Synthesis of rigid polyimides containing pendant norbornadiene moieties and their valence isomerization between norbornadiene and quadricyclane

Abstract

Polymers having pendant norbornadiene (NBD) moieties and rigid main chains were prepared from reactions in DMF of pendant bromomethyl groups present in polyimides with a potassium carboxylate or phenolate derivatives of NBD using a phase transfer catalyst. The substitution was quantitative using tetrabutylammonium bromide at room temperature. The photochemical valence isomerization of pendant NBD to quadricyclane (QC) moieties occurred in films upon irradiation with 365 nm light. NBD polyimide (P-1a) prepared from hex afluoroisopropylidene-2-bis(phthalic anhydride) and 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane showed a higher photochemical reactivity than the other NBD polyimides when the polymers were irradiated with 365 nm light. While usual polyimides have absorptions around 365 nm that hinder the photo-isomerization of NBD and also cause the polymer to decompose because of the irradiation, the P-1a matrix has no absorption at wavelengths above 340 nm and irradiation hardly affected the rate of isomerization. When the irradiated NBD polymers were heated, the reversion isomerization of the QC moieties to the original NBD moieties occurred easily concurrent with their decomposition. The rigid polyimide structure stabilizes the QC moieties and protects them from degradation during cyclic isomerization between NBD and QC. For example, the half-life of a pendant QC moiety in a polyimide is estimated to be 6 years at 25°C.