The synthesis of poly(acrylic acid-co-acrylamide) was investigated to enhance its rheological properties. Syntheses were conducted in both aqueous and supercritical fluid media, with and without the incorporation of a novel star-shaped macromonomer. The macromonomer, synthesized from a Boltorn H30 core with PEGMA⁵⁰⁰ arms and modified to contain a single vinyl group, was copolymerized with acrylic acid and acrylamide. Comprehensive polymer characterization was performed using FTIR, NMR, and SEC-MALS-dRI techniques. Rheological assessments revealed that copolymers containing the star-shaped monomer exhibited significantly higher viscosities than those lacking the hyperbranched component, a result attributed to the inter- and intrachain interactions facilitated by the PEGMA⁵⁰⁰ arms. Additionally, purification studies demonstrated that dialysis was necessary to remove short-chain polymers, particularly for samples synthesized in supercritical media, to achieve optimal rheological performance. Polymers synthesized in a supercritical CO₂–ethyl acetate mixture exhibited higher viscosities compared to their water-synthesized counterparts. The integration of the novel star-shaped macromonomer into HPAM-like polymers offers substantial potential for enhanced oil recovery applications.