PHOTO-CROSS-LINKING REACTION OF BENZOPHENONE-CONTAINING POLYIMIDE AND ITS MODEL COMPOUNDS

The quantum yield for the photo-cross-linking reaction of the polyimide-containing benzophenones as well as alkyl-substituted diphenylmethane structures in the main chain was determined to be 2.0 x 10-3 at room temperature from gel permeation chromatography measurements. The Arrhenius plot of the quantum yields for the cross-linking is linear without any break near the T(g) of the polymer and gives an apparent activation energy of 2.4 kcal mol-1. From the change in the IR spectra, the key process of photo-cross-linking was determined to be hydrogen abstraction by the triple benzophenone moiety from the pendant ethyl groups and subsequent combination of the radicals produced. The rate constant for the hydrogen abstraction from ethylbenzene by the triple excited state of the model bisimides is about an order of magnitude smaller than that by triple benzophenone. This difference is thought to be due to the different contribution of n-pi* excited triplet state between model bisimide and benzophenone.