The photochemical initiation reaction

The absorption of light by a molecule of a chemical compound added or by the polymer is a necessary condition for the occurrence of the initiation reaction. The absorption in the majority of polymers is, however, very low since they are transparent to visible and UV light (2300—4000 A). The absorption spectra of three quite different polymers are shown in Fig. 5. Pure saturated polyolefins are not expected to show UV absorption beyond 2000 A [473, 474] and consequently should be unaffected by atmospheric sunlight (wavelengths greater than 2900 A). 

The recent observation of the formation of charge transfer complexes between a number of polymers and oxygen explains the bathochromic shift of their absorption [622, 623]. Such complexes absorb the radiation in the range up to 3400 A and may be excited by it. The absorbed energy may cause the dissociation of bonds in the polymer. This problem is not 

R nby et al. [518, 519] considered that the UV light causing the scission of C—C and C—H bonds in the polymer is a primary effect since the polymer absorbs the radiation that has enough energy to break these bonds. The most damaging wavelengths that affect C—C bonds are 

Two kinds of impurities are usually present in every polymer  

The mechanism of the excitation energy transfer from the sensitizer to the polymer molecule has been investigated by many authors for example, Geuskens et al. [178—180, 240], Cozzens and Fox [165, 229] and others [Refs. 25, 41, 285, 352, 381, 476]. 

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Fig. 14.26. Mechanisms of the photochemicatty initiated and Ag(I)-catatyzed Wotff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazota-tion of the diazoketene D in the presence of catalytic amounts ofAg(I). E and F are converted into G via a [1,2]-shift of the alkyl group R1. N2 and a carbene C are formed in the photochemically initiated reaction. The carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclo-propene A. The [l,2-]-shift of B also leads to the ketene G.

What makes gases react in the atmosphere It turns out that most of the trace gases listed in Table 3.3 are not very reactive with the major components of air. In fact, the most important reactive entity in the atmosphere is a fragment of a water molecule, the hydroxyl (OH) radical. This radical (a reactive molecular fragment) is formed by the photochemically initiated reaction sequence, started by the photon of light, hv ... 

The photochemically initiated reaction can be explained by the mechanism shown below. 

The initiation step and the termination step of the photochemically initiated reaction... 

Figure 16.12 shows the overall reaction scheme for smog formation, which is based upon the photochemically initiated reactions that occur in an atmosphere containing nitrogen oxides, reactive hydrocarbons, and oxygen. The time variations in levels of hydrocarbons, ozone, NO, and NO2 are explained by the following overall reactions ... 

Addition of iodine to alkenes can be accomplished by a photochemically initiated reaction. Elimination of iodine is catalyzed by excess iodine, but the diiodo compounds can be obtained if unreacted iodine is removed.52... 

The photodegradation of synthetic polymers can be considerably reduced upon addition of ultraviolet stabilizers. The UV stabilizers (preferably derivatives of o-hydroxy-benzophenone or of 2-(2 -hydroxys -methylphenyl)benzotriazole (Tinuvin) transform the absorbed light energy into thermal energy thus preventing all sorts of photochemically initiated reactions. For review articles see the papers of Otter-stedt (.l), Heller and Blattmann ( 2, 2), Kloepffer (jl, j ), Gysling (JS) and Trozzolo (19 ) ... 

F . 9. Data ftom a mass spectrometric study of the photochemical exchange reaction of (-BHNH-)3 with D2. Initial conditions are indicated... 

Tetralin hydroperoxide (1,2,3,4-tetrahydro-l-naphthyl hydroperoxide) and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidizing the two hydrocarbons and purified by recrystallization. Commercial cumene hydroperoxide was purified by successive conversions to its sodium salt until it no longer increased the rate of oxidation of cumene at 56°C. All three hydroperoxides were 100% pure by iodometric titration. They all initiated oxidations both thermally (possibly by the bi-molecular reaction, R OOH + RH — R O + H20 + R (33)) and photochemically. The experimental conditions were chosen so that the rate of the thermally initiated reaction was less than 10% of the rate of the photoreaction. The rates of chain initiation were measured with the inhibitors 2,6-di-ter -butyl-4-methylphenol and 2,6-di-fer -butyl-4-meth-oxyphenol. None of the hydroperoxides introduced any kinetically first-order chain termination process into the over-all reaction. 

At a later date elegant studies by Fackler and coworkers on 34S-enriched Ni(PhDta)2 S2 reaffirmed the initial conclusions. Mass spectral intensities of the C6HsCS+ fragment demonstrated that the photochemically initiated sulfur-addition reaction was highly specific (20, 232). (235). 

The heat for Reaction (4.49) is 88 kcal/mol, and for Reaction (4.50) it is about 50 kcal/mol. The latter reaction predominates in uncatalyzed vapor-phase decomposition and photochemically initiated reactions. 

The photochemically initiated substitution reactions depicted in Scheme 1 (paths e,f,i) can also be performed with Ru(Pc) systems [211]. 

Sloan et al. [93, 141] noted the formation of reverse adducts when triflu-oroethylene was telomerised either photochemically with fluorotribromo-methane [93] or in the presence of radical initiator with CBr4 or CBr3H [141]. Haszeldine et al. [252-255] investigated the telomerisation of such a monomer with different perfluoroalkyl iodides (CF3I, i-C3F7I) and they showed that the thermal initiation led to a higher amount of reverse adduct in contrast to the photochemical induced reaction. Recently, we have shown that a real telomerisation occurs when the reaction was initiated thermally since the first five adducts were formed. 

The photochemically induced reaction of F2C=CHC1 with CF3I offered 92% of monoadduct but much lower yields were observed by Haszeldine s team [297] for thermal initiation from 225 °C. Besides, no reaction occurred at 190 °C for lOOh. In successful reactions, both isomers were obtained (Table 19) whereas only one was produced from the photochemical addition of HBr to such an olefin. But, this team did not observe such a result for similar reaction of F2C = CHCF3 which formed the three following isomers [299] ... 

Photochemically initiated reactions and quantum yields can yield vital clues in finding the mechanism see Section 6.8. 

Since all of the model compounds examined were monomeric lignin-like structures, our current studies do not address the possibility of an intramolecular photochemical initiated reaction contributing to brightness reversion. Investigations directed at examining this latter issue are ongoing. 

A first study on the combination of transition metal catalysis with radical chemistry was published in 2002 by Ryu [158], Under CO pressure (40 atm), and in the presence of a palladium catalyst, cyclopentanones were formed from 4-pentenyl iodide in a photochemically initiated reaction. 

The photoinduced initiation reaction may have the disadvantage of poor quantum yields, arising from a fast backward ET which annihilates the ion pair before its cage separation. This means a poorly efficient source of radicals. However, if the photochemical ET is the initiation of a very efficient radical chain process, a poor quantum yield in the reaction may turn to an advantage because small extent of production of the intermediates (Ar, ArX- , ArNu ) will disfavour the proposed termination steps of the mechanism. 

Further studies on the photochemistry of friedelin have led to the isolation of the unsaturated aldehyde (130).105 Silver oxide oxidation of (130) gave the known putranjivic acid. Irradiation of friedelin in the presence of acetone afforded the hydroxy-ketone (131).106 Photochemically initiated reaction of 7/3-hydroxyfriedelane and 3/3,7/3-dihydroxyfriedelane with lead tetra-acetate-iodine... 

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