We studied a simultaneous living anionic copolymerization of styrene (S) and isoprene (1) monomers in a dilute solution with deuterated benzene as a solvent by means of combined time-resolved measurements of small-angle neutron scattering (SANS), size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), and ultraviolet-visible spectroscopy (UV-vis). We observed structural change of the living chains during the polymerization process in three different length scales on the same solution and in a single batch, which enabled us to explore simultaneously the time changes in the local structure (living chain ends), the primary Structure (propagating chains), and the higher order structure (the star-like local aggregates of the living chains). We found the copolymerization process is divided into two time regions, defined by regions I and II. In region I, the copolymerization of S and I monomers occurred and all I monomers were consumed at the end of region I, while in region II. pure polystyrene (PS) block chains are formed. In the beginning of region II (region IIa), living polymers having isoprenyl anion (PI-) started to change rapidly into those having styryl anion (PS-), and followed by a slow change from PI- to PS- in the latter part of region II (region IIIb). As a consequence of the polymerization of S monomers under the coexistence of PI- and PS-, we found the following two phenomena: (i) an increase in M-w/M-n (ii) the effective conversion rate of S monomers being slow compared with the corresponding homopolymerization of S monomers. The theoretical analysis of the time-resolved SANS profiles elucidated the time change in the association number of the living chain ends n: n similar to 4 in region I, it decreases from 4 to 2 in region IIa and n similar to 2 in region IIb.