The development of well-control led synthetic schemes for growing polymer brushes directly from Surfaces has recently enabled very high polymer grafting densities to be explored. Here, we report on the structure of polystyrene brushes as a function of molecular weight. The brushes were prepared by atom transfer radical polymerization (ATRP) at a high grafting, density of 0.44 chains/nm(2). The dry Film thickness scaled linearly with chain molecular weight. Under good solvent conditions, strongly stretched brushes Of moderate molecular weight were found to maintain a parabolic density distribution consistent with theoretical predictions, anomalous behavior was observed for higher molecular weights, Suggesting that entanglements are much more pronounced in "grafted from" systems.