Photopolymerisation Initiating Systems

Jacques Lalevée, Jean-Pierre Fouassier

• Publisher: Royal Society of Chemistry
• Year: 2018
• eBook ISBN: 9781788014946
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Physics
• Index: Physics

Photoinitiating systems play a key role in the starting point of a polymerization reaction under exposure to a UV or a visible light. The number of publications discussing photoinitiating systems for polymerization has seen a significant growth in recent years and this book provides an update on their latest research developments.

The book covers different types of photoinitiating systems including UV radical photoinitiators, long wavelength sensitive radical photoinitiators, cationic photoinitiators and water soluble photoinitiators as well as a chapter on how to design novel photoinitiators. The book then focusses on the applications of the photoinitiators from nanoparticles and materials to ionic liquids and solar cells.

Edited by leading names in the field, the book is suitable for postgraduate students and researchers in academia and industry interested in polymer chemistry, organic chemistry, materials science and the applications of the materials.

CHAPTER 1

Thioxanthone Photoinitiators with Heterocyclic Extended Chromophores

NURCAN KARACA^a,b, NUKET OCAL^a, NERGIS ARSU^*a AND STEFFEN JOCKUSCH^*c

^a Department of Chemistry, Yildiz Technical University, Davutpasa Campus, Istanbul 34210, Turkey;

^b Yalova University, Central Research Laboratory, Yalova 77200, Turkey;

^c Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, USA
*Email: [email protected]; [email protected]

1.1 Introduction

For many decades, photopolymerization has been the basis for commercial applications in coatings, adhesives, dental composites, inks, printing plates, and microelectronics.^1,2 The key ingredient in these photopolymerization formulations is the photoinitiator.³ A good match of the absorption spectrum of the photoinitiator with the light source is of major importance. In the past, photo-curing tools with light sources emitting in the near-UV spectral region (350–400 nm) have dominated. For that reason, most commercially used photoinitiators are tailored to the near-UV spectral region. For example, thioxanthone-based photoinitiators show some of the best light absorption properties in the near-UV spectral region and are therefore widely used.⁴ However, UV light penetration of pigmented coatings and composite materials is poor and can lead to incomplete curing, especially for thicker layers. Shifting the initiator absorbance and light source into the visible, blue spectral region has been shown to improve the light penetration problem. In addition, recent progress in the development and mass production of blue LEDs has made these low-cost, highly energy efficient light sources attractive in photo-curing tools, which has increased the demand for blue-light absorbing photoinitiators.

Bathochromatically shifted absorption of thioxanthone can be achieved with appropriate substituents. Table 1.1 lists such examples, including thioxanthone derivatives, that have been used as photoinitiators for polymerization. Only small bathochromic shifts were observed for most substituents. Dibromination (6) shifts the absorption by ∼15 nm while the triplet energy and photoreactivity remains high.⁵ A much larger bathochromic shift was observed with amino substituents (7). However, amino substitution alters the thioxanthone photoreactivity.

Table 1.1 Thioxanthone derivatives with electron-donating substituents: Absorption maximum at the longest wavelength peak (λ_max) and molar absorptivity (ε).

Thioxanthone derivative	λmax (nm)	ε at λmax (M^−1 cm^−1)
	380 (benzene)^6 381 (DMF)^5	6600 (benzene)^6 6328 (DMF)^5
	386 (benzene)^6	6900 (benzene)^6
	383 (THF)^7	3857 (THF)^7
	388 (benzene)^6	6700 (benzene)^6
	388 (DMF)^5	4941 (DMF)^5
	396 (DMF)^5	5234 (DMF)^5
	438 (THF)^8	4470 (THF)^8

In this chapter, we explore heterocyclic extended thioxanthone derivatives, which shift the absorption bathochromically more significantly than simple substitutions. The structures are shown in Scheme 1.1. We discuss their photophysical properties, photoreactivity and the mechanisms to initiate polymerization.

Scheme 1.1 Heterocyclic extended thioxanthones.

Heterocyclic extended thioxanthones are synthetically accessible by the condensation reaction of thiosalicylic acid with heterocyclic compounds such as benzothiophene, dibenzothiophene, carbazole and benzotriazole in the presence of concentrated H₂SO₄. Examples are shown in Scheme 1.2.^9–12

Scheme 1.2 Synthesis of heterocyclic extended thioxanthones.

1.2 Photophysical Properties of Heterocyclic Extended Thioxanthones

The absorption spectra of thioxanthone and heterocyclic extended thioxanthones are shown in Figure 1.1. The spectra reveal that the cyclic extension causes major bathochromic shifts of up to 100 nm in addition to large variations in molar absorptivities. Extension of thioxanthone with thiophene (8) has little impact on...

Table of contents

• Cover
• Title
• Copyright Page
• Preface
• Contents
• Chapter 1 Thioxanthone Photoinitiators with Heterocyclic Extended Chromophores
• Chapter 2 Long-wavelength-sensitive Radical Photoinitiators
• Chapter 3 Cationic Photoinitiators
• Chapter 4 Monomeric and Polymeric Photoinitiators
• Chapter 5 Photoinitiators for Blue to Red LED Exposures
• Chapter 6 How to Design Novel Photoinitiators for Blue Light
• Chapter 7 Photocatalysts as Photoinitiators
• Chapter 8 Controlled Reversible Deactivation Radical Photopolymerization
• Chapter 9 Photosynthesis of Polymeric Particles
• Chapter 10 Photoinitiators in Ionic Liquids
• Chapter 11 Photoinitiators in Dentistry: Challenges and Advances
• Chapter 12 ZnO Nanoparticle-based Photoinitiators
• Chapter 13 Water-soluble Photoinitiators: Present and Future
• Chapter 14 NIR Light for Initiation of Photopolymerization
• Chapter 15 D–π–A-type Sulfonium Salt Photoinitiators for Photopolymerizations Under Near-UV and Visible Light-emitting Diodes
• Chapter 16 Photopolymers for Third-generation Solar Cells
• Chapter 17 Photopolymerization of Amphiphilic Molecule Self-assemblies
• Chapter 18 Emulsion Photopolymerization
• Conclusion
• Subject Index