Radicals formed from photolysis of photoinitiators

Photodegradation (photo-oxidative degradation) of polymers (PH) can be photochemically initiated by free radicals formed from a photoinitiator added to a polymer sample [1756-1758, 1760, 1766, 1804, 1809]. 

Photoinitiators which intramolecularly cleave into radicals (R [, R2). which, by abstraction of hydrogen from a polymer (PH) molecule may initiate the formation of a polymer alkyl radical (P )  

To this group belong such photoinitiators as [662,1344,1350,1756,1766] Benzoyl peroxide  

Photoinitiators which abstract hydrogen intermolecularly from a polymer (PH) molecule  

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Radicals formed from photolysis of photoinitiators 383 Biacetyl ... 

A study of chemically induced dynamic electron polarization, CIDEP (see Section 12.3.3) on F and G pairs of radicals formed under photolysis of a common termo- and photoinitiator 2,2 -azobis(2-methylpropionitrile) (AIBN) led to a tentative conclusion that initial spatial separation of 2-cyano-2-propyl radicals does not depend upon viscosity However, it is plausible that the diamagnetic dinitrogen molecule formed under photolysis of AIBN (and is invisible by ESR) separates further from a contact RP under photolysis in solvents of lower viscosity. The problem of initial spatial separation and mutual orientation ofradicals under photolysis still waits experimental elucidation. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

Simultaneous Radical and Acid Catalyzed Condensation Polymerization. As shown in Equations 1-7, the photolysis of diaryliodonium and triarylsulfonium salts produces in addition to strong protonic acids, a variety of radical fragments. These photoinitiators are, therefore, capable of initiating free radical polymerizations. A number of hybrid imaging systems which take advantage of both radical and acidic species formed from the photolysis of these salts have been designed. For example, Equation 26 illustrates one such system based on simultaneous radical and acid catalyzed condensation polymerizations which has been explored in our laboratory. 

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