Norrish I reaction

Norrish type I chemistry is claimed to be responsible for about 15% of the chain scission of ethylene—carbon monoxide polymers at room temperature, whereas at 120°C it promotes 59% of the degradation. Norrish I reactions are independent of temperature and oxygen concentration at temperatures above the T of the polymer (50). 

Whether a ketone can undergo a-cleavage depends on a complex function of the dissociation energy of the bond being broken and the excitation energy of the excited state, and competition with other physical and chemical decay processes available to the excited states. Suitably designed ketones which eschew Norrish I reactions in favor of the Norrish II pathways are plentiful and their photochemical behavior has been studied in depth. 

Reddy et al. have examined the influence of j -CD on a series of alkanophenones 97 and 103 for which Norrish I reactions in solution do not... 

Intramolecular photochemical reactions need the presence of light sensitive groups such as carbonyls to induce transformations by oc-cleavage (Norrish I reaction) or by y-H-abstraction (Norrish II reaction). 

To avoid Norrish II photolysis of pyranosid-2-uloses derivatives containing aglycones derived from primary and secondary alcohols, t-butoxy derivatives 58 can be successfully photolyzed by a Norrish I reaction to yield, by C-l, C-2 cleavage and diradical formation, compounds 59 [30 %) and 60 [40 %) (Scheme 32) [62],... 

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 ... 

A typical odour-intensive breakdown product generated by a-cleavage (Norrish I reaction) is benzaldehyde. Owing to recombination and reduction processes further secondary products such as benzil and acetone [50] result from the fragmentation. In principle, odourless photoinitiators can therefore certainly cause significant odour problems via their degradation products. 

This is the well-known Norrish I reaction. The carbonyl radical can abstract hydrogen to form an aldehydic chain end, or split off CO to form a phenyl radical chain end. 

Several studies were carried on the relationship between the chemical structures of ketones and of amines and the rates and efficiencies of photoreductions. It was observed, for instance, that Norrish I reaction of aromatic ketones is affeeted by electron donating and electron withdrawing substituents of the aromatie portions of the ketones. That, of course, is a result of stabilization or destabilization of the partial eharges in the transition state by the substituents. The rates of eleetron transfer reaetions in photo reduction of various ketones, like benzophenone or thioxanthones and their derivatives by some amines can be found in the literature. Some examples are listed in Table 2.5. 

Although there is little doubt that all can be important in particular circumstances, it is generally agreed that hydroperoxides are normally the key to the initial photoinstability in hydrocarbon polymers. Hydroperoxides are present in commercial PP at levels of around 10 - 10 mol/kg and have molar extinction coefficients at 310 nm of around 0.4 l/(mol cm). Carbonyl groups are typically present at around 10 mol/kg and have molar extinction coefficients at 310 nm of around 10 l/(mol cm) (112). However, the quantum efficiency for radical generation in hydroperoxide photolysis is 1, whereas it is of the order of 0.05 for carbonyl groups, partly because radicals formed by Norrish I reaction recombine rather easily and partly because of the preference for Norrish II chemistry, which does not give radicals. 

A Norrish Type I free radical reaction, is also believed to occur with E/CO polymers, notably at elevated temperatures. At room temperatures, about 10 per cent of E/CO chain scission has been estimated to be via Norrish I reaction, while at 120°C, it becomes closer to 50% [3]. The Norrish I reaction for E/CO is ... 

Scheme 18.4 Norrish I reaction of carbonyl (X=CH2), ester (X=0), or amide (X=NH) containing polymers.

As previously discussed in the introductory chapter, the Norrish type reactions account for photodecomposition of polymers with carbonyl moieties, such as polyacrylates, polyesters and polyketones, and through abstraction of the hydrogen atom [21, 22]. After forming of free radicals through Norrish I reactions, photodegradation further proceeds via autoxidation. 

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