Sequence-controlled polymers are macromolecules in which monomer units of different chemical nature are arranged in an ordered fashion. The most prominent examples are biological and have been studied and used primarily by molecular biologists and biochemists. However, recent progress in protein- and DNA-based nanotechnologies has shown the relevance of sequence-controlled polymers to nonbiological applications, including data storage, nanoelectronics, and catalysis. In addition, synthetic polymer chemistry has provided interesting routes for preparing nonnatural sequence-controlled polymers. Although these synthetic macromolecules do not yet compare in functional scope with their natural counterparts, they open up opportunities for controlling the structure, self-assembly, and macroscopic properties of polymer materials.

Introduction 4964 2. Design of the Initiating Systems 4965 2.1. Required Initiating Systems 4965 2.1.1. Synthesis of Controlled Polymers Free from Catalyst Residues 4966 2.1.2. Environmentally Friendly and Inexpensive Catalysts 4966 2.1.3. Suppression of Side Reactions for High Molecular Weight Polymers and Perfect Block Copolymerization 4966

Fundamental polymer science is undergoing a profound transformation. As a result of recent progress in macromolecular chemistry and physics, synthetic polymer chains are becoming much more than just the modest building blocks of traditional 'plastics'. Promising options for controlling the primary and secondary structures of synthetic polymers have been proposed and, therefore, similarly to biopolymers, synthetic macromolecules may now be exploited as discrete objects with carefully engineered structures and functions. Although it is not possible today to reach the high level of complexity found in biomaterials, these new chemical possibilities open interesting avenues for applications in microelectronics, photovoltaics, catalysis and biotechnology. Here, we describe in detail these recent advances in macromolecular science and emphasize the possible emergence of technologies based on single-chain devices.

In the past decade, living radical polymerization has provided one of the most versatile methods to precisely construct designed polymer architectures with complexity and polar functionality. This process takes advantage of carbon-radical intermediates, which tolerate a variety of functional groups in monomers and reaction media. "Transition metal-catalyzed living radical polymerization", one of these living systems, has widely been employed for precision polymer synthesis. Not only can this process produce well-defined functional polymers, but it can also generate hybrids or conjugates with other (often biological) materials. Metal-catalyzed systems retain the advantages of conventional radical polymerization but distinguish themselves through a catalytic reversible halogen exchange equilibrium: the growing radical exists alongside a dormant speciesa covalent precursor capped with a terminal halogen from an initiator. The catalyst dictates the selectivity, exchange rate, and control over the polymerization. This Account provides an updated overview of our group's efforts in transition metal-catalyzed living radical polymerization with specific emphasis on the design of metal catalysts and the resulting precision polymer syntheses. With increasing use of the living processes as convenient tools for materials synthesis, researchers are currently seeking more active and versatile metal catalysts that are tolerant to functional groups. Such catalysts would enable a wider range of applications and target products, would have low metal content, could be readily removed from products, and would allow recycling. Since we first developed the "transition metal-catalyzed living radical polymerization" with RuCl 2(PPh 3) 3, FeCl 2(PPh 3) 2, and NiBr 2(PPh 3) 2, we have strived to systematically design metal catalysts to meet these new demands. For example, we have enhanced catalytic activity and control through several modifications: electron-donating or resonance-enhancing groups, moderate bulkiness, heterochelation via a ligand, and halogen-donor additives. For some catalysts, the use of amphiphilic and polymeric ligands allow efficient recovery of catalysts and convenient use in aqueous media. We have also used ligand design (phosphines) and other methods to improve the thermal stability of iron- and nickel-based catalysts and their tolerance to functional groups.