Photoinitiators free radical types

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. 

The methods used to prepare NC gels are simple and versatile, i.e., injection of reaction solutions into closed vessels followed by polymerization at ambient temperature. Hence, NC gels can be readily formed in various shapes and sizes, such as films, sheets, rods, spheres, hollow tubes, etc. (Fig. 4a) [21,29]. NC gels can also be prepared by photoinitiated free-radical polymerization using very low concentrations (e.g., 0.1 wt% relative to the monomer) of a hydrophobic photoinitiator in aqueous systems (Fig. 4b) [48], Furthermore, the other type of NC gel, i.e., tetra-PEG-based NC gels, with good biocompatibility can be prepared by incorporating clay nanoparticles into the tetra-PEG network [30],... 

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... 

TAS 07a] Tasdelen M.A., Demirel A.L., Yagci Y., Poly(propylene imine) dendrimers as hydrogen donor in Type II photoinitiated free radical polymerization , European Polymer Journal, vol. 43, pp. 4423-4430, 2007. 

The second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

Two types of organometallic photoinitiators for free radical vinyl polymerization are considered (1) transi-... 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Depending on the type of reactive species generated upon exposure to UV light, photoinitiators are classified as free radical, cationic, and anionic. 

Free-radical photoinitiators are classified by their chemical nature as type I and type II however, there are a few systems with different chemistry, e.g., borate salt initiators that depend on inter-/intramolecular electron transfer,i that do not fit into either category. 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

Cationic Cure Epoxies. As with free radical uv chemistry, the same principles of formulation component selection apply to cationic cured uv epoxy adhesives. Although different monomers and oligomers are normally required for this type of chemistry, the main difference lies with the photoinitiator system. 

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 ... 

Moreover, the polymerization of MMA, photoinitiated by poly(AVBP-co-St), proceeds at a lower rate than that promoted by the low-molecular-weight analogue AIBP and is enhanced in solvents where the macromolecules are more expanded, thus reducing free radical recombinations [60]. The presence of cage recombinations in this type of systems is also substantiated by the occurrence of a very efficient crosslinking when films of poly(VBP-ca-DMAS) are subjected to UV irradiation [54]. 

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]. 

---