We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the-basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model.