ON THE CROSS-LINKING MECHANISM OF BENZOPHENONE-CONTAINING POLYIMIDES

A group of fully imidized polyimides containing benzophenone as well as alkyl-substituted biphenylmethane in the main chain were prepared. In these materials cross-links are formed on exposure to UV radiation through hydrogen abstraction by triplet benzophenone from the alkyl groups acting as hydrogen donors and subsequent coupling of the radicals so formed. The involvement of radicals in the process is supported by ESR spectra of the films under irradiation. The quantum yield of cross-linking is phi equals 0. 027, and less than half of the benzophenone units in the solids are reactive. Even at the reactive sites the reaction probability is a mere 3%.