Atom transfer radical reactions—encompassing atom transfer radical addition (ATRA)¹ and atom transfer radical polymerization (ATRP)²—provide a versatile route to synthesize functionalized small molecules and polymers. Using readily available unactivated alkyl chlorides³ instead of activated organohalides represents the state-of-the-art in this field, as this requires cleaving stronger C–Cl bonds in the substrates while preserving weaker ones in the products. Here we describe a dual-copper relay catalytic strategy to overcome this limitation. A dinuclear copper complex selectively activates the robust C–Cl bond of unactivated alkyl chlorides, while a mononuclear copper catalyst mediates controllable C–Cl bond reconstruction. The reaction pathway can be switched between ATRA and ATRP-type reactions in the presence or absence of the mononuclear copper partner, thus providing a switchable platform for chlorine atom transfer radical reactions. This protocol can not only enable synthesis of structurally diverse ATRA products and α-end-functionalized polyacrylates, but allow for direct modification of poly(vinyl chloride) (PVC) under mild reaction conditions. It demonstrates the successful utility of unactivated organochlorides in organic synthesis and polymer science.