The synthesis of well-defined high molecular weight block copolymers by sequential in situ chain extensions via Cu(0)-mediated living radical polymerization is reported. Optimal conditions for iterative high molecular weight block formation were determined using model homopolymer quasiblock systems, including methyl acrylate (MA), ethyl acrylate (EA), and n-butyl acrylate (nBA; each block DPn ? 100). The PDI after each chain extension was below 1.2, with good agreement between theoretical and experimental molecular weights, while the conversion of monomer incorporation into each distinct block was 95-100 (up to 6 blocks). To demonstrate this approach for true block copolymer materials, well-defined block polymers containing MA, ethylene glycol methyl ether acrylate (EGMEA), and tert-butyl acrylate (tBA) were prepared in high purity: diblock P(MA-b-EGMEA) and triblock P(MA-b-tBA-b-MA). These were prepared in high yields, on multigram scales, and with purification only required at the final step. To the best of our knowledge, this is the first time that high molecular weight block copolymers have been reported using this novel technique. ? 2013 American Chemical Society.