Fourier-transform infrared (FTIR) spectra show the presence of characteristic absorption peaks of the triarylborane building block in all SBP samples, and hence, they hint at successful copolymerization (Error! Reference source not found.A). Additional peaks are assigned to the respective thiophene building block (Table -S5). 1 H- C solid-state cross-polarization magic angle spinning nuclear magnetic resonance ( 1 H- C CP-MAS NMR) spectra show signals between 108 and 155 ppm assigned to the arylic subunits of the triarylborane as well as thiophene-based building blocks (Error! Reference source not found.B). A characteristic, strong signal at 21-22 ppm originates from the methyl groups shielding the central boron atom. The 11 B nucleus has a half-integer spin of 3/2 and a quadrupole moment that gives rise to an unusual peak shape in solid-state NMR spectra (Figure ). We observe this peak in the Therefore, all bandgaps are in the range suitable for visible-light photocatalytic proton reduction. Photocatalytic hydrogen evolution reactions were carried out using incident light above 395 nm, and the amount of evolved hydrogen was determined by injection of headspace samples into a gas chromatograph equipped with a thermal conductivity detector. To obtain sufficient dispersion of the hydrophobic SBPs in aqueous medium, the catalytic tests were performed in a solvent mixture comprising equal volumes of water and acetonitrile. To evaluate the catalytic activities, we screened different reaction parameters such as catalyst loading and type of sacrificial agents, and we found that both factors had a strong influence on the catalytic activity. The lowest tested concentration of 1 mg of catalyst per 16 mL of solvent gave the highest activity per mass of catalyst (Figure ). Even at such low loadings, we observed vigorous formation of hydrogen bubbles (see video in the supporting information). It has been noted previously that low catalyst concentrations can result in exceptionally high catalytic activity. However, despite the low catalyst loadings tested here, we observe smooth hydrogen evolution curves and good reproducibility. L-ascorbic acid, triethanolamine, and triethylamine were evaluated as sacrificial electron donors (Figure ). Similar to other recently reported microporous polymer photocatalysts, L-ascorbic acid yielded the highest activities by far. Using L-ascorbic acid as the sacrificial agent, TT-TAB, BDT-TAB, and BT-TAB showed hydrogen evolution rates of 113.9, 87.7, and 74.2 mmol h -1 g -1 , respectively, under addition of 3 wt% of platinum and averaged over five hours (Error! Reference source not found.B). No hydrogen was detected when experiments were performed without the organic catalyst, water, or sacrificial electron donor.