Benzoin ether

Benzoin ethers (e.g., benzoin isopropyl, benzoin isobutyl)... 

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 benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

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). 

The extent of cage recombination is of very practical importance, since benzoin ethers are among the most common initiators in several commercial photo-initiated polymerizations 52K The excited states of these benzoin ethers are so reactive that they undergo no measurable bimolecular reactions. 

The reactions of benzoin ether-onium salt systems are illustrated in Scheme 12. All expected products have been found. The quantitative formation of diketones also in presence of onium salts indicates that the electrophilic benzoyl radical does not efficiently react with RnX+. In consequence of the recombination of acyl and... 

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

Radicals can be produced by ultraviolet irradiation of a monomer like styrene which absorbs sufficiently strongly in this wavelength region. Photochemical initiation may also be provided by thermal initiators or by compounds like benzoin ethers ... 

Photolysis of benzoin and benzoin methyl ether in the presence ctf the phenoxynitrone gave rise to e.sj. signals from both phenoxy and nitroxide radicals. Hydrogen abstraction by benzoin and benzoin ethers would yield radicals of the type... 

All the members of this class of compounds contain an aromatic ketone group, the most representative systems being benzoin [71], benzoin ethers [72], dialkoxy-acetophenones [73], hydroxyalkylphenones [74], benzoyloxime esters [75] and, more recently, benzoylphosphine oxides [76], a-hydroxymethylbenzoin sulfonic esters [77], sulfonyl ketones [78] and morpholino ketones [70],... 

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. 

One way of minimizing the detrimental effect caused by chain transfer on decreasing molecular weight and crosslink density of UV curable coatings has been successfully applied [115] by using copolymerizable methacrylic monomers containing the tertiary amine moiety as components of reactive acrylic formulations in the presence of benzoin ether derivatives. 

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. 

The photochemistry of benzoin ethers (a-alkoxy-a-phenyl-acetophenones) has been examined in considerable detail, recently, by quenching, sensitization (, CIDNP (, and radical scavenging studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This abscission is not retarded by conventional triplet... 

Initiator activities (a) together with average values for polymer activities (A) are presented in Table II. In marked contrast to the earlier reports (10. 11). it is seen that polymer activities are less than that of the corresponding initiator in each case. Unfortunately, we can offer no rationale for this discrepancy. However, the present data are readily interpreted in terms of benzoin ether piho to cleavage into benzoyl and benzyl ether radicals (eqn 1), which initiate MMA polymerization. The incorporation data may be analyzed by eqns 2-4, wherein ai, and a3 refer to the specific activities of initiators BEi, BE2 and BE3, respectively, Ai, Aj and A3 represent the corresponding polymer activities, while Band E are the number... 

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. 

Several studies of a related nature have appeared on the photocrosslinking of polyethylene by benzophenone identifying benzopinacol as the main product by FT NMR analysis.Benzoin ethers have been used for the same reaction while in polypropylene little damage was found in terms of the morphological structure of the polymer. ... 

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