Benzil ketals

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. 

Oligomeric benzil ketals (DTGPA), prepared by reaction of 2,2-dimethoxy-2-phenyl-acetophenone (DMPA) with triethyleneglycol, in the presence of p-toluen-sulphonic acid, are reported [88] to be efficient in the UV curing of styrene/ unsaturated polyester formulations giving rise to coatings with very low odour. 

II), benzil (III), and benzil ketals (IV). These and related compounds are extensively used as photoinitiators in commercial practice, usually in combination with hydrogen donors. 

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. 

Trapping Reactions with 2-t-Butylacrylic Acid Methylester XIV. This trapping reaction, which mimics the initiation step of the polymerisation process, has been used to obtain information on the reactivity of the primary radicals formed upon irradiation [ 1 ] [26]. Photolysis of BOMB in the presence of a threefold excess of XIV affords the benzoyl derivative XV in 87 % yield (Figure 9). Again VII was etlso isolated (64 %), whereas no stable addition product of -aminoalkyl radical could be identified. This result suggests that the benzoyl radical is mainly responsible for the polymerisation of vinylic monomers and is in agreement with previous studies on benzil ketals and benzoin ethers [ ]. But, as -aminoalkyl radicals have also been shown to initiate acrylate polymerization [ ], further investigations will be devoted to the elucidation of the role of this primary photoproduct in the overall polymerisation process. 

Table 2.3. Some Benzil Ketals and Acetophenone Based Photoinitiators ...

Also, side reactions of the products form photodecomposition of benzil ketals that can detract from their initiation efficiency are recombination reactions of radicals... 

Those that undergo intramolecular bond cleavage, known as homo-lytic fragmentation type or Type 1 photoinitiators (a-cleavage type). Examples are benzoin ethers, substituted acetophenone derivatives, acyloxime esters, benzil ketals, and cyclic benzoin and benzils. Representative examples are shown in Figure 5.15. 

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