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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Fachliteratur
- Sprache: Englisch
Dieses in seiner Ausführlichkeit einzigartige Referenzwerk stellt Photoinitiatoren (PI) und Photoiniatorsysteme sowie deren Rolle bei verschiedenen Polymerisierungsreaktionen im Detail vor. Beispiele unterschiedlichster Anwendungen in der Industrie werden ebenfalls behandelt.
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Table of Contents
Cover
Title Page
Copyright
Introduction
Volume 1
Part I: Photopolymerization Reactions and Photoinitiators: Backgrounds
1 Backgrounds in Photopolymerization Reactions: A Short Overview
1.1 Photopolymerization and Photo‐cross‐linking
1.2 Photopolymerization Reactions
1.3 Implementation of Photopolymerization Reactions and Brief Overview of the Applications
References
2 Photoinitiating System
2.1 Characteristics of a Photoinitiating System
2.2 Approach of Photochemical and Chemical Reactivity
2.3 Reactivity of a Photosensitive System
2.4 Efficiency vs. Reactivity
References
Part II: Photoinitiators: Structures, Excited States, Reactivity, and Efficiency
3 Cleavable Radical Photoinitiators
3.1 Benzoyl Chromophore‐Based Photoinitiators
3.2 Hydroxy Alkyl Heterocyclic Ketones
3.3 Benzophenone and Thioxanthone Moiety‐Based Cleavable Systems
3.4 Benzoyl Phosphine Oxide Derivatives: a C—P Bond Breaking
3.5 Trichloromethyl Triazines
3.6 Biradical Generating Ketones
3.7 Diketones
3.8 Silyl Glyoxylates
3.9 Peroxides
3.10 Peresters
3.11 Azides and Aromatic Bis‐azides
3.12 Carbon–Germanium Cleavable Bond‐Based Derivatives
3.13 Carbon–Tin Cleavable Bond‐Based PIs
3.14 Carbon–Silicon Cleavable Bond‐Based PIs
3.15 Carbon–Nitrogen Cleavable Bond Containing PIs
3.16 Boron–Sulfur Cleavable Bond Containing PIs
3.17 Boron–Nitrogen Cleavable Bond Containing PIs
3.18 Disilane Derivatives
3.19 Diselenide and Diphenylditelluride Derivatives
3.20 Sulfur–Carbon Cleavable Bond‐Based Derivatives
3.21 Disulfide Derivatives
3.22 Oxyamines
3.23 Barton's Ester Derivatives
3.24 Hydroxamic and Thiohydroxamic Acids and Esters
3.25 Ion Pair PIs
3.26 Organometallic Compounds
3.27 Metal Salts and Metallic Salt Complexes
3.28 Miscellaneous Systems
References
Note
4 Two‐Component Radical Photoinitiators
4.1 Ketone/Hydrogen Donor and Ketone/Electron/Proton Donor Couples
4.2 Ketone/Electron Acceptor Systems
4.3 Ketone/Diethoxyacetate Salt
4.4 Well‐Known and Novel Type II Ketones
4.5 Dye‐Based Systems
4.6 Organometallic Compound‐Based Systems
4.7 Ketone/Ketone‐Based Systems
4.8 Photoinitiator/Peroxide (or Hydroperoxide)‐Based Systems
4.9 Type I Photoinitiator/Additive
4.10 Donor/Acceptor Charge Transfer Systems
References
5 Cationic Photoinitiating Systems
5.1 Diazonium Salts
5.2 Onium Salts
5.3 Organometallic Derivatives
5.4 Photosensitized Decomposition of Onium Salts
5.5 Unconventional Cationic Systems
References
6 Anionic, Photoacid, and Photobase Initiating Systems
6.1 Anionic Photoinitiators
6.2 Nonionic Photoacid Generators Systems
6.3 Photobase Generators Systems
References
7 Reactivity of Radicals Toward Various Substrates: Understanding and Discussion
7.1 Backgrounds
7.2 Reactivity of Radicals Toward Oxygen, Hydrogen Donors, Monomers, and Additives
References
8 Role of Experimental Conditions on the Performance of a Radical Photoinitiator
8.1 Role of Viscosity
8.2 Role of the Surrounding Atmosphere
8.3 Role of the Light Source
8.4 Role of Monomer Matrix: An Example
References
9 Reactivity and Efficiency of Radical Photoinitiators
9.1.1 Reactivity of Photoinitiators
9.2 Reactivity/Efficiency of Photoinitiators: Examples of Structural Effects
9.3 Up‐to‐date Approach of the Reactivity and the Structure/Property Relationships
References
Volume 2
Part III: High Performance Photoinitiating Systems: Achievements, Trends, Challenges, Opportunities and Applications
10 Design of Photoinitiators for Enhanced Performance: A Mechanistic Approach
10.1 Traditional Search of Substituent Effects in Radical Photoinitiators
10.2 Search of Novel Cleavable Bonds
10.3 Multifunctional Photoinitiators for an Increased Visible Light Absorption
10.4 Photoinitiators with a Thermally Activated Delayed Fluorescence (TADF)
10.5 Amine or Phosphine/Iodonium Salt Charge Transfer Complexes
10.6 Mechanosynthesized Photoinitiators
References
11 Multicomponent Radical Photoinitiating Systems for Enhanced Reactivity
11.1 Photoinitiator/Amine/Iodonium Salt
11.2 Photoinitiator/Amine/Organic Halide
11.3 Photoinitiator/Amine/Other Additive
11.4 Photoinitiator/Borate/Additive
11.5 Photoinitiator/Cl‐HABI/Thiol
11.6 Photoinitiator/Silane/Iodonium Salt: A Very Versatile System
11.7 Photoinitiator/N‐Vinylcarbazole/Iodonium Salt
11.8 Photoinitiator/Germane/Iodonium Salt
11.9 Photoinitiator/CARET/Iodonium Salt
11.10 Photoinitiator/Triphenylphosphine Derivative/Iodonium Salt
11.11 Miscellaneous Photoinitiator/Additive/Iodonium Salt
11.12 Photoinitiator/Additive/Sulfonium Salt
11.13 Charge Transfer Complex/Peroxide
11.14 Photoinitiator/Additive/Hydroperoxide
11.15 Other Miscellaneous Three‐Component Systems
11.16 Four‐Component Systems: Examples
References
12 Photoinitiating Systems for Free Radical Promoted Cationic Polymerization
12.1 FRPCP Process
12.2 Type I Photoinitiator/Iodonium Salt Two‐Component Systems
12.3 Type I Photoinitiator/Sulfonium Salt
12.4 Type I Photoinitiator/Pyridinium Salt
12.5 Type I Photoinitiator/Zinc Salt
12.6 Type II Photoinitiator/Iodonium Salt Three‐Component Systems
12.7 Addition/Fragmentation Reaction
12.8 Photoinitiator/Metal Salt/Additive‐Based Systems
References
13 Photoinitiators for Novel Specific Properties
13.1 One‐Component Type II Photoinitiators
13.2 Macrophotoinitiatiors
13.3 Water‐Soluble Photoinitiators
13.4 Photoredox Catalysts as Photoinitiators
13.5 Two‐Photon Absorption Photoinitiators
13.6 Photoinitiators for Overcoming Oxygen Inhibition
13.7 Photoinitiators with Other Miscellaneous Properties
13.8 Eco‐friendly Photoinitiators
References
14 Industrial Photoinitiators: A Brief Overview
14.1 Radical Industrial Photoinitiators
14.2 Cationic Industrial Photoinitiators
14.3 Tailor‐Made Formulations of Industrial Photoinitiators
14.4 Toxicity and REACH Registration of Photoinitiators
References
Part IV: Photoinitiators for Specific Reactions and Traditional or Emerging Innovative Applications
15 Photoinitiators and Light Sources: Novel Developments
15.1 Photoinitiators Under Soft Irradiation Conditions
15.2 Photoinitiators Under Sunlight
15.3 Photoinitiators Under LED and Laser Diode Irradiation Conditions
15.4 NIR Light‐Induced Polymerization
15.5 NIR Light‐Induced Thermal Polymerization
15.6 Photoinitiators and Pulsed Light Irradiation
15.7 Photoinitiators Under Visible Lights: Residual Coloration of the Coating
15.8 Examples of Efficient Photoinitiator Structures vs. the Irradiation Wavelengths
References
16 Photoinitiators for Controlled/Living Polymerization Reactions
16.1 Dithiocarbamates
16.2 O‐Alkoxyamines
16.3 Organometallic Compounds
16.4 Metal‐Free Compounds
16.5 TiO2–graphitic Carbon Nitride Composite
16.6 Iron Oxides and Salts
16.7 Photoinitiators in Living Cationic Polymerization
16.8 Photoinitiators in Living Anionic Polymerization
References
17 Photoinitiators in Specific Polymerization Processes
17.1 Photoinitiators for Thiol–Ene Reactions
17.2 Photoinitiators in Related Thiol–Ene Chemistries
17.3 Photoinitiators for Interpenetrating Polymer Networks Synthesis
17.4 Photoinitiators in Photoactivated Redox Polymerization
17.5 Photoinitiators in Photo‐CuAAC Reactions
17.6 Photoinitiators in Hydrosilylation Reactions
17.7 Photoinitiators in Hybrid Sol–Gel Photopolymerization
17.8 Photoinitiators for the In Situ Generation of Nanoparticles
17.9 Photoinitiators in Particular Experimental Conditions
17.10 Photoinitiators in Miscellaneous Novel Reactions
References
18 Photoinitiators for the Curing of Thick or Filled Samples
18.1 Penetration of Light in a Thick Sample
18.2 Use of Bleachable Photoinitiators
18.3 Use of High Irradiances
18.4 Use of Temperature Effects
18.5 UCNP‐Assisted Photopolymerization Under NIR Lights
18.6 Use of Catalytic Photoredox Processes
18.7 Design of Novel Photoinitiating Systems
References
19 Photoinitiators in Various Sectors of Industrial Applications
19.1 Photoinitiators in the Radiation Curing Area
19.2 Photoinitiators in Graphic Arts
19.3 Photoinitiators in 3D Printing Technologies
19.4 Photoinitiators for Biomaterials
19.5 Photoinitiators for Dentistry Applications
19.6 Photoinitiators in (Micro)Electronics
19.7 Photoinitiators in the Optics Area
19.8 Photoinitiators in Organic Electronics
References
Index
Conclusion
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Effect of various photoinitiating systems (at the same w/w concentr...
Chapter 3
Table 3.1 Examples of available data concerning cleavable photoinitiators (PI...
Table 3.2 Absorption properties of different PIs in n‐hexane.
Table 3.3 Examples of available data concerning cleavable PIs derived from am...
Table 3.4 Examples of available data concerning cleavable PIs derived from hy...
Table 3.5 Examples of available data concerning cleavable PIs derived from va...
Chapter 4
Table 4.1 Typical hydrogen donors (HD) and rate constants for H‐transfer to t...
Table 4.2 Examples of available data concerning usual non‐cleavable photoinit...
Table 4.3 Benzophenone derivatives applicable to near‐UV/visible lights. Mola...
Table 4.4 Examples of available data concerning thioxanthone TX derivatives: ...
Table 4.5 Triplet state absorption maxima
λ
m
and interaction rate constan...
Table 4.6 Triplet state lifetimes τ
T
of 1‐methylester thioxanthone in differe...
Table 4.7 Thioxanthone derivatives applicable to visible lights. Some of them...
Table 4.8 Examples of available data concerning other usual non‐cleavable pho...
Table 4.9 Redox and excited state properties (singlet and triplet energy leve...
Table 4.10. Newly developed purple or blue light sensitive dyes applicable in...
Table 4.11 Newly developed green light‐sensitive dyes applicable in PISs.
Table 4.12 A red light‐sensitive dye applicable in PISs. It also works in cat...
Table 4.13 Dyes applicable in multicolor (or panchromatic) PISs.
Table 4.14 Other novel dyes in photopolymerization.
Table 4.15 Some characteristics of the excited states and thermodynamical val...
Chapter 5
Table 5.1 Substitution effects on the absorption maxima and molar extinction ...
Table 5.2 Examples of structural effects on the absorption maxima and molar e...
Table 5.3 Free energy change (Δ
G
) calculated from the Rehm–Weller equation fo...
Table 5.4 Electron transfer rate constants
k
(in 10
−7
mol
−1
l s
−1
...
Table 5.5 Electron transfer rate constants (
k
e
) in various PS/diphenyliodoniu...
Table 5.6 Epoxy function conversion in EPOX in the presence of ketone/iodoniu...
Chapter 7
Table 7.1 Addition rate constants
k
i
of different alkyl (and derived) radicals...
Table 7.2 Rate constants characterizing the reactivity of different benzoyl r...
Table 7.3 Interaction rate constants of monomer‐derived radicals (methylacryl...
Table 7.4 Interaction rate constants between aminoalkyl radicals and O
2
, TEMP...
Table 7.5 Rate constants characterizing the reactivity of different phosphoru...
Table 7.6 Addition rate constants
k
i
(in mol
−1
l s
−1
) of thiyl rad...
Table 7.7 Interaction rate constants
k
H
(in mol
−1
l s
−1
) of silane...
Table 7.8 Addition reaction rate constants
k
i
(in mol
−1
l s
−1
) of ...
Table 7.9 Reaction rate constants (
k
) in (mol
−1
l s
−1
) of the rad...
Table 7.10 Interaction rate constants between
t
BuO
⋅
and different H‐dono...
Table 7.11 Interaction rate constants (
and
) in mol
−1
l s
−1
be...
Table 7.12
t
BuOO
⋅
/H‐donor interaction rate constants and BDE values for the...
Table 7.13 Rate constants and radical quantum yields in
3
BP (or
t
BuO
⋅
,
t
Table 7.14 Reaction rate constants of the germyl and stannyl radicals with O
2
Table 7.15 Formation rate constants
k
H
of L → BH
2
⋅
, oxidation potentials...
Table 7.16 Interaction rate constants of L → BH
2
⋅
radicals with various...
Table 7.17 Interaction rate constants k (in mol
−1
l s
−1
) between ...
Table 7.18 Reaction rate constants
k
(in mol
−1
l s
−1
) of metal‐cen...
Table 7.19 Addition rate constants
k
1
(in mol
−1
l s
−1
) of monomer‐...
Table 7.20 Reactivity of different sulfur‐centered radical as control agents:...
Table 7.21 Parameters characterizing the cleavage processes for A–D.
Table 7.22 Ionization potentials (IPs) and oxidation rate constants (mol
−1
...
Chapter 9
Table 9.1 Interaction rate constants
k
H
of typical hydrogen donors with the be...
Table 9.2 Interaction rate constants
k
q
(in 10
9
mol
−1
l s
−1
) of so...
Table 9.3 Calculated yields of electron transfer (Φ
eT
) between some dyes and ...
Table 9.4 Adiabatic ionization potentials IPa of the studied monomers and que...
Table 9.5 Examples of available data concerning cleavable photoinitiators: (i...
Table 9.6 Relative initiation efficiency ϕ
rel
values as a function of the pol...
Table 9.7 Relative rates of polymerization of (i) acrylamide AA (0.7 mol l
−1
...
Table 9.8 Examples of available data concerning radical photoinitiators in mi...
Table 9.9 Polymerization rates of TMPTA under air and (superscript
d
) in lamin...
Table 9.10 Polymerization rates of TMPTA under air (
R
p
/[M
0
]*100 in s
−1
)...
Table 9.11 Comparative performance of various two‐component PISs: final conve...
Table 9.12 Final conversions of Bis‐GMA/TEGDMA blend (70%/30%, w/w) obtained ...
Table 9.13 Relative initiation quantum yields (
) for the cationic photopolym...
Chapter 11
Table 11.1 Final TMPTA conversions using dye/amine (MDEA)/chlorotriazine (TZ)...
Table 11.2 Final TMPTA conversions using metal complex based PISs.
Table 11.3 Final TMPTA conversions using dyes/NVK/Iod PISs.
Chapter 12
Table 12.1 EPOX and divinylether (DVE‐3) photopolymerization efficiency of dy...
Table 12.2 Examples of final conversions of EPOX using organometallic complex...
Table 12.3 Examples of final conversions of EPOX using dye/NVK/Iod PISs.
Chapter 13
Table 13.1 Examples of available data for water‐soluble photoinitiators: inte...
Chapter 15
Table 15.1 Examples of conventional and novel efficient radical systems based...
Table 15.2 Examples of conventional and novel efficient cationic systems base...
List of Illustrations
Chapter 1
Scheme 1.1 Crosslinking mechanisms.
Scheme 1.2 Multifunctional monomer.
Scheme 1.3 Different processes in imaging area.
Scheme 1.4 Chemical mechanisms.
Scheme 1.5 Light activation of dormant species.
Figure 1.1 Typical time evolution of the radical photopolymerization reactio...
Figure 1.2 Typical ground‐state absorption spectra of a UVA, a HALS, and a g...
Figure 1.3 Typical absorption of a white pigment, a photoinitiator PI, and a...
Figure 1.4 (a) Example of an emission spectrum (of sun). Schematic transmiss...
Figure 1.5 (a,b) Typical emission spectra of different microwave powdered la...
Figure 1.6 (A) Typical emission spectra of (a) 365 nm LED, (b) 532 nm diode ...
Chapter 2
Figure 2.1 Photophysical processes involved in an electronically excited mol...
Figure 2.2 Typical absorption spectra of various compounds usable as PI.
Scheme 2.1 Initiation pathway.
Figure 2.3 Schematic representation of the evolution of
R
p
,
k
diff
, and
k
add
...
Scheme 2.2 Overall diagram of evolution of the excited states.
Chapter 3
Figure 3.1 Typical absorption spectrum of a benzoyl chromophore containing a...
Scheme 3.1 Representation of the molecular orbitals.
Scheme 3.2 Electronic configurations.
Figure 3.2 HOMO of the benzoyl chromophore for Y = Mor and X = C[(CH
3
)
2
]‐Mor...
Figure 3.3 (a) HOMO and LUMOs of DMPA. (b) Potential energy surface PES for ...
Figure 3.4 HOMO and LUMO in TPO at DFT level.
Figure 3.5 Reaction in alcoholic media.
Figure 3.6 Highest occupied molecular orbital (HOMO)and lowest unoccupied mo...
Figure 3.7 MOs involved in a silyl glyoxylate (a) and a phenyl glyoxylate (a...
Figure 3.8 HOMO and LUMO for BisGe (3.24) at DFT level. The germanium atoms ...
Figure 3.9 HOMO and LUMO of BSK at UB3LYP/6‐31G* level (isovalue = 0.02). Si...
Figure 3.10 HOMO and LUMO of B—N bond containing PIs (calculated at UB3LYP/6...
Figure 3.11 (A) UV–vis absorption spectra of Naphth‐Iod (3) vs. Iod (1) and ...
Chapter 4
Figure 4.1 Photopolymerization profiles (methacrylate function conversion vs...
Scheme 4.1 Energetic diagram.
Figure 4.2 Photopolymerization profiles of (left) TMPTA in laminate upon hal...
Scheme 4.2 Chemicals mechanisms.
Figure 4.3 UV–visible absorption spectra of (a) 2‐isopropylthioxanthone ITX ...
Scheme 4.3 Energetic diagram.
Scheme 4.4 Energetic diagram.
Scheme 4.5 .
Scheme 4.6
Figure 4.4 Contour plots of HOMOs and LUMOs for the structures optimized at ...
Figure 4.5 Conversion vs. time curves for the photopolymerization of (A) an ...
Figure 4.6 (A) Calculated absorption spectra. (B) UV–vis spectra in DCM of: ...
Chapter 5
Figure 5.1 Typical absorption spectra of onium salts: (a) Ph–S–PhS
+
Ph
2
i...
Figure 5.2 (a) HOMO and (b) LUMO for Ph
3
S
+
(DFT level).
Scheme 5.1 Reactivity of ferrocenium salt.
Figure 5.3 Carbazole‐bound ferrocenium salt.
Scheme 5.2 Expected chemical mechanisms.
Chapter 6
Scheme 6.1 Proposed chemical mechanisms..
Chapter 7
Figure 7.1 State correlation diagram for the addition of a radical to a doub...
Scheme 7.1 Fragmentation mechanims.
Figure 7.2 SOMO of a benzoyl radical calculated at DFT level; the radical ce...
Figure 7.3 (a) Time‐resolved IR spectrum of PhC(=O)
⋅
,.
Figure 7.4 Transient absorption spectra for the TEA–BA
⋅
, TEA–MMA
⋅
...
Scheme 7.2 Molecular orbitals involved.
Scheme 7.3 Oxygen inhibition..
Figure 7.5 TR‐ESR spectra recorded (a) 50–250 ns and (b) 750–1050 ns followi...
Figure 7.6 Absorption spectrum of radical S
3
(generated by a
t
‐BuO
⋅
/th...
Figure 7.7 Spin densities on the thiyl radical formed in Barton esters (comp...
Figure 7.8 ESR‐spin trapping experiments for
t
‐BuO
⋅
/(TMS)
3
SiH with PBN...
Figure 7.9 Single occupied molecular orbital SOMO and structure of the tris(...
Scheme 7.4 Chemical mechanisms..
Figure 7.10 ESR spectrum and decay of
t
BuOO
⋅
at RT (for more detail, s...
Figure 7.11 SOMOs for representative alkoxyl or peroxyl radicals (DFT level)...
Figure 7.12 SOMOs for representative boryls at DFT level. Boron is indicated...
Scheme 7.5
β
‐fragmentation in carbine‐borane.
Scheme 7.6 Boryl radical reactivity with O
2
.
Figure 7.13 SOMOs for H
2
C=N
⋅
at DFT level (two different views). Nitro...
Scheme 7.7 Reaction of dithiocarbamyl radicals..
Scheme 7.8 Alkoxyamine reactivity.
Chapter 8
Figure 8.1 Conversion vs. time curves for the photopolymerization of EpAc (A...
Scheme 8.1 Competitive pathways.
Figure 8.2 (A) Conversion vs. time curves for the photopolymerization of EpA...
Figure 8.3 Conversion vs. time curves for the photopolymerization of TMPTA u...
Figure 8.4 Photopolymerization of EpAc using BAPO (phosphine oxide, phenyl b...
Chapter 9
Figure 9.1 MO calculations for the cleavage of a coumarin‐based iodonium sal...
Figure 9.2 Addition of different representative initiating radicals (CH
3
)
2
NC...
Scheme 9.1 Energetic pathway.
Figure 9.3 Plot of ϕ
rel
vs. ϕ
diss
for the photopolymerization of
A
) bulk EpA...
Chapter 10
Scheme 10.1 Design of new PIs.
Figure 10.1 Molecular orbitals involved in the S
0
–S
1
and S
0
–S
2
absorption of...
Figure 10.2 Double‐bond conversion of TMPTA vs. time of formulations contain...
Figure 10.3 Highest occupied molecular orbital (HOMO) and lowest unoccupied ...
Figure 10.4 Calculated molecular orbitals (HOMO and LUMO) at UB3LYP/6‐31G* l...
Figure 10.5 Molecular orbitals involved in a mono‐ and a HAP‐based difunctio...
Figure 10.6 UV–visible absorption spectra of Tr_DMPA and DMPA in toluene; In...
Figure 10.7 Examples of Push–Pull dyes used in photoinitiating systems. Invo...
Scheme 10.2 TADF properties.
Figure 10.8 Photopolymerization profiles of methacrylate functions (BisGMA‐T...
Figure 10.9 Frontier molecular orbital (calculated at UB3LYP/LANL2DZ level) ...
Chapter 11
Figure 11.1 Contour plots of HOMOs and LUMOs for A‐2DPA and A‐2PTz structure...
Figure 11.2 (a) Polymerization profiles of TMPTA (acrylate function conversi...
Figure 11.3 Photopolymerization profile of TMPTA in laminate in the presence...
Scheme 11.1 Chemical mechanisms in photoinitiator/silane/iodonium salt syste...
Scheme 11.2 Catalytic cycle involving silyl radicals.
Figure 11.4 Photopolymerization profiles of TMPTA in laminate in the presenc...
Figure 11.5 The SOMO orbital of CARET
(−H)
⋅
; the radical center i...
Figure 11.6 Redox photoactivated polymerization profiles of a methacrylate r...
Figure 11.7 Photopolymerization profiles of a methacrylate film (1.4 mm thic...
Chapter 12
Scheme 12.1 Expected chemical mechanisms.
Figure 12.1 Polymerization profiles of an epoxide EPOX under air in the pres...
Figure 12.2 (a) UV–vis absorption spectra of ND10 and ND4; (b) photopolymeri...
Figure 12.3 Panchromatic photopolymerizable cationic films using indoline an...
Figure 12.4 Photopolymerization profiles of an industrial diepoxide formulat...
Chapter 13
Scheme 13.1 New thioxanthone reactivity.
Figure 13.1 Highest occupied molecular orbital (HOMO) and lowest unoccupied ...
Scheme 13.2 Chemical mechanisms with aldehyde.
Scheme 13.3 Structures of macrophotoinitiators.
Scheme 13.4 PI architecture.
Scheme 13.5 Example of catalytic cycle.
Scheme 13.6 Cycles involving Cu complexes. Source: Bouzrati‐Zerelli et al. [...
Scheme 13.7 Cycle of organic photoredox catalysts. Source: Tehfe et al. [260...
Scheme 13.8 Cycle for Ru complexes.
Figure 13.2 EPOX polymerization profiles (epoxy function conversion vs. irra...
Figure 13.3 Typical photopolymerization profiles of TMPTA in the presence of...
Scheme 13.9 Oxygen inhibition.
Figure 13.4 (A) Conversion vs. time curves for the photopolymerization of TM...
Figure 13.5 Conversion vs. time curves for the photopolymerization of an epo...
Figure 13.6 (a) Absorption spectra of the A6H investigated compounds in acet...
Chapter 15
Figure 15.1 IR spectra recorded during the photopolymerization of an epoxide...
Figure 15.2 (a) UV–vis absorption spectrum for OBN and (b) cationic photopol...
Figure 15.3 Temperature profiles of a methacrylate formulation under air (te...
Figure 15.4 Photopolymerization of metharylate resin under air in the presen...
Figure 15.5 (a) Steady‐state photolysis in the presence of DKPP1/Iod/EDB (0....
Chapter 17
Figure 17.1 Conversion for the double bond (1) and the SH function (2) for (...
Figure 17.2 Photopolymerization profiles of an EPOX/TMPTA blend (50%/50%, w/...
Figure 17.3 Polymerization profiles of a methacrylate resin using (a) amine/...
Scheme 17.1 Chain functionalized PIs.
Scheme 17.2 Synthesis of block co‐polymers..
Scheme 17.3 Block copolymer on gold.
Chapter 18
Figure 18.1 (a) Surface topology of the calculated curing space. 2‐Benzyl‐2‐
Figure 18.2 Photopolymerization profiles of methacrylate resins under air in...
Figure 18.3 Photopolymerization experiments under air using an LED at 405 nm...
Figure 18.4 Number of passes required to have tack‐free glass fiber‐impregna...
Figure 18.5 Photocomposites produced upon near‐UV light (LED @395 nm), belt ...
Chapter 19
Figure 19.1 Free radical photopolymerization experiments for laser write: (a...
Guide
Conclusion
Cover Page
Photoinitiators
Photoinitiators
Copyright
Introduction
Table of Contents
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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Photoinitiators
Structures, Reactivity and Applications in Polymerization
Volume 1
Written by
Jean-Pierre Fouassier
Jacques Lalevée
Photoinitiators
Structures, Reactivity and Applications in Polymerization
Volume 2
Written by
Jean-Pierre Fouassier
Jacques Lalevée
Authors
Prof. Jean‐Pierre Fouassier
15, rue du Château
68590 Saint Hippolyte
France
Prof. Jacques Lalevée
University of Haute Alsace
UMR CNRS 7361
15, rue Jean Starcky
68057 Mulhouse Cedex
France
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Introduction
As already known, a photoinitiator (PI) or aphotoinitiating system (PIS), where a PI is introduced, allows the initiation of a polymerization reaction under exposure to a light source.
Light‐induced polymerization reactions are largely encountered in many industrial applications or in promising laboratory developments. The basic idea is to readily transform a liquid monomer (or a soft film) into a solid material (or a solid film) upon light exposure at ambient temperature. The huge sectors of applications are found in both traditional and high tech areas such as radiation curing, laser imaging, 3D printing, microelectronics, optics, biosciences, dentistry, nanotechnology, etc. Radiation curing is considered as a green technology that continues its rapid development.
Photopolymerization reactions can be carried out in various experimental conditions, such as in films, gas phase, aerosols, multilayers, (micro)heterogeneous media or solid state, on surface, in ionic liquids, in situ in the manufacturing of microfluidic devices, in vivo, even under magnetic field, etc. Very different aspects are concerned in gradient, template, frontal, controlled, sol–gel, two‐photon, redox, laser‐induced, or spatially controlled photopolymerizations, etc.
A photopolymerization reaction involves a PI or a PIS, a polymerizable medium, and a light source. The photoinitiator plays a crucial role as it absorbs the light and starts the reaction. Its reactivity governs the efficiency of the polymerization. The literature shows that a considerable number of works are devoted to the design of photosensitive systems being able to operate in many various (and sometimes exotic) experimental conditions.
Fantastic developments have occurred all along the past five decades. Significant achievements have been done since the early works on photopolymerization in the 1960s and the traditional developments of the radiation curing area. Today, high tech applications are continuously emerging. A tailor‐made photochemistry and chemistry for the design of high‐performance systems (that can operate at any wavelength, under low light intensity and under air, ensure a better safety, and provide novel handling or end use properties) have appeared in this area and has achieved remarkable success. The search for a safe and green technology is growing. Interesting items first relate to the polymer science and technology field but also to the photochemistry, physical chemistry, and organic chemistry areas.
In the past 40 years, many aspects of light‐induced polymerization reactions have been discussed in books and review papers. Each of these books, however, usually cover more deeply selected aspects depending first on the origin (university and industry) and activity sector of the author/editor (photochemistry, polymer chemistry, applications, etc) and second on the goals of the book (general presentation of the technology, guide for end users, and academic scope, etc). Our previous general book published more than 20 years ago (Hanser, 1995) and devoted to the three photoinitiation/photopolymerization/photocuring complementary aspects already provided a first account on the photosensitive systems. Unfortunately, for obvious reasons, all these three fascinating aspects that continuously appeared in the literature could no longer be developed in detail in a single monograph because of the rapid growth of the research. This was the reason why our second book ( Photoinitiators for Polymer Synthesis) published in 2012 (Wiley) was only focused on the photosensitive systems that are used to initiate the photopolymerization reaction, their adaptation to the light sources, their excited state processes, their interaction with the different available monomers, their working out mechanisms, and the approach for a complete understanding of the (photo)chemical reactivity. This second book showed the huge progress made between 1995 and 2012.
Why a new book? Indeed, by the end of 2010, one could have had the feeling that almost everything had been foreseen and verified in the design of photoinitiators and PISs. However, science is ever going on! Thanks to novel methods of investigation (both experimental and theoretical) and novel ideas accompanied by searches in other areas (fine product catalogs, natural compounds, optoelectronics, organic light‐emitting diodes (OLEDs), solar cells, composites: lightweight materials, etc), a huge progress has been done in the past 10 years! A lot of novel structures previously unimaginable and usable in novel and promising applications have been published and have opened new unsuspected horizons.
Huge challenges remain in the area of PIs and PISs, but a constant development is noted in (i) nontoxic (or less toxic) systems, e.g. biological and medical applications, food packaging, and for the safety of the end users (the toxicity of many classical PIs has been re‐evaluated in the context of the REACH registration); (ii) high‐performance systems for the access to larger objects in 3D printing, the high productivity for the curing of inks, coatings, paints, composites, the ability to photopolymerize pigmented or highly filled thick samples for composites with improved mechanical properties, and the working out in hydrogels, eco‐friendly water‐based formulations, or self‐assembled systems; (iii) systems operating under safe irradiation devices and visible, red, and near‐infrared (NIR) light sources; and (iv) systems for new photopolymerization processes, e.g. controlled polymerization, hybrid polymerization to reduce polymerization stress and shrinkage, dual cure polymerization combining redox and light activation for reactions in shadow areas, polymerization of biosourced monomers, two‐photon photopolymerization, and emulsion photopolymerization upon visible lights.
The following picture schematically depicts 60 years of evolution of the research and development in this area. Originally, the screening of the available chemicals, the synthesis of suitable derivatives, and the large use of trial and error experiments allowed to satisfactorily photocure monomer/oligomer formulations according to the rather undemanding requested conditions at this period. Then, the investigation of the excited state processes and the proposal of mechanistic schemes helped to design more powerful systems for an improved performance. Finally, the molecular orbital (MO) calculations provided new solutions for the construction of efficient molecules and the interpretation of the chemical reactivity. Questions arising from end users or researchers for the development of novel applications lead to a demand of novel “five‐legged” photoinitiators that, along time, are more and more sophisticated. Moreover, good fortune, close attention, or brilliant ideas, even without any heavy experimental or theoretical approach, have also played a key role in the discovery of novel structures somewhere on our planet.
The above considerations prompted us to write today a new book on photoinitiators in which the most recent developments have been stressed. Here, we intend (i) to give the best up‐to‐date situation of the subject and take time to briefly define a lot of basic principles and concepts, mechanistic reaction schemes, and examples of studies that remain true and are not submitted to a significant aging on a 10 year timescale, (ii) to keep a complete presentation of the encountered PISs together with a discussion of the structure/excited state processes/reactivity/efficiency relationships, (iii) to focus on the involved mechanisms (the role of the basic research toward the progress of the applied research being absolutely necessary), and (iv) to outline the latest developments and trends in the design of novel tailor‐made photoinitiators/PISs (based on experimental and theoretical approaches) as well as the corresponding existing, emerging, promising, or challenging applications where current or new systems are employed. To our opinion, such an extensive and complete book within this philosophy remains totally original today.
The book is divided into four parts including 19 chapters. As mentioned before, we decided to provide a basic thorough description of the processes and mechanisms together with an in‐depth treatment of the design of PIs and PISs and a general presentation of the current applications.
In Part I, we deliver a general but concise presentation of the basic principles of photopolymerization reactions with a description of the available light sources, the different monomers, the characteristics, kinetics, and monitoring of the reactions together with a few words on the application areas (Chapter 1). As it specially concerns the polymer science point of view, this chapter will only focus on what is necessary to clearly understand the following chapters. The characteristics, the role, the different basic processes, and the reactivity/efficiency of the PISs are described in Chapter 2 together with a few photochemistry checks.
Part II is devoted to the structures, excited state processes, reactivity, and efficiency of photoinitiators. In a first step, we provide (i) the most exhaustive presentation of the commercially or academically used or potentially interesting PISs developed in the literature so far (photoinitiators, co‐initiators, and photosensitizers), (ii) the characteristics of the excited states, (iii) the involved reaction mechanisms, and (iv) a complete presentation of the experimental and theoretical reactivity of more than 15 kinds of radicals. Being aware of the importance of a total homogeneity throughout the book, we keep a complete qualitative overview of all the available systems developed along years, but we will focus our attention on newly developed photoinitiators, recently reported studies, and novel data on previous well‐known systems. All this information is provided for radical photoinitiators (Chapters 3 and 4); cationic photoinitiators (Chapter 5); anionic photoinitiators, photoacids, and photobases (Chapter 6); and initiating radicals (Chapter 7). In a second step, we discuss the role of the experimental conditions, e.g. formulation viscosity, surrounding atmosphere, light source emission spectrum, and intensity on the efficiency of a photopolymerization reaction (Chapter 8). The reactivity and efficiency of PISs in solution, bulk, or film, under air, in high/low‐viscosity media or under high/low light intensities as well as the elaboration of structure/property relationships (Chapter 9) will be restricted to radical structures as almost nothing concerns cationic systems; the reactivity/efficiency in microheterogeneous media will also be evoked.
In Part III, we introduce specific chapters (which did not appear at all in the previous 2012 book) to deal with the design of high‐performance PISs: these chapters allow to understand the novel directions of research, the building up of novel chemical structures of photoinitiators, the search of additives with more suitable bond dissociation energies (BDEs) and redox properties, the specific role of PIs and PISs in up‐to‐date possibilities of applications and end uses, etc. We successively present the synthesis and mechanistic approach of novel architectures of photoinitiators (Chapter 10); the design of multicomponent radical PISs for an enhanced reactivity (Chapter 11); the recent huge progress made in PISs for free radical promoted cationic photopolymerization (Chapter 12); the proposal of photoinitiators for novel specific and improved properties such as one‐component, water‐soluble, two‐photon absorption or oxygen‐tolerant compounds, orthogonal photoinitiators, safe photoinitiators, and natural photoinitiators (Chapter 13); and the industrial photoinitiators (Chapter 14).
Part IV is devoted to the role of photoinitiators in specific reactions and current or emerging innovative applications. The development of photoinitiators adapted to novel light sources (LEDs, sun, pulsed sources, NIR lights, etc.) is shown in Chapter 15. Chapter 16 reviews photoinitiators in controlled polymerization reactions. Photoinitiators usable in specific polymerization processes (thiol–ene, interpenetrating polymer network [IPN], redox, polymerization‐induced self‐assembly [PISA], copper‐catalyzed azide‐alkyne cycloaddition [CUAAC], ionic liquids, hydrogels, incorporation of nanoparticles, etc.) are detailed in Chapter 17. The recent progress in the design of photoinitiators and PISs for the polymerization of thick films is described in Chapter 18. Finally, Chapter 19 covers some aspects of photoinitiators in industrial applications, e.g. coatings, graphic arts, 3D printing, medical, optics, and electronics.
When questioning the Chemical Abstract database, many references appear. We have not intended to give here an exhaustive list of references or a survey of the patent literature. More than 1000 references appeared each year. Pioneer works are cited, but our present list of references mainly refers now to papers dispatched during the past 30/20 years with an emphasis on the 2010–2019 period. The selection of the articles is most of the time a rather hard and sensitive task. We have done our best and beg forgiveness for possible omission.
This research field continuously knows a fantastic evolution. We would like now to share the real pleasure we had (and still have) in participating and contributing to this area. Writing this second book was really a great pleasure. We hope that our readers, R&D researchers, engineers, technicians, University people, and students, involved in various scientific and/or technical areas such as photochemistry, polymer chemistry, organic chemistry, radical chemistry, physical chemistry, radiation curing, imaging, physics, optics, medicine, and nanotechnology, will appreciate this book and enjoy its content.
Part IPhotopolymerization Reactions and Photoinitiators: Backgrounds
As mentioned in the Introduction chapter, a photopolymerization reaction is a strong interplay between a photoinitiating system, a polymerizable medium, and a light source. In this first part of the book, we start with a brief overview of the photopolymerization reactions, used monomer/oligomers, and light sources and show a large variety of practical situations where photoinitiating systems are employed. Some of the topics that receive a deeper analysis in books, reviews, or papers and are not strictly concerned with the photoinitiating systems are not discussed here. In Chapter 1, we summarize the necessary backgrounds in photopolymerization reactions. In Chapter 2, we provide a general discussion on the role of photoinitiating systems, evaluation of the practical efficiency of a photopolymerizable medium, and the approach of photochemical and chemical reactivity.
1Backgrounds in Photopolymerization Reactions: A Short Overview
The goal of this chapter is to provide the reader with the backgrounds of the involved scientific and technical aspects in photopolymerization reactions (characteristics, processes, kinetics, light sources, polymerizable media, properties of the resulting materials, and different areas of applications) but not to detail the “polymer” point of view of this field. In the present case, we will only give a basic and rather brief description that should allow an easy understanding of the following chapters of this book.
Examples of general books dealing with this “polymer aspect” can be found in Refs. [1–40]. In the past 10 years, we observed a huge development of applications involving light in the different facets of the polymerization area (see Parts III and IV) such that a complete description would require in the writing of a single book on this subject! This is not the goal of the present monograph where the general concern is the photosensitive systems. Therefore, in the present chapter, we decided to only include the references of some typical (review) papers.
1.1 Photopolymerization and Photo‐cross‐linking
1.1.1 Reactions
It is known that a polymerization reaction consists in adding many monomer units M to each other, thereby creating a macromolecule (1.1). The initiation step of this reaction corresponds to the decomposition of a molecule (an initiator I) usually obtained through a thermal process (1.2). This produces an initiating species (X‧) being able to attack the first monomer unit. Other units add further to form the macromolecule.
Instead of thermal activation of polymerization, other stimuli such as light, electron beam, X‐rays, γ‐rays, plasma, microwaves, or even pressure can be used (see Ref. [32]). Among them, the exposure of a resin (monomer/oligomer matrix) to a suitable light is found to be a very convenient way for the initiation step: in that case, the reaction is called a photopolymerization (1.3).
1.1.2 Photoinitiation Step
Because of their absorption properties, monomers or oligomers are usually not sensitive to the available lights (except a few cases involving specifically designed light‐absorbing structures). The addition of a photoinitiator (PI) or a photoinitiating system containing a PI and other compounds (PIS) is necessary (1.4). Excited states are generated under the light exposure of PI (see Chapters 2–4). Then, an initiating species is produced. Its nature – radical (R‧), cationic (C+), and anionic (A−) – is dependent on the starting molecule.
1.1.3 Different Kinds of Photopolymerization Reactions
Accordingly, the usual types of photopolymerization reactions – free radical polymerization (FRP), cationic polymerization (CP), anionic photopolymerization (AP), or acid‐ and base‐catalyzed photo‐cross‐linking reactions – can be encountered, e.g. (1.5), in suitable resins.
The term “photopolymerization” is very general and relates to two different concepts ( Scheme 1.1). A photoinduced polymerization reaction is a chain reaction where one photon yields one initiating species and induces the incorporation of a large number of monomer units. A photo‐cross‐linking reaction refers to a process involving a prepolymer or a polymer backbone in which a cross‐link is formed between two macromolecular chains: each chain propagation requires the absorption of a photon. This kind of polymer can be designed in such a way that it contains pendent (e.g. in polyvinyl cinnamates) or in‐chain photo‐cross‐linkable moieties (e.g. in chalcone‐type chromophore‐based polymers).
1.1.4 Monomers and Oligomers
A monomer is a rather small molecule having usually one or several chemical reactive functions (e.g. acrylates), whereas an oligomer is a large molecular structure consisting of repetitive units of a given chemical structure, which constitutes the backbone and contains one or more reactive chemical functions (1.6). The oligomer skeleton governs the final physical and chemical properties of cured coating.
Scheme 1.1 Crosslinking mechanisms.
When using multifunctional monomers or oligomers, the photoinduced polymerization reaction does not obviously proceed to form a linear polymer. As it develops in the three directions of space, it also leads to a cross‐linking reaction, thereby creating a polymer network (see Scheme 1.2 for a free radical reaction). Sometimes, the reaction is depicted as a cross‐linking photopolymerization.
Scheme 1.2 Multifunctional monomer.
1.1.5 Photopolymerizable Formulations
A photopolymerizable formulation consists in (i) a monomer/oligomer matrix (the monomer plays the role of a reactive diluent to adjust the viscosity of the formulation; it readily copolymerizes), (ii) a PI or a PIS, and (iii) various additives, e.g. flow, slip, mist, wetting, dispersion agents, inhibitors for handling and fillers, plasticizers, matting or gloss agents, pigments, and light stabilizers according to the applications.
1.1.6 UV Curing
Originally, UV curing is a word that designs an ever‐expanding industrial field [7,11,21,41–45] where light is used to transform a liquid photosensitive formulation into an insoluble solid film for coating applications through a photopolymerization reaction. Although it also includes electron beam curing, the term “radiation curing” is often used instead of “UV curing.” “Photocuring” is a practical word that refers to the use of light to induce this rapid conversion of the resin into a cured and dried solid film. Film thicknesses typically range from a few micrometers to a few 100 μm (and even more; high thicknesses can now be photopolymerized) depending on the applications. In 3D printing technologies, the idea consists in building up a solid object through a layer‐by‐layer photopolymerization procedure. Today, many novel applications are found in other areas.
1.1.7 Imaging
In the imaging area, an image is obtained according to a process largely described in the literature, e.g. [9, 10, 35]. The resin layer is irradiated through a mask. A reaction takes place in the irradiated areas. Two basically different reactions can occur (Scheme 1.3): (i) a photopolymerization or a photo‐cross‐linking reaction that renders the film insoluble (using a suitable solvent allows to dissolve the monomer present in the shadow areas; after etching of the unprotected surface and bake‐out of the polymerized film, a negative image is thus formed), and (ii) a depolymerization or a hydrophobicity/hydrophilicity change (that leads to solubilization of the illuminated areas, thereby forming a positive image).
Scheme 1.3 Different processes in imaging area.
Free radical photopolymerization (FRP) and photo‐cross‐linking (Scheme 1.4) lead to a negative image through (1). The acid‐ or base‐catalyzed reaction (Scheme 1.4) leads to either a negative image (1) or a positive image (2). FRP and photo‐cross‐linking lead to a negative image through (1). The acid‐ or base‐catalyzed reaction leads to either a negative image (1) or a positive image (2).
Scheme 1.4 Chemical mechanisms.
In microelectronics, such a monomer/oligomer or polymer matrix sensitive to a light source is named a photoresist. In the imaging technology, the organic matrix is called a photopolymer. Strictly speaking, a photopolymer refers to a polymer sensitive to light, but this word is often used to design a monomer/oligomer matrix that polymerizes under light exposure. The term “photomaterial” refers to an organic photosensitive matrix that leads, upon irradiation, to a polymer material exhibiting specific properties useful in the nanotechnology field; it could also design the final material formed through this photochemical route.
1.1.8 Controlled Photopolymerization
Conventional photopolymerization reactions lead to a dead polymer. On the opposite, a controlled photopolymerization is a living reaction where a formed polymer chain (considered as a dormant species) can be reactivated upon light exposure in order to start a novel polymerization (Scheme 1.5).
Scheme 1.5 Light activation of dormant species.
1.2 Photopolymerization Reactions
1.2.1 Monomers and Oligomers in Photopolymerization Reactions
Details can be found in Refs. [1–41]. The following sections only focus on the examples of usual and representative monomers/oligomers used in the photopolymerization area as well as some considerations on the reactions where they are involved.
1.2.1.1 Monomers/Oligomers in Radical Photopolymerization
Typical monomers and oligomers are based on acrylates and methacrylates such as trimethylolpropane triacrylate (TMPTA), 2‐hydroxy ethyl methacrylate (HEMA), 2‐hydroxyethyl acrylate (HEA), hexanediol diacrylate (HDDA), tetrapropyleneglycol diacrylate (TPGDA), 2‐(dimethyl‐amino)ethyl methacrylate (DMAEMA), poly(ethylene glycol)methyl ether methacrylate (PEGMA), poly(ethylene glycol)diacrylate (PEGDA), triethyleneglycol dimethacrylate (TEGDMA), epoxy diacrylate oligomer diluted with 25% of tripropyleneglycol diacrylate monomer (EpAc), bisphenol A‐glycidyl methacrylate (Bis‐GMA), TEGDMA, and so on (see e.g. (1.7)). They possess sometimes a strong odor and exhibit skin irritating properties. Unsaturated polyester resins dissolved in styrene were well known in the curing of glass‐reinforced materials. Owing to the current pressure of new environmental regulations, the interest for solventless systems, e.g. water‐soluble media, emulsions, dispersions, and increases; reactive monomer/oligomer cross‐linkers can be added (Chapter 17). Many novel matrices are continuously proposed. A growing interest is noted in the use of new starting monomer structures for getting novel end use properties, low‐toxicity monomers, renewable monomers, and reworkable materials.
The FRP of multifunctional monomers and oligomers in film is very fast (in the one second time range under high‐power lamps in industrial conditions) and generally sensitive to the presence of oxygen. It presents a relatively slow post‐effect. Under continuous laser sources, the exposure time can drop down to 1 ms or less. The role of the photoinitiator is important for getting a suitable photosensitivity of the formulations under UV to NIR lights, LED exposures, and air (Chapters 3, 7, 10, 11, 13, and 15). Oxygen inhibition significantly affects the top surface layer and the network structure in terms of physical properties. Photopolymerization of thick clear coatings (centimeter range or several tens of centimeter as can be seen in Chapter 18) can be achieved, provided a suitable photoinitiating system is used.
These radical polymerizations can be carried out not only in traditional reactions in film as mentioned above but also in many other experimental conditions such as in the following examples. Polymerization reactions of acrylates under red/NIR lights are accompanied by the production of heat that likely ensures an acceleration of the monomer conversion and gives the opportunity to get an efficient in‐depth curing (see Chapter 18). Using a photochemical process, the polymerization of (meth)acrylate‐based self‐assembled systems allowing fast initiation rates at room temperature was developed (see Chapter 17). Acrylate‐functionalized polyesters or unsaturated polyester/acrylic functional polyurethane have been considered attractive compared to liquid finishes in photopolymerization of powder formulations. Cross‐linked copolymers can be formed as interpenetrating polymer networks (IPNs) combining the properties of two polymer backbones (e.g. acrylate/epoxide and acrylate/vinylether) using the hybrid cure technique (see Chapter 17). Radical reactions involving acrylates or methacrylates are largely encountered in controlled or living radical photopolymerizations (Chapter 16) and are highly interesting as the activation and reactivation arise under exposure to a light source (on/off procedure) at room temperature. Acrylates are also involved in hybrid sol–gel photopolymerization (Chapter 17). Photoactivated redox polymerization of acrylate‐ or methacrylate‐based monomer structures is emerging as an elegant solution to combine the top surface efficiency and temporal control of a photopolymerization process with the interstitial robustness of a redox polymerization (Chapter 17). Radical matrices (basedon acrylates) are also largely used in two‐photon absorption‐induced polymerization (Chapters 13 and 19) as well as in dual‐cure photopolymerization reactions (pregelification followed by complete curing, surface cure followed by body cure, UV irradiation, and thermal drying or UV curing followed by air drying).
1.2.1.2 Monomers/Oligomers in Cationic Photopolymerization
The range of available cationic monomers and oligomers, e.g. (1.8), has been largely expanded. THF, lactones, acetals, and cyclohexene oxide (CHO) are examples of monofunctional monomers. Epoxides, e.g. 3,4‐epoxycyclohexane methyl 3,4‐epoxycyclohexyl carboxylate (EPOX), bisphenol A diglycidyl ether (DEGBA), and vinyl ethers, e.g. triethylene glycol divinyl ether (DVE‐3) and N‐vinylcarbazole (NVC), are very widely used and are known to give coatings with high thermal capability, excellent adhesion, good chemical resistance, and environmentally friendly characteristic. Vinyl ethers polymerize faster than epoxides (e.g. cycloaliphatic diepoxide). Cyclic ethers such as oxetanes can be alternatives to epoxides for getting faster curing speeds in industrial lines. A proper selection of novel cationic monomers, organic–inorganic hybrid resins, modified usual cationic monomers, renewable monomers (e.g. (1.9)), or dual‐cure formulations helps in the design of high‐performance systems (see Chapters 5 and 19).
The cationic photopolymerization (CP) in a film is fast, usually insensitive to oxygen (except in free radical promoted cationic photopolymerization (FRPCP) where novel photoinitiating systems have to be used to overcome this drawback; see Chapter 12), and sensitive to moisture and water. An important dark post‐effect is noted (see Chapter 5). The molecular origin of the induction period is discussed in Ref. [46].
Cationic monomers are also used in living/controlled polymerizations, IPN synthesis, sol–gel polymerizations, and two‐photon polymerizations (Chapters 16, 17, and 19). UV deblockable acid releasing systems (known as photolatent systems) allow to start a cross‐linking reaction (of epoxide groups) on demand as the generation of the acid is triggered by light exposure (Chapter 6).
1.2.1.3 Monomers in Thiol–ene Photopolymerization
A lot of olefins (allyl ethers, vinyl ethers, allylic urethanes, ureas and phosphazenes, vinyl‐functionalized silicones, norbornenes, etc.) and di‐, tri‐, and tetra‐functional thiols have been proposed. Examples are shown in (1.10).
Thiol–ene photopolymerization shows some interesting features: very fast process, low or even no oxygen inhibition effect, and formation of highly cross‐linked networks with good adhesion, reduced shrinkage and stress, and improved physical and mechanical properties. The reaction refers to a step‐growth addition of a thiol to the double bond (vinyl, allyl, acrylate, methacrylate, etc.). The recent huge developments in thiol–ene and related thiol–ene chemistries are presented in Chapter 17.
1.2.1.4 Monomers in Charge Transfer Photopolymerization
Maleimides (absorption around 300–350) and vinyl ethers (absorption below 250 nm) are representative monomers (1.11). A photoinitiator‐free charge transfer polymerization (1.12) refers to a reaction between a monomer containing an electron‐poor moiety A and a monomer bearing an electron‐rich moiety D that forms a charge transfer complex CTC whose absorption is red shifted (Chapters 4 and 17). Such CTCs absorb above 350 nm. A photoinitiator can also be added (homopolymerization and cross‐propagation occur).
