Herein we report on the solvent-free free radical grafting of styrene homo- and copolymers onto functionalized graphene nanosheets without requiring initiator addition. The key intermediate is an organophilic stearylamine-modified graphite oxide (Stearyl-GO), containing both olefinic unsaturation and stable graphene radicals. According to the online tracking of the C-centered graphene radicals by time-resolved EPR spectroscopy, the graphene radical concentration markedly increases during the early stage of polymerization and is depleted as the viscosity builds up with progressing styrene conversion. "Grafting-to" by addition of polystyrene radicals to graphene and "grafting-from" by graphene-initiated free radical polymerization afford high graft yields, as determined by ultracentrifugal separation of polystyrene-grafted functionalized graphene (PS-g-FG) from the styrene homopolymer byproduct. The addition of TEMPO enables control of the further grafting yields by a controlled radical "grafting-from" polymerization. In sharp contrast to graphene/polystyrene solution blends (PS/Stearyl-GO), morphological and rheological investigations reveal that the in situ formation of PS-g-FG accounts for much more uniform graphene dispersion in polystyrene melts and also in various organic solvents. This is paralleled by enhanced elasticity of the graphene particle network and markedly improved electrical conductivity of the resulting graphene/polystyrene nanocomposites. Owing to the effective polymer radical addition to FG, both Stearyl-GO and PS-g-FG significantly improve the thermal stability of polystyrene, as measured by thermogravimetric analysis of thermal decomposition during prolonged heating at 250 degrees C. This versatile in situ self-initiated polymerization process enables grafting of styrene copolymers with butyl acrylate and acrylonitrile.