Thioxanthone-amine system

In addition, when used to photoinitiate the polymerization of acrylic monomers, low concentrations of N-substituted maleimide coupled with isopropylthioxanthone and a tertiary amine system result in markedly increased rates of polymerization, faster than the traditional isopropyl-thioxanthone/amine system alone (see the next section). Maleimides can also be used in combinations with a diarylketones and amines to initiate polymerization. ... 

The photoinitiating efficiency is pH dependent, a feature which has been capitalized upon with the tethered thioxanthone-amine co-initiator system 635, which at pH < 6 fails to generate free radicals as a result of protonation of the tertiary amino group, but at higher pH regains its normal initiator activity (Scheme 244) <1998PSA2563, B-2002MI1>. 

Thioxanthones (Table 11) in conjunction with tertiary amines are efficient photoinitiators [140] with absorption characteristics that compare favorably with benzophenones absorption maxima are in the range between 380 and 420 nm (e = lO L moP cm ), depending on the substitution pattern. The reaction mechanism has been extensively investigated by spectroscopic and laser flash photolysis techniques [64,141-143]. It was found that in conjunction with tertiary amines, reactions similar to that of benzophenone-amine systems take place. 

Type 2 initiators are bimolecular systems and consist of a photosensitizer and a hydrogen donor. The most well known system is benzophenone/tertiary amine. Because of the relatively weak absorption of benzophenone at 360 nm the efficiency of this system is rather low. A more efficient aromatic ketone is thioxanthone (TX) and its derivatives simply because of the increased at the exposure wave length. 

Other Mechanisms Other three-component systems involve different mechanisms (see Refs [li,m] and references therein). For example, in the dye (crystal violet, phenosaffanine, methylene blue, thiopyronine)/amine/ketone (acetophenone, benzo-phenone, thioxanthone, 4,4 -bis-dimethylamino benzophenone) combi-nation, the mechanism is quite complex. In the camphorquinone CQ/amino-benzophenone ABP two-component system, both CQ and ABP absorbs the light an aminoalkyl radical on ABP and a ketyl radical on CQ are formed. 

Thioxanthones (XXXVII) behave like aromatic ketones,and their lifetimes are relatively long, which makes their H-abstraction process efficient. Popular derivatives of this system include 2-chlorothioxanthone and 2-isopropylthioxanthone. They are typically used in conjuction with various tertiary amines. ... 

Photoinitiator systems from hydrogen abstraction or electron transfer usually contain two components a photoinitiator (typically an aromatic ketone) and a co-initiator with a weak covalent bond. Some examples for this system include benzophenones (1), thioxanthones (2), benzyls (3), camphor-quinones (CQs) (4), and ketocoumarins (5), which can be used in the presence of H donors (alcohols, THE, and thiols) or electron donors (such as amines) (Table 2). One of the drawbacks of photoinitiation requiring electron transfer is back electron transfer that limits their practical utility. For this reason, the acidity of the C-H bond of the co-initiator is of great importance. A few initiators are also able to undergo cationic and radical photoinitiation such as iodium and sodium salts and arene complexes. ... 

Valderas et al., also studied the photopolymerization of methyl methacrylate initiated by 2-chlorothioxanthone in the presence of various amines of different structures. Here too, the photoinitiation efficiency of these systems was found to be highly dependent on the structure of the amine. The polymerization rate increases with the amine concentration and reaches a constant value at an amine concentration range of 10—30 mm. At these amine concentrations, aliphatic hydroxyaUcyl amines are more efficient photoinitiators than the corresponding trialkyl-substituted compounds. Dimethylanilines with electron acceptor substituents in the 4-position give higher polymerization rates than electron donor substituted anilines. Their data also show that the singlet and triplet excited states of thioxanthones are efficiently deactivated by the amines. Rate constants correlate well with the oxidation potentials of the amines. The effects of the chemical structure of the amine on the polymerization rates of 2-... 

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