Benzoin derivatives radical generation

Electron donating radicals may be produced thermally or photochemically. Benzoin derivatives and phenylazotriphenylmethane were found to be effective to generate electron donating radicals, directly by a photochemical process (Scheme 9). 

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. 

Two types of photoinitiation, the so-called Norrish type I and Norrish type II, are known. In the former, two initiating radicals are generated by a-cleavage when the initiator is exposed to UV tight. Benzoin derivatives serve efficiently for this purpose (Scheme 13.1). Such chromophores anchored to the polymer trunk, in chain or side chain, will afford block and graft copolymers, respectively. Here, homopolymer formation is an intrinsic outcome of this kind of initiation that originates from the low molar mass initiator fragment. 

Scheme 7.11 Generation of free radicals by a-cleavage in electronically excited acetophenone and benzoin derivatives and subsequent formation of macroradicals P by hydrogen abstraction from macromolecules, PH.

Another benzoin derivative, desoxybenzoin, undergoes a-scission with rather low quantum yields, too. However, it becomes an efficient initiator when utilized together with a hydrogen donor, such as tertiary amine or tetrahydrofuran. In this case, initiating radicals are generated by hydrogen abstraction with comparatively high yields [31,32] ... 

Ester derivatives of benzoin are known to display usually quite a low photoinitiation activity in the polymerization of vinyl monomers [85,104]. By contrast, ben2 in alkyl ethers are claimed to generate [17], by a photofragmentation mechanism, benzoyl and a-alkoxy benzyl radicals resulting in a much more active polymerization and crosslinking initiating species (Scheme 25). Thus, polymeric systems having the above moieties in the side chains have been prepared and their photoreactivity studied in more detail [105-107],... 

Photoinitiators. Derivatives of benzoin and benzil are added in amounts of 1-3% as photoinitiators in UV-curing systems. These have differing effectiveness in the UP systems [2.99], Special initiators are available for pigmented systems. The UV light splits them into radicals which in turn initiate polymerisation. The UV radiation is generated using superactinic fluorescent lamps and/or high-pressure mercury vapour lamps [2.109]. 

This problem prompted the development of second-generation photoinitiators for radical polymerization, which do not possess benzylic H atoms, such as a,a-dimethoxy-a-phenylacetophenone (1), a,a-diethoxyacetophenone (2), and the a-hydroxyacetophenone derivatives (3) and (4). These photoinitiators undergo photocleavage, in an analogous manner to benzoin ethers, as shown by various studies, " including free-radical trapping and laser flash photolysis. 

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