Cleavage-type photoinitiators

SCHEME 13.9 Homogeneous synthesis of graft copolymers by a-cleavage type photoinitiation. 

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] ... 

Those that undergo intramolecular bond cleavage, known as homo-lytic fragmentation type or Type 1 photoinitiators (a-cleavage type). Examples are benzoin ethers, substituted acetophenone derivatives, acyloxime esters, benzil ketals, and cyclic benzoin and benzils. Representative examples are shown in Figure 5.15. 

The classical photochemical free-radical source is a compound that is photoexcited and then undergoes bond cleavage to yield active radicals. Benzoin and its ethers are efficient radical sources and are the most commonly employed photoinitiators in industrial photopolymerization processes (42,43). The reaction is a simple Norrish type I cleavage ... 

Changing the substitution at the p C atom leads to a more efficient radical photoinitiator (10.23) it allows to favor the a-cleavage process (yielding a benzoyl radical and a second benzyl type radical that liberates an oxysulfonyl radical through a subsequent cleavage). 

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... 

Recently, Yagci and coworkers investigated the initiation mechanism of a new type of cationic photoinitiator, namely, /V-phenacyl-/V,/V-dimethylanilinium hexafluoroan-timonate (PDA+ Sbl fi ) which initiates the polymerization of appropriate monomers [116]. The proposed mechanism includes irreversible fragmentation of the absorbent salt to yield the initiating species either via a heterolytic cleavage or via a homolytic cleavage followed by subsequent electron transfer between the preformed species still, forming the same cation that initiates cationic polymerization (Scheme 11.30). 

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. 

Although the compounds and preferred modes of decom-position/recombination vary with the overall systems, diethoxyacetophenone in acetone exhibits primarily Nor-rish Type I cleavage as evidenced by the formation of benzaldehyde, ethyl benzoate, acetophenone and benzll. Although acetalydehyde may indicate Norrish Type II cleavage is observed, it is likely that this compound is derived from the solvent and not the photoinitiator (Scheme 1). 

However, in the presence of chlorendic anhydride, the decomposition of diethoxyacetophenone occurs only by Norrish I type cleavage, yielding benzoyl chloride, acetophenone and benzll. No Norrish Type II cleavage products were observed. The 24 hour photolysis of chlorendic anhydride in acetone using diethoxyacetophenone as the photoinitiator shows carbon-chlorine homolytic bond cleavage and the generation of chlorendic radicals as evidenced by the appearance of benzoyl chloride, chloro-... 

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. 

Benzil dialkyl ketals are also very efficient photoinitiators. They too are believed to decompose by the Norrish Type I cleavage ... 

Two classes are known based on free radicals and hydrogen abstraction techniques, respectively. Free residual types are receptive to UV light by absorbing radiation energy such that free radicals result and these produce a chain polymerization reaction and eventually a solid polymer matrix. An example of the photoinitiation reaction sequence, which follows a Norrish I-type cleavage, is given in Fig. 12.10. Another photoinitiator would be benzoin butyl ether shown in Fig. 12.11. 

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... 

The photoinitiator makes the resin formulation UV-curable. Only the development of thermally stable but photolabile compounds enabled the development of UV curing. For an efficient UV curing, the absorption of the photoinitiator should match, at least partly, the emission spectrum of the light source used. Two main types of photoinitiators are used photoinitiators based on an a-deavage process (Norrish type I), and photoinitiators based on an electron transfer followed by a hydrogen abstraction process (Norrish type II). a-Cleavage photoinitiators are mono-molecular initiators, of which a-hydroxy or a-alkoxy ketones and benzoyl phosphine oxides are well known examples shown in Scheme 16.33. 

Cationic photoinitiators are usually based on iodonium salts [e.g. Phal ) and sulfonium salts (e.g. PhsS" ") which liberate a proton upon exposure to UV light (after subsequent rearrangement of the primary pairs formed, cleavage of a C-I or C-S bond and/or hydrogen abstraction on a H-donating structure). Photoinitiators of radical polymerization were originally classified as Type I PI (cleavable systems e.g. aryl allq l ketones or phosphine oxides mostly through a Norrish I scission, Scheme 2a) and Type II PI (PI and a co-initiator such as an amine AH or electron/proton transfer process, Scheme 2b PI stands e.g. for benzophenones. 

Phosphine oxides acyl and biacylphosphine oxides are photoinitiators which, if used alone or in combination with other photoinitiators, absorb well into the visible part of the spectrum and are useful to cure highly pigmented systems. Irradiation leads to highly reactive phosphinoyl and phosphinyl radicals generated by a Norrish type-1 cleavage. 

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