Norrish type I split

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 first two processes occur with quantum yields very close to unity, probably through a dissociative excited state. The photolysis of ketones is more complex and it is suggested that the Norrish type I split partly occurs through excitation above the dissociation limit of an upper state. 

As the only potential source of carbon monoxide in this case is carbonyl groups initially present in the polymer, it is clear that the Norrish type I split of ketonic structures is involved in the photolysis of polyethylene at liquid nitrogen temperature. 

Photolysis of polyphenylvinylketone at 365 nm in benzene solution or in the solid state results in a decrease in the molecular weight of the polymer [6], The quantum yield of this process (irradiation at 313 nm) has been estimated to be 0.25 [Ref. 60] or 0.22 [Ref. 61] in solution. Since no carbon monoxide resulting from the Norrish type I split could be detected, the Norrish type II scission is considered to be responsible for the photodegradation of polyphenylvinylketone [6] namely,... 

It is wmih pointii out that, from the photodegradation point of view, the NorrisJi t5q>e II split of macroaldehydes is not important because it produces splitting of two chain carbon atoms only on the contrary, the Norrish type I split appears much more harmful since the radicals formed can induce further oxidative d adation according to the schemes previoudy described. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

Spin-Spin Coupling, and ss character, 16 Spiropentadiene, 273 Split valence basis set, 24-25 State correlation diagrams, 203-208 carbene to alkene, 207 Dauben-Salern-Turro, 212-213 Norrish type I, 215-217 Norrish type II, 213-215 from orbital correlation, 203-206 rules for, 206 Stationary point, 209 Steric interaction, 47... 

The mechanism of photopolymerization in many application, generally involves the light-induced activation of a sensitizer material, such as an aromatic ketone, for example, Michler s ketone or a polyaromatic, to its singlet state, followed by crossover to form its triplet-excited state, followed by energy transfer to an initiator. The activated initiator usually splits into two radicals, one or both of which react witir tire monomer causing polymerization to occur. Norrish type I and type II reaction mechanisms are examples of this science [3]. Acrylates and methacrylates are commonly the monomers of choice because of their high polymerization rate, low cost, and acceptable toxicity (Figure 7.5). Additionally, acrylates and methacrylates... 

Norrish and co-workers in a series of early papers (2) showed that the chemical products of decomposition of aliphatic ketones could be explained on the basis of two primary reactions, the first designated Type I (Reaction 1)—a homolytic split of the molecule to give two free radicals—... 

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