# Effect of the Sulfonation on the Swollen State Morphology of Styrene Cross-Linked Polymers
###### Abstract  
The chemical structure and morphology of a set of sulfonic gel-type poly(styrene-divinylbenzene) resins (2 mol% DVB) prepared with different synthetic approaches were investigated by solid state NMR, Inverse Size Exclusion Chromatography (ISEC), FT-IR and elemental analysis to compare their swollen state structure. FT-IR and solid state NMR clearly show that the sulfonation mainly occurs in the para- position with respect the main polymer chain. Sensible proportions of sulfone bridges were found in the materials obtained with oleum and chlorosulfonic acid. With oleum, the presence of the sulfone bridges is clearly associated to a reduced ability to swell in the water medium relative to the proton exchange capacity. This highlights the cross-linking action of the sulfone bridges according to ISEC results, showing a high proportion of a dense polymer fraction in the swollen material. An even higher degree of sulfone-bridging, lower swelling ability, and a high proportion of a dense polymer fraction in the swollen material are found in the resin obtained with chlorosulfonic acid. As a matter of fact, Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (CP-MAS \({}^{13}\)C-NMR), elemental analysis, and ion exchange capacity, show that oleum and chlorosulfonic acid produced resins with remarkably smaller pores and lower swollen gel volume in polar solvents, with respect to concentrated sulfuric acid.  
s 2020  
one by [PERSON] [16]. However, they were both devoted to the formation of cross-links upon Friedel-Crafts reactions catalyzed by Lewis acids such as FeCl\({}_{3}\) and ZnCl\({}_{2}\). In one case [16], this was the unpredicted result of the chloromethylation of PS-DVB with (chloromethyl)methylether during the production of anion-exchange resins. Solid state \({}^{13}\)C-NMR allowed to characterize the parent resin, all the intermediate products and the final ion-exchanger. The degree of post cross-linking (methylene bridges between phenyl ring pairs) was quantified in 50-60%. As solid state \({}^{13}\)C-NMR still represents a state-of-the-art technique for the characterization of polymers [17] in the solid state, we decided to take a similar approach to address the problem of sulfone bridges formation during PS-DVB sulfonation with different agents. In this way, a semi-quantitative evaluation of the content of sulfone bridges was achieved. Moreover, it is well known that changes in the cross-linking degree of resins affects their swelling behaviour (as a rule of thumb, the higher the former, the smaller the latter). Therefore, we also investigated on the effect of the additional sulfone-bridge cross-links on the swollen state morphology of the ion-exchangers. For this purpose, the swollen sulfonated resins were characterized by ISEC in order to disclose differences in the porosity of ion-exchangers prepared with different sulfonating agents. All these pieces of information might be useful for better controlling the production of sulfonated ion-exchange resins and tailoring their morphology to specific practical applications under solid-liquid condition processes.