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CommuniCation
1900047 (1 of 5) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.mrc-journal.de
Near-Infrared-Induced Cationic Polymerization Initiated by
Using Upconverting Nanoparticles and Titanocene
Zhiquan Li, Junzhe Zhu, Xin Guan, Ren Liu,* and Yusuf Yagci*
Dr. Z. Li, Prof. R. Liu
International Research Center for Photoresponsive Molecules and
Materials
Jiangnan University
Wuxi, Jiangsu 214122, China
E-mail: liuren@jiangnan.edu.cn
Dr. Z. Li, Prof. R. Liu
Key Laboratory of Food Colloids and Biotechnology
School of Chemical & Materials Engineering
Jiangnan University
Wuxi, Jiangsu 214122, China
J. Zhu, X. Guan, Prof. Y. Yagci
School of Chemical & Materials Engineering
Jiangnan University
Wuxi, Jiangsu 214122, China
E-mail: yusuf@itu.edu.tr
Prof. Y. Yagci
Istanbul Technical University
Department of Chemistry
Maslak, Istanbul 34469, Turkey
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/marc.201900047.
DOI: 10.1002/marc.201900047
Although most of the technologically
applied photochemical processes are
based on free radical polymerization, after
the discovery of onium salt photochem-
istry by Crivello,[11] photoinitiated cationic
polymerization has gained interest due to
their thermal stability, solubility in most of
the cationically polymerizable monomers,
insensitivity to oxygen and high quantum
efficiency to generate reactive species.[3]
The existing onium salt–based photoiniti-
ating systems for cationic polymerization
include iodonium,[11] sulfonium,[12] alkoxy
pyridinium,[13] and phosphonium[14] salts.
However, unless additional chromophores
are incorporated[15] into the salt structure the absorption of these
salts at above 300 nm is rather low. In order to match with the
wavelength requirements for certain practical applications, the
sensitivity range of onium salts was extended to higher wave-
lengths by using free radical photoinitiators,[16] charge transfer
complexes,[17] and singlet and triplet photosensitizers.[18] It was
also shown that nanoparticles[19] and highly conjugated mole-
cules[20] can also sensitize cationic polymerization by photoin-
duced electron transfer reactions. Among all these strategies,
so-called free radical promoted cationic polymerization outlined
in Scheme 1 is most widely used methodology as many free
radical photointiators acting at broad wavelength range are
readily available.
In this process, photochemically formed electron donor radi-
cals are oxidized by onium salts. The cations thus generated are
used as initiating species for cationic polymerizations. As a con-
sequence of the redox reaction, aryl radicals are also generated
that can undergo various subsequent reactions resulting in the
chemical amplification of the photons absorbed by the system.
Free radical photoinitiators including aromatic carbonyl com-
pounds,[21] acyl phosphine oxides,[22] acyl germanes,[23] organo-
tellurium,[24] manganese carbonyl compounds,[25] and silanes[26]
were successfully used to promote cationic polymerizations
Despite the fact that the above-mentioned indirect sys-
tems can successfully initiate cationic polymerization at the
near UV and visible range,[3] it is highly desirable to extend
the spectral sensitivity to lower energy irradiation, such as
nonharmful near-infrared (NIR) region which also limits side
reactions resulting in self-initiation of monomers and degra-
dation of already-formed polymers and oligomers. However,
until recently, reports describing NIR sensitized photopoly-
merizations are exceedingly scarce. Several examples reported
in the literature are mostly based on cyanine dyes and poly-
methines.[27] Besides their use in conventional photopolymeri-
zation formulations, polymethines with zwitter ionic structure
Photopolymerization
A new near-infrared (NIR)-sensitized photoinitiating system for free-radical-
promoted cationic polymerization of oxirane and vinyl monomers such as
cyclohexene oxide, and n-butyl vinyl ether (BVE), and N-vinyl carbazole (NVC)
is described. A three-component photoinitiating system consists of upcon-
verting nanoparticles (UCNPs), titanium-complex free radical photoinitiator
(Irgacure 784, titanocene), and diphenyl iodonium hexafluorophosphate
(Ph2I+PF6−). Upon NIR laser irradiation at 980 nm, the radicals generated
from titanocene by the visible light emitted by UCNP abstract hydrogen or
add to the monomer, forming electron donor radicals that can be oxidized by
iodonium salt to initiate cationic polymerization.
Photoinitiated polymerizations[1] have led to significant
advances in various technologically important fields including
coatings, inks, adhesives, varnishes, microelectronics, 3D
printing, and microlithography. In these applications, polymers
were synthesized by various photochemical reactions involving
free radical,[2] cationic,[3] anionic,[4] and step-growth[5] polymeri-
zations as well as copper catalyzed click processes,[6] which pro-
vides formation of films with different physical and chemical
properties. Recently, light-induced reactions have also become
important methodology for controlled/living radical polymeri-
zations,[7] nanocomposites,[8] and hyperbranched polymers,[9]
which were useful materials for drug delivery applications.[10]
Macromol. Rapid Commun. 2019, 1900047
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were also shown to sensitize atom transfer radical polymeriza-
tion (ATRP) with the ppm level of copper catalyst.[28] The other
controlled/living radical polymerization, radical addition frag-
mentation transfer (RAFT) polymerization can also be initi-
ated by NIR sensitization.[29] Lanthanide-doped upconverting
nanoparticles (UCNP)s convert NIR light into ultraviolet (UV)
or visible light with high photoluminescence efficiency, sharp
emission and absorption spectra, and long lifetimes.[30] It was
shown that free radical photopolymerization can be initiated
through generation of visible or UV light by the emission of
the NIR-excited UCNPs. The initiating radicals are then formed
from the appropriately selected photoinitiators with matching
absorption characteristics present in the system.[31] In a recent
paper from one of the authors’ laboratory,[32] it was shown that
UCNP films can induce both free radical and free radical pro-
moted cationic polymerizations by NIR irradiation.
The commercially available photoinitiator, Irgacure 784
(bis(
η
5-cyclopentadienyl)bis[2,6-difluoro-3-(1-H-pyrrol-1-yl)
phenyl]titanium, titanocene) which possesses strong absorp-
tion bands at 405 and 480 nm is used in various applications
involving visible light-induced free radical polymerization.[33]
Recently, we reported[34] a facile method to realize deep pho-
topolymerization using UCNP and titanocene by taking advan-
tage of deeper penetration ability of NIR light.
This paper outlines a new NIR photoinitiating system for
cationic polymerization that utilizes UCNP. Thus, herein-pre-
sented methodology for free radical promoted cationic polym-
erization entails the added advantage of insource lightning and
applicability to oxirane and vinyl monomers.
We establish our NIR sensitized cationic polymerization
concept by using UCNP, titanocene, and diphenyl iodonium
hexafluorophosphate (Ph2I+PF6−) as emitter, visible light pho-
toinitiator, and oxidant, respectively. The first
critical issue concerns appropriate matching
of the emission and absorption character-
istics of the components. As can be seen
from Figure 1, the visible light emitted from
the NIR excited UCNP perfectly match with
the absorption of titanocene photoinitiator.
Ph2I+PF6− can act as oxidant in the ground
state as it is transparent at both NIR and vis-
ible region.
The ternary photoinitiating system effi-
ciently initiates the cationic polymeriza-
tion of representative monomers, namely
cyclohexeneoxide (CHO), n-butyl vinyl ether
(BVE), and N-vinyl carbazole (NVC). Typical
results are shown in Table 1.
It is notable that polymerizations do not
proceed in the absence of the either mol-
ecules; the three components of the ini-
tiating systems are indispensable for the
polymerization to occur under the identical
reaction conditions. Upon NIR excitation,
visible light emitted from UCNP is absorbed
by titanocene to generate free radicals
(Scheme 2). Initially, we considered these
species are responsible for the generation
of the initiating cations. However, they do
not undergo efficient oxidation as found for
diaryliodonium or N-alkoxypyridinium salts
by a direct redox process.[35] It is quite likely
that these species abstract hydrogen from
CHO monomer to form oxidizable radicals.
The rate constant for the hydrogen abstrac-
tion from CHO by triplet excited states was
found[18a] to be kabs = 3.6 × 104 mol−1 L s−1.
Monomer derived carbon centered radicals
are then oxidized to the cations to initiate
Macromol. Rapid Commun. 2019, 1900047
Scheme 1. General mechanism for free-radical-promoted photoinitiated
cationic polymerization.
Figure 1. Emission spectrum of UCNP (cyan); absorption spectra of titanocene (pink), and
Ph2I+PF6− (purple).
Table 1. NIR-photoinitiated cationic polymerization of various monomers by using UCNP,
titanocene, and Ph2I+PF6− in CH2Cl2 at room temperature.
Monomer Titanocene [mol
L−1]
Ph2I+PF6
[mol L−1]
MeO-Bz
[mol L−1]
Conversion
[%]
Mn x 10−3
[g mol−1]
CHO — 5 × 10−3—0—
CHO 5 × 10−3— — 0 —
CHO 5 × 10−35 × 10−3— 66.9 16.0
CHO 5 × 10−35 × 10−35 × 10−359.6 13.3
BVE 5 × 10−35 × 10−3— 64.7 14.1
NVC 5 × 10−35 × 10−3— 94.2 31.4
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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the cationic polymerization. In the presence of methoxybenzyl
alcohol (MeO-BzOH), a different situation was encountered
and these additive appear to be an appropriate promoter. Sim-
ilar behavior was observed by Crivello[36] in the conventional
visible light initiating sysem. Initially formed radicals abstract
hydrogen from the MeO-BzOH to form benzylic radical which
can be oxidized. Thus formed
α
-hydroxybenzyl cations release
protons capable of initiating cationic polymerization by spon-
taneous deprotonation. Proposed mechanisms both systems
were also proved for by H-NMR analysis. As can be seen from
Figure S2, Supporting Information, the spectra of the polymers
do not exhibit any aromatic signals since aromatic radicals
formed from the initiator or MeO-BzOH are not directly oxi-
dized to the initiating cations.
In Figure 2, FTIR epoxy conversion of CHO is plotted versus
time. Evidently, photopolymerizations with MeO-BnzOH are
more efficient in accordance with Crivello's findings. The pla-
teau value corresponds to the time when the components of the
initiating system are consumed.
With vinyl monomers, BVE and NVC a different mechanism
is operative. In accordance with the high reactivity of the radi-
calic species generated from titanocene to the double bonds,[34]
the adduct radicals responsible for the reduction of the diaryli-
odonium salt to initiate cationic polymerization as described in
Equation (9) in Scheme 2.
Notably, in the case of NVC, partially cross-linked poly-
mers are obtained as a consequence of the participation of
side chain carbazole groups in radical coupling reactions[37]
Macromol. Rapid Commun. 2019, 1900047
Scheme 2. NIR-photoinduced free radical promoted cationic polymerization of epoxy and vinyl monomers by using UCNP, titanocene, and Ph2I+PF6− in
the absence and presence of MeO-BzOH.
Figure 2. The conversion of epoxy group versus the irradiation time as
monitored by ATR-FTIR. [UCNP] = 100 g L−1, [titanocene] = 5 × 10−3 mol
L−1, and [Ph2I+PF6−] = 5 × 10−3 mol L−1,
λ
irr. = 980 nm.
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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through electron transfer reactions followed by proton release.
The enhanced conjugation as a result of coupling can be visual-
ized by the dark blue coloring of the polymers (Figure S3, Sup-
porting Information).
In conclusion, the system consisting of UCNP, titanocene,
and Ph2I+PF6− is found to be highly suitable for NIR-photoini-
tiated cationic polymerizations. The initiation efficiency was
demonstrated for various monomers. Depending upon the
nature of monomers and the free radicals formed, two different
mechanisms are feasible. Electron donor radicals formed from
the monomer by hydrogen abstraction or addition are oxidized
forming cations capable of initiating cationic polymerizations.
Thanks to the nonharmful NIR irradiation condition, which
will enable the described approach to open a new way to com-
bine both radical and cationic hybrid systems and have a poten-
tial for various industrial applications.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors acknowledge the financial support by the National
Nature Science Foundation of China (51673086 and 21404048), the
Fundamental Research Funds for the Central Universities (JUSRP
51719A), and National First-class Discipline Program of Food Science
and Technology (JUFSTR20180301). Y.Y. is indebted to the Local Minister
of Jiangsu Province, China for providing a visiting scientist grant through
the International Collaboration program (MOE & SAFEA, 111 Project
(B13025))
Conflict of Interest
The authors declare no conflict of interest.
Keywords
cationic polymerization, near infrared, photopolymerization, titanocene,
upconverting nanoparticles
Received: January 25, 2019
Revised: February 20, 2019
Published online:
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