An in situ composite synthesis technique has been developed by grafting polystyrene chains onto single-wall carbon nanotubes (SWNTs) via a single-step debundling/polymerization scheme. The method, based on established anionic polymerization techniques, eliminates the need for nanotube pretreatment prior to functionalization and allows attachment of polymer molecules to pristine tubes without altering their original structure. The composites obtained contain well-dispersed SWNTs with good nanotube−matrix interaction. The scheme is quite general in nature and can be applied to different polymer systems. The simplicity and scalability of the process can lead to the realization of superior nanotube-based polymer composites for applications as advanced multifunctional structural materials.