We report the synthesis by RAFT polymerization of well-defined diblock copolymers bearing carbamoylmethylphosphonate moieties which proved to sorb gadolinium. These poly(diethyl-6-(acrylamido)hexylcarbamoylmethylphosphonate-b-acrylic acid) (P(CPAAm6C-b-AA)) copolymers were able to self-assemble as a function of temperature, above their cloud point value.