Norrish Type I and

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. 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

Reactions are known where both Norrish Type I and Norrish Type II reactions compete, and the substituents on and nature of the substrate will determine which leads to the major product." ... 

Norrish Type I and Norrish Type II Reactions in the Isolated Molecule... 

Table 1.3 provides the reaction yields of methylated pinonic acid with one/five water molecules at 300 K. Although this section mainly focuses on Norrish type I and Norrish type II reactions, the percentage of other reaction channels are given for completeness. 

The statistics of Norrish type I and Norrish type II reactions occurring in one pentanal molecule in the cluster (i.e., not followed by subsequent reactions) can be compared with the previously discussed statistics in the bare pentanal, in di-pinonic acid, and hydrated c/i -pinonic acid Norrish type I reaction is most pronounced in c/Y-pinonic acid, with 37% of yield, much more than for the bare pentanal (14% only). The water and the pentanal cluster reduce in both systems the percentage of Norrish type I reaction (PA-H2O - 18%, PA-(H20)5 - 24%, pentanal cluster - 9%). On the contrary, Norrish type II reaction is mostly observed in the bare pentanal (27%), compared to 10% for the bare pinonic acid. Hydration of the cA-pinonic acid further reduces the percentage of Norrish type II reactions. The existence of the pentanal cluster also decreases the percentage of Norrish type II reaction (PA-H2O - 2%, PA-(H20)5 - 3%, pentanal cluster - 1 %). 

Measures of the sensitivity were made in two ways, (l) Loss of ketone carbonyl was determined by FTIR on the exposed samples by measuring the relative absorbance A at 1700 cm-1. The ratio (Aq/A))7oo, was adjusted for film thickness using the styrene bands at 1600, 1495, and 1455 cm-1. This value is proportional to the rates of the Norrish type I and photoreduction processes in the copolymer (2). Changes in molecular weight result from scission in the backbone of the polymer chain. A measure, Z, of the sensitivity to main-chain scission can be derived as follows. 

A variety of photodissociation reactions have been found to take place with ketones, but the products almost always can be explained as the result of Norrish type I and/or II cleavage. Examples are ... 

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... 

Here we discuss only reactions in solution. For an example of Norrish Type I and Norrish Type II reactions in the gas phase, see H. E. O Neal, R. G. Miller, and E. Gunderson, J. Amer. Chem. Soc., 96,3351 (1974). 

If an aldehyde or ketone has an abstractable y hydrogen, Norrish Type I and Norrish Type II reactions are, of course, competitive. Aliphatic ketones, which have no a side chains (63), undergo exclusively Type II reaction in solution 97 but aliphatic t-butyl ketones (64), which can form a stable t-butyl radical on a cleavage, undergo predominantly Type I reaction.98 Phenyl aliphatic ketones (65) form Type I products much more slowly than 64. The probable reason is that the low triplet energy of 65 makes a cleavage of it almost a thermoneutral reaction, whereas a cleavage of 64 is exothermic by ca. 5 kcal."... 

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). 

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. 

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. - ... 

In this report we examine the effects of several vinyl ketone monomers on the photodegradation of polystyrene in the solid phase. Previous work Sj has indicated that copolymers containing vinyl ketones undergo photolysis by the Norrish type I and type n primary reactions. Studies by Golemba and Gulllet and by Kato and Yone-shiga 0 have shown that these processes also occur in copolymers of styrene with meUiyl vinyl ketone and widi phenyl vinyl ketone. 

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. 

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