Photoinitiation, of free-radical

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Photoinitiation of Free Radical Polymerization by Organometallic Compounds... 

Thus, a mixture of simple carbonyls Me(CO)n and halides should behave as a photoinitiator of free radical polymerization. Many such systems have been found to function in this way. Complexes formed by irradiation of Fe(CO)5 in the presence of a vinyl monomer (M) (such as MMA, styrene, vinyl acetate, propylene, and vinyl ether) have been studied by Koerner Von Grustrof and colleagues [12,13] and shown to have the chemical struc-... 

Photoinitiation of free-radical reactions.2 Use of thermal initiators for radical sources, such as AIBN or dibenzoyl peroxide, requires temperatures >50°. This perester, in contrast, decomposes at room temperature or below on irradiation at 360 nm. This mode of initiation can be useful when stereoselectivity is enhanced at lower temperatures. 

A variety of amines (Table 8) and various xanthene dyes (Figure 25) were tested as visible-light photoinitiators of free-radical polymerization [108, 121]. The rate of photoinitiated polymerization depends on the type of amine used as the electron donor (Figure 26) [122, 123]. 

Figure 25. Xanthene dyes tested as visible-light photoinitiators of free-radical polymerization.

Finally, diphenylsulfonium-bis-(methoxycarbonyl)methylide, IX [87], and salts of p,p -bis-[(triphenylphosphonio)methyl]-benzophenone, X [88], have been claimed as photoinitiators of free radical polymerizations [89]. They presumably photolyze homolytically in the case of X, intramolecular triplet energy transfer from the benzophenone chromophore to the phosphonium moieties may likely be involved. 

D.C. Neckers and I.I. Abu-abdoun, Para-para -bis((triphenylphosphonio)methyl) benzophenone salts as photoinitiators of free-radical and cationic polymerization. Macromolecules 1984, 77(12), 2468-2473. 

Two types of compounds are employed as photoinitiators of free radical polymerizations, which differ in their mode of action of generating reactive free radicals. Type I initiators undergo a very rapid bond cleavage after absorption of a photon. On the other hand, type II initiators form relatively long-Hved excited triplet states capable of undergoing hydrogen-abstraction or electron-transfer reactions with co-initiator molecules that are deliberately added to the monomer-containing system. 

Yagci, Tnrro and coworkers also reported a mechanistic study of photoinitiation of free radical polymerization with thioxanthone thioacetic acid as a one-component, Type II, photoinitiator. 

The photoinitiation of free radical abstraction of hydrogen from a polymer molecule (PH) occurs in two steps ... 

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