Free radical initiators benzoin derivatives

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]. 

Recently, Ledwith described combined systemscomposed of a radical initiator and a cationic photoinitiator, which are very suitable for hybrid systems. It is particularly convenient to employ common photochemical sources of free radicals for this purpose. Since many of them possess aromatic carbonyl groups (e.g. benzoin and acetophenone derivatives), these groups provide an extension of the absorption to longer wavelengths and promote the initiation of cationic polymerization. Figure 19 shows the proposed formation mechanism of free-radical and cationic active species by electron transfer ... 

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. 

In systems 1-4 radical polymerization takes place. Decomposition of the photoinitiator (e.g., benzoin and its derivatives) leads to formation of free radicals which react with the carbon-carbon double bonds. In system 5 acidic reactive species are formed which react with cycloaliphatic epoxy compounds and vinyl ethers to form a cross-linked network. Sulfonium, and iodonium salts are used to initiate this cationic polymerization, a typical example is PhjS PFg. ... 

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