Reaction of Merocyanines with Free Radicals

To the list of interactions that may lead to degradation must be added the easy reaction of free radicals with the merocyanine forms. We have recently reported48 that photochromic PMMA and polycarbonate cast matrices develop a violet-reddish color on aging, due to the formation of free-radical adducts (FRA), 

FRAs are compounds characterized by large molar absorptivities (monoadduct max = 55,000, kmax = 540nm biadduct Gmax = 20,000, Xmax = 560nm). 

The isolated yields are not high (5-20%), but because of the deep color, formation of FRAs gives a permanently colored background to the photochromic system and limits its practical applications. 

---