Norrish I and II reactions

In the case of polyamides and polyesters the most important photolytic reactions are the Norrish I and II reactions (see Scheme 1). The Norrish I reaction leads to chain cleavage and radicals that might initiate oxidation, the Norrish II reaction only leads to chain cleavage. The main question for these polymers is What is the relative importance of photolysis and photo-oxidation ... 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

The Norrish I and II reactions may occur from the excited singlet (S ) or triplet (T ) states however the triplet state is much more favoured because of its longer lifetime (Table 1.1). Both Norrish reactions are responsible for the photodegradation of polymeric ketones (cf. section 3.2.1) and polymers containing main chain carbonyl groups. 

This chapter is divided up in the same manner as in previous years with sections dealing with Norrish Types I and II reactions, oxetane formation and miscellaneous reactions relating to carbonyl compounds and related species. 

He, H. Y., Fang, W.H., Phillips, D. L., Photochemistry of Butyrophenone Combined Complete active space Self consistent Field and Density Functional Theory Study of Norrish Type I and II Reactions, J. Phys. Chem. A 2004, 108, 5386 5392. 

Several recent reviews have explored PCET [1, 5, 23, 24, 29, 30] and specifically HAT [1,16, 17] from reaction chemistry and theoretical perspectives. This chapter will not exhaustively re-examine this material, but rather introduce a descriptive framework for the electron and proton that adequately depicts both the geometric and mechanistic complexities of PCET and its relation to HAT. Most examples will be restricted to systems in which kinetics have been measured and discussed within a PCET framework. Accordingly, more classical topics, such as radical organic photochemistry (e.g., Norrish Type I and II reactions) will not be considered. 

A new coarse grained molecular dynamics model was developed to study the role of thermal, mechanical and chemical reactions in the onset of the ablation process of PMMA [58]. In this model, the laser energy is absorbed in different ways, i.e. pure heating and Norrish type I and II reactions. Mechanical stresses and pressure are dominant for very short pulses in the stress confinement regime and can initiate... 

As in past years this chapter describes the photochemistry of those carbonyl compounds where the reaction type is dictated by the carbonyl function. Thus Norrish Type I and II reactions, rearrangements, and cycloadditions are dealt with in this chapter, but reductions and reactions of enones will be covered in later chapters in Part III. The shift in emphasis away from the study of simple carbonyl compounds which was pointed out in Volume 6 has been noticeable throughout this year. 

If a polymer contains structures that can absorb terrestrial sunUght wavelengths (>290-300 nm), which leads to chemistry causing changes in molecular structure, photolysis can be an important degradation mechanism. Norrish I and II of carbonyl, ester, or amide containing polymers (e.g., oxidized PE, PES, and PAs) and the photo-Fries reaction in bi-sphenol A polycarbonate (BPA-PC) are well-known photolytic reactions [52]. These reactions are shown in Schemes 18.4-18.6. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Norrish Type I fission of the side chain carbonyl group again at C-4. - Laser flash irradiation has been used as a aethod for the production of n-butylkotene from cyclohexanone. The chemistry of this ketene was studied in detail. The cyclohexanones (9a) undergo both Norrish Type I and II processes on irradiation. The fluorinated compounds (9b) showed a preference for Norrish Type II behaviour. Within the Norrish Type II biradical fluorine substitution leads to a preference for cyclization rather than cleavage. The Norrish Type I biradical afforded a ketene rather than an alkenal. A study of the photochemical reactivity of the diones (10) has shown that both Norrish Typo I and Type II reactivity can take place. The Typo I Type II product ratio is dependent upon ring size. Thus dione (10a) affords the Type II products (11) and (12) while dione (10c) yields the Norrish type I products (I3c-15c) and low yields of the Norrish Type II products (11) and (12). Compound (10b) is intermediate between these results affording a Type I Type II ratio of 0.3. A mechanistic study of the reactions was carried out. - ... 

Not excluding the possibility of more complex mechanism of PETP photodestmction and photooxidation it is supposed that photodestmction of the given polymer takes place according to the Norrish reaction of I and II type [2]. 

The UV radiation affects aldehyde groups in polymers in the same way as ketone groups that is, Norrish reactions of type I and II are involved in the photochemical process [116]. These reactions are not very important in the photo-oxidative degradation of polymers, because the aldehyde groups are exclusively found at chain ends namely,... 

On irradiation with ultraviolet light, the activated ketone groups present can take part in two different types of free radical, bond-breaking reactions. In organic photochemistry, these two reactions are referred to as Norrish I and Norrish II Reactions, and their mechanisms are shown below for the degradation of copolymers of ethylene and carbon monoxide [46, 47] ... 

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