Macroscopically, homopolymer 2 and block copolymers 3−7were soluble in water only when the pH of the medium was 9−11. However, upon addition of fructose (0.2 M) and glucose(0.5 M), the polymers became completely soluble in neutralphosphate buffer (pH 7.4), indicating the sugar-responsivesolubility change of the PBOx domain triggered by complex-ation between the boroxoles and monosaccharides (Figure 1).To chracterize this binding, we performed Wang’s competitivebinding assay to quantitate the binding of PBOx tomonosaccharides (for details, see the SI).19The absorption at452 nm of the initial PBOx/Alizarin red S (ARS) complex([boroxole]:[ARS] = 250:1) in a 9:1 (v/v) phosphate buffer/dioxane mixture (pH 7.4) shifted to 520 nm upon addition offructose (0.5 M) and glucose (0.5 M), indicating thereplacement of boroxole-bound ARS molecules with mono-saccharides (Figures S6 and S7). The association constant KaofPBOx was assessed by measuring the decrease in fluorescenceemission of the PBOx/ARS complex caused by the replacementof ARS molecules bound to PBOx by monosaccharides. Themeasured Kafor homopolymer 2 was 643.3 M−1for fructoseand 14.5 M−1for glucose; for the representative blockcopolymer 7, Ka= 420.1 M−1for fructose and 9.9 M−1forglucose. These Kavalues are comparable to the results reportedfor benzoboroxole- and boroxole-functionalized polymers,17,18indicating that boroxoles incoporated into the polymericbackbone exhibit binding to monosaccharides comparable tothat of phenylboroxole