Increasing amounts of chlorosulfonic acid in the initial formulation appear to increase the ratio of substituted to unsubstituted aromatic carbon. Considering the increase in C-S peak as well as the disappearance of the C-Cl peak with higher initial concentrations of chlorosulfonic acid, this is in line with expectation. We also confirmed successful polymer formation and sulfonation using Fourier-transform infrared spectroscopy (Figure ). There, the presence of bands assigned to both SO3H and C-S bonds emerged with increasing ratios of catalyst to monomer, confirming increasing sulfonation. The diminishing of the C-Cl band across the series was also observed, in good agreement with ssNMR. We used X-ray photoelectron spectroscopy (XPS) to gain a more in-depth understanding of SHCPs' chemical compositions (XPS derived quantitative data for all polymers is provided in Table ). The main component of the high-resolution C 1s spectra observed at a binding energy of 284.8 eV is attributed to C-C bonding, encompassing both sp2 aromatic carbon and sp3 carbon in methylene crosslinks (SHCP-10 shown in Figure ). A peak of lower intensity is observed at 286.8 eV, corresponding to C-S. A broad, low intensity π-π* shake-up feature is also observed at 291 eV. High resolution S 2p spectra showed a typical asymmetrical peak for a sulfonic acid moiety (SHCP-10 shown in Figure ). The peaks at binding energies of 168.5 eV and 169.5 eV are assigned to S 2p3/2 and 2p1/2, respectively. This peak is present throughout all sulfonated networks, confirming the vast majority of the sulfur present exists in sulfonic acid groups. Further evidence for this is provided by a consistent ratio of 1:3 for S to O across all samples, as would be expected for sulfonic acid groups. Increasing catalyst to monomer ratios led to higher concentrations of sulfonic acid sites in the networks, reaching a maximum sulfur content of 4.6 mmol•g -1 in SHCP-10, as determined by XPS. This is amongst the highest degrees of sulfonation reported for hypercrosslinked polymers, with another similarly high example reported for carbazole based networks (3.7 mmol/g, as determined by elemental analysis). We hypothesize that polymerisation occurs more rapidly than sulfonation, leading to the uniform distribution of chlorosulfonic acid throughout the HCP structure. This dispersion facilitates excellent acid densities throughout networks upon sulfonation, as it eliminates the need for the sulfonation agent to permeate into a pre-formed structure.