A series of chains of triblock amphiphilic copolymers were synthesized by atom transfer radical polymerization (ATRP) techniques and post modified to polymeric dispersant for waterborne paint. Poly(butyl acrylate (BA))-b-poly(hydroxy ethyl methacrylate (HEMA))-b-poly(methyl methacrylate (MMA)) triblock copolymers having predetermined molecular weights were synthesized by ATRP using CuBr, 2-bromoisobutyrate, and pentamethyldiethylenetriamine as a catalytic system in dioxane at 80 °C. The copolymers were further reacted with cyclic chlorophosphate and triethyl amine to form dispersible modified poly(BA-HEMA-MMA). The synthesized copolymers were structurally evaluated by Fourier transform infrared, ¹H NMR, ³¹P NMR, gel permeation chromatography (GPC), energy dispersive X-ray spectroscopy, and their hydroxyl equivalent, respectively. The surface activity of modified copolymers as dispersing additives was investigated by the surface tension analysis and wetting ability test. The ability of additives to function as wetting and dispersing agents was evaluated by analyzing their mechanical, optical, chemical, and rheological properties of water-based paints at different pigment volume concentrations. The effects of the chain length of copolymers on dispersibility and optical properties were studied. The optical properties of paints suggested that the dispersibility of modified poly (BA)-b-poly (HEMA)-b-poly (MMA) (MPBHM) increased with an increase in the molecular weight of the copolymer.