In the end, we monitored the change in paramagnetic behavior of prepared polymers upon protonation using electron paramagnetic resonance (EPR) spectroscopy (Figure , S47). All pristine materials show the presence of paramagnetic species typical for polymeric networks with extended π-conjugation. The EPR signals are centered around g-values ranging from 2.0035 to 2.0045 (linewidth of about 0.47 to 1.26 mT, Table ) indicating the contribution of heteroatoms to overall charge localization. After exposure to HCl vapors all SNPs show a decrease in EPR signal intensity with the exception of SNP-NDT1 -a polymer that is made up of directly coupled, strongly interacting D-A domains. This supports the hypothesis that charge localization is enhanced by stronger donor-acceptor interactions (Figure ). Again, the homocoupled, sulfur-containing SP-BTT network is the outlier: there is a marked increase in EPR signal intensity after protonation (Figure ). Quantitative analysis of the EPR spectra of SP-BTT shows a 2-times increase of radical concentration from (7±0.2)*10 13 radicals mg -1 (0.12±0.02 nmol mg -1 ) to (1.4±0.2)*10 14 radicals mg -1 (0.23±0.03 nmol mg -1 ) in the protonated state. In the light of the I-V conductivity results, this suggests that protonation leads to an increased delocalization of paramagnetic species and that these paramagnetic species could be contributing to the conductivity of HCl-treated SP-BTT. and the protonated (orange) state.