Mechanistic insights into the novel glucose-sensitive behavior of P(NIPAM- co -2-AAPBA)

A glucose-sensitive polymer, poly( N -isopropylacrylamide- co -2-acrylamidophenylboronic acid) (P(NIPAM- co -2-AAPBA)), was synthesized by reversible addition fragmentation chain transfer (RAFT) copolymerization. Addition of glucose results in reduced solubility and hence increased turbidity, rather than the normal increase in solubility (decreased turbidity) observed for other PBA-based glucose-sensitive polymers. The novel glucose-sensitive behavior is explained by a new mechanism, in which glucose acts as an additive and depresses the lower critical solution temperature (LCST) of the polymer, instead of increasing solubility by increasing the degree of ionization of the PBA groups. Experimental and theoretic analysis for the influence of glucose on the thermal behavior of P(NIPAM- co -2-AAPBA) reveals that glucose depresses the LCST of P(NIPAM- co -2- AAPBA) copolymers in a two-stage manner, a fast decrease at low glucose concentrations followed by a slow decrease at high glucose concentrations. For low glucose concentrations, the binding of glucose with PBA groups on the polymer chain increases the number of glucose molecules proximal to the polymer which influences the thermal behavior of the polymer, causing a rapid decrease in LCST. Importantly, the transition occurs at a glucose concentration equal to the reciprocal of the binding constant between PBA and glucose, thus providing a novel method to determine the binding constant. Other saccharides, including mannose, galactose and fructose, also depress the LCST of P(NIPAM- co -2-AAPBA) copolymer in the same way.