New polyelectrolytes based on 4-vinyl-1,2,3-triazole and 1-vinylimidazole

Article abstract of DOI:10.1002/pola.25921

Copolymers of 4-vinyl-1,2,3-triazole and 1-vinylimidazole (VI) were obtained by radical copolymerization of (4-vinyl-1H-1,2,3-triazol-1-yl)methyl pivalate with VI followed by alkali hydrolysis. Reactivity ratios of the triazole and imidazole monomers are 0.51 and 0.30, respectively. Theoretical quantum-chemical calculations by the PM3 semiempirical method give close values, which show that the obtained reactivity ratios reflect the activity of the vinyl groups. Polyelectrolyte properties of the copolymers were studied by potentiometric titration. Hydrogen bonds between the protonated triazole cycle and the triazole or imidazole units were found to considerably influence the solubility and solution properties of the copolymers.

Full text of DOI:10.1002/pola.25921

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New Polyelectrolytes Based on 4-Vinyl-1,2,3-triazole and 1-Vinylimidazole
Elena N. Danilovtseva, Mikhail A. Chafeev, Vadim V. Annenkov
Group of silica nanostructures chemistry, Limnological Institute SB RAS, Irkutsk, Russian Federation
Correspondence to: V. V. Annenkov (E-mail: annenkov@lin.irk.ru)
Received 21 September 2011; accepted 5 January 2012; published online 28 January 2012
DOI: 10.1002/pola.25921
ABSTRACT: Copolymers of 4-vinyl-1,2,3-triazole and 1-vinyli-
studied by potentiometric titration. Hydrogen bonds
between the protonated triazole cycle and the triazole or
imidazole units were found to considerably influence
the solubility and solution properties of the copolymers.
VC 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
midazole (VI) were obtained by radical copolymerization of
(
4-vinyl-1H-1,2,3-triazol-1-yl)methyl pivalate with VI followed
by alkali hydrolysis. Reactivity ratios of the triazole and
imidazole monomers are 0.51 and 0.30, respectively. Theo-
retical quantum-chemical calculations by the PM3 semiem-
pirical method give close values, which show that the
obtained reactivity ratios reflect the activity of the vinyl
groups. Polyelectrolyte properties of the copolymers were
Chem 50: 1539–1546, 2012
KEYWORDS: light scattering; quantum chemistry; radical poly-
merization; vinylimidazole; vinyl triazole
INTRODUCTION Azole-containing vinyl polymers have been
studied since the 1960s. Among such polymers, the most
investigated polymers to date have been the derivatives of
methods has resulted in the facilitation of the synthesis of
VT and its derivatives.
1
2–14
This work is devoted to the copolymerization of VT with VI
and to the study of the acid-base properties of the copolymers
in comparison with their homopolymers. VI is a weak base
and was used in the synthesis of carboxylic polyampholytes
with a precisely adjustable isoelectric point (IEP) between 2.8
1
vinylimidazoles (see ref. and references mentioned in it)
2
and vinyltetrazoles. The attention gained by polymers of 1-
vinylimidazole (VI) is understandable, especially because of
the ready commercial availability of the monomer. Tetra-
zole-containing polymers were found to be attractive mainly
as components of solid rocket propellants and explosives.
Some hydrophilic poly(vinylazoles) were later studied as
15
and 6.7. The triazole cycle is a weaker acid compared to the
carboxylic group, so we expect that the IEP range of imidaz-
ole-containing polyampholytes can be expanded up to the al-
kali area. VT has been introduced in copolymerization in a
protected form as (4-vinyl-1H-1,2,3-triazol-1-yl)methyl piva-
late (VTp), which is the precursor of VT:
2
–4
potential biologically active substances.
Crosslinked poly-
mers are effective sorbents of metal cations due to the elec-
tron-donor properties of pyridinic nitrogen atoms in the az-
ole cycles. Polymers of N-unsubstituted C-vinylazoles
contain basic nitrogen atoms as well as an acidic N-H group
in one heterocycle. This combination explains some unusual
properties of these polymers, such as the low water solubil-
ity of poly(5-vinyltetrazole), despite the high hydrophilicity
5
of the corresponding monomer and high proton mobility
6
–10
in films containing N-H imidazole or triazole moieties.
Polymers of C-vinyltriazoles have been the least studied
group of such compounds because of difficulties in synthe-
sizing the monomers. 4-Vinyl-1,2,3-triazole (VT) was
1
1
obtained by a multistage procedure, and until recently,
this method was the sole available for C-vinyltriazole.
Poly(4-vinyl-1,2,3-triazole) (PVT) is a promising polymer
for the design of proton-conducting membranes; its conduc-
tivity is 100 times higher than that of the imidazole-con-
This substance is preferable to unprotected 4-vinyl-1H-1,2,3-tria-
zole for two reasons: absence of acid-base interactions with VI,
which can complicate copolymerization mechanisms, and the
easy and quantitative removal of the protective group from the
final polymer. It is interesting to note that the abovementioned
7
,8
14
taining polymer.
The development of ‘‘click chemistry’’
protection group is not easy to remove from the monomer.
Additional Supporting Information may be found in the online version of this article.
2012 Wiley Periodicals, Inc.
V
C
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EXPERIMENTAL
The residue after evaporation was subjected to column chro-
matography eluting with ethyl acetate–hexanes mixture (1:1)
to afford VTp (2.01 g, 66%) as an off-white solid.
Materials
Ethanol, diethyl ether, hexane, sodium hydroxide (NaOH),
and 0.1 M and 1 M solutions of NaOH and HCl were pur-
chased from Sigma Aldrich, Fisher, or Acros Chemicals and
used without further treatment. VI (Sigma Aldrich) was puri-
fied from stabilizers by vacuum distillation before use. 2,2 -
Azobis(isobutyronitrile) (AIBN, Sigma Aldrich) was crystal-
lized from ethanol.
1
H NMR (300 MHz, CDCl ) d 7.74 (s, Ar, 1H), 6.67 (dd, J ¼
3
28.9, 11.1 Hz, CH ¼¼ CH , 1H), 6.63 (s, NCH O, 2H), 5.91 (dd,
2
2
J ¼ 17.7, 1.0 Hz, cis-CH ¼¼ CH , 1H), 5.35 (dd, J ¼ 12.4, 1.2 Hz,
2
0
trans-CH ¼¼ CH , 1H), 3.68(s, CO(CH ) , 9H), (ESþ) m/z
2
3 3
210.26 (Mþ1).
Synthesis of Copolymers
1
2
VTp was synthesized according to the procedure starting
from 3-butyn-1-ol with some minor modifications. To a solu-
tion of 3-butyn-1-ol (10.0 g, 142.6 mmol) and triethylamine
PVTp and VTp–VI copolymers were synthesized by radical
polymerization in ethanol (8 mL of 25% solution) under the
ꢀ
action of AIBN (1% from the monomer mass) at 60 C under
ꢀ
(
43.4 g, 428 mmol) in dichloromethane (900 mL) at 0 C was
an argon atmosphere in hermetically sealed 30-mL vials. The
resulting solutions in the case of PVTp were precipitated
into diethyl ether–hexane (2:1) or, in the case of copolymers,
into diethyl ether. The solution–precipitant ratio was 1:10.
The products were then reprecipitated from ethanol, washed
with the precipitant, and dried in vacuum. The composition
of the monomer mixtures, duration of the reaction, and yield
are summarized in Table 1.
added methanesulfonyl chloride (21.2 g, 184.6 mmol) drop-
wise over 1 h under nitrogen. The reaction mixture was
allowed to warm up to ambient temperature and was stirred
under nitrogen for 16 h at ambient temperature. On comple-
tion, the reaction mixture was washed with 1 M HCl (2 ꢁ
3
00 mL) and with Brine (1 ꢁ 300 mL). The organic extract
was dried over MgSO , filtered, and concentrated under
4
reduced pressure. The residue after evaporation was sub-
jected to column chromatography eluting with dichlorome-
thane to obtain but-3-yn-1-yl methanesulfonate (15.9 g, 75%)
as an yellow liquid(ESþ) m/z 149.04 (Mþ1). The compound
was used in the next step without further characterization.
The obtained polymers were used for the synthesis of PVT
and VT–VI copolymers by alkali hydrolysis as outlined below:
PVTp (1 g, 4.79 mmol) was added to 1 M NaOH solution (10
mL, 10 mmol NaOH) and ethanol (5 mL). The mixture was
ꢀ
heated at 40 C for 2 h with continual stirring, purified by
A 500-mL flask equipped with magnetic stir bar was charged
dialysis against water through a cellophane membrane (8
kDa cutoff), and freeze dried. The yield was 0.385 g (86%).
VTp–VI copolymers were hydrolyzed using a similar proce-
dure, by decreasing the amount of alkali in proportion to the
VTp content in the copolymer.
1
6
with azidomethyl pivalate (6.83 g, 40.6 mmol), but-3-yn-1-
yl methanesulfonate (6.10 g, 40.6 mmol), and 60 mL of t-bu-
tanol. In a separate flask, sodium ascorbate (2.40 g, 12.14
mmol) and CuSO (0.324 g, 2.02 mmol) were introduced to
4
60 mL of deionized water. The aqueous mixture was then
added in one portion to the t-butanol mixture in the first
flask. The reaction mixture was vigorously stirred under
nitrogen at ambient temperature for 16 h. On completion,
the reaction mixture was concentrated under reduced pres-
sure and then extracted with dichloromethane (3 ꢁ 60 mL).
Characterization
H and C NMR spectra were recorded on a Bruker AVANCE
400 spectrometer in D O. Typical spectra of the polymers
and their assignment can be found in the Supporting Infor-
mation. Infrared spectra were recorded on an Infralum FT-
801 spectrophotometer (SIMEX, Russia). Potentiometry
measurements were performed on a ‘‘Multitest’’ ionometer
using a combined pH electrode in a temperature-controlled
1
13
2
The organic fractions were combined, dried over MgSO , and
4
filtered, and the filtrate was concentrated under reduced
pressure. The residue after evaporation was subjected to col-
umn chromatography eluting with ethyl acetate–hexanes
mixture (1:1) to obtain (4-(2-((methylsulfonyl)oxy)ethyl)-1H-
ꢀ
cell at 20 6 0.02 C under an argon atmosphere. The pH of
the solutions was adjusted up to 11.5 using 0.1 M NaOH; 0.1
M HCl was used as a titrant. The concentration of the copoly-
mers was 1.5 mg/mL.
1,2,3-triazol-1-yl)methyl pivalate (9.17 g, 74%) as a white
solid. (ESþ) m/z 306.36 (Mþ1). The compound was used in
the next step without further characterization.
Dynamic light scattering (DLS) experiments were performed
using a LAD-079 instrument built at The Institute of Ther-
mophysics (Novosibirsk, Russia). All solutions were purified
of dust using filter units with 0.45 lm pore size (Sartorius
16555-Q Minisart syringe filters). The experiments were
A 250-mL round-bottom flask equipped with magnetic stir
bar was charged with (4-(2-((methylsulfonyl)oxy)ethyl)-1H-
1
,2,3-triazol-1-yl)methyl pivalate (4.43 g, 14.5 mmol) and 1,2-
dimethoxyethane (145.0 mL). To this solution, NaI (6.53 g,
3.6 mmol) was added, followed by 1,8-Diazabicyclo[5.4.0]un-
ꢀ
4
performed at 20 6 0.02 C. Measurements were performed
using a 650-nm solid-state laser at 36 , 54 , 72 , and 90
ꢀ
ꢀ
ꢀ
ꢀ
dec-7-ene (DBU) (4.42 g, 29.1 mmol). After addition of all
reagents, the reaction mixture was heated to reflux for 30
min. On completion, the reaction mixture was participated
between dichloromethane and water (100 mL:100 mL), and
the aqueous layer was extracted with dichloromethane (3 ꢁ
scattering angles. Correlation functions were analyzed with
a polymodal model using a random-centroid optimization
1
7
ꢀ
method. Results obtained at 90 are presented in this ar-
ticle; the data at other angles were analogous. Viscosity
measurements were performed in a capillary viscometer at
100 mL). The organic fractions were combined, dried over
ꢀ
MgSO , filtered, and concentrated under reduced pressure.
20 6 0.02 C.
4
1
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TABLE 1 Copolymerization of VTp with VI
a
b
VTp in Initial
Time
(h)
Yield
(%)
VTp in Copolymer
(mol %)
[g]
Rh (nm)
pK
o
Turbidity
Run
Mixture (mol %)
(dL/g)
[Intensity (%)]
(VT units)
Interval (pH)
1
2
3
4
5
6
7
8
9
100
100
88.7
70.0
50.3
30.2
9.5
19
45
26
26
26
26
26
6
62
100
100
86.5
67.4
52.4
37.2
15.2
81.8
64.0
58.2
46.9
20.3
0
0.58
0.55
0.54
0.52
0.49
0.54
0.43
–
88 (47), 670 (53)
9.8
9.8
9.6
9.5
9.2
9.3
8.9
–
2.5–9.2
2.5–9.2
1.5–9
1.5–9
1.5–9
6–9
6.6–9
–
100
88
–
54 (35), 1,100 (65)
84
38 (41), 360 (59)
71
14 (6), 270 (94)
69
52 (27), 340 (73)
53
66 (77), 600 (23)
89.7
70.1
48.8
29.1
9.4
51.1
50.6
18.5
26.0
12.5
3.6
–
6
–
–
–
–
1
1
1
1
a
0
1
2
3
6
–
–
–
–
6
–
–
–
–
6
–
–
–
–
0
6
0.23
71
b
in 0.1 M NaOH þ 1 M KBr.
in 0.1 M HCl þ 0.1 M NaCl.
2
3
Quantum-chemical calculations were performed with the
HyperChem program.
the reactivity ratios. Previously, we have shown that theo-
retical reactivity ratios can be obtained from activation ener-
gies of all elementary stages. These values were calculated as
differences between the total energy of the activated complex
and the summary energy of the free radical and monomer
1
8
RESULTS AND DISCUSSION
(
see Supporting Information). Reactivity ratios were obtained
Synthesis of Copolymers
EiiꢂEij
from the equation ri ¼ expðꢂ
Þ, where Eii and Eij are the
VTp is easily polymerizable under the action of AIBN. Yields of
the copolymers decrease with an increase in the VI content in
the initial monomer mixture (Table 1); the yield reduced to 53
at 9.5% VTp. The compositions of the copolymers are calcu-
lated from NMR data. Yields of copolymers 3–7 (Table 1) were
unduly high for the calculation of reactivity ratios, so we
obtained samples at less than 50% yields. Reactivity ratios of
the monomers were calculated by the integral method
RT
activation energies of the corresponding elementary reactions.
The quantum chemical calculations by the PM3 semiempirical
method result in rVTp ¼ 0.54 and r ¼ 0.26; these values are
VI
close to the values obtained from the experimental data.
Thus, the copolymerization in the VTp–VI system complies
with the Mayo-Lewis equation and the obtained reactivity
ratios reflect the activity of the vinyl groups.
1
9
described by Kuo and Chen, which was modified according
2
0
21
to to obtain an equation symmetrically relative to r and r :
1
2
.
pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffiffiffi
pffiffiffiffiffiffiffi
1
kx ꢂ kx ¼ k=xr2 ꢂ x=kr1;
m20M1
m10M2
m1
where k ¼
; x ¼ ; and m , m , and M are the mono-
i0
i
i
m2
mer fractions in the initial monomer mixture, monomer mix-
ture after reaction, and copolymer, respectively.
The following values are obtained: rVTp ¼ 0.51 6 0.03 and r
VI
¼
0.30 6 0.02. These reactivity ratios are typical for systems
having an azeotropic point that corresponds to 59.3 mol %
VTp, and experimental data confirm these values (Fig. 1).
Reactivity ratios calculated according to the classical Mayo-
2
2
Lewis equation are often effective values because in addi-
tion to the activity of vinyl groups, they depend on many
other factors: specific interactions between functional groups
of monomers, preferential adsorption of a monomer by a mac-
romolecular coil, the effect of the penultimate unit, and so on.
One way to verify the correlation between the activities of the
monomers with the electronic effect of substituents on the
vinyl group is through the quantum chemical simulation of
FIGURE 1 Dependence of VTp content in copolymer (M , mol
1
1
%) on VTp in monomer mixture (m ). Azeotropic point (black
triangle) is calculated from reactivity ratios.
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FIGURE 2 IR spectra of (co)polymers.
5
ꢂ1
VTp–VI copolymers were converted into VT–VI copolymers by
alkali hydrolysis and purified by dialysis, followed by freeze
drying. The reaction can be easily monitored by the disap-
to poly(5-vinyltetrazole). The wide band at 2300–3700 cm
5
corresponds to associated NH moieties. The spectra of VT–VI
copolymers are superpositions of the homopolymers’ spectra.
It must be mentioned that these spectra are observed for sam-
ples obtained from water solutions at pH < 9.5. The isolation
of PVT or copolymers by freeze-drying solutions with pH
10–11 results in samples with interesting IR spectra, in which
ꢂ1
pearance of the C ¼¼ O band at 1740 cm in the IR spectra
1
3
(
Fig. 2), and lines in the C NMR spectra at 70 and 177 ppm
correspond to CH2 and C(Me)3 of the protective group. The
obtained copolymers are white solids soluble in water, giving
solutions with pH 9–9.5. A decrease in the pH results in the
precipitation of the copolymers (Table 1). VT–VI copolymers
are soluble in methanol and insoluble in ethanol, dimethylfor-
mamide (DMFA), and dimethyl sulfoxide (DMSO), with the
exception of a copolymer containing 15.2% VT, which is solu-
ble in both ethanol and DMSO.
ꢂ1
an extremely strong band at 1450 cm is observed (Fig. 2).
The relative intensity of this band increases with an increase in
the content of ionized VT units to more than 0.2–0.3 (Fig. 3),
independent of the copolymer composition. Experimental and
theoretical studies of IR spectra of triazole and tetrazole anions
show that bands near 1450 cm
ꢂ1
are attributed to CAN
25
stretching and CAH deformation vibrations of the cycle. The
The IR spectrum of poly(1-vinylimidazole) (PVI), according to
ref. 24, contains bands of stretching vibrations of imidazole
intensity of this band is low in the case of the 1,2,3-triazole
26
anion but increases drastically for the 1,2,4-triazole cycle. We
can suppose that the extreme intensive absorption of PVT and
ꢂ1
cycles (1500, 1415, 1284, and 1228 cm ), stretching vibra-
ꢂ1
ꢂ1
tions of azole CAH (1109 and 1083 cm ), and bending vibra-
copolymers at 1450 cm in samples obtained at pH> 10 is
ꢂ1
tions of the heterocycle (912, 820, 745, 661, and 635 cm
)
related to the vibrations of the triazole anionic cycle, which are
strengthened by some peculiarities of the polymer structure in
the solid state. The detailed clarification of this phenomenon is
possible by thorough simulation experiments and calculations,
which go beyond the scope of this work.
(
1
Fig. 2). The main bands in the PVT spectrum are at 1576,
ꢂ1
547, 1450, 1353, 1218, 1113, 980, and 835 cm . The band
ꢂ1
at 1650 cm can be assigned to vibrations of water molecules,
which are strongly coordinated with NH–triazole cycles, similar
1
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with DLS shows the multimodal size distribution of the mac-
romolecules. An increase in the ionic strength did not result
in the destruction of the associates, which did not allow us to
measure the molecular weight of the copolymers with static
light scattering. The solutions of the copolymers and PVT con-
tain particles from two size intervals (Table 1): 14–90 nm,
which may be attributed to individual macromolecules, and
particles with diameters of more than 500 nm. The presence
of the large aggregates is at variance with the filtration of the
samples before DLS using 450 nm filters, which is probably
related to the reversible destruction of large particles during
filtration. Previously, we observed a similar phenomenon in
1
5
the case of VI–acrylic acid copolymers.
Potentiometry titration curves of PVT and copolymers
Fig. 4) contain a sharp inflection corresponding to the full
(
neutralization of the VT–Na units. Using data on the copoly-
mer composition, we calculated points that corresponded to
the presence of VT units in the VT–Na state and points of
full VI and VT unit protonation. These neutralization points
were used to obtain the ionization degree (a) of the VT units
FIGURE 3 Dependence of relative intensity of the bands at
ꢂ1
1450–830 cm in IR spectra of PVT and copolymers on molar
fraction of VT anions in the polymeric chain. Lines labeling cor-
responds to Table 1.
(
Fig. 5) and conjugated acids from protonated VT and VI
units (Fig. 6). The a values were calculated by taking into
account self-ionization and hydrolysis of the acidic units by
Properties of the Copolymers in Water Solution
The PVT and VT–VI copolymers are soluble in water under
acidic or alkali conditions (Table 1). Study of these solutions
2
7
conventional equations. The pK values for the acidic ioniza-
tion of VT units and for ionization of conjugated acids in the
titration area after inflection were obtained using the Hen-
2
8
derson–Hasselbalch equation.
FIGURE 4 Typical titration curves of PVT and copolymers. The
inflection corresponds to the neutralization of the VT–Na units
according to the scheme. Concentration: 1.5 mg/mL, solution
volume: 20 mL, in the presence of 0.1 M NaCl. Titrant: 0.1 M
HCl. pH was adjusted up to 11.5 with 0.1 M NaOH before titra-
tion. Lines labeling corresponds to Table 1.
FIGURE 5 Dependence of pK of triazole units on ionization
degree a. Triazole species corresponded to a ¼ 0 and 1 are pre-
sented in the figure. Lines labeling corresponds to Table 1.
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of the straight parts of the pK–a curves to a ¼ 0. The
obtained data (Table 1) are close to the pK of 1,2,3-triazole
2
9
(
9.26) and decrease only slightly with an increase in the VI
content. The viscosity of the polymer solutions increases
with pH (Fig. 7), and the most pronounced growth is
observed up to a content of ionized VT units of 0.2–0.3,
ꢂ1
which corresponds to the appearance of a 1450 cm band
in the IR spectra and confirms the assignment of this band
to the ionized triazole cycles.
The other titration region is placed in the acidic area and
corresponds to the protonation of VT and VI units. The PVI
titration curve (Fig. 6) shows an increase in pK with a, simi-
3
0
lar to data published previously, and the characteristic pK
at a ¼ 1 (protonation of the first VI unit) is equal to 5.9. The
behavior of PVT and copolymers 3–5 is unusual: relatively
high pK near a ¼ 1, followed by a sharp decrease up to a ¼
0
.7–0.9. The extrapolation of pK to a ¼ 1 for PVT and
copolymers is irrelevant owing to their insolubility at a ¼
0
.9–1.
The observed high pK values for PVT near a ¼ 1 are
anomalously high, taking into account pK ¼ 1.17 for the
2
9
protonation of 1,2,3-triazole. A similar effect in the case
3
0
of poly(1-vinyl-1,2,4-triazole)
was explained by the
involvement of protons in intramolecular donor–acceptor
bonds between the polymer units. In the case of PVT, we
FIGURE 6 Dependence of pK on ionization degree a of conju-
gated acid from VT and VI units. Triazole and imidazole species
corresponded to a ¼ 0 and 1 are presented in the figure. Lines
labeling corresponds to Table 1.
a
TABLE 2 Enthalpy of Hydrogen Bond Formation Between
Triazole and Imidazole Units
Calculated energy of
Structures
hydrogen bonds (kJ/mol)
In the case of the acidic ionization of VT units (Fig. 5), we
observe an expected increase in pK with a, which is related to
a decrease in VT acidity owing to the known electrostatic
effect. The inclination of the curves decreases with an increase
in the VI content in the copolymers, probably because of the
dilution of the VT units with nonionized imidazole groups and
the corresponding decrease in the electrostatic effect. Copoly-
mer samples 4–7 show a horizontal part at a < 0.2–0.3, which
is attributed to the destruction of the hydrogen bonds between
VT and VI units under neutralization:
ꢂ53.4
ꢂ107.0
ꢂ49.2
ꢂ133.5
**
This hydrogen bonding results in the precipitation of PVT
and copolymers at pH < 9.2, which corresponds to a < 0.1.
a
Calculated by ab initio Hartree–Fock method using 6-31G basis set
with full geometry optimization. Triazole tautomers with the lowest
energy of formation were used.
Characteristic pK values were obtained by the extrapolation
o
1
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FIGURE 7 Dependence of reduced viscosity on pH and molar fraction of VT anions in the polymeic chain. Lines labeling corre-
sponds to Table 1. Polymer concentration: 1.5 mg/mL, in 0.1 M NaCl.
observe precipitation at pH 9.2 and below; the precipitate
CONCLUSIONS
dissolves again at pH 2.5, which corresponds to a ¼ 0.9
The compound VTp gives statistical copolymers with VI, and
the radical copolymerization of the monomers complies with
the Mayo-Lewis equation. The alkali hydrolysis of these
copolymers results in VT–VI copolymers. Triazole cycles are
capable of acidic dissociation at pH> 9 and protonation in
an acidic medium. PVT and copolymers containing greater
than 37% VT units are insoluble in water at pH 2–9 owing
to hydrogen bonding between acidic NAH of the VT and pyr-
idinic nitrogens in the VT and VI cycles. These polymers
show anomalously high basicity of triazole units with proto-
nation up to 10%, which is attributed to triazole cycles that
are hydrogen-bonded with other VT or VI units. The ability
of PVT to eagerly accept protons probably explains the high
(
Fig. 6). We can assume that the insolubility of PVT is a
result of hydrogen bonding. Quantum-chemical calculation
Table 2) shows relatively high energy of this bonding,
(
possibly due to the involvement of two hydrogen atoms
from both the triazole units. The protonation of one of the
azole cycles enhances the bonding drastically: DH
increases from ꢂ53.4 to ꢂ107.0 kJ/mol. This strengthen-
ing of the hydrogen bonds explains the insolubility of the
polymers in acidic media, down to pH 2–2.5. VI units are
also able to interact with the NAH bonds of the VT cycles
(
Table 2), and the protonation of triazole units results in a
very stable hydrogen bond (DH ¼ ꢂ133.5 kJ/mol), whose
length (1.57 Å) is lower than the typical value for hydro-
gen bonds (2 Å). Thus, copolymers containing up to 60%
VI units show behavior similar to PVT: high basicity and
insolubility at low protonation degrees and solubility after
some critical positive charge of the chain, followed by a
drop in basicity. Copolymers bearing greater than 60% VI
units contain imidazole groups unbounded by triazole
cycles and are protonated similar to PVI.
8
–10
proton conductivity of triazole-containing membranes.
The introduction of imidazole units into the PVT chain
increases this effect. Thus, VT–VI copolymers are promising
substances for designing membranes to be used for polymer
electrolyte membrane fuel cells.
The authors acknowledge support from Siberian Branch of the
Russian Academy of Sciences (project #39).
WWW.MATERIALSVIEWS.COM
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