Norrish type I and II processes

There are two basic methods for making polymer materials photo-chemically degradable.1,2 One method is to chemically incorporate a chromo-phore into the polymer chains. One commercially successful chromophore is the carbonyl group.1,2,7 Absorption of UV radiation leads to degradation by the Norrish type I and II processes or by an atom abstraction process (Scheme 1). Note that once radicals are introduced into the system, chain degradation occurs by the autoxidation mechanism (Scheme 2). 

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. - ... 

Our results in Figure 6 show that during photooxidation of the polymers the ,/ -unsaturated carbonyl groups are converted into saturated ketonic/aldehydic groups that can themselves be converted subsequently to nonluminescent products by Norrish Type I and II processes, e.g., carboxylic acids (2). For the two light-sensitive polyolefins considered here, Reaction 1 is likely to be the more important, because the concentration of the species is very low. 

Tension on molecular chains may either inhibit or promote reaction rates, for example, poly(ethylene-co-carbon monoxide) photolysed by competing Norrish Type I and II processes (cf. section 3.1.5). 

Pitts and Osborne have compared the y-radiolyses and UV photolyses of low molecular weight methyl ketones, which undergo both Norrish Type I and II processes. They were unable to formulate a detailed mechanism for the y-radiolyses due to the multiphcity of excitation processes and the large number of possible excited species present. However, the gross pathway of reaction appears to be similar to that initiated by UV photons (i.e., predominantly Norrish Type 1 and II processes). From the product ratios, the authors estimated that carbonyl groups are 4 times more reactive than methylene groups when exposed to y-rays (Table 3.3). 

Tadid, J.M., I.O. Juranic, and G.K. Moortgat (2002), Photooxidation of n-heptanal in air Norrish type I and II processes and quantum yield total pressure dependency, J. Chem. Soc., Perkin Trans., 2, 135-140. 

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... 

Most aldehydes and ketones in inert solvents or in the gas phase undergo one or two photoreactions, called Norrish Type I and Norrish Type II processes.81 The Norrish Type I reaction, shown in Equation 13.62, may originate... 

Steps I and II are often referred to as Norrish type I and Norrish type II processes, respectively. Free radicals are formed in the former, while the latter leads to stable molecules without the participation of free radicals. The terms, type I and type II, were suggested by Bamford and Norrish following the discovery of primary decomposition mode II by Norrish and Appleyard in the photolysis of 2-hexanone. 

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,... 

The detrimental environmental degradation of unstabilized commercial polymeric products consisting of polyethylene, polypropylene, poly (vinyl chloride), etc., is frequently due to very small amounts of ketonic carbonyl groups. Electronically excited ketone groups can undergo different processes, in particular the so-called Norrish type I and Norrish type II reactions, as illustrated in Scheme 7.5 for the case of a copolymer of ethylene and carbon monoxide ... 

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