Norrish type I mechanism

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. 

Taking into account [81,82] that photodegradation of poly(4,4-dimethyl-l-penten-3-one) [poly(BVK)] and poly(3-methyl-3-buten-2-one) [poly(MIK)] proceeds predominantly through a Norrish type I mechanism via the triplet state (Scheme 15), the above homopolymers have been studied as initiators in the photoinduced polymerization of vinyl monomers such as MMA, St, AN and VAc [83]. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

With ketones, various photoinduced processes can occur. In what is known as the Norrish type I mechanism, a free radical reaction occurs ... 

If the monomer itself cannot form free radicals by this process, then a photoinitiator must be added. Bisulfides form two free radicals RS on irradiation. The azo group of azobisisobutyronitrile absorbs light at 350 nm and then forms free radicals [reaction (20-3)]. Certain aliphatic ketones decompose according to a Norrish type I mechanism into two free radicals ... 

High-field NMR measurements show the formation of the acrylic-ended poly(methyl methacrylate), which can only be achieved via the Norrish Type I mechanism [275] ... 

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. 

The mechanism probably involves a Norrish type I cleavage (p. 318), loss of CO from the resulting radical, and recombination of the radical fragments. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

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). 

There are at least three types of mechanisms discussed in order to explain the ODPM rearrangement. The first mechanism (Sch. 6) is radicaloid in nature in involves a Norrish type I cleavage leading to the formation of acyl and allyl radical and recombination of these radicaloid species to the ODPM rearrangement product in two ways. 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

The hydroxycyclopentenone (145) yields the lactone (146) on irradiation. The reaction is thought to involve a Norrish Type I path via biradical (147), The authors believe that a concerted mechanism is operative. 

Meanwhile new criteria to distinguish between thermal activation and nuclear tunnelling mechanisms in the photochemistry of ketones was proposed [74]. This criteria is based on the influence of spin-orbit coupling on the crossing probability, P, at the crossing w. The experimental test of this criteria showed nuclear tunnelling is the dominant mechanism in H abstractions [75]. TET was then applied to Norrish type I reactions [76]. 

A mechanism involving Norrish type I cleavage followed by radical cyclization has been proposed to explain this photolytically induced rearrangement.An alternative possibility might involve bonding from the carbonyl carbon to the 6-carbon followed by alkoxy radical expulsion and ring closure. 

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