 (C-2), 79.08 (C-b), 78.15 (C-4), 71.76 (C-5), 71.66
(C-a), 60.40, 59.40, 59.30 (OCH3), 53.80 (C-6), 28.85 (C-c).
Anal. Calcd for C12H20N4O51/3H2O: C, 47.05; H, 6.80; N,
18.29. Found: C, 47.13; H, 6.86; N, 17.98.
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Spain). Optical rotations were measured at 20 6 5 C with a
Perkin-Elmer 341 polarimeter. Chemical ionization and fast-
atom bombardment mass spectra were performed on a
micromass Autospec spectrometer. FABMS spectra were
obtained using thioglycerol-NaI as a matrix. The diffracto-
grams were obtained using a Bruker D8 Advance diffractom-
eter. Cu Ka (nickel-filtered) radiation was generated using a
Kristalloflex 780 generator. The operating voltage and cur-
rent were 40 kV and 40 mA. The radial scans on the samples
were carried out in the region 2h 5 3ꢀ–70ꢀ. The data were
collected at 0.02ꢀ intervals with counting for 10 s at each
step. The thermal properties of the polymers were deter-
mined by differential scanning calorimetry (DSC) using a TA
Instruments DSC Q200, calibrated with indium. Samples of
6-Azido-6-deoxy-2,3,4-tri-O-benzyl-N-(prop-2-yn-1-yl)-D-
gluconamide (11)
A solution of 6-azido-6-deoxy-2,3,4-tri-O-benzyl-D-glucono-1,5-
lactone16 (9) (2.16 g, 4.56 mmol) and propargylamine (0.3 mL,
4.68 mmol) in dry methanol (3 mL) was refluxed for 2 h. The
solvent was evaporated under reduced pressure and the resi-
due was chromatographed on a silica gel column (eluent 2:1
hexane/ethyl acetate) to give 11 as a syrup (1.92 g, 80%).
IR: mmax 3600–3100 (OH, NH), 3065, 3035 (H-Csp and aro-
matic), 2100 (N3, CBC), 1672 and 1518 cm21 (Amide); NMR
1
data (CDCl3): H, d 7.43–7.28 (m, 15H, Ar), 6.86 (t, 1H, NH),
21
4.67–4.45 (m, 6H, CH2Ph), 4.32 (d, 1H, J2,3 2.2 Hz, H-2), 4.12
(dd, 1H, J3,4 5.0 Hz, H-3), 4.05 (dd, 1H, Ja,c 2.5, Jc,c’ 5.6 Hz,
H-c), 4.03 (dd, 1H, Ja,c’ 2.5 Hz, H-c’), 4.00 (m, 1H, H-5), 3.72
ꢀ
about 2–3 mg were heated at a rate of 10 C min under a
nitrogen atmosphere (flow rate of 20 mL min21) and cooled
to room temperature. Thermogravimetric analyses (TGAs)
were performed with a SDTQ600 TA instrument at a heating
0
(dd, 1H, J4,5 8.5 Hz, H-4), 3.50 (dd, 1H, J5,6 2.8, J6,6 12.7 Hz,
21
rate of 10 C min under a nitrogen flow of 100 mL min21
.
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0
0
H-6), 3.34 (d, 1H, J 2.6 Hz, OH), 3.50 (dd, 1H, J5,6 5.40, J6,6
12.7 Hz, H-60), 2.24 (t, 1H, Ja,c 2.6 Hz, H-a). 13C, d 170.85 (C-
1), 137.31, 136.98, 136.09, 128.82, 128.60, 128.55, 128.28,
128.14 (CH2Ph), 79.36 (C-3), 79.07 (C-2,b), 75.76 (C-4),
74.53, 74.08, 73.50 (CH2Ph), 72.09 (C-5), 71.81 (C-a), 53.43
(C-6), 28.87 (C-c). HRMS: m/z 529.2462 (calcd for [M11]1:
529.2451). Anal. Calcd for C30H32N4O5 H2O: C, 65.92; H,
6.27; N, 10.25. Found: C, 66.09; H, 6.08; N, 10.08.
Gel permeation chromatography (GPC) analyses were per-
formed using a Waters apparatus equipped with a Waters
2414 refractive-index detector and two Styragel HR columns
(7.8 3 300 mm) linked in series, thermostatted at 60 ꢀC,
using N,N-dimethylformamide-LiBr as the mobile phase, at a
flow rate of 1.0 mL min21. Calibration was performed using
12 polystyrene samples of narrow molecular-weight distribu-
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TABLE 1 Properties of Polytriazoles 12–15
137.44, 137.28, 136.67, 128.19, 128.12, 128.00, 127.88,
127.71, 127.68, 127.64, 127.36, 127.31, 123.88 (CH2Ph),
132.22 (C-a0), 123.88 (C-a), 81.29–81.12 (C-2, 20), 80.73,
80.48, 80.30, 80.13 (C-3, 30, 4, 40), 74.75, 74.69, 73.80, 72.74,
72.59 (CH2Ph), 71.30, 70.55–70.50 (C-5, 50), 52.62 (C-6),
50.60–50.40 (C-60), 34.27 (C-c), 31.84 (C-c0). Anal. Calcd for
Polymera
Yield (%)
1,4/1,5b Ratio
Mn
Mw
c
c
12 (A)
13 (A)
14 (B)
15 (B)
100
100
93
63/35
67/33
100/0
100/0
90,700
57,300
33,600
101,000
112,900
72,400
44,800
129,100
C24H42N2O10: C, 48.90; H, 7.18; N, 4.75. Found: C, 48.83; H,
97
7.47; N, 4.46.
a
A: neat, thermal polymerization; B: tBuOH-H2O, CuSO4, sodium
Poly(amide triazole) 14
ascorbate.
IR: mmax 3316 (OH, NH), 1651, 1525 cm21 (Amide); NMR
data (D2O): 1H, d 8.19 (bs, 1H, H-a), 4.80–4.30 (m, 4H, H-c,
H-6), 4.15–4.00 (m, 1H, H-5), 4.00–3.85 (m, H-2), 3.80–3.60
(m, 1H, H-3), 3.60–3.20 (m, 1H, H-4), 3.46, 3.37, 3.31 (3s,
9H, 3 OCH3). 13C, d 172.45 (C-1), 82.20, 81.72, 81.58 (C-2, 3,
4), 70.13 (C-5), 60.65, 60.54, 58.81 (OMe), 53.04 (C-6),
34.61 (C-c).
b
From 1H NMR.
c
Determined by GPC analysis with polystyrene standards. Mobile
phase: DMF-LiBr.
General Procedure for the Synthesis of Poly(amide
triazole)s
Procedure A: Thermal Polymerization
Monomer 10 or 11 was heated under argon at 100 ꢀC for
24 h to obtain the polymer 12 or 13, respectively, as a
glassy material.
Poly(amide triazole) 15
IR: mmax 3400 (OH, NH), 3060, 3031 (Ar), 1655, 1522 cm21
1
(Amide); NMR data (DMSO-d6): H, d 8.50 (bs, 1H, NH), 7.83
(bs, 1H, H-a), 7.40–7.06 (m, 15H, CH2Ph), 5.48 (d, 1H, J 5.8
Hz, OH), 4.82–4.23 (m, 10H, 3 CH2Ph, H-6, H-c), 4.19 (d, 1H,
J2,3 4.7 Hz, H-2), 4.12–3.94 (m, 2H, H-3, H-5), 3.66 (m, 1H, H-
4). 13C, d 169.75 (C-1), 144.21 (C-b), 138.55, 138.41, 137.41,
128.17, 128.09, 128.00, 127.66, 127.61, 127.34, 127.69
(CH2Ph), 123.89 (C-a), 81.11 (C-2), 80.46, 80.30 (C-3, 4),
74.68, 73.74, 72.57 (CH2Ph), 70.54 (C-5), 52.63 (C-6), 34.28
(C-c).
Procedure B: Copper-Catalyzed Reaction
The above bifunctional monomers were polymerized in (1:1)
tert-butanol-H2O under argon at 50 ꢀC for 24 h in the pres-
ence of 5% CuSO4 and 10% sodium ascorbate. Poly(amide
triazole) 14 was isolated and washed sequentially with ether,
dichloromethane, and tert-butanol. The residue was dis-
solved in water, dialyzed against water, and finally lyophi-
lized. Poly(amide triazole) 15 was isolated and purified as
follows: after the polymerization period, the suspended poly-
mer was recovered by filtration on a glass filter and washing
with (1:1) tert-butanol-H2O. The isolated yields and some
characteristics of the polymers are given in Table 1. Their
spectroscopic properties (IR and NMR spectra) are described
below.
Hydrolytic Degradation
To a solution of poly(amide triazole) 12 in D2O (0.6 mL)
was added 20 lL of 40 wt % NaOD in D2O. The resultant
mixture was stirred at 44 ꢀC for different times. The degra-
dation of the polymer was monitored by NMR spectroscopy.
General Procedure for Monitoring the Polyaddition
Reaction by DSC
Poly(amide triazole) 12
IR: mmax 3323 (OH, NH), 1659, 1520 cm21 (Amide); NMR
1
DSC capsules containing about 3 mg of monomer 10 or 11
were sealed off under ambient conditions. A first capsule of
each monomer was heated in the DSC apparatus from 240
to 250 ꢀC at a heating rate of 10 ꢀC min21 to obtain the
monomer glass transition temperature (Tg) and the total
bulk polyaddition enthalpy (DHtotal). Then, different capsules
data (D2O): H, d 7.94 (bs, 1H, H-a), 7.72 (bs, 1H, H-a0),
4.80–4.40 (m, 8H, H-c, c0, 6, 60), 4.15–4.00 (m, 2H, H-5, 50),
4.00–3.90 (m, 2H, H-2, 20), 3.75–3.65 (m, 2H, H-3, 30), 3.55–
3.25 (m, 2H, H-4, 40), 3.52, 3.47, 3.39, 3.38, 3.34, 3.32 (6s,
13
18H, 6 OCH3). C, d 172.79 (C-10), 172.45 (C-1), 144.28 (C-
b), 136.38 (C-b0), 133.26 (C-a0), 125.03 (C-a), 82.16, 82.02,
81.61, 81.50 (C-2, 20, 3, 30, 4, 40), 70.55 (C-50), 70.30 (C-5),
60.65, 60.62, 60.55, 59.03, 58.83 (OMe), 52.72 (C-6), 50.70
(C-60), 34.15 (C-c), 31.96 (C-c0). Anal. Calcd for C12H20N4O5:
C, 47.99; H, 6.71; N, 18.65. Found: C, 47.85; H, 6.72; N,
18.20.
ꢀ
were heated in an oven at 100 C for different times before
being quenched at room temperature and directly analyzed
by DSC to access Tg of the formed polymers and the residual
polyaddition enthalpy (DHresidual).
RESULTS AND DISCUSSION
Poly(amide triazole) 13
Novel carbohydrate-derived poly(amide triazole)s 12–15
(Fig. 1) were prepared from 6-azido-6-deoxy-2,3,4-tri-O-
methyl-N-(prop-2-yn-1-yl)-D-gluconamide (10) and 6-azido-
6-deoxy-2,3,4-tri-O-benzyl-N-(prop-2-yn-1-yl)-D-gluconamide
(11).
IR: mmax 3416 (OH, NH), 3061, 3030 (Ar), 1666, 1519 cm21
1
(Amide); NMR data (DMSO-d6): H, d 8.68 (bs, 1H, NH0), 8.52
(bs, 1H, NH), 7.84 (bs, 1H, H-a), 7.54 (bs, 1H, H-a0), 7.45–
7.10 (m, 15H, CH2Ph), 5.68–5.46 (4d, 1H, OH), 4.90–4.26 (m,
10H, 3 CH2Ph, H-6, 60, c, c0), 4.21 (m, 1H, H-2), 4.15–4.00 (m,
13
2H, H-3, H-5), 3.80–3.60 (m, 1H, H-4). C, d 170.27 (C-10),
The syntheses of these monomers were carried out in six steps
169.75 (C-1), 144.21 (C-b), 138.58, 138.51, 138.44, 138.35,
from methyl a-D-glucopyranoside by previously described
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FIGURE 1 Structures of the monomers and the poly(amide triazole)s.
methods.15,16 Briefly (Scheme 1), selective 6-tosylation of 1 fol-
lowed by treatment with sodium azide in N,N-dimethylforma-
mide gave 3. Treatment of 3 with methyl iodide-potassium
hydroxide in methyl sulfoxide or with sodium hydride and ben-
zyl bromide in THF gave the tri-O-methyl derivative 4 or the
tri-O-benzyl derivative 5. Acid hydrolysis of 4 and 5 followed
by oxidation17 with a mixture of acetic anhydride and methyl
sulfoxide gave 6-azido-6-deoxy-2,3,4-tri-O-methyl-D-glucono-
1,5-lactone15 (8) and 6-azido-6-deoxy-2,3,4-tri-O-benzyl-D-glu-
cono-1,5-lactone16 (9), respectively.
intermediates to prepare triazole-fused bicyclic com-
pounds,20,21 and they found that these intermediates are
formed with the concomitant generation of significant
amounts of the cyclization compounds, which in some cases
could be isolated in moderate yields. Second, there is the
issue of stability: once isolated the a-azido-x-alkyne com-
pounds must be stable enough to be handled and stored
without polymerizing.
In the present work, we were able to open the lactone ring,
with propargylamine in dry methanol at reflux, to obtain the
monomers 10 and 11 in good to high yield (75–80%). The
monomers could be handled at roꢀom temperature for hours or
stored at low temperature (220 C) for several months with-
out traces of coupling reactions as determined by 1H NMR.
Opening of the lactone ring with propargylamine should pro-
duce a-azide-x-alkyne compounds, which could be used as
monomers for the preparation of linear polytriazoles. How-
ever, this procedure has two potential drawbacks. First,
intramolecular reactions can occur, affording cyclic species.18
This could be limited by employing high monomer concen-
trations.19 Other authors have prepared azidoalkynes as
In order to examine the influence of polyaddition regiospeci-
ficity (1,4- or 1,4/1,5-triazoles) on the physicochemical
SCHEME 1 Synthesis of monomers.
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clearly shown by 1H NMR spectroscopy. For example, poly(-
amide triazole) 14 displayed only one broad singlet at 8.19
ppm [Fig. 2(B)] characteristic of 1,4-disubstituted 1,2,3-tria-
zoles. In contrast, its 13C NMR spectrum [Fig. 3(B)] did not
show the expected signals for the triazole ring. However, the
solid-state 13C NMR spectrum of poly(amide triazole) 14
[Fig. 3(C)] exhibited chemical shifts almost the same as its
liquid-state counterpart, including two broad signals at
125.03 and 144.27 ppm, which had not appeared before
[Fig. 3(B)], and were assigned to C-a and C-b, respectively.
Metal- and solvent-free click polymerization was also per-
formed to rule out the presence of copper ions in the result-
ing polymers, and thereby avoid the disadvantages caused by
the presence of residual metal catalysts in polymers, espe-
cially those that could be used as biomedical materials.22,23
Following this strategy, monomers 10 and 11 were heated at
100 ꢀC for 24 h to synthesize poly(amide triazole)s 12 and
13, respectively. The polymers were obtained as glassy color-
less materials in quantitative yield (Table 1). As is known,
under these reaction conditions, the polyaddition is nonre-
gioselective, as could be observed by NMR spectroscopies.
1
Both polymers (12 and 13) displayed in their H NMR spec-
trum two signals in a ratio of about 2:1. For example, for poly
(amide triazole) 12 the peaks appeared at 7.94 and 7.72 ppm,
which were assigned to 1,4- and 1,5-disubstituted 1,2,3-tria-
zoles, respectively [Fig. 2(A)]. 13C NMR data also support the
nonregioselective nature of the cycloaddition reaction. The
spectrum showed four signals for triazole rings. Signals
appearing at 144.28 and 125.03 ppm were assigned to C-b and
C-a, respectively [Fig. 3(A)], of the 1,4-disubstituted 1,2,3-tria-
zole. Two more signals at 136.38 and 133.26 ppm were found
in the spectrum, and were assigned to C-b0 and C-a0, respec-
tively, of the 1,5-disubstituted 1,2,3-triazole.
SCHEME 2 Synthesis of polymers 12–15.
properties of the resulting polymers, the synthesis of the
poly(amide triazole)s was carried out by CuAAC polyaddition
in solution or by catalyst- and solvent-free 1,3-dipolar Huis-
gen cycloaddition reaction (Scheme 2). Monomers 10 and 11
were first polymerized by CuAAC following the Sharpless
classical conditions. Thus, polyaddition was carried out in
DMSO-H2O mixtures, and catalyzed by copper sulfate in the
presence of sodium ascorbate (Scheme 2, Procedure B). Poly-
mers 14 and 15 were obtained, after purification, in high
yield (Table 1), but they still had some residual amounts of
copper. The presence of this ion produces the widening of
the proton of the triazole ring, and also lowers resolution of
Qualitative solubilities24 of polymers in various solvents are
listed in Table 2. The poly(amide triazole)s 12 and 14, which
contain hydrophilic methoxy groups along the polymer back-
bone, are soluble in highly polar aprotic solvents, such as
NMP, DMSO, or DMF, and are also readily soluble in water.
They are not soluble in nonaggressive oxygenated solvents
1
the signals in the H NMR spectrum. Nevertheless, the regio-
specificity expected for this cycloaddition reaction was
FIGURE 2 1H NMR spectra of poly(amide triazole)s: (A) 12; (B) 14.
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FIGURE 3 13C NMR spectra of poly(amide triazole)s: (A) 12; (B) 14; (C) 14 CPMAS.
such as acetone, ethyl acetate, or ether. It is worth mention-
ing the observed difference in solubility in methanol
between the two polymers.
correspond to the amide I and amide II band, respectively.
As an example, Figure 4 shows the IR spectra of 10 and 12.
The absorption band appearing at 2100 cm21 [Fig. 4(A)]
associated with the a-azido-x-alkyne functions of the mono-
mer disappeared in the IR spectrum of poly(amide triazole)
12 [Fig. 4(B)]. This indicates that the alkyne and azide
groups present in the monomer have undergone the polycy-
cloaddition reactions to give the triazole rings in the
polymer.
Poly(amide triazole) 14, which is regioregular, is not soluble
in methanol, despite having a lower molecular weight than
polymer 12. The poly(amide triazole)s 13 and 15, which
contain benzyl groups along the polymer chain, are less
hydrophilic than the polymers 12 or 14. So, they are soluble
in polar solvents such as dimethylsulfoxide or dimethylfor-
mamide, and also in acetone or ethyl acetate, but they are
not soluble in polar protic solvents such as methanol or
water. However, they still have some hydrophilicity. Thus, the
moisture sorption of poly(amide triazole) 15 determined14
at room temperature under a relative humidity of 100% was
7.5% after 48 h.
The molecular-weight distributions were studied by GPC
using lStyragel columns with DMF-LiBr as the mobile phase.
Relatively high values of weight-averaged molecular weights
(Mw) were obtained, ranging from 45,000 to 129,000
(Table 1). All the chromatograms were unimodal; as an
example, Figure 5 shows the evolution of the conversion of
monomer 11 to poly(amide triazole) 13. As can be seen,
after 3 h, the peak corresponding to monomer 11 becomes
very weak, and after 9 h it completely disappears.
IR spectra showed the characteristic absorption bands of the
amide and alcohol functions. The broad band around 3323
cm21, was attributed to the stretching band of the NH and
OH functions. The peaks appearing at 1659 and 1520 cm21
TABLE 2 Qualitative Solubilities of Polymers
Solventa
12
13
14
15
Ether
2
2
2
2
Acetone
Dichloromethane
Methanol
Water
2
11
11
2
2
2
2
2
2
11
11
11
11
11
2
2
2
11
11
11
11
2
DMSO
11
11
11
11
11
11
DMF
NMP
a
Estimated according to the method of Braun24: (2) insoluble and (11)
FIGURE 4 IR spectra of: (A) 10; (B) poly(amide triazole) 12,
between 2300 and 900 cm21
.
soluble at room temperature.
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The signal at 2.22 ppm [Fig. 6(A)] was assigned to H-a of
the monomer 11. This signal appears as a triplet due to its
coupling with protons H-c. The integration of this signal
decreases as the polymerization proceeds. Thus, it can be
seen that after 3 h of heating, the signal virtually disap-
peared [Fig. 6(C)]. The integral corresponding to this signal
was used to determine the monomer conversion versus time
[Fig. 8(B,C)]. Concomitant with the decreases in the H-a sig-
nal of the monomer, the appearance could be observed of
new peaks corresponding to the resonances of triazole pro-
tons in the region 7.58–7.45 ppm, which confirmed that the
1,3-dipolar cycloaddition has occurred and that the triple
bond and the azide group of 11 have been transformed to
the triazole ring. The methylene protons (H-6) adjacent to
the azide group in 11 appeared at 3.45 and 3.26 ppm as
double doublets. In the polymer spectrum, these protons res-
onate to lower field. In fact, of all the protons of the mono-
mer, H-6 are those that suffer the greatest displacement (ca.
1 ppm to downfield) as the polymerization progresses.
Another signal that is clearly affected by the cycloaddition
reaction is that of methylene H-c, adjacent to the alkyne
group, which also suffers a low-field displacement. Both
methylenes are neighbors to triazole rings formed in the pol-
yaddition reaction. The signal corresponding to H-4 in the
monomer is the only one that is shifted upfield with poly-
merization. Thus, in the spectrum of the monomer [Fig.
6(A)], H-4 is found at 3.70 ppm as a double doublet, while
in the polymer spectrum it resonates in the 3.64–3.56 ppm
region. 13C NMR spectroscopy also revealed the shift differ-
ences between signals of the monomer and those of the
polymer. The greatest differences occurred in C-4, C-6, and
C-c. For example, the signal of C-c in the monomer appears
at 28.87 ppm. When the alkyne group neighbor to C-c is con-
verted to the triazole ring, the C-c signal resonates at 34.75
and 31.74 ppm for the 1,4- and 1,5-disubstituted 1,2,3-tria-
zoles, respectively. Unlike the C-c signal, C-6 suffers a
FIGURE 5 Evolution of GPC during thermal polymerization of
11: (A) t 5 0 h; (B) t 5 1 h 40 min; (C) t 5 3 h; (D) t 5 9 h;
(E) t 5 24 h.
The evolution of the conversion of monomers to polymers
during metal- and solvent-free click polymerization was bet-
ter followed using other techniques, such as NMR and DSC.
The nonregioselective character of this polyaddition process
has already been highlighted by NMR (Figs. 2 and 3). To per-
form these studies, similar amounts of monomer were
weighed and heated under inert atmosphere at 100 ꢀC for
different times. Upon completion of the heating time, the
sample was dissolved in deuterochloroform and analyzed by
NMR spectroscopies. As can be seen from Figure 6, as the
polymerization progresses the resolution of the signals
decreases, becoming wider and less intense, as expected.
Some of the signals present in the monomer disappear. Thus,
the signal appearing at 6.86 ppm as a triplet assigned to the
NH of the amide function of the monomer gradually disap-
pears. In the polymer, the signal corresponding to the NH of
the amide function is shifted downfield, appearing in the
zone of the signals corresponding to the aromatic rings,
7.40–7.00 ppm. Its chemical shift, 7.24 ppm, was determined
by means of an H-15N HSQC experiment.
FIGURE 6 Evolution of 1H NMR spectra during thermal polymerization of 11: (A) t 5 0 h; (B) t 5 1 h 40 min; (C) t 5 3 h.
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TABLE 3 Thermal Properties of 12–15
Procedurea
T10% (ꢀC)
Tde (ꢀC)
b
Tg (ꢀC)
c
c
Polymer
12
13
14
15
A
A
B
B
105
75
237
243
191
229
250
249
244
254
107
88
a
A: neat, thermal polymerization; B: tBuOH-H2O, CuSO4, sodium
ascorbate.
b
Glass-transition temperature measured by DSC, second heating after
rapid cooling.
c
FIGURE 8 Monomer conversion versus reaction time for the
catalyst-free polyaddition of monomer 11, in bulk: (A) by DSC;
(B) by 1H NMR. In DMSO-d6 solution: (C) by 1H NMR.
Temperature at which a 10% weight loss (T10%) and temperature of
maximum degradation (Tde) as determinate by TGA.
displacement to higher field. Thus, C-6 in the monomer
appears at 53.43 ppm, while in the polymer it appears at
52.79 and 50.38 ppm for 1,4- and 1,5-disubstituted 1,2,3-tri-
azoles, respectively.
tuted 1,2,3-triazoles through the thermal 1,3-dipolar
cycloaddition reaction was studied by DSC at a heating rate
of 10 ꢀC min21. A first heating scan was performed from
ꢀ
240 to 250 C to determine the Tg of the monomer and the
DHtotal [Fig. 7(A)]. Then, a second heating run of previously
The thermal behavior of the resulting poly(amide triazole)s
was studied by DSC, and the thermal values obtained in this
study are given in Table 3. Poly(amide triazole)s 12–15 did
not show any endothermal transition on the DSC thermo-
grams. This result is in line with previously reported studies
made on carbohydrate-derived polyamides, polyesteramides,
and polyesters, in which the thermal behavior clearly
appeared to depend on the sugar residue used as mono-
mer25. Only well-defined slope changes due to second-order
thermal transitions (Tg) were observed for all the polymers.
heated samples was performed. Typically, the thermal 1,3-
ꢀ
dipolar cycloaddition starts at about 60 C and ends at about
250 ꢀC, with the peakꢀ of the exothermal reaction (Tp) being
ꢀ
observed at 140–150 C (at a constant heating rate of 10 C
min21). Similar values of Tp have been reported in the
literature.26
As an example, Figure 7 shows the evolution of DSC traces
with time for monomer 11. The evolution of monomer con-
version versus reaction time for the catalyst-free polyaddi-
tion is displayed in Figure 8(A).
Annealing experiments at different temperatures did not
result in the crystallization of the samples. From the glass
transition temperatures of polymers given in Table 3, we can
see that, although the values for 12 and 14 are very similar,
in general the Tg values of the poly(amide triazole)s obtained
by the catalytic method are higher than those obtained by
the thermal procedure. This could also be observed for other
polytriazoles19 prepared by the two procedures. The conver-
sion of alkyne and azide functions to 1,4- and 1,5-disubsti-
Monomer conversion (q) was calculated using the equation:
DHtotal 2DHresidual
q5
DHtotal
Powder X-ray diffraction analysis of polymers corroborated
the DSC data. The broad distribution of 2h values (Fig. 9)
indicates that the nature of the samples is mainly
amorphous.
FIGURE 7 Evolution of DSC traces during the thermal polymer-
ization of 11: (A) t 5 0 h; (B) t 5 1 h 40 min; (C) t 5 3 h 20 min;
(D) t 5 7 h.
FIGURE 9 Powder X-ray profiles for poly(amide triazole)s: (A)
12; (B) 13.
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CuAAC method has no advantage over the thermal polymer-
ization, although regioregular polymers having higher Tg val-
ues were obtained by the first method. Compared with
previous studies on thermal Huisgen polyaddition, the par-
ticularly high reactivity of these monomers afforded the
completion of the reaction after short reaction times at
moderate temperature. The polymers exhibit similar fea-
tures, but the thermal polymerization process has the
advantage of being less toxic and more environmentally
friendly, since it does not use any metal catalyst. Thus, the
polymers prepared by this method are free from damage
caused by metal residues.
SCHEME 3 Hydrolysis of 12.
ACKNOWLEDGMENTS
Thermal stabilities of poly(amide triazole)s 12–15 were eval-
uated by TGAs by heating the sample at a rate of 10 ꢀC
min21 under a nitrogen atmosphere. All polymers were sta-
ꢀ
We thank the Ministerio de Economıa y Competitividad of Spain
for financial support (grant MAT2012-38044-C03-01).
ꢀ
ble to thermal degradation to about 250 C. The decomposi-
tion temperature corresponding to 10% weight loss and the
temperature for the maximum degradation rate are listed in
Table 3.
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