Sulfonation of poly(ether ether ketone): An evidence for di-substitution of the repeat unit

Sulfonation of poly(ether ether ketone) (PEEK) is an electrophilic substitution reaction that takes place on the aromatic ring flanked by the two ether groups. The degree of sulfonation (DS) reached up to 100% with only one sulfone group is attached to the activated aromatic rings. In the present work, a spectroscopic evidence for the attachment of a second sulfone group on the para position rather the meta position with respect to the first sulfone group in the activated ring is provided. With the help of 1H-, 13C- nuclear magnetic resonance (NMR) and two-dimensional correlation (2D-COS) spectroscopies, it was possible to show that the intensity of the proton in the para position with respect to the newly introduced sulfone group changed before the proton in the meta position with increasing DS. Further evidence was obtained from the analysis of 2D-NMR spectra namely; 1H-1H correlation spectroscopy (1H-1H-COSY), 1H-13C heteronuclear single quantum coherence spectroscopy (1H-13C-HSQC) and 1H-13C heteronuclear multiple bond correlation spectroscopy (1H-13C-HMBC) of highly sulfonated PEEK by the disappearance of proton 12` and the confirmation of the presence of other protons, i.e. 10` and 13`. 2D-COS is a powerful tool to analyze NMR spectra and extract information related to the structure of final products of complicated reactions such as sulfonation of polymers.