Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency

Photoinitiating systems for polymerization reactions are largely encountered in a variety of traditional and high–tech sectors, such as radiation curing, (laser) imaging, (micro)electronics, optics, and medicine. This book extensively covers radical and nonradical photoinitiating systems and is divided into four parts: Basic principles in photopolymerization reactions Radical photoinitiating systems Nonradical photoinitiating systems Reactivity of the photoinitiating system The four parts present the basic concepts of photopolymerization reactions, review all of the available photoinitiating systems and deliver a thorough description of the encountered mechanisms. A large amount of experimental and theoretical data has been collected herein. This book allows the reader to gain a clear understanding by providing a general discussion of the photochemistry and chemistry involved. The most recent and exciting developments, as well as the promising prospects for new applications, are outlined.

Abbreviations XIX Introduction XXV Part I Basic Principles and Applications of Photopolymerization Reactions 1 1 Photopolymerization and Photo–Cross–Linking 3 References 6 2 Light Sources 11 2.1 Electromagnetic Radiation 11 2.2 Characteristics of a Light Source 12 2.3 Conventional and Unconventional Light Sources 13 References 20 3 Experimental Devices and Examples of Applications 21 3.1 UV Curing Area: Coatings, Inks, Varnishes, Paints, and Adhesives 21 3.2 Conventional Printing Plates 25 3.3 Manufacture of Objects and Composites 25 3.4 Stereolithography 25 3.5 Applications in Microelectronics 26 3.6 Laser Direct Imaging 26 3.7 Computer–to–Plate Technology 27 3.8 Holography 27 3.9 Optics 28 3.10 Medical Applications 28 3.11 Fabrication of Nano–Objects through a Two–Photon Absorption Polymerization 29 3.12 Photopolymerization Using Near–Field Optical Techniques 29 3.13 Search for New Properties and New End Uses 30 3.14 Photopolymerization and Nanotechnology 32 3.15 Search for a Green Chemistry 33 References 34 4 Photopolymerization Reactions 41 4.1 Encountered Reactions, Media, and Experimental Conditions 41 4.2 Typical Characteristics of Selected Photopolymerization Reactions 45 4.3 Two–Photon Absorption–Induced Polymerization 60 4.4 Remote Curing: Photopolymerization without Light 60 4.5 Photoactivated Hydrosilylation Reactions 61 References 61 5 Photosensitive Systems 73 5.1 General Properties 73 5.2 Absorption of Light by a Molecule 74 5.3 Jablonski’s Diagram 78 5.4 Kinetics of the Excited State Processes 78 5.5 Photoinitiator and Photosensitizer 80 5.6 Absorption of a Photosensitive System 81 5.7 Initiation Step of a Photoinduced Polymerization 82 5.8 Reactivity of a Photosensitive System 86 References 87 6 Approach of the Photochemical and Chemical Reactivity 89 6.1 Analysis of the Excited–State Processes 89 6.2 Quantum Mechanical Calculations 93 6.3 Cleavage Process 94 6.4 Hydrogen Transfer Processes 95 6.5 Energy Transfer 96 6.6 Reactivity of Radicals 98 References 99 7 Efficiency of a Photopolymerization Reaction 103 7.1 Kinetic Laws 103 7.2 Monitoring the Photopolymerization Reaction 109 7.3 Efficiency versus Reactivity 111 7.4 Absorption of Light by a Pigment 112 7.5 Oxygen Inhibition 114 7.6 Absorption of Light Stabilizers 115 7.7 Role of the Environment 117 References 118 Part II Radical Photoinitiating Systems 123 8 One–Component Photoinitiating Systems 127 8.1 Benzoyl–Chromophore–Based Photoinitiators 127 8.2 Substituted Benzoyl–Chromophore–Based Photoinitiators 148 8.3 Hydroxy Alkyl Heterocyclic Ketones 157 8.4 Hydroxy Alkyl Conjugated Ketones 158 8.5 Benzophenone– and Thioxanthone–Moiety–Based Cleavable Systems 158 8.6 Benzoyl Phosphine Oxide Derivatives 161 8.7 Phosphine Oxide Derivatives 165 8.8 Trichloromethyl Triazines 165 8.9 Biradical–Generating Ketones 166 8.10 Peroxides 166 8.11 Diketones 167 8.12 Azides and Aromatic Bis–Azides 168 8.13 Azo Derivatives 168 8.14 Disulfide Derivatives 168 8.15 Disilane Derivatives 169 8.16 Diselenide and Diphenylditelluride Derivatives 170 8.17 Digermane and Distannane Derivatives 170 8.18 Carbon–Germanium Cleavable–Bond–Based Derivatives 170 8.19 Carbon–Silicon and Germanium–Silicon Cleavable–Bond–Based Derivatives 172 8.20 Silicon Chemistry and Conventional Cleavable Photoinitiators 172 8.21 Sulfur–Carbon Cleavable–Bond–Based Derivatives 173 8.22 Sulfur–Silicon Cleavable–Bond–Based Derivatives 173 8.23 Peresters 173 8.24 Barton’s Ester Derivatives 174 8.25 Hydroxamic and Thiohydroxamic Acids and Esters 174 8.26 Organoborates 176 8.27 Organometallic Compounds 176 8.28 Metal Salts and Metallic Salt Complexes 178 8.29 Metal–Releasing Compound 178 8.30 Cleavable Photoinitiators in Living Polymerization 179 8.31 Oxyamines 183 8.32 Cleavable Photoinitiators for Two–Photon Absorption 184 8.33 Nanoparticle–Formation–Mediated Cleavable Photoinitiators 185 8.34 Miscellaneous Systems 185 8.35 Tentatively Explored UV–Light–Cleavable Bonds 185 References 187 9 Two–Component Photoinitiating Systems 199 9.1 Ketone–/Hydrogen–Donor–Based Systems 199 9.2 Dye–Based Systems 238 9.3 Other Type II Photoinitiating Systems 241 References 258 10 Multicomponent Photoinitiating Systems 269 10.1 Generally Encountered Mechanism 269 10.2 Other Mechanisms 271 10.3 Type II Photoinitiator/Silane: Search for New Properties 279 10.4 Miscellaneous Multicomponent Systems 281 References 281 11 Other Photoinitiating Systems 283 11.1 Photoinitiator–Free Systems or Self–Initiating Monomers 283 11.2 Semiconductor Nanoparticles 284 11.3 Self–Assembled Photoinitiator Monolayers 284 References 285 Part III Nonradical Photoinitiating Systems 287 12 Cationic Photoinitiating Systems 289 12.1 Diazonium Salts 289 12.2 Onium Salts 289 12.3 Organometallic Derivatives 308 12.4 Onium Salt/Photosensitizer Systems 311 12.5 Free–Radical–Promoted Cationic Photopolymerization 318 12.6 Miscellaneous Systems 332 12.7 Photosensitive Systems for Living Cationic Polymerization 332 12.8 Photosensitive Systems for Hybrid Cure 333 References 333 13 Anionic Photoinitiators 343 13.1 Inorganic Complexes 343 13.2 Organometallic Complexes 343 13.3 Cyano Derivative/Amine System 344 13.4 Photosensitive Systems for Living Anionic Polymerization 344 References 345 14 Photoacid Generators (PAG) Systems 347 14.1 Iminosulfonates and Oximesulfonates 347 14.2 Naphthalimides 348 14.3 Photoacids and Chemical Amplification 349 References 349 15 Photobase Generators (PBG) Systems 351 15.1 Oxime Esters 351 15.2 Carbamates 351 15.3 Ammonium Tetraorganyl Borate Salts 351 15.4 N–Benzylated–Structure–Based Photobases 352 15.5 Other Miscellaneous Systems 353 15.6 Photobases and Base Proliferation Processes 354 References 354 Part IV Reactivity of the Photoinitiating System 357 16 Role of the Experimental Conditions in the Performance of a Radical Photoinitiator 359 16.1 Role of Viscosity 360 16.2 Role of the Surrounding Atmosphere 361 16.3 Role of the Light Intensity 361 References 364 17 Reactivity and Efficiency of Radical Photoinitiators 367 17.1 Relative Efficiency of Photoinitiators 367 17.2 Role of the Excited–State Reactivity 377 17.3 Role of the Medium on the Photoinitiator Reactivity 381 17.4 Structure/Property Relationships in Photoinitiating Systems 388 References 394 18 Reactivity of Radicals toward Oxygen, Hydrogen Donors, Monomers, and Additives: Understanding and Discussion 399 18.1 Alkyl and Related Carbon–Centered Radicals 399 18.2 Aryl Radicals 401 18.3 Benzoyl Radicals 402 18.4 Acrylate and Methacrylate Radicals 403 18.5 Aminoalkyl Radicals 404 18.6 Phosphorus–Centered Radicals 412 18.7 Thiyl Radicals 413 18.8 Sulfonyl and Sulfonyloxy Radicals 417 18.9 Silyl Radicals 418 18.10 Oxyl Radicals 425 18.11 Peroxyl Radicals 426 18.12 Aminyl Radicals 431 18.13 Germyl and Stannyl Radicals 432 18.14 Boryl Radicals 435 18.15 Lophyl Radicals 439 18.16 Iminyl Radicals 439 18.17 Metal–Centered Radicals 440 18.18 Propagating Radicals 442 18.19 Radicals in Controlled Photopolymerization Reactions 443 18.20 Radicals in Hydrosilylation Reactions 446 References 447 19 Reactivity of Radicals: Towards the Oxidation Process 455 19.1 Reactivity of Radicals toward Metal Salts 455 19.2 Radical/Onium Salt Reactivity in Free–Radical–Promoted Cationic Photopolymerization 456 References 459 Conclusion 461 Index 465

Jean–Pierre Fouasssie r is Professor Emeritus of Physical Chemistry and Photochemistry at the University of Haute Alsace (UHA) in Mulhouse, France. He received his Ph.D at the University of Strasbourg, France, in 1975 and became a full Professor in 1978. From 1980 –1993 he was the Head of the Laboratory of General Photochemistry at CNRS–URA 431, Mulhouse, and then Head of the Molecular Photochemistry and Photopolymerization team at the Department of Photochemistry at UHA (1994–2010). He was the Director of the Ecole Nationale Supérieure de Chimie de Mulhouse (1993–1998), a Member of the National Committee of CNRS (1991–1995), Member of the National Committee of University (1989–2009), French Representative at the Steering Committee of the European Photochemistry Association (EPA, 1992–2000) and Coordinator of the Academic Committee of RadTech Europe (1988–2001). His research interests include time–resolved laser spectroscopy, photopolymerization reactions, as well as photosensitive systems for UV curing and imaging. He has authored 500 research articles, 32 book chapters as well as several books. Jacques Lalevée received his Ph.D at the University of Mulhouse, France, in 2002. From 2004–2009 he was Associate Professor in Physical Chemistry and became a full Professor of Physical Chemistry at Ecole Nationale Supérieure de Chimie at University of Haute Alsace, Mulhouse, France, in 2009. Jacques Lalevée was a Member of the National Committee of University (2007–2009) and has authored more than 90 research articles. His research is focused on free radical chemistry, photochemistry and photopolymerization reactions, electron spin resonance (ESR), and time–resolved laser spectroscopy.