We confirmed the successful formation of SHCPs using C cross-polarisation/magic angle spinning solid-state NMR (CP/MAS ssNMR) (Figure ). Signals at ~38 ppm are assigned to methylene bridges from newly formed crosslinks in all cases. A weak signal assigned to C-Cl at ~42 ppm can be seen in SHCP-1 and decreases in intensity through to SHCP-3, indicative of incomplete crosslinking due to low concentrations of chlorosulfonic acid. A signal at ~120 ppm is found in the shoulder of a larger peak in samples SHCP-5 -SHCP-10, assigned to the C-S bond formed during sulfonation. At lower ratios of catalyst to monomer this peak can no longer be seen due to reduced degrees of sulfonation. All SHCPs showed strong signals at ~129 and ~139 ppm, corresponding to aromatic (CAr-H) and quaternary, or substituted, aromatic carbons (CAr-R), respectively. Although difficult to quantify due to overlapping peaks, it is clear that the ratio between substituted and unsubstituted aromatic peaks changes across the polymer set.