The self-assembly in toulene of linear polystyrene functionalized in alpha-position by benzoic acid moieties into active micelle-like structures was exploited to trap trimethylaluminium and thus generate encapsulated methylaluminoxane-like species. These nanimicellar reactors were subsequently used as such or in the presence of free benzoic acid for supporting the tridentate bis(imino)pyridinyl iron catalyst for ethylene polymerization. In this way, high catalytic activities and polyethylenes (PEs) with unimodal chain distribution and controlled morphology Were obtained. Remarkably well-defined spherical PE beads with an average diameter of 2 mm were produced when free benzoic acid was added to the micellar polymeric supports.