Counteranion effect on the RAFT polymerization of methacrylate-g-pentaarginine and its UCST-type thermoresponsive behavior

• The synthesis conditions of methacrylate- g -pentaarginine have been optimized. • The nature of the counteranion effects the polymerization of methacrylate- g -pentaarginine. • The bridging effect of citrate ions with the polymer is defying its expected kosmotropic effect. Poly(methacrylate- g -pentaarginine) prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization exhibits an upper critical solution temperature (UCST) behavior with a sharp transition in pure water that can be affected by the presence of salts. In this work after optimizing the conditions for the synthesis of methacylate- g -pentaarginine (MA–hR5) and its RAFT polymerization, the effect of kosmotropic anions on the polymerization and the thermoresponsive behavior of these polymers in aqueous solutions was investigated experimentally and computationally. The use of chloride, phosphate, and sulfate anions lowered the phase transition temperature, while citrate ones increased it. The latter phenomenon was attributed to the ability of citrate anions to form bridges with the guanidinium groups present on the polymers according the protonation state of the citrate anions.