Photoinitiator benzophenone

The W(CO)4(4-Me-phen) complex also displays a number of spectroscopic features that lend themselves to utility as a probe [106], Figure 20 depicts electronic absorption spectra recorded from a 0.25-mm thickness film of a photosensitive acrylate system comprising a 1 1 (by weight) ratio of TMPTA and medium weight PMMA. Incorporated are the photoinitiators benzophenone (4% by weight) and 4,4/-bis(dimethylamino)benzophenone (0.5% by weight), and the probe complex W(CO)4(4-Me-phen) (0.3% by weight). The major absorption feature in this acrylate film is observed at 353 nm and is due to the photoini-... 

The abstraction of a hydrogen molecnle from the co-initiator or the transfer of an electron between the two initiating molecnles takes place once the photoinitiator is in the excited state. In both cases, free radicals result, one or more of which may actnally begin the photopolymerization. Initiation by hydrogen transfer is common in diaryl ketones. The co-initiator is usually an ether or an alcohol with an abstractable or-hydrogen, such as 2-propanol. Equation 4 demonstrates the H-abstraction reaction between the photoinitiator benzophenone and the hydrogen donor tetrahydrofiiran. In this case, the ether radical initiates polymerization, and the ketyl radical only participates in termination. 

Fig. 2. Electron-transfer reaction with subsequent proton-transfer reaction between the photoinitiator benzophenone and the co-initiator triethylamine.

Keywords UV-photoinitiators Benzophenone BP 4-methylbenzophenone 4 MBP 4-hydroxybenzophenone 4-OH BP Benzhydrol Amine synergists Michler s ketone 4,4 -bis(dimethylamino)benzophenone MK 4,4 -bis(diethylamino)benzophenone DEAB 4-(dimethylamino)benzophenone ... 

PLA pellets 3001D were supplied by Natureworks LLC, l-vinyl-2-pyrrolidinone (Aldrich) was purified by vacuum distillation before using. The photoinitiator benzophenone (BP) was analytical grade used without fiuther purification,... 

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 ... 

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]. 

Real-Time FTIR. For our IR studies, we utilized a stoichiometrically equivalent amount of a trifunctional thiol, trimethylolpropane tris(2-mercaptoacetate), with a difunctional allyl, trimethylolpropane diallyl ether. The thiols were protected from oxidative polymerization by the addition of hydroquinone. The monomers and hydroquinone were purchased from Aldrich Chemicals and were used as received. This formulation was mixed for five minutes and then a commercial photoinitiator, Esacure TZT (Sartomer Inc.), which contained a blend of methyl benzophenones, was added at a level of 1.0% by weight of monomers to the formulation. Stirring was maintained for a further five minutes following the addition of the photoinitiator. The final formulation contained 2.0% by weight of hydroquinone. The samples were prepared prior to each experiment in order to ensure reproducibility of sample history. 

The efficiency of the 2P-induced polymerization process in SR9008 using q.2 as an initiator has been compared to that using conventional radical photoinitiators, including benzil, benzophenone, bis(M,W-dimethylamino)benzil, 4,4 -bis(J f,Ar-dimethylamino)benzophenone, c3, and c4. The threshold power for... 

Recently, there has been an issue with the detection of 4-methylbenzophe-none (MBP) in breakfast cereal in Europe, leading to publicity concerning the presence of this photoinitiator in the food. Based on the close similarity between MBP and benzophenone, a photoinitiator for inks and coatings that are cured with UV light, the European Food Safety Authority (EFSA) has concluded that short-term consumption of breakfast cereals containing ppm traces of MBP should not pose a risk to most people, but recommended that this substance undergo more detailed evaluation if it will continue to be used in food packaging. ... 

Figure 20 Electronic absorption spectrum of a 1 1 (by weight) TMPTA/PMMA 0.25-mm thin film containing benzophenone photoinitiators and W(CO)4(4-Me-phen) at 293 K. (From Ref. 106.)...

Figure 22 (a) Plots of emission intensity at 715 nm of W(CO)4(4-Me-phen) in a 1 1 (by weight) TMPTA/PMMA 0.25-mm thin film as a function of UV-irradiation time (A) with and ( ) without the benzophenone photoinitiators. Excitation wavelength is 400 nm in each case, (b) Plot of the normalized area of acrylate monomer vibration at 808 cm-1 in a 1 1 (by weight) TMPTA/PMMA 0.25-mm thin film as a function of UV irradiation time. (From Ref. 106.)... 

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... 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

R CHORj. Such radicals have been formed by hydrogen atom abstraction from the ether by radicals produced from thermal decomposition of peroxides (67, 75, 76). Similar radicals may be produced in photochemical processes, either by direct irradiation (29, 54), or by the use of a photosensitizer or a photoinitiator, such as acetone or benzophenone (21, 64, 66). The ether radicals once produced, participate in a variety of chemical reactions. It might be noted that resonance forms as illustrated... 

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... 

When using photoinitiators such as phenyl acetophenone derivatives, addition of an hydrogen donating solvent (Table IV) to the system improved ultimately the percent grafting and, in addition, the concentrations of initiator required were lower. Moreover, the presence of THF is necessary with benzophenone derivatives. These results are in agreement with similar observations with respect to MMA polymerization in solution in fact, the rate of polymerization was enhanced in the presence of THF. 

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

Similarly, 4-dimethylamino-4 -isopropyl benzophenone functions as a photoinitiator [104]. 

In principle, the photoreactions of CT s are able to offer a great number of photoinitiator systems for radical polymerization. But, so far, this subject has only received little attention, and the current knowledge relative to the photochemistry of such complexes is poor. In addition to the amine complexes mentioned above, chinoline-bromine [124-127], chinoline-chlorine [128], 2-methylpyridine-chlorine [129], pyridine-bromine [130], IV-vinylpyrrolidone-bromine [131], acridone-bro-mine [132], acridone-chlorine [133], benzophenone-S02 [134], isoquinoline-S02 [135, 136], and 2-methylquinoline-S02 [136] combinations are used for radical polymerization of AN, alkyl methacrylates, acrylic and methacrylic acid, and for... 

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