Benzophenone, as photoinitiator

Lapin [33] suggested that photoinitiated cationic polymerization can proceed through reactions of free radicals, as shown below for benzophenone sensitized photoinitiation ... 

The reaction between Cell—O radical and vinyl monomers leads to the formation of grafted cellulose. In the presence of photosensitizers generally used as photoinitiators, such as benzophenone and phenylace-tophenone derivatives, the photoinitiator absorbs the UV radiation and transforms to its singlet (S ) and then triplet (T ) states. After that it may decompose into free radicals or transfer its energy to cellulose or any other molecules in the system. Take benzophenone as an example ... 

Choi and co-workers [56] modified BC with cation-exchangeable acrylic acid units by UV graft polymerization. For this purpose, the BC was dried at 80 °C, immersed in methanol with benzophenone as a photoinitiator for... 

The possibility that the ether radical results from the collapse of an excited ether molecule generated through energy transfer from the excited carbonyl compound seems less probable. The isopropanol isolated from the reaction mixtures when acetone was used as photoinitiator, and the formation of considerable amounts of benzpinacol when benzophenone... 

Bradley [3] coated circuit boards with a reactive substrate as a method of activating a latent photoinitiator, (V), for subsequent crosslinkable reactions as illustrated below. Multi-layered circuits were also prepared by Kawasaki [4] using benzophenone as the photoinitiator. 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

Block, Ledwith and Taylor (46) have reported the results of a detailed investigation into the efficiencies of benzof enone, 3,3, 4,4 -benzo-phenone tetracarboxylic dianhydride (BTDA) and 3,3, 4,4 -tetramethoxy-carbonyl benzophenone (TMCB) as photoinitiators for the polymerization of methyl methacrylate in tetrahydrofuran at C in vacuo. 

The photo-induced polymerization of acrylonitrile, again in tetrahydrofuran solution, is greatly enhanced by the presence of benzophenone (46), which in this case exhibits a higher efficiency than TCMB. Both these compounds retarded the photoinduced polymerization of vinyl acetate in tetrahydrofuran, but did act as photoinitiators when toluene was used as solvent (46), possibly reflecting the differing abilities of the solvent-derived radicals to add to vinyl acetate. 

On the basis of the above described results, tailor-made benzophenone-containing polymeric photoinitiators have been studied in order to establish the effect of the relative distance from the polymer chain of the side-chain benzophenone groups. Thus, the homopolymer of Uvecryl P36 [poly(UP36)] and its copolymers with MtA [poly(UP36-co-MtA)s], bearing the benzophenone moiety at quite a large distance from the backbone, have been checked as photoinitiators in the polymerization of the HDDA/BA equimolar mixture against poly(ABP), poly(VBP) and poly(VBP-co-MtA)s [20-22]. 

One of the earliest examples of photodecomposable, but thermally stable, polymeric systems contains pendant perester substituted benzophenone moieties, the photodissociation of the former group being promoted by the benzophenone chromophore which behaves as a triplet sensitizer. Indeed, the homopolymer of 4-vinylbenzoyl-4 -tert-butyl perbenzoate and its copolymers with styrene [poly(VBPE) and poly(VBPE-co-St), respectively] have been prepared and checked as photoinitiators in the polymerization of styrene and MMA by irradiation at 366 nm [79,80]. 

Finally, diphenylsulfonium-bis-(methoxycarbonyl)methylide, IX [87], and salts of p,p -bis-[(triphenylphosphonio)methyl]-benzophenone, X [88], have been claimed as photoinitiators of free radical polymerizations [89]. They presumably photolyze homolytically in the case of X, intramolecular triplet energy transfer from the benzophenone chromophore to the phosphonium moieties may likely be involved. 

D.C. Neckers and I.I. Abu-abdoun, Para-para -bis((triphenylphosphonio)methyl) benzophenone salts as photoinitiators of free-radical and cationic polymerization. Macromolecules 1984, 77(12), 2468-2473. 

Koch and A.H. Jones, A photochemical exchange reaction of Michler s ketone, J. Am. Chem. Soc. 92, 7503 (1970) D.I. Schuster and M.D. Goldstein, Photochemistry of ketones in sol ution. XXXVII. Plash photolysis of Michler s ketone in solution. Rate constants for decay and triplet excimer formation, J. Am. Chem. Soc. 95, 986 (1973) V.D. McGinnis and D.M. Dusek, Photopolymerization of methyl methacrylate with the use of 4,4 BIS (diethylamion) benzophenone as the photoinitiator, ACS Polym. Prepr. 15(1), 480 (1974). 

Formulated coatings were made at a UE/ae ratio of 4/1 using 6% benzophenone as the photoinitiator. The degree of cure of the films after exposure to dosages of 1 and 2 J/sq-cm was again measured as the number of MEK double rubs. The results of the testing of these systems are shown in Table v. 

The kinetics of ultrafast polymerization of acrylic monomers exposed to UV radiation or laser beams has been investigated by IR spectroscopy. An 8 fold increase of the cure speed was observed by using diphenoxybenzophenone as photoinitiator instead of benzophenone. llie reactivity of polyurethane-acrylate or epoxyacrylate systems was markedly improved by adding acrylic monomers that contain carbamate or oxazolidone groups and which impart both hardness and flexibility to the cured polymer. Time-resolved infirared spectroscopy was used to directly record the actual polymerization profile for reactions taking place within a fraction of a second upon UV or laser exposure. Comparison with other techniques of real-time analysis show the distinct advantages of this method for an accurate evaluation of the important kinetic parameters and of the dark polymerization which develops just after the irradiation. 

The soluble prepolymers of diallyl isophthalate may be cured not only by conventional thermal methods but also by photocross-linking techniques. The use of aryl diazides as photoinitiator of solutions of poly(diallyl isophthalate) in aryl ketones has been suggested. With synergistically active sensitizers such as benzil or benzophenone with Michler s ketone curing is possible at 300-400 nm [111]. 

An alkynyliodonium salt, namely, phenyl(phenylethynyl)iodonium hexafluorophosphate, has been tested for application as cationic photoinitiator [38]. The high activity of phenyl(phenylethynyl)iodonium salt as a photoinitiator was verified by photo differential scanning calorimetry (photo-DSC) experiments in direct irradiation and in photosensitized initiation using 9,10-dibutylanthracene, 2-isopropylthioxanthone and benzophenone as sensitizers [38]. 

Piletsky et al. [81] had found that using a coated hydrogen abstraction photoinitiator (see Scheme 4e) very thin MIP films, which were covalently anchored and covered the entire surface of the base material, could be synthesized by a photo-initiated cross-linking graft copolymerization. This approach had been first explored with benzophenone as photo-initiator and a membrane from polypropylene as support. MIP synthesis and recognition were possible in/from water, and significantly less cross-linker than with bulk preparations was necessary to obtain the highest template specificity. Both effects were explained by a contribution of the soUd polymer support to the stabilization of the imprinted sites. The approach is very flexible because no premodification is necessary. 

A patent was issued to BASF that describes two component photoinitiators containing mono- or a diacylphosphine oxide, which is described as R1R2P(0)C(0)R3. The other component is mono or polysubsituted benzophenone. These photoinitiators are low in volatility. It is claimed that they are not inhibited by air and are particularly suitable for ultraviolet light curing of coatings. 

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