CIDNP studies of macromolecular systems. I. Photolysis of poly(methyl isopropenyl ketone) and its model compounds in solution

Abstract The photoreactions of poly(methyl isopropenyl ketone) (PMIK) and two low‐molecular‐weight model compounds have been investigated in solution with proton NMR spectroscopy. Chemically induced dynamic nuclear polarization (CIDNP) has been observed in all three systems, but the CIDNP intensities have been found to depend on the chain lengths of the reaction products: long‐chain products fail to show CIDNP at room temperature, whereas simultaneously formed small fragments exhibit nuclear spin polarization. The CIDNP originates from the Norrish type I decomposition of the ketones, which for the polymer occurs from the triplet state in all solvents. In the model compounds this cleavage can occur from the triplet or singlet state. The main photoreaction of PMIK, namely, scission of the polymer backbone, does not give rise to CIDNP. The failure of long‐chain products to display CIDNP is attributed in part to a relaxation phenomenon: the short nuclear spin relaxation times of the long‐chain product molecules destroy the CIDNP before it can be detected. In high‐temperature studies above 150°C, however, in which the relaxation times are longer, CIDNP has been detected in long‐chain molecules as well.