Poly(2-ureidoethylmethacrylate) (PUEMn) was synthesized via reversible addition-fragmentation chain transfer (RAFT) radical polymerization and following polymer reaction. We prepared two PUEMn samples with different degrees of polymerization (n=100 and 49). The polymers exhibited upper critical solution temperature (UCST) in phosphate-buffered saline (PBS) solution. The phase separation temperature (T-p) in PBS can be controlled ranging from 17 to 55 degrees C by changing molecular weight of the polymer, polymer concentration, and adding NaCl concentration. The polymers in PBS formed coacervate drops by liquid-liquid phase separations below T-p. Results of the dielectric relaxation measurement, the hydration number per monomeric unit was 5 above T-p. Based on a fluorescence study, the polymer formed slightly hydrophobic environments below T-p. The liquid-liquid phase separation was occurred presumably because of weak hydrophobic interactions and intermolecularly hydrogen bonding interactions between the pendant ureido groups. (C) 2016 Wiley Periodicals, Inc.