Photo/thermal reactions, mechanism

Photo/Thermal Reactions. The fifth basic class of photopolymer chemistry that can be used in commercial applications is based more on physical changes in a polymer-based matrix than on chemical reactions. A recent application of this technology is the laser ablation (77) of an organic coating on a flat support to directly produce a printing plate. The availability of newer high energy lasers will allow more applications to be based on the photo/thermal mechanism. 

Instead of using thermal energy to trigger the hydrogen abstraction mechanism, photo-induced reactions can be also be used to successfully crosslink acrylic PSAs [74-76], In this case, photoactive compounds, such as for example those containing benzophenone, anthraquinone or triazine nuclei are compounded with the polymer or copolymerized as one of the monomers. After drying, the adhesive... 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

Both these catalytic centers and the reaction mechanism on them have been better than average well-characterized by the spectroscopic methods discussed and applied in this volume. The comparison between anticipated locations of the catalytic centers for these materials also points up the importance of methodology for determining not only the number, but the accessibility of catalytic sites. Therefore, it is thought that comparisons and contrasts between thermal and photo chemistry, as well as many aspects of the state of present comprehension of mineral spectroscopy and mineral-mediated catalysis will be well-illustrated by a reexamination of published studies of change, even though this reaction is not directly relevant to applications of spectroscopy discussed in the succeeding papers. 

The reactions of both alkoxy and amino complexes are highly stereoselective and give the unnatural epimer at the C-6 position in the penicillin analogs, but methods are known for inversion at this center, llie mechanism of these reactions is thought to involve the intermediacy of a metal-4cetene complex whose formation is photo-induced.Early indications that this was the case came when it was found these reactions fail to give cyclic products of any kind under thermal conditions. More recently, vi-nylketene complexes have been trapped from these reactions.With the recent realization that metal-ketene intermediates are likely to be involved in these reactions further development of the photo-induced reactions of Fischer carbene complexes can be anticipated. 

The outstanding thermal reaction of the acyl azides is their rearrangement to isocyanates. This Curtius rearrangement occurs also in the photodecomposition of the acyl azides, but the yield of isocyanate is lower than in the corresponding thermal process, and other products appear which can be accounted for by a nitrene precursor. A considerable amount of work has been directed towards an elucidation of the mechanism of the photo-Curtius rearrangement and the bulk of the evidence seems to favour the view that it ° is a concerted process not involving a discrete nitrene intermediate. Nitrenes are nevertheless formed in the photolysis of acyl azides and the products of this alternative route will now be discussed. 

Interest in the susceptibility of polypropylene (PP) to degradation, especially by heat, light, oxygen and combinations of these agencies, has kept pace with the increasing commercial application of this material. The purely photo (1-5) and thermal reactions (0 which occur, have been studied in isolation from one another, and it is clear that similar radicals are involved as intermediates in the overall processes. Previous work on acrylate and methacrylate polymers and copolymers (7-8) has demonstrated that the superficial differences in overall degradation mechanism may be attributed to the different ways in which the same primarily produced radicals react in the solid and liquid... 

The reaction mechanism has been described in ref, (23), The reaction is carried out at temperatures <-10°C where no dark thermal activity of the metal can be detected. It only occurs when near-UV light is admitted onto the solid, thus showing that the active phase is constituted by the support. However, the presence of a metal (Pt,Ni) is required to confer a catalytic character to the reaction since, otherwise, the reaction carried out on the bare support declines and stops after a certain time corresponding to an exhaustion of non-renewable active sites. These sites have been identified as deuterated hydroxyl groups since naked titania either dehydroxylated or pretreated in instead of is totally photo-inactive,... 

Exposure of polymers to sunlight or to artificial light sources results in a more or less rapid deterioration of their physical and mechanical properties as a consequence of either primary and secondary photochemical processes or of subsequent photo-initiated thermal reactions. 

In the photoageing of polycarbonates, photo-oxidation has been shown to be a more important process than the photo-Fries reaction. The photo-oxidation of epoxy-resins has been found to depend on the type of hardener and its concentration. A correlation has been observed between the changes in chemical structure and thermally stimulated current of poly(vinyl alcohol) during photo-oxidation. Other studies of interest include the weathering of polymers outdoors,effect of ozonation on polymers, effect of drawing, effect of biodegradation, and changes in mechanical properties of thermoplastics. ... 

Olefins containing at least one allylic hydrogen are suitable substrates and are of special importance and interest with regard to the intrinsic mechanism involved in their reactions with singlet oxygen. Allylic hydroperoxides are formed, but the mechanism of their formation is clearly distinct from that by which allylic hydroperoxides are produced in thermal or photochemically initiated (see example above for a Type I process) autoxidation reactions. This has unequivocally been shown with optically active limonene as a substrate, which gives rise to different products in free radical and Type II photo-oxygenation reactions (22, 57, 61). 

In contrast to thermal reactions, any photochemical reaction is accompanied by a number of photophysical processes which all have to be taken into account in the reaction scheme. Most of these processes are thermal reactions. The mechanism of the photoreaction and - as we will see in the examples - the photochemical quantum yield depends on these photophysical steps. One of the most simple photoreactions is a photo-isomerisation... 

The mechanism of the thermal Claisen rearrangement is discussed in Chapter 11. Photo-dissociative reactions are discussed in Chapter 12. 

Photo-induced aromatic substitution reactions occur through an electron transfer process, which creates an aromatic radical anion or aromatic radical cation as intermediate. This intermediate couples with the electrophile or nucleophile radical to give the product. This mechanism is called Sr I (where the abbreviations stand for substitution, radical, nucleophilic, and first order). Photoirradiation of aromatic compounds in the presence of nucleophiles gives nucleophilic-substituted products different from those of thermal reaction. For example, 3,4-dimethoxynitrobenzene on UV irradiation in presence of hydroxide ion gives 3-hydroxy-substituted product, while on heating gives 4-hydroxy-substituted product [57]. 

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