Active diazonium groups are attached onto surfaces via oxidative grafting of the 4-phenylacetic diazonium salt. The fraction of the anchored aryldiazonium accessible to (electro)chemical transformation is determined by electrochemical interrogation. Because enough sites are available to react with radical traps, the ability of the aryldiazonium-anchored surfaces to induce radical surface chemistry is tested. Particularly, a new route to grow polymer brushes from surfaces under ATRP conditions is presented. Such an example of a controlled coupling reaction demonstrates how aryldiazoniums, which are irreversible sources of Ar-center dot radicals, can sustain long-term radical cross-coupling reactions. Such a strategy of generating dormant radical sources from diazonium precursors is fruitful to the simple, versatile, and sustainable chemical decoration of materials.