Cast polyurethane thermosets have been modified with randomly oriented exfoliated and intercalated montmorillonite clay usinghyperbranched polymers (HBP) as reactive additives to promote dispersion of the clay platelets. The influence of the clay content on thestiffness of the nanocomposites is discussed in terms of simplified classical micromechanical models that allow correlations to be establishedbetween the properties of the final polyurethane and the limiting high strain rate shear viscosity of the HBP or HBP/polyethylene glycolnanocomposite precursors. Such models imply certain restrictions on the potential for mechanical reinforcement with unoriented plateletsowing to crowding effects. Moreover, they appear unable to account simultaneously for the observed degrees of reinforcement in the glassyand the rubbery states, if currently accepted values for the stiffness of the MMT are assumed. Possible reasons for this are discussed.