We report the fabrication of shortened electrospun polymer fibers with a well-defined concentrated polymer brush. We first prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate, with number-average molecular weight M-n = 105 200 and weight-average molecular weight M-w = 296 700 (M-w/M-n = 2.82). The fibers had a diameter of 593 +/- 74 nm and contained initiating sites for surface-initiated atom transfer radical polymerization (SI-ATRP). Then, SI-ATRP of hydrophilic styrene sodium sulfonate (SSNa) was carried out in the presence of a free initiator and the hydrophobic fibers. Gel permeation chromatography confirmed that M-n and M-w/M-n values were almost the same for free polymers and graft polymers. M-n agreed well with the theoretical prediction, and M-w/M-n was relatively low (<1.3) in all the examined cases, indicating that this polymerization proceeded in a living manner. Using the values of the graft amount measured by Fourier transform infrared spectroscopy, the surface area, and M-n, we calculated the graft density as sigma 0.22 chains nm(-2). This value was nearly equal to the density obtained on silicon wafers (sigma = 0.24 chains nm(-2)), which is categorized into the concentrated brush regime. Finally, we mechanically cut the fibers with a concentrated poly(SSNa) brush by a homogenizer. With increasing cutting time, the fiber length became shorter and more homogenous (11 +/- 17 mu m after 3 h). The shortened fibers exhibited excellent water dispersibility owing to the hydrophilic poly(SSNa) brush layer.