Abstraction-type photoinitiators

SCHEME 13.11 Homogeneous synthesis of graft copolymers by hydrogen abstraction type photoinitiation. 

The abstraction type photoinitiator acts by abstraction of a l drogen atom from a donor compound, the most common of which are aliphatic amines such as triethylamine, methyl diethanol amine (MDEA), or dimetl l ethanol amine. The radical produced (R) is the initiator of the free radical polymerization process. The semi-pinacol radical is unreactive and simply couples to give the pinacol. The abstraction process is, however, highly dependent on the amine co-initiator. Examples of abstraction type photoinitiators are given below ... 

It should finally be emphasized that the use of DPB is not restricted to acrylic compounds since it proved to be also a very efficient photoinitiator for the polymerization of vinyl monomers, like N-vinylpyrrolidone (NVP). In addition, DPB appeared to be particularly well-suited to photo-cure systems that need hydrogen abstraction type photoinitiators, like the thiol-polyenes resins (2), since it is then to be compared to the poor-performing benzophenone. 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

Michler s ketone, 4,4 -bis(dimethylamino)benzophenone, is another efficient hydrogen-abstraction-type photoinitiator that possesses both chromophoric aromatic ketone and tertiary amine groups in its structure. It absorbs much... 

Photoinitiators are classified according to the type of photopolymerization system they initiate (i.e., radical or ionic). The basic photochemical routes that produce radicals are photocleavage (type I), intermolecular hydrogen abstraction (type II), and electron transfer followed by proton transfer. Schematics of these photochemical routes are given in Scheme 5. The efficiency of these routes directly determines the monomer conversion, molecular weight of the polymer, and degree of polymerization, and hence the structural, physical, and mechanical properties of the final product. The general relationships of Pp, Pi, and DP were provided in the previous section. 

Abstract The photoinitiated polymerization of different types of multifunctional... 

Radical polymerization is induced by photoinitiators, cleaverage type, and hydrogen abstraction type. The merits of radical polymerization are its higher cure rate and availability of a variety of materials. Most acrylate and methacrylate monomers are applied for radical polymerization. Figure 4.7 shows typical polymerization of a monomer. Mono-, bi-, and multifunctional vinyl monomers are used for polymerization. Figure 4.8 shows typical initiators for photopolymerization. Benzophenone is a hydrogen abstract type initiator. 2,2-Diethoxy-l,2-diphenylethane-l-one is a cleavage type initiator. 

Most of the radical-type photoinitiators used in UV-curable adhesives consist of aromatic ketones which are known to generate free radicals upon UV-exposure, either hy homolytical cleavage of C-C bonds, or by hydrogen abstraction from a H-donor molecule [11] ... 

Radical generation by hydrogen abstraction some photoinitiators tend to abstract a hydrogen atom from a H-donor molecnle via an exciplex, to generate a ketyl radical and the donor radical. The H-donor radical initiates the polymerization, the inactive ketyl radical disappears by a radical coupling process (5). This type of photoinitiators includes benzophenone and thiox-anthone. 

Those that undergo intermolecular H-abstraction from an H-donor, known as H-abstraction type or Type II photoinitiators (non-fragmentation type). Tertiary amines with abstractable a-H atoms have been shown to be particularly effective as synergists (H-donors). Type II photoinitiators include benzophenone, Michler s ketone. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

The primary reaction of Type 2 photoinitiators is a hydrogen abstraction from the tertiary amine by a triplet excited ketone. The amino radical thus formed is sufficiently active to initiate the polymerization of vinyl monomers Scheme 2. 

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. 

II photoinitiators and react with the plastic surface through hydrogen abstraction from COC surfaces. It is shown that type I photoinitiators can simultaneously act as type II photoinitiators. The mechanism allows the use of a single initiator to functionalize the COC surfaces and to synthesize the organic monolith simultaneously. The only requirement is to have a polymerizing formulation containing a sufficient photoinitiator concentration. This simple approach is expected to facilitate in the development of organic monolith as a stationary phases inside thermoplastic microchannels. 

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