Synthesis of Arsenocholine-Type Polycation Brush and Its Hydration Structure in Aqueous Solution

Arsenocholine-containing methacrylate (MTAsB) inspired by marine organisms was synthesized by the reaction of 2-bromoethyl methacrylate and trimethylarsine to investigate its polymerization behavior and the fundamental properties of the resulting polymer. Controlled radical polymerization of MTAsB proceeded in the presence of a copper catalyst and imidazolium chloride at 60 °C for 8 h to give a water-soluble polycation with a 94% yield. The smaller amount of nonfreezing water and intermediate water of poly(MTAsB) was observed compared with that of the ammonium-containing polycations. A poly(MTAsB) brush was also prepared on a silicon substrate to investigate its swelling structure in aqueous salt solution by scanning probe microscopy and neutron reflectivity measurements. The brush chains adopted a relatively extended conformation in pure water because of the Coulombic repulsion among the arsenic cation groups of the polyelectrolyte, while the brush formed a collapsed structure in aqueous solutions of Hofmeister series anions as a result of the screening effect by salt ions. In particular, thiocyanate ions induced a significant reduction in the swollen thickness of the brush, probably caused by the attractive interaction between arsenic cations and chaotropic thiocyanate ions. The salt concentration dependency of the poly(MTAsB) brush was similar to that of the ammonium-cation-type polyelectrolyte brushes.