We report the unusually low swelling capacity of highly stretched polystyrene chains grown on a silicon surface using "living" free radical polymerization. The control over the lateral density leads to brushes with a very high grafting density (up to > 1. 0 chain/nm(2)). Neutron reflectivity measurements have evidenced two specific features of such brushes: G) the controlled nature of the surface polymerization through a very homogeneous growth of homopolymer and copolymer chains; (ii) the highest volume fraction in good solvent &UPhi;(z=0) = 0.85 ever seen due to the highly stretched configuration of the dry chains, hindering a complete swelling of the layers and suggesting a significant deviation from predicted scaling laws.