A long-standing challenge in polymer chemistry has been to prepare synthetic polymers with not only well-defined molecular weight, but also precisely controlled microstructure in terms of the distribution of monomeric units along the chain. Here we describe a simple and scalable method that enables the synthesis of sequence-controlled multiblock copolymers with precisely defined high-order structures, covering a wide range of functional groups. We develop a one-pot, multistep sequential polymerization process with yields >99%, giving access to a wide range of such multifunctional multiblock copolymers. To illustrate the enormous potential of this approach, we describe the synthesis of a dodecablock copolymer, a functional hexablock copolymer and an icosablock (20 blocks) copolymer, which represents the largest number of blocks seen to date, all of very narrow molecular weight distribution for such complex structures. We believe this approach paves the way to the design and synthesis of a new generation of synthetic polymers.