Photopolymerization initiators

Finally, we should indicate that we have not ruled out the possibility that there is a contribution to initiating photopolymerization in maleimide/vinyl ether systems from an exciplex type complex between an excited state maleimide and ground state vinyl ether. A biradical formed from such a complex might initiate free radical polymerization in lieu of cyclization to form a 2 + 2 adduct. However, we note that at present we have no evidence for such a reactive exciplex. 

It is interesting that there is little change in the time required to cure the polymer as the concentration of BP is increased from 0.2 to 1.0 wt.%. These results suggest that the reaction proceeds to completion even before the light penetrates deep into the sample, perhaps by the propagation of a thermal front created by the initial photopolymerization at the leading surface. In any case, these results illustrate that the addition of a thermal initiator drastically reduces the polymerization time for thick systems. 

General Electric Co.) were added to the acrylic and epoxy monomers, respectively, to initiate photopolymerization. The acrylic monomers that polymerized by a free-radical mechanism were irradiated under nitrogen. The output of the lamp measured at the substrate was about 4 mW/cm2 at 260 nm and about 11 mW/cm2 at 310 nm. 

It was found that conventional equation can be applied for description of the process. The process is the first order with respect to monomer and the order with respect to initiator is 0.25. The energy of activation was estimated at 43 kJ/mol. Moreover, it was found that without initiator, photopolymerization also occurs when the system is illuminated by UV light. 

Initiator systems used to initiate photopolymerization with UV light generate radicals mostly by homolytic dissociation. Ketone acetals (e.g., Irgacure 651) absorb in the mid-range UV, dissociating to a benzoyl radical which initiates acrylate polymerization. The benzoyl group remains as part of the formed polymer (Eqs. 14 and 15). 

Just as the oxidizing power of a dye increases on excitation, so does the reducing power. However, examples of the use of dye excited states to initiate photopolymerization via dye photooxidation are less common than photoreducible dye sensitization. 

Multiphoton microscopy has been used to initiate photopolymerization of a photoresist material in three dimensions with resolution in the hundred-nanometer range. 

We found that all samples prepared were effective in initiating photopolymerization, and on returning from Europe, I submitted samples of several of these new HABIs to Photo Products Department for evaluation, but no response was received. Of course, it is difficult to develop much enthusiasm for incorporating yet another new material into existing coating recipes, as this would entail a long round of testing, examination, etc. 

A 3D optical memory device can be based on different principles. Among them are the write once and read many times principle, based on TP initiated photopolymerization resulting in a photopolymer structure [242] or photo-bleaching of a fluorescent material [250]. The rewritable principle is based mainly on TP induced molecular change. This TP induced molecular change can be an isomerization reaction in the writing process and OP induced fluorescence in the reading process [119, 144, 247, 258],... 

This review describes some of the recent developments in materials which exhibit enhanced two-photon absorption that can initiate photopolymerization or up-converted emission. Various optical methods including femtosecond time-resolved pump-probe experiments to characterize the two-photon properties are discussed. Finally, the applications of two-photon processes to optical power limiting, up-converted lasing, 3-D data storage, 3-D micro-fabrication, two-photon fluorescence microscopy and bio-imaging, and two-photon photodynamic therapy are presented. 

Photoinitiated Addition Polymerization.—The role of carbonyl compounds in initiating photopolymerization continues to be an area of prolific research. Laser flash photolysis has provided a valuable insight into both the photophysical and photochemical processes involved in carbonyl-initiated photopolymerizations. For example, Fouassier and co-workers have found that carbonyl-initiated vinyl polymerization proceeds faster in a micellar system and an example of this interesting effect is shown in Table 1. Laser flash photolysis has shown that... 

Essentially electron-donor substituents retarded polymerization, whereas electron-acceptor substituents accelerated the process. In fact, these workers obtained a good correlation between the Hammett a values of the substituents and the polymerization rate. Remaining with benzophenone, the photopolymerization of 1,3,5-trithiane has been found to be inhibited by amines and also the absence of oxygen,whereas in the hydrogen peroxide-initiated photopolymerization of methyl methacrylate-benzophenone has been found to be a powerful accelerator. In the latter study different solvents were found to have different effects on initiation. In solvents giving low conversions, degradative initiator transfer was found to be a dominant process. 

Abstraction of hydrogen by the excited state molecules makes possible the generation of radicals from molecules with low excitation energies, which effectively extends photoionization that can initiate photopolymerization of reactive monomers into the visible part of the spectrum. Examples of H-abstraction reactions... 

Many aromatic ketones are even more efficient in initiating photopolymerizations as a result of photoreductions in the triplet state. The mechanism of photoreduction is now conunonly accepted to be one of electron transfer. Upon irradiation, one of the n, nonbonding, electrons of the oxygen atom is promoted into either a a or n antibonding orbital. These are n- <7 and n- r transitions. They are the most readily observed transitions of carbonyl compounds, requiring less energy than n- n transitions, and can be illustrated as follows" ... 

This initiating system is reported as being capable of initiating photopolymerizations of thiek pigmented eoatings, useable as paints in wood furniture industry. 

Gosh and Mukherjee reported that monochloroacetic, dichloroacetic and trichloroacetic acids readily initiate photopolymerization of methyl methacrylate at 40 °C in visible light when used in the presence of dimethylaniline. The inhibition period decreases with an increase in the number of chlorine substitution of acetic acid. Addition of benzophenone decreases the induction period. They believe that free-radical initiation is a result of a collapse of the exiplex (acid-dimethylaniline) and an electron transfer from the nitrogen to the carbonyl oxygen of the acid. ... 

LUO 02a] Luo N., Hutchison J.B., Anseth K.S. et al., Surface-Initiated Photopolymerization of Poly(ethylene glycol) Methyl Ether Methacrylate on a Diethyldithiocarbamate-Mediated Polymer Substrate , Macromolecules, vol. 35, pp. 2487-2493, 2002,... 

Ma, D., Chen, H., Shi, D., Li, Z., Wang, J. (2009). Preparation and characterization of thermo-responsive PDMS surfaces grafted with poly(A -isopropylacrylamide) by benzo-phenone-initiated photopolymerization. Journai of Coiioid and Interface Science, 332, 85-90. 

Gam-Derouich, S., S. Mahouche-Chergui, M. Turmine, J.-Y. J.-Y. Piquemal, D. Ben Hassen-Chehimi, M. Omastova, and M. M. Chehimi. A versatile route for surface modification of carbon, metals and semi-conductors by diazonium salt-initiated photopolymerization. Suif. Sci. 605, 2011 1889-1899. 

Hume, P. S., Anseth, K. S., Inducing Local T Cell Apostosis with Anti-Fas-FimctionaUzed Polymer Coatings Fabricated via Surface-Initiated Photopolymerizations, Biom eri 2010,31, 3166-3174. 

Photosensitizers - lsen(t)-so- tIz- n (ca. 1923) A substance which, by absorbing light, passes its energy to another substance which then reacts. (Eouassier J-P (1995) Photo initiation, photopolymerization and photocuring. Hanser-Gardner, New York)... 

Papavasiliou, G., Songprawat, R, Perez-Luna, V., Hammes, E., Morris, M., Chiu, Y. C., and Brey, E. 2008. Three-dimensional pattering of poly (ethylene Glycol) hydrogels through surface-initiated photopolymerization. Tissue Eng C Methods, 14,129-40. 

UV irradiation of 44 (Figure 15) readily initiates photopolymerization of its acrylic pendants. Development of the exposed films gives well-defined photoresist patterns. The patterned lines glow under UV illumination, because the poly... 

Fig. 10 Schematic illustration for the strategy of hydrogel-cartilage integration by tissue-initiated photopolymerization. Individual steps of the process are (1) clearance of the proteoglycans in cartilage by chondroitinase to expose the collagen network (2) in situ generation of tyrosyl radicals by photo-oxidation of tyrosine residues on collagen with H2O2 under low intensity UV irradiation and (3) introduction of a macromer solution and in situ photo-gelation via tyrosyl radical initiation and UV-excitation (Reprinted with permission Advanced Functional Materials [321])...

Acylphosphine oxide and acylphosphonates with different structures have been used as photoinitiators for free-radical-initiated photopolymerization (see Table 6). 

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