Degradable and salt-responsive random copolymers

We report a class of degradable stimuli-responsive random copolymers that exhibit high sensitivity to ionic strength (salt concentration). Cationic random copolymers, poly(epsilon-caprolactone)-co-poly(epsilon-caprolactone-graft-quaternary ammonium) (PCL-co-P(CL-g-QA)), were synthesized by a combination of ring-opening polymerization and copper-catalyzed click chemistry. Random copolymers with various compositions of QA were prepared by adjusting the ratio of CL and substituted CL. Due to the presence of cationic QA groups at the polymer side chain, these random copolymers showed salt concentration (or ionic strength)-dependent solubility. In salt-free water or water with low ionic strength of salt (NaCl, CaCl2), random copolymers were soluble due to the overwhelming domination of electrostatic repulsive Coulomb interactions between cationic QA species over attractive hydrophobic interactions between CL segments. The solubility of copolymers decreased with the increase of ionic strength of salt solution due to the screening effect of free ions to reduce the repulsive Coulomb interactions between QA species and thus a significant change of macromolecular conformations. It was found that the salt responsiveness of synthesized random copolymers was maximized when the CL-g-QA fraction was similar to 15 mol% in the copolymers. These random copolymers were readily degradable under dilute acidic conditions.