Photoinitiators-benzoin ethers

If the rate of addition to monomer is low, primary radical termination may achieve greater importance. For example, in photoinitiation by the benzoin ether 12 both a fast initiating species (13, high k) and a slow initiating species (14, low... 

The chemistry involved in LfV-curable resin systems has been extensively investigated and thoroughly surveyed [88-94]. LfV-radiation polymerization, is in principle, completely analogous to the conventional addition polymerization. A photoinitiator is used in UV polymerization. Its function is the same as the free-radical initiator. A conventional initiator possesses a thermally labile bond which is cleaved to form free-radical species, but the photoinitiator has a bond which breaks upon absorption of radiant energy. Benzoin ethers, benzyldialkyl ketals, benzophenone, and acetophenone derivatives are the important LfV-photoinitiators [95-99]. 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

The chromophore in this type of photoinitiator is frequently an aromatic carbonyl. The benzoyl radical is the major initiating species, while the other fragment may also contribute to the initiation, in some cases. The most efficient type I initiators are benzoin ether derivatives, benzil ketals, hydroxyl-alkylphenones, a-aminoketones, and acylphosphine oxides. Substituents on the aromatic carbonyl influence the absorption. 

Based on the known photoreduction chemistry of Rose Bengal [275], one would anticipate that electron transfer would reduce the xanthene skeleton of RBAX and that the radical anion thence formed might decay by the elimination of an acetyl radical. Acetyl is totally analogous to benzoyl, the radical that initiates chains in the case of most Norrish type I UV photoinitiators, that is, benzoin ethers or acetophenone acetals. The putative scheme is shown in Scheme 7. 

Benzoic ethers, dioxolane, and sulfur derivatives are photoinitiators which undergo photochemical dissociation reactions into pairs of radicals. The polymerization of vinyl (monomer) into the polymers of the polyvinyl series can be photoinitiated by an ammonium salt of a benzoin ether derivative. In this case the active species is the benzoyl radical (Figure 6.11). 

Polymeric systems containing side-chain benzoin moieties, recently appearing [100,102] in the patent literature, have been applied as photoinitiators to UV curing of prepolymers of a different nature. In particular, siloxane-compatible photocrosslinking initiators (PSBME) are manufactured [100] by reacting silox-anes with benzoin ether derivatives (Scheme 23) and used in UV curing of acryloxypropyl-terminated dimethylsiloxane prepolymers. 

Our interest in the photochemistry of benzoin ethers was prompted by two major reasons (1) a discrepancy existed between recent mechanistic studies on the photochemistry of benzoin ethers and earlier reports on the benzoin ether photo-initiated polymerization of reactive monomers, and (2) the extensive, commercial utilization of benzoin ethers as photoinitiators in uv curable coatings and printing inks warranted further investigation of this discrepancy. 

UvlmerTM 530 and the other members of the UvlmerTM series may be cured with benzoin ether type photoinitiators and with diethoxy acetophenone. Table III presents data on the cure speed of UvlmerTM 530 formulated with butyl benzoin ether. These data were obtained in air without the use of Inert blanketing. Films of 3 mil thickness were irradiated on a moving belt using a 5000 watt Addalux uv lamp (Berkey Photo, Inc.) focussed on one linear Inch, This Is a low pressure lamp Incorporated In an experimental unit. Faster cure speeds (up to 200 ft. per mlnute/lamp) were obtained in later studies with a unit employing two medium pressure, 200 watts/llnear inch Hanovia lamps. 

Photoinitiation System. In most cases the curing is initiated by free radicals generated by UV light and photoinitiators. The free radicals can be formed from photocleavage or hydrogen abstraction mechanisms. Benzoin ethers, benzil ketals, and acetophenone derivatives are typical photoinitia,tors. 

Benzoin Ethers Benzoin ethers were the most widely used photoinitiators for a long time [13-15]. The cleavage process in the triplet state (10.8) is very fast compared to that of some related structures (>1010 s ). The cleavage yield is almost unity for 2,2-dimethoxy—2 phenyl-acetophenone DMPA or 2-methoxy -2 phenyl acetophenone BME. A secondary photochemical cleavage (the possible thermal cleavage is less efficient) can arise in the dimethoxy benzyl radical and leads to a methyl R" radical and an alkyl benzoate. 

Photoinitiators for Multiphotonic Absorption Multiphoton three-dimensional micro- and nanofabrication is an emerging technology where a laser beam activates the medium by a multiphoton excitation of the photoinitiator. Conventional one-photon UV sensitive cleavable photoinitiators such as bis-acyl phosphine oxides or benzoin ethers can be activated at 700-900 nm (which is thought to lead to the generation of the same usual initiating radicals) but their two-photon absorption cross sections are rather low. New developments are under way [106-111]. 

Vicure . [Akzo] Benzoin ether < r methyl phenylglyoxylate photo sensitizer ex photoinitiator ftnr uv-curable systems. 

Escacure . [Sartomer] Benzyl ketal or benzoin ethers photoinitiators for composites, coatings, inks, photopolymers, electrcmic photoresists. 

Photoinitiation of Addition Polymerization.—Much of the research reported in this section has arisen from commercial and environmental needs for surface coatings which do not rely on polymerization in solvent media. One approach to this problem has been to initiate the addition polymerization of monomer/ oligomer/pigment formulations with u.v. light and suitable photoinitiators. The initiators most commonly used are benzophenone, Michler s ketone (1), and benzoin ethers, but the search for other efficient photoinitiators continues. 

Other acetophenone derivatives that have been proposed include 1-benzo-cylcohexanol, 4-methylmercapto-a,a-dimethyl-morpholino-acetophenone, which has a much stronger absorption at 365 nm than other benzoin ethers (s lO L/ mole cm) and is therefore suitable as a photoinitiator for highly pigmented coat-... 

Photoinitiators that produce radicals by intramolecular photocleavage (e.g., benzoin ethers). 

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