A new copper catalyst containing chlorine and a photo-labile diethylthiocarbamoylthiyl group was successfully employed in the reverse ATRP of methyl methacrylate (MMA). The polymeric chains were end-capped wuth S2CNEt2, due to pseudo-halogen atom-transfer reaction between active and dormant species. Photopolymerization of this PMMA in the presence of fresh MMA and styrene monomers at ambient temperature yielded chain-extended PMMA and MMA/styrene block copolymers, respectively.