This study reports on the time-resolved polymer propagation of thermal-sensitive latex nanoparticles containing Fe3O4 cores. The latex shells are made with poly(N-isopropylacrylamide-co-acrylic acid) (Fe3O4/P(NIPAAm-co-AAc)) at different reaction times. The Fe3O4 particles are first modified using AAc monomers. The AAc-modified Fe3O4 cores are then copolymerized with NIPAAm to form the latex shell. The Fe3O4 cores in the latex nanoparticles are confirmed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), and thermo gravimetric analyzer (TGA). As the reaction time is increased from 0.5 h to 2 h, the particle size enlarges from 100 to 250 nm and the Fe3O4 content decreases from 46.4% to 2.6%. The thermal responses are more pronounced in the 2 h sample with the phase transition temperature (lower critical solution temperature, LCST) about 35 degrees C. The nanoparticles show a gradient concentration distribution of AAc as the particles propagate. A higher AAc concentration is observed near'the Fe3O4 core and the AAc content deceases as the degree of polymerization increases in the latex particles. This declining AAc concentration is beneficial for profound thermal responses in the synthesized nanoparticle.