The surface of polyethylene (PE) film was sulfonated by reaction with gaseous SO3. The structure of the sulfonated PE films was determined on the basis of spectroscopic data such as IR, UV, and resonance Raman spectra. It was confirmed that a PE film and SO3 gave unsaturated sulfonic acids and that, as the reaction proceeded, the elimination of sulfurous acids took place to form sulfonic acids having highly conjugated CC unsaturated bonds.

Conjugated polyenesulfonic acids were found to be sensitive not only to UV but also to visible light, affording stable radical species. The photoreaction included the cleavage of CS bonds to eliminate sulfonic acid groups.

As a model for the reaction between polyethylene films and gaseous SO3, n‐hexane was allowed to react with gaseous SO3 to give 1,3,5‐hexatriene‐1,6‐disulfonic acid (HTDS), which was isolated from the reaction products by column chromatography. Furthermore, it was found that an aqueous solution of HTDS‐Na was sensitive to UV, which eliminated one of the sulfonic acid groups by a radical mechanism. sulfonic acids having less than three conjugated double bonds were not sensitive to UV.

Infrared-active SO stretching bands [ν(SO)] observed for the sulfonated polyethylene (PE) samples were found to change their vibrational frequencies systematically depending on the degree of the π-conjugated chain structure generated through the sulfonation reaction and also on the sulfonate ion species SO3X in the structure [−CHCHCHCHCHC(SO3X)CC−...], where X = H, Na, Li, K, Ca, and Zn. For the sulfonated PE samples with SO3Na groups, the ν(SO) band was found to shift to the higher frequency side as the conjugated chain length was increased. When the π-conjugated chain structure was modified by bromination reaction (...−CCCCCCCC−... + Br2 → ...−CCCCCBrCBr−...), such a high-frequency shift was canceled. The sulfonated PE samples with SO3H groups, on the other hand, did not show any high-frequency shift even for longer π-conjugated chain structure but gave the ν(SO) bands at the position rather close to those of the SO3Na species coupled with a short π-conjugated chain. These spectral changes were confirmed also by measuring the spectral changes for a series of the model compounds having the π-conjugated structure combined with the SO3Na groups.