The chemical composition of the materials was determined by combustion elemental analysis, and the observed amounts of carbon, hydrogen and sulfur are in good agreement with the theoretical values (Table ). The contents of boron, tin and palladium were determined via inductively coupled plasma optical emission spectroscopy (ICP-OES). The amounts of boron range between 1.50 and 1.59 wt% (inst. error ± 10%). These are slightly lower than the theoretical values (between 1.90 and 2.04 wt%) but are still commensurate considering the minuscule contribution of boron to the overall composition. Residual amounts of tin originating from unreacted end groups range between 0.01 and 0.03 wt%, corroborating the elemental analysis results and confirming a high coupling efficiency. As is common for polymer networks obtained via palladium-catalyzed cross-coupling reactions, we detected small amounts of immobilized palladium (between 1.1 and 1.5 wt%). Detailed ICP-OES results can be found in the supporting information (Table ). After thermogravimetric analysis (TGA) up to 1000 °C under oxygen atmosphere, we observe residual masses of around 6 wt% for each of the SBP samples (Figure ). After full oxidation of carbon, sulfur and hydrogen to volatile CO2, H2O, and SO3, we attribute this non-volatile residue to the expected mass of B2O3 including the residual mass of palladium (Table ).