N -Vinyltriazoles: A new functional monomer family through click chemistry

Article abstract of DOI:10.1021/ma100011x

The synthesis, polymerization behavior, and physical properties of a third vinyltriazole monomer family, the isomeric 1,4-N-vinyltriazoles, are reported. Xanthate-based reversible addition-fragmentation and transfer (RAFT) procedures were developed to obtain higher levels of N-vinyltrizaole incorporation and block copolymers based on trizaole homopolymers. Thermogravimetric analysis (TGA) of all the functionalized polymers demonstrated the influence of the aromatic triazole ring with similar onsets of weight loss around 350°C. The T_g's of the newly synthesized poly(1,4-N-vinyltriazole)s are found to be higher than the corresponding poly(1,4-Cvinyltriazole)s but lower than the poly(1,5-C-vinyltriazole)s. The polymer is found to exhibit high solubility in polar solvents such as dimethyl sulfoxide (DMSO) and N,N-dimethylformamide(DMF) and also in acidic and 5 wt % NaCl aqueous solutions.

Full text of DOI:10.1021/ma100011x

5474 Macromolecules 2010, 43, 5474–5477
DOI: 10.1021/ma100011x
N-Vinyltriazoles: A New Functional Monomer Family
through Click Chemistry
Hunaid Nulwala,† Daniel J. Burke,† Anzar Khan,‡ Abigail Serrano,†,§ and Craig J. Hawker*,†
†Materials Research Laboratosry, Materials Department, and Department of Chemistry and Biochemistry,
University of California, Santa Barbara, California 93106, ‡Department of Materials, Institute of Polymers,
§
ETH-Zurich, Wolfgang-Pauli-Strasse 10, HCl H-520, 8093 Zurich, Switzerland, and Department of
Inorganic Chemistry, Pontificia Universidad Catolica de Chile, Santiago, Chile
Received January 4, 2010; Revised Manuscript Received May 13, 2010
Introduction
The modularity of this alkyne/azide click reaction allows for
incorporation of a library of functional groups while the effi-
The design of new building blocks and functional units for the
construction of macromolecules with novel and/or improved
properties is a constant theme in advanced materials.1 While a
goal in itself, an underlying theme in this arena is the development
of synthetic strategies for accessing these new building blocks/
functional units that are modular and efficient. This allows the
efficient incorporation of a wide variety of functional groups and
permits a diverse spectrum of researchers to employ these systems.
The recent focus on click chemistry has both drawn attention to
these concepts as well as facilitated the preparation of a large
number of functionalized polymeric materials in an efficient
manner.2 The Cu(I)-mediated azide/alkyne coupling reaction
(CuAAC) is the most widely studied click reaction and results
in the formation of a 1,4-disubstituted 1,2,3-triazole moiety.3,4
This triazole core is not simply a passive linker but can present
a range of interesting structural and electronic features such as an
electron-rich aromatic ring capable of forming π-π interactions,
nitrogen atoms available for metal-ion coordination and hydro-
gen bonding, and a strong dipole moment. While considering the
1,2,3-triazole moiety as a potential building block for macromo-
lecular synthesis, the regioselectivity of the azide-alkyne coupling
chemistry allows a library of different vinyl-based monomers to
be envisaged (Scheme 1). These monomers are analogous to the
widely used styrene, vinylpyridine, and acrylates considering
their aromaticity, presence of heteroatoms, and functional group
diversity. In this context, we have previously reported novel
1,4- and 1,5-C-vinyltriazole-based monomer families which are
readily prepared in multigram quantities.5-7 These C-vinyl sys-
tems displayed a range of novel properties while at the same time
exhibiting polymerization profiles similar to styrenic/acrylate
systems. In this Note, the synthesis, polymerization behavior,
and physical properties of a third vinyltriazole monomer family,
namely the isomeric 1,4-N-vinyltriazoles, are reported.
ciency and simplified nature of the reaction allow for the pre-
paration of the triazole derivatives on a multigram scale. Finally,
base-catalyzed elimination of 3a-h delivers the desired monomer
series, 4a-h, in overall yields of 50-70% starting from 1. Initi-
ally, eight monomers carrying diverse functional side chains have
been synthesized and characterized by 1H and 13C NMR spectro-
scopy, ESI mass spectrometry, and IR spectroscopy. The crystal-
line nature of monomer 4e also allowed for X-ray analysis,
further confirming the isomeric N-vinyl structure of these mono-
mers (Figure 1).
The electronic structure of the CdC double bond in the N-vinyl
monomers differs significantly from the corresponding C-vinyl
derivatives. Consequently, the vinyl group is not conjugated to a
radical stabilizing resonance structure as in the C-vinyl case, but
rather to a lone pair bearing nitrogen atom, which results in
significantly altered radical polymerization kinetics. To address
this issue, xanthate-based reversible addition-fragmentation and
transfer (RAFT) procedures have been developed for controlling
the polymerization of N-vinyl monomers.10 Initial investigations
to determine suitable polymerization conditions for this new class
of monomers were conducted with 4a under a variety of different
living free radical polymerization procedures. This includes nitr-
oxide-mediated polymerization11 as well RAFT procedures in the
presence of xanthate-based RAFT agent, 5, or the commercially
available dithiocarbamate derivative 712 with azobis(isobutyro-
nitrile) (AIBN) as the initiator in both RAFT cases (Scheme 3).
A detailed screening of the polymerization reaction conditions
suggested N,N-dimethylformamide (DMF) as a good solvent and
60 °C to be an optimal polymerization temperature. The optimal
AIBN:5 ratio was found to be 1:4, which resulted in polymers
with moderate polydispersity but failed to yield high molecular
weight polymers when compared to the polymers obtained by
conventional free radical polymerization using AIBN as an initi-
ator. The latter traditional system consistently gave molecular
weights over 50000 g/mol but resulted in high-polydispersity,
bimodal size exclusion chromatography traces. In a further effort
to efficiently polymerize N-vinyltrizaole systems, substitution of
the xanthate, 5, for the dithiocarbamate, 7, afforded faster kin-
etics with the polymerizations reaching 80þ% conversion after
6 h (cf. ca. 30% conversion for 5). The use of 7 allows higher
molecular weight, low-polydispersity materials to be obtained.
To demonstrate that these monomers are amenable to the reli-
able construction of polymers with advanced architectures, the for-
mation of random and block copolymers using living free radical
techniques was examined. Random and block copolymerization,
Results and Discussion
A modular synthetic strategy is established for accessing the
desired 1,4-N-vinyl-based monomers (Scheme 2). 2-Chloroetha-
nol was treated with sodium azide to give 2-azidoethanol, 1,8
followed by conversion of the hydroxyl group in a tosylate or
mesylate group. Coupling of the azide 2 with a variety of func-
tionalized acetylene molecules in the presence of a heterogeneous
copper (copper-in-charcoal) catalyst under established click
conditions leads to the establishment of the trizaole nucleus.9
*Corresponding author. E-mail: hawker@mrl.ucsb.edu.
pubs.acs.org/Macromolecules
Published on Web 05/25/2010
r 2010 American Chemical Society

Note
Macromolecules, Vol. 43, No. 12, 2010 5475
Figure 2. Structure of the N-vinyltrizaole homopolymers, 6a-h.
Scheme 3. Polymerization of Monomers 4a-h To Give the
Corresponding Polymers 6a-h
Figure 1. Crystal structure of monomer 4e.
Scheme 1. Chemical Structures of Three Possible Monomers That Can
Be Synthesized by Azide/Alkyne Click Chemistry
Scheme 2. Synthesis of 1,4-N-Vinyltriazole Monomer Family
Scheme 4. Synthesis of Random and Block Copolymers,
9, Based on the Benzyl-Substituted N-Vinyltrizaole, 4e, and
Nitroxide-Mediated Polymerization
analysis (TGA) of all the functionalized polymers demonstrated
the influence of the aromatic triazole ring with similar onsets of
weight loss around 350 °C, except for polymer 6h, which shows
increased thermal stability (onset temperature=400 °C). Major
property differences for the library of N-vinyltriazole polymers,
6a-h, was observed on examination of the glass transition
temperature and solubility characteristics. The glass transition
temperatures could be tuned depending on the functionality pre-
sent in the 4-position of the triazole ring. For example, polymer
6a appended with a long and flexible alkyl chain exhibits the
lowest Tg of 70 °C while polymer 6d bearing a phenyl group
shows the highest Tg of 183 °C. The availability of 6a and 6e also
allowed for a direct comparison of their physical properties with
their C-vinyl structural isomers reported previously.7 Interest-
ingly, the Tg’s of the newly synthesized poly(1,4-N-vinyltriazole)s
were found to be higher than the corresponding poly(1,4-C-
vinyltriazole)s but lower than the poly(1,5-C-vinyltriazole)s. The
Tg of 6a is 70 °C while Tg of poly(1-octyl-4-vinyltriazole) and
poly(1-octyl-5-vinyltriazole) is 22 and 95 °C, respectively. The
influence of the substitution pattern was also observed on the
solubility behavior of these polymers. Polymer 6a shows good
solubility in chloroform and tetrahydrofuran (THF) but was
insoluble in ethanol, dimethyl sulfoxide (DMSO), and water at
room temperature, whereas polymers 6b-g exhibit good solubility
mediated by the alkoxyamines, 8, showed facile incorporation of
the N-vinyltrizaole, 4e, with low polydispersities and high mole-
cular weights being observed for feed ratios of 4e up to 20 mol %.
For example, copolymerization of 500 equiv of a 1:9 molar
mixture of 4e and styrene with the alkoxyamine 8 (R=H) affor-
ded a random copolymer (Mn = 42 000; PDI = 1.17) with the
incorporation of the trizaole repeat unit matching the feed ratio
(9.5 mol % product ratio compared with 10 mol % feed ratio). At
loadings of above 20%, a progressive loss of living character
was observed. Similar results were observed for block copolymers
derived from alkoxyamine-terminated polystyrene or poly-
(ethylene glycol) macroinitiators (Scheme 4). Higher levels of
N-vinyltrizaole incorporation and block copolymers based on
trizaole homopolymers could be obtained through RAFT
procedures using the dithiocarbamate, 7, though the polydis-
persity observed for these materials was increased to ca. 1.4.
The range of homopolymers shown in Figure 2 and related
random and block copolymers were thoroughly characterized
using a combination of analytical techniques. Thermogravimetric

5476 Macromolecules, Vol. 43, No. 12, 2010
Note
in almost all solvents except water. Interestingly, polymer 6h
exhibits high solubility in polar solvents such as DMSO and
DMF and also in acidic and 5 wt % NaCl aqueous solutions,
which suggest the possible use of the parent N-vinyltriazole poly-
mer in proton exchange membrane or biomaterial applicat-
ions. Deprotection of the protected derivatives, 6f and 6g, under
standard conditions gave the corresponding hydroxymethyl and
aminomethyl polymers which proved to have high solubility in
water.
structures were solved by direct methods and refined on F2 by
full-matrix least-squares techniques. Mass spectral data were
collected on a Micromass QTOF2 quadrupole/time-of-flight
tandem mass spectrometer (ESI-MS)
General Click Procedure: Synthesis of 2-(4-Propyl-1H-1,2,3-
triazol-1-yl)ethyl Methanesulfonate (3b). Pent-1-yne (1.99 g, 29.2
mmol), 2-azidoethyl methanesulfonate (4.55 g, 27.5 mmol), and
triethylamine (2.32 g, 22.9 mmol) were dissolved in 50 mL of
1,4-dioxane. Then 200 mg of Cu/C was added, resulting in the
formation of a black suspension. The reaction mixture was
heated to 30 °C for 24 h, after which the reaction mixture was
filtered through a Celite plug, and solvent was removed using
a rotary evaporator to yield 5.30 g (83%) of 3b. 1H NMR
(500 MHz, CDCl3): δ=7.36 (s, 1H), 4.56 (br s, 2H), 4.50 (br s,
2H), 2.96 (s, 3H), 2.55 (t, J=7.6 Hz, 2H), 1.56 (m, J=7.6 Hz,
2H), 0.83 (t, J=7.6 Hz, 3H). ESI-mass spectra for C8H15N3O3S:
Calculated: 233.29; Found (M þ H)þ: 234.08. IR (ATR):
ν (cm-1) 3445, 2962, 1667, 1552.
General Elimination Procedure: Synthesis of1-Ethenyl-4-propyl-
1H-1,2,3-triazole (4b). In a 500 mL round-bottom flask equipped
with cooling condenser was added 3b (10.6 g, 45.3 mmol) and NaI
(2.00 g, 13.6 mmol) in 250 mL of dimethoxyethane, followed by
DBU (13.8 g, 90.8 mmol). After addition of all the reagents, the
mixture was heated to 130 °C for 1 h and the dimethoxyethane
removed under reduced pressure. The crude product was then
dissolved in CH2Cl2 and washed with water. The aqueous layer
was extracted three times with CH2Cl2, the organic fractions were
collected, and the crude product was absorbed on silica gel. This
crude product was then purified via flash column chromatography
(2:3 ethyl acetate:hexanes) to yield 4.04 g (65%) of 4b. 1H NMR
(500 MHz, CDCl3): δ = 7.53 (s, 1H), 7.15-7.20 (dd, J = 15.76,
8.83 Hz, 1H), 5.50 (dd, J = 15.76, 1.89 Hz, 1H), 4.92-4.96
(m, 1H), 2.54-2.57 (t, 2H), 1.52-1.60 (m, 3H), 0.81-0.83 (t,
3H). 13C NMR (126 MHz, CDCl3): δ=148.3, 130.2, 117.7, 103.8,
27.4, 22.4, 13.5. IR (ATR): ν (cm-1) 3137, 2961, 1647, 1043, 959,
715. ESI-mass spectra for C7H11N3: Calculated: 137.10; Found
(M þ H)þ: 138.11.
Conclusions
In conclusion, we have introduced a new monomer family
based on N-vinyltriazoles. The modular synthetic strategy allows
for incorporation of a variety of groups into the monomer struc-
ture with controlled polymerization by either nitroxide or RAFT
procedures leading toa library of functionalized macromolecules.
The polymer properties are governed by the nature of the
functional group present along the backbone with moderate to
high glass transition temperatures and variable solubility being
observed. The most striking comparison with the corresponding
C-vinyl systems is the significantly different polymerization be-
havior, which illustrates the importance of isomer structure in
these 1,2,3-trizole-based systems. This study also further demon-
strates the use of efficient and modular chemistry for the pre-
paration of novel materials with tunable properties.
Experimental Section
Materials and Instrumental Details. Unless otherwise stated,
ACS reagent grade chemicals and solvents were purchased from
Aldrich and used without further purification. All reactions
were carried out under ambient conditions unless specified.
Analytical thin layer chromatography (TLC) was performed
on commercial Merck Plates coated with silica gel GF254
(0.24 mm thick). Flash column chromatography was carried
out either by using silica gel (Merck Kieselgel 60 (230-400
mesh, ASTM)) or on Biotage SP1 system using 40þM or 65i
columns. 1H NMR (500 and 200 MHz) and 13C NMR (162 and
60 MHz) analyses were done using Bruker AC 500 and 200
spectrometers, respectively, with the chemical shift reported in
ppm and referenced to signals from residual protons of the
solvents. NMR data were processed on ACD laboratories
Version 11 software. Size exclusion chromatography (SEC)
was carried out at room temperature on a Waters chromato-
graph connected to a Waters 410 differential refractometer
and six Waters Styragel columns (five HR-5 μm and one
HMW-20 μm) using THF as eluent (flow rate: 1 mL/min).
A Waters 410 differential refractometer and a 996 photodiode
array detector were employed. The molecular weights of the
polymers were calculated relative to linear polystyrene stan-
dards. Differential scanning calorimetry (DSC) measurements
took place using a TA Instruments DCS 2920 at a ramp rate of
10 deg/min. A Mettler TGA/DTA 851e was used for thermal
gravimetric analysis (TGA) measurements with the experi-
ments being run under nitrogen at a ramp rate of 10 deg/min.
Infrared spectroscopic experiments were completed on a
Perkin-Elmer Spectrum 100 Fourier transform infrared spec-
trometer (FTIR). Typically, 4 scans at a resolution of 4 cm-1
were recorded on each sample by using an attenuated total
reflection (ATR) apparatus. The crystal structures were de-
termined by mounting crystals on glass fibers and transference
to a Bruker CCD platform diffractometer. The SMART1
program was used to determine the unite cell parameters and
data collections (15 s/frame, 0.3 deg/frame for a sphere of
diffraction data). The data were collected at room temperature
with Mo KR radiation. The raw frame data were processed
using the SAINT2 program. Empirical absorption corrections
were applied based on Psi scan method. Subsequent calcula-
tions were carried out using SHELXTL3 program, and the
General Polymerization Procedure Using CTA 5. Ampules
equipped with a magnetic stirring bar, monomer 4, CTA 5,
AIBN (CTA:AIBN=4:1), and DMF (10 wt % monomer con-
centration) were subjected to four cycles of freeze-pump-thaw
and then sealed under vacuum. The monomer (typically 200 mg)
to initiator ratio was 200:1. In a typical run, polymerization
reaction was conducted at 60 °C for 6 h. The viscous reaction
mixture was then cooled to room temperature, and monomer
conversion was calculated by using 1H NMR spectroscopy.
Polymers were then precipitated in ethanol or diethyl ether as
a white solid. The polymers were dried under vacuum at 50 °C
for 12 h. Raft polymerizations with the dithiocarbamate, 7, were
conducted under the same procedure.
General Polymerization Procedure Using AIBN. Free radical
polymerization was carried out in 1:1 vol % DMF:DMSO using
AIBN as an initiator in ampules equipped with magnetic stirring
bar. AIBN to monomer ratio was kept 1:200 for all the poly-
merizations. In a typical polymerization run, the ampules were
subjected to four cycles of freeze-pump-thaw then sealed
under vacuum and then placed in an oil bath at 60 °C. The
run were generally completed in 2 h with almost complete con-
sumption of the monomers.
General Polymerization Procedure Using Alkoxyamine.
Nitroxide-mediated radical polymerizations were performed using
the alkoxyamines, 8, under standard conditions. In a typical
polymerization run, the ampules were subjected to four cycles of
freeze-pump-thaw, then sealed under vacuum, and placed in an
oil bath at 120-125 °C. The polymerizations were generally
completed in 12 h.
Acknowledgment. Financial support from the NSF (CHE-
0957492) and the MRSEC Program DMR-0520415 (MRL-
UCSB) is greatly acknowledged. Professor Bongjin Moon

Note
Macromolecules, Vol. 43, No. 12, 2010 5477
(Sogang University, Seoul, Korea) is also thanked for
valuable scientific discussions.
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