Poly(phenylacetylene) with rationally designed urea groups was demonstrated to be a superior anion receptor possessing both a calorimetric response ability and a positive homotropic allosteric binding mode. The target polymer, poly(phenylacetylene) with [bis(trifluoromethyl)phenyflurea pendants (poly-1), was synthesized by the stereoselective polymerization with Rh(nbd)BPh4 (nbd = norbornadiene) in high yield. The anion sensing ability of poly-1 was evaluated using the tetra-n-butylammonium (rBA) salts of a series of anions in TH F. Upon the addition of anions such as CH3CO2-, C6H5CO2-, F-, Cl-, Br-, NO3-, N-3(-), and HSO4-, the yellow color of the THF solution of poly-1 immediately turned to a different color depending on the types of anions, indicating the anion recognition capability of poly-1. The 111 NM R Otrations of poly-1 by using increasing amounts of CH3CO2- revealed that the colorimetric response of poly] was considered to be the direct consequence of the hydrogen-bonding complex formation between the urea functionality and the anions. Interestingly, during the course of the determination of the anion-binding affinity, we encountered highly cooperative binding ability of poly-1. The Hill plot analysis provided an exceptional positive cooperativity, e.g., 8.4 of Hill coefficient in the complexation between poly-I and C6H5CO2-. This result clearly indicated that this binding mode was based on a positive homotropic altosterism, in which the partially formed urea/anion complex units in the polymer chain should produce a change in the whole main chain conformation, facilitating further anion binding. Additionally, by comparing the anion binding of poly-I with that of previously prepared urea-functionalized poly(phenylacetylene) (poly-2), an enhanced anion binding ability based on two electron-withdrawing -CF3 groups was found to be essential to realize such cooperativity.