This Thesis tackles the synthesis of a functional viscosity modifier made from loosely cross-linked poly(acrylic acid), utilising a CO2 solvent based approach, whilst developing understanding of the composition of the polymers created. Chapter 1 gives an extensive introduction to the topic, and details the polymerisation as well as the importance of the cross-linked network, and why CO2 should be used for the synthesis. Chapter 2 describes the high pressure equipment used within the Thesis, and includes details on the analytical techniques employed. Chapter 3 targets the development of the polymerisation of poly(acrylic acid) in CO2, and attempts to understand the how the conditions affect the cross-link density of the final product. Techniques to measure thecross-link density of the polymer are also used and critiqued. Knowing the effect of the conditions on the cross-link density, Chapter 4 investigates the optimisation of the cross-linker and initiator concentrations in the development of a high swelling polymer. A novel incorporation of salt acrylates is attempted to further the optimisation. The Chapter also demonstrates the possibility of purifying the product with CO2. A series of optimised polymers are then investigated with a range of rheological experiments in Chapter 5. Specific studies are focused on the viscosity of gels produced with the CO2 synthesised poly(acrylic acid) powders in comparison with a commercially used Carbomer made in benzene. Finally, oscillatory experiments are used to determine the structural aspects of the gels against the commercial competitor.