Thiol-isocyanate-ene ternary networks by sequential and simultaneous thiol click reactions

Article abstract of DOI:10.1021/cm903856n

Thiol-isocyanate-ene ternary networks with systematic variations (100/100/0, 100/80/20, 100/60/40, 100/40/60, 100/20/80, and 100/0/100) were prepared by sequential and simultaneous thiol-ene and thiol-isocyanate click reactions. The thiol-isocyanate coupling reaction was triggered thermally or photolytically to control the sequence with the thiol-ene photopolymerization. Triethyl amine (TEA) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were used for the sequential thermally induced thiol-isocyanate coupling and photochemically initiated thiol-ene reaction, respectively. A thermally stable photolatent base catalyst (tributylaminetetraphenylborate salt, TBAHBPh ₄) capable of in situ generation of tributylamine by UV light was used with isopropylthioxanthone (ITX) for the simultaneous thiol-isocyanate/ thiol-ene curing systems. The kinetics of the hybrid networks investigated using real-time IR indicate that both thiol-isocyanate and thiol-ene reactions were quantitatively rapid and efficient (>90% of conversion in a matter of minutes and seconds, respectively). The T_g of the thiourethane/thiol-ene hybrid networks progressively increases (-5 to 35 °C by DSC) as a function of the thiourethane content due to the higher extent of hydrogen bonding, also resulting in enhanced mechanical properties. Highly uniform and dense network structures exhibiting narrow full width at half-maximum (～10 °C) were obtained for both the sequential and the simultaneous thiol click reactions, resulting in identical thermal properties that are independent of the sequence of the curing processes.

Full text of DOI:10.1021/cm903856n

2616 Chem. Mater. 2010, 22, 2616–2625
DOI:10.1021/cm903856n
Thiol-Isocyanate-Ene Ternary Networks by Sequential and
Simultaneous Thiol Click Reactions
Junghwan Shin,† Hironori Matsushima,‡ Christopher M. Comer,‡
Christopher N. Bowman,*,§ and Charles E. Hoyle‡
†Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE,
Minneapolis, Minnesota 55455, ‡School of Polymers and High Performance Materials, University of
Southern Mississippi, 118 College Drive, Hattiesburg, Mississippi 39406, and §Chemical and Biological
Engineering, University of Colorado at Boulder, Engineering Center, ECCH 111, Boulder, Colorado 80309
Received December 25, 2009. Revised Manuscript Received February 11, 2010
Thiol-isocyanate-ene ternary networks with systematic variations (100/100/0, 100/80/20, 100/
60/40, 100/40/60, 100/20/80, and 100/0/100) were prepared by sequential and simultaneous thiol-ene
and thiol-isocyanate click reactions. The thiol-isocyanate coupling reaction was triggered ther-
mally or photolytically to control the sequence with the thiol-ene photopolymerization. Triethyl
amine (TEA) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were used for the sequential
thermally induced thiol-isocyanate coupling and photochemically initiated thiol-ene reaction,
respectively. A thermally stable photolatent base catalyst (tributylamine tetraphenylborate salt,
3
TBA HBPh4) capable of in situ generation of tributylamine by UV light was used with isopro-
3
pylthioxanthone (ITX) for the simultaneous thiol-isocyanate/thiol-ene curing systems. The
kinetics of the hybrid networks investigated using real-time IR indicate that both thiol-isocyanate
and thiol-ene reactions were quantitatively rapid and efficient (>90% of conversion in a matter of
minutes and seconds, respectively). The Tg of the thiourethane/thiol-ene hybrid networks progres-
sively increases (-5 to 35 °C by DSC) as a function of the thiourethane content due to the higher
extent of hydrogen bonding, also resulting in enhanced mechanical properties. Highly uniform and
dense network structures exhibiting narrow full width at half-maximum (∼10 °C) were obtained for
both the sequential and the simultaneous thiol click reactions, resulting in identical thermal proper-
ties that are independent of the sequence of the curing processes.
Introduction
base catalyst (Scheme 2) is also reported to give 100% yield
in a matter of seconds with few, if any, side products.11
These two thiol-based reactions proceed by free-radical or
anionic chain processes,7-11 respectively. It has been re-
ported that base catalysts have a pronounced effect on the
thiol-isocyanate reaction since the base catalyst makes the
carbonyl carbon of isocyanate more electron deficient while
also producing a very strong nucleophilic thiolate ion11-15
that results in rapid thiol-isocyanate coupling reactions.
We recently reported that highly elastic novel segmented
polythiourethane elastomers could be synthesized by react-
ing a dithiol, an oligomeric dithiol, and a diisocyanate in the
presence of triethylamine (TEA).11 High conversions of
greater than 95% were achieved within 10 min at room
temperature, and the nanophase separation of the segmen-
ted structure was controllable by the thiol-isocyanate
reaction.
Thiol chemistry continues to attract attention due to its
efficiency and versatility with numerous thiol reactions
being regarded as click reactions.1 In particular, thiol-ene
free-radical photopolymerizations (Scheme 1) proceed via a
highly robust, oxygen resistant reaction that has been widely
used in making polymeric materials and in functionalizing
small molecules.2-10 Additionally, the nucleophilic addition
of thiols to isocyanate groups in the presence of a strong
*Corresponding author. E-mail: christopher.bowman@colorado.edu.
(1) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci.: Part A, Polym.
Chem. 2004, 42, 5301.
(2) Killops, K. L.; Campos, L. M.; Hawker, C. J. J. Am. Chem. Soc.
2008, 130, 5062.
(3) Fairbanks, B. D.; Scott, T, F.; Kloxin, C. J.; Anseth, K. S.;
Bowman, C. N. Macromolecules 2009, 42, 211.
(4) Chan, J. W.; Zhou, H.; Hoyle, C. E.; Lowe, A. B. Chem. Mater.
2009, 21, 1579.
(5) Campos, L. M.; Killops, K. L.; Sakai, R.; Paulusse, J. M. S.;
Damiron, D.; Drockenmuller, E.; Messmore, B. W.; Hawker, C. J.
Macromolecules 2008, 41, 7063.
(6) Dondoni, A. Angew. Chem., Int. Ed. 2008, 47, 2.
(7) Chan, J. W.; Hoyle, C. E.; Lowe, A. B. J. Am. Chem. Soc. 2009, 131,
5751.
(8) Chan, J. W.; Yu, B.; Hoyle, C. E.; Lowe, A. B. Chem. Commun.
2008, 4959.
(9) Hoyle, C. E.; Lowe, A. B.; Bowman, C. N. Chem. Soc. Rev. 2010,
DOI:10.1039/B901979K.
(10) Hoyle, C. E.; Bowman, C. N. Angew. Chem. 2010, 122, 1584.
There have been several multicomponent simultaneous
or sequential systems based on thiol Click reactions. Wei
et al. demonstrated that thiol-ene free-radical and vinyl
(11) Shin, J.; Matsushima, H.; Chan., J. W.; Hoyle, C. E. Macromole-
cules 2009, 42, 3294.
(12) Dyer, E.; Glenn, J. F.; Lendrat, E. G. J. Org. Chem. 1961, 26, 2919.
(13) Dyer, E.; Osborne, D. W. J. Polym. Sci. 1960, 47, 361.
(14) Dyer, E.; Glenn, J. F. J. Am. Chem. Soc. 1957, 79, 366.
(15) Klemm, E.; Stockl, C. Macromol. Chem. 1991, 192, 153.
r
pubs.acs.org/cm
Published on Web 03/04/2010
2010 American Chemical Society

Article
Chem. Mater., Vol. 22, No. 8, 2010 2617
Scheme 1. Free-Radical Step Growth Mechanism of Thiol-Ene
Photopolymerization
of free radicals and tertiary amines by irradiating with UV
light lead to simultaneous reactions of thiols with enes
and isocyanates. Controlling the reaction sequence and
timing provides an exciting opportunity for implementing
a suitable strategy for fabricating photocurable materials
for both thin films and thick cross-linked materials. In
addition, the inclusion of thiourethane groups in
thiol-ene networks offers numerous advantages such as
enhanced physical and mechanical properties resulting
from extensive hydrogen bonding. The relationships
between the chemical composition and the physical/
mechanical properties of thiol-isocyanate-ene based
ternary networks are established in terms of calorimetry,
thermomechanical properties, refractive index, hardness,
and tensile properties. The results reported are indicative
of the vast range of properties achievable with thiol-
isocyanate-ene based ternary networks.
ether cationic photopolymerization resulted in cross-
linked networks that effectively combined the thermal
and mechanical properties inherent to each system
while also taking advantage of the kinetic nature of
each reaction.16 The combination of thiol-ene and
thiol-epoxy reactions to form hybrid networks indicated
that highly cross-linked and high Tg polymer materials
could be formed with significantly reduced shrinkage and
stress when the polymerization kinetics of each reaction
were manipulated appropriately.17 Recently, incorpora-
tion of urethane functional groups into thiol-ene net-
works to introduce strong hydrogen bonding for
enhancing physical and mechanical properties has been
reported.1,18,19 However, these urethane-modified thiol-
ene networks were essentially based on the simple pho-
topolymerization of thiols and urethane-modified ene
oligomers that had been prepared separately by an iso-
cyanate coupling reaction with hydroxyl-terminated enes.
This approach leads to significant difficulties since the
tetra-functional ene monomers are highly viscous with
extensive hydrogen bonding occurring prior to the pho-
topolymerization process. To overcome this problem and
generate polymer networks with significant hydrogen
bonding while still beginning with low viscosity mono-
mers, sequential or simultaneous thiol-isocyanate and
thiol-ene reactions were performed on a single formula-
tion that contained all necessary components.
Experimental Section
Materials. Hexanethiol, butyl 3-mercaptopropionate, butyl
thioglycolate, benzenethiol, hexyl isocyanate, phenyl isocya-
nate, cyclohexyl isocyanate, 1,5-diazabicyclo[4.3.0]-5-nonene
(DBN), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-di-
azabicyclo[2.2.2]octane (DABCO), 4-dimethylaminopyridine
(DMAP), triethylamine (TEA), tributylamine (TBA), 1,8-bis-
(dimethylamino)naphthalene (proton sponge)), and 1,3,5-trial-
lyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione (TATAT) were
purchased from Aldrich. Dithiol (glycol di-3-mercaptopropio-
nate (GDMP)) and hexamethylene diisocyanate trimer
(Desmodur N3600) were supplied by Bruno Bock Thio-Chemi-
cals-S and Bayer Materials Science, respectively. Photoinitia-
tors, 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and
isopropylthioxanthone (2 and 4-isomer mixture) (ITX), were
obtained from Ciba Specialty Chemicals and Albemarle. The
photogenerated amine (tributylamine tetraphenylborate salt,
TBA HBPh4) was synthesized by reacting tributylamine and
3
3
sodium tetraphenylborate in a hydrocholic acid (HCl) aqueous
solution as reported in the literature.20 The structures of all
materials used are shown in Schemes 3 and 4. All materials were
used as received.
Herein, we report thiol-isocyanate-ene reaction sys-
tems that polymerize by two thiol Click reactions that are
performed either sequentially or simultaneously, that is, a
thiol-isocyanate coupling reaction and a thiol-ene reac-
tion. The sequence of the two reactions is controlled by
the applied reaction triggers and their timing. Using a
thermally active base catalyst results in sequential reac-
tions, while using a photolatent base catalyst which
generates a tertiary amine upon exposure to light results
in simultaneous curing. In the first process the
thiol-isocyanate precured in the presence of an exter-
nally added tertiary amine and the subsequent thiol-ene
photopolymerization result in a quantitatively controlled
sequential dual curing process. Simultaneous production
Kinetics. Real-time infrared (RTIR) spectra were recorded on
a modified Bruker 88 spectrometer to obtain kinetic profiles of
thiol-isocyanate model reactions in dilute benzene solution as
well as thiol-isocyanate-ene network formation. For
thiol-isocyanate model reactions, all samples were prepared
by adding isocyanates to the thiol solutions with catalyst present
(for detailed concentration and measurement conditions see
figure and table captions). Thin samples (25 μm) between two
salt plates sealed with silicon were placed immediately in the
RTIR after mixing the samples. The conversion of thiol and
isocyanate as a function of time were measured by monitoring
the peaks at 2570 and 2250 cm-1, respectively. For stoichio-
metric reactions, the results measured from both peaks were
identical; thus, only isocyanate conversions are reported here. It
should be noted that there is some error in the measurements due
to delay (∼10 s) in initiating the RTIR measurements. For the
sequential thiol-isocyanate/thiol-ene curing process, samples
were prepared by dissolving the photoinitiator (DMPA, 1 wt %)
and base catalyst (TEA, 0.1 wt %) into GDMP followed by the
add-on of TATAT and N3600. Mixtures were immediately
(16) Wei, H.; Li, Q.; Ojelade, M.; Madbouly, S.; Otaigbe, J. U.; Hoyle,
C. E. Macromolecules 2007, 40, 8788.
(17) Carioscia, J. A.; Stansbury, J. W.; Bowman, C. N. Polymer 2007,
48, 1526.
(18) Li, Q.; Zhou, H.; Wicks, D. A.; Hoyle, C. E. J. Polym. Sci., Part A:
Polym. Chem. 2007, 45, 5103.
(19) Senyurt, A. F.; Hoyle, C. E.; Wei, H.; Piland, S. G.; Gould, T. E
Macromolecules 2007, 40, 3174.
(20) Sun, X.; Gao, J. P.; Wang, Z. Y. J. Am. Chem. Soc. 2008, 130, 8130.

2618 Chem. Mater., Vol. 22, No. 8, 2010
Scheme 2. Tertiary Amine Catalyzed Thiol-Isocyanate Nucleophilic Coupling Reaction Mechanism
Shin et al.
Scheme 3. Molecular Structures of Thiol, Ene, Isocyanate, Photointitiator, and Photo-Generated Amine Catalyst
The thiol-isocyanate-ene network films were prepared on
glass plates using a 200 μm draw down bar. For sequential
thiol-isocyanate/thiol-ene reaction processes, cast films were
precured at ambient temperature for 1 h to induce selectively
GDMP-N3600 reactions followed by initiation of GDMP-TTT
photopolymerization with a medium pressure mercury lamp
(intensity = 10.2 mW/cm2 for 5 min). Thiol-isocyanate cou-
pling reactions and thiol-ene photopolymerizations were per-
formed at the same time for simultaneous reaction systems by
irradiating with a medium pressure mercury lamp passed
through a 365 nm optical filter (1.5 mW/cm2) for 30 min. All
samples were postcured at 80 °C for 24 h to obtain approxi-
mately quantitative 100% conversion and remove any effects of
the extent of reaction on the physical and mechanical properties.
Scheme 4. Molecular Structures of Tertiary Amine Catalysts
placed in the RTIR and retained for 20 min before irradiating
with UV light to selectively induce the TEA catalyzed
thiol-isocyanate coupling reactions. The thiol-ene free-radical
reaction was subsequently initiated by irradiating with light
(20.5 mW/cm2) from an Oriel lamp system equipped with a 200
W, high-pressure mercury-xenon bulb channeled through an
electric shutter and optical fiber cable into the sample chamber.
Samples for the simultaneous thiol-isocyanate/thiol-ene reac-
The same concentrations of TEA, DMPA, TBA HBPh4, and
ITX as used for RTIR measurements were employed in the
preparation of films for mechanical and other testing.
3
tions were prepared by dissolving TBA HBPh4 (0.61 wt %) and
3
ITX (0.36 wt %) in GDMP followed by mixing the monomers,
TATAT and N3600. Both thiol-isocyanate and thiol-ene
reactions were triggered simultaneously by irradiating with
UV light (1.41 mW/cm2) passed through a 365-nm filter. The
conversions of GDMP, N3600, and TATAT as a function of
time were measured during the sequential and simultaneous
thiol-isocyanate and thiol-ene reactions by monitoring peaks
at 2570, 2250, and 3080 cm-1 for thiol, isocyanate, and ene,
respectively.
Thermal properties of the thiol-isocyanate-ene films pre-
pared by both sequential and simultaneous reaction systems
were characterized by differential scanning calorimetry (DSC,
TA Instruments Q 1000, equipped with RCS 90 as a refrigerated
cooling system). Sample weights were 8.0 ( 1.0 mg, and all
experiments were carried out under nitrogen with a flow rate of
50 mL/min. DSC scans were conducted over a temperature
range from -50 to 100 °C using 10 °C/min heating and cooling
rates. On the first heating scan, samples were equilibrated at
100 °C (higher than the Tg of the thiourethane neat film
(GDMP/TTT/N3600 = 100:0:100)) for 10 min to erase any
thermal history, followed by cooling to -50 °C and subsequent
heating to 100 °C. The second heating scans were recorded and
used here for all thiol-isocyanate-ene network films.
Characterization. 1H NMR spectra were obtained to confirm
the dissociation of TBA HBPh4 salt and generation of TBA
3
before and after irradiation with light at 365 nm (1.5 mW/cm2)
and full arc (10.2 mW/cm2) for 30 min in the presence and
absence of ITX. All spectra were recorded on a Varian 300 MHz
NMR for samples in CDCl3 with tetramethysilane (TMS) as the
internal reference.
Dynamic thermal mechanical properties of sequentially pre-
pared thiol-isocyanate-ene samples were measured using a

Article
Chem. Mater., Vol. 22, No. 8, 2010 2619
Table 1. Second Order Rate Constant (k2) of Amine Catalyzed Thiourethane Reactions with Mono-Functional Thiols and Isocyanates in Benzene
isocyanate
thiol (pKa)
hexanethiol (10.53)
catalyst (pKa)
DBU (11.6)
DBN (13.5)
DABCO (8.2)
DMAP (9.2)
TEA (10.75)
k2 ꢀ 102 (L mol-1 s-1
)
3
3
hexyl isocyanate
10700a
8300a
3.0b
0.6b
0.7b
TBA (10.7)
proton sponge (12.1)
0.3b
slowb
hexyl isocyanate
hexanethiol (10.53)
TEA (10.75)
3.0c
21.7c
67.8c
2097c
butyl 3-mercaptopropionate (9.33)
butyl thioglycolate (7.91)
benzenethiol (6.43)
hexyl isocyanate
cyclohexyl isocyanate
phenyl isocyanate
benzenethiol (6.43)
DABCO (8.2)
4.7d
4.7d
212d
a [SH] = [NCO] = 570 mM, [Catalyst] = 0.24 mM. b [SH] = [NCO] = 730 mM, [Catalyst] = 80 mM. c [SH] = [NCO] = 730 mM, [TEA] = 2.3 mM.
d [SH] = [NCO] = 570 mM, [DABCO] = 0.12 mM.
DMTA (MK VI, Rheometrics). Measurements were conducted
using the vertical tension mode on samples with dimensions of
10 ꢀ 8.0 ꢀ 0.2 mm (L ꢀ W ꢀ T) from -50 to 150 °C at a 5 °C/min
heating rate and 1 Hz frequency with 0.05% strain.
Tensile property measurements were conducted with a me-
chanical testing machine (MTS, Alliance RT/10) according to
ASTM D882 using a 100 N load cell with a specimen gauge
length of 20 mm at a crosshead speed of 100 mm/min.
Refractive index was obtained using a Bausch&Lomb ABBE-
3 L refractometer at 24 °C. 1-Bromonaphthalene was applied
between the sample film and the prism shield.
the nitrogen and the steric hindrance.12,21-23 The kinetic
profiles of the thiol-isocyanate reactions of a monofunc-
tional thiol (hexanethiol) and an aliphatic isocyanate
(hexyl isocyanate) in dilute acetonitrile in the presence
of different types of tertiary amines were obtained by real-
time IR (see Supporting Information Figure S1 for kinetic
plots), and the second order rate constants are summar-
ized in Table 1. Note that the rate constants at the lower
catalyst concentrations (Supporting Information Figure
S1a) could only be given for DBU and DBN since the
ratesfor the other catalysts were too slow to measure by real
time IR. The rate constants for the 1,5-diazabicyclo[4.3.0]-
5-nonene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) catalyzed model system are significantly greater
than for 1,4-diazabicyclo[2.2.2]octane (DABCO), 4-di-
methylaminopyridine (DMAP), triethylamine (TEA), tri-
butylamine (TBA), and 1,8-bis(dimethylamino)naphtha-
lene (proton sponge). As stated above, this outcome is
due to the higher basicity (see Table 1) of DBN and DBU as
compared to DABCO, DMAP, TEA, TBA, and the proton
sponge as determined by the pKa values of the conjugate
acids of each amine listed in Table 1.24-26 Though a strong
base catalyst, the proton sponge is effectively a noncatalyst
for the thiol-isocyanate coupling reaction due to the
significantly crowded environment of nitrogen atoms so
that it is very difficult for electron lone pairs to take part in
the thiol-isocyanate nucleophilic addition reactions.
It is well-known that the addition of thiols to electron
deficient carbons depends on the acid dissociation con-
stant (pKa) of the thiol because this nucleophilic addition
reaction proceeds through the thiolate anion chain pro-
cess as shown in Scheme 2.12,27-30 In Table 1 the rate
Pencil and Persoz pendulum hardness (ASTM D-4366 using a
BYK-Gardner pendulum hardness tester with a square frame
pendulum) were measured. The results reported are the average
of 10 tests.
Results and Discussion
Model Kinetics for Tertiary Amine Catalyzed
Thiol-Isocyanate Coupling Reactions. To enhance under-
standing of the thermally and photolytically induced
tertiary amine catalyzed thiol-isocyanate polymeriza-
tion reactions, extensive evaluation of reaction kinetics
in model compounds was performed, and the results were
evaluated in terms of the structures of the tertiary amine
catalysts, thiols, and isocyanates. It is well-known that the
reactions between hydrogen-containing compounds such
as alcohols, thiols, and primary (or secondary) amines
and isocyanates occur via a nucleophilic addition process
such that the reactivity is generally proportional to the
nucleophilicity of -OH, -SH, and -NH2 and the elec-
tron deficiency of the isocyanate carbon.12,21
The catalytic activity of tertiary amines results from the
coordination of the nitrogen electron lone pair with both
active hydrogen containing compounds and isocyanates,
with the result being an increase in the nucleophilicity of
hydrogen donors and a more electron susceptible carbon,
respectively. The catalytic efficiency of tertiary amines is
dependent upon the availability of the electron pair for
complex formation as determined by both the basicity of
(23) Szycher, M. Szycher’s Handbook of Polyurethanes; CRC Press LLC:
Boca Raton, FL, 1999; Chapter 3.
(24) Li, J.-N.; Fu, Y.; Liu, L.; Guo, Q.-X. Tetrahedron 2006, 62, 11801.
€
€
€
(25) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.;
Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019.
(26) Serjeant, E. P.; Dempsey, B. Ionisation Constants of Organic Acids
in Aqueous Solution; Pergamon Press: New York, 1979.
(27) Bernasconi, C. F.; Killion, R. B. J. Am. Chem. Soc. 1988, 110, 7506.
(28) Bordwell, F. G; Hughes, D. L. J. Org. Chem. 1982, 47, 3224.
(29) Kreevoy, M. M.; Harper, E. T.; Duvall, R. E.; Wilgus, H. S., III;
Ditsch, L. T. J. Am. Chem. Soc. 1960, 82, 4899.
(21) Randall, D.; Lee, S. The polyurethanes book; John Wiley & Sons
Ltd.: The United Kingdom, 2002.
(22) Hepburn, C. Polyurethane Elastomers; Elsevier: Essex, 1992.
(30) Hupe, D. J.; Jencks, W. P. J. Am. Chem. Soc. 1977, 99, 451.

2620 Chem. Mater., Vol. 22, No. 8, 2010
Shin et al.
constants for the reaction of hexyl isocyanate with four
different thiols in the presence of TEA (2.3 mM) are
shown (see Supporting Information Figure S2 for kinetic
plots). The rate constants increase as the pKa of the thiol
decreases due to the increase in the formation rate of the
corresponding thiolate anion, which is a strong nucleo-
phile.
As discussed above, the reactivity of isocyanates with
active hydrogen-containing compounds is affected by the
electron deficiency of the carbon atom in isocyanates so
that the substituents that stabilize positive character of
the carbon atom enhance the isocyanate reactivity with
the corresponding thiolate anion. Table 1 (see Supporting
Information Figure S3 for kinetic plots) shows the
isocyanate structural effect on the thiol-isocyanate cou-
pling reaction. The reaction of the aromatic isocyanate
(phenyl isocyanate) with benzenethiol is significantly
faster than the reaction between the aliphatic isocyanates
(hexyl and cyclohexyl isocyanate) and benzenethiol due
to carbocation formation in the resonance structures of
the aromatic isocyanate.21
Having established the relationship between structure
and reactivity for the basic components (thiol, isocyanate,
and catalyst) in thiol-isocyanate nucleophilic coupling
reactions, results for the polymerization of a ternary
mixture of thiol, isocyanate, and ene are evaluated. To
demonstrate the efficiency of this ternary approach in
Figure 1. Kineticprofilesfor(a)thiol-ene(SH/CdC= 100:100;DMPA
1 wt %, intensity of high pressure mercury lamp: 20.5 mW/cm2) and (b)
thiol-isocyanate networks (SH/NCO = 100:100; TEA 0.1 wt %).
Figure 2. Kinetic profiles of thiol-isocyanate-ene ternary networks formed by the sequential curing process; SH/CdC/NCO = (a) 100:80:20, (b)
100:60:40, (c) 100:40:60, and (d) 100:20:80 (DMPA 1 wt %; TEA 0.1 wt %; intensity of high pressure mercury lamp: 20.5 mW/cm2).

Article
Chem. Mater., Vol. 22, No. 8, 2010 2621
Figure 3. 1H NMR spectra (CHCl3-d) of (a) tributylamine (TBA) and
TBA HBPh4 (b) before, (c) after irradiation at365nm(1.5 mW/cm2), and
3
(d) full arc (10.2 mW/cm2) in CHCl3-d solution (5 wt %).
Figure 5. 1H NMR spectra (CHCl3-d ) of (a) tributylamine (TBA) and
TBA HBPh4 with ITX ((b) before and (c) after irradiation at 365 nm (1.5
3
mW/cm2) in CHCl3-d solution (5 wt %; ITX/TBA HBPh4 = 0.36:0.61)).
3
polymerization indicate that each reaction is independent
of the other and can be controlled separately through the
initial dark period, through light intensity, and through
the catalyst and photoinitiator concentration. In
Figure 2a-d, kinetic profiles are presented for thiol-
isocyanate-ene ternary mixtures in which the sequential
dual curing process is performed on formulations with
systematic variations of the relative thiol-ene to
thiol-isocyanate ratio. The concentrations of TEA and
DMPA were fixed at 0.1 and 1.0 wt % based on the total
amount of the mixture. GDMP-N3600 coupling reac-
tions forming thiourethane networks were allowed to
proceed for 20 min in the dark followed by irradiation
with UV light to trigger photopolymerization of GDMP
and TATAT. N3600 conversions reached 90-91% in 20
min during selective TEA catalyzed thiol-isocyanate
coupling reactions, and the corresponding GDMP con-
versions were 19, 36, 54, and 73% (∼90% of theoretical
conversion of thiols based on stoichiometry with NCO) as
the N3600 molar ratio was increased (20, 40, 60, and
80%). TATAT and the remaining GDMP were reacted
by photopolymerization,and nearly quantitative conver-
sions were obtained in just a few seconds. It should be
noted that, for all compositions, the final conversions of
GDMP, N3600, and TATAT obtained by RTIR were not
100% due to molecular mobility restrictions at higher
Figure 4. UV absorption spectra of ITX in acetonitrile (concentration:
5.8 ꢀ 10-5 M).
making networks highlighted by the presence of extensive
thiourethane groups, we selected a dithiol based on the
diester of mercaptopropionic acid and ethylene glycol, a
triisocyanate which is the trimer (isocyanurate) of 1,6-
hexane diisocyanate, and a corresponding triallyl triene
with the same isocyanurate cyclic ring structure as the
triisocyanate.
Kinetics of Thiol-Isocyanate-Ene Ternary Systems.
The kinetics of thiol-isocyanate-ene polymerizations
were investigated by systematically varying the reactive
composition and the reaction sequence. Figures 1 and 2
present the resultant functional group conversions as a
function of reaction time for the separate and simulta-
neous thiol-isocyanate-ene ternary networks. In
Figure 1a,b, results are presented for bulk polymeriza-
tions of each type. Here, it is seen that each reaction is
relatively fast and essentially quantitative; the GDMP-
TATAT photopolymerization occurs in a few seconds
while the GDMP-N3600 coupling reaction takes ap-
proximately 20-30 min. These results on the distinct

2622 Chem. Mater., Vol. 22, No. 8, 2010
Shin et al.
Figure 6. Kinetic profiles of thiol-isocyanate-ene ternary networks formed by the simultaneous curing process; SH/CdC/NCO = (a) 100:100:0, (b)
100:80:20, (c) 100:60:40, (d) 100:40:60, (e) 100:20:80, and (f) 100:0:100 (ITX 0.36 wt %; TBA HBPh4 0.61 wt %; intensity of high pressure mercury lamp:
1.41 mW/cm2 through 365-nm filter).
3
Table 2. Glass Transition Temperatures of Thiol-Isocyanate-Ene
Ternary Networks through the Sequential and Simultaneous Dual Cure
Systems Obtained by DSC
conversions. As shown in the model studies, the
thiol-isocyanate reaction rate is controlled by both
catalyst type and concentration while the thiol-ene
photopolymerization can be initiated at any desired
intensity and at any time prior to, during, or after the
thiol-isocyanate coupling reactions. Consequently, the
sequence of the thiol-isocyanate and thiol-ene reactions
involving different reaction mechanisms is readily pro-
grammable based on whatever property requirements
exist for the final polymers.
SH/CdC/NCO (molar
ratio)
TEA þ DMPA
(°C)
TBA HBPh4 þ ITX
3
(°C)
100:100:0
100:80:20
100:60:40
100:40:60
100:20:80
100:0:100
-6
4
10
16
30
36
-5
3
8
16
28
35
nm gives TBA and other byproducts.20 Accordingly, in
Figure 3 are shown the NMR spectra of tributylamine
and TBA HBPh4 before and after irradiation with UV
For simultaneous thiol-isocyanate-ene ternary net-
work formation, both the thiol-isocyanate coupling and
the thiol-ene free-radical reactions are triggered by UV
light at 365 nm using the photogenerated amine, tributyl-
amine tetraphenylborate salt, (TBA HBPh4, 0.61 wt %),
3
light at 365 nm and broad spectrum of UV that contains
wavelengths below 300 nm. The generation of TBA from
TBA HBPh4 only occurs when TBA HBPh4 is photo-
3
3
and the photosensitizer, isopropylthioxanthone (ITX, 0.36
wt %), which generates both free-radicals and tributyl-
amine catalyst upon photolytic decomposition.
3
3
lyzed with the full arc of the unfiltered UV light. It is also
well-known that ITX efficiently photosensitizes upon UV
irradiation through photoexcitation and subsequent en-
ergy transfer to other molecules. The sensitization poten-
tial of ITX is highly dependent on the wavelength of the
First, considering photolysis of only TBA HBPh4 in
solution, TBA HBPh4 absorbs UV light below 300 nm
and upon direct photolysis at wavelengths less than 300
3
3

Article
Chem. Mater., Vol. 22, No. 8, 2010 2623
Figure 7. DSC thermograms of thiol-isocyanate-ene hybrid networks
formed by the sequential curing process; SH/CdC/NCO = (a) 100:100:0,
(b) 100:80:20, (c) 100:60:40, (d) 100:40:60, (e) 100:80:20, and (f) 100:0:100
(DMPA 1 wt %; TEA 0.1 wt %; precure at room temperature for 1 h;
intensity of medium pressure mercury lamp 10.2 mW/cm2; irradiation
time, 5 min; postcure at 80 °C for 24 h).
irradiating UV light; that is, the higher extinction coeffi-
cient, the greater the amount of light absorbed as shown
in Figure 4. Upon irradiation at 365 nm, ITX abstracts
hydrogen from thiols (hydrogen donor) to produce thiyl
radicals, thus initiating the radical thiol-ene photopo-
lymerization. If ITX and TBA HBPh4 are both present
3
and irradiated at wavelengths greater than 300 nm, the
excited triplet state of ITX will also transfer energy to
TBA HBPh4 which subsequently decomposes to give
TBA, which catalyzes the thiol-isocyanate reaction and
other byproducts. In Figure 5, the appearance of TBA is
indeed confirmed by NMR with peaks at 2.6 ppm occur-
3
Figure 8. Dynamic mechanical behavior of thiol-isocyanate-ene tern-
ary networks formed by the sequential curing process; SH/CdC/NCO =
(a) 100:100:0, (b) 100:80:20, (c) 100:60:40, (d) 100:40:60, (e) 100:80:20, and
(f) 100:0:100 (DMPA 1 wt %; TEA 0.1 wt %; precure at room tempera-
ture for 1 h; intensity of medium pressure mercury lamp 10.2 mW/cm2;
irradiation time, 5 min; postcure at 80 °C for 24 h).
ring following photolysis at 365 nm of a TBA HBPh4
solution containing ITX.
3
Ultimately, this combination results in the thiol-
isocyanate coupling reaction and thiol-ene free-radical
photopolymerization being triggered simultaneously when
ITX is used. Consequently, both the thiourethane and
thiol-ene reactions will be initiated by ITX. In Figure 6,
kinetic profiles of simultaneous thiol-isocyanate-ene tern-
ary networks are presented as a function of the SH/CdC/
NCO ratio during UV irradiation at 365 nm in the presence
borate or by changing the relative amounts of the ITX and
the TBA HBPh4. Of course, the combined rate of both
reactions is easily controlled simply by varying the amount
of ITX.
3
Thermal Properties. DSC measurements were con-
ducted to investigate the effect of the compositional ratio
and reaction sequence of the thiourethane/thiol-ene
hybrid networks on thermal properties. DSC thermo-
grams of thiourethane/thiol-ene hybrid networks pre-
pared by both sequential and simultaneous curing
systems were almost identical. Glass transition tempera-
tures obtained by DSC are summarized in Table 2, and a
representative second heating scan of the thiourethane/
thiol-ene hybrid networks prepared by sequential curing
system is shown in Figure 7. With an increase in the
N3600 concentration, Tg progressively increases due to
the higher extent of hydrogen bonding resulting from the
increase in the thiourethane linkages in the thiol-
isocyanate-ene ternary networks. In addition, as shown
in Figure 7, the glass transition range (Tg,e-Tg,i) is very
narrow (∼5 °C) for all compositions. It is clear that thiol-
isocyanate-ene ternary networks are highly uniform as
of TBA HBPh4 and ITX. Both the thiol-isocyanate cou-
3
pling reaction and thiol-ene photopolymerization are
successfully initiated, and the conversion of GDMP, TA-
TAT, and N3600 is nearly quantitative based on the stoichio-
metry; that is, ConversionSH ∼ ConversionCdC þ Con-
versionNCO. However, it should be noted that the lower
overall conversion of all components by the simultaneous
process is due to the lower light intensity at 365 nm (1.41
mW/cm2) as compared to the full arc (20.5 mW/cm2) for the
sequential process. In addition, under these conditions, the
thiol-ene photopolymerization proceeds more rapidly than
the thiol-isocyanate coupling reaction. The different reac-
tion rates can be synchronized either by changing the type of
tertiary amine used to make the salt with the tetraphenyl

2624 Chem. Mater., Vol. 22, No. 8, 2010
Shin et al.
Table 3. Glass Transition Temperatures and Rubbery Modulus (E0 at 80 °C) of Thiol-Isocyanate-Ene Ternary Networks through the Sequential and
Simultaneous Dual Cure Systems Obtained by DMTA
TEA þ DMPA
E0 (MPa)b
TBA HBPh4 þ ITX
3
SH/CdC/NCO (molar ratio)
Tg (°C)a
fwhm (°C)
Tg (°C)a
E0 (MPa)b
fwhm (°C)
100:100:0
100:80:20
100:60:40
100:40:60
100:20:80
100:0:100
11
23
32
41
50
56
5.1
7.4
8.4
9.5
9.6
13.5
12
12
12
11
11
10
11
20
33
38
52
54
4.9
7.6
8.1
9.3
10.2
12.1
12
13
12
12
12
11
a The peak temperature of tan δ. b The storage modulus (E0) at 80 °C.
Table 4. Mechanical Properties and Refractive Index of
Thiol-Isocyanate-Ene Ternary Networks by the Sequential Dual Cure
System
SH/CdC/NCO
(molar ratio)
pendulum pencil
hardness hardness
refractive index (sulfur
content, wt %)
100:100:0
100:80:20
100:60:40
100:40:60
100:20:80
100:0:100
30
23
16
18
82
220
3B
HB
H
2H
4H
8H
1.5535 (15.8)
1.5510 (14.4)
1.5490 (13.2)
1.5475 (12.2)
1.5460 (11.3)
1.5440 (10.6)
found for typical thiol-ene based systems.1,31 Also, the
sequence of the thiol-isocyanate and thiol-ene reactions
does not affect the uniformity or thermal transition of the
network structure since both reactions are essentially
independent of each other with no side products, resulting
in homogeneous chemical structures as long as the con-
ditions used ensure quantitative conversions for all com-
ponents. This outcome would not be expected for highly
glassy systems polymerized well below their Tg where
diffusion limitations would limit the extent of reaction for
the slower or last reaction.
Figure 9. Tensile properties of thiourethane/thiol-ene hybrid networks
bysequentialdualcure;SH/CdC/NCO=(a)100:100:0, (b) 100:80:20, (c)
100:60:40, (d) 100:40:60, (e) 100:80:20, and (f) 100:0:100 (DMPA 1 wt %;
TEA 0.1 wt %; precure at room temperature for 1 h; intensity of medium
pressure mercury lamp 10.2 mW/cm2; irradiation time, 5 min; postcure at
80 °C for 24 h).
Thermal Mechanical Properties. In addition to thermal
properties, dynamic mechanical properties of thiour-
ethane/thiol-ene hybrid networks are largely dictated
by the compositional ratio. In Figure 8, the storage
modulus and tan δ of thiol-isocyanate-ene ternary net-
works are shown. The Tg determined by tan δ peak (see
Table 3) increases as a function of the N3600 content due
to increased hydrogen bonding consistent with DSC
results. The rubbery plateau modulus (Table 3) also
increases monotonically with increasing thiourethane
content indicating that hydrogen bonding acts to physi-
cally cross-link the thiol-isocyanate-ene ternary net-
works so that the apparent network cross-linking
density increases. The broadness of the glass transition
determined by fwhm (full width at half-maximum) of the
tan δ plots is about 10 °C for all compositions (Table 3)
implying that the distribution of relaxation times of
thiol-isocyanate-ene ternary networks is narrow due
to uniformity of the chemical structure. Consequently,
the Tg of the thiol-isocyanate-ene ternary networks can
be tuned simply by varying the composition over a wide
temperature range without sacrificing the characteristic
uniform network structure typically associated with
thiol-ene based materials.
Mechanical Properties and Refractive Index. Tensile
properties, pencil hardness, and pendulum hardness of
thiourethane/thiol-ene hybrid networks prepared by the
sequential curing process are shown in Figure 9 and
Table 4. It is well-known that the mechanical properties
of thiol-ene based materials may be enhanced by using
(thio)urethane modified thiols or enes for the preparation
of thiol-ene networks to improve hardness and increase
Tg.18,19 However, (thio)urethane linkages have strong
hydrogen bonding and, if present in the thiol or ene
monomers, increase the viscosity of the unpolymerized
system. For thiol-isocyanate-ene ternary networks
cured by both the sequential and the simultaneous curing
methods, thiourethane linkages are formed during the
curing process, thus eliminating the initial viscosity issues
and promoting high functional group conversion. As seen
in Figure 9, the Young’s modulus and the stress at break
are significantly improved by increasing the thiourethane
content due to the large extent of hydrogen bonding in the
hybrid networks, consistent with the DMA results
(storage modulus at room temperature) in Figure 8 and
Table 3. The elongation at break also increases from 50%
for the thiol-ene neat network (Figure 9a) to 230% for
(31) Cramer, N. B.; Scott, J. P.; Bowman, C. N. Macromolecules 2002,
35, 5361.

Article
Chem. Mater., Vol. 22, No. 8, 2010 2625
the thiourethane-thiolene 60:40 hybrid network
(Figure 9d). Surface hardness was also measured using
pencil hardness. As shown in Table 4, the pencil hardness
increases with increases in the thiourethane content. The
pendulum hardness, which simply measures energy
damping, has a minimum for the thiourethane/thiol-ene
at 40:60 composition, which has a glass transition tem-
perature near ambient temperature. Finally, the refrac-
tive index is one of the most important physical properties
of thiol based materials. It has been reported that the
refractive index of thiol-ene, thiol-yne, and thiour-
ethane systems is largely determined by the sulfur content
although other factors related to additional components
in the networks are also important.4,32 The refractive
indexes of the thiol-isocyanate-ene ternary networks
shown in Table 4 are all relatively high and increase
slightly with increasing sulfur content.
thiol-ene reactions are quantitative. Thermal properties
of the hybrid network films prepared by both methods
were identical regardless of the sequence. Consequently,
thiourethane/thiol-ene hybrid systems can form the uni-
form and densely cross-linked network structure that is
the characteristic of thiol click type reaction based mate-
rials. Very narrow glass transition temperature ranges
were observed for all compositions as measured by DSC
and DMA. Compositional variation of the ratio between
polythiourethanes and thiol-ene networks enabled us to
control the mechanical properties such as tensile, pencil,
and pendulum hardness. Refractive index was more
dependent on the sulfur content than the amount of
hydrogen bonding in the hybrid networks.
The thiourethane/thiol-ene hybrid polymerization
strategy offers many advantages in tailoring physical/
mechanical properties and in the manufacturing process
for materials based on the thiol click reactions. Since
thiols undergo various chemical reactions with corre-
sponding reactants and initiators (or catalysts) associated
with different mechanisms such as free-radical and anio-
nic step reactions, it is possible to envision that many
different combinations of thiol click type reactions will be
appropriately used to form hybrid networks which meet
varied requirements in chemistry, materials processing,
and physical/mechanical properties.
Conclusions
In this study, thiol-isocyanate coupling and thiol-ene
free radical step reactions, which are independent of each
other, were sequentially and simultaneously employed to
form thiourethane/thiol-ene hybrid networks with tun-
able network properties. A conventional base catalyst,
TEA, and photoinitiator, DMPA, were used for sequen-
tial curing of thiourethane/thiol-ene hybrid networks.
For simultaneous systems a photogenerated amine cata-
Acknowledgment. We dedicate this article to the late Dr.
Charles E. Hoyle who passed during the preparation of the
publication. This work is a small part of his significant
scientific contributions to thiol click reactions. His endea-
vors and enthusiasm will long be remembered.
We also acknowledge Perstorp, Bruno Bock Thio-Che-
micals-S, Bayer MaterialScience, Ciba Specialty Chemicals,
and Albemarle for supplying chemicals and Fusion UV
Systems for the light source.
lyst (TBA HBPh4) was used in the presence of ITX to
3
trigger the thiol-isocyanate coupling and the thiol-ene
free radical reaction simultaneously by UV light. TBA
was successfully generated by irradiating UV light at
365 nm due to the sensitization by ITX and efficiently
catalyzed thiol-isocyanate coupling reaction. The
thiol-ene free radical reaction was also initiated by ITX
at 365 nm without any additional photoinitiator. Kinetic
profiles clearly showed that both thiol-isocyanate and
Supporting Information Available: Kinetic profiles (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(32) Li, Q.; Zhou, H.; Wicks, D. A.; Hoyle, C. E.; Magers, D. H.;
McAlexander, H. R. Macromolecules 2009, 42, 1824.