Side-chain micellization in the copolymers of sodium 2-(acrylamido)-2-methylpropane-sulfonate and methacrylates substituted with HO(CH2CH2O)(m)C12H25 (C12Em), where m = 2, 6, and 25, was investigated in 0.1 M NaCl aqueous solutions focusing on the effect of the ethylene oxide (EO) spacer length (m) on the apparent critical micelle concentration (cmc), aggregation number (N-agg) of the sidechain hydrophobes in polymer-hound micelles, hydrodynamic radius of polymer assemblies, steady-shear viscosity, and viscoelastic behavior. The cmc decreased with increasing m, an opposite trend to that of the corresponding free C12Em surfactant molecules, whereas N-agg decreased with increasing m, a similar trend to that of the free surfactant molecules. N-agg values for the copolymers with m = 6 and 25 were virtually the same as those for micelles formed from free C12E6 and C12E25 molecules, respectively. Steady-shear viscosity increased gradually at low or intermediate polymer concentrations (C-p) followed by a drastic increase at higher C-p. Viscosities for the copolymers with m = 25 are roughly 2 and 3 orders of magnitude higher than those of the copolymers with m = 6 and 2, respectively, at a given C-p. Dynamic viscoelastic measurements revealed that the number density of mechanically active chains for the copolymer with m = 25 is roughly 1 and 3 orders of magnitude larger than those for the copolymers with m = 6 and 2, respectively, at a given C-p, indicating that interpolymer side-chain micellization occurs more favorably for the copolymer with longer EO spacer length leading to a network structure of a higher cross-linking density. The rheological terminal relaxation times decreased with increasing m, indicating that the lifetime of the transient network is shorter for larger m.