CIDNP.‐Investigation of Photoinduced Polymerization

The photochemistry and some initiator characteristics of the polymerization initiators ω,ω‐dimethoxy‐ω‐phenyl‐acetophenone (A), ω,ω‐diethoxy‐acetophenone (B) and ω,ω‐diisopropoxy‐acetophenone (C) have been studied by 1H‐ and 13C‐chemically‐induced nuclear polarization (CIDNP.) experiments. The primary reaction of initiator A is a Norrish‐type I cleavage, while for B and C Norrish‐type I and Norrish‐type II cleavages are of comparable importance. Three different recombination products could be detected for initiator A which correspond to the three canonical resonance forms of the substituted benzyl radical. In a number of polymerization experiments the competition between reactions among initiator radicals and reactions of initiator radicals with the monomer acrylic acid methyl ester was studied at low concentrations of the monomer. These experiments give insight into the first steps of the polymerization process. The positive sign of the α‐hydrogen hyperfine couplings of the dimethoxy‐methyl‐ and the diisoproproxymethyl‐radicals could be established, in agreement with the deviation of these radicals from planarity. A slow square‐wave light‐modulation technique has been employed in 13C‐FT.‐experiments to measure absolute CIDNP.‐intensities.