Thermomechanical Formation-Structure-Property Relationships in Photopolymerized Copper-Catalyzed Azide-Alkyne (CuAAC) Networks

Article abstract of DOI:10.1021/acs.macromol.6b00137

Bulk photopolymerization of a library of synthesized multifunctional azides and alkynes was carried out toward developing structure-property relationships for CuAAC-based polymer networks. Multifunctional azides and alkynes were formulated with a copper catalyst and a photoinitiator, cured, and analyzed for their mechanical properties. Material properties such as the glass transition temperatures (T_g) show a strong dependence on monomer structure with T_g values ranging from 41 to 90 °C for the series of CuAAC monomers synthesized in this study. Compared to the triazoles, analogous thioether-based polymer networks exhibit a 45-49 °C lower T_g whereas analogous monomers composed of ethers in place of carbamates exhibit a 40 °C lower T_g. Here, the formation of the triazole moiety during the polymerization represents a critical component in dictating the material properties of the ultimate polymer network where material properties such as the rubbery modulus, cross-link density, and T_g all exhibit strong dependence on polymerization conversion, monomer composition, and structure postgelation.

Full text of DOI:10.1021/acs.macromol.6b00137

Article
pubs.acs.org/Macromolecules
Thermomechanical Formation−Structure−Property Relationships in
Photopolymerized Copper-Catalyzed Azide−Alkyne (CuAAC)
Networks
Austin Baranek, Han Byul Song, Mathew McBride, Patricia Finnegan, and Christopher N. Bowman*
Department of Chemical and Biological Engineering and Materials Science and Engineering Program, University of Colorado
Boulder, Boulder, Colorado 80309-0596, United States
S
* Supporting Information
ABSTRACT: Bulk photopolymerization of a library of synthesized multifunc-
tional azides and alkynes was carried out toward developing structure−property
relationships for CuAAC-based polymer networks. Multifunctional azides and
alkynes were formulated with a copper catalyst and a photoinitiator, cured, and
analyzed for their mechanical properties. Material properties such as the glass
transition temperatures (Tg) show a strong dependence on monomer structure
with Tg values ranging from 41 to 90 °C for the series of CuAAC monomers
synthesized in this study. Compared to the triazoles, analogous thioether-based
polymer networks exhibit a 45−49 °C lower Tg whereas analogous monomers
composed of ethers in place of carbamates exhibit a 40 °C lower Tg. Here, the
formation of the triazole moiety during the polymerization represents a critical
component in dictating the material properties of the ultimate polymer network
where material properties such as the rubbery modulus, cross-link density, and Tg
all exhibit strong dependence on polymerization conversion, monomer composition, and structure postgelation.
been used for in synthetic chemistry, these reactions have been
generally performed in a solvent to solubilize the copper(II)
salts and with a reducing agent such as sodium ascorbate used
to reduce the copper from a +2 to a +1 oxidation state. The use
of a homogeneously soluble reducing agent precludes spatial
and temporal control of the reaction, as the reduction of copper
will generally commence upon addition of the reducing agent.
Successfully implementing photochemical approaches to
reduce the copper to its +1 oxidation state enables enhanced
control over the reaction that is particularly useful in materials
science. Broadly, two approaches have been used to photo-
chemically reduce Cu(II) to Cu(I) and initiate the CuAAC
reaction.
Both ligand metal charge transfer utilizing an amine complex
and the photochemical formation of radicals from a conven-
tional radical photoinitiator have been used to facilitate
photoinitiation of the CuAAC reaction.8,9,12−19 In particular,
with the use of a photoinitiator, the reduction of copper can be
triggered upon exposure to either UV or visible light (Figure
1A), depending on the specific photoinitiator used and its
absorption spectra.1,20 Adzima et al. showed that the photo-
CuAAC reaction can be applied in an aqueous environment to
perform photolithography and postpolymerization modification
of hydrogels.1 However, for this reaction to be truly considered
as one of only a handful of photopolymerization techniques,
INTRODUCTION
■
The development of the photoinduced copper-catalyzed azide−
alkyne cycloaddition, or photo-CuAAC reaction, represented a
significant milestone as an important new photopolymerization
technique.1 The advantages of photoinitiating this reaction
complement its already significant value within synthetic
methodologies, bioconjugation, labeling, surface functionaliza-
tion, dendrimer synthesis, polymer synthesis, and polymer
modification2−7 by adding spatial and temporal control.8,9
Scheme 1 illustrates the traditional CuAAC reaction, where the
key component allowing this reaction to occur on a reasonable
time scale is the copper(I) catalyst. In the absence of Cu(I), the
reaction proceeds as much as 7 orders of magnitude more
slowly.10,11 Traditionally, by the nature of what this reaction has
Scheme 1. General Scheme for the Proposed Mechanism of
a
the CuAAC Reaction
a
A reducing agent is first used to reduce Cu(II) to Cu(I) followed by
Cu(I) catalysis of the 1,3-dipolar cycloaddition. Various photo-
reduction methodologies have been developed to enable photo-
chemical control of the Cu+1 generation.
Received: January 20, 2016
Revised: January 25, 2016
© XXXX American Chemical Society
A
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of the copper but also necessary to enable the polymerization
to occur within a reasonable time scale. The same can be said
for the photoinitiator, as both are required to create the
copper(I) catalyst. Figure 1 is a schematic of the CuAAC
photopolymerization accompanied by an illustration of a
structural element of the network formed from a 2−3
functional azide−alkyne resin system.
Despite several published examples of bulk CuAAC
reactions, little is known about the materials produced using
this technique, particularly the formation−structure−property
relationships that underlie the behavior. Given the novelty and
potential for these materials, it is critical to assess and
understand these relationships. In particular, the triazole
adducts formed during polymerization have been shown to
improve mechanical properties;25 however, the extent of this
phenomenon is still unknown as is an ability to design materials
with desired behavior. Here, we report simple protocols for
synthesis of a variety of both multifunctional azide and alkyne
monomers with different structural characteristics (Figure 2)
that constitute a library of monomers from which to achieve a
wide range of properties. Systematically integrating different
moieties into the monomers and comparing the differences in
the resulting polymers aid in determining the impact of each
moiety on the overall mechanical behavior of the network. To
polymerize these materials, CuCl2[PMDETA] (vide infra) was
used as the catalyst because of its high solubility in the
monomer resins and 2,2-dimethoxy-2-phenylacetophenone was
chosen as the radical initiator due to its high stability under
visible light allowing initiation only under high-intensity UV
irradiation. The CuAAC photopolymerization of these
monomers results in thermosets with tunable properties,
where the thermomechanical properties and their dependence
on specific chemical structural elements are reported.
Figure 1. General scheme for the proposed mechanism of the
photocatalyzed CuAAC polymerization. A photoinitiator is first used
to generate radicals, which reduce Cu(II) to Cu(I). The transiently
generated Cu(I) then catalyzes the 1,3-dipolar cycloaddition. In the
presence of the multifunctional azide and alkyne monomers, the
●
photo-CuAAC reaction forms a cross-linked network, where “ ”
represents a triazole linkage.
bulk polymerizations must be considered. Few examples of this
reaction exist in a bulk system due to the heat of reaction of the
CuAAC reaction and the safety concerns with neat azides.21
Both Diaz et al. and Sheng et al. pioneered bulk CuAAC
polymerizations with significant success; however, because
polymerization occurred spontaneously upon mixing, these
systems lacked spatial and temporal control.22−24 Gong et al.
studied a small group of synthesized multifunctional azide and
alkyne monomers and successfully formed networks using light
as the stimulus for polymerization (Figure 1), forming highly
cross-linked, high glass transition temperature networks.25
Additionally, photo-CuAAC has been accelerated using tertiary
aliphatic amine ligands as an electron transfer species to reduce
Cu(II) upon irradiation while also functioning as an
accelerating agent and as protecting ligands for the Cu(I).12
The advantages of the CuAAC photopolymerization include
not only facile control over the reaction with light but also the
benefits of having large quantities of the triazole adduct present
within the material postpolymerization. Triazoles formed
during this reaction have excellent thermal and chemical
stability, and the added rigidity from the heterocycle leads to
higher glass transition temperatures, as shown previously.25 In
addition, the step-growth nature of the CuAAC polymerization
allows higher conversion of the monomers, as gelation occurs
much later in the reaction, forming a more homogeneous
network with fewer unreacted chain ends. This behavior leads
to a more uniformly cross-linked material with a sharp glassy to
rubbery transition. Finally, the triazoles are also able to
participate in numerous secondary-bonding interactions such
as π−π stacking and hydrogen bonding which may also
improve the toughness of these materials.27
In other considerations of the photo-CuAAC reaction,
compared to other catalysts, the copper catalyst is both long-
lived and may freely diffuse previtrification. In the case of a
radical-mediated reaction, even if provided sufficient mobility,
radicals will only continue to react for a short time due to the
prevalence of rapid termination reactions. In contrast, the
copper catalyst remains in the +1 activated state for much
longer, extending the dark cure potential and enabling
continued polymerization without continuing irradiation. One
major concern in bulk CuAAC polymerizations is the solubility
of the copper catalyst in the resin. Finding an appropriate
ligand, which allows dissolution of the catalyst in the resin, is
not only desirable to prevent phase separation or precipitation
MATERIALS AND METHODS
■
1,3-Bis(isocyanatomethyl)cyclohexane, 1,3-bis(2-isocyanatopropan-2-
yl)benzene, 4,4-methylenebis(cyclohexyl isocyanate), 4,4′-methylene-
bis(phenyl isocyanate), bis(4-hydroxyphenyl)methane, 6-chloro-1-
hexanol, 8-chloro-1-octanol, dibutyltin dilaurate sodium azide, 1,1,1-
tris(hydroxymethyl)propane, tris-1,3,5-bromomethylbenzene, phloro-
glucinol, propargyl alcohol, propargyl bromide, allyl bromide, sodium
hydride (NaH), sodium sulfate (Na2SO4), potassium thioacetate,
diethyl azodicarboxylate (DEAD), tetrabutylammonium iodide,
N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA), copper(II)
chloride, 2,2-dimethoxy-2-phenylacetophenone (DMPA), triphenyl-
phosphine (TPP), tetrahydrofuran (THF), dimethylformamide
(DMF), and dimethyl sulfoxide (DMSO) were all purchased from
Sigma-Aldrich and used without further purification. Potassium
carbonate (K2CO3) and hydrochloric acid (HCl) were purchased
from Fisher Scientific and used without further purification.
Azide Monomer Synthesis (AZ-(1−4)). Note: organic azides are
potentially explosive substances that can decompose with the slightest input
of energy from external sources. Additionally, small molecules containing
the azido functionality tend to decompose violently which may result in
injury if proper safety precautions are not utilized. When designing a
target azide, one must keep in mind the azide rule represented in the
equation (NC + NO)/NN ≥ 3. This equation takes into account all
nitrogen atoms in your azide, not just those in the azido group where
N signifies the number of atoms.10,26
Synthesis of Bis(6-chlorohexyl)dicarbamates. A round-bottom
flask purged with argon was charged with the diisocyanate (1,3-
bis(isocyanatomethyl)cyclohexane, 1,3-bis(2-isocyanatopropan-2-yl)-
benzene, 4,4-methylenebis(cyclohexyl isocyanate), 4,4′-methylenebis-
(phenyl isocyanate) (0.004 09 mol), THF (3 mL), and dibutyltin
dilaurate (3−5 drops). The reaction flask was then cooled to 0 °C by
placing in an ice bath. The 6-chloro-1-hexanol (1.17 g; 0.008 60 mol)
B
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m, CH2, CH), 3.03 (4H, t, CH2−carbamate), 3.56 (4H, t, CH2−Cl),
4.05 (4H, t, CH2−carbamate), 4.78 (2H, s, NH, carbamate). 13C NMR
(CDCl3), ppm: δ 25.19, 25.23, 26.54, 28.89, 28.89, 20.42, 32.46, 34.61,
(12C, CH2), 37.95 (2C, CH), 45.00 (2C, CH2−NH, carbamate),
47.19 (2C, CH2−Cl), 64.68 (2C, CH2−O, carbamate), 156.85 (2C,
CO, carbamate).
Bis(6-chlorohexyl)(1,3-phenylenebis(propane-2,2-diyl))-
dicarbamate. Yield 98%. 1H NMR (CDCl3), ppm: δ 1.15−1.85 (28H,
m, CH2, CH3), 3.52 (4H, t, CH2−Cl), 3.97 (4H, t, CH2−carbamate),
5.07 (2H, s, NH, carbamate), 7.27 (3H, s, CH−aromatic), 7.42 (1H, s,
CH−aromatic). 13C NMR (CDCl3), ppm: δ 26.26, 26.57, 28.95, 29.35
(8C, CH2), 32.51 (4C, CH3), 45.04 (2C, CH2−Cl), 55.37 (2C, C,
aliphatic), 64.25 (2C, CH2−O, carbamate), 121.28, 123.25, 128.37
(4C, CH, aromatic), 147.20 (2C, C, aromatic) 154.87 (2C, CO,
carbamate).
Bis(6-chlorohexyl) (methylenebis(cyclohexane-4,1-diyl))-
dicarbamate. Yield 91%. 1H NMR (CDCl3), ppm: δ 0.85−2.05
(36H, m, CH2, CH), 3.40 [mixture of isomers] (2H, m, NH,
carbamate), 3.54 (4H, t, CH2−Cl), 4.04 (4H, t, CH2−O, carbamate),
4.54, 4.80 [mixture of isomers] (2H, CH−NH, carbamate). 13C NMR
(CDCl3), ppm: δ 25.24, 25.60, 26.53, 28.03, 28.89, 29.69, 33.44, 33.62,
33.72 [mixture if isomers] (18C, CH, CH2), 44.04 (1C, cyclohex−
CH2−cyclohex), 44.97 (2C, CH2−N3), 53.45 (2C, CH−NH,
carbamate), 64.47 (2C, CH2−O, carbamate), 155.91 (2C, CO,
carbamate).
Bis(6-chlorohexyl)(methylenebis(4,1-phenylene))dicarbamate.
1
Yield 94%. H NMR (d-DMSO), ppm: δ 1.31−1.77 (16H, m, CH2),
3.63 (4H, t, CH2−Cl), 3.86 (2H, s, Aryl−CH2−Aryl), 4.05 (4H, t,
CH2−O, carbamate), 7.10 (4H, d, CH−aromatic), 7.36 (4H, d, CH−
aromatic), 9.51 (2H, s, CH2−NH, carbamate). 13C NMR (d-DMSO),
ppm: δ 25.21, 26.51, 28.79, 32.43 (8C, CH2), 40.53 (1C, Aryl−CH2−
Aryl), 44.97 (2C, CH2−Cl), 65.02 (2C, CH2−O, carbamate), 118.89,
136.07 (8C, CH, aromatic), 129.38, 136.23 (4C, C, aromatic) 153.78
(2C, CO, carbamate).
Synthesis of Bis(6-azidohexyl)dicarbamates. A round-bottom
flask fitted with a reflux condenser was charged with bis(6-
chlorohexyl)dicarbamates (0.004 15 mol) and dissolved in 30 mL of
DMF. Sodium azide (1.08 g; 0.0166 mol) was then added and allowed
to react overnight at 80 °C. The reaction flask was then cooled to
ambient temperature, and the product was extracted with ethyl acetate
followed by water washes to remove DMF and the excess sodium
azide. The organic phase was dried with Na2SO4 and dried in vacuo to
colorless oil and, if necessary, purified using column chromatography.
Bis(6-azidohexyl)(cyclohexane-1,3-diylbis(methylene))-
1
dicarbamate (AZ-1). Yield 89%. H NMR (CDCl3), ppm: δ 0.50−
1.90 (26H, m, CH2, CH), 3.04 (4H, t, CH2−carbamate), 3.29 (4H, t,
CH2−N3), 4.06 (4H, t, CH2−carbamate), 4.76 (2H, s, NH,
carbamate). 13C NMR (CDCl3), ppm: δ 25.19, 25.50, 26.40, 28.75,
28.91, 30.42, 34.62 (12C, CH2), 37.96 (2C, CH), 47.19 (2C, CH2−
NH, carbamate), 51.35 (2C, CH2−N3), 64.66 (2C, CH2−O,
carbamate), 156.85 (2C, CO, carbamate).
Bis(6-azidohexyl)(1,3-phenylenebis(propane-2,2-diyl))-
1
dicarbamate (AZ-2). Yield 93%. H NMR (CDCl3), ppm: δ 1.15−
1.75 (28H, m, CH2, CH3), 3.28 (4H, t, CH2−N3), 3.99 (4H, t, CH2−
carbamate), 5.10 (2H, s, NH, carbamate), 7.30 (3H, s, CH−aromatic),
7.44 (1H, s, CH−aromatic). 13C NMR (CDCl3), ppm: δ 25.46, 26.36,
28.89, 29.27 (8C, CH2), 28.72 (4C, CH3), 51.33 (2C, CH2−N3),
55.30 (2C, C, aliphatic), 64.17 (2C, CH2−O, carbamate), 121.22,
123.20, 128.31 (4C, CH, aromatic), 147.15 (2C, C, aromatic), 154.82
(2C, CO, carbamate).
Figure 2. Library of azide, alkyne, thiol, and alkene monomers in
addition to the catalyst and initiator system used in the CuAAC
polymerization (CuCl2[PMDETA] and DMPA).
Bis(6-azidohexyl)(methylenebis(cyclohexane-4,1-diyl))-
1
dicarbamate (AZ-3). Yield 88%. H NMR (CDCl3), ppm: δ 0.90−
was then added dropwise to the reaction, removed from the ice bath,
allowed to equilibrate to room temperature, and reacted for 1−2 h
(note: minimal heat (40−50 °C) may be used to reduce the reaction
time). Following completion of the reaction, the mixture was diluted
with excess THF and flowed through a silica plug to remove the tin
catalyst. The product was dried in vacuo to a colorless oil and, if
necessary, purified using column chromatography.
2.05 (36H, m, CH2, CH), 3.25 (4H, t, CH2−N3), 3.27, 3.75 [mixture
of isomers] (2H, s, NH, carbamate), 4.02 (4H, t, CH2−carbamate),
4.60, 4.83 [mixture of isomers] (2H, CH−NH). 13C NMR (CDCl3),
ppm: δ 25.49, 26.38, 28.02, 28.72, 28.90, 29.68, 32.00, 32.55, 33.41,
33.60, 33.70 [mixture of isomers] (18C, CH, CH2), 46.89 (1C,
cyclohex−CH2−cyclohex), 50.28 (2C, CH2−N3), 51.31 (2C, CH−
NH, carbamate), 64.42 (2C, CH2−O, carbamate), 155.92 (2C, CO,
carbamate).
Bis(6-chlorohexyl)(cyclohexane-1,3-diylbis(methylene))-
dicarbamate. Yield 95%. 1H NMR (CDCl3), ppm: δ 0.50−1.85 (26H,
C
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1
Bis(6-azidohexyl)(methylenebis(4,1-phenylene))dicarbamate
oil. Yield 88%. H NMR (CDCl3), ppm: δ 0.84, 0.92 (6H, t, CH3),
1.29 (8H, m, CH2) 2.41 (3H, t, alkyne−H), 3.33 (4H, s, CH2), 4.14
(4H, d, CH2−alkyne). 13C NMR (CDCl3), ppm: δ 7.23 (1C, CH3),
14.16 (1C, CH3), 23.54, 23.67, 25.81, 30.56 (4C, CH2), 40.70 (1C, C,
core), 58.48 (2C, CH2−alkyne), 72.29 (2C, CH, alkyne) 73.82 (2C,
CH2), 80.33 (2C, C, alkyne).
1
(AZ-4). Yield 85%. H NMR (CDCl3), ppm: δ 1.38−1.74 (16H, m,
CH2), 3.29 (4H, t, CH2−N3) 3.91 (2H, s, Aryl−CH2−Aryl), 4.17 (4H,
t, CH2−O, carbamate), 6.63 (2H, s, CH2−NH, carbamate), 7.12 (4H,
d, CH−aromatic), 7.31 (4H, d, CH−aromatic). 13C NMR (CDCl3),
ppm: δ 25.50, 26.40, 28.75, 28.80 (8C, CH2), 40.54 (1C, Aryl−CH2−
Aryl), 51.36 (2C, CH2−N3), 65.01 (2C, CH2−O, carbamate), 118.86,
135.99 (8C, CH, aromatic), 129.42, 136.29 (4C, C, aromatic) 153.74
(2C, CO, carbamate).
Thiol Monomer Synthesis. Bis(6-mercaptohexyl)(1,3-
phenylenebis(propane-2,2-diyl))dicarbamate (2-SH). To a round-
bottom flask fitted with a reflux condenser was charged potassium
thioacetate (0.47 g, 4.15 mmol) dissolved in 10 mL of DMF.
Following the dissolution of the potassium thioacetate, a solution of
bis(6-chlorohexyl)(1,3-phenylenebis(propane-2,2-diyl))dicarbamate
(1 g, 1.88 mmol) in 3 mL of DMF was slowly added and heated to 60
°C for 12 h. The solution was cooled to ambient temperature, and the
product was extracted with ethyl acetate followed by washing with
excess water. The organic layer was then concentrated and redissolved
in ethanol (5 mL of ethanol per gram of thioacetate). An equal molar
equivalence of NaOH to thioacetate was added using a 7 M NaOH
solution in water. The reaction was heated at 80 °C for 3 h before
allowing it to be cooled to room temperature and neutralized using a 2
M HCl solution in water. Once neutralized, the reaction was extracted
with diethyl ether and washed with excess water. After the separation,
the organic phase was gathered, dried with anhydrous Na2SO4, filtered,
and concentrated. The crude product was then purified using column
chromatography to obtain a colorless oil. Yield 69%. 1H NMR
(CDCl3), ppm: δ 1.25−1.75 (30H, m, CH2, CH3, SH), 2.53 (4H, q,
CH2−SH), 3.99 (4H, t, CH2−carbamate), 5.07 (2H, s, NH,
carbamate), 7.30 (3H, s, CH−aromatic), 7.44 (1H, s, CH−aromatic).
13C NMR (CDCl3), ppm: δ 24.53, 25.36, 27.96, 28.92, 29.30 (10C,
Alkyne Monomer Synthesis (AL-(1−4)). 1-(Prop-2-yn-1-yloxy)-
2,2-bis((prop-2-yn-1-yloxy)methyl)butane (AL-1). To a round-
bottom flask 1,1,1-tris(hydroxymethyl)propane (1.972 g; 14.7 mmol)
and 15 mL of dimethyl sulfoxide (DMSO) were charged. Following
dissolution of the 1,1-tris(hydroxymethyl)propane, 10 mL of a 40% w/
w solution of NaOH in water was added and allowed to stir for 1 h at
ambient temperature. Propargyl bromide (8.9 mL of an 80% solution
in toluene; 44.8 mmol) was then slowly added, and the reaction was
allowed to stir for 3−5 days. The reaction was extracted with diethyl
ether and washed with excess water. After the separation, the organic
phase was gathered, dried with anhydrous Na2SO4, filtered, and
concentrated. The crude product was then purified using column
1
chromatography to obtain a clear oil. Yield 72%. H NMR (CDCl3),
ppm: δ 0.90 (3H, t, CH3), 1.45 (2H, q, CH2−CH3) 2.42 (3H, t,
alkyne−H), 3.43 (6H, s, CH2), 4.14 (6H, d, CH2−alkyne). 13C NMR
(CDCl3), ppm: δ 7.48 (1C, CH3), 22.71 (1C, CH2), 42.73 (1C, C,
core), 58.58 (3C, CH2−alkyne), 70.26 (3C, CH2), 74.05 (3C, CH,
alkyne) 80.13 (3C, C, alkyne).
1,3,5-tris((prop-2-yn-1-yloxy)methyl)benzene (AL-2). A 60% NaH
(0.18 g, 4.51 mmol) oil dispersion was added to a solution of
propargyl alcohol (0.25 g, 4.51 mmol) in DMF (15 mL) at 0 °C under
N2. After 10 min of mixing at 0 °C, tris-1,3,5-bromomethylbenzene
(0.5 g, 1.4 mmol) was added, and the resulting solution was stirred
overnight at ambient temperature. The reaction was quenched with
methanol and water in that order to neutralize the excess NaH. The
reaction mixture was extracted with CH2Cl2 and washed with excess
water. After the separation, the organic phase was gathered, dried with
anhydrous Na2SO2, filtered, and concentrated. The crude product was
then purified using column chromatography to obtain a colorless oil.
Yield 67%. 1H NMR (CDCl3), ppm: δ 2.50 (3H, t, CH, alkyne), 4.21
(6H, d, CH2−alkyne), 4.63 (6H, CH2−aromatic) 7.32 (3H, CH,
aromatic). 13C NMR (CDCl3), ppm: δ 57.32 (3C, CH2−alkyne),
71.28 (3C, CH2−O), 74.79 (3C, CH, alkyne), 79.54 (3C, C, alkyne),
127.17 (3C, CH, aromatic), 137.91 (3C, C, aromatic).
1,3,5-Tris(prop-2-yn-1-yloxy)benzene (AL-3). Phloroglucinol (10 g,
0.0793 mol) and K2CO3 (65.8 g, 0.476 mol) were charged into a two-
neck round-bottom fitted with a reflux condenser and dissolved in
DMF (500 mL). The mixture was purged with N2 followed by the
slow addition of propargyl bromide (53 mL of an 80% w/w solution in
toluene). The reaction was heated to 80 °C for 24 h before extracting
with ethyl acetate and washing with a 1 M NaOH solution followed by
excess water. After the separation, the organic phase was gathered,
dried with anhydrous Na2SO4, filtered, and concentrated. The crude
product was then purified using recrystallization in methanol to obtain
a white crystalline powder. Yield 82%. 1H NMR (CDCl3), ppm: δ 2.56
(3H, t, CH, alkyne), 4.67 (6H, d, CH2−alkyne), 6.29 (3H, CH,
aromatic). 13C NMR (CDCl3), ppm: δ 55.96 (3C, CH2−alkyne),
77.75 (3C, CH, alkyne) 78.25 (3C, C, alkyne), 95.44 (3C, CH,
aromatic), 159.34 (3C, C, aromatic).
CH2), 33.85 (4C, CH3), 55.32 (2C, C−NH, carbamate), 64.27 (2C,
CH2−O, carbamate), 121.21, 123.21, 128.34 (4C, CH, aromatic),
147.14 (2C, C, aromatic) 154.85 (2C, CO, carbamate).
Trialkene Synthesis. 1-(Allyloxy)-2,2-bis((allyloxy)methyl)-
butane (1-ene). To a round-bottom was charged 1,1,1-tris(hydroxy-
methyl)propane (1.972 g, 14.7 mmol) and 15 mL of dimethyl
sulfoxide (DMSO). Following dissolution of the 1,1-tris(hydroxy-
methyl)propane, 10 mL of a 40% w/w solution of NaOH in water was
added and allowed to stir for 1 h at room temperature. Allyl bromide
(5.1 mL, 58.8 mmol) was then slowly added, and the reaction was
allowed to stir for 3−5 days. The reaction was extracted with diethyl
ether and washed with excess water. After the separation, the organic
phase was gathered, dried with anhydrous Na2SO4, filtered, and
concentrated. The crude product was then purified using column
1
chromatography to obtain a clear oil. Yield 79%. H NMR (CDCl3),
ppm: δ 0.88 (3H, t, CH3), 1.46 (2H, q, CH2−CH3), 3.35 (6H, s,
CH2), 3.97 (6H, 2t, CH2−alkene), 5.15−5.31 (6H, m, CH2, alkene),
5.86−5.96 (3H, m, CH, alkene). 13C NMR (CDCl3), ppm: δ 7.72 (1C,
CH3), 23.03 (1C, CH2), 43.16 (1C, C, core), 70.71 (3C, CH2−
alkene), 72.24 (3C, CH2−O), 116.12 (3C, CH2, alkene) 135.31 (3C,
CH, alkene).
1,3,5-Tris((allyloxy)methyl)benzene (2-ene). A 60% NaH (0.18 g,
4.51 mmol) oil dispersion was slowly added to a solution of allyl
alcohol (0.25 g, 4.51 mmol) in DMF (15 mL) at 0 °C under N2. After
10 min of mixing at 0 °C, tris-1,3,5-bromomethylbenzene (0.5 g, 1.4
mmol) was added, and the resulting solution was stirred overnight at
ambient temperature. The reaction was quenched with methanol and
water in that order to neutralize the excess NaH. The reaction mixture
was extracted with CH2Cl2 and washed with excess water. After the
separation, the organic phase was gathered, dried with anhydrous
Na2SO4, filtered, and concentrated. The crude product was then
purified using column chromatography to obtain a colorless oil. Yield
86%. 1H NMR (CDCl3), ppm: δ 4.05, 4.07 (6H, t, CH2), 4.55 (6H, s,
CH2), 5.22−5.36 (6H, CH2−alkene) 5.97 (3H, m, CH, alkene) 7.29
(3H, s, CH−aromatic). 13C NMR (CDCl3), ppm: δ 71.28, 71.95 (6C,
CH2), 117.22 (3C, CH2−ene), 126.29 (3C, CH, aromatic), 134.70
(3C, CH−ene), 138.69 (3C, C, aromatic).
3,3-Bis((prop-2-yn-1-yloxy)methyl)heptane (AL-4). To a round-
bottom flask 2-butyl-2-ethyl-1,3-propanediol (11.77 g, 73.45 mmol),
tetrabutylammonium iodide (300 mg, 0.81 mmol), potassium
hydroxide (27 g, 481 mmol), and 250 mL of THF were charged
and fitted with a reflux condenser. Following dissolution of the 2-butyl-
2-ethyl-1,3-propanediol, 32.7 mL of propargyl bromide (80% solution
in toluene; 293.8 mmol) was then slowly added, and the reaction was
allowed to stir for 24 h at 70 °C. The reaction was cooled to room
temperature and extracted with ethyl acetate and washed with excess
water. After the separation, the organic phase was gathered, dried with
anhydrous Na2SO4, filtered, and concentrated. The crude product was
then purified using column chromatography to obtain a light yellow
Diazide−Ether Synthesis. Bis(4-((8-azidooctyl)oxy)phenyl)-
methane (AZ-4-ether). To a round-bottom flask was charged bis(4-
hydroxyphenyl)methane (1 g, 9.98 mmol), 8-chloro-1-octanol (2.47 g,
14.98 mmol), and triphenylphosphine (TPP) (3.93 g, 14.98 mmol)
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Figure 3. Plots of (a) tan δ vs temperature and (b) storage modulus vs temperature for a series of azide monomers (AZ-(1−4)) with a single alkyne
monomer (AL-1). All resins were composed 2 mol % CuCl2[PMDETA] and 4 mol % DMPA and were cured with 10 mW/cm2 UV light.
Representative curves are presented as the third cycle in the DMA.
dissolved in 5 mL of THF. Diethyl azodicarboxylate (DEAD) (6.82
mL of a 40% w/w solution in toluene, 14.98 mmol) was added
dropwise while under sonication at 0 °C. Once all the DEAD was
added, the reaction was allowed to equilibrate to room temperature
and sonicated for an additional 2 h. The reaction was extracted with
ethyl acetate and washed with excess water. After the separation, the
organic phase was gathered, dried with anhydrous Na2SO4, filtered,
and concentrated. The crude product was then purified by first
crystallizing out the excess TPP followed by column chromatography.
Following purification, the dichloro intermediate was then exchanged
to the diazide using the previously reported procedure for AZ-(1−4).
Yield 93%. 1H NMR (CDCl3), ppm: δ 1.35−1.84 (24H, m, CH2), 3.29
(4H, t, CH2−N3) 3.89 (2H, s, Aryl−CH2−Aryl), 3.95 (4H, t, CH2−
O), 6.84 (4H, d, CH−aromatic), 7.11 (4H, d, CH−aromatic). 13C
NMR (CDCl3), ppm: δ 26.00, 26.67, 28.84, 29.10, 29.25, 29.29 (12C,
CH2), 40.16 (1C, Aryl−CH2−Aryl), 51.47 (2C, CH2−N3), 67.89 (2C,
CH2−O), 114.43, 129.71 (8C, CH, aromatic), 133.59, 157.42 (4C, C,
aromatic).
between 2300 and 2000 cm−1 having the alkane C−H stretching
bonds as a reference peak between 2980 and 2840 cm−1 with 12 scans
s−1 and 2 cm−1 resolution. In addition, grazing-angle attenuated total
reflectance mode FTIR (gATR-FTIR) with a VariGATR accessory
(grazing angle 65°, zinc selenide crystal; Harrick Scientific) was used
to provide an indication of the functional group conversions of thick
samples (100−250 μm) postannealing. Spectra were collected with a
resolution of 4 cm−1 by accumulating a minimum of 64 scans per
sample (spectra shown in Supporting Information, Figures S13−S16).
Dynamic mechanical analysis (DMA) was performed on a TA
Instruments Q800 DMA in tension film mode with a heating rate of 3
°C min−1 to a maximum temperature of 150 °C at a frequency of 1 Hz.
E″/E′, i.e., the ratio of the loss and storage moduli, gives the tan δ, a
dampening term, which relates to the energy dissipation relative to the
energy stored in the material upon deformations where the Tg is
assigned here as the peak of the tan δ curve. Samples were prepared
using the sandwich method previously described to create 250 μm
thick films, which were cut into bars ranging from 5 to 7 mm wide.
DMA experiments were thermally cycled and replicated three times,
and the representative curves are presented as the third heating cycle.
Note: DMA experiments for samples annealed at lower temperatures
(30 °C) can be found in the Supporting Information (Figures S17−
S20) and represent the progression of the Tg over each of the three
DMA cycles.
CuCl2[PMDETA] Complex. A 1:1 molar mixture of CuCl2 and
PMDETA (N,N,N′,N′,N″-pentamethyldiethylenetriamine) in acetoni-
trile was stirred overnight at room temperature and dried under
vacuum to a blue-green solid.
Film Preparation. Resins were mixed in equal molar ratios of azide
to alkyne based on functional groups with 2 mol % CuCl2[PMDETA]
and 4 mol % DMPA. Methanol was used to homogenize the resins but
was removed prior to polymerization to below 0.5% w/w to limit
plasticization. 75−125 mg of the resins was placed in the center of a
RainX coated 75 × 50 mm glass slide. Spacers (250 μm) were inserted
on the sides of the glass slide, and a second RainX coated 75 × 50 mm
glass slide was gently placed on the top, making sure bubbles were
excluded when “sandwiching” the monomer. The glass slides were
clamped together, and the sandwiched resin was placed under 5−10
mW cm−2 UV light. The resins were irradiated for 3−5 min, followed
by annealing in a preheated oven at 75 °C for 24 h.
RESULTS AND DISCUSSION
■
Thermomechanical Properties. Thermomechanical tran-
sitions of the cured CuAAC resins were investigated using
dynamic mechanical analysis (DMA) in tension mode. Figure 3
shows the DMA results for a series of four synthesized azide
monomers (AZ-(1−4)) with a single trialkyne cross-linker
(AL-1). Relatively cyclohexane-based monomers show a
consistently lower Tg as compared to their aromatic analogues.
In both examples, similar aromatic and nonaromatic cyclic
monomers show a 14−16 °C difference in the Tg as a result of
the increased stiffness and additional secondary interactions
associated with π−π stacking. Interestingly, the increase in Tg
going from a single aromatic to a double aliphatic cyclic core,
specifically comparing azide monomers AZ-2 and AZ-3, cannot
be explained with additional secondary interactions, as
cyclohexane rings do not contribute to such forces. A possible
explanation is that there is an increase in the stiffness of the
monomer and polymer structures with azide monomer (AZ-3)
as compared to AZ-2. Additional data including the glassy and
rubbery modulus along with the estimated molecular weight
between cross-links (Mc) are summarized in Table 1. In the
glassy state all these materials exhibit high modulus values
1
Characterization and Measurements. H NMR and 13C NMR
measurements were performed in deuterated chloroform (CDCl3) and
dimethyl sulfoxide (d-DMSO) to determine the purity and degree of
functionalization of the synthesized molecules, using a Bruker Avance-
1
III-400 NMR spectrometer. The number of transients for H and 13C
is 16 and 256, respectively, and a relaxation time of 1 s was used for the
1
integrated intensity determination of H NMR spectra. NMR spectra
of all the synthesized monomers can be found in the Supporting
Information (Figures S1−S12).
Polymerization conversions were analyzed using a Fourier transform
infrared spectroscopy (FTIR) instrument (Nicolet 8700) incorporated
with a heating stage to monitor the real-time functional group
conversions in transmission mode. Irradiation was performed using a
light guide connected to a mercury lamp (Acticure 4000) with a 365
nm bandgap or other filter, as noted. Samples were placed between
NaCl plates, and the azide peak was monitored in the absorption range
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A similar trend is observed in the Tg values for the series of
synthesized trialkyne monomers AL-(1−3) when polymerized
with a single azide monomer (AZ-2) (Figure 4a). The material
with the lowest Tg of this series is made with the larger
molecular weight aromatic trialkyne (AL-2). One would expect
that having an aromatic core would result in a higher Tg;
however, the opposite is observed, likely as a result of a higher
molecular weight monomer and reduced π−π interactions. This
correlation also holds true for trialkyne (AL-3) as it has the
lowest molecular weight, which leads to the greatest cross-link
density and shows the highest Tg. The opposite is observed
when comparing trialkyne monomers AL-1 and AL-2 with
azide monomer AZ-4 (Figure 4b). An increase in the Tg with
trialkyne AL-2 is likely a result of improved π−π stacking from
the azide monomer AZ-4 that contains two aromatic rings
incorporated through the para position. Additionally, as shown
in Table 2, in these materials the glassy moduli ranges from 1.4
to 2.2 GPa; however, changing the alkyne monomer for AZ-2
had little effect on the rubbery modulus with values for the
various alkynes used here ranging only 300 MPa. One would
expect that more significant changes to the equivalent weight
per alkyne functional group in the monomer would beyond the
limited range used here would lead to a change in the cross-link
density and corresponding rubbery modulus value. In contrast,
for AZ-4 the rubbery modulus significantly increases from ∼2.1
Table 1. Summary of DMA Results for a Series of Azide
Monomers (AZ-(1−4)) with a Single Alkyne Monomer (AL-
1) Composed 2 mol % CuCl2[PMDETA] and 4 mol %
a
DMPA and Cured with 10 mW/Cm2 UV Light
alkyne cross-linker (AL-1)
azide monomer
Tg (°C)
AZ-1
57
AZ-2
67
AZ-3
73
AZ-4
81
E30 °C (GPa)
1.7
2.2
1.1
1.4
E at Tg + 40 °C (MPa)
ρ (g/cm3)
3.9
3.4
1.8
2.1
∼1.1
∼1200
∼1.2
∼1500
∼1.1
∼2600
∼1.1
∼2300
Mc (g/mol)
a
ρ is the density of the material estimated from the dimension and
mass of the sample; Mc is the molecular weight between cross-links
calculated from the equation MC = 3ρRT/2E where R is the gas
constant.
between 1.1 and 2.2 GPa; however, the lower MW azides (AZ-
1, AZ-2) have a higher rubbery modulus as compared to the
higher MW azides (AZ-3, AZ-4) due to a corresponding
reduction in cross-link density. To ensure these material
properties are accurate and not a result of different conversions
of the monomers, FTIR spectra of the resin and annealed
polymers are included in the Supporting Information and show
near-quantitative conversion of the azide moiety as highlighted
by the peak disappearance at 2100 cm−1.
Figure 4. Plots of tan δ vs temperature and storage modulus for a series of alkyne monomers AL-(1−3) with a single azide monomer ((a) AZ-2, (b)
AZ-4). All resins composed 2 mol % CuCl2[PMDETA] and 4 mol % DMPA and were cured with 10 mW/cm2 UV light. Representative curves are
presented as the third cycle in the DMA.
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Lastly, a single resin was formulated without the addition of
the radical initiator (DMPA), and instead, the copper was
chemically reduced using hexylamine. Through this process, no
spatial or temporal control was achieved; however, the
mechanical properties are within error of the same resin
cured photolytically (i.e., through a radical reduction of copper)
(Figure S21). This outcome verifies that the structure of the
triazole-containing polymer is dictating the behavior in contrast
to the initiation method used to form the polymer.
In addition to the cores of the monomer structures imparting
mechanical performance and characteristics, the triazoles
themselves can interact with both π−π stacking and H-
bonding27−29 that is unique to the CuAAC polymerization. To
study the effect of the triazole secondary interactions on
mechanical behavior, alternative analogous monomers (2-SH
and X-ene, shown in Figure 5b) were synthesized to create
thiol−ene networks in which all else (i.e., backbone, cross-link
density, etc.) is equivalent to the CuAAC networks, but the
triazoles are effectively replaced by thioethers. It is hypothe-
sized that a CuAAC-based network has the potential for
improved thermomechanical behavior as compared to other
“click” and especially photo-“click”-based polymeric systems.
The triazole adduct, compared to a thioether produced from a
thiol−ene or thiol−Michael “click” reaction, for example,
produces a stiffer material; however, only minimal attempts
to observe this have been conducted. Gong et al. compared a
CuAAC-based network to a thiol−Michael network of similar
cross-link density; however, the structural dissimilarities of the
Table 2. Summary of DMA Results for a Series of Alkyne
Monomers AL-(1−3) with a Single Azide Monomer (AZ-2
or AZ-4) Composed 2 mol % CuCl2[PMDETA] and 4 mol %
a
DMPA and Cured with 10 mW/Cm2 UV Light
azide monomer (AZ-2)
alkyne cross-linker
Tg (°C)
AL-1
67
AL-2
63
AL-3
73
E30 °C (GPa)
2.2
1.9
1.8
E at Tg + 40 °C (MPa)
ρ (g/cm3)
3.4
2.9
∼1.1
∼1600
2.1
∼1.2
∼1500
∼1
∼2100
Mc (g/mol)
azide monomer (AZ-4)
alkyne cross-linker
AL-1
AL-2
AL-3
Tg (°C)
81
90
−
−
−
−
−
E30 °C (GPa)
E at Tg + 40 °C (MPa)
ρ (g/cm3)
1.4
1.8
2.1
6.7
∼1.1
∼2300
∼1.3
∼900
Mc (g/mol)
a
ρ is the density of the material estimated from the dimension and
mass of the sample; Mc is the molecular weight between cross-links
calculated from the equation MC = 3ρRT/2E, where R is the gas
constant; (−) represents no data collected because AZ-4 and Al-3 are
both solid monomers and no resin was formulated.
MPa for AL-1 to ∼6.7 MPa for AL-2 likely due to improved
π−π stacking as discussed above.
Figure 5. (a) Plots of tan δ vs temperature and storage modulus vs temperature for analogous materials made from either the (a) thiol−ene or
CuAAC reactions and (b) the generalized polymeric structure. The thiol−ene resin composed 4 mol % DMPA and the CuAAC resin composed 2
mol % CuCl2[PMDETA] and 4 mol % DMPA, and both were cured with 10 mW/cm2 UV light. Representative curves are presented as the third
cycle in the DMA.
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Table 3. Functional Group (FG) Ratios of Three Different Monomers (AZ-2, AL-1, and AL-4) Used To Create an Off-
Stoichiometric Resin with Excess Azide Functionality with the Resulting Azide Conversions and Material Properties Including
Tg and Molecular Weight between Cross-Links (Mc) As Calculated from Rubber Elasticity Included
AZ-2 monomer (FG eq)
AL-4 monomer (FG eq) AL-1 monomer (FG eq)
theor azide conv (%)
measd azide conv (%)
Tg (°C) Mc (g/mol)
1 eq
1 eq
1 eq
1 eq
0.5 eq alk
0.4 eq alk
0.2 eq alk
0 eq alk
0.25 eq alk
0.4 eq alk
0.7 eq alk
1 eq alk
75
80
74
83
46
51
59
67
∼85000
∼20000
∼9000
∼1600
90
91
100
∼99
Figure 6. (a) Plots of tan δ vs temperature and storage modulus vs temperature for comparing analogues materials made from monomers with and
without carbamates and (b) the generalized polymeric structure. Both resins composed 2 mol % CuCl2[PMDETA] and 4 mol % DMPA, and both
were cured with 10 mW/cm2 UV light. Representative curves are presented as the third cycle in the DMA.
monomers limit the comparison25 and as shown previously
have a significant impact on material properties. Here, a thiol
and ene monomer are synthesized with the specific intent of
directly comparing the relative impact of the triazole and
thioether moieties. Figure 5 shows the comparison between
nearly identical CuAAC and thiol−ene-based networks, where a
significant increase in Tg of nearly 50 °C was observed as a
result of the triazole presence. Additionally, an additional
thiol−ene-based resin was formulated, cured, and analyzed
using the alkene analogue of AL-2 and showed a similar drop in
Tg of about 45 °C when comparing the CuAAC network to a
thiol−ene analogue (Figure S22). These results point to the
dramatic impact of the triazole moiety within the network
structure.
unique as a significant portion of the desirable material
performance arises directly from the moiety formed during
the photopolymerization. In particular, it would be desirable to
perform polymerizations at different conversions and observe
the evolution of the mechanical behavior of the material at
those different extents of reaction. Unfortunately, this approach
is dificult in CuAAC-based networks since, as the partially
cured networks are heated, the catalyst becomes more mobile
allowing conversion to increase. Here, we use off-stoichiometric
experiments as mimics of the evolution of conversion. In
particular, we synthesized AL-4, which is a difunctional
analogue of the trialkyne, AL-1, but with one alkyne replaced
with a saturated alkyl chain. By systematically replacing a
portion of the AL-1 with AL-4, we alter the azide−alkyne
stoichiometry such that at complete conversion of the alkyne,
the network mimics a network with incomplete conversion.
Here, however, no additional CuAAC reaction occurs upon
heating since no residual alkyne is present. Thus, these off-
stoichiometry studies were designed and performed to observe
The difference in material performance is purely a result of
the polymerization, forming the triazole adducts which are rigid
and capable of both π−π and H-bonding interactions. Few
polymerization techniques and no other photopolymerization
techniques possess this capability, making these materials
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the buildup of material properties as the concentration of
triazoles increases during polymerization within a network
structure that remains largely the same other than the extent of
cross-linking/triazole formation. Table 3 presents the resulting
material properties achieved when AL-4 is systematically
substituted for AL-1 in such a manner that mimic a 75%,
80%, and 90% azide conversions were achieved. As the AL-4
monomer is reduced and the ratio of alkyne to azide functional
groups approaches a 1:1 molar ratio, the Tg increases as a result
of the increasing triazole concentration and the molecular
weight between cross-links (Mc) significantly decreasing. The
DMA curves for these samples are provided in the Supporting
Information (Figures S23−S25).
Lastly, a major contributor to the mechanical performance
likely results from the carbamate structures inherently
incorporated into the network via the monomer synthesis
approach broadly used here. To investigate this relationship, a
monomer containing no carbamates (4-ether, shown in Figure
6b) that is structurally similar to AZ-4 was synthesized. Figure 6
shows the modulus and glass transition temperature for
networks both with and without the carbamates. A significant
dependence on the carbamate presence is observed as a
decrease in glass transition temperature of ∼40 °C occurred
upon removal of the carbamates from the analogous networks.
Within these networks, the carbamates improve the mechanical
performance through hydrogen bonding and the formation of
hard segments, generally resulting in an increase in Tg. For all
of these resins, FTIR was used to confirm near-quantitative
conversion of the polymerizable functional groups following the
thermal cycles.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail christopher.bowman@colorado.edu (C.N.B.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Institutes of Health (NIH: 5UO1DE023774) and the
National Science Foundation (NSF: CHE1214109).
REFERENCES
■
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CONCLUSIONS
■
In summary, the materials developed using the photoinitiated
CuAAC polymerization show vast tailorability of mechanical
properties, and the unique ability to form a high glass transition
temperature material, which is shown to occur from the step
growth photopolymerization itself. Both the monomer
structure and the polymerization technique utilized have a
significant impact on the material properties where the
monomers are easily changed and tailored as a result of the
simple and scalable synthesis techniques utilized. Lastly, the
triazoles generated during the polymerization and the
carbamates incorporated into the monomers drastically increase
the Tg by 45−49 and 40 °C, respectively, due to secondary
interactions (H-bonding, π−π stacking, and chain stiffness).
Few polymerization techniques and particularly no other
photopolymerization techniques possess the capability for
altering the material properties so directly from the moiety
formed during the photopolymerization. Of all the photo-
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show the highest potential for creating glassy step growth
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b00137.
1H NMR and 13C NMR along with supporting FTIR and
DMA results are detailed (PDF)
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DOI: 10.1021/acs.macromol.6b00137
Macromolecules XXXX, XXX, XXX−XXX