Photoinitiator, 2,2 -azobis

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. 

Our application of the rotating sector technique to hydrocarbon oxidations has been described (14,15,18). Oxidation rates were measured at the longest convenient chain lengths and corrected for the absorption and evolution of gas in initiation and in peroxy radical-peroxy radical reactions. ,a -Azobiscyclohexylnitrile (ACHN) was used as the photoinitiator at 30°C. and a,a -azobis-l-propanol diacetate as the photoinitiator at 56°C. ,a -Azobisisobutyronitrile (AIBN) was used as a thermal initiator at 30° and 56°C. 

In a further, and more comprehensive investigation into radical-based thiol-ene chemistry. Hawker and colleagues prepared a series of polymers with either single or multiple alkene functionality and clicked on a variety of commercially available mercaptans under both photoinitiation (with trace amounts of DMPA) and thermal initiation (in the presence of radical initiator 2,2 -azobis(2-methylpropionitrile)) (Campos et al, 2008). In aU cases studied, the photoinitiated thiol-ene reactions (all performed at ambient temperature) could be made to proceed to completion in times ranging from 30 min to 2h. 

Regarding azonitriles, the famous thermal initiator 2,2 -azobisisobu-tyronitrile (AIBN) has also shown some potential for photochemical initiation. It was used to polymerize vinyl acetate [102-104], styrene [103,105], vinyl chloride [106], methyl methacrylate [103], and acrylonitrile [107]. Also of certain interest as photoinitiators for vinyl polymerization are a-azobis-1-cyclohexanecarbonitrile [108-110] and l, l -azodicyclohexanecarboni-trile [111]. 

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