Type I photoinitiators

Type I initiators are compounds that upon irradiation undergo a cleavage reaction (a- or 3-cleavage) to generate two radicals  

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. 

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When using a suitable type I photoinitiator, e.g., acylphosphine oxides and non-amine-containing acetophenones in this system, the polymerization process is very efficient and the resulting product is a 1 1 alternating copolymer. This system is susceptible to oxygen inhibition, but to a much lesser extent than acrylate polymerization. An important advantage of this system is its low toxicity. Typical components include a variety of vinyl ethers and unsaturated esters, such as maleate, fumarate, citra-conate, imides (maleimides), or N-vinylformamides. The system is also... 

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

Observations The commercial Norrish Type I photoinitiator Irgacure -819, bis(2,4,6-... 

Kinetics of the Cleavage Process in Type I Photoinitiators Working now on the picosecond timescale (with a pump-probe laser setup) shows the shortlived transient absorptions observed upon light excitation [250], For example [251], the cleavage process of DMPA (2,2-dimethoxy -2 phenyl-acetophenone) occurs... 

In spite of the large number of available photoinitiators [4], the search for new initiators is ongoing. For example, S-(4-benzoyl)phenylthiobenzoate, BpSBz, has been found to be a type I photoinitiator. Upon exposure to light it is cleaved into free radicals (quantum yield 0.45), which initiate the polymerization of methyl methacrylate. In contrast, BpOBz (see Chart 10.1) is not cleaved. It forms a long-lived triplet state rather than free radicals [43]. 

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. 

On the basis of the mechanism by which initiating radicals are formed, photoinitiators are generally divided into two classes Type I photoinitiators nndergo a unimolecular bond cleavage upon irradiation to yield free radicals. Type II... 

Type I Photoinitiators undergo a unimoiecuiar bond cleavage upon irradiation to yield free radicals. 

There are large differences in the reactivity of the free radicals generated from type I photoinitiators towards unsaturated monomers. For example, the rate constants for the addition of diphenylphosphinoyl radicals to vinyl monomers are of the order of 10 to 10 mol 1 s -that is, several orders of magnitude larger than those for benzoyl or other carbon-centered radicals [18b]. 

Interestingly, ketones containing germanium act as visible light initiators [20]. In this case, a coinitiator is not needed because these ketones are type I photoinitiators - that is, they undergo a-cleavage upon irradiation (Scheme 3.5). 

Type I Photoinitiators Unimolecular Photoinitiators. These substances undergo an homolytic bond cleavage upon absorption of light. The fragmentation that leads to the formation of radicals is, from the point of view of chemical kinetics, a unimolecular reaction. The number of initiating radicals formed upon absorption of one photon is termed the quantum yield of radical formation... 

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