Generation of Radicals by Direct Photofragmentation

As an example of this class we will describe the benzoin ethers. These are cleaved in the following way  

Benzoin ethers have a weak absorption band with a maximum near 350 nm and a molar extinction coefficient of about 200 cm The photochemistry of the methyl ether has been studied by Adam et al. the quantum yield for photodissociation at 365 nm is 0.5. 

There has been some dispute in the literature whether or not the ether radical contributes to the initiation but recently the exclusive generation of ether radicals has been reported and it turned out that polymerization still occurred, albeit that the rate was very low, even at an unusually high concentration of the radical precursor 

Under conditions of relatively higji radical and low monomer concentrations the ether radical predominantly acts as a terminating radical. 

Benzoin ethers are efficient photoinitiators. However, the pot-life of solutions in monomer is rather restricted presumably due to the easy formation of hydroperoxides which undergo subsequent decomposition into initiating radicals 

---