Earlier experimental studies on the emulsion polymerized methacrylic acid/ethyl acrylate co-polymer indicated that the ethyl acrylate (EA) segments are sufficiently blocky to induce hydrophobic association between the EA blocks once the methacylic acid groups are neutralized by a base. Detailed rheological characterization on semi-dilute solutions suggests that the viscoelastic property is caused by the transient network assembled through hydrophobic associations, rather than by physical chain entanglements. In the semi-dilute solution regime, the hydrophobically modified associative polymer exhibits higher viscosities when compared to the unmodified analogue. This is due to the formation of higher proportion of intermolecular association between the polymer clusters, which enhances the hydrophobic interaction between the hydrophobic macromonomers of different polymer chains. A mechanistic model is proposed to describe the nature of associations between the blocky EA and the hydrophobic segments of the polymer.