Norrish Type II

Other Types of Protecting Croup 13.2.3.1 Norrish Type II 

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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. 

A Norrish type I reaction for side-chain carbonyl functionahty is equation 7, and a Norrish type II reaction for side-chain carbonyl functionahty is equation 8. 

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

This reaction, called Norrish Type II cleavage, involves intramolecular abstraction of the y hydrogen followed by cleavage of the resulting diradical (a... 

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." ... 

If there are hydrogen atoms in the y-position relative to the acyl group, irradiation of an imidazolide leads to a 1,2-shift of the acyl group (step one) followed by a Norrish type II or type I fragmentation (step two) [41,[51... 

Conformational factors in the photolysis of N-acylimidazoles leading to Norrish type II products or cyclobutanols have been discussed in reference [7]. 7V-Acylimidazoles have been irradiated in tetrahydrofuran using a low-pressure mercury lamp (quartz well,... 

The photolysis of the following steroid system resulted in two products corresponding to the Norrish type II reaction and one product due to a-cleavage (Norrish type I cleavage)<101) ... 

The photolysis of an azetidine has been found to yield pyrole derivatives by the Norrish type II reaction/1075... 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

If the intersystem crossing process is efficient at this excitation, then the Norrish type II rearrangement must occur from the triplet state. This is further substantiated by a reduction in loss of tenacity with increasing concentration of triplet state quencher. The reduction in loss of tenacity may be equated with interruptions of the chain scission process(es). 

We conclude that the Norrish type II rearrangement in PET proceeds, for the most part, via the lowest triplet state. 

UV stabilisers protect polymers by restricting UV penetration to the surface and therefore confine the damage to surface layers. Protection is important because the energy possessed by UV radiation is sufficient to break chemical bonds. The initial breakage can either be by a radical (Norrish type I) or non-radical (Norrish type II) pathway. The effects are similar to degradation of the polymer by oxidation routes the radical intermediates can be neutralised by anti-oxidants. 

Norrish type II mechanism for photo-oxidation of polyolefins. 

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