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.