Degree of sulfonation and microstructure of post-sulfonated polyethersulfone studied by NMR spectroscopy

Commercial polyethersulfone (Udel® 1800) based on 4,4′-dihydroxy-2,2-diphenylpropane (bisphenol-A) and dichlorodiphenylsulfone was post-sulfonated using trimethylsilyl chlorosulfonate under mild conditions. The partially sulfonated polyethersulfones were investigated by 1H and 13C NMR in their acid and/or sodium salt form. Several 13C NMR signals could be assigned up to the triad level. 13C-T1 relaxation times were determined by the inversion recovery method for all carbons to ensure conditions for quantitative 13C NMR measurements. Additionally, the nuclear Overhauser enhancements are given. The degree of sulfonation (DS) of the samples covers the range from 13.6 to 100% as determined both by 1H and 13C NMR spectroscopy. The NMR spectra confirm that the sulfonation occurs solely as mono-substitution of each phenyl ring of the dioxy-2,2-diphenylpropane unit in ortho-position to the ether bond. The microstructure of all polymers was evaluated from the content of dioxy-2,2-diphenylpropane-based diads and diphenylsulfone-centred triads. From the development of non-, mono- and disulfonated dioxy-2,2-diphenylpropane units with increasing DS it can be concluded that the sulfonation of the first phenyl ring retards the sulfonation of the second one. Thus, the non-sulfonated units deplete faster and the monosulfonated units are enriched compared with a random sulfonation. There is no reactivity influence between dioxy-2,2-diphenylpropane units which are separated by a diphenylsulfone unit because the content of diphenylsulfone-centred units follows the random distribution.