Photochemistry of acrylates at 222Â nm

Excimer lamps as monochromatic UV sources with an intense short-wavelength emission (especially KrCl∗, 222 nm) allow a photoinitiator-free initiation of the acrylate polymerisation. Laser photolysis (KrCl∗ excimer laser, pulse width 20 ns, up to 5 mJ per pulse) gives rise to similar transient spectra (λmax ≈ 280 nm) for all acrylates studied. As the rather unspecific spectra do not allow conclusions as to the main reaction channel, a product study has been performed by GC-MS following steady-state photolysis of acrylate solutions in acetonitrile, methanol and n-hexane. Somewhat unexpected, α-cleavage seems to be a main reaction channel, and quantum chemical calculations show that such a reaction can occur from either the excited singlet state or the unrelaxed triplet state, but not from the relaxed triplet state that is observed spectroscopically. A reaction scheme accounting for the observed products is presented.