Photoinitiators-Peroxides

Photoinitiator/Peroxide or Hydroperoxide-Based Systems In these systems where useful effects occur [li], oxygen-centered radicals are generated the mechanism is not clear. In the same way, in the 1-benzoyl-cyclohexanol HAP/benzophenone BP system under air, the radicals that are formed after the a-cleavage of HAP consume oxygen and further allow the generation of a hydroperoxide ROOH whose decomposition is sensitized by BP, and so on. 

The ptincipal commercial initiators used to generate radicals are peroxides and a2o compounds. Lesser amounts of carbon—carbon initiators and photoinitiators, and high energy ionising radiation are also employed commercially to generate radicals. 

Although a variety of methods for generating radicals by one or more of these three methods are reported in the Hterature, commercial initiators are primarily organic and inorganic peroxides, aUphatic a2o compounds, certain organic compounds with labile carbon—carbon bonds, and photoinitiators. 

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. 

The photoinitiation of vinyl polymerization by organic compounds (carbonyl, azo, peroxide, disulphide compounds, etc.) or inorganic salts (e.g., metal halides and their ion pairs, etc.) will not be discussed here, since these type of photoinitiators are beyond the scope of the present chapter. 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

General concepts have been discussed in Section 3.1.8. General reviews on photoinitiation include those by Pappas,"6" 264 Bassi,26 Mishra300 and Osier and Yang267 and Gruber.268 The applications of azo-compounds and peroxides as photoinitiators are considered in the sections on those initiators (see 3.3.1.1.2, 3.3.2.1.2, 3.3.2.3.2). References to reviews on specific pholoinilialors are given in the appropriate section below. 

The last-mentioned compound has been chosen in preference to benzoyl peroxide in many of the more recent kinetic investigations on account primarily of the freedom of its decomposition from side reactions such as radical-induced decomposition (see p. 113). It may also serve as a photoinitiator under the influence of near ultraviolet radiation, which... 

Tetrahydrobenzyl alcohol (( )3-cyclohexenene-l-methanol) and 30% aqueous hydrogen peroxide were purchased from Fluka, AG. 3-Cyclohexene-1-carboxylic acid and cis-4-cyclohexene-l,2-dicarboxylic acid were used as purchased from Lancaster Chemical Co. Methyl iodide, acetic anhydride, Oxone (potassium peroxymonosulfate), Aliquot 336 (methyl tri-n-octylammonium chloride), sodium tungstate dihydrate and N,N-dimethylaminopyridine (DMAP) were purchased from Aldrich Chemical Co. and used as received. 3,4-Epoxycyclohexylmethyl 3, 4 -epoxycyclohexane carboxylate (ERL 4221) and 4-vinylcyclohexene dioxide were used as purchased from the Union Carbide Corp. (4-n-Octyloxyphenyl)phenyliodonium hexafluoroantimonate used as a photoinitiator was prepared by a procedure described previously (4). 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Based on the kinetic results of experiments with photoinitiated peroxyoxalate chemiluminescence, Milofsky and Birks proposed, for the first time, the involvement of a six-membered cyclic peroxide (51) as HEI. On the basis of this suggestion, Hadd and coworkers, using conventional chemiluminescent kinetic studies with 47, also proposed the involvement of two HEIs, 48 and another six-membered cyclic peroxide 52 similar to 51. 

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...

An alternative photo-SIP approach was described by Kang and coworkers, where they used an argon plasma to oxidize alkylthiolate SAMs on planar gold [55]. The plasma treatment oxidized carboxy-terminated SAMs to peroxide moieties. UV irradiation in the presence of acryhc acid and allylpentafluorobenzene yielded ultra-thin graft layers of 6-7nm. The poly(acrylic acid) layers were found to adsorb Fe " ions from solution. This particular photoinitiation method yields low-density polymer brush films. 

As initiators, common peroxides, such as dicumyl peroxide, di-benzoylperoxide can be used, but also photo initiators have been reported. Photoinitiating systems are the combinations of benzo-phenone with amines, such as n-butylamine, trimethylamine and triethylamine (43). 

Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 311 radical pairs in, 311, 313 stress generated in, 313 diundecanyl peroxide (UP), 313 derivatives of, 317 EPR reaction scheme for, 313 IR reaction scheme for, 316 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 71, 96 CH3C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer s law and, 294 bimolecular processes in, 291 concepts of, 283... 

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. 

The major impurities which are found in any polymer are the unreacted monomer itself, unreacted initiator (peroxides and all types of photoinitiators) and catalysts used in the polymerization process, as well as traces of the solvent and of water. Within the polymer chain itself there will be some defects or impurity sites which result essentially from oxidation reactions during the making of the polymer. The polymerization process on an industrial scale cannot be performed in the absence of atmospheric oxygen, and this will attack the growing polymer chain at random points to produce... 

The peroxide and azo thermal initiators also are photochemically unstable and have been used as radical sources at well below their normal thermal decomposition temperatures. However, their industrial use as photoinitiators has been limited because their light-absorption characteristics frequently are unsuitable and because of the obvious potential complication owing to their slow thermal decomposition, which leads to poor shelf-life and nonreproducible photoactivity in given formulations (88). Further information on photoinitiators can be found in the literature (92). 

If a swelling agent is added to the reaction mixture, a maximum rate is usually noted for some fairly low agent-to-monomer ratio. DMF, which is merely a swelling agent when mixed with monomer, gives a maximum rate at 10 mol-% in photoinitiated polymerization, at about 25 mol-% in polymerization catalyzed by benzoyl peroxide (9), and at about 30 mol-% with gamma rays (114), all near room temperature. On the other hand, as little as 10 mol-% of DMF reduces the rate at 60° by a factor of about 15. It decreases also the ratio of the fast reaction at 60° to the 25° rate. 

R CHORj. Such radicals have been formed by hydrogen atom abstraction from the ether by radicals produced from thermal decomposition of peroxides (67, 75, 76). Similar radicals may be produced in photochemical processes, either by direct irradiation (29, 54), or by the use of a photosensitizer or a photoinitiator, such as acetone or benzophenone (21, 64, 66). The ether radicals once produced, participate in a variety of chemical reactions. It might be noted that resonance forms as illustrated... 

Lunar S, Sedlak P (1992) Photoinitiated Reactions of Hydrogen Peroxide in the liquid Phase, J. Photochem. Photobiol. A ... 

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